Compact and reliable middle-infrared fiber-bulk hybrid lasers based on Er-/Tm- doped fiber pump sources and post-growth transition metal doped polycrystalline II–VI semiconductor gain media (Cr²⁺:ZnS, Cr²⁺:ZnSe, Cr²⁺:CdSe) enable lasing over 1.9–3.6 µm spectral region, which is important for a number of applications ranging from a high resolution molecular spectroscopy to industrial processing of advanced materials [1]. CW lasers with output powers exceeding 20 W and optical efficiency exceeding 50% have been demonstrated at 2.4–2.5 µm, near the central wavelength of the Cr²⁺:ZnSe/ZnS laser tuning range [2]. Very broad emission bands of those materials offer unique opportunities for generation of ultra-short middle-infrared (mid-IR) pulses. Furthermore, ultrafast 2–3 µm lasers are convenient pump sources for femtosecond systems operating over 4–8 µm spectral range based on synchronously pumped OPOs [3, 4] or Fe²⁺ doped II–VI media [5].

First femtosecond Cr²⁺:ZnSe/ZnS lasers relied on SESAM mode-lockers [6–10]. The use of SESAM ensured stable self-starting mode-locking in single-crystal as well as in polycrystalline Cr²⁺:ZnSe/ZnS laser media with the pulse duration as short as ~100 fs [7]. The power scaling of SESAM mode-locked polycrystalline Cr²⁺:ZnSe fs oscillator in chirped-pulse regenerative amplifier has been demonstrated resulting in 300 fs pulses with 1 GW peak power [10].

Absorption in the SESAM imposes a limit on the average output power of Cr²⁺:ZnSe/ZnS oscillators, which typically does not exceed 100 – 200 mW. Therefore, recent effort on ultrafast Cr²⁺:ZnSe/ZnS sources has been concentrated on the development of pure Kerr-lens mode-locked (KLM) oscillators. There have been a number of reports on KLM lasers based on a single crystal Cr²⁺:ZnSe/ZnS [11–13]. The 1 W output power level at 75 fs pulse duration has been recently achieved [14].

Currently, the high quality Cr²⁺:ZnS and Cr²⁺:ZnSe are not readily available in the single-crystal form. Crystal sublimation during the growth process results in poor uniformity of the single-crystal samples and limits the dopant concentration. Important advantage of polycrystalline Cr²⁺:ZnS/ZnSe laser media is post-growth diffusion doping technology, which enables mass production of large-size laser gain elements with high dopant concentration, uniform dopant distribution, and low losses. Therefore, the polycrystalline Cr²⁺:ZnS/ZnSe gradually superseded the single-crystals in cw, gain-switched, and SESAM-mode-locked regimes of laser operation. First result on Kerr-lens mode-locked polycrystalline Cr²⁺:ZnSe laser was reported in [15]. The pulse duration of 300 fs was estimated from the output spectra of the laser. However, the fs pulse duration was not confirmed by the measurement of the nonlinear autocorrelation function. Furthermore, according to the private communication with the authors, the result was not confirmed in further experiments. Hence, the only laser regime, which has not been successfully realized with the use of Cr²⁺:ZnS/ZnSe polycrystals is pure Kerr-lens mode-lock regime. Therefore, the demonstration of KLM in polycrystalline Cr²⁺:ZnS and Cr²⁺:ZnSe is important and opens a new venue for mid-IR fs laser development.

In this letter we report on the first polycrystalline Cr²⁺:ZnS KLM laser and stable, reproducible KLM regime of polycrystalline Cr²⁺:ZnSe laser. Schematic of the laser setup is shown in Fig. 1. As a pump source we used a linearly polarized Er- doped fiber amplifier (EDFA) seeded by a low noise 1550 nm narrowband semiconductor laser. The pump laser was coupled to the standard astigmatism compensated asymmetric Z-folded resonator consisting of two curved high reflecting (HR) mirrors (ROC = 50 mm), plane HR mirror and plane output coupler (OC, R = 99%). The length of the laser cavity was about 94 cm (160 MHz spacing between the longitudinal modes). The output coupler was mounted on a translation stage. The distance AB between the output coupler and the curved mirror was about 33 cm. The distance CD between plane HR mirror and another curved mirror was 56 cm. The experiments were carried out using two types of the laser media: polycrystalline Cr²⁺:ZnS (2.0 mm thick, 43% low-signal transmission at 1550 nm) and polycrystalline Cr²⁺:ZnSe (2.4 mm thick, 15% transmission). Gain elements were produced in house by thermal diffusion doping of polycrystalline ZnS and ZnSe grown by chemical vapor deposition process (CVD) [16]. Post-growth diffusion doping of CVD-ZnS/ZnSe retains the polycrystalline zinc-blend structure of the material with the grain size of 50-100 µm. Gain elements were plane-parallel polished, uncoated and Brewster mounted on a copper heat sink without forced cooling.

