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J. Eur. Opt. Society-Rapid Publ. 22, 4( 2026)
Figure 2. Spectral tuning of the narrowband NRO with the 1-mm thick PPLN and two TC-VBGs:( a) signal wave and( c) idler wave with linear fitting, and the corresponding spectra( b, d) recorded with a spectral resolution of 0.5 nm.
Figure 3. Input – output performance of the narrowband NRO with the 1-mm thick PPLN at different repetition rates:( a) 25 kHz,( b) 30 kHz,( c) 35 kHz, and( d) 40 kHz.
< 0.5 % andfortheidlerwave < 0.75 % persurface. Forthe thick PPLN, spectral narrowing was achieved by substituting the idler outcoupler by a home-made VBG centered near 1922 nm. This VBG had an aperture of 6 6.72 mm 2 and a thickness of 10.72 mm to ensure higher reflectivity. The estimated reflectivity bandwidth and diffraction efficiency were 0.5 nm and 99 %, respectively. The fixed wavelength VBG was also double-side AR-coated for the signal and idler wavelengths and the measured residual reflectivity was 0.45 % per surface for either of them.
The NRO cavity length( the physical distance between the two output couplers) was kept at minimum but this depended also on the crystal heater mount: The cavity length was ~ 50 mm for the 20-mm long PPLN and 135 mm for the 50-mm long PPLN. The dichroic 45 ° pump mirrors were mounted as close as possible to the PPLN crystal, and the retro-reflecting pump mirror – as close as possible to the second of them. All idler and signal powers were characterized using long-pass filters to eliminate residual light at shorter wavelengths.
3 1-mm thick PPLN: tuning and power scaling
The tuning performance and the power scaling potential of the PPLN NRO were first studied with the same 1-mm thick PPLN employed previously in [ 13 ]. Initially, a temperature of 169 ° C was selected on the temperature controller corresponding to parametric down-conversion to
~ 1880 nm for the signal wave and 2453 nm for the idler
wave. The average pump power was set at 3 W for a repetition rate of 20 kHz. At this pump level, the average output power of the signal wave amounted to 0.33 W and that of the idler wave to 0.29 W.
Spectral tuning was accomplished by transversal shifting the TC-VBGs across their width with a precision translation stage. The output power could be kept constant by adjusting the crystal temperature within roughly ± 5 ° C. As can be seen in Figure 2, gapless tuning across 40 nm( 1860 – 1900 nm) for the signal wave, Figure 2a, andacross 66 nm( 2420 – 2486 nm) for the idler wave, Figure 2c, was achieved. Linear fitting of the signal tuning data in Figure 2a gave a chirp rate of 0.96 nm / mm for the shorter wave TC-VBG and 0.97 nm / mm for the longer wave TC-VBG, in close agreement with the specifications. These values translate into 1.7 and 1.6 nm / mm in the corresponding idler wavelength tuning ranges. The simultaneously recorded signal spectra reveal a bandwidth( Full Width at Half Maximum, FWHM) of 1.4 – 1.9 nm, cf. Figure 2b, and for the idler the FWHM is in the 1.5 – 2.1 nm range, cf. Figure 2d. It can be seen that the spectral narrowing imposed to the signal wave by the TC-VBG is effectively