High Repetition-Rate 2.3–2.7 µm Acousto-Optically Tuned Narrow-Line Laser System Comprising Two Master Oscillators and Power Amplifiers Based on Polycrystalline Cr2+:ZnSe with the 2.1 µm Ho3+:YAG Pulsed Pumping

1. Introduction

High average power tunable solid-state lasers in the wavelength range from 2 to 3 μm are attractive light sources for many applications, such as medical surgery and lasertripsy, laser processing, environmental remote sensing, wireless energy transmission technology, pumping mid-infrared optical parametric oscillators and solid-state lasers based on Fe²⁺-doped crystals [1,2,3,4,5,6,7]. Cr²⁺-doped zinc selenide (Cr²⁺:ZnSe) crystals are among the most attractive active media for high-average power lasers in the 2-3 µm wavelength range due to their wide tunability, broad absorption bands, and large stimulated-emission cross section [8,9,10,11,12,13]. Gain-switched operation in nanosecond pulse mode of the Cr²⁺:ZnSe lasers and a wide wavelength tuning of their outputs were previously reported [10,11,12,13,14,15,16,17]. Various wavelength tuning techniques have been used in Cr²⁺:ZnSe lasers: intracavity wavelength-selective elements such as diffraction gratings [11,12,19,20,21], birefringent Liot filters [16,22], prisms [23,24], acousto-optic tunable filters (AOTF) [10,14,17,18,25,26], liquid crystal etalon [27], or injection seeding [28]. The AOTF enabled the Cr²⁺:ZnSe laser to electronically tune every pulse in the broad-wavelength tuning range [29]. In the narrow-line widely-tunable Cr²⁺:ZnSe laser systems pumped by the Tm³⁺:YAP lasers two pulsed regimes were reported: the pulse energy of up to 50 mJ in slow pulse regime at the pulse repetition rate (PRR) of 10 Hz [18], or the average power of up to 5.1 W in the faster operating regime at 7 kHz PRR [15]. However, the scaling of the average and peak power and the pulse energy of the laser systems is hindered by the laser induced damage (LID) of both polycrystalline and single-crystalline Cr²⁺:ZnSe [12,14,15,18,30,31,32].

In this paper we present a high-average-power high-PRR AOTF-tunable solid-state laser system based on the polycrystalline Cr²⁺:ZnSe pumped at 2091 nm by the repetitively-pulsed fiber-laser-pumped Ho³⁺:YAG lasers. The laser system architecture comprised a tunable Cr²⁺:ZnSe master oscillator and Cr²⁺:ZnSe power amplifier pumped with 10-40 kHz PRR by a Ho³⁺:YAG mater oscillator and a Ho³⁺:YAG power amplifier, respectively. An intracavity AOTF provided 2.3-2.7 μm tunability of narrow-band generation of the Cr²⁺:ZnSe laser pulses. An advanced procedure of Cr²⁺:ZnSe-element doping and surface improvement was applied to increase the LID threshold, which resulted in an increase of the output power of the Cr²⁺:ZnSe laser system.

2. Cr²⁺:ZnSe Active Elements

The Cr²⁺:ZnSe active elements were made from laser-quality zinc selenide (ZnSe) obtained by chemical vapor deposition (CVD) and cut into blanks with a cross-section of up to 4.5×4.5 mm².The length of the blanks, and thus of the laser elements, varied from 15 mm to 33 mm. A chromium film was vacuum deposited on the side surface of the blanks. Subsequently, the samples were placed in a quartz ampoule, washed with high purity argon, and vacuumed 5 times. Metallic zinc (Zn) was also added to the ampoule; the amount of zinc was adjusted to equalize the Zn vapor pressure at the annealing temperature to atmospheric pressure. The ampoule was vacuumed, sealed, and held in a muffle furnace at 1050°C for 7 days. The ampoule was then unsealed and the elements were processed in a hot isostatic press for 20 h at 1100°C in argon to restore the ZnSe stoichiometry (to remove excess Zn). In preliminary testing the higher LID-threshold was found in Cr²⁺:ZnSe samples annealed in argon (Ar) atmosphere compared to samples annealed in Zn atmosphere or with hot isostatic pressing [32]. However, annealing in a sealed ampoule does not provide confidence that the stoichiometry of the material has been restored. In this work hot isostatic pressing was used, but compared to the procedure described in [32], the samples were placed in a graphite container to reduce contamination. As a result of this doping procedure, chromium ions (Cr²⁺) were uniformly distributed throughout the sample volume and the elements had an increased LID threshold. The side surfaces of the samples were finely ground to prevent stray reflections and to reduce the probability of parasitic generation; all right angles were chamfered at ~0.2 mm.

