Ultrabroadband infrared solid-state lasers - IEEE Xplore

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IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 3, MAY/JUNE 2005

Ultrabroadband Infrared Solid-State Lasers Evgeni Sorokin, Sergey Naumov, and Irina T. Sorokina (Invited Paper)

Abstract—Ultrabroadband infrared transition metal ion-doped solid-state lasers have come of age and are increasingly being used in trace gas monitoring, remote sensing, telecommunications, ophthalmology, and neurosurgery. Operating at room temperature, they are stable, versatile, and easy to handle successors to the color center lasers. They are becoming the critical components in optical frequency standards, space-based remote sensing systems, and may soon find application in femtochemistry and attosecond science. The article reviews the principles and basic physics of these types of lasers, which are distinguished by their ability to support the shortest pulses down to single optical cycle durations and the ultimately broad tuning ranges. The paper further reviews the state of the art in the existing diode-pumped sources of broadly tunable continuous wave, and ultrashort pulsed radiation in the infrared, and provides examples of their successful application to supercontinuum generation, trace gas measurements, and ultrasensitive intracavity spectroscopy. Developments in such lasers as Cr:YAG, Cr:ZnSe, Cr:ZnS, as well as the recently proposed mixed Cr:ZnSx Se1−x laser, are discussed in more detail. These lasers nearly continuously cover the infrared spectral region between 1.3 and 3.1 µm. The gain spectra of these lasers perfectly match and extend toward the infrared spectra of such established ultrabroadband lasers, operating at shorter wavelengths between ∼0.7–1.3 µm, as Ti:sapphire, Cr:LiSAF/Cr:LiSGaF and Cr:forsterite. This opens up new opportunities for synthesis of single-cycle optical pulses and frequency combs in the infrared. Index Terms—Laser, mid-infrared, near-infrared, tunable.

I. INTRODUCTION EMARKABLE progress has been achieved in the last decade in ultrabroadband transition metal doped crystalline continuous wave (CW) and ultrashort pulse lasers, operating between 1–3 µm. The driving force for this progress was rapidly developing application fields in environmental, engineering, medical, biological, and chemical sciences, and the strong demand for compact and versatile sources in telecommunications (around 1.3 and 1.5 µm) and molecular fingerprint (roughly between 2–5 µm) wavelength regions. Going to the infrared region also brings an important advantage of rapidly decreasing Raleigh scattering losses in propagation, which is important in telecommunications and for imaging techniques in turbid media such as optical coherence tomography. Infrared ultrabroadband lasers are becoming critical components in op-

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Manuscript received December 3, 2004; revised April 19, 2005. This work was supported by the Austrian Science Fund (projects P14704-TPH and F-016), by the Austrian Ministry of Science, by the Austrian-French exchange program Amadeus, and by the Austrian-Italian exchange program. The authors are with the Institut für Photonik, Vienna University of Technology, A-1040 Vienna, Austria (e-mail: [email protected]). Digital Object Identifier 10.1109/JSTQE.2005.850255

tical frequency standards, space-based remote sensing systems, femtochemistry, and may even find application in X ray generation and attosecond science. Indeed, the cutoff frequency of the high harmonic generation scales as the inverse square of laser frequency hνmax = Ip +3.17E 2 /4ω 2 [1]. Using the infrared source allows reaching the required XUV frequency with significantly lower intensity sources, as compared to currently available Ti:Sapphire-based systems, making the high harmonic generation closer to real life applications. At the same time, there exist established technologies for generating tunable radiation and even few-cycle pulses in the mid-infrared, using the frequency conversion techniques [2]. To compete with the available techniques, the solid-state lasers should be more convenient to operate, more environmentally stable, more efficient, simpler, and more compact in design. In this review, we therefore limit our consideration to the systems that operate at room temperature and allow direct diode pumping. The first part of the paper reviews the principles of ultrabroadband (otherwise called vibronic, which is an abbreviation from vibrational-electronic and refers to the phonon-broadened electronic transitions) solid-state infrared lasers. The latter are distinguished by their ability to support the shortest, down to single optical cycle, pulse durations as well as ultimately broad single line tuning ranges. In particular, the paper discusses issues which are specific to infrared lasers: wavelength scaling rules, material issues, and nonlinear optical properties in the infrared. The second part of the article focuses on the developments in such lasers as Cr4+ :YAG, Cr2+ :ZnSe, Cr2+ :ZnS, as well as the recently proposed mixed Cr2+ :ZnSx Se1−x laser. The rapid advances during the last decade in laser materials and semiconductor lasers have led to the development of the two directly diode-pumped ultrabroadband lasers: Cr4+ :YAG [3] operating around 1.5 µm, and Cr2+ :ZnSe lasers [4], [5] for the 2–3 µm domain. In the Cr4+ :YAG system, stable self starting generation of few optical cycle pulses and continuum generation spreading over more than two octaves were achieved. A diode-pumped Cr4+ :YAG femtosecond laser has also been demonstrated. After the Cr3+ -doped lasers (LiSAF, LiSGaF, and LiCAF), this is the second vibronic laser system for which diode-pumped femtosecond operation has been achieved. In the Cr2+ -ZnSe system, kW peak power picosecond pulses and a tuning range of 1100-nm continuous wave regime were demonstrated. Due to the broadest gain among all solid-state laser materials, Cr2+ -based lasers hold promise for the generation of ultimately short pulses down to single optical cycle. Altogether, the lasers based on Cr3+ , Cr4+ , and Cr2+ active ions, together with the Tm3+ -based lasers [6]–[9], continuously cover

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SOROKIN et al.: ULTRABROADBAND INFRARED SOLID-STATE LASERS

Fig. 1. Upper graph: emission cross section spectra of the infrared vibronic active media. Lower graph: demonstrated tuning ranges of lasers using broadband crystalline active media at room temperature. The logarithmic wavelength and frequency scale, keeping the ∆λ/λ ratio constant, allows correct comparison of the bandwidths in different spectral regions.

the whole infrared spectral region between ∼0.7–3.1 µm (see Fig. 1) with only a small gap between 1.65 and 1.75 µm. As an outlook toward future developments, we present recent demonstration of the first infrared nanocrystalline random lasers. These works open a wide field of unstudied laser physics on the nanoscale, and provide potential for future broadband nanocrystalline integrated laser devices. Finally, we describe examples of successful application of these lasers to supercontinuum generation, trace gas monitoring, and intracavity spectroscopy. A. Historical Perspective The first tunable continuous wave solid-state lasers were demonstrated in 1963 [10]. It was a flash lamp pumped Ni2+ :MgF2 laser operating around 1.6 µm. A year later, laser operation of a Co2+ :MgF2 and a Co2+ :ZnF2 was realized by the same authors in the wavelength range between 1.75– 2.16 µm [11]. Operation of Ni2+ and Co2+ in several other hosts has also been reported [12], [13]. However, all these lasers required cryogenic cooling for operation. At approximately the same time the color center lasers were invented, representing a good alternative in the infrared region. They have low thresholds, relatively high output powers, and are smoothly and broadly tunable, covering the vast wavelength ranges from the visible to the midinfrared [14]. In the single mode CW regime, color center lasers have extremely narrow spectral line widths; in the mode-locked regime they can provide ultrashort pulses (down to 60% [4], [49]–[51], [118]) and power levels in CW regime in excess of 1.8 W [124] in TEM00 mode. In a pulsed regime, 18 W of output power at 30 W absorbed power pumped by the Q-switched Tm:YALO laser at 7-kHz repetition rate, as well as 65% slope efficiency (59% optical-tooptical efficiency), has been recently demonstrated [201]. In this experiment tunability between 2.1–2.85 µm was achieved at up to 10 W output power. Based on the analysis of the mechanical, thermal, spectroscopic, and laser properties of Cr:ZnSe, the output powers over 10 W in CW regime and several Watts in the mode-locked or amplifier regime are envisioned. Cr:ZnSe material is also suitable for the thin disk laser design. The lifetime quenching does not exceed 25% up to the concentration levels of 1 × 1019 cm−3 [125], corresponding to >10 cm−1 peak absorption coefficient, a typical figure for Yb:YAG thin disk lasers. In the pulsed mode, Schepler et al. have demonstrated in the thin disk configuration 4.2 W of output power at 10 kHz repetition rate [203], [204]. The laser yielded up to 1.4 W in continuous-wave mode. The pump wavelength of 1.89 µm required relatively thick samples for good absorption. An optimized Cr:ZnSe thin disk laser design with reduced disk thickness and proper pump wavelength should be able to produce much higher output power in a good transversal mode. To this point, the following record parameters were also demonstrated from this laser: 1) the broadest tuning bandwidth of 1100 nm between 2000 and 3100 nm in CW regime (Fig. 10); 2) narrow linewidth 600 MHz operation without any intracavity etalons [53] as well as single longitudinal mode operation with 20 MHz linewidth, using intracavity etalons [202]; 3) 350

Fig. 10. Continuous wave tuning of a Cr:ZnSe laser, using a broadband mirror set (circles) and an infrared mirror set (squares). The effective gain curve is computed from the fluorescence and absorption cross sections at real ion concentration N t and threshold inversion n th .

nm tuning range at 65-mW output power in the diode-pumped regime (450 nm in Cr:ZnS [106]); and 4), the active [120], [121] and passive mode locking [121] with pulses as short as 4 ps at 80 mW and 400 mW of output power, respectively. The n2 value of 1.7 × 10−14 cm2 /W at 1.8 µm in Cr:ZnSe is a factor of 50 higher than in Ti:sapphire [206], making the nonlinearoptical mechanisms of mode locking and pulse shortening feasible [145]. Very recently the first passively mode-locked Cr:ZnSe laser producing 11-ps pulses using an InGaSb based SESAM was realized [205]. Further optimization of the dispersion compensation in this laser should lead to stable self starting femtosecond pulses. The symmetry properties of the Cr-doped chalcogenides (especially Cr:ZnS) produce the pronounced second-order nonlinearity [53], which is absent in such crystals as Ti:sapphire. The interaction of the cascaded second-order nonlinearity with the third-order nonlinearity is another interesting research field in connection with ultrashort-pulsed Cr:ZnSe and Cr:ZnS lasers. The most impressive results have been obtained thus far using the Cr2+ :ZnSe crystals. However, there exist other promising Cr2+ -doped crystals. One of these is Cr2+ :ZnS [43], [44]. This crystal has remained less studied as a laser material due to the lack of good optical quality single crystals. Having similar spectroscopic properties to Cr:ZnSe, Cr:ZnS crystal is known to have a larger bandgap (compare 3.84 eV in ZnS and 2.83 eV in ZnSe [131]), better hardness, a higher thermal shock parameter (compare 7.1 and 5.3 W/m1/2 in Cr:ZnS and Cr:ZnSe, respectively [43]), and the lower dn/dT (46 versus 70 × 10−6 K−1 [43]) than Cr:ZnSe. In addition, the temperature quenching of the Cr:ZnS lifetime starts at lower temperatures than Cr:ZnSe, which might be a serious disadvantage, especially in CW applications. With proper cooling, however, the power handling capability of this material should be on par or better than that of Cr:ZnSe, making Cr:ZnS attractive for high power applications. Our experiments with equally doped Cr:ZnS and Cr:ZnSe (e.g., with the same thermal load per unit length) showed that Cr:ZnS performed at least as well as Cr:ZnSe. The pulsed laser operation of Cr:ZnS laser has first been reported in [43], [44], and [132]. The spectroscopic study and the first continuous wave operation was recently reported in [143]. Using Er-fiber pumping up to 700 mW at room temperature tunable over 700 nm (between 2.1–2.8 µm), CW operation was

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TABLE I MAIN SPECTROSCOPICAL DATA OF Cr:ZnSSe IN COMPARISON WITH Cr:ZnSe AND Cr:ZnS [141]

ceramic laser could be created. This would be a most practical source of few-cycle light pulses. B. Laser Using a Solid Solution of Cr2+ :ZnSe and Cr2+ :ZnS

