J. Eur. Opt. Society-Rapid Publ. 22, 4( 2026) 35
Figure 4. Comparison of narrowband and broadband NRO operation with the 1-mm thick PPLN in terms of measured input-output( a, c) and spectral( b, d) characteristics. The repetition rate is 30 kHz and the spectra in( b, d) were recorded at an average pump power of 7 W.
Figure 5. Spatial characteristics of the tunable, narrowband NRO with the 1-mm thick PPLN: measured( a) signal and( b) idler caustics in the horizontal plane with fitting for the evaluation of the M 2 propagation factor. The average pump power is 16 W at 30 kHz and the oven temperature is set to 169 ° C corresponding to 1880 nm( signal) and 2453 nm( idler). The insets show recorded near-field beam profiles.
transferred to the idler wave, even though each of the waves is completely extracted in every cavity round trip in the NRO.
Power scaling of the narrowband NRO with the 1-mm thick PPLN was investigated at fixed signal( 1880 nm) and idler( 2453 nm) wavelengths, at an oven temperature of 169 ° C. To utilize the full power available from the pump laser we had to increase the repetition rate, see Figure 3. Nevertheless, at 25 kHz we did not increase the pump level beyond 12 W, cf. Figure 3a, to avoid optical damage problems. In general, both the spatial and temporal characteristics of the pump laser change with the repetition rate. The shorter pulse duration at lower repetition rates, however, provides the best conditions for highest conversion efficiency and output powers due to the higher pump intensity. Thus, a maximum total average power of 9.84 W( 5.32 W for the signal output and 4.52 W for the idler output) was obtained at the maximum pump level for a pulse repetition rate of 30 kHz, cf. Figure 3b. Atanaveragepump power of 20 W, this corresponds to an optical conversion efficiency of 49.2 % and a slope efficiency of 56.4 %. The RMS stability of the average output power for the signal wave at 30 kHz, measured at a pump power of 16 W for 30 min, was 0.71 %.
At the same pump conditions, substituting the TC – VBG by a signal reflecting mirror( idler outcoupler), as specified in the previous section, the output powers and efficiencies were higher in the broadband regime. Figure 4 shows a comparison at a repetition rate of 30 kHz. The
Figure 6. Temporal characteristics of the narrowband NRO with the 1-mm thick PPLN recorded with a 200-ps, extended InGaAs photodetector( FWHM indicated) at an oven temperature of 169 ° C corresponding to emission wavelengths of 1880 nm( signal) and 2453 nm( idler). The average pump power is 16 W at 30 kHz.
maximum total average power in the broadband regime reached 12.16 W( 7.07 W for the signal output and 5.09 W for the idler output). At an average pump power of 20 W, this corresponds to an optical conversion efficiency of 60.8 % and a slope efficiency of 69.2 %, see Figure 4c.
The signal and idler spectra for broadband and narrowband operation are compared in Figures 4b, 4d. Roughly 10-fold narrowing for the signal FWHM can be seen and about 20-fold for the idler FWHM.
The beam profiles for the signal and idler NRO outputs were measured by a Pyrocam PY-III-C-B camera