Objective High power and low noise continuous-wave (CW) single-frequency all-solid-state lasers at 1.5 μm have important applications in laser interferometry, coherent Doppler lidar, optical frequency standard, cold atom physics, continuous variable (CV) quantum information, and basic research of quantum optics. 1.5 μm lasers with power more than 500 mW and an intensity noise spectrum down to the shot-noise limit are indispensable in the development of telecom band quantum light sources, e.g., CV entangled states. The diode-pumped all-solid-state CW single-frequency 1.5 μm lasers were mainly developed using the Er ³⁺ and Yb ³⁺ co-doped gain mediums and the longitudinal mode selection techniques, such as intracavity etalon, microchip cavity, or twisted-mode cavity. This kind of laser suffers from the relatively low thermal fracture threshold and significant thermal-induced depolarization, and the deterioration of beam quality and longitudinal mode structure. There is no report on a CW single-frequency all-solid-state laser at 1.5 μm providing low noise output more than 500 mW.

Methods To achieve high power CW single-frequency laser operation at 1.5 μm, the heat deposition inside the gain medium was firstly reduced by a dual-end face-cooling scheme, i.e., using two polished sapphire plates acting as transparent heat spreaders to improve the axial heat conduction in the Er,Yb∶YAB crystal. Secondly, to realize a relatively homogeneous distribution of pump absorption inside the gain medium, and to raise the maximum permitted incident pump power and achieve the best mode-matching, a long depth-of-focus polarized dual-end-pump structure was employed (Fig. 1, Fig. 2). Finally, two additional half-wave plates were used to control the pump polarization. Due to the dependence of absorption on the polarization, the pump power axial absorption can be tuned to be more uniform, and the pump saturation effect can be prevented.

The designed ring resonator was based on the precise measurements of the thermal focal lengths of both the laser crystal and the bismuth iron garnet (BIG) magneto-optical crystal. The former was measured using a knife method, and the latter was measured based on the theoretical analysis of the resonator’s dynamical behaviors depending on the varied thermal focal length of the BIG crystal. Considering the thermal lens of the laser and BIG crystals, the resonator length was optimized by balancing the thermal insensitive region, the mode-matching between the laser and pump beams, and astigmatism (Fig. 5). Figure 1 shows the resonator design of the CW single-frequency 1.5 μm laser based on unidirectional traveling-wave ring cavity.

Compared with the single-end face-cooled Er,Yb∶YAB crystal under σ-polarization single-end-pumping, the dual-end face-cooling scheme combined with the long depth-of-focus tunable polarization dual-end-pump structure reduced the thermal lens effect of the laser crystal significantly. The thermal focal length of Er,Yb∶YAB crystal was lengthened from 45 to 78.2 mm under 4.5 W pumping after the optimizations (Fig. 3). Besides, the functional relationship between the intracavity laser power and the thermal focal length of the BIG crystal was experimentally determined [Figs. 4(a) and (b)]. It can be found that the BIG crystal thermal effect was severe, and the thermal focal length was only 58 mm in the case of 25.3 W intracavity laser power. Based on the developments of the cooling and pumping schemes, using the measured thermal focal lengths to optimized the resonator length, 755 mW CW single-frequency 1.5 μm laser was generated from the ring cavity [Figs. 6(a) and (b)], and the power fluctuation within 2 hours was less than ±1.2% (Fig. 7). The intensity noise of the 1.5 μm laser was measured using a balanced homodyne detection system. The intensity noise reached the shot-noise limit for frequencies above 5 MHz (Fig. 8).

Results and Conclusions A low noise CW single-frequency 1.5 μm laser based on Er,Yb∶YAB crystal and unidirectional traveling-wave ring cavity was demonstrated. By measuring the thermal focal lengths of the sapphire-Er,Yb∶YAB-sapphire laser crystal and the BIG magneto-optic crystal, as well as adopting the long depth-of-focus tunable polarization dual-end-pump structure to reduce the thermal effects of laser crystal and raise the maximum permitted incident pump power, CW single-frequency 1.5 μm laser was realized using the unidirectional traveling-wave cavity technique. The laser power was scaled up to 755 mW with the power fluctuation less than ±1.2%, and the laser intensity noise reached the shot-noise limit beyond the analysis frequency of 5 MHz. This laser source can be used to generate a CV entangled light source in the telecom band. The power of the CW single-frequency 1.5 μm laser can be further scaled up by employing low-doped Er,Yb∶YAB crystal as the gain medium, using the magneto-optical crystals with lower absorption loss, as well as designing a 6-mirror ring cavity with weaker astigmatism.