Fig. 11. Comparison of the absorption (upper graph) and fluorescence (lower graph) spectra of Cr:ZnSe, Cr:ZnS, and Cr:ZnSSe [141]. The emission spectra are corrected for the detector and spectrometer response.

demonstrated [127]. Tuning over 400 nm between 2250–2650 nm in the directly diode-pumped configuration [127] as well as an Er-fiber pumped CW microchip laser at 2320 nm were recently demonstrated [133]. An advantage of Cr:ZnS is the shift of the absorption peak by about 100 nm to the blue (Fig. 11), allowing a convenient pumping of this material with available 1.6-µm telecommunications diodes [127]. Another important issue is broadening the operation range of the Cr2+ -doped lasers, especially beyond 3-µm wavelength. This could be obtained by using other II–VI compounds with a larger lattice constant and hence lower crystalline field. For example, hosts such as CdSe [45], [46], CdTe [134], and CdMnTe [135]–[137] also allow room temperature operation with Cr2+ ion. Tuning up to the record 3.4 µm in the pulsed regime was demonstrated in Cr:CdSe [138]. In this way, it makes sense to also consider the mixed ternary and quaternary compounds that would provide both a control over the central wavelength and additional inhomogeneous broadening of the spectrum, as will be shown in the next section. Finally, maybe one of the most interesting advantages of Cr:ZnSe is the availability of technologically developed and low cost polycrystalline material. The existing technologies of producing ceramic ZnSe, such as a chemical vapor deposition (CVD) method or the hot-press method of powders, result in high optical quality low cost substrates of arbitrary size. Recently, the first ceramic directly diode-pumped CW tunable and actively mode-locked laser has been developed [139], [142]. With proper optimization, a directly diode-pumped femtosecond

Having described the comparative merits of both Cr:ZnSe and Cr:ZnS laser crystals, we now consider the possibilities opened by the well known technology and crystal field design possibilities of the ternary II–VI compounds. Our aim is the development, spectroscopic, and laser investigation of a novel mixed crystal, Cr2+ :ZnSx Se1−x . Undoped solid solutions of ZnSx Se1−x [140] are being used as substrates for epitaxial growth of blue emitting diodes, as well as active media for e-beam longitudinally pumped lasers. In this work, we realized diffusion doping of this crystal grown by seeded chemical vapor transport. Based on the Raman and infrared absorption spectra, we determined the content of ZnSx to be 0.42 (crystal composition ZnS0.42 Se0.58 ). The 4 × 3 × 1.5 mm crystal plate was placed in a clean quartz ampoule together with metallic high purity Cr powder. The ampoule was evacuated and sealed off. The doping was obtained by leaving the ampoule in an oven at a temperature of about 825 ◦ C for twenty days. The results of the absorption and room temperature luminescence measurements are given in Fig. 11 and summarized in Table I. As seen in Fig. 11, the high-quality absorption due to predominantly Cr2+ ions could be obtained in this crystal with peak absorption coefficient of 9.5 cm−1 in the maximum of the absorption band around 1.69 µm. The room-temperature lifetime was measured to be 3.7 µs, which is close to the corresponding value measured in concentrated Cr2+ :ZnS [127] and Cr2+ :ZnSe, as are the corresponding values for absorption and emission cross sections. However, emission bandwidth is noticeably broader than in Cr:ZnS or Cr:ZnSe and is peaked at the same wavelength as in Cr:ZnSe. Thus, Cr2+ :ZnSx Se1−x represents an interesting alternative to pure selenides and sulphides. There are many reasons for studying these crystals, including: 1) the possibility of optical and nonlinear property variation by changing the chemical composition and lattice parameter of the crystal—indeed, the bandgap of Cr2+ :ZnSx Se1−x could be increased by ∼0.4 eV by varying x between 1–0.42, leading to the decrease of the third-order nonlinearity and 2) the larger lattice parameter relative to Cr2+ :ZnS leads to the shift of the emission spectrum by ∼100 nm toward infrared (Fig. 11). At the same time, the Cr2+ absorption peaks around 1.68 µm, allowing for convenient pumping with the available Er-fiber and telecom diode lasers around 1.6 µm [127]. The only disadvantage may be


Fig. 12.

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Output characteristics of the Cr:ZnSSE laser [86].

the higher maximum phonon frequency in this crystal (compare ∼350 cm−1 in ZnS0.42 Se0.58 [141] with ∼250 cm−1 in ZnSe). Similar to Cr2+ :ZnS, this may lead to the more rapid onset of nonradiative decay in this crystal relative to Cr2+ :ZnSe. In laser experiments, we used a 1-mm-thick polished plate of polycrystalline Cr:ZnS0.42 Se0,58 in the conventional threemirror configuration (for experimental details see, e.g., [127]). The sample absorbed ∼38% of the Er-fiber pump radiation at 1.607 µm. Without additional cooling, the laser operated at room temperature in continuous wave regime, producing ∼30 mW of output power at 3% output coupling with 170-mW threshold pump power. These results could be further improved using the Co:MgF2 laser at 1.67 µm. The laser output characteristics are given in Fig. 12. The threshold was measured to be less than 80 mW of absorbed power at 2% output coupling. In the similar cavity, Cr:ZnS exhibits 210 mW and Cr:ZnSe a few tens of milliwatts threshold. Without additional cooling, the laser operated at room temperature in the continuous wave regime, producing ∼50 mW of output power with 4% output coupling at 600 mW of incident pump power. This corresponds to slope efficiency of 24% with respect to absorbed pump power, taking into account the pump reflection from uncoated surfaces of the lens and the sample. The laser operated at three wavelengths simultaneously (2430, 2455, and 2480 nm) due to the intracavity etalon effect, with 2480 nm being the strongest laser line. Using a dry fused silica Brewster prism as a tuning element, we were able to demonstrate tunability over ∼560 nm: from 2099 to 2658 nm (Fig. 13). In order to provide a fair comparison, a tuning curve of the polycrystalline Cr:ZnSe sample in similar conditions is given. The tuning range of Cr:ZnS0.42 Se0,58 significantly exceeds that of the Cr:ZnSe on the short wavelength side. The long wavelength cutoff for both samples was due to the water vapor absorption in the cavity, as shown by the air transmission curve. C. Cr2+ :ZnSe and Cr2+ :ZnS Random Nanolasers As noted, the Cr2+ -doped laser materials are characterized by high gain and an intrinsically low lasing threshold, as well as by such remarkable spectroscopic features as the absence of

Fig. 13. Tuning of the ceramic Cr:ZnSSe laser. For comparison, tuning curve of the ceramic cr:ZnSe is given [86].

excited state absorption and negligible nonradiative decay at room temperature. It is, therefore, not a surprise that material such as Cr2+ :ZnSe successfully operates not only as a single crystal, but also in the ceramic form. The ceramic Cr2+ :ZnSe has been demonstrated in tunable, diode-pumped, and even modelocked regimes [53], [194], [195]. The active media were obtained in this case by diffusion doping of metallic chromium into the ceramic ZnSe. Along with several other techniques of producing ceramic ZnSe, the latter is often obtained by hot pressing the micro- and nanocrystalline ZnSe powder. A somewhat odd (but not without good reason) question arises as to whether it would be possible to get laser action from Cr2+ :ZnSe, or any other Cr2+ -doped II–VI compound in the powder form. Indeed, random powder lasers is a hot topic in modern photonics research. For extensive reviews on this subject, see [188]–[190]. This type of laser has been extensively studied since the first proposal in 1966 by Ambartsumyan et al. [191] of lasers with nonresonant feedback, and demonstrated in ZnO by Nikitenko et al. [192] and in Nd3+ :LaMoO4 by Markushev et al. [193]. During the last decade, a great variety of random powder lasers have been developed, all of them emitting in the UV to near infrared wavelength range. Recently, we reported the first eye-safe midinfrared ion-doped semiconductor random lasers based Cr2+ :ZnSe [196] and Cr2+ :ZnS [197] powders operating around 2.4 µm and 2.3 µm, correspondingly. Experimentally, several samples of Cr:ZnSe and Cr:ZnS powders made by mechanically grinding the Cr:ZnSe single crystal with a concentration of Cr2+ ions varying between 5 × 1018 cm−3 and 2 × 1019 cm−3 have been studied. The average size of the nanoparticles in different samples ranged from hundreds of nanometers to tens of micrometers (Fig. 14). Each glass ampoule with the powder with inner diameter equal to 1 mm and outer diameter equal to 1.5 mm was illuminated by 15ns sub-millijoule pulses of an OPO near the absorption peak of Cr2+ at 1780 nm in the case of Cr:ZnSe powder, and at 1700 nm in the case of Cr:ZnS powder, in order to measure the lifetime dependence on pump pulse energy. In the spectral studies, the samples were pumped at 1594 and 1570 nm, respectively. The

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Fig. 14. A scanning electron microscope image of the Cr:ZnSe powder [196].

pumping beam was focused on the powder into a spot size between 0.7–1.1 mm. At pump energy flux comparable to the absorption saturation flux Jsat of bulk Cr:ZnSe and Cr:ZnS (i.e., between 0.3Jsat – 0.5Jsat ), we observe the dramatic shortening of the emission lifetime (Fig. 15), the threshold-like behavior of emission intensity (Fig. 15), and the radical narrowing of the emission spectrum at gain peak (Fig. 16). The maxima of the narrowed spectra at 2400 nm and 2300 nm correspond to the gain maxima of both crystals, and are shifted 100 nm from each other. The spectral linewidth in Fig. 16 is limited by the resolution of the apparatus. To avoid leaking of the pump light, the spectra in Fig. 16 have been recorded under shorter wavelength pumping as compared to the data in Fig. 15. Due to the different absorption, the apparent oscillation threshold energy density is not the same in the two figures. Threshold pump energy density as low as ∼20 mJ/cm2 could be observed in Cr:ZnSe and a factor of 1.5 higher in Cr:ZnS, which corresponds to the higher threshold of Cr:ZnS in the bulk form [127]. An interesting feature of this new class of midinfrared random lasers is a remarkably low threshold. In fact, the threshold pump intensities in the powder and bulk samples are comparably low (4–6 kW/cm2 in powder versus 3–4 kW/cm2 in bulk samples), and significantly lower than in such undoped semiconductor random lasers as ZnO, where the threshold pump intensity Ith is ∼ 80 MW/cm2 [198]. This makes the Cr2+ -doped ZnSe random lasers very attractive for real applications. Finally, it should be noted that the stimulated emission in Cr2+ :ZnSe and Cr2+ :ZnS powders is eye safe and eye safepumped. This opens a broad range of applications for midinfrared random nanolasers in aero- and space technologies, marking and identification, search and rescue, etc. The demonstrated extremely low laser threshold in both lasers renders continuous wave operation in these powder materials feasible. Recently, a sensitization of induced radiation in these crystals in the presence of charge transfer processes [5], [53], [209] has been observed, allowing to pump the upper laser level of Cr2+

Fig. 15. Upper graph: decay time dependence on pump energy in the Cr2+ :ZnSe powder (excitation wavelength 1780 nm, pump spot diameter 0.7 mm). Lower graph: emission intensity versus pump energy for Cr2+ :ZnSe and Cr2+ :ZnS (excitation wavelength 1780 nm, pump spot diameter 1.1 mm) [197].

through the charge transfer mechanism. This phenomenon opens the way toward electrically pumped active ion-doped nanocrystalline ZnSe lasers. V. APPLICATIONS OF ULTRABROADBAND LASERS In this section we consider some recent demonstrations utilizing the bandwidth capability of infrared vibronic lasers that have potential to become established techniques for real-world applications. In particular, we shall describe low-threshold supercontinuum generation, trace gas analysis, and high sensitivity spectroscopy. There also exist a number of other applications, such as optical coherence tomography [156], which will not be discussed here, as they are adequately described elsewhere. A. Supercontinuum Generation With Cr:YAG Laser Infrared spectral supercontinuum (SC) generation is of great interest for numerous applications such as frequency metrology [170], femtosecond pulse phase stabilization [171], ultrashort pulse compression [172], optical coherence tomography [173], and high resolution spectroscopy [174]. In the near infrared wavelength range around 800 nm, photonic crystal fibers (PCFs) [175] and tapered fibers [176] provide extremely broad


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Fig. 17. Cross section of the SF6 fiber with 4.5-µm core size. The inset shows the enlarged core area.

