J. Eur. Opt. Society-Rapid Publ. 22, 4( 2026) 39
Figure 12. NRO spectra for the 3-mm thick PPLN calculated using the QSG approach for narrowband operation by averaging over 10 pulses( a) and with additional smoothing to simulate the experimental spectral resolution of 0.5 nm( b).
dramatically narrows both the signal and idler spectra, cf. Figure 12b. The spectral FWHMs estimated from the computational raw data in the narrowband case were 0.44 and 1.09 nm for the signal and idler, respectively. Then the actual spectral narrowing factor for the signal exceeds 40 while for the idler it is close to 30. The calculated bandwidth values increased to 0.74 and 1.25 nm, respectively, as a result of the smoothing with the spectrometer instrumental function, in good agreement with the recorded spectra shown in Figure 8b for the NRO signal and idler outputs. Somewhat larger deviation from the experimental spectra is observed for the idler spectral bandwidth, both in the broadband and in the narrowband cases, which is larger in the simulations. Finally, within the framework of the plane-wave model approach, it is not possible to make reliable statements on the absolute values of the output powers. Further, predictions of the beam profiles( beam qualities) are completely out of the scope of this numerical method and the present work.
6 Conclusion
Our recent studies with different samples of PPLN revealed that this material is ideally suited for application in 1-lm pumped non-resonant parametric oscillators to ensure maximum energy extraction in the nanosecond regime for both signal and idler. VBGs present a simple, robust and effective solution to narrow the bandwidths of both outputs although they act only on the signal wave. This is important not only for the spectral selectivity in direct applications but also for achieving high efficiency and narrow bandwidths in the mid-IR part of the spectrum by adding a second, cascade frequency conversion stage based on a non-oxide nonlinear crystal.
Using a 1-mm thin and 20-mm long PPLN sample and TC-VBGs for spectral narrowing, we achieved power scalable and simultaneously wavelength tunable operation of the NRO with a maximum total( signal plus idler) average power of 9.84 W at 30 kHz. Spectral tuning ranges of 40 nm for the signal and 66 nm for the idler wave were covered combining two such TC-VBGs. The spectral narrowing effect in this case depends on the beam size on the 20 mm wide TC-VBGs and the measured spectral bandwidths did not exceed 2 nm in the entire tuning range both for the signal and the idler outputs.
Aconversionefficiency of 63 % with a total( signal plus idler) output power of 11.35 W were achieved from the NRO in the narrowband regime using a 50-mm long PPLN crystal pumped at 20 kHz and a home-made, narrowband, fixed wavelength VBG. The larger aperture( 3 3mm 2) of this crystal enabled more flexible operation in terms of repetition rates to utilize the full available power from the specific pump source and the average powers obtained correspond to single pulse energies of 312.5 and 255 lJ for the signal and idler, respectively. At maximum pump / output levels, the bandwidths of both outputs were narrowed in this case to less than 1 nm although the signal and idler wavelengths( ~ 1922 and ~ 2384 nm) were not far from degeneracy.
The NRO was modelled using a split-step method approach in the plane wave approximation applicable for broadband radiation, with excellent agreement between the numerical and experimental results in terms of spectral narrowing achieved by using VBGs. The actual spectral narrowing calculated for the signal wave exceeds a factor of 40. Spatial intensity distributions were considered in a simplified QSG extension of the same code which provided better agreement with the experimental data in terms of output energies and efficiency.
The maximum on-axis pump fluence applied in the experiments with the large aperture( 3 3mm 2) 50-mm long PPLN crystal was below 0.32 J / cm 2. For this estimation we took into account a factor of 2 for the double pass pumping which is inherent to the NRO concept. The above value is three times lower compared to the experiments with the 1-mm thick PPLN, in which comparable average output powers were achieved for lower( 49.2 %) conversion efficiency at a higher( 30 kHz) repetition rate chosen for safe operation. The maximum on-axis fluence of 0.32 J / cm 2 translates into less than 100 MW / cm 2 for the peak on-axis intensity which is expected to ensure safe long-term operation for further extension of the present experiments towards a cascade frequency conversion configuration covering the mid-IR part of the spectrum beyond 5 lm. The achieved spectrally narrow outputs and the high average powers are prerequisites for efficient frequency conversion from the 2-lm range to the mid-IR spectral range.