IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 42, NO. 2, FEBRUARY 2006
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Midinfrared Holmium Fiber Lasers Stuart D. Jackson
Abstract—The use of the high-power Tm3+ -doped silica fiber laser as a pump source for Ho3+ -doped silica and Ho3+ -doped fluoride fiber lasers for the generation of 2.1- m radiation is demonstrated. The Ho3+ -doped silica fiber laser produced a maximum output power of 1.5 W at a slope efficiency of 82%; one of the highest slope efficiencies measured for a fiber laser. In a nonoptimized but similar fiber laser arrangement, a Ho3+ -doped fluoride fiber laser produced an output power of 0.38 W at 2.08 m at a slope efficiency of 50%. A Raman fiber laser operating at 1160 nm was also used to pump a Ho3+ -doped fluoride fiber laser operating at a wavelength of 2.86 m. An output power of 0.31 W was produced at a slope efficiency of 10%. The energy transfer upconversion process that depopulates the lower laser level in this case operates at a higher efficiency when the pump wavelength is closer to the absorption peak of the 5 6 energy level, however, this energy transfer process does not impede to a great extent the performance of the Ho3+ -doped fluoride fiber laser based on the 2.1- m laser transition. Index Terms—Fiber lasers, high efficiency, high power, holmium, midinfrared, Raman fiber lasers.
I. INTRODUCTION
T
HE STRONG interest in fiber laser research arises from the high efficiency and high-power particular examples such as the Yb -doped silica fiber laser [1] and the Yb , Er -doped silica fiber laser [2], [3] have recently demonstrated. As a consequence of the high overall nonlinearity present in silica-based fibers of sufficient length, fiber lasers are also convenient generators of ultrafast pulses [4]. Alternatively, when the nonlinearities are minimized, high-power single frequency emission from fiber lasers is also possible [5]. Perhaps, as a consequence of the now well-accepted utility of fiber lasers, these lasers will soon be ubiquitous sources of laser radiation. Extending the wavelength range of the output emitted from fiber lasers into the midinfrared region is an active pursuit within fiber laser research. Recently a milestone was reached when an output power of 85 W at 2.05 m was demonstrated from a directly diode-pumped Tm -doped silica fiber laser [6]; a milestone which has cemented the Tm -doped silica fiber laser as one of the most convenient generators of midinfrared laser radiation. In efforts to extend the wavelength still further, the Er ion was shown some time ago to provide efficient fiber laser
Manuscript received June 27, 2005; revised September 14, 2005. This work was supported in part by the Australian Research Council and the Australian Photonics Co-operative Research Centre. The author is with the Optical Fiber Technology Centre, Australian Photonics CRC, The University of Sydney, 1430 Sydney, Australia (e-mail:
[email protected]). Digital Object Identifier 10.1109/JQE.2005.861824
Fig. 1. Simplified energy level diagram of Ho showing the pump and laser mechanisms for the 2046-nm Tm -doped silica fiber laser pumped Ho -based fiber lasers, the pump and laser processes for the Raman fiber laser pumped Ho -doped fluoride fiber laser and the ETU processes ETU1 and ETU2.
output in the 3- m region of the spectrum albeit at power levels much lower than their 2- m counterparts [7]–[9]. The Ho ion, in addition to the Tm ion [10], offers high power output in the 2.1- m region of the spectrum [11], [12] while also providing 3 m output [13], [14] and 3 m 2.1 m laser transition of output [15]–[17]. The Ho ion when doped into silica also exhibits a broad tuning range, extending from 2019 to 2163 nm [18] and it offers extended scalability due to the possibility of optical excitation with a Yb -doped silica fiber laser [19], [20]. The Ho ion is perhaps the last rare earth ion that can enable high-power infrared output to be generated from a silica-based fiber laser configuration. -doped silica In this paper, we explore the use of the Tm fiber laser as a pump source for direct upper laser level exci2.1 m laser transition of Ho , tation of the see Fig. 1. Whilst previous reports have shown the utility of the -doped silica fiber laser as a pump source for Ho :YAG Tm lasers [21] we show in this report the highly efficient excitation of both Ho -doped silica and Ho -doped fluoride fiber lasers for the generation of 2.1- m output. We also show that Raman fiber lasers that produce 1160 nm output are useful pump sources for singly Ho -doped fluoride fiber lasers for 3- m output. The increased rate of enthe generation of ergy transfer upconversion (ETU) resulting from the increased excited Ho ion density associated with this pump method allows more effective depletion of the lower laser level of the 3 m laser transition of Ho .
