We report the demonstration of a wavelength-agile coherent tunable mid-infrared (IR) ZnGeP2 (ZGP) optical parametric
oscillator (OPO). The mid-IR wavelength was tuned by varying the KTiOPO4 (KTP) OPO pumping wavelength, while
the ZGP crystal angle remained fixed. The wavelength of pump OPO was controlled by changing the KTP crystals angle
using a Galvano-optical beam scanner. Our mid-IR source can jump to a arbitrary wavelength without scanning through
the intermediate wavelengths.
A nonlinear optical crystal ZGP is suitable for OPOs in the mid-IR region. In most cases of mid-IR light generation
using an OPO, the wavelength tuning is achieved by controlling the phase-matching angle or temperature of the
nonlinear optical crystal. However, there are several disadvantages of this method including the walk-off angle, beam
pass instability, refraction losses due to the high refractive index of ZGP (n~3.1), slow tuning rate, and so forth.
Therefore, we developed a 2-&mgr;m-band pump-wavelength tunable mid-IR ZGP-OPO source. The mid-IR wavelength
from the ZGP-OPO could be tuned from 5 to 9.8 &mgr;m, when the pump wavelength was controlled from 1.95 to 2.3 &mgr;m.
The output pulse energy at 8&mgr;m was 1.3mJ/pulse at repetition rate of 30Hz. As an application of random wavelength
accessibility, we could achieve the real-time measurement of phase change (solidification) of candle wax by measuring
the change of absorption at two arbitrary wavelengths. We selected two wavelengths on the spectrum where significant
changes of absorption upon solidification were detected and performed two-wavelength absorption measurements as the
sample is allowed to cool from melted state.