Objective With their high sensitivity to Ge and InGaAs and excellent transparency in the atmosphere, eye-safe lasers emitting in the 1.5--1.6 μm spectral range have great application prospects in the lidar, rangefinder, and three-dimensional imaging. Vehicular lidars operating at 1.5 μm have attracted wide attention in the recent development of unmanned aerial vehicles (UAVs). Therefore, high-performance 1.5-μm lasers with high repetition rate, high pulse energy, and narrow pulse duration are commercially valuable. Passively Q-switched lasers operating at 1.5 μm in different gain media, such as Er ³⁺/Yb ³⁺co-doped borated crystals and phosphate, have been often reported. Owing to their high thermal conductivity, high Yb ³⁺→ Er ³⁺ energy-transfer efficiency, and weak up-conversion loss, Er, Yb∶YAB crystals are considered as an excellent 1.5 μm laser material. However, all previous reports on Er, Yb∶YAB lasers have employed c-cut Er, Yb∶YAB crystals. This paper reports a passively Q-switched microchip laser with an a-cut Er, Yb∶YAB crystal.

Methods This paper explores a passively Q-switched microchip laser with an a-cut Er, Yb∶YAB laser crystal as the gain medium. The experimental setup is shown in Fig. 2(b). The detailed performance of a laser with an a-cut Er (atomic fraction of 1.5%)∶Yb (atomic fraction of 12%)∶YAB crystal was reported in our previous work. In the present experiment, a Co ²⁺∶MgAl₂O₄ crystal with an initial transmission of 96% at 1.5 μm was employed as the saturable absorber. An input mirror and an output coupler were separately coated on the surfaces of two sapphire crystals, which acted as heat sinks in the microchip laser. The input-mirror material was antireflective in the 800--1000 nm range and reflected 99.8% of the light in the 1500--1600 nm range. Meanwhile, the output coupler transmitted 2.5% of the light in the 1500--1600 nm range. The input mirror and output coupler were tightly attached to the surfaces of the Er∶Yb∶YAB and Co ²⁺∶MgAl₂O₄ crystal, respectively. The resonator length was 2.7 mm. The pump source was a 976-nm diode laser with a central wavelength of 975.5 nm, a core diameter of 105 μm, and a numerical aperture of 0.22. After passing the lens assembly, the pump beam was focused into the laser crystal (with approximate radius of 60 μm). The microchip laser was maintained at 19 ℃ by water-cooling.

Results and Discussions Pumped by the 976 nm diode laser, the microchip laser successfully generated 1.5-μm pulses. The output power of the laser was measured by a PM 100D power meter with an S314C thermal power head. The pulse profile was recorded by an InGaAs photodiode connected to a digital oscilloscope with a 1.0-GHz bandwidth, and the laser spectrum was recorded by a wavescan laser-spectrometer. From the plotted dependence of average output power on the incident power (Fig. 3), the laser threshold was determined as 4 W. The average output power increased as the incident pumped power increased from 4 to 6 W. At higher incident powers (> 6 W), the average output power was not obviously increased and bends appeared in the output characteristics. The average output power was maximized at 275 mW. The repetition rate increased with incident pump power, and was maximized at 127 kHz. The pulse train [Fig. 4(a)] was stable and the pulse-amplitude variations and time jittering remained at 5%. The pulse duration [Fig. 4(b)] was 12 ns, narrower than that in previous reports. The laser wavelength was 1530 nm, consistent with previously reported emission spectra of Er, Yb∶YAB crystals. The pulse energy and pulse duration remained at approximately 2 μJ and 13 ns, respectively. As the Er∶Yb∶YAB crystal is optically uniaxial, it was characterized for two principal light polarizations, E∥c(π) and E^c(σ) (in which the optical axis is parallel and perpendicular to the c axis, respectively). The emission coefficient was much higher in σ-polarization than that in π-polarization, implying linear polarization of the pulses generated from the a-cut Er, Yb∶YAB microchip laser. In pulse-polarization tests (Fig. 5), the extinction ratio of the pulse laser was 44∶1. The spatial profile of the output beam presented high ellipticity and an even energy distribution. Therefore, a high-repetition-rate linear polarization laser was successfully fabricated from the microchip.

Conclusions This paper reports a microchip laser based on an a-cut Er, Yb∶YAB crystal. A Co ²⁺∶MgAl₂O₄crystal with an initial transmission of 96% was employed as the saturable absorber. The cavity length and maximum repetition rate of the microchip laser were 2.7 mm and 127 kHz, respectively, corresponding to the pulse energy and duration of 1.8 μJ and 12 ns, respectively. The emission coefficient was much larger in σ-polarization than that in π-polarization, indicating linear polarization of the pulse laser. In summary, a high-repetition-rate linearly polarized 1.5-μm pulse laser was fabricated by a simple and reliable method.