Optical parametric oscillators(OPOs)have been proven to be effective,coherent light sources that can expand the wavelengths of commercial lasers(typically limited to narrow emission lines and bands)to a broad range from visible to far-infraredbands. Q-switched lasers with high peak powers have significantly promoted the development and applications of OPOs with the following advantageous characteristics:system compactness(for example,two cavity mirrors and a nonlinear crystal),relatively high conversion efficiency,singly resonant operation,and frequency-agile tunability(for example,angle tuning).A plane-parallel cavity with a large mode volume is well-suited for Q-switched laser pumps.This type of OPO is widely adopted,for example,as a pump/seed source in nonlinear terahertz or mid-infrared(MIR)generation or directly as an MIR source,owing to the wide tuning range and ease of construction.As the earliest configuration in a laser resonator,a plane-parallel cavity is critically stable and sensitive to mirror misalignment.The misalignment of laser cavities,including those of argon ion,CO₂,and Nd∶YAG lasers,has been analyzed previously;however,studies on OPO cavities have rarely been reported.In this study,we performed an experimental investigation on the misalignment characteristics of a plane-parallel cavity-based OPO.

In this study,an OPO based on a plane-parallel cavity structure was developed.A potassium titanyl phosphate(KTP)crystal was utilized as the nonlinear medium(cut at θ=60°,φ=0°,and 10 mm×7 mm×20 mm,anti-reflection(AR)-coated at 532 nm/800-900 nm/1300 1600 nm).A frequency-doubled Nd∶YAG laser(532 nm,10 ns,and 10 Hz)was employed as the pump source.Two flat mirrors(AR-coated at 532 nm/1300-1600 nm and highly reflection-coated at 800 900 nm)formed a singly resonant cavity.The ns-pulsed OPO was operated at a wavelength of 1514 nm via o→e(signal)+o(idler)critical phase matching.The cavity mirrors were precisely controlled using piezoelectric optical mounts for alignment.Each mount was equipped with two piezo actuators,which could provide a two-dimensional(2D)adjustment(axes 1 and 2 for output mirror M1 and axes 3 and 4 for input mirror M2)with an angular resolution of ≤0.7 μrad.

Typical output results(pulse envelopes and beam profile)of the KTP-OPO are presented in Fig.2.The piezoelectric optical mounts with a motion controller module facilitate quantitative analysis of the influence of mirror misalignment on the OPO output.The variation in the output pulse energy with angular tilt δ_xis measured while scanning each cavity mirror along two directions around the well-aligned position(δ_x=0),as presented by the 2D graphs in Fig.3.The subscripts x=1–4 correspond to the four actuators,axes 1–4,respectively.The four curves presented in Fig.3 present envelopes along the principal axis(with the other three δ=0).The full widths at half maximum of the curves(called alignment tolerance)from axes 1 to 4 are 0.171,1.861,0.177,and 1.933 mrad,respectively,which are determined at a pump beamdiameter Φ=4 mm,cavity length L=65 mm,and output pulse energy=6.6 mJ.The discrepancy between the two mirrors along the same direction(axes 1 and 3 and axes 2 and 4)is minimal,which is verified by alternating the two mounts.The tolerances along the horizontal direction(axes 2 and 4, y-principal dielectric axis)are approximately 10 times those along the vertical direction(axes 1 and 3, x-z-principal plane).This can be attributed to the critical phase matching configuration(Fig.4).As presented in Fig.5(a),the alignment tolerance increases with the beam size at a specific pump intensity and cavity length because a larger interaction region(cross-section)can provide more effective round trips for misaligned signal beams.The relationship between the tolerance andoutput energy,shown in Fig.5(b),demonstrates an increasing trend because a higheroutput energycorresponds to a higher single-pass gain(easier to build up).The alignment tolerances of different cavity lengths are compared at fixed input[Fig.6(a)]and output pulse energies[Fig.6(b)].In addition,the other output characteristics vary with the OPO cavity length.A longer cavity length results in a higher threshold and lower energy conversion efficiency(Fig.7).The divergent angles decrease with the cavity length at approximately equal output energies and beam sizes(left y-axis of Fig.8).Better beam quality and worse stability can be obtained with a more extended cavity(right y-axis of Fig.8),and the root-mean-square(RMS)of pulse energy fluctuation increases(3.09%→3.61%→3.96%).

Herein,we quantitatively characterize the mirror misalignment of a plane-parallel cavity-based OPO,which has been widely utilized as a convenient coherent light source with a desired wavelength.A green laser-pumped KTP-OPO equipped with piezoelectric optical mounts is constructed.An almost circular Gaussian beam with a wavelength of 1514 nm is delivered with a slope efficiency of ≥25% and a pulse energy fluctuation(RMS)of ≤4%. The output shrinkage is measured by scanning the cavity mirrors around a well-aligned position.The alignment appears to be significantly more sensitive in the critical direction than in the noncritical direction,which can be explained based on the phase-matching configuration.The alignment tolerance increases with the beam size and input intensity.In addition,the cavity length dependence is analyzed at specific input and output pulse energies.This paper presents a type of ns-pulsed,singly resonant,and critical phase-matched OPO with a wide-angle tuning capability.