Precisely Targeting Molecular Absorption Lines in 2 μm Region by Optical Parametric Oscillator using Type-II PPRKTP

Greenhouse gas (GHG) monitoring by differential absorption LIDARs using airborne and space-based platforms requires nanosecond transform limited pulses in the region of 2 μm, with the precision of the central frequency better than 1 MHz, while providing capability for frequency hopping by tens of GHz between on- and off-line signals. Far from degeneracy, doubly resonant OPOs employing type-0 PPLN crystals provide possibility for such performance [1] . However, close to degeneracy, type-II interaction must be used for gain bandwidth narrowing and signal-idler dispersive mode separation. For-type-II interaction, periodically poled RKTP (PPRKTP) has advantage owing to almost two-times larger figure of merit, FOM = d eff 2 / (n p n s n i ), where d eff is effective nonlinear coefficient, n j (j=p,s,i) are refraction indices for the pump, signal and idler, respectively. Moreover, large damage threshold and large periods for type-II QPM in PPRKTP simplify output energy scaling by using higher pump energies and larger aperture crystals. Very similar refractive index dispersion and thermooptic coefficients for n y and n z indices in KTP mandate high accuracy Sellmeier equations for targeting GHG lines with required precision. However, the equations published in literature so far give widely different QPM periods, making type-II PPRKTP OPO design impossible in the 2 μm region.