The Cr²⁺-ion doping concentration (N_d) and element length (L) were chosen so that the single-pass small-signal absorption coefficient (α_abs×L = σ_abs×N_d×L, where σ_abs is the absorption cross section) of the Cr²⁺:ZnSe element at 2091 nm was in the range of 1-1.5. The Cr²⁺-ion doping concentration was kept low to avoid strong thermal aberrations in the active elements. The length of Cr²⁺:ZnSe elements was optimized based on the preliminary absorption measurements; the final length of the active elements with different Cr²⁺-ion doping concentration was in the range of 14 to 32 mm. Assuming the absorption cross section of Cr²⁺:ZnSe at 2091 nm of σ_abs = 1.4×10⁻¹⁹ cm², the final concentration of the Cr²⁺ ions in the laser elements was estimated to be N_d = 3-6×10¹⁸ cm⁻³.

The cross section of the active element was 3×3 mm² or 3.5×3.5 mm² in the 14-20 mm long samples 4×4 and 4.3×4.3 mm² in the longer active elements. A broadband (2.0-2.7 µm) antireflection coating of yttrium fluoride and zinc sulfide layers was applied on the working surfaces of the active elements. A sample active element is shown in Figure 1.

The absorption spectrum of Cr²⁺:ZnSe crystals enables the low-quantum defect pumping of the 2.3-2.7-µm laser using a Ho³⁺:YAG laser at 2091 nm (Figure 2). At 2091 nm the absorption cross section σabs of the Cr²⁺:ZnSe crystal is less than the emission cross section σem [8,12]. Since the Cr²⁺ ion in a ZnSe crystal field is a 4-level system with fast relaxation in the vibronic sublevels [8,19], the sum of the populations of the 5E upper state Nup at the 2091-nm pumping and the 5T2 ground state, Ng, effectively equals to the doping concentration of Cr2+ ions in the active sites Nda (neglecting an exited state absorption and an up-conversion). In this case the effective gain cross section G(λ) of the crystal at a wavelength λ can be calculated for a given inversion fraction β using the following expression:

G(λ) = σem(λ)×β - σabs(λ)×(1 - β) –γ(λ)/Nda,

(1)

where β = Nup/Nda, γ(λ) is the passive losses at a wavelength λ.

3. Laser System Architecture

The laser system comprised the double master oscillator – power amplifier: a Ho³⁺:YAG master oscillator pumping a tunable Cr²⁺:ZnSe master oscillator, and a Ho³⁺:YAG power amplifier pumping a Cr²⁺:ZnSe power amplifier (Figure 3).

3.1. Ho³⁺:YAG Laser Oscillator

An in-house made repetitively-pulsed Ho³⁺:YAG laser at 2091 nm was used as the pump source of the Cr²⁺:ZnSe laser [33]. The Ho³⁺:YAG laser was pumped, in turn, by a CW Tm-doped fiber laser at 1908 nm (“NTO IRE-Polus”, Fryazino, Russia). The repetitively-pulsed regime was achieved by active Q-switching the cavity with a quartz acousto-optical modulator (“NII Polyus”, Moscow, Russia). The operation wavelength of ~2091 nm was selected and stabilized by a rotated silica etalon of 90-µm thickness (Figure 4a). The Q-switched laser operated with PRR varied from 10 to 50 kHz. The FWHM pulse width (τ) of 15–45 ns depended on the PRR and the pump power.