 

Fig. 1 Schematic of the Kerr-lens mode-locked polycrystalline Cr²⁺:ZnSe/ZnS laser. HR – high reflectors, OC – output coupler, FS, YAG – dispersion compensation plates, L – pump focusing lens. The laser is pumped at 1550 nm by a radiation of narrowband semiconductor laser (seed) amplified to 1 W in Er- doped fiber amplifier (EDFA). AB – distance between output coupler and curved mirror, CD – distance between plane HR mirror and another curved mirror, BC – distance between curved mirrors.

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A combination of Brewster mounted fused silica plate (2 mm thick, IR grade) and YAG plate (4 mm thick) was used for the dispersion compensation. The exact data on mirror’s dispersion was not available by the time of the experiments. According to the information provided by the coaters, the mirrors are non-dispersive. Estimated group delay dispersion of the resonator at 2400 nm, near the middle of the 2200 – 2700 nm HR band of the mirrors, was about −1000 fs². The experimental setup was arranged on the optical table and was operated in a standard lab environment.

The resonator was at first aligned for maximum CW output power from the laser and then the distance BC between the curved mirrors was fine-adjusted in order to obtain KLM regime. The mode-locked laser oscillation was initiated by the OC translation. Occasionally, self-starting of KLM regime was observed in the laser equipped with Cr²⁺:ZnSe gain element.

Best stability of the Cr²⁺:ZnSe laser in KLM regime was reached at 1 W pump power and 60 mW laser output power. Multi-hour uninterrupted single-pulse oscillations were observed in Cr²⁺:ZnSe at this pump power level. Further increase of the pump power resulted in multi-pulsing and more frequent interruptions of the mode-lock.

Maximum stability of Cr²⁺:ZnS KLM laser was observed at 1.25 W pumping and 30 mW output power (1-2 hours of uninterrupted single-pulse oscillations). Compared to Cr²⁺:ZnSe KLM laser, KLM regime in Cr²⁺:ZnS was less readily initiated and was more sensitive to the environmental disturbances (e.g. an acoustical noise or air flows).

Figure 2 compares the emission spectra and autocorrelation traces obtained for Cr²⁺:ZnS and Cr²⁺:ZnSe lasers in KLM regime (we used a grating monochromator with ~0.5 nm resolution and an interferometric autocorrelator equipped with a two-photon Ge detector). The measurements were carried out for single pulse oscillations at 160 MHz pulse repetition rate (1.25 W pumping, 30 mW output for Cr²⁺:ZnS and 1 W pumping, 60 mW output for Cr²⁺:ZnSe). The shape of the autocorrelation trace and of the emission spectrum for Cr²⁺:ZnS laser allows us to assume sech² transform limited pulses: 125 fs pulse duration (FWHM) was derived from the autocorrelation trace assuming sech² profile and 126 fs pulse duration was calculated from the emission spectrum assuming ∆τ∆ν = 0.315 time-bandwidth product (∆λ = 45 nm, ∆ν = 2.5 THz, ∆τ = 0.315/∆ν = 126 fs). On the other hand, the shape of the autocorrelation trace for Cr²⁺:ZnSe laser reveals chirped pulses. Emission spectrum of the laser is distorted and, hence, the time-bandwidth product is increased. We roughly estimate the pulse duration of Cr²⁺:ZnSe laser as 100 – 130 fs. A spike in the spectrum of mode-locked Cr²⁺:ZnSe laser was attributed to a leakage of a fraction of the laser output to high-order transverse modes.

 

Fig. 2 Emission spectra (left) and autocorrelation traces (right) for Kerr-lens mode-locked polycrystalline Cr²⁺:ZnS and Cr²⁺:ZnSe lasers. FWHM bandwidth of the spectra are 75 nm for Cr²⁺:ZnSe and 45 nm for Cr²⁺:ZnS. 125 fs pulse duration can be derived from the autocorrelation trace, emission spectrum of Cr²⁺:ZnS laser assuming sech² pulse profile and time-bandwidth product ∆τ∆ν = 0.315. Distortions of the autocorrelation trace and the spectrum of Cr²⁺:ZnSe laser indicate a chirped pulse and an increase of the time-bandwidth product. We roughly estimate the pulse duration of Cr²⁺:ZnSe laser as 100 – 130 fs.

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The ability of the laser to operate in KLM regime was tested in a range of the cavity parameters (total length of the cavity L, a ratio of the lengths of the cavity legs AB/CD), using Cr²⁺:ZnSe gain element. The mode-locked laser oscillations were obtained for 83 cm < L < 125 cm and 1/4 < AB/CD < 2/3. Autocorrelation traces obtained for Cr²⁺:ZnSe KLM laser at the pulse repetition rate of 120, 160, and 180 MHz (cavity length 125, 93.8, 83.3 cm respectively) are shown in Fig. 3.As can be seen, variation of the cavity length by a factor of 1.5 did not significantly affect the autocorrelation trace and hence the pulse duration of the laser.

 

Fig. 3 Autocorrelation traces for Kerr-lens mode-locked polycrystalline Cr²⁺:ZnSe laser at different pulse repetition rates.