Fig. 16. Emission spectra of the Cr2+ :ZnSe powder (upper graph) and Cr2+ :ZnS powder (lower graph). The experimental conditions and pump energies are marked on the graphs [196], [197].

SC generation at low input energy (≤1 nJ). This is possible due to the small fiber core areas and the proximity of the zero dispersion wavelength (ZDW) to the pulse central wavelength. Especially, the strong and controllable contribution of the holey structure (for the PCFs) or of the surrounding air (for the tapered fibers) to the fiber dispersion [177], [178] allows precise trimming of the fiber dispersion properties. However, this approach to SC generation does not scale automatically with the wavelength. First, we should note the intrinsic λ−3 dependence of the effective nonlinearity, as discussed in Section II-C. Second, the first ZDW in silica PCFs and tapered fibers typically lies around ∼800 nm [179]. For the infrared SC generation, it is therefore necessary to use the second ZDW of fibers with core diameters of 1–1.5 µm [180]. In this case, however, we encounter much higher third-order dispersion (TOD) of inverse sign (Fig. 18). As a result, one expects significant spectral restructuring, in particular, due to an unusual interplay between the TOD and the stimulated Raman scattering. Additional potential problems are confinement loss and launch problems resulting from the small core sizes ≤ λ, and the need for an adiabatic transition region between single-mode and tapered sectors of the fiber [207]. A way to compensate for the intrinsic λ−3 dependence of the effective nonlinearity may be to use fibers with very low dispersion (dispersion shifted fibers) that would allow longer interaction lengths. Using conventional silica based fiber technology with a GeO2 -doped core, one can achieve values of the nonlinear coefficient γ = 21 W−1 km−1 [181]. Such fibers with 1–5 m length and with relatively low dispersion have been used

for supercontinuum generation with input energies of 500 pJ [182]. Another way would be to increase the effective nonlinearity by using the fiber material with high nonlinear index n2 . Recently, fibers from SF6 glass with n2 value of 2.2 × 10−15 cm2 /W have been manufactured [183] (Fig. 17). By varying the core size in the PCF structure it is possible to position the first ZDW in the 1–2-µm range. This allows optimization of the dispersion for the given pump pulse wavelength, and allows utilizing an efficient spectral broadening due to so-called highorder soliton fission mechanism [184]. The high refractive index n ≈ 1.76 gives a large core to cladding index gradient that provides an efficient fiber-mode confinement in the infrared even for the small fiber core sizes, reducing the effective mode area and increasing the nonlinear coefficient γ. Experiments with the fibers of 2.5-µm core diameter have demonstrated SC generation using an ultrashort pulse optical parametric oscillator (375 pJ pulses with 100-fs width, SC spanning 0.7–2 µm at −30 dB level) [183], the Er-fiber oscillatoramplifier system (octave-spanning SC excited by 200-pJ pulses with 60 fs width) [185], and Cr4+ :YAG laser (≈40-pJ 60-fs pulses) [186]. At 1.5-µm wavelength, the calculated effective mode areas [187] increase from 3.7 to 9.4 µm2 for the lowest modes of the PCFs with 2.5- and 4.5-µm core sizes, respectively. With these effective mode areas, the nonlinear coefficient becomes γ = 280 W−1 km−1 and 98 W−1 km−1 for the 2.5- and 4.5-µm cores, respectively. This is a significant improvement over the γ = 21 W−1 km−1 value for the highly nonlinear silica fiber [181]. The applicability of these fibers as structures for continuum generation depends not only on their effective nonlinear properties, but also their dispersion, which has been calculated from the dependence of the effective indexes on the wavelength (Fig. 18). The dispersion characteristics of the PCF are significantly more appropriate for SC generation in the IR range than those of the tapered fibers [207]. The decisive factor here is the small value of the TOD that preserves spectral homogeneity and coherence. Simulation of the pulse propagation in a nonlinear fiber of this type predicts [207] that there exist optimal propagation lengths for the smooth and broad spectrum of the pulse. By

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Fig. 18. Group-delay dispersions for fundamental modes of the silica TFs and lowest order modes of the soft glass PCFs with different core sizes. Filled regions show the spectral range of Ti:sapphire and Cr:YAG lasers. Tangents to the dispersion curves in the operational points visualize the TOD.

choosing the right propagation length, it is possible to obtain a practically flat 3-dB coherent spectrum (Figs. 19, 20). The spectrum is a superposition of a broadened spectrum in a zero-order mode and a fraction of power in a first-order mode. With only 35 mW of launched power, it is possible to obtain a spectrum that covers the 1.25–1.63-µm range (Fig. 19). The alternative approach using the Raman broadening in a holey fiber requires a 15-W Yb fiber amplifier to generate a flat spectrum in the 1.12–1.33-µm range [208].

B. Trace Gas Analysis The monitoring and analysis of gases at low concentration has become an essential environmental, medical, industrial, and chemical issue. Numerous techniques and instruments focus on different aspects of gas sensing such as sensitivity, selectivity, multicomponent capability, and sample preparation requirements. Optical measurements offer some unique advantages with respect to these features. In particular, the optical measurements can be performed without direct contact, on a number of components simultaneously, are specimen-independent, etc. To fully exploit these advantages, the laser source should demonstrate broad tunability in the wavelength range of po-

tential specimens, sufficiently narrow oscillation linewidth for good selectivity, and power for higher sensitivity. The main absorption features of many relevant gases lie in the midinfrared (2–15 µm) region with overtones and combination vibrational-rotational bands in the near-IR (0.8–2 µm) spectral range. The atmospheric window between 2–5 µm is especially interesting, because it is characterized by the presence of the strong fundamental vibrational absorption lines of atmospheric constituents, vapors and other gases. Those include water vapor (H2 O), filling the whole range between 2.5 and 3 µm with maximum around 2.7 µm; carbon monoxide (CO) with strong features around 2.3–2.4 µm; carbon dioxide (CO2 ) absorbing around 2.7–2.8 µm; nitrous oxide (N2 O), having several absorption features all through the 2–4-µm range; and many other species. Detection of low concentrations of these and other molecules, constituting air pollutants or greenhouse gases for the purpose of environmental diagnostics or even the human breath for the purpose of medical diagnostics, is currently done using laser systems, which are based mainly on nonlinear optical conversion techniques and include optical parametric oscillators, difference frequency generators, and tunable semiconductor lasers [157], [158]. Except for the quite complicated and expensive optical parametric oscillators, these sources provide low power output of the order of 1 mW or less and a narrow tuning range. A broadly tunable laser such as Cr:ZnSe, would provide an interesting alternative with important advantages: operation directly in the wavelength range of interest, and significant output power. For registration, we adopted the photoacoustic scheme, which is also widely specimen- and wavelength-independent [159], [160]. The source was the tunable Cr2+ :ZnSe laser [50], pumped by an Er3+ -fibre laser at 1607 nm. The tuning is achieved by a tandem of CaF2 or fused silica prisms by rotating the end mirror. With a single broadband mirror set, a wavelength range between 2000–2937 nm (3405–5000 cm−1 ) can be covered (Fig. 10). With special infrared optics, it is possible to extend the tuning range to 3100 nm (Fig. 10) and probably even farther, as the long wavelength cutoff is still defined by the mirror transmission. The typical experimental results are presented in Fig. 21. Using the same source it was possible to perform measurements in two different wavelength regions: 2.3 and 2.9 µm. The laser output power of 100–500 mW is orders of magnitude higher than the powers achieved with difference frequency generation in the same wavelength range. This allows sensitive measurement using even the relatively weak absorption lines in the spectrum. Using the certified gas mixtures, one can calibrate the sensitivity of the whole setup and perform quantitative measurements [159]. The minimum detectable absorbance at a power level of 300 mW was 1.6 × 10−5 . The resulting minimum detectable concentrations for a number of gases are plotted in Fig. 22, [159], [161]. For many important molecules, the detection limit lies well in the parts-per-billion (ppb) region. The linewidth of the laser is also an important factor, as it defines the selectivity of the technique. It was measured using the gas lines themselves and was found to be ∼0.2 cm−1 with


Fig. 19.

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Supercontinuum generation in the the SF6 fiber. (a) Dependence of the output spectrum on fiber length and (b) on the launched power.

Fig. 20. Smooth supercontinuum spectrum, generated by a Cr:YAG laser in the SF6 fiber.

fused silica prisms, and ∼1.2 cm−1 with CaF2 prisms, due to their lower dispersion [159]. This linewidth is dominated by the thermal and mechanical instability of the setup used, because the short-term linewidth was measured to be less than 0.02 cm−1 [53]. The Cr:ZnSe laser is thus a versatile laser source that can be used for trace gas measurements in the whole range between 2–3.1 µm. The detection limits achieved allow multicomponent measurement both for monitoring and detection of trace species in ambient air and at working places. C. Intracavity Laser Spectroscopy Another method of ultrasensitive spectroscopic absorption measurements, where the gain bandwidth plays a crucial role, is time resolved intracavity laser spectroscopy (ICLAS) [162]. The highest sensitivity obtained in this method is due to the very long effective propagation path (many kilometers) that can

be achieved at the initial stages of laser operation. At the same time, the initially broad spectrum continues to narrow during the laser evolution, so that the spectral coverage and long absorption path have to be traded off. It can be shown that the laser bandwidth at time t after the onset of oscillation follows the rule ∆λ ∝ ∆λ0 × t−1/2 [162]; i.e., it is proportional to the initial bandwidth ∆λ0 of the gain spectrum. Reversing this relation, we may see that for a given final bandwidth, the effective absorption path length scales quadratically with the initial bandwidth: labs = ct ∝ ∆λ20 /∆λ2 . The material bandwidth thus becomes a crucial parameter in intracavity spectroscopy. Only a few experiments with ICLAS using broadband sources in the infrared were performed: atmospheric spectra were obtained in the 2636–2640 nm region using the KCl:Li FA (II) color center laser [163]; in the 2035–2055 nm region using the Co:MgF2 laser [164]; and in the 1770–1950 nm region using the Tm-doped fiber laser [165]. Quite recently, the Cr:ZnSe laser has been used for the intracavity spectroscopy in the 2410–2460 nm region [166], and a Cr4+ :YAG laser has been applied in the 1350–1610 nm range [167]. However, the extreme sensitivity of the intracavity spectroscopy becomes a problem if it is used in the infrared region, where air constituents possess main and combination absorption lines. As a result, in most of the above quoted experiments, the spectra were completely oversaturated by the atmosphere, predominantly CO2 and water vapor. Fig. 23, where only water lines are seen, illustrates this situation. This is despite the fact that Cr4+ :YAG laser was purged for 10 hours by dry nitrogen and then sealed, yet the spectrum is still obscured by the water vapor. In this configuration, the experiment serves as a water vapor detector with a 0.2-ppb

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Fig. 23. Two time components (32- and 195-µs generation times) extracted from a time-resolved spectrum of the dynamics of a Cr4+ :YAG laser, operated in a sealed box after 10 h of dry nitrogen purging. The apodized spectral resolution is 0.07 cm−1 . Due to the long effective absorption path length, the spectra look totally saturated by the residual water vapor [167].

Fig. 21. Gas absorption using the tunable Cr:ZnSe laser: Spectrum of methane and carbon monoxide around 2.3 µm (upper graph) [159]; spectrum of CO2 and water around 2.9 µm (lower graph) [161]. All measurements performed at normal pressure and room temperature.

Fig. 24. Atmospheric absorption spectra around 1.53 µm at up to 28-km path length. The spectral resolution is 0.36 cm−1 . Despite the lower water absorption at this wavelength range, the water vapor lines still dominate [167].

