Room-Temperature Mid-Infrared Single-Photon Imaging Using Upconversion

The mid-wave infrared (MWIR) region is a fast developing research area due to many possible applications. Indeed a lot of research has been put into the development of novel light sources in the MWIR. This has led to very powerful sources such as quantum cascade lasers (QCL) and optical parametric oscillators (OPO). Even super-continuum MWIR sources have been developed and are readily becoming commercial availability. However, on the detector side fundamental issues are limiting the sensitivity of particularly uncooled devices. Specifically, very large dark noise is hampering even most cooled MWIR detectors, when compared to silicon based detectors available for the visible and near visible spectral range. In fact, camera sensitivities down to the single photon level have been developed for sub-μm wavelengths. This discrepancy in sensitivity makes it attractive to perform wavelength upconversion in order to shift the information from MWIR to sub-μm wavelengths. However, historically this dream has been riddled by low conversion efficiency and large dark noise. We present a virtually dark noise free, high quantum conversion efficiency device, which when combined with a sensitive visible light camera paves the path to single-photon sensitive imaging device for the MWIR. The device is based on sum-frequency mixing of the MWIR signal with a 1064 nm laser. Thus a signal at 3 μm is upconverted to 0.785 μm which is easily detectable with low noise detectors. In order to obtain low power consumption and a portable device, while having high conversion efficiency, the wavelength conversion is based on an intra-cavity design. In this way a few Watts of pump power results in a circulating field of about 100 W in the nonlinear material. Using a 20 mm long MgO:PPLN nonlinear crystal intra-cavity enables upconversion efficiencies of 20 % for polarized collinear MWIR light. To make the module truly portable the laser cavity is assembled in a closed mechanical unit which ensures that visible light cannot enter from the outside, and provides a very stable mount for the optical components. Figure 1 depicts the actual conversion device and a drawing of the conversion module.