J. Eur. Opt. Society-Rapid Publ. 22, 55( 2026) 537
OPO system based on a c-mount LD pump source and PPMgLN as the nonlinear crystal. The output characteristics of the system at two distinct wavelength bands – 1.5 lm and 3.8 lm – are systematically investigated.
2 Experimental setup
Schematic diagram of the experimental setup is shown in Figure 1. An 807.6 nm c-mount LD is adopted as the pump source. This LD, combined with a fiber-based beam shaping and focusing configuration, uses a fiber to homogenize and condition the raw output beam into a uniform and stable near-Gaussian spot, greatly improving the spatial quality and pointing stability of the pump light. At the same time, this structure achieves efficient heat dissipation through c-mount packaging, supporting the stable operation of the LD at high power. Compared to the structure utilizing a fiber-coupled LD array, the present design offers greater compactness and simplicity.
The laser medium is a Nd: YVO 4 crystal with a size of 3 3 5mm 3 and 0.2 at. % Nd 3 +-doping. The left side of the crystal, M in, is coated with an 807.6 nm high-transmission( HT) coating( T > 98 %) and a 1064 nm, 1.4 – 1.6 lm high-reflection( HR) coating( R > 99 %). The right side of the crystal is coated with a 1064 nm, 1.4 – 1.6 lm anti-reflection( AR) coating( R < 0.5 %). The output mirror, M oc, has a radius of curvature of 100 mm. Its left side is coated with a 1064 nm HR coating( R > 99 %), a1.4 – 1.7 HR coating( R = 98 %), and a 3 – 4 lm AR coating. The right side is coated with a 1.4 – 1.6 lm and3 – 4 lm HT coating. The material of the output mirror is CaF 2, which has a relatively small absorption coefficient in the mid-infrared range, facilitating the output of mid-infrared idler light. Both M in and M oc have the HR coating at 1064 nm, forming a resonant cavity for the fundamental light at 1064 nm; they also have the HR coating for the signal light, forming a resonant cavity for the OPO system. The resonant cavity length is 65 mm, and the total length including the LD is only 70 mm.
The PPMgLN crystal is 50 mm in length, and both sides are coated with a 1.4 – 1.6 lm and3 – 4 lm HTcoating. The thickness of the PPMgLN crystal is 1 mm. The PPMgLN crystal is mounted on a copper heat sink with a thermoelectric cooler( TEC) maintaining a setpoint of 25 ± 0.1 ° C, and the MgO doping can effectively suppresses photorefractive damage under our experimental conditions. We selected five periods of 28.6, 28.8, 29.0, 29.2, and 28.4 lm, which can achieve tuning of the signal light from 1435 to 1473 nm and the idler light from 4114 to 3833 nm.
The shared-cavity OPO resonates both the fundamental and signal lights within a single optical cavity, leading to a fundamental enhancement in system performance. This architecture not only significantly increases the intracavity power of the fundamental light – thereby providing strongerdrivingpowerfornonlinearconversion – but also, and more importantly, ensures perfect spatial mode matching between the two beams. Across the entire cavity, especially
Figure 1. Schematic diagram of the experimental setup.
within the nonlinear crystal, the spot sizes and wavefront curvatures of the fundamental and signal modes are automatically aligned. This enables highly efficient parametric oscillation and fundamentally overcomes the conversionefficiency limitations imposed by mode mismatch in conventional external-cavity designs.
In terms of mechanical structure, all beams share the same cavity mirrors and mechanical mounts, which greatly reduces sensitivity to vibration, thermal drift, and mechanical misalignment. This configuration endows the system with good short-term stability over a two-hour period and beam pointing stability. Meanwhile, the inherent modeselection property of the resonator guarantees that the output signal beam possesses a narrow spectral linewidth. This structure is simple and compact, providing a compact and low-cost architecture for mid-infrared continuous-wave lasers.
Using ABCD matrix analysis [ 14 – 16 ], the fundamental mode waist radius at the plane mirror M in is calculated to be 122 lm in the cold cavity as shown in Figure 2. Themeasured LD spot radius at 807.6 nm is about 100 lm, giving an overlap integral of approximately 0.96, which confirms excellent mode matching at 1064 nm. Under high-power pumping, thermal lenses in both Nd: YVO 4 and PPMgLN are considered [ 17 ]. At 5.0 W pump power, the calculated thermal focal lengths are 48 mm and 96 mm, respectively. Including these thermal lenses, the waist radius at M in decreases to 105 lm, still well above 100 lm, and the fundamental overlap integral reaches approximately 0.99. The cavity stability parameter remains between 0.21 and 0.35 for all pump powers( 0 – 5.0 W), far below the stability limit of 1. For the signal light, taking 1500 nm as an example, at the pump power of 0 W, at the center of the PPLN crystal, the beam waists of the fundamental and signal lights are 124 lm and147lm, respectively, with an overlap integral of 0.97. At the pump power of 5.0 W, the corresponding beam waists become 130 lm and 108 lm, with an overlap integral of 0.96. Over the all pump powers( 0 – 5.0 W), the signal beam waist remains less than the actual fundamental beam waist, maintaining good mode matching.