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Vol. 40, No. 18 / September 15 2015 / Optics Letters
Ultrafast pulses from a mid-infrared fiber laser TOMONORI HU,1,* STUART. D. JACKSON,2
AND
DARREN. D. HUDSON1
1
Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS), Institute of Photonics and Optical Sciences, School of Physics, University of Sydney, New South Wales 2006, Australia 2 MQ Photonics, Department of Engineering, Macquarie University, New South Wales 2109, Australia *Corresponding author:
[email protected] Received 11 March 2015; accepted 10 August 2015; posted 18 August 2015 (Doc. ID 246376); published 7 September 2015
Ultrafast laser pulses at mid-infrared wavelengths (2–20 μm) interact strongly with molecules due to the resonance with their vibration modes. This enables their application in frequency comb-based sensing and laser tissue surgery. Fiber lasers are ideal to achieve these pulses, as they are compact, stable, and efficient. We extend the performance of these lasers with the production of 6.4 kW at a wavelength of 2.8 μm with complete electric field retrieval using frequency-resolved optical gating techniques. Contrary to the problems associated with achieving a high average power, fluoride fibers have now shown the capability of operating in the ultrafast, high-peak-power regime. © 2015 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.4050) Mode-locked lasers; (140.3070) Infrared and far-infrared lasers. http://dx.doi.org/10.1364/OL.40.004226
Light absorption by molecules is increased by orders of magnitude at mid-infrared (mid-IR) wavelengths due to the fundamental resonance of molecular vibrational modes. This has raised interest in fields such as frequency combs for precision molecular spectroscopy [1], and in laser surgery by targeting the strongest water absorption at 2.9 μm [2] in human tissue, leading to a reduced surgical damage area by more than an order of magnitude compared to conventional methods [3]. These highimpact applications have spurred the development of mid-IR sources, with the most dominant being the quantum cascade laser, which has a wavelength coverage from approximately 3 to 20 μm, and with which mode locking [4] and frequency comb generation [5] were recently demonstrated. Another platform that has enabled much progress in the near-IR is fiber lasers. These provide efficient, high-power systems in a compact and robust arrangement that is more suitable for field-deployable devices. For frequency comb generation, a fiber laser-based comb remains stable even when subject to mechanical vibrations with accelerations of up to 1.6 g [6]. A major challenge, however, for the generation of longer wavelengths is that standard silica-based optical fibers become opaque beyond 2.2 μm due to a multiphonon edge. Alternatively, the low-phonon energy fluoride glass family, such as the ZrF4 -BrF2 -LaF3 AlF3 -NaF (ZBLAN) composition, are able to remain 0146-9592/15/184226-03$15/0$15.00 © 2015 Optical Society of America
transmitting up to 4 μm, and can be doped with rare-earth elements. Despite their success as moderate-power continuouswave sources [7], their ultrafast performance via mode locking is only recent, and improvements in their measurements need to be made. Previous reports of pulse widths from passively mode-locked fluoride fiber lasers with emission between 2.8 and 2.9 μm have varied from 6 ps [8], to a series claim of 5–60 ps systems [9–11], and most recently with the demonstration of intensity autocorrelation-measured 207 fs pulses with a peak power of 3.4 kW [12]. To extend to the field, we demonstrate 6.4 kW pulses from a 2.8 μm mode-locked fiber laser, and perform a complete field retrieval using a mid-IR compatible frequency-resolved optical gating (FROG) analysis. This higher level of measurement allows for a better understand of the pulses to be used in the previously mentioned applications. Mode locking was carried out using nonlinear polarization rotation (NPR) inside a fluoride fiber that allows transmission into the mid-IR. As opposed to using saturable absorbers, which eventually suffer optical damage, the shortest pulses have been produced via nonlinear optical effects [13–16]. Our fiber ring laser (see Fig. 1) used the Kerr nonlinearity of the fluoride fiber (n2 2.1 × 10−20 m2 ∕W [17]) to create an intensity-dependent rotation of the polarization state inside the fiber which, when combined with polarizing optics, makes an effective saturable absorber for mode locking, a well-known effect in silica fibers [18]. Our ZBLAN fiber had a 16 μm diameter 7 mol. % doped core (with an NA of 0.12), and a second truncated cladding to guide the pump light at 980 nm, the same geometry as that used in [19]. We exploited the 4 I11=2 → 4 I13=2 transition of erbium, centered at 2750 nm. This system allows the use of standard telecommunication pump diodes and is consequently the most power-efficient 3 μm-class fiber laser [19]. Spectral measurements were made using a Fourier-transform infrared (FTIR) spectrometer, and temporal measurements were made using a mercury cadmium telluride (MCT) detector and electronic amplifier with a bandwidth of 400 MHz. A 26.7 GHz RF spectrum analyzer was used to measure the beat strength of the mode-locked pulses. The fiber laser was not encased in a dry nitrogen environment. At a launched pump power of 1.62 W, the specific orientations of the waveplates created various temporal outputs. For given configurations, Q-switched and Q-switched modelocked outputs were observed; however, their precise power
Vol. 40, No. 18 / September 15 2015 / Optics Letters
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Detection Spectrometer (FTIR)
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Fig. 1. Experimental setup for generating and measuring modelocked pulses from a ZBLAN fiber laser. A ring cavity is formed with a free-space section containing polarization optics to provide the NPR mode locking. The output is then sent to a mid-IR FROG, the FTIR spectrometer, and the MCT detector.
thresholds were not clear. More importantly, cw mode locking was achieved at the correct waveplate orientation, producing a 56.7 MHz stable pulse train with an average power of 206 mW [see Fig. 2(a)]. The RF beat of the pulses had a signal-to-noise ratio of 73 dB [see Fig. 2(b)], which also shows the evenly spaced higher harmonics with a smooth detector bandwidth roll-off, indicating no multiple pulse beating effects. The optical spectrum had a rms bandwidth that was 20.1 nm wide and
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Fig. 3. Optical spectrum of laser output; ASE spectrum (red, shifted above for clarity) and mode-locked spectrum (black, below).