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Fig. 2. Schematic diagram of the experimental setup of the Tm fiber laser-pumped Ho -based fiber lasers.
-doped silica
II. EXPERIMENT Two holmium-doped fibers were used for the experiments. An in-house fabricated 0.5 wt.% (4 10 m ) Ho -doped silica fiber with a mode cut-off wavelength near 1950 nm was used for experiments relating to 2.1- m emission and Tm -doped silica fiber laser pumping. The Ho -doped silica fiber contained 5 mol.% GeO and 11.5 wt.% Al so that a Ho concentration ratio of 15:1 could be obtained. Al Ho concentration ratio ensured that clusThe high Al tering-enhanced energy transfer processes were minimized. The Ho -doped fluoride fiber (nominally ZBLAN and fabricated by FiberLabs, Japan) had a 4.2 wt.% (i.e., 7 10 m ) m, a numerical Ho concentration, a core diameter of 0.02 and measured background losses of aperture of 0.16 40 and 130 dB/km at 800 and 1400 nm, respectively. At worst, this fiber supported single transverse mode propagation down to 3.29 m. The Tm -doped silica fiber laser was an in-house built system that was diode cladding pumped with a 50-W 805-nm diode laser system manufactured by Fisba Optik, Switzerland. The fiber used for the Tm -doped silica fiber laser was optimized for maximum cross-relaxation and minimum ETU [22] and was used recently for high-power fiber laser experiments [6]. The output from the Tm -doped silica 4.1 and was capable of fiber laser was multimode 13 W output power at a wavelength of 2046 nm. The light from the Tm -doped silica fiber laser was collimated with a 0.25 NA 10 microscope objective ( 75% transmitting at 2046 nm) and was directed toward a second identical microscope objective for focussing onto the Ho -doped fibers; see experimental setup shown schematically in Fig. 2. The focussed pump light was steered onto the Ho -doped fibers with the use of a 45 mirror that was 90% transmitting at 2 m. The Ho -doped silica fiber laser cavity comprised of Fresnel reflection at the output end of the fiber and a highly reflecting 2- m mirror at the unpumped end to the fiber. The output power was measured with a power meter (Melles Griot 13PEM001) and the wavelength of the fiber laser output measured with the use of a thermo-electrically-cooled InAs photodiode in conjunction with a lock-in amplifier and a manually controlled monochromator. For the Raman fiber laser pumping experiments, a high-power Yb -doped silica fiber laser, which was pumped with a commercial 975 nm diode laser system (Laserline, Germany)
Fig. 3. Schematic diagram of the experimental setup of the Raman fiber laser-pumped Ho -doped fluoride fiber laser.
pumped a Raman fiber laser that utilized moderately Ge-doped silica fiber [23]. The Yb -doped silica fiber laser employed a 20% reflecting fiber Bragg grating centred at 1104 nm and the Raman fiber laser employed a 20% reflecting fiber Bragg grating centred at 1160 nm; see Fig. 3. The broad-band dichroic at the pump input end to the Yb -doped silica fiber laser served as the high reflector for both the 1104- and 1160-nm-based fiber laser resonators. The 1160-nm output was collimated and focussed with a pair of microscope objective lenses and was launched at an efficiency of 70% into the core of the Ho -doped fluoride fiber after reflection from a 45 dichroic mirror. The Ho -doped fluoride fiber laser resonator comprised of Fresnel reflection at the pump input end 2.9 m) dielectric mirror butted and a highly reflecting (at against the distal end to the fiber; see Fig. 3. III. RESULTS The first experiments were carried out to determine the absorption coefficients and absorption cross sections of Ho at the 2046-nm pump wavelength from the Tm -doped silica fiber laser. Using the standard cutback technique, the absorpfor the Ho -doped silica and fluoride tion coefficient fibers was 1.8 and 40 m , respectively. The absorption cross sections at 2046 nm were calculated to be 5 10 m and m for the Ho -doped silica and fluoride fibers, 6 10 respectively. The measured cross section for Ho -doped fluoride is lower than previous measurements which have the 1.5 10 m corresponding absorption cross section at [24] and 1 10 m [25] for Ho -doped ZBLAN and fluorozircoaluminate glass, respectively. The reason for the inconsistency is currently unidentified, however, we did not know exact composition of the fluoride fiber used in our investigation. The Ho -doped silica fiber laser output as a function of absorbed 2046-nm pump power is displayed in Fig. 4. The maximum output power of 1.5 W was generated from a 0.71-m-long ) at a slope efficiency of 82% with refiber (i.e., spect to the launched pump power. Note that pump retro-reflection allows complete absorption of the launched pump light. ) The slope efficiency for a 0.93-m-long fiber (i.e.,
JACKSON: MIDINFRARED HOLMIUM FIBER LASERS
Fig. 4. Measured output power of the 2.1-m emission from the Ho -doped silica fiber laser as a function the launched (and absorbed) power from the Tm -doped silica fiber laser. The output characteristics for two Ho -doped silica fiber lengths are shown.