The average power of the Ho³⁺:YAG laser measured by a power meter (Coherent PM 10 with FieldMax detector, Coherent, USA) was up to 34 W at 30 kHz PRR (at 55-W fiber laser pump power) in a linearly polarized high-quality Gaussian beam. The laser beam was monitored using a Pyrocam IV camera (Ophir-Spiricon, North Logan, USA) (Figure 4b). The M2 parameter (determined by ISO 11146-1:2021 standard with the knife-edge method [34]) was less than 1.2-1.3.

To avoid the influence of a back reflection on the Ho³⁺:YAG laser, the Faraday isolator (EOT, Traverse City, USA) was used (Figure 3). A Galilean telescope (T1), a half-wavelength plate (λ/2) and a polarizer (P1) were placed in the Ho³⁺:YAG laser beam path; the last two optical elements controlled the power of the linearly-polarized 2091 nm output and provided two polarization separation. Finally, the horizontally-polarized beam from the Ho³⁺:YAG laser was focused into a Cr²⁺:ZnSe active element, but the vertically polarized beam at 2091 nm propagated to the Ho³⁺:YAG laser amplifier.

3.2. Ho³⁺:YAG Power Amplifier

The power amplifier of the beam at 2091 nm was based on Ho³⁺:YAG rods (“NII Polyus, Moscow, Russia) with the length of 50 mm or 65 mm and diameter of 5 mm or 6 mm, respectively. The Ho³⁺-ion concentration was 0.5 at. %. The end faces of the Ho³⁺:YAG rods were AR coated at 2050-2150 nm. The Ho3+:YAG rod was pumped by a CW Tm-doped fiber laser at 1908 nm (“NTO IRE-Polus”, Fryazino, Russia) with the power up to 53 W. The diameter of the pumping beam at 1908 nm in the Ho³⁺:YAG crystal was tuned by the Galilean telescope T2 to be equal to ~0.8-0.9 mm, when measured at e-2 peak intensity by the knife-edge method [34]. The diameter of the repetitively-pulsed amplified beam at 2091 nm was tuned by the Galilean telescope T3 to be equal also to ~0.8-0.9 mm.

At the repetitively-pulsed input power of 3.1 W, the maximum output power of the Ho³⁺:YAG amplifier based on a 50 mm long crystal was ~5.5 W. When the input power was increased to 21.6 W the output power increased to 36 W (Figure 5a). The maximum output of the Ho³⁺:YAG amplifier based on a 65 mm long crystal was 7.5 W at 3.1 W input power, and with the input power of 21.6 W output power increased to 43 W (Figure 5b).

The decrease in gain of the 65-mm long amplifier with increasing input pulsed power can be explained by the saturation effect (Figure 6). With PRR increasing, the gain increased at low input pulsed power, but the same gain decreased at high input pulsed power (Figure 6b). The gain increase in a large signal at higher PRR can be explained by the influence of the thermal lens and the electronic self-focusing effect, which reduced the diameters of the pumping and amplified beams in the laser amplifier.

The beam quality of the amplified beam in the 65 mm rod remained high enough up to the highest power: the M2 parameter (measured using the knife-edge method [34]) was ≤1.8 at 30 W and ≤2.5 at 43 W output power (Figure 7).

The amplified beam at 2091 nm was propagated through a half-wave-plate polarization rotator (“λ/2 - plate”) and a dichroic mirror M4 with a high p-polarization transmission at 2091 nm and a high s-polarization reflection at 2.3-2.7 µm before being directed into the second Cr²⁺:ZnSe crystal (Figure 3). The 2091-nm beam was the pumping beam of the Cr²⁺:ZnSe amplifier of the waves at 2.3-2.7 µm.

3.3. Cr²⁺:ZnSe Tunable Laser Oscillator

Multiple Cr²⁺:ZnSe crystals with varying lengths between 14 mm and 26 mm were tested as active elements of the laser oscillator. A part of the Ho³⁺:YAG laser beam was sent as the pump into the Cr²⁺:ZnSe laser oscillator. To avoid the LID on the Cr²⁺:ZnSe crystal surface the pump energy fluence was kept under 1.5 J/cm². For this reason, the pump beam diameter in the active element was 0.7–0.9 mm (at e⁻² peak intensity) depending on the PRR that was varied from 10 to 40 kHz.

