Acousto-Optic Q-Switched Ho:YLF Ring Laser Based on Anti-Misalignment Resonant Cavity

Abstract

An acousto-optic Q-switching Ho:YLF ring oscillator at 2066.33 nm with two anti-misalignment corner cube reflectors (CCRs) pumped by a 1940 nm thulium-doped fiber laser is demonstrated. The depolarization effect of the CCR is expressed in the form of equivalent transmission, and the transmission from two output directions of the oscillator is changing synchronously and periodically as the waveplate angle changes. In the experiment, under the pump power of 21.76 W, the optimum bidirectional energy of 3.13 mJ for a pulse duration of 122 ns at a repetition rate of 50 Hz is realized. The pulse energy reduction percentage at 50 Hz is 8% by changing the horizontal drift angle of one of the CCRs to 4.5°. The beam quality factor M² is calculated to be about 1.10, revealing that the Ho laser is working in a fundamental transverse mode.

1. Introduction

Nanosecond pulsed solid-state lasers at 2 μm eye-safe region generated by acousto-optic Q-switching operation have become irreplaceable components in different kinds of applications, such as master oscillators of the injection-locked laser in coherent laser lidars for long-range wind field measurement and atmospheric trace gas detection [1,2,3,4], medical treatment for laser lithotripsy and soft tissue ablation [5,6], and excitation sources for optical parametric oscillators [7,8]. In particular, the tuned Ho pulsed lasers based on YLF host material can serve as the light sources of both coherent wind lidars and differential absorption lidars due to the high atmospheric transmission and strong trace gas absorption peaks at 2.05 μm or 2.06 μm [9,10,11]. Furthermore, Ho:YLF crystal is an excellent crystal to produce high-energy pulses with high beam quality since it has a longer upper state lifetime, larger gain cross section, and smaller thermal lens effect in comparison to the Ho:YAG crystal [12,13,14,15]. Strauss et al. realized a 330 mJ pulse energy for 350 ns pulse duration with an optical beam quality factor M² of 1.5 at 50 Hz in a 2064 nm narrow-linewidth double-pass Ho:YLF slab amplifier [16].

In airborne and even space-borne missions, the compactness and stability of the light source seriously affect the environmental adaptability of the lidar system. With the emergence of the 1.9 μm high-power thulium fiber lasers, the robust and efficient Ho:YLF lasers pumped by thulium-doped fibers have been proven [17,18]. However, at present, the high-energy Ho:YLF oscillators are all composed of ordinary spatial cavity mirror structures, and their stability is facing challenges. Laser oscillators with corner cube retroflectors (CCRs) have anti-misalignment properties because the output light of the CCR is opposite and parallel to the incident light, which can improve the stability of solid-state lasers. In passive Q-switched lasers, Gao et al. applied a CCR to the Nd:YAG laser, reaching a maximum pulse energy of 174.4 mJ for 2.95 ns pulse duration in passive Q-switching operation [19]. In continuous wave single-longitudinal-mode lasers, Wu et al. achieved greater than 700 mW single-longitudinal-mode power in a ring oscillator with double CCRs by placing a Faraday rotator into the oscillator to work in unidirectional operation [20]. The output power decreased by 30% when the angle of the CCR was shifted by 11.8°, proving the anti-misalignment of the ring oscillator with CCRs. Wu et al. demonstrated a narrow linewidth laser with two CCRs at 2.09 μm with a power of 728 mW, the corresponding slope efficiency was 23.42% [21]. In injection-locked lasers, Zhang et al. reported a 1.6 μm injection-locked laser system at a repetition rate of 143 Hz [22], which employed a ring oscillator with double CCRs as the slave laser, delivering the energy of approximately 2 mJ. Yan et al. achieved 32.3 mJ pulse energy for 161.6 ns pulses in a 2.09 μm injection-locked amplifier [23]. In coherent combined solid-state lasers, Cheng et al. proposed a coherent combination oscillator comprised of six crystal rods based on a corner cube [24], which delivered 15.3 J pulse energy with 95.6% combining efficiency. From the abovementioned analysis, it is clear that oscillators with CCRs pumped by fiber lasers are very promising for improving the stability, compactness, and energy of laser oscillator.

