Objective Stable single-frequency and high-energy lasers in the eye-safe band are important light sources for lidars and coherent detection. Currently, many studies have reported a human eye-safe 1645-nm single-frequency pulsed laser output. Er∶YAG crystals and Er∶YAG ceramics are two main types of common-gain media for realizing 1645-nm lasers. Compared with crystalline materials, ceramic materials possess the advantages of short growth time, large-scale production, and flexible doping concentration. In the present study, we report the results of an engineered prototype of a single-frequency pulsed Er∶YAG ceramic laser. The volume of this laser system is reduced, and the frequency stability is improved. An optimized symmetrically pumped double Er∶YAG ceramic structure is designed to solve the problem of performance degradation caused by limited space. Such a single-frequency laser light source with a smaller volume and higher stability is more helpful for a practical application.

Methods The single-frequency Er∶YAG pulsed laser system with seed injection mainly includes three parts: an Er∶YAG master laser, a symmetrically pumped dual Er∶YAG ceramic ring-cavity slave laser, and a detection-control system. To improve the stability of the pulsed laser, a single-frequency continuous Er∶YAG non-planar ring-cavity laser is employed as a master laser. To improve the mode-matching efficiency and reduce the laser volume, the slave-laser cavity is designed with a multiple-folding structure that adopts total reflecting mirrors. This structure is a symmetrical one of dual laser-diode(LD)-pumped dual Er∶YAG ceramics. To simultaneously satisfy the requirements of pulse energy and pulse width for lidars, the total cavity length is set to 2.3 m. By using multiple folding mirrors, the volume of the cavity is reduced, and a space is reserved for the seed laser at the center position to realize a reasonable use of space. The working process of the seed injection and laser output is described as follows: a p-polarized seed light is reflected by the injection mirror and enters the slave-laser cavity. The seed laser is then injected through the path of the first diffraction order of the acousto-optic modulator (AOM). In this manner, resonance is realized in the slave laser, and the AOM shuts off the optical path during this period. The photodetector detects the leaked resonant signal and transmits it to the control system. Simultaneously, the control system loads a triangular wave to the piezoelectric ceramic (lead zirconate titanate) to allow it to continuously scan the cavity length. When the peak of the resonant signal has been scanned, the AOM is stopped from turning off the optical path to establish the laser pulse.

Results and Discussions The realization of the stable single-frequency laser output with seed injection is shown. An experimental study of the injection-locked laser is conducted at a repetition rate of 200 Hz. The energy and pulse duration of the single-frequency laser linearly change with the increase in the incident pump power [Fig. 4(a)]. The obtained maximum pulse energy is 22.75 mJ, and the corresponding pulse duration is 223.1 ns. The envelope of the single-frequency pulsed waveform becomes very smooth [Fig. 4(b)] because only one longitudinal mode is present in the pulse after the seed injection. The heterodyne beat-frequency technique is used to detect the single-frequency characteristics of the obtained pulsed laser. The heterodyne beat frequency is obtained at the maximum output energy [Fig. 5(a)], and a fast Fourier transform is performed on the beat-frequency result [Fig. 5(b)]. The spectrogram shows that the output laser pulse has a single-frequency, and the center frequency of the heterodyne signal is 39.09 MHz, which is similar to the acousto-optic frequency shift of 40.68 Hz. The full width at half maximum of the spectrum is 2.46 MHz, 1.2 times the Fourier transform limit, which corresponds to a pulse duration of 223.1 ns. We measure the energy and frequency stabilities when the output energy of the laser pulse is kept at the highest level. The standard deviation of the energy jitter within 30 min is approximately 0.118 mJ [Fig. 6(a)], and the standard deviation of the frequency drift is approximately 0.578 MHz [Fig. 6(b)]. The spot-radius values of the pulsed laser with the highest energy are recorded using an infrared camera at different positions. Subsequently, the results are fitted and calculated, and the beam quality factors in the x and y directions are 1.16 and 1.15, respectively (Fig. 7).

Conclusions In this study, a single-frequency laser is designed and developed based on the seed-injection technology, in which the dual Er∶YAG ceramic symmetrical structure is end-pumped by a dual 1470-nm LD. Through the theoretical analysis, it is found that the main factors determining the seed-injection effect are the coupling of the seed with oscillating lasers, the power, and the frequency detuning of the seed laser. In the experiment, the obtained maximum average pulse energy is 22.75 mJ at a repetition rate of 200 Hz, and the corresponding pulse duration is 223.1 ns. The beam quality factors of the single-frequency pulsed laser are 1.16 and 1.15 in the x and y directions, respectively. The spectral width of this Q-switched single-frequency laser is 2.46 MHz and is 1.2 times of the Fourier transform limit. The standard deviation of the energy jitter is 0.118 mJ, and the standard deviation of the center frequency drift is 578 kHz. This stable and compact single-frequency Er∶YAG pulsed laser system can be used as a laser source for wind lidar and coherent detection.