Opt. Express 23, 33460–33465 (2015)

Cascaded optical parametric oscillators (OPOs) covering a broad spectral range from 5.8 to 18 μm have been demonstrated by Andrey Boyko and co-workers from Germany, Russia and Sweden. The first stage of the cascade is an OPO composed of a Rb-doped periodically poled KTiOPO4 crystal, which was excited by a 1.064 μm laser with a pulse width of 8 ns to generate signal (S1) and idler (I1) beams. Subsequently, the S1 beam excited the second OPO, composed of a AgGaSe2 (AGSe) crystal, to generate another set of signal (S2) and idler (I2) beams. The second OPO had a single-resonant cavity set-up for the S2 beam, while the first OPO had a double-resonant cavity set-up for both the S1 and I1 beams. To achieve highly efficient wavelength conversion, the international team prepared two AGSe samples — one cut for short-wavelength conversion (5.8–8.3 μm) and the other for long-wavelength conversion (8–18 μm). The overall external conversion efficiency from the 1.064 μm pump laser was around 4% and is limited by the effective second-order nonlinear optical coefficient and the transparency of the AGSe crystals.