Fig. 5. Measured wavelength of the output from the Ho -doped silica fiber laser as a function of the length of Ho -doped silica fiber.
was only 58%. The reduced efficiency in this case relates to the comparatively lower absorption efficiency introduced with the longer fiber length. For shorter, i.e., 0.71 m fiber lengths, a residual amount of unabsorbed pump light was detected and a lower overall efficiency was measured. The wavelength of the Ho -doped silica fiber laser output as a function of the fiber length is shown in Fig. 5. The wavelength of the output varies in an essentially linear manner with the change in the length Ho -doped silica fiber. The measured output power from the Ho -doped fluoride fiber laser as a function of absorbed 2046-nm pump power is shown in Fig. 6. In this case, the length of fiber was limited ) because of mechanical restrictions. to 0.1 m (i.e., L Using the above results as a guide, we would expect higher slope efficiencies than the 50% measured if shorter fiber lengths were tested. The output wavelength was measured to be 2084 nm, which is commensurate with previous demonstrations of Ho -doped fluoride fiber lasers, which have been shown to lase in the range 2050–2080 nm [11], [12], [26]. In general, optimally configured Ho -doped fluoride fiber lasers emit light at shorter wavelengths than their Ho -doped silica fiber laser equivalents.
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Fig. 6. Measured output power of the 2.1-m emission from the Ho -doped fluoride fiber laser as a function the launched (and absorbed) power from the Tm -doped silica fiber laser.
Fig. 7. Measured output power of the 2.9 m emission from the Ho -doped fluoride fiber laser as a function the launched (and absorbed) power from the Raman fiber laser. Included is the output characteristics for the Ho , Pr -doped fluoride fiber laser as a function the launched (and absorbed) power from the Raman fiber laser.
Fig. 7 displays the output power from the Ho -doped fluoride fiber laser as a function of launched power at 1160 nm from the Raman fiber laser. A maximum output power of 89 mW was generated at a slope efficiency of 10% after the threshold of 190 mW was reached. This slope efficiency is approximately double the slope efficiency measured for 1100-nm pumping [27] which indicates that the comparatively stronger absorption associated with 1160-nm pumping leads to an increase in the rate of ETU which acts to deplete the lower laser level of the 2.9 m laser transition of Ho . As a consequence, the Ho ion population density with electrons in the energy level is lower for 1160 nm pumping and hence the laser transition operates at comparatively shorter wavelengths; the measured wavelength was 2.86 m as compared with 2.92 m which was associated with 1100 nm pumping [27]. For comparison, a Ho , Pr -doped fluoride fiber [28] was also pumped with the 1160-nm Raman fiber laser output and the results are also displayed in Fig. 7. The performance was essentially unchanged compared to the performance associated with 1100 nm pumping [28].
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IV. DISCUSSION The concept of directly exciting the upper laser level of fiber lasers operating on ground state based laser transitions was first applied some time ago to 1.48- m-pumped Er doped silica fiber amplifiers [29] and lasers [30]. Later, the F upper laser level of the Tm ion when doped into fluoride [31] and silica [32] was directly pumped with the output from an Er -doped silica fiber laser and 1.6 m diode laser [33] to provide ef2 m output. Direct upper laser pumping or resoficient nant pumping of fiber lasers provides very efficient and stable output [34]. The current slope efficiency of 82% is higher than the slope efficiencies measured in the above cited demonstrations involving direct upper laser level pumping but is significantly lower than the Stokes efficiency limit of 97%. Background losses in the Ho -doped silica fiber and the use of a nonoptimized resonator configuration are possible reasons for the lower efficiency. In a practical device, however, it would be desirable to excite the Ho ion at shorter wavelengths i.e., closer to 1.9 m, as is used for direct upper laser level-excited Ho -doped bulk solid state lasers [35], [36]. More efficient extraction of the Ho -based fiber laser output will take place in this case using a dichroic mirror. With the use of fiber Bragg gratings, however, an all fiber based system could be configured for the Ho -doped silica fiber laser arrangement. The development of tandem pumping arrangements is more complicated than the development of single fiber lasers. The diode-pumped Tm , Ho -doped silica fiber laser for example has been shown to offer a high efficiency of 44% with respect to the absorbed pump power [37]. Obviously, with further development of the ion concentrations, higher efficiencies are