Fig. 22. Minimum detectable concentrations for a number of gases using the photoacoustic registration. The horizontal bars show the tuning ranges of Cr:ZnSe and Cr:CdSe lasers.

detection limit. To avoid water absorption, one can make use of the large available bandwidth and tune the laser to another spectral region, e.g., by a pellicle. Fig. 24 presents three slices from a time resolved spectrum, recorded around 6550 cm−1 (1.53 µm) with the cavity in the open air. Despite the much lower absorption cross section, water vapor still dominates the spectrum, obscuring any other useful information. Recently, another ICLAS experiment using a Cr:ZnSe laser in the 2.5-µm region under Er-fiber laser pumping has been

performed [168]. To avoid water absorption, the whole laser except the pump source has been placed into a sealed box, which was evacuated (Fig. 25). The box was then filled with a gas of interest at pressures between 0.1–70 mbar, to avoid the collisional broadening. The recording and retrieval procedure, involving the high resolution Fourier-Transform spectrometer, has been described in [169]. Fig. 26 shows the laser gain curve and the reflection curves of the mirrors in the cavity. A typical ICLAS spectrum of CO2 is shown, spanning over 100 nm bandwidth, at 2.6-km effective propagation distance. Fig. 27 shows a portion of the CO2 spectrum, corresponding to 4.9 km of effective propagation distance. The effective


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in the infrared region, special care has to be taken to eliminate natural constituents of the atmosphere, especially water vapor. VI. CONCLUSION

Fig. 25. Schematic diagram of the time-resolved FT spectrometer. The dashed rectangle shows the vacuum chamber [168].

Fig. 26. Fluorescence spectrum of Cr:ZnSe (gray) and spectral losses due to the output coupler (OC) and five high reflectors (HRs) on a round trip. A typical output spectrum at 8.9-µs delay (2.6-km effective propagation distance) is shown in black, with the CO2 absorption lines [168].

Rapid advances during the last decade in laser materials and semiconductor lasers have led to the development of the two major directly diode-pumped ultrabroadband and ultrashort pulsed lasers operating around 1.5 and 2.5 µm: Cr4+ :YAG and Cr2+ :ZnSe lasers. The Cr4+ :YAG laser operates in the few-cycle ultrashortpulsed regime and supports directly diode-pumped operation, tunable as well as mode-locked. In the diode-pumped regime, the laser generates 65-fs pulses at 30-mW output power. Combined with the novel highly nonlinear PCF fibers, this allows generation of smooth and coherent supercontinuum in the 1–2-µm region with threshold energies of the order of 100 pJ. The Cr2+ -doped II–VI lasers represent a prospective family of lasers, which currently master applications such as high sensitivity and high resolution spectroscopy, trace gas monitoring, and remote sensing. Their unsurpassed bandwidth exceeding 1000 nm suggests a bright future for few-cycle pulse and frequency comb generation, optical standards, etc. Moreover, similar upcoming materials in this wavelength region, e.g., Cr2+ :ZnSSe, promise even broader bandwidth for ultrashort pulse generation with extended tunability ranges. ACKNOWLEDGMENT

Fig. 27. Doppler limited spectrum of CO2 at 4.9-km propagation distance [168].

absorption is as high as 90% for the strongest lines, corresponding to a sensitivity of 6 × 10−8 cm−1 . The explored spectral domain is the location of the two weak vibration-rotation bands 2ν3 − ν2 (Fig. 27) and 2ν3 + ν2 − 2ν2 . The maximum absorption of the line profiles reaches 100% with a pressure-absorption path condition equal to 66 mbar and 30 km, respectively. It is worth noting that previously, these spectra could only be detected by astronomic measurements in the atmosphere of Venus [199], [200] which is 96.5% CO2 . Concluding this section, we note that ICLAS spectroscopy in the infrared region can be successfully used for measuring and detecting gas constituents with extreme sensitivity. The Cr2+ doped lasers such as ZnS, ZnSe, CdSe, etc., provide additional flexibility, allowing one to set the observed region from 2.2 to beyond 2.8 µm by selecting the proper material. In order to make use of these possibilities and obtain useful information

The authors would like to express their sincere gratitude to all colleagues and collaborators, without whose participation this work would have been impossible: V. Kalashnikov (TU Vienna); A. Di Lieto and M. Tonelli (Pisa University); N. Kuleshov, V. Levchenko, and V. Scherbitsky (International Laser Center Minsk); E. Vinogradov (Institute of Spectroscopy, Troitsk); A. Shestakov (JSC POLUS); R. Kanth Kumar and J. Knight (University of Bath); and C. Fischer and M. Sigrist (ETH Zurich). Special thanks go to N. Picqué and G. Guelachvili (University Paris Sud, Orsay), not only for their collaboration in the Cr:ZnSe ICLAS experiments, but also for communicating their unpublished results on Cr:YAG intracavity spectroscopy. REFERENCES [1] P. B. Corkum, “Plasma perspective on strong field multiphoton ionization,” Phys. Rev. Lett., vol. 71, pp. 1994–1997, 1993. [2] M. Ebrahimzadeh, “Midinfrared ultrafast and continuous-wave optical parametric oscillators,” in Solid-State Midinfrared Laser Sources, Topics Applied Physics, vol. 79. Berlin, Germany: Springer, 2003, pp. 179– 219. [3] I. T. Sorokina, S. Naumov, E. Sorokin, E. Wintner, and A. V. Shestakov, “Directly diode-pumped tunable continuous-wave room-temperature Cr4 :YAG laser,” Opt. Lett., vol. 24, pp. 1578–1580, 1999. [4] A. V. Podlipensky, V. G. Shcherbitsky, N. V. Kuleshov, V. I. Levchenko, V. N. Yakimovich, M. Mond, E. Heumann, G. Huber, H. Kretschmann, and S. Kück, “Efficient laser operation and continuous-wave diode pumping of Cr2+ :ZnSe single crystals,” Appl. Phys. B., vol. 72, pp. 253–255, 2001. [5] E. Sorokin and I. T. Sorokina, “Tunable diode-pumped continuous-wave Cr2+ :ZnSe laser,” Appl. Phys. Lett., vol. 80, pp. 3289–3291, 2002. [6] F. Cornacchia, E. Sani, A. Toncelli, M. Tonelli, M. Marano, S. Taccheo, G. Galzerano, and P. Laporta, “Optical spectroscopy and diode-

708

[7]

[8]

[9] [10] [11] [12] [13] [14] [15] [16] [17]

[18] [19] [20] [21]

[22]

[23] [24] [25] [26] [27] [28] [29]

[30]


pumped laser characteristics of codoped Tm-Ho:YLF and Tm-Ho:BaYF: A comparative analysis,” Appl. Phys. B, vol. 75, pp. 817–822, 2002. E. Sorokin, A. N. Alpatiev, I. T. Sorokina, A. I. Zagumennyi, and I. A. Shcherbakov, “Tunable efficient continuous-wave room-temperature Tm3+ :GdVO4 laser,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 68. Washington, D.C.: Optical Society of America, 2002, pp. 347–350. V. Petrov, U. Griebner, F. Gell, J. Massons, J. Gavalda, R. Sole, M. Aguilo, and F. Diaz, “Room temperature cw lasing of Tm:KGd(WO4 )2 in the 2 µm spectral range,” presented at the Europhoton 2004. F. Cornacchia, D. Parisi, C. Bernardini, A. Toncelli, and M. Tonelli, “Efficient, diode-pumped Tm3+ :BaY2 F8 vibronic laser,” Opt. Expr., vol. 12, pp. 1982–1989, 2004. L. F. Johnson, R. E. Dietz, and H. J. Guggenheim, “Optical maser oscillation from Ni2+ in MgF2 involving simultaneous emission of phonons,” Phys. Rev. Lett., vol. 11, p. 318, 1963. , “Spontaneous and stimulated emission from Co2+ ions in MgF2 and ZnF2 ,” Appl. Phys. Lett., vol. 5, pp. 21–22, 1964. L. F. Johnson, H. J. Guggenheim, and R. A. Thomas, “Phonon terminated optical masers,” Phys. Rev., vol. 149, p. 179, 1966. L. F. Johnson, H. J. Guggenheim, and D. H. Bahnck, “Phonon terminated laser emission from Ni2+ ions in KMgF3 ,” Opt. Lett., vol. 8, p. 371, 1983. W. Gellermann, “Color center lasers,” J. Phys. Chem. Solids, vol. 52, pp. 249–297, 1991. L. F. Mollenauer and R. H. Stolen, “The soliton laser,” Opt. Lett., vol. 9, p. 14, 1984. S. B. Mirov and T. Basiev, “Progress in color center lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 1, pp. 22–30, 1995. A. V. Kirpichnikov, E. V. Pestryakov, V. V. Petrov, and V. I. Trunov, “Generation of picosecond pulses in the near-infrared range using color centers stable at room temperature in a NaCl:OH crystal,” Izv. Akad. Nauk Ser. Fiz., vol. 58, pp. 157–160, 1994. Translated Bull. Russ. Acad. Sci. Phys., vol. 58, pp. 1047–1050, 1994. I. T. Sorokina, “Crystalline midinfrared lasers,” in Solid-State Midinfrared Laser Sources, Topics in Applied Physics, vol. 79. Berlin, Germany: Springer, 2003, pp. 255–350. C. R. Pollock, D. B. Barber, J. L. Mass, and S. Markgraf, “Cr4+ -lasers: Present performance and prospects for new host lattices,” IEEE J. Sel. Topics Quantum Electron., vol. 1, pp. 62–66, 1995. A. Sennarogli, “Broadly tunable Cr4+ -doped solid-state lasers in the near-infrared and visible,” Prog. Quantum Electron., vol. 26, pp. 287– 352, 2002. E. Sorokin, “Solid-state materials for few-cycle pulse generation and amplification,” in Few-Cycle Laser Pulse Generation and Its Applications, Topics in Applied Physics, vol. 95. Berlin, Germany: Springer, 2004, pp. 3–73. N. B. Angert, N. I. Borodin, V. M. Garmash, V. A. Zhitnyuk, A. G. Okhrimchuk, O. G. Siyuchenko, and A. V. Shestakov, “Lasing due to impurity color centers in yttrium alluminum garnet crystals in the range 1.35–145 µm,” Sov. J. Quantum Electron., vol. 18, pp. 73–74, 1988. G. M. Zverev and A. V. Shestakov, “Tunable near-infrared oxide crystal lasers,” OSA Proc. Adv. Solid-State Lasers, vol. 5, pp. 66–70, 1989. H. Eilers, U. Hommerich, S. M. Jacobsen, W. M. Yen, and M. Kokta, “Spectroscopic properties of Cr4+ :Lu3 Al5 O12 ,” Opt. Lett., vol. 18, pp. 1928–1930, 1993. S. Kück, K. Petermann, U. Pohlmann, U. Schönhoff, and G. Huber, “Tunable room-temperature laser action of Cr4+ -doped Y3 Scx Al5−x O12 Cr,” Appl. Phys. B, vol. 58, p. 153, 1994. V. Petricevic, S. K. Gayen, R. R. Alfano, K. Yamagashi, H. Anzai, K. Yamagashi, H. Anzai, and Y. Yamaguchi, “Laser action in chromiumdoped forsterite,” Appl. Phys. Lett., vol. 52, p. 1040, 1988. V. Petricevic, S. K. Gayen, and R. R. Alfano, “Near infrared tunable operation of chromium doped forsterite laser,” Appl. Opt., vol. 28, p. 1609, 1988. , “Continuous-wave laser operation of chromium-doped forsterite,” Opt. Lett., vol. 14, pp. 612–614, 1989. J. Koetke, S. Kück, K. Petermann, G. Huber, G. Cerullo, M. Danailov, V. Magni, L. F. Qian, and O. Svelto, “Quasi-continuous wave laser operation of Cr4+ -doped Y2 SiO5 at room-temperature,” Opt. Commun., vol. 101, p. 195, 1993. A. G. Avanesov, V. A. Lebedev, B. Denker, B. I. Galagan, V. V. Osiko,

[31] [32]

[33] [34] [35]

[36] [37] [38] [39] [40]

[41] [42]

[43]

[44]

[45] [46] [47]

[48]

[49] [50] [51] [52] [53] [54]

