Abstract

Non-linear image up-conversion [1] based on intra-cavity sum-frequency mixing an external image with a laser beam in continuous wave (CW) solid-state lasers has proven to be a useful resource to display real-time infrared images in standard video cameras based on CCD or CMOS 2-D silicon focal plane array sensors [2]. The up-converted image resolution achievable with this technique is known to improve by using the largest laser beam size compatible with the nonlinear crystal cross sectional aperture [2,3]. Whilst quasi-phase matching in non-linear poled crystals like PPLN or PPKTP provide higher up-conversion efficiencies than perfect phase matching in bulk crystals of lower effective non-linear coefficients, the present fabrication techniques for poled crystals limit their cross-sectional apertures for the mixing laser beam. High CW infrared-to-green intra-cavity second harmonic generation conversion efficiencies at modest diode pump powers are however, routinely achieved in widespread commercially available composite lasers (green laser pointers). They are based on Type II phase-matching in bulk KTP crystals of potentially greater cross sectional aperture, by gluing the KTP crystal to a Nd³⁺:YVO₄ laser crystal, and by depositing dielectric mirrors directly on the flat crystal facets. A similar performance can well be expected in the case of sum-frequency mixing (SFM). In particular, Type II phase-matching allows for perhaps the most compact realization of intra-cavity SFM image up-conversion systems of this type, as a simple polarizing beam splitter eases coupling the incoming image with the laser, in contrast to present usual techniques based on dichroic beam splitters [2,3]. Here, we present a first example of such an image up-conversion system in a miniaturized architecture. In our case, and only for illustration purposes, we have intra-cavity SFM a pulsed 1064 nm infrared image with a 1342 nm CW laser in a KTP crystal to yield a yellow up-converted image at around 593 nm.

© 2017 IEEE