Experimental investigation of optical feedback from periodically poled crystals for nonlinear frequency conversion

Crystals with nonlinear optical properties are widely used for frequency conversion of laser light. Thereby wavelength ranges beyond the laser’s emission spectrum can be reached [1] enabling new applications [2] . In order to achieve efficient frequency conversion quasi phase matching (QPM) [3] by periodically poling ferroelectric domains of the crystal is a well known technique. However, the variation of the orientation of the ferroelectric domains causes disturbances of local refractive index [4] that reflect or scatter the incident pump light in the crystal which is a source of optical feedback [5] . Due to the periodicity the QPM structure can act as a Bragg-grating. For crystals with a waveguide structure characteristic resonances at distinct wavelengths with significant optical feedback strength have been reported [6] . In [5] the reflectivity of bulk crystals is measured at a fixed wavelength for optimal frequency conversion to be of the order of 10 −4 but not going into detail on the reflectivity spectrum of the crystal. Since optical feedback can deteriorate the emission characteristics of lasers used for pumping the nonlinear crystal, the optical feedback of the crystal has to be considered in the development of laser systems.