J. Eur. Opt. Society-Rapid Publ. 21, 25( 2025) 247
Fig. 4. Conversion efficiency for a ns BWOPO( left) and spectrum for the BWOPO( red) compared with an SRO( blue) pumped with the same laser( right) [ 28 ].
Fig
. 5. Sketch of the BWOPO amplifier setup( left). It was pumped with 8 ns pulses at 1064 nm. Part of the pump beam was used to pump the PPKTP OPA crystals. The backward beam had a bandwidth of 274 MHz at 1856 nm and was tunable with temperature of the BWOPO crystal, as seen in the graph to the right [ 30 ].
PPRKTP sample with a QPM period of 455 nm with stretched pulses from a Ti: Sapphire regenerative amplifier.
The BWOPO reached degeneracy when pumped at 798.35 nm and the parametric outputs were centered at a wavelength of 1596.7 nm for both forward and backward waves. At a pump energy of 20.5 lJ, a pump depletion of 47.3 % was measured with a conversion efficiency into the backward and forward waves of 40.7 %. An interesting feature was that the signal and idler phase-locked stably at the degeneracy point. It was verified by doing an interference measurement between the two beams [ 32 ].
3.3 BWOPO in waveguides
The first waveguide based BWOPO was recently demonstrated and leveraged the benefits of waveguide technology to enhance nonlinear optical interactions [ 33 ]. By confining light within a small cross-sectional area, the waveguide maintains a high optical intensity over a long length, enabling efficient parametric oscillation at significantly lower pump power compared to bulk systems. Waveguides was for this purpose fabricated on the polar surface of a 20 mm long RKTP crystal utilizing a two-step process. Initially, a segmented ion-exchanged periodic structure was formed, serving as waveguides and a coercive field grating.
Subsequently, the QPM grating was inscribed via electric field poling. A Ti: Sapphire regenerative amplifier was used as the pump source, providing linearly chirped pulses with a duration of 215 ps at a repetition rate of 1 kHz. The pulses had a centre wavelength of 798.9 nm and a spectral bandwidth of 470 GHz. The light was coupled into the waveguides through an objective lens with a numerical aperture of 0.28, and a low loss of 0.2 dB / cm was measured, similar to the lowest values achieved for KTP waveguides [ 34, 35 ]. The backward wave was generated at 1515 nm and the forward wave at 1689 nm. The lowest oscillation threshold was 325 nJ obtained for a 9.8 lm widewaveguide, see Figure 6, which was 19 times lower than what was obtained in the bulk of the same sample. At a pump energy of 714 nJ, a peak conversion efficiency of 8.4 % was observed, while it declined at larger pump energies due to the emergence of backward stimulated polariton scattering( BSPS) [ 36 ].
4 Applications
The unique properties of the BWOPO can be exploited in many different applications where narrow linewidth and stable coherent radiation is required. The possibility for fine tuning provides additional advantages of this new device.