The input dichroic mirror M2 of the Cr²⁺:ZnSe laser cavity (Figure 3) was chosen to enable high transmission of the p-polarized pump beam at 2091 nm and high reflectivity of the lasing at 2.3-2.7 μm, which had the s-polarization. The rear mirror M1 had over 99% reflectivity both at the lasing wavelength of 2.3–2.7 μm and the pumping wavelength of 2091 nm. The M1 mirrors with the radius of curvature of 300 mm or 500 mm were tested in the experiments. The transmittance of the output coupler M3 was varied from 30% to 70% at the lasing wavelengths. The physical cavity length was also changed from 110 to 190 mm.

An acousto-optical tunable filter (AOTF) (“NII Polyus”, Moscow, Russia) inserted into the output leg of the laser cavity provided fine wavelength tuning on the deflecting acoustic wave. The driving voltage frequency was varied from 36.5 to 43 MHz. The acousto-optical element based on TeO₂ crystal had dimensions of 4×4×25 mm³. The entrance and exit surfaces of the acousto-optical element were antireflection coated at 2.3-2.7 µm.

3.3.1. Numerical Simulation of the Thermal Lens and Optimization of the Cr²⁺:ZnSe Laser Cavity

The Cr²⁺:ZnSe-laser cavity was numerically modelled to ensure a good overlap of a fundamental mode with the pump beam. To calculate the radius of the fundamental mode in the “hot” laser cavity the thermal lens induced in the active element by the absorbed pump beam must be estimated.

A transient thermal lens was induced in the Cr²⁺:ZnSe active element under each pumping pulse. The dioptric power D_T of the transient thermal lens can be estimated by the following expression [35]:

$$D_T=-2\times \left(\frac{\partial \mathrm{n}}{\partial \mathrm{T}}\right)\mathrm{e}\mathrm{f}\mathrm{f}\times \frac{\partial }{{\partial r}^2}\underset{0}{\overset{L}{\int }}T\left(r,z,t\right)dz⃒(r=0),$$

(2)

where T(r,z,t) is the temperature distribution, r and z are the transverse and longitudinal coordinates, t is the time, (∂n/∂T)_eff is the effective thermo-optic coefficient of the Cr²⁺:ZnSe crystal, including the volume coefficient (∂n/∂T), bulging of end faces and the photoelastic effect, and given for an active element by the well-known expression:

$$(\partial \mathrm{n}/\partial \mathrm{T}{)}_{\mathrm{eff}} \approx (\partial \mathrm{n}/\partial \mathrm{T})+\left(n-1\right){\beta }_T\left(1+\nu \right),$$

(3)

where n is the refractive index, β_T is the thermal expansion coefficient, ν is the Poisson’s ratio. The transient temperature change during each pump pulse can be estimated by

$$T\left(r,z,t\right)=\frac{{\sigma }_{abs}\left({\lambda }_p\right)(1-{\raisebox{1ex}{${\lambda }_p$}\!\left/ \!\raisebox{-1ex}{$\lambda $}\right.}_S)}{\rho c_p}\underset{0}{\overset{t}{\int }}{I}_p(r,z,t)\times n_{up}(r,z,t)dt=\frac{{N_{da}\sigma }_{abs}\left({\lambda }_p\right)(1-{\raisebox{1ex}{${\lambda }_p$}\!\left/ \!\raisebox{-1ex}{$\lambda $}\right.}_S)}{\rho c_p}\underset{0}{\overset{w_p\left(r,z,t\right)}{\int }}d{w}_p\left[1-\frac{{\sigma }_{abs}\left({\lambda }_p\right)}{\left({\sigma }_{abs}\left({\lambda }_p\right)+{\sigma }_{em}\left({\lambda }_p\right)\right)}\left(1-e^{-\frac{w_p\left({\sigma }_{abs}\left({\lambda }_p\right)+{\sigma }_{em}\left({\lambda }_p\right)\right)}{h\mathrm{*}{\nu }_p}}\right)\right],$$

