Due to its small construction, low-intensity noise, and narrow linewidth, all-solid-state continuous-wave (CW) single-frequency lasers have been widely employed in scientific research, military, and medical applications. The emission wavelengths of lasers can be confined to many particular ranges due to the constraints of the fluorescence spectra of laser crystals, which cannot match the demands of rapidly increasing scientific research. As an effective laser wavelength conversion technology, the nonlinear frequency conversion process including optical parametric oscillation (OPO), sum frequency (SF), different frequency (DF), etc., provides multi-watt CW output powers in the deep-ultraviolet (DUV) to mid-infrared and further expands the applied field of lasers. The single-frequency 1550 nm laser is frequently utilized in the formation of quantum squeezed and entangled states because its wavelength matches the low dispersion and low loss window of fibers, allowing for long-distance and steady laser transmission through the fiber. At present, there are several methods to generate a 1550 nm laser. Firstly, the 1550 nm laser is produced by a laser crystal co-doped with ytterbium-erbium (Yb³⁺ , Er³⁺ ) directly pumped by a 976 nm semiconductor laser. The low cost, small construction, and ease of downsizing draw a lot of interest, however, the gain crystal’s intrinsic excited state absorption restricts the laser output power. Secondly, the 1550 nm laser is also produced by an erbium (Er³⁺ )-doped fiber laser. The erbium-doped fiber laser’s waveguide structure is advantageous for achieving high-power output, although the output laser’s noise is rather high. In comparison to the previous approaches, the OPO process combines the benefits of low noise, small line width, and high stability to make the single-frequency 1550 nm laser the ideal contender. Especially, when we would like to generate the 1550 nm squeezed and entangled states, it is needed to use 775 nm and 1550 nm lasers as the pump and signal lasers of the optical parametric amplifier (OPA), respectively. As a result, an intra-cavity frequency-doubled (FD) singly resonant optical parametric oscillator (SRO) made of four mirrors is created and reported in this research, with simultaneous watt-level CW single-frequency lasers at 775 nm, 1550 nm, and 3393 nm.

In the experiment, to achieve high power signal laser and its frequency-doubled laser, it is important to acquire two focus waists in the cavity. The thermal lens effect of the MgO∶PPLN crystal induced by the pump, signal, and idler lasers was initially estimated for this purpose. The thermal lens effect of MgO∶PPLN is mostly due to its absorption of high-power intra-cavity signal lasers, according to theoretical simulations. On this basis, a ring resonator including two small waists was designed and built, which consists of four concave mirrors (radius of curvature of mirrors M1 and M2 is 60 mm and that of mirrors M3 and M3 is 40 mm), and whose whole length was optimized to 406 mm. The waist radii of the signal lasers at the OPO and second harmonic generation (SHG) crystals were 70 μm and 52 μm, respectively, in this scenario. To ensure a singly resonant optical parametric oscillator for the signal and single-pass transmission for the pump and idler lasers, the input coupler M1 was coated with high reflection (HR, reflectivity R>99.8%) film for the signal laser across 1400-1700 nm and high transmitting (HT, transmittivity T>97%) film for the pump 1064 nm laser. Mirror M2 was coated with HR(R>99.8%) film for the signal laser and HT(T>95%) film for the idler laser (3000-4200 nm). Mirror M3 was coated with HR(R>99.8%) film for the signal laser. Mirror M4 was coated with 1% transmission film for signal laser and HT (T>95%) film for frequency-doubled laser across 730-850 nm. The pump source was a handmade all-solid-state CW single-frequency 1064 nm laser with good performance. A coupling system consisting of an optical isolator (OI), two half wave-plates (HWP), and two lenses oriented and focused the output laser beam on the OPO. A 40 mm long 5% MgO-doped periodically poled PPLN crystal (polarization cycle Λ=30.49 μm) was used as the OPO crystal owing to its wide transparent window and low absorption loss, which was placed at the focus point between the mirrors M1 and M2 to generate the high efficiency and high-power signal and idler lasers. For the intra-cavity SHG crystal, a PPKTP crystal (Λ=24.7μm) with the size of 2 mm×2 mm×15 mm was used and placed at the other waist between mirrors M3 and M4. For signal and frequency-doubled lasers, both sides of the crystals were covered with antireflection coatings. Both crystals were kept in separate ovens, each controlled by a 0.01 ℃ high precision temperature controller. The high-quality watts single-frequency infrared to mid-infrared laser output was generated by controlling the temperature of MgO∶PPLN and PPKTP to 51 ℃ and 40.2 ℃, respectively.

Using a small double waists single resonance oscillation and periodically polarized crystal, a single-frequency CW three-wavelength laser output from the near-infrared to the mid-infrared at the watt level was obtained. Figure 1 depicts the effect of the pump, signal, and idler lasers on the thermal lens focal length of MgO∶PPLN crystal. It is shown that the effect of signal laser on the thermal lens focal length of MgO∶PPLN crystal is much larger than that of idle and pump lasers. A compact four-mirror ring SRO was designed as Fig. 2. We obtained 4.1 W of 1550 nm signal laser output power and 2.1 W of 3393 nm idler laser output power when the input pump power was 21 W (Fig.4), the quantity factor M² of 1550 nm laser was better than 1.05 (Fig.5). The measured output power of signal and SHG lasers vs the input pump power is shown in Fig. 7. The pump threshold was discovered to be 8.3 W. When the incident pump power was raised to 16 W, more nonlinear effects appeared in the crystal as the signal laser power in the cavity rose, resulting in a reduction in FD conversion efficiency. When the pump power was increased to 21 W, the laser was operating stable and the output powers of 1550, 775, and 3393 nm lasers were 2.1, 1.1, and 1.7 W, respectively.The root-mean-square (RMS) variations of the output power during 5 h are less than 2.5% for the 1550 nm laser, 0.8% for the 3393 nm laser, and 1.6% for the 775 nm laser, respectively (Fig.8). The overall efficiency of light-to-light conversion was 23.3%. The 775 nm lasers’ measured beam quality was better than 1.13. (Fig.10). The output laser operates in a single longitudinal mode.

The experimental findings of the creation of near-infrared to mid-infrared lasers employing an SRO made up of four-mirror ring resonators and period poled crystals were described in this study. To begin, we looked at OPO’s focusing characteristics as well as the impact of the nonlinear crystal’s thermal lens effect on the laser stable area and waist size. On this basis, a four-mirror ring resonator structure with double small waists was designed. Then, as the OPO and frequency-doubling crystals, a MgO∶PPLN and a PPKTP were used to create the signal and frequency-doubled lasers, as well as the idler laser. When the pump power was 21 W, the output power of the 1550 nm signal laser could reach up to 2.1 W. The output powers of the 775 nm frequency-doubled laser and 3393 nm idler lasers were 1.1 W and 1.7 W, respectively, at the same time. The pump threshold was 8.3 W, the overall light-to-light conversion efficiency was 23.3%, and the beam quality was greater than 1.05 and 1.13 at 1550 and 775 nm, respectively. The RMS fluctuations of the power during 5 h were less than 2.5% of 1550 nm, 0.8% of 3393 nm, and 1.6% of 775 nm. The 775 nm and 1550 nm lasers created can be utilized as the pump and seed lasers of OPO and OPA, respectively, in quantum experiments to generate a 1550 nm compressed light field. It gives a trustworthy assurance for the development of a multi-component quantum light source, and it is viewed as a novel technological technique of attaining compact quantum squeezed state laser source manufacture.