Objective As a space active photoelectric remote sensing technology, lidar is of great significance to high-precision three-dimensional imaging, ground detection with high vertical resolution, and deep space exploration with high spatio-temporal resolution. For traditional spaceborne and high-altitude airborne lidars, laser signals with low repetition rate and high pulse energy and linear photoelectric detection technology were mostly used, which had problems such as high power consumption, large size, and low surface resolution. The development of single-photon detection technology can simplify the lidar system, and improve the detection sensitivity and detection efficiency. However, it also requires laser signals with different performance parameters. Laser with high repetition rates can increase the sampling frequency and describe the sampling target more accurately. And the laser signal with a narrower pulse width can reduce the detection error and improve the detection accuracy of the lidar. And the narrower linewidth laser, combined with the corresponding narrowband filter, can reduce the influence of background noise on the detector and improve the sensitivity of the detection system. In this paper, we report a compact single-frequency laser with high repetition rate, high pulse energy, and narrow pulse width output. We hope that our laser will be helpful to space active detection lidar based on single-photon detection technology.
Methods In order to achieve narrow pulse laser output, electro-optic Q-switch is selected to obtain narrow pulses under 10 ns and generate high peak power laser. According to the theory of electro-optic Q-switched laser, the factors affecting the pulse width are analyzed: Nd∶YVO4 crystal with higher σ21τ value is selected to obtain a higher small signal gain; the cavity length is shortened and the pump power is increased to obtain a narrower laser pulse. In order to achieve a narrow linewidth laser output, a volume Bragg grating mode selection method is used to build a solid single-frequency laser. The laser is end-pumped by a continuous-wave laser diode. The semiconductor laser with a pigtail output has a center wavelength of 808 nm, which can realize an adjustable continuous output with a power of 0--15 W. The core diameter and numerical aperture of the fiber are 200 μm and 0.22. The collimating and focusing system is a combination of two plano-convex lenses with focal lengths of 15 mm and 23 mm, respectively. The 808 nm pump light is focused on the gain crystal. The actual spot radius of the focal point is about 300 μm. The 0 ° total reflection plane mirror M1 coated with 808 nm high-transmittance film and 1064 nm high-reflection film forms a flat cavity structure with the coupling output element reflective volume Bragg grating (RBG), and the physical cavity length is 48 mm. The polarization beam splitting is used as a polarizing element. The rubidium titanyl phosphate (RTP) crystal pair is used as an electro-optic Q-switch. PBS, RTP and the 1/4 wave plate together constitute the Q-switch of the laser. It adopts a voltage-increased electro-optic Q-switched method, and is driven by a high-frequency and high-voltage signal to realize the on and off switch of the optical circuit, forming a Q-switched giant pulse output.
Results and Discussions At a repetition frequency of 10 kHz, when the pump power is 9.67 W, a laser output with an average power of 1.68 W is obtained (Fig. 4). The power instability within 3 h is 0.32% (Fig. 5). The output laser pulse width is 1.3 ns, and the pulse waveform is smooth (Fig. 6). The output wavelength is 1064.355 nm, and the line width is 1.0 pm (Fig. 8). According to the longitudinal mode interval formula Δλ=λ02/(2l'), the longitudinal mode interval is 8.6 pm in the condition of 65.7 mm optical cavity length in this experiment, which is larger than line width of the output laser. So the laser realizes single longitudinal mode output. The beam quality factor of two directions is Mx2=1.22 and My2=1.18 (Fig. 9).
Conclusions A single-frequency solid-state laser with high repetition rate and narrow pulse width is introduced in this paper. The laser is end-pumped by a continuous-wave laser diode, Nd∶YVO4 crystal as gain medium, the RTP crystal pair as the electro-optic Q-switch, and RBG as output mirror. In a resonant cavity with an optical cavity length of 65.7 mm, the single-frequency laser output with a wavelength of 1064.355 nm is locked. And at a repetition frequency of 10 kHz, the laser has a pulse width of 1.3 ns, an average power of 1.68 W, and the beam quality of Mx2=1.22 and My2=1.18. The laser has a compact structure and achieves a narrow pulse width, a narrow line width, and a large energy laser output at a high repetition rate. It can be used as a laser radar emission source for single-photon detection, and can also be used as a seed source of the main oscillation power amplification system for amplification to achieve more long-distance detection.