(4)

where ${\sigma }_{abs}\left({\lambda }_p\right)$and ${\sigma }_{em}\left({\lambda }_p\right)$ are the absorption and emission cross sections at the pump wavelength ${\lambda }_p$, ν_p is the pump frequency, ${\lambda }_s$ is the laser wavelength, h is the Plank constant, ρ and C_p are the density and the thermal capacity of the Cr²⁺:ZnSe crystal, I_p and w_p are the pump intensity and fluence. The fluence of the short pumping pulse (with the pulse width τ_p much less than the excited-state life time) w_p(r,z,t) can be given by Franz-Nodvig solution [36]:

$$w_p\left(r,z,t\right)=\frac{h\mathrm{*}{\nu }_p}{\left({\sigma }_{abs}\left({\lambda }_p\right)+{\sigma }_{em}\left({\lambda }_p\right)\right)}\ln \left(e^{-{\alpha }_{abs}\left({\lambda }_p\right)z}\left(e^{-\frac{w_p\left(r,0,t\right)\times \left({\sigma }_{abs}\left({\lambda }_p\right)+{\sigma }_{em}\left({\lambda }_p\right)\right)}{h\mathrm{*}{\nu }_p}}-1\right)+1\right)$$

(5)

The expressions (2)-(5) determined the focal length of the transient thermal lens at the end of a single pulse.

The focal length of the steady-state thermal lens f_T was estimated by the following expression [37,38]:

$$f_T=\frac{\pi K_T{a_p}^2}{P_{in}\left(1-\frac{{\lambda }_p}{{\lambda }_S}\right)\left(\left(\frac{\delta n}{\delta T}\right)+\left(n-1\right){\beta }_T\left(1+\nu \right)\right)\left(1-\exp \left(-{\sigma }_{abs}\left({\lambda }_p\right)L\right)\right)},$$

(6)

where P_in is the average incident pump power with a Gaussian beam radius of a_p. The other parameters of the crystal, pump beam, and laser cavity (including the TeO₂ AOTF) are shown in Table 1.

The focal length of the thermal lenses at the end of each pulse (calculated by the expressions (2)-(5)) and the steady-state thermal lens (calculated by expression (6)) are presented in the Figure 8a.

The effective inversion fraction at the axis of the Cr²⁺:ZnSe active element at the end of each pumping pulse can be estimated by the following expression

$${\beta }_{ef}=\frac{1}{L}\underset{0}{\overset{L}{\int }}\frac{n_{ex}\left(0,z,{\tau }_p\right)}{N_{da}}dz=\frac{1}{L}\underset{0}{\overset{L}{\int }}\left[1-\frac{{\sigma }_p}{\left({\sigma }_p+{\sigma }_{emp}\right)}\left(1-e^{-\frac{w_p\left(o,z,{\tau }_p\right)\times \left({\sigma }_p+{\sigma }_{emp}\right)}{h\mathrm{*}{\nu }_p}}\right)\right]dz$$

(7)

Expressions (5) and (7) provide a way to estimate the inversion fraction for the average pump power at the active element input:

P_in = W_p(0)×R_rate,

(8)

where W_p(0) is the input pump pulse energy, R_rate is the PRR. In this way, the gain cross-section can be plotted for each value of the input pump power (Figure 8b).

The radius of the fundamental mode in the laser cavity with the steady-state thermal lens was calculated by the ”Rezonator” software package [46]. The optimal overlap of the fundamental mode at 2400 nm and the pumping beam in the active element was achieved at the pump power of 28 W in the cavity with the physical length of 140 mm (Figure 6a). The optimal cavity length for the stable mode operation decreased with the increase in the pump power (Figure 6b). The rear mirror M1 with the 300 mm radius of curvature was found to be more suitable for the used pump power and the active elements due to the good overlap of the pumping beam and the laser mode.

3.3.2. Cr²⁺:ZnSe Laser Experimental Results

In the first series of experiments the Cr²⁺:ZnSe laser was examined without the intracavity AOTF. The repetitively-pulsed operation with the highest average output power of 3.8 W at 23 W pumping power and 30 kHz PRR was obtained in the 150-mm long cavity with the 300-mm radius of curvature on the rear mirror M1 and the 35% output coupler M3 (Figure 7). The highest output power was achieved with the 16.5-mm long Cr²⁺:ZnSe element.