In this paper, we have proposed a Ho:YLF ring laser applying double CCRs as the resonant cavity mirrors. The depolarization characteristic of the CCRs on the output transmission is first investigated, and the optimal output performance is obtained by rotating the waveplate angle. At 50 Hz repetition rate, the light energy of 3.13 mJ is realized for a 122 ns pulse duration at 21.76 W pump power. The spectrum peak of the Ho laser is located at 2066.33 nm as measured by the zoomed-in spectrum. The beam quality factor M² is calculated to be approximately 1.10.

2. Materials and Methods

The structure of the Q-switching Ho laser with double anti-misalignment corner cube retroflectors (CCRs) is depicted in Figure 1. The 1.94 μm pump source is a thulium fiber oscillator possessing a highest power of 30 W and a beam quality factor M² of approximately 1.52, which is comprised of a pair of high-reflectivity and low-reflectivity fiber Bragg gratings (FBGs), and Tm active fibers with core/cladding diameter of 25/400 μm. The temperature of the low-reflectivity FBG is stabilized by a thermoelectric cooler (TEC) with an accuracy of ±0.03 °C, resulting in a wavelength stability of 0.05 nm for the pump source. The pump waist, with a radius of about 0.3 mm, is located at the center of the Ho:YLF crystal, which is expanded and condensed through two planoconvex lenses: F1, with f = 10 mm, and F2, with f = 300 mm. The ring resonator is composed of double fused-silica CCRs (CCR1 and CCR2) with a 35 mm height from the vertex to the bottom; the physical length between two CCRs is 520 mm.

The input surface of the CCRs have high transmittance films at 1.9~2.1 μm band, and three reflective surfaces do not have any films. TFP1 and TFP2 are 45° polarizers, possessing p-polarized high-transmittance films at 2.06 μm and high-reflectivity films for both pumping light and s-polarized light at 2.06 μm, so the pumping light is poured into the cavity through TFP1, and the rest is dumped from TFP2. The Ho³⁺-doping concentration of the gain medium of the 4 × 4 × 50 mm³ Ho crystal is 0.5 at.%, and the Ho crystal assembled on a red copper heat sink is restricted to 15 °C by a TEC. The two plane convex lenses F3 and F4 with f = 400 mm are placed symmetrically on both sides of the Ho medium to compensate for the negative thermal lensing effect. An acousto-optic Q-switch module is located at the resonant cavity to deliver laser pulses with different repetition rates. Equivalent output transmissions of two polarizers (TFP1 and TFP2) from two directions (clockwise and counterclockwise direction) of the ring oscillator with double CCRs are adjusted by the half-wave plate.

3. Results and Discussion

Polarizers TFP1 and TFP2 are used as the output mirrors in the Q-switching Ho resonant cavity, with double CCRs ensuring that the p-polarized light transmits from the polarizer and oscillates in the cavity, and the s-polarized light reflects from the polarizer. Considering the depolarization effect of the CCRs, in order to prevent the occurrence of a ‘no light output’ phenomenon, it is necessary to utilize an intra-cavity waveplate to vary the polarization states (or equivalent output transmission) before the two polarizers to generate the laser.

When two CCRs are mirror symmetric and have a cross angle of 30°, the polarization state components of oscillating light arriving at the polarizers are simulated in clockwise and counterclockwise direction by the Jones matrices. Therefore, the equivalent output transmission is calculated by the expression of T = $E_y^2/\left(E_x^2{+E}_y^2\right)$, where $E_x$ and $E_y$ are the intensity components along and perpendicular to the optical axis of oscillation light. More details about the simulation of the light polarization state of the resonant cavity with two CCRs can be found in the literature [19]. Equivalent output transmissions at the waveplate rotation angles of 0° to 360° are displayed in Figure 2. The equivalent output transmissions in both directions vary synchronously and periodically, indicating that there are two laser output directions for the oscillator. In the experiment, the optimum waveplate angle based on the maximum output power is obtained. The zoomed-in view of the output spectrum of the Ho laser with double CCRs is illustrated in Figure 3, revealing that the center wavelength of the oscillating laser is at 2066.33 nm.