S. E. Sverchkov, and A. V. Shestakov, “Room-temperature laser action of Y2 SiO5 :Cr4+ crystal,” OSA Proc. Adv. Solid-State Lasers, vol. 20, pp. 185–187, 1994. V. Petricevic, A. B. Bykov, J. M. Evans, and R. R. Alfano, “Roomtemperature near-infrared tunable laser operation of Cr4+ :Ca2 GeO4 ,” Opt. Lett., vol. 21, pp. 1750–1752, 1996. M. H. Garrett, V. H. Chan, H. P. Jenssen, M. H. Whitmore, A. Sacra, D. Singel, and D. J. Simkin, “Comparison of Cr-doped forsterite and akermanite laser host crystals,” OSA Proc. Adv. Solid-State Lasers, vol. 10, pp. 76–81, 1991. H. Eilers, U. Hömmerich, S. M. Jacobsen, and W. M. Yen, “The near infrared emission of Mn2 SiO4 and MgCaSiO4 ,” Chem. Phys. Lett., vol. 212, pp. 109–112, 1993. L. D. Merkle, T. H. Alik, and B. H. T. Chai, “Crystal growth and spectroscopic properties of Cr4+ in Ca2 Al2 SiO7 and Ca2 Ga2 SiO7 ,” Opt. Mater., vol. 1, pp. 91–100, 1992. M. A. Scott, T. P. J. Han, H. G. Gallagher, and B. Henderson, “Nearinfrared laser crystals based on 3d2+ ions, spectroscopic studies of 3d2+ ions in oxide, melilite, and apatite crystals,” J. Lumin., vol. 72–74, pp. 260–262, 1997. S. Kück, “Laser-related spectroscopy of ion-doped crystals for tunable solid-state lasers,” Appl. Phys. B., vol. 72, pp. 515–562, 2001. J. Kalisky, “Cr4+ -doped crystals: Their use as lasers and passive Qswitches,” Prog. Quantum Electron., vol. 28, pp. 249–303, 2004. S. Naumov, E. Sorokin, V. L. Kalashnikov, G. Tempea, and I. T. Sorokina, “Self-starting five optical cycle pulse generation in Cr4+ :YAG laser,” Appl. Phys. B, vol. 75, p. 1, 2003. A. Agnesi, E. Piccinini, and G. C. Reali, “Influence of thermal effects in Kerr-lens mode-locked femtosecond Cr4+ : Forsterite lasers,” Opt. Commun., vol. 135, p. 77, 1997. S. Naumov, E. Sorokin, and I. T. Sorokina, “Directly diode-pumped femtosecond Cr4+ :YAG laser,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 83. Washington, D.C.: Optical Society of America, 2003, pp. 163–166. , “Directly diode-pumped Kerr-lens mode-locked Cr4+ :YAG laser,” Opt. Lett., vol. 29, pp. 1276–1278, 2004. R. H. Page, L. D. DeLoach, G. D. Wilke, S. A. Payne, R. J. Beach, and W. F. Krupke, “Cr2+ -doped II-VI crystals: New widely tunable, room-temperature mid-IR lasers,” Proc. IEEE Lasers and Electro-Optics Society Annu. Meeting, vol. 1, pp. 449–450, 1995. L. D. DeLoach, R. H. Page, G. D. Wilke, S. A. Payne, and W. P. Krupke, “Transition metal-doped Zinc chalcogenides: Spectroscopy and laser demonstration of a new class of gain media,” IEEE J. Quantum Electron., vol. 32, no. 6, pp. 885–895, Jun. 1996. R. H. Page, K. I. Schaffers, L. D. DeLoach, G. D. Wilke, F. D. Patel, J. B. Tassano, S. A. Payne, W. F. Krupke, K. T. Chen, and A. Burger, “Cr2+ -doped zinc chalcogenids as efficient widely tunable midinfrared lasers,” IEEE J. Quantum Electron., vol. 33, no. 4, pp. 609–619, Apr. 1997. K. L. Schepler, S. Kueck, and L. Shiozawa, “Cr2+ emission spectroscopy in CdSe,” J. Lumin., vol. 72–74, pp. 116–117, 1997. J. McKay, K. L. Schepler, and G. C. Catella, “Efficient grating-tuned midinfrared Cr2+ :CdSe laser,” Opt. Lett., vol. 24, pp. 1575–1577, 1999. U. Hömmerich, X. Wu, V. R. Davis, S. B. Trivedi, K. Grasze, R. J. Chen, and S. W. Kutcher, “Demonstration of room-temperature laser action at 2.5 µm from Cr2+ :Cd0. 85 Mn015 Te,” Opt. Lett., vol. 22, pp. 1180–1182, 1997. S. B. Trivedi, K. Grasze, R. J. Chen, S. W. Kutcher, U. Hömmerich, X. Wu, and J. T. Seo, “Chromium doped cadmium manganese telluride: A novel solid-gain media for midinfrared lasers,” Phys. Semicond. Devices, vol. 2, pp. 762–767, 1998. G. J. Wagner, T. J. Carrig, R. H. Page, K. I. Schaffers, J. Ndap, X. Ma, and A. Burger, “Continuous-wave broadly tunable Cr2+ :ZnSe laser,” Opt. Lett., vol. 24, pp. 19–21, 1999. I. T. Sorokina, E. Sorokin, A. Di Lieto, M. Tonelli, R. H. Page, and K. I. Schaffers, “Efficient broadly tunable continuous-wave Cr2+ :ZnSe laser,” J. Opt. Soc. Amer. B, vol. 18, pp. 926–930, 2001. A. Sennaroglu, A. O. Konza, and C. R. Pollock, “Continuous-wave power performance of a 2.47-µm Cr2+ ZnSe laser: Experiment and modelling,” IEEE J. Quantum Electron., vol. 36, pp. 1199–1205, 2000. T. J. Carrig, “Transition-metal-doped chalcogenide lasers,” J. Electron. Mater., Jul. 2002. I. T. Sorokina, “Cr2+ -doped II-VI materials for lasers and nonlinear optics,” Opt. Mater., vol. 26, pp. 395–412, 2004. K. Vodopyanov, “Pulsed mid-IR optical parametric oscillators,” in Solid-


[55] [56]

[57]

[58]

[59] [60] [61] [62] [63]

[64]

[65] [66] [67] [68] [69]

[70] [71] [72] [73] [74] [75] [76] [77] [78]

State Midinfrared Laser Sources, Topics in Applied Physics, vol. 79. Berlin, Germany: Springer, 2003, pp. 141–180. C. Fischer and M. W. Sigrist, “Mid-IR difference frequency generation, in solid-state midinfrared,” in Laser Sources, Topics in Applied Physics, vol. 79. Berlin, Germany: Springer, 2003, pp. 97–143. T. T. Basiev, V. V. Osiko, A. M. Prokhorov, and E. M. Dianov, “Crystalline and fiber Raman lasers,” in Solid-State Midinfrared Laser Sources, Topics in Applied Physics, vol. 79. Berlin, Germany: Springer, 2003, pp. 351–400. E. Sorokin, I. T. Sorokina, A. Unterhuber, E. Wintner, A. I. Zagumenny, I. A. Shcherbakov, V. Carozza, A. Toncelli, and M. Tonelli, “A novel cw tunable and mode-locked 2 µm Cr, Tm, Ho:YSGG:GSAG laser,” in OSA Trends in Optics and Photonics, vol. 19, W. R. Bosenberg and M. M. Fejer, Eds Washington, D.C.: Optical Society of America, 1998, pp. 197–200. E. Sorokin, I. T. Sorokina, A. Unterhuber, E. Wintner, A. I. Zagumenny, I. A. Shcherbakov, V. Carozzo, and M. Tonelli, “A Novel cw modelocked µm Cr, Tm, Ho:YSGG:GSAG laser,” in Conf. on Lasers and Electro-Optics, Tech. Dig., 1998, p. 296, paper CWM1. B. T. McGuckin and R. T. Menzies, “Efficient CW diode-pumped Tm, Ho:YLF laser with tunability near 2.067 µm,” IEEE J. Quantum Electron., vol. 28, no. 4, pp. 1025–1028, Apr. 1992. A. Di Lieto, P. Minguzzi, A. Toncelli, M. Tonelli, and H. P. Jenssen, “A Diode-laser-pumped tunable Ho:YLF laser in the 2 µm region,” Appl. Phys. B, vol. 57, pp. 3172–3175, 1993. , “Efficient, broadly tunable, laser-pumped Tm:YAG and Tm:YSGG CW lasers,” Opt. Lett., vol. 15, p. 486, 1990. R. C. Stonemann and L. Esterowitz, “Efficient 1.94-µm Tm:YALO laser,” IEEE J. Sel. Topics Quantum Electron., vol. 1, pp. 78–80, 1995. L. Fornasiero, N. Berner, B.-M. Dicks, E. Mix, V. Peters, K. Petermann, and G. Huber, “Broadly tunable laser emission from Tm:Y2 O3 and Tm:Sc2 O3 at µm,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 26. Washington, D.C.: Optical Society of America, 1999, pp. 450–453. G. Galzerano, S. Taccheo, P. Laporta, F. Cornacchia, D. Parisi, A. Toncelli, and M. Tonelli, “Widely tunable 2-µm Tm:BaY2 F8 vibronic laser,” presented at the Adv. Solid-State Photonics Topical Meeting, 2005. B. Henderson and G. F. Imbusch, Optical Spectroscopy of Inorganic Solids. Oxford, U.K.: Clarendon, 1989. R. C. Powell, Physics of Solid-State Laser Materials. New York: Springer, 1998. B. Henderson and R. H. Bartram, Crystal-Field Engineering of SolidState Laser Materials. Cambridge, U.K: Cambridge Univ. Press, 2000, p. 398. W. H. Fonger and C. W. Struck, “Unified model of energy transfer for arbitrary Franck-Condon offset and temperature,” J. Lumin., vol. 17, pp. 241–261, 1978. M. Yamaga, Y. Gao, F. Rasheed, K. P. O’Donnel, B. Henderson, and B. Cockayne, “Radiative and non-radiative decays from the excited state Ti3+ ions in oxide crystals,” Appl. Phys. B, vol. 51, p. 329, 1990. M. D. Sturge, “Temperature dependence of multiphonon nonradiative decay at an isolated impurity center,” Phys. Rev. B., vol. 8, pp. 6–14, 1973. R. Englman and J. Jortner,, “The energy gap law for radiationless transitions in large molecules,” Mol. Phys., vol. 18, pp. 145–164, 1970. L. F. Johnson and H. J. Guggenheim, “Electronic-and phonon-terminated laser emission from Ho3+ in BaY2 F8 ,” IEEE J. Quantum Electron., vol. 10, no. 4, pp. 442–449, Apr. 1974. O. G. Peterson, S. A. Tuccio, and B. B. Snavely, “CW operation of an organic dye solution laser,” Appl. Phys. Lett., vol. 17, pp. 245–247, 1970. L. F. Mollenauer and D. H. Olson, “A broadly tunable cw laser using color centers,” Appl. Phys. Lett., vol. 24, p. 386, 1974. B. Struve, G. Huber, V. V. Laptev, I. A. Scherbakov, and E. V. Zharikov, “Tunable room-temperature cw laser action in Cr3+ :GdScGa-garnet,” Appl. Phys. B, vol. 30, pp. 117–120, 1983. P. Albers, E. Stark, and G. Huber, “Continuous-wave laser operation and quantum efficiency of titanium-doped sapphire,” J. Opt. Soc. Amer. B, vol. 3, pp. 134–139, 1986. S. Basu and R. L. Byer, “Continuous-wave mode-locked Nd:glass laser pumped by a laser diode,” Opt. Lett., vol. 13, pp. 458–460, 1988. R. Mellish, N. P. Barry, S. C. W. Hyde, R. Jones, P. M. W. French, C. J. Van der Poel, and A. Valster, “Diode-pumped Cr:LiSAF all-solid-state

709

[79]

[80] [81] [82] [83]

[84] [85] [86]

[87] [88] [89]

[90] [91] [92] [93] [94] [95] [96] [97]

