538 J. Eur. Opt. Society-Rapid Publ. 22, 55( 2026)
3 Results and discussion
3.1 Output spectral characteristics
Firstly, when the PPMgLN’ s period is 29.4 lm, under different pump powers( 1.0, 3.0 and 5.0 W), we used the Yokogawa AQ6370( resolution 0.02 nm) optical spectrum analyzer to measure the output spectra of the fundamental and signal lights of the OPO system as shown in Figure 3. With the increase of pump power, the output intensity of the fundamental light at 1063.9 nm remains basically unchanged, while the intensity of the signal light at 1472.9 nm increases significantly. During the nonlinear conversion operation of the OPO system, the power of the fundamental light in the cavity is always maintained near the oscillation threshold. When the externally injected pump energy increases, any additional pump energy is not used to further enhance the intensity of the fundamental light in the cavity, but is almost entirely used to drive the parametric down-conversion OPG process, efficiently converting into signal light at 1472.9 nm and idler light, and outputting through the output mirror to the outside of the cavity.
Then, we measure the output spectra of the signal and idler lights of the OPO system using a mid-infrared spectrometer( Bristol 771 Spectrum Analyzer, resolution 4 GHz), when the PPMgLN’ speriodis29.4lm, as shown in Figure 4. The wavelength of the signal light is 1472.9 nm, and that of the idler light is 3833.2 nm. The linewidth of the fundamental light is 1.8 nm, and that of the signal light is 0.9 nm. Interestingly, the linewidth of the idler light is relatively wide, approximately 6.4 nm. In this OPO system, the asymmetry in the linewidths of the fundamental, signal, and idler lights originates from the differences in their mode selection and gain mechanisms within the resonant cavity. In this study, the signal light is designed as an intracavity resonant mode, and its wavelength is constrained by the longitudinal mode structure of the optical OPO cavity. The high reflectivity of the cavity mirrors and the multiple round-trip interference effect significantly narrow its linewidth and improve its spectral purity. In contrast, the gain of the idler light does not come from resonant amplification but is entirely dependent on the parametric down-conversion process in the nonlinear crystal.
The output wavelengths of the signal light and the idler light are very stable, as shown in Figure 5. Within a twohour period, the signal light varies between 1472.79 nm and 1473.03 nm, with a fluctuation range of only 0.24 nm, while the idler light varies between 3832.51 nm and 3834.25 nm, with a fluctuation range of only 1.74 nm.
3.2 Parasitic visible light
We also measure the output spectra of parasitic visible light, pump light and signal light using a micro fiber optic spectrometer( Avantes AvaSpec-2048FT-SPU), as shown in Figure 6. In the visible light section, visible lights at 532.0 nm, 616.6 nm and 736.5 nm appear. Actually, the green light at 532.0 nm is the second harmonic of the fundamental light at 1063.9 nm, the red light at 736.5 nm is
Figure 2. LD, fundamental and signal beam waists vs. pump power( 0 – 5.0 W).
the second harmonic of the signal light at 1472.9 nm, while the orange light at 616.6 nm is the sum frequency of the fundamental and signal lights [ 18 ]. In this OPO system, the relative intensities of the visible lights( second harmonic and sum frequency) are mainly determined by the field intensities of the corresponding nonlinear processes and the degree of phase matching optimization. Among them, the sum frequency of the fundamental and signal lights is usually the strongest, as it is driven by the strongest fundamental and signal lights in the shared cavity and its phase matching condition is often associated with the tuning of the main parametric process, making it easy to achieve a high conversion efficiency [ 19 ]. The second harmonic of the fundamental light is often weaker, as the phase matching of this process is actively suppressed during the OPO design stage through crystal period to avoid energy loss of the fundamental light and ensure the efficiency of parametric down-conversion. The second harmonic of the signal light has the weakest intensity, although it is driven by a strong signal light, its phase matching condition is independent of the main OPO design and is usually not in the optimal state. These parasitic visible lights reduce the energy of both the fundamental and signal lights.
The total power of visible light is low, with a maximum power of only 30 mW. Although the power is very low, it can still affect the stability of the signal light power. The total parasitic power is less than 1 % of the pump power, confirming a negligible impact on the overall conversion efficiency. Possible suppression strategies include: designing the poling period to detune parasitic phase-matching, inserting anti-reflection coatings for visible wavelengths inside the cavity, or moderately reducing the pump focusing. In the present work, no extra suppression is applied because the parasitic power is already negligible.
3.3
Output power characteristics
We measure the output powers of the signal light, idler light and total OPO under different pump powers, as shown in Figure 7. The threshold of the OPO system at 807.6 nm