At the optimum waveplate angle, the Q-switched performance including pulse energy and pulse duration of the Ho laser with double CCRs at pulse repetition frequencies of 50 Hz, 100 Hz and 200 Hz is shown in Figure 4 and Figure 5. Laser pulse train at 50 Hz emits from the Ho laser with a bidirectional pulse energy of 3.13 mJ when the pump power reaches 21.76 W. Additionally, 2.22 mJ pulse energy is achieved with a repetition rate of 200 Hz. The pulse duration decreases as the pump power increases at the same repetition rate, and the pulse duration increases as the repetition rate increases for the identical pump power. Consequently, the narrowest pulse duration of 122 ns is realized, corresponding to a 50 Hz repetition rate and 21.76 W pump power.

Compared to the traditional space resonant cavity structures, the laser oscillators with a CCR have an anti-misalignment feature because the light emitted from the CCR is always parallel to the incident light. The anti-misalignment feature of the Q-switched Ho oscillator with two CCRs is studied by changing the drift angle of one of the CCRs along the horizontal direction, as shown in Figure 6. At 50 Hz repetition rate, the maximum pulse energy becomes 2.85 mJ when the horizontal drift angle of the CCR1 in the experimental setup is up to 4.5°, which corresponds to an 8% energy reduction percentage. If a conventional space cavity with ordinary mirrors also has a 4.5° drift angle, the oscillator will not produce light. The spot sizes at different locations after focusing the output beam from the Ho laser are measured through the 90% to 10% knife-edge device, as displayed in Figure 7. By applying a Gauss fitting to the spot sizes at different positions, a beam quality factor M² of 1.10 for the Ho:YLF ring oscillator is calculated.

The 2.05 μm or 2.06 μm nanosecond injection locking lasers can not only serve as light sources for coherent wind lidars, but also as light sources for differential absorption lidars. Lasers in this band are prone to high energy and high beam quality, which is crucial for boosting the detection range of the laser lidars. In the nanosecond injection locking systems, the Q-switching pulsed slave oscillator plays a role in energy output, and the anti-misalignment feature of the slave oscillator has a critical impact on the energy and frequency stability of the entire system. Wang et al. achieved a 2.09 μm Q-switching Ho:YAG resonator using a solid-state Tm pumping module [25]. The Ho oscillator worked stably when the drift angle of the CCR reached about 12°. According to the percentage reduction in energy or power, the anti-misalignment angle in this work is better than previously obtained results [25]. Furthermore, the Ho oscillator in this work is pumped by a fiber laser, once again improving the compactness of the laser system. Therefore, the acousto-optic Q-switching Ho:YLF ring oscillator comprised of anti-misalignment CCRs is a meaningful oscillator for improving the stability of 2.05 μm or 2.06 μm light sources of lidars.

4. Conclusions

In conclusion, a compact and anti-misalignment Ho:YLF ring oscillators with double CCRs pumped by a 1940 nm thulium fiber laser was demonstrated in this paper. The equivalent output transmissions of the Ho oscillator were stimulated through Jones matrices, and the waveplate was rotated from 0° to 360° to search for the optimum transmission and maximum output power. A bidirectional pulse energy of 3.3 mJ for a pulse duration of 122 ns was realized at the repetition rate of 50 Hz under the pump power of 21.76 W. By changing the horizontal drift angle of one of CCRs to 4.5°, the energy reduction percentage at 50 Hz was 8%, proving that the oscillator with CCRs is anti-misalignment. The beam quality factor M² of the Ho laser was calculated to be approximately 1.10 by measuring spot sizes at different positions through the 90% to 10% knife-edge method, indicating that the oscillating light was operating in a fundamental transverse mode.