Double Fabry–Pérot etalons assistant injection-seeded single-frequency Tm,Ho:YLF laser

In this paper, we report a diode side-pumped injection-seeded single-frequency Tm,Ho:YLF laser, which operated at lasing threshold and emitted at 2051.2 nm. The assistance of Fabry–Pérot (FP) etalons to seed injection via mode selection was investigated. An acousto-optic Q-switched output of 6.7 mJ was achieved at 5 Hz. A temporal pulse duration of 310 ns with spectral linewidth of 2.0 MHz and frequency stability of 0.8 MHz root-mean-square (rms) was detected. The beam profile was a high-quality Gaussian TEM00 mode with M2x/y of ~ 1.15/1.14. This 2 μm laser source would be a good candidate for ground-based wind-detection coherent Lidar system.


Introduction
Since the beginning of this century, pulsed 2 μm lasers have received growing interest from the remote-sensing community due to their eye-safe properties, efficient pumping mechanism, matching with the absorption lines of multiple trace gases, and etc. [1][2][3][4]. To achieve 2 μm lasing, diodepumped solid-state lasers based on the trivalent thulium, holmium (hereafter Tm, Ho) or Tm,Ho co-doped materials are one of the most common methods [5]. Among them, the co-doped materials are more attractive for Q-switched pulse generation, benefitting from both the higher energy storage capability of Ho ions and the advantage of Tm ions being directly pumped by the commercially available laser diodes around 793 nm [6]. As a fluoride host material, the LiYF 4 (YLF) stands out for its long radiative lifetime and low phonon energy, when it is doped with Tm,Ho [7]. These properties are especially preferred in the Tm,Ho co-doped system.
Aiming at coherent Lidar applications, 2 μm all-solidstate laser has several advantages, such as its eye-safe range, high atmosphere transmittance and so on, which should have fine narrow linewidth narrower than a few megahertz (MHz) and high beam quality nearly reaching the diffraction limitation to ensure Lidar's sensitivity and accuracy [8]. Injection seeding is the mostly reported approach to achieve these properties. U. N. Sigh et al. accomplished in 1997 a 500 mJ, 10 Hz, 2 μm laser employing a Tm,Ho:YLF microchip injection seeder [9]. Based on the same mechanism, J. Yu et al. achieved 125 mJ Q-switched output at 6 Hz in 1998, and increased the optic-optic efficiency to 3% as well as optimized the beam quality for Lidar application [8]. In 2008, the NASA LaRC group demonstrated a 350 mJ 2 μm laser prototype for ground, airborne, and space-based Lidar systems [10]. In 2019, S. Ishii et al. developed a conductively cooled single-frequency injection-seeded 2 µm MOPA with an output energy of 125 mJ/pulse at the laser rod temperature of 233 K [11].
In all above-mentioned reports, a high output energy of over hundred-mJ is obtained and injection seeding is realized in the far beyond lasing threshold regime, where its criteria are much more accessible [12]. However, for some ground-based Lidar or testing systems, a much lower output energy (e.g., several-mJ) can already meet their requirements 3 Page 2 of 6 [13,14]. Thereon, the at-threshold operation of 2 μm lasers is obliged, where injection seeding is still a challenging task mainly due to the highly desired spectral purity [12]. To increase the spectral purity, the Fabry-Pérot (FP) etalon can be a good assistance, which is by now barely reported as mode-selector in Q-switched injection-seeded 2 μm laser systems but commonly used in the low power seeders [15][16][17].
In this paper, we present an injection-seeded single-frequency 2 μm laser employing a double-FP mode-selector. This laser can provide several-mJ Q-switched output energy, high beam quality and several-MHz linewidth to fulfill the coherent Lidar applications.

