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J. Eur. Opt. Society-Rapid Publ. 22, 4( 2026)
Figure 7. Signal and idler NRO average output powers for the 3-mm thick PPLN versus pump power at 20 kHz with the corresponding total conversion efficiency( signal + idler) for broadband( a) and narrowband( b) NRO operation.
( Ophir-Spiricon, USA) using a 300-mm CaF 2 focusing lens. The beam propagation factors( M 2) of the narrowband NRO with the 1-mm thick PPLN shown in Figure 5 were estimated from the fitted Gaussian diameters. We obtained M 2 = 2.09 for the signal beam and M 2 = 2.04 for the idler beam.
The pulse durations were measured at a pump level of 16 W, giving a FWHM value of 10.5 ns for the signal wave and 11.7 ns for the idler wave, slightly shorter than the pump pulse duration of 12 ns( Fig. 6).
4 3-mm thick PPLN: power / energy scaling at 20 kHz
In contrast to the previous section, using larger beam diameters in the 3-mm thick PPLN, it was possible to optimize the performance of the NRO at lower repetition rates which result in higher single pulse energies. Figure 7 shows the average signal and idler output powers at 20 kHz( corrected for the filters employed) versus pump power measured in frontoftheNRO( max: 17.95W) atanoventemperature of 32 ° C( chosen to match the VBG wavelength). This temperature was not changed further in the experiment.
The maximum signal( ~ 1922 nm) and idler( ~ 2384 nm) average powers at 20 kHz using the VBG reached 6.25 and 5.1 W, respectively, see Figure 7b. These average powers correspond to single pulse energies of 312.5 and 255 lJ, respectively. These results can be compared with [ 15 ] where we optimized the NRO at a repetition rate of 10 kHz for a similar 3-mm thick but only 25-mm long PPLN. The present threshold of 2.2 W is two times lower compared to [ 15 ] in terms of pump fluence for a similar number of cavity round trips. The maximum conversion efficiency( 63 %) is also almost two times higher compared to the 32.8 % in [ 15 ]. The present single pulse energies at 20 kHz in fact exceed the maximum pulse energies achieved in [ 15 ] at 10 kHz. Comparing the NRO threshold for narrowband operation in Figures 7b and 4a, it can be seen that the values are similar in terms of single pump pulse energy but the threshold fluence is roughly four times lower for the 3-mm PPLN due to the larger beam sizes.
In the broadband regime( using the signal reflecting mirror instead of the VBG) at 20 kHz, the maximum conversion efficiency of the NRO with the 3-mm thick PPLN reached 72.8 %, primarily due to increased signal output( 7.5 W) with the idler almost unchanged( 5.3 W) at a maximum pump level of 17.57 W, but the saturation behavior was more pronounced, see Figure 7a. The pump threshold for broadband operation was very similar( 2.1 W).
All further NRO characteristics presented in this section were measured at maximum pump level. The measured spectral bandwidths( FWHM) recorded with the VBG in Figure 8b for the narrowband case, were limited by the spectrometer resolution. They were more than 20 times narrower compared to the broadband configuration with the signal reflecting mirror, cf. Figure 8a. In fact this estimation represents a lower limit because of the finite experimental spectral resolution in the narrowband case.
The fits for evaluation of the M 2 factors were performed with the second moment diameters. In the narrowband regime, at the maximum pump level at 20 kHz, we obtained M 2 = 4.1 for the signal and M 2 = 2.7 – 2.8 for the idler( see Fig. 9). The beam quality was inferior in broadband operation with measured M 2 = 4.5( H)– 5.4( V) for the signal and M 2 = 3.6( V) – 4.1( H) for the idler.
The signal pulse duration in narrowband operation roughly reproduced the pump pulse duration( 7.5 ns) at 20 kHz, as shown in Figure 10. The pump temporal profile in Figure 10 shows some longitudinal mode-beating because the oscilloscope sampling bandwidth has not been reduced in these measurements, in contrast to Figure 6. The idler pulse was slightly longer( 8.5 ns). In broadband operation with a signal reflecting mirror instead of the VBG, the output pulses were somewhat longer: 8.3 ns( signal) and 9.3 ns( idler).
5 Numerical modeling
The spectral distribution of the NRO output beams was simulated by a code widely following the split-step method # 1 presented in [ 20 ] for plane waves. The mixing equations are integrated in the propagation direction in the time domain to account for the nonlinear parametric amplification ignoring dispersion, followed by a step taking into account the linear effects and in particular the temporal