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

Abstract

High-average-power narrow-linewidth tunable solid-state lasers in the wavelength region between 2 and 3 μm are attractive light sources for many applications. This paper reports a narrow-linewidth widely tunable laser system based on the polycrystalline Cr²⁺:ZnSe elements pumped by repetitively pulsed 2.1 µm Ho³⁺:YAG laser operating at a pulse rate of tens of kilohertz. An advanced procedure of ZnSe element doping and surface improvement was applied to increase the laser-induced damage threshold, which resulted in an increase in the output power of the Cr²⁺:ZnSe laser system. The high-average-power laser system comprised double master oscillators and power amplifiers: Ho³⁺:YAG and Cr²⁺:ZnSe laser oscillators, and Ho³⁺:YAG and Cr²⁺:ZnSe power amplifiers. The output wavelength was widely tuned within 2.3–2.7 µm by means of an acousto-optical tunable filter inside a Cr²⁺:ZnSe master oscillator cavity. The narrow-linewidth operation at the pulse repetition rate of 20–40 kHz in a high-quality beam with an average output power of up to 9.7 W was demonstrated.

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 [1,2,3,4,5,6,7,8,9,10,11]. 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 [12,13,14,15,16,17,18,19,20]. The gain-switched operation in nanosecond pulse mode of the Cr²⁺:ZnSe lasers and a wide wavelength tuning of their outputs were previously reported [18,19,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36]. Various wavelength tuning techniques have been used in Cr²⁺:ZnSe lasers: intracavity wavelength-selective elements such as diffraction gratings [18,19,21,27,28], birefringent Liot filters [24,30], prisms [31,32], acousto-optic tunable filters (AOTFs) [22,23,25,26,29,33,34], liquid crystal etalon [35], or injection seeding [36]. The AOTF wavelength tuning is one of the most attractive technique: the AOTF enabled the laser to electronically tune every pulse in the broad-wavelength tuning range [37]. 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 a slow pulse regime at a pulse repetition rate (PRR) of 10 Hz [26], or the average power of up to 5.1 W in a faster operating regime at 7 kHz PRR [23]. 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 [19,22,23,26,38,39,40,41].

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 master oscillator and a Ho³⁺:YAG power amplifier, respectively. An intracavity AOTF provided a 2.3–2.7 μm tunability of the 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 in 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 means of 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 [15]. 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 highest 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 [40,42]. 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 [40], 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 [12,19], 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, and 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 Cr²⁺:ZnSe active elements were wrapped in indium foil and inserted into the copper radiators with the temperature stabilized at 5 ± 0.1 °C.

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 σ_abs of the Cr²⁺:ZnSe crystal is less than the emission cross section σ_em [12,19]. Since the Cr²⁺ ion in a ZnSe crystal field is a 4-level system with fast relaxation in the vibronic sublevels [12,27], the sum of the populations of the ⁵E upper state N_up at the 2091 nm pumping and the ⁵T₂ ground state, N_g, effectively equals to the doping concentration of Cr²⁺ ions in the active sites N_da (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) = σ_em(λ) × β − σ_abs(λ) × (1 − β) − γ(λ)/N_da,

(1)

where β = N_up/N_da, and γ(λ) are the passive losses at a wavelength λ.

The calculation of the effective gain cross section through Exp. 1 showed an increase in the operation wavelength span of the Cr²⁺:ZnSe laser (where the total laser gain G(λ) × N_da × L can exceed the cavity losses) with an increase in the inversion fraction (Figure 2). However, these calculations also showed that at high pump power (when the inversion fraction exceeds 0.2), pump absorption at 2091 nm ceased (where G(λ = 2091) = 0) due to the mutual compensation of up and down transitions ⁵E ↔ ⁵T₂. The Cr²⁺:ZnSe laser crystal can bleach at the high pump power. Therefore, the low-quantum defect pump of the Cr²⁺:ZnSe laser at 2091 nm demonstrated a self-limiting behavior.

3. Laser System Architecture

The laser system comprised a 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 [43]. 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 estimated 3 dB linewidth of the Ho:YAG laser was less than 0.3 nm (the measurement accuracy was limited by the spectrometer used). 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 using a power meter (Coherent PM 10 with FieldMax detector, Coherent, Saxonburg, PA, USA) was up to 34 W at the 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, UT, USA) (Figure 4b). The M2 parameter (determined by the ISO 11146-1:2021 standard with the knife-edge method [44]) 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, MI, 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 a length of 50 mm or 65 mm and a 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 rods were wrapped in indium foil and inserted into the copper radiators, with the temperature stabilized at 5 ± 0.1 °C. The Ho³⁺:YAG elements were pumped by a CW Tm-doped fiber laser at 1908 nm (“NTO IRE-Polus”, Fryazino, Russia) with a power up to 53 W. The diameter of the pumping beam at 1908 nm in the Ho³⁺:YAG crystal was tuned by Galilean telescope T2 to be equal to ~0.8–0.9 mm, when measured at e⁻² peak intensity with the knife-edge method [44]. The diameter of the repetitively pulsed amplified beam at 2091 nm was also tuned by Galilean telescope T3 to be equal to ~0.8–0.9 mm.