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We also found out that KLM laser regime is not sensitive to the location of the laser beam in the polycrystalline gain element. For instance, it was possible to initiate KLM regime after translation of the gain element across the resonator (i.e. in the direction perpendicular to the plane of Fig. 1). This confirms the high uniformity of optical and laser properties of polycrystalline Cr²⁺:ZnS/ZnSe materials. This feature of polycrystalline II–VI laser materials is of importance for practical use as it simplifies the fs laser alignment and maintenance.

In the next experiment we amplified the femtosecond pulse train of Cr²⁺:ZnSe oscillator (160 MHz pulse repetition rate) in a single pass CW pumped laser amplifier, schematically shown in Fig. 4.We used the EDFA, described above, as a pump source for the amplifier, while the Cr²⁺:ZnSe oscillator was pumped at 1645 nm by a CW radiation of Er:YAG laser. The Er:YAG laser itself was pumped at 1532 nm by Er-fiber laser. The output beam of the femtosecond Cr²⁺:ZnSe laser and the CW pump beam were superimposed on a dichroic mirror (DM). Diameters of both beams were about 1 mm. Combined beam was focused by a CaF₂ lens with f = 20 mm focal length (~60 µm beam diameter at the focal point). 9 mm thick polycrystalline Cr²⁺:ZnS with 5% low-signal transmission at 1550 nm was used as a gain element. The facets of the gain element were broad-band AR coated and it was mounted at normal incidence on a copper heat sink. The laser beam at the output of the amplifier was collimated by a second CaF₂ lens. The transmitted CW pump and amplified fs signal were separated by a second dichroic mirror. The fs laser power at the amplifier output was about 38 mW when the amplifier’s pump was off.

 

Fig. 4 Schematic of polycrystalline Cr²⁺:ZnS fs laser amplifier. HR – high reflectors, DM – dichroic mirrors, L – focusing and collimation lenses. The amplifier is pumped by cw output of EDFA at 1550 nm.

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The dependence of the fs laser power at the amplifier output on applied CW pump power is shown in the right part of Fig. 5 by a solid line. The maximum fs laser power at the amplifier output was 250 mW at 10-W pumping that corresponds to the amplifier gain of 6.5. No CW background was detected in the amplified fs pulse train (we used a DC coupled mid-IR VIGO detector with <ns response time for this measurement). Dashed line in the Fig. 5 corresponds to the amplification of the CW seed signal in the same amplifier (we brought the fs oscillator to CW regime and adjusted the distance between the curved mirrors in order to obtain the CW beam with the same size to carry out this measurement). As can be seen, the characteristics of the amplifier in fs regime and in CW regime are very similar. We explain this similarity by a low pulse energy Ep ≈0.3 nJ, which is much smaller than the saturation energy in Cr²⁺:ZnS (Esat ≈2 µJ at 60 µm beam diameter). This condition reduces equations for the pulsed amplifier to formula, which is similar to that of for the steady-state amplifier, see Eqs. (3)-(4) in [17].

 

Fig. 5 Left: Output power of fs laser amplifier vs cw pump power. The amplifier was seeded by fs pulses at 160 MHz repetition rate (solid curve/symbols) and by cw laser with the same beam parameters, for comparison (dashed line, open symbols) Right: Autocorrelation traces of the fs pulses at the amplifier input (a), at the amplifier output with the amplifier’s pump turned off (b), and at the amplifier output at 10-W cw amplifier’s pump (c).

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Thus, very compact and robust single-pass amplifier arrangement allows conversion of ‘low-cost’ emission of a CW fiber laser to mid-IR femtosecond pulses as efficiently as in CW regime of the amplifier. Our experience shows that multi-Watt output power with ~50% pump extraction efficiency at the amplifier’s gain of 2-3 can be obtained in saturated single-pass Cr²⁺:ZnS/ZnSe CW amplifiers.

Left part of Fig. 5 shows autocorrelation traces of the femtosecond pulses at the amplifier input (a) and at the amplifier output with the amplifier’s pump turned off (b) and at 10-W pumping of the amplifier (c). A chirp introduced to the fs pulses by the dispersion in the polycrystalline ZnS matrix and optical components can be easily compensated using the standard techniques (e.g. dispersive mirrors). Comparison of the autocorrelation traces for ‘cold’ and ‘hot’ amplifier reveals a significant distortion of the fs pulses due to the laser interaction of the fs beam with the gain medium. Further study of the single-pass Cr²⁺:ZnS/ZnSe fs amplifiers is required in order to evaluate and minimize those distortions.

In conclusion, we have demonstrated the Kerr-lens mode-locked laser regime using standard polycrystalline Cr²⁺:ZnSe and Cr²⁺:ZnS materials. Transform limited pulses with 125 fs duration at 30 mW output power were obtained for Cr²⁺:ZnS laser and chirped pulses with approximate duration of 100 – 130 fs and 60 mW output power were obtained for Cr²⁺:ZnSe laser. We expect that better management of the dispersion in the laser resonator, optimization of the OC transmission and of the gain element parameters will allow us to significantly increase the laser output power in KLM regime and to produce shorter pulses. Those optimizations should also allow to obtain transform-limited pulses for polycrystalline Cr²⁺:ZnSe KLM laser and to eliminate the spikes in the laser’s emission spectrum.