Experimental
A schematic diagram of the laser setup is illustrated in Fig. 1. The slave-oscillator (SO) is an 8-shape ring resonator with optical length of 1.71 m. It consists of two curved mirrors (M1 and M2 with Roc of 4 m) and two plane mirrors (M3 and M4). M1-3 are high-reflectively (HR) coated and M4 has 27% output coupling transmission at 2.051 μm. M3 is attached to a piezoelectric actuator (PZT), which works as a cavity-length actuator as described in [8,18]. These mirrors are angled ~ 8.5° off axis.
The laser crystal is an a-cut, φ4*21.4 mm Tm,Ho:YLF crystal rod (5 and 0.5 at.% doped), which is mounted in a home-designed side-pumping laser head [19,20]. The laser diodes employed emit at 793 nm and provide maximum 3.6 J pump energy at 5 Hz with pump duration of 1.2 ms. To achieve Q-switched pulse generation, an acousto-optic modulator (AOM) of quartz is placed between M3 and M4.
Alongside, two FP etalons of thickness 0.32 and 1 mm are inserted, functioning as longitudinal mode-selectors to increase the spectral purity of SO.
As the master laser (ML), a commercial single-frequency laser of 200 mW at 2051.2 nm is used. The emission from ML is directed to the SO by two 45° HR mirrors (M5 and M6) after passing two isolators, a λ/2-plate and a collimation lens system. The isolators protect the seeder from the backpropagated pulses of SO, the λ/2-plate adjusts the polarization of ML light to match with that of SO, while the collimation system helps to optimize the spatial mode matching between ML and SO for a maximum seeding efficiency.
The Q-switch trigger of SO is controlled by the resonance signal detected behind M2, where an InGaAs detector is placed to collect the leaking signal of the injected ML light. This resonance signal is created by the axial scanning movement of the PZT (0.12 μm/V), which can be driven by a voltage up to 150 V. As shown in Fig. 2, once the peak of the resonance signal is detected, where the SO frequency matches well with that of ML, the PZT is held for 30 μs and the Q-switch is triggered there. Thus, a high spectral purity of the output is guaranteed for an optimized seeding efficiency. Figure 3 shows the output/pump energy characteristics of SO in the free-running and Q-switched regimes without injection-seeding. The chiller temperature for the crystal rod is set to 15 ℃. A free-running/Q-switched output energy of 9.6/6.5 mJ is achieved under a pump energy of 1.98 J, which is just above the lasing threshold pump energy of 1.81 J. The output ratio of Q-switching to free-running amounts to 0.68. M 2 measurement and data fitting for the Q-switched output are carried out to evaluate the beam quality, as shown in Fig. 4. The beam profile is a high quality Gaussian TEM 00 mode and the M 2 values along x-and y-axes which are about 1.15 and 1.14, respectively. This performance meets the requirements of Lidar application very well.

Results and discussion
To ensure the injection seeding operation, a wavelength profile of the SO output was characterized with an optical spectrum analyzer (AQ6375B, YOKOGAWA). As shown in Fig. 5a, the Q-switched SO pulses possess a broad bandwidth ranging from ~ 2051.1 to ~ 2053.8 nm with FWHM (Full Width Half Maximum) of 0.77 nm. This results in a poor spectral purity at the seeder wavelength (2051.2 nm), which hinders the effective coupling between SO and ML. To improve the spectral purity, two FP etalons with different thicknesses are placed besides AOM in SO and the modeselection results are shown in Fig. 5b, c, respectively. One can see that the 0.32 mm-thickness FP etalon picks up two peaks at the wavelengths of 2051.3 and 2054.8 nm. The  Fig. 5d. A successful seed injection is observed. The averaged output energy is 6.7 mJ under the pump energy of 1.98 J, corresponding to an optic-optic efficiency of 0.3%.
The successful seeding is confirmed by the smooth shape of the output pulse compared with the unseeded condition, as shown in Fig. 6a, b, respectively. The signals are detected by an InGaAs photodiode (Newport 818-BB-51F) with 10-GHz bandwidth. A temporal pulse duration of 310 ns is acquired. In addition, auto Fast-Fourier-Transform is analyzed by the oscilloscope (Tektronix, TDS3052C). Under the seeded condition, a single frequency is observed, while multiple frequencies with division of 175 MHz are observed under unseeded condition. This value matches well with the optical cavity length of SO.
To measure the spectral linewidth, the optical heterodyne method is employed [21]. The ML light, which has narrow linewidth and high frequency stability, can be used as the reference signal. Part of the ML light is frequency-shifted by 160 MHz, and then it is mixed with the Q-switched output pulses. The single-short heterodyne beat signal is captured by the high speed photodiode and recorded by an oscilloscope (Keysight, DSOX4104A, 1-GHz, 5 GSamples/s). A single-shot beat-note signal and its FFT are plotted in Fig. 7a, b, respectively. A symmetric spectrum with a linewidth of 2.0 MHz can be read from the FFT result, fulfilling the requirements of Lidar measurements. The frequency stability is analyzed via a 3 min-period data acquisition of the heterodyne beat-note signals. As shown in Fig. 8, a frequency stability of 0.8 MHz rms is achieved at an averaged beat frequency of 150.5 MHz. This 9.5-MHz deviation from the 160-MHz-shift is mainly resulted from the reaction time of the ramp-hold-fire control system.

Summary
In this paper, we present an at-threshold operated injection-seeded 2 μm laser with assistance of two FP etalons. At the threshold, the SO exhibits a very broad emission band, which leads to a poor spectral purity at the seeding wavelength and results in unsuccessful seed injection. The use of two FPs narrows the bandwidth of SO from 0.77 nm down to 0.1 nm and improved the seeding efficiency x/y of 1.15/1.14. All these performances meet well the demands of coherent wind detection Lidar measurements. Moreover, it is approved as a promising approach using FPs as longitudinal mode selectors in an injection-seeded Q-switched SO to optimize seeding efficiency.