The Ophir-Spiricon “PYROCAM IV” beam profiler was used to analyze the Cr²⁺:ZnSe laser output. The diameter of the output Cr²⁺:ZnSe laser beam was estimated to be 800-900 µm. The divergence angle and beam quality were analyzed in the focal zone of a lens with the 70 cm focal length using the knife-edge method [34]. The beam quality was M² ≤ 1.5 at the output power of 3.8 W in the single-transverse-mode regime (Figure 7).

Waveforms of the Cr²⁺:ZnSe laser pulses were registered by an in-house-made photodetector based on a PD 36-02 (or 01)-PR(TO18) photodiode manufactured by IBSG Co., Ltd., St. Petersburg, Russia (spectral response range is 2.3–3.6 μm) in comparison with the pumping pulses of the Ho³⁺:YAG laser at 2091 nm, which were measured by an in-house-made photodetector based on HAMAMATSU InGaAs PIN G8422-03 photodiode (spectral response range is 0.9–2.1 μm). The Cr²⁺:ZnSe laser operated in the gain-switch mode: the laser pulse delayed with respect to the pumping pulse, the delay time decreased with an increase the pump pulse energy (Figure 8). The laser pulse width decreased from 50-70 ns near threshold to 20-25 ns far above threshold.

The output spectrum was measured by an IR Fourier spectrometer FT-801 (Simex, Russia). The wavelength of the power maximum and the linewidth were found to depend on both the pump power and the cavity mirrors. The wavelength of the spectral maximum tuned from 2400 nm to 2450 nm with an increase of the average pump power from 8 W to 22 W. The self-tune of the spectral maximum wavelength can be explained by tuning of the laser operation from the mirror-reflection maximum at the low pump power to the gain maximum at the higher pump power. The linewidth at half maximum was measured to be ~70 nm at 18 W pump power (Figure 9a), and decreased as the pump power decreased.

The operating wavelength of the Cr²⁺:ZnSe laser with the intracavity AOTF was tuned by frequency tuning of the HF driver: from 2300 nm at ~42.6 MHz, to 2700 nm at ~36.6 MHz (Figure 12). The discreteness of the wavelength tuning was determined by the discreteness of the AOTF-driver frequency tuning, which was ~1 kHz (corresponding to the discreteness of the laser wavelength tuning of ~0.2 nm). The linewidth of the Cr²⁺:ZnSe laser with the intracavity AOTF was measured to be strongly ≤0.8 nm (the measurement accuracy was limited by the spectrometer used). The average output power of the electronically-tuned Cr²⁺:ZnSe laser at 23 W pumping power and 30 kHz PRR had a maximum of 3.1-3.3W at 2410 nm, and decreased to 1.1-1.2 W at 2300 nm and to 0.6-0.8 W at 2700 nm (Figure 10). The beam quality of the tuned Cr²⁺:ZnSe laser remained high, M²≤1.5.

The Cr²⁺:ZnSe laser wave had linear polarization due to the polarization selectivity of the cavity mirror M2 and the AOTF. The polarization contrast (measured by a single-crystal silica wedge) was ~70:1 at the highest output power. The output beam of the Cr²⁺:ZnSe laser was directed to the Cr²⁺:ZnSe power amplifier by the dichroic mirror M4 (Figure 3).

3.4. Cr²⁺:ZnSe Power Amplifier

The Cr²⁺:ZnSe crystals with length varied from 16 to 32 mm were tested as active elements of the Cr²⁺:ZnSe amplifiers. The highest output power at 2.3-2.7 µm was achieved with the longer active element of 26-32 mm length.

The diameter of the pump beam at 2091 nm in the Cr²⁺:ZnSe amplifier was determined by the telescope T2 and the thermal lens inside the Ho³⁺:YAG amplifier. It was chosen to be approximately equal to the diameter of the amplified laser beam in the Cr²⁺:ZnSe crystal and had a value of 0.9-1.0 mm. To avoid LID in Cr²⁺:ZnSe the PRR of the master Ho³⁺:YAG oscillator was varied only within 20-40 kHz.

