Highly sensitive liquid crystal optically addressed spatial light modulator for infrared-to-visible image up-conversion

A liquid crystal optically addressed spatial light modulator based on an InGaAs photodiode array operating at low light levels is investigated in the short wavelength infrared (SWIR) spectral band to serve as a SWIR-to-visible imaging upconversion device. It consists of InGaAs/InP heterojunction photodetectors array sandwiched with a nematic LC layer. The photodiode array is composed of a 640×512 InGaAs/Inp heterojunctions, grown on InP substrate with a 15 μm pitch. Full up-converted visible images in stills and video modes were demonstrated with SWIR light intensities as low as 70 nW/cm2 or less than pW/pixel. The influence of operation frequency on the performance of the device was found theoretically and experimentally to be crucial for a proper operation of the device. The optimum sensitivity and contrast of the device was found at a frequency around 70 Hz. To the best of our knowledge, this is the first time that such a high performance upconversion device is presented and that actual visible images are obtained.