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

摘要

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 up conversion 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 up converted 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 up conversion efficiencies of 20% for polarized collinear MWIR light. To make the module truly portable the las- r 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. Wavelength filtering is an important part of the up conversion detector technology in order to reach the single-photon limit. The conversion module is separated into two compartments where residual light from the pump diode is prevented from reaching the second compartment. Furthermore all the mirrors in the module are highly reflecting for 1064 nm, and highly transmitting for the pump diode. The fluorescence from the mixing laser, which tends to come over a very wide bandwidth interval, also needs to be considered. In this manner it is possible to spectrally clean up the mixing light. In order to achieve single-photon performance, a narrow bandwidth, high transmission band pass filter is used to remove residual noise components. Since the crystal has about 0.1% absorption at 3 μm it will emit thermal radiation with an emissivity of 0.1%. Only the phase matched part of this light, emitted by the nonlinear crystal itself, will be up converted. For these reasons, the noise generated by the converter unit is very low even at room temperature. In fact, the noise is so low that in order to measure the system generated noise, we have to heat the nonlinear crystal thus increasing the crystal's thermal emission and thus dark noise. The wavelength conversion takes place in the Fourier plane relative to the camera. This means that image information is contained in the angle of the light. The object can then be placed far away from this plane, or Fourier transformed to the nonlinear crystal e.g. By a 2f lens configuration. If no imaging is desired, but rather a spectrum of a small area is preferred, one can reconfigure the imaging optics before the wavelength converter so that a point in the object is i