Erbium doped ZBLAN fiber (3m)
0.0 -60
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Fig. 2. (a) Slow temporal pulse train measurement on an MCT detector, showing a pulse train at 56.7 MHz. (b) RF spectrum of pulses with a fundamental beat signal at 56.7 MHz and a signal-to-noise ratio of 73 dB (resolution bandwidth of 1 kHz).
centered at 2793 nm (see Fig. 3). For comparison, the amplified spontaneous emission (ASE) spectrum of the erbium transition is also shown (red, vertically shifted for clarity), spanning approximately 2700 to 2800 nm. The sharp absorption lines across the ASE spectrum, some of which are imprinted on the mode-locked spectrum [see inset of Fig. 2(c)], originated from OH molecular absorption lines in air. Despite the small interaction length with the air, at these mid-IR wavelengths, these effects are obvious and may be limiting the available spectral width of the laser. Kelly sidebands are visible on either side of the mode-locked spectrum, hinting at the soliton dynamics of the laser, which result from the anomalous dispersion of the ZBLAN glass [20,21]. The only reported direct measurement of pulses from a midIR mode-locked fiber laser has involved an intensity autocorrelation [8,12], which is known to be ambiguous [22,23] and does not provide phase information. To overcome this problem, we constructed a mid-IR-compatible FROG measurement system known to provide a unique pulse measurement with full phase retrieval [24,25]. A background-free second-harmonic generation FROG was built using a 2 mm-long AgGaS2 crystal, cut for phase matching at 2.9 μm. The results, which can be seen in Fig. 4, show the measured and retrieved spectrograms [Figs. 4(a) and 4(b)], and the temporal and spectral profiles [Figs. 4(c) and 4(d)]. Good agreement between the measured and retrieved spectrograms is described by the low FROG algorithm error of 1.8 × 10−4 . The retrieved pulse had a temporal width of 497 fs, which yields a calculation of the peak power of 6.4 kW. We note that the rms spectral bandwidth of the laser is calculated to be 20.1 nm, and this gives a time bandwidth product of 0.4. This slight excess is thought to be due to the chirping of the pulse, as revealed from a polynomial fit to the spectral phase, which shows a second-order chirp of 5.8 × 10−2 ps2 , and a third-order chirp of 6.7 × 10−2 ps3 . The relatively large positive cubic term gives rise to the trailing satellite pulse in Fig. 4(c) [26]. The corresponding energy of the mode-locked pulses based on the average power and pulse width is 3.62 nJ. This value is consistent with the soliton area theorem, which states that the pulse energy (E p ) for a specific dispersion and nonlinearity is given by
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Letter We have shown that fluoride fibers can underpin the development of an ultrafast mid-IR laser source, by demonstrating 497 fs pulses using NPR mode locking, fully characterized using a FROG technique. With the continued development of in-fiber components such as couplers and splitters using fluoride fibers, fluoride fiber lasers in “all-fiber” formats could perform as well as silica fiber lasers have done in the near-IR, and be a leading platform for the generation of highly functional mid-IR light. Funding. Australian Research (CE110001018, DE130101033).
Council
(ARC)
REFERENCES
Fig. 4. FROG measurements: (a) experimentally measured spectrogram, (b) retrieved spectrogram, and (c) retrieved electric field intensity (black) and temporal phase (purple, dotted), giving a pulse width of 497 fs. (d) Retrieved spectral intensity (black) and spectral phase (purple, dotted).
2jβ2 j . (1) γτ Using the previously published value of β2 for the ZBLAN glass [21] of -−8.1 × 10−26 s2 ∕m, and a nonlinearity of γ 1.67 × 10−4 Wm−1 (based on a mode area of 284 μm2 ), the soliton pulse energy is calculated to be 3.44 nJ, with a peak power of 6.13 kW, which is in close agreement with the previously calculated values. The peak intensity on the fluoride fiber facet was >3.2 GW∕cm2 (a lower estimate based on the output intensity at cw mode locking, but we expect the in-cavity intensity to be higher, especially during the startup time, which is where the Q-switched mode locking occurs), which is the highest reported peak intensity a fluoride fiber has been exposed to at these wavelengths without suffering damage. Thus, the fluoride fiber can operate in the highpeak-power regime, which was unclear from the previous damage threshold measurements (25 MW∕cm2 using 10 ms pulses [27]). These newly derived parameters in ZBLAN glass at midIR wavelengths will allow better designs for improved performance. In particular, a greater portion of the erbium bandwidth could be accessed by removing absorptions in the air using a dry nitrogen environment, which could theoretically support sub100-fs pulses [20]. These new performance levels from femtosecond-level pulses open new areas for future research. There is significant interest in the generation of frequency combs in mid-infrared wavelengths [1]. In order for these combs to be stabilized, a coherent octave spanning spectrum is required [28], which requires the use of a high-peak-power source. This laser promises to have the potential to do this by pumping, for example, the chalcogenide family of glasses, including As2 S3 and As2 Se3 , due to their long wavelength transmissions (i.e., >6 and >10 μm, respectively) and high nonlinearity (n2 3 × 10−18 m2 ∕W). In particular, tapered chalcogenide fibers allow for lowthreshold, octave-spanning spectra with short propagation lengths [29]. Ep
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