expected, however, as a result of ETU, which promotes electrons in the level to the level, the pump power at threshold will always be high in the Tm , Ho system. Tandem-pumped Ho -doped silica fiber lasers on the other hand enable a separation of the two ions thus allowing a reduced threshold pump power. -switched or mode-locked operation of the Ho -doped silica fiber laser is also made simpler in an arrangement using a separate fiber laser as a pump source. One of the main limitations to the high-power operation of fluoride-based fiber lasers is the possibility of melting the pumped end of the fiber. This is a particular problem for double-clad fluoride fibers, which generally have a large heated volume and a thick polymer second cladding. It has been shown [38] that fluoride double-clad fibers melt much more readily than silica double-clad fibers at similar pump levels and to power scale fluoride-based fiber lasers, cooling of the pumped end will be required. This method is already used in silica based high-power fiber laser development. The larger surface area to volume ratio associated with single clad fibers offers a better geometry for effective cooling, however, the concentration of the rare earth dopant must be judiciously chosen so as to avoid a very short absorption depth. Incidentally, we did not observe any optical or thermally induced damage in our experiments. (abbreviated as The ETU processes (abbreviated as ETU2), see ETU1) and Fig. 1, have been identified as possible mechanisms for depop3 m laser ulating the lower laser level of the
transition of Ho -doped fluoride [27]. It is clear from the comparison between 1160-nm pumping and 1100-nm pumping that ETU is working more effectively with the longer pump wavelength, which is closer to the absorption peak at 1150 nm. A recent investigation [39] highlighted the importance of ETU2 to the operation of bulk Ho 2 m lasers, however, an earlier YAG [40] showed that ETU1 has a microscopic study of Ho rate that is approximately 17 times higher than the microscopic rate for ETU2, as one would expect since ETU1 is exothermic and ETU2 is endothermic. One could use similar arguments to show that ETU1 is also dominant in fluoride glasses. The high slope efficiency of the Ho -doped silica fiber laser indicates that ETU had a minor effect on the overall performance of this laser which is expected given the Ho concentration in the silica fiber was only 0.5 wt.%. One would expect significantly reduced slope efficiencies when higher Ho concentrations are used, especially in light of the fact that silica has a higher maximum phonon energy and wider fluorescence and absorption spectra compared to fluoride glass. Clearly, detailed spectroscopic measurements on Ho -doped fluoride and Ho -doped silica glasses are required in order to elucidate the important ETU and cross relaxation processes present in these materials. V. CONCLUSION In this investigation we have shown that the Tm -doped silica fiber laser is an efficient and convenient pump source for Ho -based fiber lasers. In these preliminary experiments, we produced a slope efficiency of 82% with respect to the absorbed pump power for a Ho -doped silica fiber laser fiber laser operating at 2.1 m. Considering the high slope efficiencies now relevant to Tm -doped silica fiber lasers, tandempumped Ho -doped silica or fluoride fiber lasers are now a serious laser arrangement for the generation of 2.1- m laser light. We have also shown that Raman fiber lasers are useful pump sources for singly Ho -doped fluoride 2.9 m fiber lasers. The emission wavelength of Raman fiber lasers can be tailored to operate at wavelengths that provide a higher absorption efficiency, which results in high rates of beneficial ETU. REFERENCES [1] Y. Jeong, J. K. Sahu, D. N. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1 kW of continuous-wave output power,” Electron. Lett., vol. 40, pp. 470–472, 2004. [2] J. K. Sahu, Y. Jeong, D. J. Richardson, and J. Nilsson, “A 103 W erbiumytterbium co-doped large-core fiber laser,” Opt. Commun., vol. 227, pp. 159–163, 2003. [3] C. Alegria, Y. Jeong, C. Codemard, J. K. Sahu, J. A. Alvarez-Chavez, L. Fu, M. Ibsen, and J. Nilsson, “83-W single-frequency narrow-linewidth MOPA using large-core erbium-ytterbium co-doped fiber,” IEEE Photon. Technol. Lett., vol. 16, no. 8, pp. 1825–1827, Aug. 2004. [4] A. Tunnermann, J. Limpert, and S. Nolte, “Ultrashort pulse fiber lasers and amplifiers,” Topics Appl. Phys., vol. 96, pp. 35–53, 2004. [5] A. Liem, J. Limpert, H. Zellmer, and A. Tunnermann, “100-W singlefrequency master-oscillator fiber power amplifier,” Opt. Lett., vol. 28, pp. 1537–1539, 2003. [6] G. Frith, D. G. Lancaster, and S. D. Jackson, “85 W Tm -doped silica fiber laser,” Electron. Lett., vol. 41, pp. 21–22, 2005. [7] S. D. Jackson, T. A. King, and M. Pollnau, “Diode-pumped 1.7-W erbium 3-m fiber laser,” Opt. Lett., vol. 24, pp. 1133–1135, 1999.