The power of the seed (amplified) beam at 2.3-2.7 µm and the pumping beam at 2091 nm was varied by adjusting the polarization beam splitter “P1 - λ/2” and the pumping power of the Ho³⁺:YAG oscillator and amplifier.

The 2.3-2.7 µm signal pulse lagged 10-30 ns behind the 2091 nm pump at the Cr²⁺:ZnSe amplifier. The delay depended on the Cr²⁺:ZnSe master oscillator pump power. However, this did not affect the amplifier gain, because the lifetime of the Cr²⁺:ZnSe upper-laser level is much longer.

The maximum average output power and pulse energy of the narrow-line wave at 2.41 µm reached 9.7 W and 0.32 mJ, respectively, at 30 kHz PRR; the total maximum amplifier gain was over a factor of 7 (Figure 14a). The maximum of average power and pulse energy was achieved using the 24-mm long Cr²⁺:ZnSe amplifier element. The beam quality of the amplified beam was M² < 2 at the maximum output power (Figure 14b). A slight deterioration in the beam quality of the amplifier output compared to the master-oscillator output can be explained by thermal effects in the long Cr²⁺:ZnSe amplifier element. The output pulse energy increased, but the average output power decreased with PRR decrease from 30 kHz to 20 kHz (Figure 14c).

4. Discussion

Power scaling of the Cr²⁺:ZnSe laser system could be achieved by adding power amplifiers [12,18]. However, several factors limit the pulse energy and the average power of the output beam in the Cr²⁺:ZnSe lasers: the LID, aberrative thermal lens or electronic self-focusing and gain saturation.

The rather high saturation fluence of the Cr²⁺:ZnSe amplifier (62 mJ/cm² at 2.45 µm) allows for further increases in the output power of the high-PRR nanosecond laser system with strong extraction efficiency. The thermal lens effect and electronic self-focusing can result in decreased beam diameter and deterioration of beam quality; these effects are more pronounced in longer Cr²⁺:ZnSe amplifiers. The thermal lens and electronic self-focusing are weaker at the lower beam fluence. For example, a B-integral estimation in the 30-mm amplifying crystal gives the value of approximately 0.34 for the input fluence w_in = 1 J/cm² at 2400 nm. Therefore, by increasing the diameter of the signal and pump beams in the amplifier and increasing the PRR, the influence of gain saturation, thermal lens could be reduced, and electronic self-focusing, and the Cr²⁺:ZnSe-crystal LID in the amplifier chain could be avoided. The use of a protective container during the hot isostatic treatment after Cr²⁺:ZnSe doping helps prevent contamination, resulting in an increase in the LID to 2.0-2.5 J/cm² at the high PRR.

5. Conclusions

The narrow-line broadly-tunable high-PRR laser system based on polycrystalline Cr²⁺:ZnSe elements pumped by repetitively-pulsed Ho³⁺:YAG lasers was developed. Wide-band wavelength electronic tuneability was achieved by using the TeO₂ AOTF. The average output power of up to 9.7 W at 30 kHz PRR in the single-stage Cr²⁺:ZnSe power amplifier was reached. The average output power and the pulse energy can be scaled by adding Cr²⁺:ZnSe amplifier stages.

The compact and powerful Cr²⁺:ZnSe laser system tunable within 2.3-2.7 µm was designed for a long-range LIDAR environmental monitoring in the transparency window of the Earth’s atmosphere. Several pollutant gases (such as CH₄, NH₃, CO) have specific absorption bands in the wavelength range of 2.3-2.7 µm and can be detected using the developed laser source [47]. The high-repatition rate tunable Cr²⁺:ZnSe laser system can be used for other applications in medicine, material processing, and pumping of mid-infrared optical parametric oscillators and Fe²⁺-doped solid-state lasers.

6. Patents

This work resulted in a patent EA 041501B1 20221031 from the Eurasian patent office: “IR Laser source based on Cr²⁺-doped chalcogenide crystals with acousto-optical wavelength tuning’, by O.L. Antipov, I.D. Eranov, Yu.A. Getmanovskiy.