[98] [99] [100]

[101] [102]

femtosecond oscillator and regenerative amplifier,” Opt. Lett., vol. 20, pp. 2312–2314, 1995. I. T. Sorokina, E. Sorokin, E. Wintner, A. Cassanho, H. P. Jenssen, and M. Noginov, “Efficient continuous-wave TEM00 and femtosecond Kerrlens mode-locked Cr:LiSrGaF laser,” Opt. Lett., vol. 21, pp. 204–206, 1996. V. P. Yanovsky, A. Korytin, F. Wise, A. Cassanho, and H. P. Jenssen, “Femtosecond diode-pumped Cr:LiSGAF lasers,” IEEE J. Sel. Top. Quantum Electron., vol. 2, no. 3, pp. 465–472, Sep. 1996. V. Petrov, V. Shcheslavskiy, T. Mirtchev, F. Noack, T. Itatani, T. Sugaya, and T. Nakagawa, “High-power self-starting femtosecond Cr:forsterite laser,” Electron. Lett., vol. 34, p. 559, 1998. R. P. Prasankumar, C. Chudoba, J. G. Fujimoto, P. Mak, and M. F. Ruane Opt. Lett., vol. 27, pp. 1564–1566, 2002. D. J. Ripin, J. T. Gopinath, H. M. Shen, A. A. Erchak, G. S. Petrich, L. A. Kolodziejski, F. X. Kärtner, and E. P. Ippen, “Oxidized GaAs/AlAs mirror with a quantum-well saturable absorber for ultrashortpulse Cr4+ :YAG laser,” Opt. Commun., vol. 214, pp. 285–289, 2002. P. Wagenblast, R. Ell, U. Morgner, F. Grawert, and F. Kärtner, “Diodepumped 10-fs Cr3+ :LiCAF laser,” Opt. Lett., vol. 28, p. 1713, 2003. S. Naumov, E. Sorokin, and I. T. Sorokina, “Measurement of the excited state absorption cross-section in Cr4+ :YAG using relaxation oscillations study,” in Adv. Solid-State Photon., Paper WB31, 2005. I. T. Sorokina, E. Sorokin, A. Di Lieto, M. Tonelli, B. N. Mavrin, and E. A. Vinogradov, “A new broadly tunable room-temperature continuouswave Cr2+ :ZnSx Se1−x laser,” in Adv. Solid-State Photon., Paper MDI, 2005. V. L. Kalashnikov, E. Sorokin, S. Naumov, and I. T. Sorokina, “Spectral properties of the Kerr-lens mode-locked Cr4+ :YAG laser,” J. Opt. Soc. Amer. B, vol. 20, pp. 2084–2092, 2003. S. A. Payne, L. L. Chase, and G. D. Wilke, “Optical spectroscopy of the new laser materials, LiSrAlF6 :Cr3+ and LiCaAlF6 :Cr3+ ,” J. Lumin., vol. 44, pp. 167–176, 1989. B. Chassagne, A. Ivanov, J. Oberlé, G. Jonusauskas, and C. Rullière, “Experimental determination of the nonlinear refractive index in an operating Cr:Forsterite femtosecond laser,” Opt. Comm., vol. 141, p. 69, 1997. P. M. W. French, N. H. Rizvi, J. R. Taylor, and A. V. Shestakov, “Continuous-wave mode-locked Cr4+ :YAG laser,” Opt. Lett., vol. 18, p. 39, 1993. A. Sennaroglu, C. R. Pollock, and H. Nathel, “Continuous-wave selfmode-locked operation of a femtosecond Cr4+ :YAG laser,” Opt. Lett., vol. 19, p. 390, 1994. P. J. Conlon, J. P. Tong, P. M. W. French, J. R. Taylor, and A. V. Shestakov, “Passive mode locking and dispersion measurement of sub-100 fs Cr4+ :YAG laser,” Opt. Lett., vol. 19, pp. 1468–1469, 1994. , “Femtosecond pulse generation from synchronously pumped selfmode-locked Cr4+ :YAG,” J. Mod. Opt., vol. 42, pp. 723–726, 1995. J. Ishida and K. Naganuma, “Compact diode-pumped all-solid-state femtosecond Cr4+ :YAG laser,” Opt. Lett., vol. 21, pp. 51–53, 1996. V. L. Kalashnikov, E. Sorokin, and I. T. Sorokina, “Multipulse operation and limits of the Kerr-lens mode locking stability,” IEEE J. Quantum Electron., vol. 39, no. 2, pp. 323–336, Feb. 2003. Y. P. Tong, P. M. W. French, J. R. Taylor, and J. O. Fujimoto, “All-Solidstate sources in the near infrared,” Opt. Comm., vol. 136, pp. 235–238, 1997. Y. P. Tong, L. M. Sutherland, P. M. W. French, J. R. Taylor, A. V. Shestakov, and B. H. T. Chai, “Self-starting Kerr-lens mode-locked femtosecond Cr4+ YAG and picosecond Pr3+ YLF solid-state lasers,” Opt. Lett., vol. 21, pp. 644–646, 1996. Y. Ishida and K. Naganuma, “Characteristics of femtosecond pulses near 1.5 mm in self-mode-locked Cr4+ :YAG laser,” Opt. Lett., vol. 19, pp. 2003–2005, 1994. J. Theimer, M. Hayduk, M. F. Krol, and J. W. Haus, “Mode-locked Cr4+ :YAG laser: Model and experiment,” Opt. Comm., vol. 142, pp. 55– 60, 1997. S. Spaelter, M. Boehm, M. Burk, B. Mikulla, R. Fluck, I. D. Jung, G. Zhang, U. Keller, A. Sizmann, and G. Leuchs, “Self-starting solitonmodelocked femtosecond Cr4+ :YAG laser using an antiresonant FabryPerot saturable absorber,” Appl. Phys. B, vol. 65, pp. 335–338, 1997. M. J. Hayduk, S. T. Johns, M. F. Krol, C. R. Pollock, and R. P. Leavitt, “Self-starting mode-locked tunable femtosecond Cr4+ :YAG laser using a saturable-absorber mirror,” Opt. Commun., vol. 137, pp. 55–58, 1997. Z. Zhang, T. Nakagawa, K. Torizuka, T. Sugaya, and K. Kobayashi,

710

[103] [104] [105] [106] [107] [108] [109] [110]

[111] [112] [113] [114] [115] [116] [117] [118]

[119]

[120] [121]

[122] [123]

[124] [125]


“Self-starting mode-locked Cr4+ :YAG laser with a low-loss broad-band semiconductor saturable-absorber mirror,” Opt. Lett., vol. 24, pp. 1768– 1770, 1999. , “Gold-reflector based semiconductor saturable absorber mirror for femtosecond mode-locked Cr4+ :YAG lasers,” Appl. Phys. B, vol. 70, pp. S59–S62, 2000. B. C. Collings, K. Bergman, and W. H. Knox, “True fundamental solitons in a passively mode-locked short-cavity Cr4+ :YAG laser,” Opt. Lett., vol. 22, pp. 1098–1100, 1997. R. Mellish, S. V. Chernikov, P. M. French, and J. R. Taylor, “All-solidstate compact high repetition rate modelocked Cr4+ :YAG laser,” Electron. Lett., vol. 34, pp. 552–553, 1998. T. Tomaru and H. Petek, “Femtosecond Cr4+ :YAG laser with an L-fold cavity operating at a 1.2-GHz repetition rate,” Opt. Lett., vol. 25, p. 584, 2000. T. Tomaru, “Two-element-cavity femtosecond Cr4+ :YAG laser operating at a 2.6-GHz repetition rate,” Opt. Lett., vol. 26, pp. 1439–1441, 2001. U. Morgner, W. Drexler, F. X. Kärtner, X. D. Li, C. Pitris, E. P. Ippen, and J. G. Fujimoto, “Spectroscopic optical coherence tomography,” Opt. Lett., vol. 25, pp. 111–113, 2000. B. Jean and T. Bende, “Mid-IR laser applications in medicine,” in SolidState Midinfrared Laser Sources, Topics in Applied Physics, Berlin, Germany: Springer, 2003, pp. 511–544. W. Pelouch, G. J. Wagner, T. J. Carrig, and W. J. Scharpf, “Mid-wave ZGP OPO pumped by a Cr:ZnSe laser,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 50, pp. 670–674, Washington, D.C.: Optical Society of America, 2001. J. T. Vallin, G. A. Slack, S. Roberts, and A. E. Hughes, “Near and far infrared absorption in Cr doped ZnSe,” Solid State Commun., vol. 7, p. 1211, 1969. , “Infrared absorption in some II-VI compounds doped with Cr,” Phys. Rev. B, vol. 2, p. 4313, 1970. M. Kaminska, J. M. Baranowski, S. M. Uba, and J. T. Vallin, “Absorption and luminescence of Cr2+ (d4+ ) in II-VI compounds,” J. Phys. C, SolidState Phys., vol. 12, pp. 2197–2214, 1979. H. Nelkowski and G. Grebe, “IR-luminescence of ZnS:Cr,” J. Lumin., vol. 1/2, pp. 88–93, 1970. G. Grebe and H. J. Schulz, “Interpretation of excitation spectra of ZnS,Cr2+ by fitting the eigenvalues of the Tanabe-Sugano matrices,” Phys. Stat. Solidi, vol. b54, pp. k69–k72, 1972. , “Luminescence of Cr2+ centers and related optical interactions involving crystal field levels of chromium ions in ZnS, Z,” Naturf., vol. 29a, p. 1803, 1974. G. Grebe, G. Roussos, and H. J. Schulz, “Infrared luminescence of ZnSe:Cr crystals,” J. Lumin., vol. 12/13, p. 701, 1976. R. H. Page, J. A. Skidmore, K. I. Schaffers, R. J. Beach, S. A. Payne, and W. F. Krupke, “Demonstration of diode-pumped and grating tuned ZnSe, Cr2+ laser,” in OSA Trends in Optics and Photonics. Washington, D.C.: Optical Society of America, 1997, pp. 208–210. E. Sorokin, I. T. Sorokina, and R. H. Page, “Room-temperature CW diode-pumped Cr2+ :ZnSe laser,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 46. Washington, D.C.: Optical Society of America, 2001, pp. 101–105. T. J. Carrig, G. J. Wagner, A. Sennaroglu, J. Y. Jeong, and C. R. Pollock, “Mode-locked Cr2+ :ZnSe laser,” Opt. Lett., vol. 25, pp. 168–170, 2000. I. T. Sorokina, E. Sorokin, A. Di Lieto, M. Tonelli, R. H. Page, and K. I. Schaffers, “Active and passive mode-locking of Cr2+ :ZnSe laser,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 46. Washington, D.C.: Optical Society of America, 2001, pp. 157–161. F. X. Kärtner, N. Matuschek, T. Schibli, U. Keller, H. A. Haus, C. Heine, R. Morf, V. Scheuer, M. Tilsch, and T. Tschudi, “Design and fabrication of double-chirped mirrors,” Opt. Lett., vol. 22, pp. 831–833, 1997. M. Mond, D. Albrecht, E. Heumann, G. Huber, S. Kück, V. I. Levchenko, V. N. Yakimovich, V. G. Shcherbitsky, V. E. Kisel, N. V. Kuleshov, M. Rattunde, J. Schmitz, R. Kiefer, and J. Wagner, “1.9-µm and 2.0µm laser diode pumping of Cr2+ :ZnSe and Cr2+ :CdMnTe,” Opt. Lett., vol. 27, pp. 1034–1036, 2002. G. J. Wagner and T. J. Carrig, “Power scaling of Cr2+ : ZnSe lasers,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 50. Washington, D.C.: Optical Society of America, 2001, p. 506. V. E. Kisel, V. G. Shcherbitsky, N. V. Kuleshov, V. I. Konstantinov, L. I. Postnova, V. I. Levchenko, E. Sorokin, and I. T. Sorokina, “Luminescence lifetime measurements in diffusion doped Cr:ZnSe,” presented at