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[27] S. D. Jackson, “Singly Ho -doped fluoride fiber laser operating at 2.92 m,” Electron. Lett., vol. 40, pp. 1400–1401, 2004. , “Single-transverse-mode 2.5-W holmium-doped fluoride fiber [28] laser operating at 2.86 m,” Opt. Lett., vol. 29, pp. 334–336, 2004. [29] M. Nakazawa, Y. Kimura, and K. Suzuki, “Efficient Er -doped optical fiber amplifier pumped by a 1.48-m InGaAsP laser diode,” Appl. Phys. Lett., vol. 54, pp. 295–297, 1989. [30] K. Suzuki, Y. Kimura, and Y. Nakazawa, “An 8-mW CW Er -doped fiber laser pumped by 1.46 m InGaAsP laser-diodes,” Jap. J. Appl. Phys. Part II-Lett., vol. 28, pp. L1000–L1002, 1989. [31] T. Yamamoto, Y. Miyajima, T. Komukai, and T. Sugawa, “1.9 m Tm-doped fluoride fiber amplifier and laser pumped at 1.58 m,” Electron. Lett., vol. 29, pp. 986–987, 1993. [32] T. Yamamoto, Y. Miyajima, and T. Komukai, “1.9 m Tm-doped silica fiber laser pumped at 1.57 m,” Electron. Lett., vol. 30, pp. 220–221, 1994. [33] R. M. Percival, D. Szebesta, C. P. Seltzer, S. D. Perrin, S. T. Davey, and M. Louka, “1.6 m semiconductor diode pumped thulium doped fluoride fiber laser and amplifier of very high efficiency,” Electron. Lett., vol. 29, pp. 2110–2111, 1993. [34] W. H. Loh, “Suppression of self-pulsing behavior in erbium-doped fiber lasers with resonant pumping,” Opt. Lett., vol. 21, pp. 734–736, 1996. [35] C. D. Nabors, J. Ochoa, T. Y. Fan, A. Sanchez, H. K. Choi, and G. W. Turner, “Ho:YAG laser pumped by 1.9-m diode lasers,” IEEE J. Quantum Electron., vol. 31, no. 9, pp. 1603–1605, Sep. 1995. [36] P. A. Budni, M. L. Lemons, J. R. Mosto, and E. P. Chicklis, “Highpower/high brightness diode-pumped 1.9-m thulium and resonantly pumped 2.1-m holmium lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 6, no. 4, pp. 629–635, Jul./Aug. 2000. [37] S. D. Jackson and S. Mossman, “High-power diode-cladding-pumped Tm , Ho -doped silica fiber laser,” Appl. Phys. B, vol. 77, pp. 489–491, 2003. [38] D. J. Coleman and T. A. King, “Pump induced thermal effects in high power Tm and Tm =Ho cladding-pumped fiber lasers,” Meas. Sci. Technol., vol. 14, pp. 998–1002, 2003. [39] N. P. Barnes, B. M. Walsh, and E. D. Filer, “Ho:Ho upconversion: applications to Ho lasers,” J. Opt. Soc. Amer. B, vol. 20, pp. 1212–1219, 2003. [40] L. B. Shaw, R. S. F. Chang, and N. Djeu, “Measurement of up-conversion energy-transfer probabilities in Ho : Y Al O and Tm : Y Al O ,” Phys. Rev. B, vol. 50, pp. 6609–6619, 1994.
Stuart D. Jackson received the B.Sc. degree and the B.Sc. (hons) degree from the University of Newcastle, Newcastle, Australia, in 1989 and 1990, respectively. In 1990, he joined the Centre for Lasers and Applications, Macquarie University, Macquarie, Australia, to undertake research toward the Ph.D. degree, which he received in 1996. In 1995, he joined the Laser Photonics Group, University of Manchester, Manchester, U.K., and initiated the research into high-power fiber lasers. In 1999, he joined the Optical Fiber Technology Centre, University of Sydney, Sydney, U.K., and is now an Australian Research Fellow sponsored directly by the Australian Research Council. His interests are in diode-pumped solid-state lasers, spectroscopy, and philosophy.