At a repetitively pulsed input power of 2.2 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 18.1 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 2.2 W input power, and with an input power of 18.1 W, the output power increased to 42 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) [45]. 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 pump-induced thermal lens and the signal self-focusing effect [46,47], 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 [44]) 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 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, which varied from 10 to 40 kHz.

The input dichroic mirror M2 of the Cr²⁺:ZnSe laser cavity (Figure 3) was chosen to enable the high transmission of the p-polarized pump beam at 2091 nm and the high reflectivity of the lasing at 2.3–2.7 μm, which had an s-polarization. Rear mirror M1 had over 99% reflectivity at both the lasing wavelength of 2.3–2.7 μm and the pumping wavelength of 2091 nm. The M1 mirrors with a radius of curvature of 300 mm or 500 mm were tested in the experiments. The transmittance of 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 the 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 using the following expression [46]:

$$D_T=-2\times {\left(\frac{\partial n}{\partial T}\right)}_{eff} \times \frac{\partial }{{\partial r}^2}{\int }_0^{L}T\left(r,z,t\right)dz\mid \left(r=0\right),$$

(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), the bulging of end faces, and the photoelastic effect, and given for an active element using the well-known expression:

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

(3)

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

$$T\left(r,z,t\right)=\frac{{\sigma }_{abs}({\lambda }_p)(1-{\raisebox{1ex}{${\lambda }_p$}\!\left/ \!\raisebox{-1ex}{$\lambda $}\right.}_S)}{\rho c_p}{\int }_0^t{I}_p(r,z,t)\times n_{up}(r,z,t)dt=\frac{{N_{da}\sigma }_{abs}({\lambda }_p)(1-{\raisebox{1ex}{${\lambda }_p$}\!\left/ \!\raisebox{-1ex}{$\lambda $}\right.}_S)}{\rho c_p}{\int }_0^{w_p\left(r,z,t\right)}d{w}_p\left[1-\frac{{\sigma }_{abs}({\lambda }_p)}{\left({\sigma }_{abs}({\lambda }_p)+{\sigma }_{em}({\lambda }_p)\right)}\left(1-e^{-\frac{w_p\left({\sigma }_{abs}({\lambda }_p)+{\sigma }_{em}({\lambda }_p)\right)}{h\ast {\nu }_p}}\right)\right],$$

(4)

where ${\sigma }_{abs}({\lambda }_p)$ and ${\sigma }_{em}({\lambda }_p)$ 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 Planck constant, ρ and C_p are the density and the thermal capacity of the Cr²⁺:ZnSe crystal, and 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 the Franz–Nodvig solution [45]:

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

(5)

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 using the following expression [48,49]:

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

(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. Key parameters of the Cr²⁺:ZnSe crystal, laser cavity, and the pump beam used for the numerical calculations, and the calculated focal length of a thermal lens in the laser element.

Parameter	Value
Density of the doping Cr2+ ions, cm−3	5 × 1018
Density of the ZnSe crystal ρ, g/cm3	5.27 [50]
Specific heat capacity of the ZnSe crystal, Cp, J/(g K)	0.34 [50]
Thermal conductivity of the Cr2+:ZnSe crystal, KT, W/(cm K)	0.18 [14,19,51]
Refractive index of the Cr2+:ZnSe crystal (at 2400 nm), n	2.43 [52]
Thermo-optic coefficient of the ZnSe crystal, (∂n/∂T), K−1	61 × 10−5 [19,53]
Thermal expansion coefficient of the ZnSe crystal, βT, K−1	7.3 × 10−6 [54]
Absorption cross section of the Cr2+:ZnSe crystal (at 2091 nm), σabs, cm2	14 × 10−20 [18]
Emission cross section of the Cr2+:ZnSe crystal (at 2450 nm), σem, cm2 Poisson’s ratio of the ZnSe crystal, ν	13 × 10−19 [18] 0.28 [55]
Length of the Cr2+:ZnSe element L, mm	16.5
Refractive index of TeO2 for the ordinary and extraordinary waves, noTeO and neTeO (at 2400 nm)	2.17 and 2.3 [56]
Length of the AOTF element, mm	25
Curvature of the M1 rear mirror, mm	300
Pump beam radius ap (at e−2 intensity), µm	450
Estimated focal length of the steady-state thermal lens in the Cr2+:ZnSe crystal (at the average input pump power, Pin), fT, mm Electronic nonlinear refractive index, n2, cm2/W	28 (at 10 W) 14 (at 20 W) 1.2 × 10−14 [11,20]

.

