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Fig
. 1. a) Wavevector diagram for a BWOPO. b) Calculation of forward and backward wavelengths as a function of the pump wavelength for a PPKTP BWOPO with a grating period of 433 nm.
Fig
. 2. Sketch of a BWOPO where the laser is focused and folded with a dichroic mirror( DM) into the nonlinear crystal, PPKTP in this case, and the backward and forward beams are measured at the two detectors PD1 and PD2. The Si window absorbs remaining pump while transmitting the forward wave.
Fig. 3. Spectra for a ps BWOPO. Undepleted( solid line) and depleted( dashed line) pump( left), forward wave( middle), and backward wave( right). The FWHM widths of the pump and signal were 1.21 THz and 410 GHz, respectively, while the deconvoluted idler was 13 GHz( FWHM) [ 27 ].
the signal generated a second BWOPO where the signal appeared at 1898 nm and the idler 2739 nm. With yet stronger pumping this second signal could drive a third BWOPO with the signal at 2762 nm and the idler at approximately 5900 nm.
3.2 Phase-locked degenerate BWOPOs
Mutter et al. demonstrated the first degenerate BWOPO, meaning that the forward and backward wave had the same frequency [ 32 ]. This was done by pumping a 7 mm long