the CLEO/Europe 2003. [126] B. C. Collings, J. B. Stark, S. Tsuda, W. H. Knox, J. E. Cunningham, W. Y. Jan, R. Pathak, and K. Bergmann, “Saturable Bragg reflector self starting passive mode-locking of a Cr4+ :YAG laser pumped with a diode-pumped Nd:YVO4 laser,” Opt. Lett., vol. 21, pp. 1171–1173, 1996. [127] I. T. Sorokina, E. Sorokin, S. Mirov, V. Fedorov, V. Badikov, V. Panyutin, and K. Schaffers, “Broadly tunable compact continuous-wave Cr2+ :ZnS laser,” Opt. Lett., vol. 27, pp. 1040–1042, 2002. [128] S. B. Mirov and T. Basiev, “Progress in color center lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 1, pp. 22–30, 1995. [129] I. T. Sorokina, E. Sorokin, E. Wintner, A. Cassanho, H. P. Jenssen, and R. Szipöcs, “Sub-20 fs pulse generation from the mirror dispersion controlled Cr:LiSGaF and Cr:LiSAF lasers,” Appl. Phys. B, vol. 65, pp. 245–255, 1997. [130] T. Brabec, Ch. Spielmann, P. F. Curley, and F. Krausz, “Kerr lens mode locking,” Opt. Lett., vol. 17, pp. 1292–1294, 1992. [131] B. Segall and D. T. F. Marple, Physics and Chemistry of II-IV Compounds, M. Aven and J. S. Prener, Eds. Amsterdam, The Netherlands: North Holland, 1967, pp. 319–381. [132] K. Graham, S. Mirov, V. Fedorov, M. E. Zvanut, A. Avanesov, V. Badikov, B. Ignat’ev, V. Panutin, and G. Shevirdyaeva, “Spectroscopic characterization and laser performance of diffusion doped Cr2+ :ZnS,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 46. Washington, D.C.: Optical Society of America, 2001, pp. 561–567. [133] S. B. Mirov, V. V. Fedorov, K. Graham, I. Moskalev, V. Badikov, and V. Panyutin, “Er-fiber laser pumped continuous-wave microchip Cr2+ :ZnS and Cr2+ :ZnSe lasers,” Opt. Lett., vol. 27, pp. 909–911, 2002. [134] A. G. Bluiett, U. Hömmerich, J. T. Seo, R. Shah, S. B. Trivedi, S. W. Kutcher, C. C. Wang, and P. R. Boyd, “Observation of lasing from Cr2+ :CdTe and compositional effects in Cr2+ doped II-VI semiconductors,” J. Electron. Mater., vol. 31, pp. 806–810, 2002. [135] J. T. Seo, U. Hömmerich, S. B. Trivedi, R. J. Chen, and S. Kutcher, “Slope efficiency and tunability of a Cr2+ -doped Cd0. 85 Mn0. 15 Te midinfrared laser,” Opt. Comm., vol. 153, pp. 267–270, 1998. [136] J. T. Seo, U. Hömmerich, H. Zong, S. B. Trivedi, S. W. Kutcher, C. C. Wang, and R. J. Chen, “Midinfrared lasing from a novel optical material: Chromium-doped Cd0. 55 Mn0. 45 Te,” Phys. Stat. Sol. (a), vol. 175, p. R3, 1999. [137] U. Hömmerich, J. T. Seo, M. Turner, A. Bluiett, S. B. Trivedi, H. Zong, S. Kutcher, C. C. Wang, and R. J. Chen, “Midinfrared laser development based on transition metal doped cadmium manganese telluride,” J. Lumin., vol. 87–89, pp. 1143–1145, 2000. [138] J. McKay, W. B. Roh, and K. L. Schepler, “Extended mid-IR tuning of a Cr2+ :CdSe laser,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 68. Washington, D.C.: Optical Society of America, 2002, pp. 371–373. [139] I. T. Sorokina, E. Sorokin, A. Di Lieto, M. Tonelli, and R. H. Page, “Tunable diode-pumped continuous-wave single crystal and ceramic Cr2+ :ZnSe lasers,” presented at the CLEO/Pacific Rim 2001, Tokyo, Japan, 2001. [140] M. J. Kozielski, “Polytype single crystals of Zn1−x Cdx S and ZnS1−x Sex solid solutions grown from the melt under high argon pressure by Bridgman’s method,” J. Cryst. Growth, vol. 30, p. 86, 1975. [141] I. T. Sorokina, E. Sorokin, A. Di Lieto, M. Tonelli, and E. A. Vinogradov, “Spectroscopy and room-temperature continuous-wave lasing from a new gain material Cr2+ :ZnSx Se1−x ,” presented at the CLEO 2004. [142] E. Sorokin, I. T. Sorokina, A. Di Lieto, M. Tonelli, and P. Minguzzi, “Mode-locked ceramic Cr2+ :ZnSe laser,” in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, vol. 83. Washington, D.C.: Optical Society of America, 2003, pp. 227–230. [143] I. T. Sorokina, E. Sorokin, S. Mirov, V. Fedorov, V. Badikov, V. Panyutin, A. Di Lieto, and M. Tonelli, “Continuous-wave tunable Cr2+ :ZnS laser,” Appl. Phys. B, vol. 74, pp. 607–611, 2002. [144] D. Kopf, K. J. Weingarten, G. Zhang, M. Moser, M. A. Emanuel, R. J. Beach, J. A. Skidmore, and U. Keller, “High-average-power diodepumped femtosecond Cr:LiSAF lasers,” Appl. Phys. B, vol. 65, pp. 235– 243, 1997. [145] V. L. Kalashnikov, E. Sorokin, and I. T. Sorokina, “Multipulse operation and limits of the Kerr-lens mode locking stability,” IEEE J. Quantum Electron., vol. 39, no. 2, pp. 323–336, Feb. 2003. [146] D. J. Ripin, C. Chudoba, J. T. Gopinath, J. G. Fujimoto, E. P. Ippen, U. Morgner, F. X. Kärtner, V. Scheuer, G. Angelow, and T. Tschudi,


[147] [148] [149]

[150]

[151]

[152] [153] [154] [155]

[156] [157] [158] [159] [160] [161] [162] [163]

[164] [165] [166] [167] [168] [169] [170]

“Generation of 20-fs pulses by a prismless Cr4+ :YAG laser,” Opt. Lett., vol. 27, p. 61, 2002. W. Sibbett, B. Agate, C. T. A. Brown, A. A. Lagatsky, C. G. Leburn, B. Stormont, and E. U. Rafailov, “Compact femtosecond oscillators,” in Ultrafast Optic IV, New York: Springer, 2003. S. Uchiyama, N. Yokouchi, and T. Ninimiya, IEEE Photonics Tech. Lett., vol. 9, p. 141, 1997. B. C. Collings, J. B. Stark, S. Tsuda, W. H. Knox, J. E. Cunningham, W. Y. Jan, R. Pathak, and K. Bergmann, “Saturable Bragg reflector self-starting passive mode-locking of a Cr4+ :YAG laser pumped with a diode-pumped Nd:YVO4 laser,” Opt. Lett., vol. 21, p. 1171, 1996. Z. Zhang, T. Nakagawa, K. Torizuka, T. Sugaya, and K. Kobayashi, “Gold-reflector based semiconductor saturable absorber mirror for femtosecond mode-locked Cr4+ :YAG lasers,” Appl. Phys. B, vol. 70, p. S59, 2000. A. J. Alcock, P. J. Poole, and B. T. Sullivan, “Hybrid semiconductor saturable absorber mirrors as passive mode-locking elements,” presented at the OPTO Canada, SPIE Regional Meeting on Optoelectronics, Photonics and Imaging, Ottawa, ON, Canada, May 2002. A. J. Alcock, P. Scorah, and K. Hnatovsky, “Broadly tunable continuouswave diode-pumped Cr4+ :YAG laser,” Opt. Comm., vol. 215, p. 153, 2003. S. Kück, K. Petermann, U. Pohlmann, and G. Huber, “Near-infrared emis-sion of Cr4+ -doped garnets, lifetimes, quantum efficiencies, and emission cross sections,” Phys. Rev. B, vol. 51, pp. 17323–17331, 1995. S. Kück, K. L. Schepler, S. Hartung, K. Petermann, and G. Huber, “Excited state absorption and its influence on the laser behaviour of Cr4+ -doped garnets,” J. Lumin., vol. 72–74, pp. 222–223, 1997. Z. Burshtein, P. Blau, Y. Kalisky, Y. Simony, and M. R. Kokta, “Excited-state absorption studies of Cr4+ Ions in several garnet host crystals,” IEEE J. Quantum Electron., vol. 34, no. 2, pp. 292–299, Feb. 1998. G. J. Tearney, B. E. Bouma, S. A. Boppart, B. Golubovic, E. A. Swanson, and J. G. Fujimoto, “Rapid acquisition of in vivo biological images by use of optical coherence tomography,” Opt. Lett., vol. 21, p. 1408, 1996. M. Seiter and M. W. Sigrist, “Trace-gas sensor based on mid-IR difference-frequency generation in PPLN with saturated output power,” Infrared Phys. Tech., vol. 41, pp. 259–269, 2000. F. K. Tittel, D. Richter, and A. Fried, “Midinfrared laser applications in spectroscopy,” in Solid-State Midinfrared Laser Sources, Topics in Applied Physics, vol. 89. Berlin, Germany: Springer, 2003, pp. 445–516. C. Fischer, E. Sorokin, I. T. Sorokina, and M. W. Sigrist, “Photoacoustic monitoring of gases using a novel laser source tunable around 2.5 µm,” Opt. Lasers Eng vol. 43, pp. 573–582, 2005. K. P. Petrov, R. F. Curl, and F. K. Tittel, “Compact laser differencefrequency spectrometer for multicomponent trace-gas detection,” Appl. Phys. B, vol. 66, pp. 531–538, 1998. E. Sorokin, I. T. Sorokina, C. Fischer, and M. W. Sigrist, “Widely tunable Cr2+ :ZnSe laser source for trace-gas sensing,” presented at the Advanced Solid-State Photonics 2005. V. M. Baev, T. Latz, and P. E. Toschek, “Laser intracavity absorption spectroscopy,” Appl. Phys. B, vol. 69, pp. 171–202, 1999. V. M. Baev, V. P. Dubov, A. N. Kireev, E. A. Sviridenkov, D. D. Toptygin, and O. I. Yushchuk, “Application of lasers with Fa(II) color centers in KCl:Li crystals in intracavity laser spectroscopy,” Sov. J. Quantum Electron., vol. 16, pp. 121–1123, 1986. M. P. Frolov and Yu. P. Podmar’kov, “Intracavity laser spectroscopy with a Co:MgF2 laser,” Opt. Comm., vol. 155, pp. 313–316, 1998. A. Stark, L. Correia, M. Teichmann, S. Salewski, C. Larsen, V. M. Baev, and P. E. Toschek, “Intracavity absorption spectroscopy with thuliumdoped fibre laser,” Opt. Comm., vol. 215, pp. 113–123, 2003. V. A. Akimov, V. I. Kozlovsky, Yu. V. Korostelin, A. I. Landman, Yu. P. Podmar’kov, and M. P. Frolov, “Intracavity laser spectroscopy using a Cr2+ :ZnSe laser,” Quantum Electron., vol. 34, pp. 185–188, 2004. F. Gueye, E. Safari, M. Chenevier, G. Guelachvili, and N. Picqué, “Timeresolved fourier transform intracavity Cr4+ :YAG-laser spectroscopy in the 1.5 µm region” submitted for publication. E. Sorokin, I. T. Sorokina, N. Picqué, F. Gueye, and G. Guelachvili, “Mid-IR high-resolution intracavity Cr2+ :ZnSe laser-based spectrometer,” presented at the Adv. Solid-State Photonics, 2005. N. Picqué and G. Guelachvili, “High-information time-resolved Fourier transform spectroscopy at work,” Appl. Opt., vol. 39, pp. 3984–3990, 2000. R. H. Holzwarth, Th. Udem, T. W. Hänsch, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, “Optical frequency synthesizer for precision spec-

