Efficient laser sources in the 3 - 5 μm wavelength range are needed for directed infrared countermeasures, but also have applications in remote-sensing, medicine and spectroscopy. We present new results on our tandem optical parametric oscillator (OPO) scheme for converting the radiation from a 1.06 μm Nd<sup>3+</sup>-laser to the mid-infrared. Multi Watt level output power in the 3-5 μm range at 20 kHz pulse repetition frequency is reported. Our setup uses a type I quasi phase-matched PPKTP crystal in a near degenerate OPO to generate 2.13 μm radiation. A volume Bragg grating resonant close to, but not exactly at the degenerate wavelength, is used as a cavity mirror to reduce the bandwidth and ensure singly resonant operation. Both signal and idler from the PPKTP OPO are used to pump a ZGP OPO generating high power radiation in the 3-5 μm region. Using this scheme for each pump photon it is possible to generate four photons for each pump photon, all in the interesting wavelength range, thus enabling high efficiency conversion.
The characterization of the coefficient of the nonlinear optical Kerr effect, the nonlinear refractive index (<i>n</i><sub>2</sub>), of several femtosecond laser crystals with compositions derived by total or partial replacement of D<sup>2+</sup> in DXO<sub>4,</sub> X = Mo or W, is presented. Tetragonal (space group <i>I</i>4) Na-based double tungstates NaT(WO<sub>4</sub>)<sub>2</sub> (T = Y, La, Gd, Lu and Bi) and double molybdate NaY(MoO<sub>4</sub>)<sub>2</sub>, as well as the monoclinic (space group C2/c) Li<sub>3</sub>Gd<sub>3</sub>Ba<sub>2</sub>(MoO<sub>4</sub>)<sub>8</sub> crystals, have been measured by the <i>z</i>-scan technique. All these crystals present structural local disorder, and among them the tetragonal ones exhibit significant <i>n</i><sub>2</sub> values, which should allow their efficient laser pulsed operation by Kerr-lens mode locking, especially NaBi(WO<sub>4</sub>)<sub>2,</sub> 68x10<sup>-16</sup> cm<sup>2</sup>/W (for σ light), which is about twice than for the others. This feature is attributed to the high polarizability associated to the lone electron pair of Bi<sup>3+</sup>.
In this work we present the development of the sub-micron ferroelectric domain structuring technology in KTiOPO<sub>4</sub>. We
used these structures to demonstrate second-order interactions involving counter-propagating waves. Of special interest
is the mirrorless optical parametric oscillator, where distributed feedback between the counter-propagating signal and
idler waves obviates the need for mirrors, surface coatings or precise cavity alignment. Mirrorless optical parametric
oscillator also demonstrates some unique and useful spectral properties. These experimental demonstrations are but first
steps towards a number of tantalizing applications which, however, require even smaller ferroelectric domain
periodicities and further work on the material structuring technology.