Passively Q-Switched Nd3+:YVO4 Laser System for Self-Illuminated and Self-Synchronized Up-Conversion of Images

Nonlinear image up-conversion is a useful technique to transfer images from those spectral regions where sensors are not well-developed or suffer severe limitations to the visible spectrum where silicon-based focal plane array sensors, such as CCD or CMOS cameras, are widespread, well-developed and affordable. Although silicon sensor technology stands out for fast and low noise operation at room temperature while offering good resolution, there is still a need to further improve the up-conversion efficiency. For this aim, image up-conversion is combined with different alternatives to overcome efficiency limitations by the use of high gain detection of up-converted images with intensifier tubes or EMCCDs, or by intra-cavity continuous-wave (CW) up-conversion with periodically poled ferroelectric crystals inside the laser cavity where the sum frequency mixing takes place [1]. Also, synchronously pulsed up-conversion has been exploited to enhance conversion efficiency significantly by high peak power pulses [2], as well as contributing to the reduction of the background noise. However, pulse synchronization based on active Q-Switching adds some complexity and cost to these systems, since accurate delay control and low jitter is required to maximize the temporal overlap between pump and illumination pulses in the nonlinear medium. In this context, the passive Q-Switched system we propose provides a simple and effective solution for short-range image up-conversion systems, because timing jitter of even nanoseconds is not a concern in our system. This configuration is particularly suited for handheld self-illuminated up-conversion systems recently proposed in [3], where IR-illuminated targets are up-converted to the second harmonic (SH) inside the same laser used for illumination.