The focal length of the thermal lenses at the end of each pulse (calculated using expressions (2)–(5)) and the steady-state thermal lens (calculated using expression (6)) are presented in 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 using the following expression

$${\beta }_{ef}=\frac{1}{L}{\int }_0^L\frac{n_{ex}\left(0,z,{\tau }_p\right)}{N_{da}}dz=\frac{1}{L}{\int }_0^L\left[1-\frac{{\sigma }_p}{\left({\sigma }_p+{\sigma }_{emp}\right)}\left(1-e^{-\frac{w_p\left(o,z,{\tau }_p\right)\times \left({\sigma }_p+{\sigma }_{emp}\right)}{h\ast {\nu }_p}}\right)\right]dz.$$

(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, and 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 using the “Rezonator” software package [57]. The optimal overlap of the fundamental mode at 2400 nm and the pumping beam in the active element was achieved at a pump power of 28 W in the cavity with a physical length of 140 mm (Figure 9a). The optimal cavity length for stable mode operation decreased with the increase in the pump power (Figure 9b). Rear mirror M1 with a 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 a 300 mm radius of curvature on rear mirror M1 and the 35% output coupler M3 (Figure 10). 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 a 70 cm focal length using the knife-edge method [44]. The beam quality was M² ≤ 1.5 at an output power of 3.8 W in the single-transverse-mode regime (Figure 10a).

Waveforms of the Cr²⁺:ZnSe laser pulses were registered using an in-house-made photodetector based on a PD 36-02 (or 01)-PR(TO18) photodiode manufactured by IBSG Co., Ltd., St. Petersburg, Russia (the 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 using an in-house-made photodetector based on a HAMAMATSU InGaAs PIN G8422-03 photodiode (the spectral response range is 0.9–2.1 μm). The Cr²⁺:ZnSe laser operated in the gain-switch mode: the laser pulse was delayed with respect to the pumping pulse, and the delay time decreased with an increase in the pump pulse energy (Figure 11). The laser pulse width decreased from 50–70 ns near the threshold to 20–25 ns far above the threshold.

The output spectrum was measured using an IR Fourier spectrometer FT-801 (Simex, Moscow, 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 in the average pump power from 8 W to 22 W. The self-tuning of the spectral maximum wavelength can be explained by switching 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 12a) and decreased as the pump power decreased.

The operating wavelength of the Cr²⁺:ZnSe laser with the intracavity AOTF was tuned through the frequency tuning of the HF driver: from 2300 nm at ~42.6 MHz to 2700 nm at ~36.6 MHz (Figure 12b–e). The discreteness of the wavelength tuning was determined by the discreteness of the AOTF-driver frequency tuning, which was ~1 kHz (corresponding to a 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 3.1–3.3 W 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 13). The beam quality of the tuned Cr²⁺:ZnSe laser remained high, M² ≤ 1.5 (the beam quality at 2.3 µm was slightly higher than at 2.5 µm, which can be explained by the decrease in heat generation with the decrease in the pump quantum defect).

The Cr²⁺:ZnSe laser wave had linear polarization due to the polarization selectivity of cavity mirror M2 and the AOTF. The polarization contrast (measured using 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 dichroic mirror M4 (Figure 3).

3.4. Cr²⁺:ZnSe Power Amplifier

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 longest active element of 26–32 mm length.

The diameter of the pump beam at 2091 nm in the Cr²⁺:ZnSe amplifier was determined by 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.

A double master oscillator provided automatic time synchronization in the Cr²⁺:ZnSe amplifying element of pumping pulses of 2091 nm radiation amplified in the Ho:YAG amplifier and the Cr²⁺:ZnSe laser pulses which came a little later. 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). At stable temperature and environmental conditions in the laboratory, the instability of output pulse peak power and average power was less than 10–15% at 20–30 kHz PRR (the instability of the Ho³⁺:YAG laser oscillator pulses was less than 5%, and the Cr²⁺:ZnSe laser oscillator instability was less than 7–10%).

4. Discussion

The power scaling of the Cr²⁺:ZnSe laser system can be achieved by adding power amplifiers [26,27]. 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 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 the 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, and self-focusing could be reduced, 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

A 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 tunability was achieved by using the TeO₂ AOTF. An 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.

A compact and powerful Cr²⁺:ZnSe laser system, tunable within 2.3–2.7 µm, was designed for long-range LIDAR -based environmental monitoring in the transparency window of the Earth’s atmosphere. Several pollutant gases, such as CH₄, NH₃, CO, and HF, have specific absorption bands in the wavelength range of 2.3–2.7 µm and can be detected using the developed laser source [58,59,60,61]. The small mass-dimensional parameters of the created Cr²⁺:ZnSe laser system, as well as the electronic control of all its components including wavelength tuning, allow its use for the remote monitoring of pollutant gases near the surface or in the upper atmosphere, both from the Earth’s surface and from an aircraft or low-orbit spacecraft. In addition, the high-average-power repetitively pulsed tunable Cr²⁺:ZnSe laser system can find applications in other fields: dental hard tissue ablation [62], precision polymer processing [63], the pumping of mid-infrared optical parametric oscillators, and Fe²⁺-doped solid-state lasers [64,65].

6. Patents

This work resulted in 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, and Yu.A. Getmanovskiy [66].