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troscopy,” Phys. Rev. Lett., vol. 85, pp. 2264–2267, 2000. [171] D. A. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, Science, vol. 288, p. 635, 2000. [172] A. Baltuska, Z. Wei, M. S. Pshenichnikov, D. A. Wiersma, and R. Szipöcs Appl. Phys. B, vol. 65, pp. 175–188, 1997. [173] D. Huang, E. A. Swanson, C. P. Lin, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science, vol. 254, pp. 1178–1181, 1991. [174] R. Holzwarth, Th. Udem, T. W. Hänsch, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, “An optical frequency synthesizer for precision spectroscopy,” Phys. Rev. Lett., vol. 85, pp. 2264–2266, 2000. [175] J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett., vol. 25, pp. 25–27, 2000. [176] T. A. Birks, W. J. Wadsworth, and P. St. J. Russell, “Supercontinuum generation in tapered fibers,” Opt. Lett., vol. 25, p. 1415, 2000. [177] D. Mogilevtsev, T. A. Birks, and P. St. J. Russel, “Group-velocity dispersion in photonic crystal fibres,” Opt. Lett., vol. 23, pp. 1662–1664, 1998. [178] P. Russell, “Photonic crystal fibres,” Nature, vol. 299, pp. 358–362, 2003. [179] W. J. Wadsworth, A. Ortigosa-Blanch, J. C. Knight, T. A. Birks, T.P. Martin Man, and P. St. J. Russell, “Supercontinuum generation in photonic crystal fibers and optical fiber tapers: A novel light source,” J. Opt. Soc. Amer. B, vol. 19, pp. 2148–2155, 2002. [180] J. M. Harbold, F. O. Ilday, F. W. Wise, T. A. Birks, W. J. Wadsworth, and Z. Chen, “Long-wavelength continuum generation about the second dispersion zero of a tapered fiber,” Opt. Lett., vol. 27, p. 1558, 2002. [181] E. Sorokin, V. L. Kalashnikov, S. Naumov, J. Teipel, F. Warken, H. Giessen, and I. T. Sorokina, “Intra-and extra-cavity spectral broadening and continuum generation at 1.5 µm using compact low-energy femtosecond Cr:YAG laser,” Appl. Phys. B, vol. 77, pp. 197–204, 2003. [182] T. Okuno, M. Onishi, T. Kashiwada, S. Ishikawa, and M. Nishimura, “Silica-based functional fibers with enhanced nonlinearity and their applications,” IEEE J. Sel. Topics Quantum Electron., vol. 5, pp. 1385– 1391, 1999. [183] N. Nishizawa and T. Goto, “Widely broadened super continuum generation using highly nonlinear dispersion shifted fibers and femtosecond fiber laser,” Jpn. J. Appl. Phys., vol. 40, p. L365, 2001. [184] V. V. Ravi Kanth Kumar, A. K. George, W. H. Reeves, J. C. Knight, P. St. J. Russell, F. G. Omenetto, and A. J. Taylor, “Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation,” Opt. Expr., vol. 10, pp. 1520–1525, 2002. [185] J. Herrmann, U. Griebner, N. Zhavoronkov, A. Husakov, D. Nickel, J. C. Knight, W. J. Wadsworth, P. St. J. Russell, and G. Korn, “Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers,” Phys. Rev. Lett., vol. 88, p. 173901, 2002. [186] H. Hundertmark, D. Kracht, D. Wandt, C. Fallnich, V. V. Ravi Kanth Kumar, A. K. George, J. C. Knight, and P. St. J. Russell, “Supercontinuum generation with 200 pJ laser pulses in an extruded SF6 fiber at 1560 nm,” Opt. Expr., vol. 11, p. 3196, 2003. [187] E. Sorokin, S. Naumov, V. Kalashnikov, I. T. Sorokina, V. V. Ravi Kanth Kumar, A. K. George, J. C. Knight, and P. St. J. Russell, “Supercontinuum generation from a Cr4+ :YAG laser using a soft-glass extruded PCF,” presented at the Advanced Solid-State Photonics Conf., Santa Fe, NM, 2004. [188] G. P. Agrawal, Nonlinear Fiber Optics. New York: Academic, 2001. [189] H. Cao, “Lasing in random media,” Waves Random Media, vol. 12, pp. R1–R39, 2003. [190] M. A. Noginov, Solid-State Random Lasers. New York: Springer, 2005. [191] Special Issue on Random Lasers, J. Opt. Soc. Am. B, vol. 21, pp. 98–222, Jan. 2004. [192] R. B. Ambartsumyan, N. G. Basov, P. G. Kryukov, and V. S. Letokhov, “A laser with a nonresonant feedback,” IEEE J. Quantum Electron., vol. 2, pp. 442–446, 1966. [193] V. A. Nikitenko, A. I. Tereschenko, I. P. Kuz’mina, and A. N. Lobachev, “Stimulated emission of ZnO at high level of single photon excitation,” Opt. Spektrosk., vol. 50, pp. 605–607, 1981. Translated Opt. Spectroscopy, vol. 50, pp. 331–332, 1981. [194] V. M. Markushev, V. F. Zolin, and Ch. M. Briskina, “Luminescence and stimulated emission of neodymium in sodium lanthanum molybdate powders,” Sov. J. Quantum Electron., vol. 16, pp. 281–283, 1986. [195] E. Sorokin, I. T. Sorokina, A. Di Lieto, M. Tonelli, and R. H. Page, “Tunable diode-pumped continuous-wave single crystal and ceramic Cr2+ :ZnSe lasers,” CLEO/Pacific Rim 2001 Tech. Dig., 2001. [196] E. Sorokin, I. T. Sorokina, A. Di Lieto, M. Tonelli, and P. Minguzzi,

712

[197]

[198]

[199] [200] [201] [202] [203] [204]

[205] [206]

[207]

[208]

[209]


“Mode-locked ceramic Cr2+ :ZnSe laser,” OSA Trends in Optics and Photonics, Advanced Solid-State Photonics Conf., vol. 83, pp. 227–203, 2003. I. T. Sorokina, E. Sorokin, V. G. Shcherbitsky, N. V. Kuleshov, G. Zhu, and M. A. Noginov, “Room-temperature lasing in nanocrystalline Cr2+ :ZnSe random laser,” OSA Trends in Optics and Photonics, Advanced Solid-State Photonics, vol. 94, pp. 376–380, 2004. I. T. Sorokina, E. Sorokin, V. Shcherbitsky, N. V. Kuleshov, G. Zhu, A. Franz, and M. A. Noginov, “Novel midinfrared random powder lasers: Cr2+ :ZnS vs. Cr2+ :ZnSe,” presented at the Int. Quantum Electronics Conf. (IQEC), 2004. C. M. Soukoulis, X. Jiang, J. Y. Xu, and H. Cao, “Dynamic response and relaxation oscillations in random lasers,” Phys. Rev. B, vol. 65, p. 041103, 2002. P. Connes and G. Michel, “High-resolution Fourier spectra of stars and planets,” Astrophys. J. vol. 190, pp. L29–L32, 1974. J.-Y. Mandin, “Interpretation of the CO2 absorption bands observed in the Venus infrared spectrum between 1 and 2.5 µm,” J. Mol. Spectrosc., vol. 67, pp. 304–321, 1977. T. J. Carrig, G. J. Wagner, W. J. Alford, and A. Zakel, “Chromiumdoped chalcogenide lasers,” in Proc. Photonics Europe 2004. Strasbourg, France, Apr. 2004, vol. 5460, pp. 74–82. G. J. Wagner, B. G. Tiemann, W. J. Alford, and T. J. Carrig, “Singlefrequency Cr:ZnSe laser,” presented at the Advanced Solid-State Photonics Conf., 2004. J. B. McKay, W. Roh, and K. L. Schepler, “4.2 W Cr2+ : ZnSe facecooled disk laser,” in OSA Trends in Optics and Photonics, Conference on Lasers and Electrooptics, vol. 73. Washington, D.C.: Optical Society of America, 2002, pp. 119–120. K. L. Schepler, R. D. Peterson, P. A. Berry, and J. B. McKay, “Thermal effects in Cr:2+ ZnSe thin disk lasers,” in this issue. C. Pollock, N. Brilliant, D. Gwin, T. J. Carrig, W. J. Alford, J. B. Heroux, W. I. Wang, I. Vurgaftman, and J. R. Meyer, “mode-locked and Qswitched Cr:ZnSe laser using a semiconductor saturable absorbing mirror (SESAM),” presented at the Advanced Solid-State Photonics Conf., 2005. A. Major, A. Major, F. Yoshino, J. S. Aitchison, and P. W. E. Smith, “Ultrafast nonresonant third-order optical nonlinearities in ZnSe for photonic switching at telecom wavelengths,” Appl. Phys. Lett., vol. 85, pp. 4606– 4608, 2004. C. J. S. de Matos, S. V. Popov, J. R. Taylor, and K. P. Hansen, “Low noise, high-brightness, broadband, all-fiber CW sources for OCT around 1300 nm,” presented at the Paper TuA3 at Adv. Solid-State Photonics Conf., 2004. A. Gallian, V. V. Fedorov, J. Kernal, S. B. Mirov, and V. V. Badikov, “Laser oscillation at 2.4 µm from Cr2+ in ZnSe optically pumped over Cr ionization transitions,” presented at the Advanced Solid-State Photonics Conf., 2005.

Evgeni Sorokin was born in Moscow, Russia, in 1962. He received the M.S. degree in physics and mathematics from the Moscow M. V. Lomonosov State University in 1986, and the Ph.D. degree in technical physics from the Vienna University of Technology, Vienna, Austria, in 1994 for spectroscopy and laser properties of disordered crystals. In 1986, he joined the research staff of the General Physics Institute of the Russian Academy of Sciences, working on high-temperature Raman spectroscopy of solids and melts, while also teaching at the Moscow Physics and Technology Institute. Since 1992, he has been with

the Quantum Electronics and Laser Technology Group of the Vienna University of Technology, and since 1999, he has been an Assistant Professor with the Photonics Institute of the same university. His current research interests include physics of diode-pumped tunable and ultrashort-pulsed lasers based on novel rare-earth and transition-metal-doped crystals, as well as spectroscopic applications of the ultrabroadband lasers. Dr. Sorokin is a Member of the Optical Society of America.

Sergey Naumov was born in 1976 in Moscow, Russia. He received the M.Sc. degree in 1999 from the Moscow Institute of Physics and Technology (State University), Russia, and the Ph.D. degree from Vienna University of Technology, Vienna, Austria for development of the directly diode-pumped modelocked Cr:YAG laser. Currently he is doing his post doctoral research on high energy Ti:sapphire oscillator at the Max-PlanckInstitute for Quantum Optics in Garching, Germany.

Irina T. Sorokina was born in Moscow, Russia, in 1963. She received the M.S. degree in physics from the Moscow Lomonosov State University in 1986 and the Ph.D. degree in laser physics from the General Physics Institute of the Russian Academy of Sciences (GPI), Moscow, in 1992. Her doctoral study was focused on the electron excitation energy transfer processes from Cr to Nd, Tm and Ho ions in scandium garnet crystals. She obtained the Habilitation degree in laser technology and quantum electronics in 2003. Since 1986, she has been a Research Staff Scientist at the GPI. In 1989, she spent a few months as a Visiting Scientist with the Quantum Electronics Department of the Vienna University of Technology (TU Wien), Vienna, Austria. She is currently with the same department of the TU Wien, where she was a Lise-Meitner Fellow of the Austrian National Science Fund (FWF) from 1992 through 1994, and a Hertha-Firnberg Assistant Professor from 1999 through 2003. Since 1999 she has been the Head of the Solid-State Lasers Group at the Photonics Institute, TU Wien. Her current research is focused on the development and characterization of the novel broad band crystalline lasers, femtosecond pulse generation, nonlinear optics, and physics of interionic processes in laser crystals. Dr. Sorokina chaired a number of international conferences. She is a member of the Austrian Physical Society, the Optical Society of America, and the Austrian Association of Scientists Wissenschaftsforum.