Objective The mid-infrared band (2.5-25 μm) has important applications in the field of spectroscopy and imaging. Spectral migration technique up-converts mid-infrared signal light to visible/near-infrared light through a non-linear frequency process, which is then detected using high-performance detectors based on wide-band gap materials such as silicon. Compared to schemes directly using traditional semiconductor detectors, this technique has the advantages of fast response and room temperature operation. Bulk crystals have large aperture to realize array detection. In particular, chirped polarized crystals have obvious advantages in imaging acceptance bandwidth and field of view due to their large phase-matching bandwidth. Previous up-conversion imaging theory, however, didn't consider the nonlinear process of signal light in the crystal to affect the propagation. Therefore, there is some deviation between the theoretical analysis and the up-conversion imaging results under the weak signal light condition. Based on the basic imaging principle, a simple physical model of the up-conversion imaging process is presented by solving the coupled wave equation using finite difference method and considering the effect of nonlinear process on the optical propagation. On this basis, a theoretical derivation for up-conversion imaging under coherent/incoherent radiation illumination conditions based on chirped polarized crystals is provided.

Methods A mid-infrared up-conversion detection imaging system based on a chirped polarized crystal is built (Fig.3). The target object is illuminated by the thermal radiation of an electric soldering iron, then the visible light in the signal is filtered out by a band pass filter (BP1). A strong 1 080 nm pump light is directed into the crystal through a dichroic mirror (DM) along with the signal beam. Through a 4f system, the up-conversion results of the target are imaged on the EMC CD. A chirped polarized lithium niobate (CPLN) crystal with a period interval of 0.01 μm and a period range of 21.6-23.4 μm is used in the experiment. The length of the crystals is 40 mm and the cross section size is 2 mm×3 mm. The temperature of CPLN crystal is controlled by a home-made temperature controller, whose fluctuation is ±0.002 ℃.

Results and Discussions By using a mature spectrometer to measure the spectrum after up-conversion and combining with the law of conservation of energy, the accepted spectrum of the up-conversion process in the corresponding mid-infrared band can be obtained (Fig.4). The corresponding mid-infrared acceptance range is 2 915-3 512 nm, and its full-width of half-max (FWHM) is 597 nm. Due to the low transmittance of the DM at wavelengths greater than 3 400 nm, the actual conversion bandwidth is larger than the direct measurement results, which is in agreement with the numerical calculation results (Fig.2). In contrast, the wavelength acceptance bandwidth of single-period polarized crystals is only on the order of nanometers. In the up-conversion imaging results (Fig.5), the largest one-dimensional size of the target is 3.62 cm, corresponding to 125 mm propagation distance, thus the full angle of the field of view is 16.59°, which is slightly smaller than the numerical calculation result in Fig.2. Under the condition of weak signal light, the background of pattern directly imaged by mid-infrared light through the mercury cadmium telluride thermal imager is full of white noise, making it difficult to identify the target contour information, while the pattern obtained by the up-conversion method with the same power of light has clean background and high signal-to-noise ratio (SNR), which also can realize high SNR of the single photon level imaging (Fig.6-7). In addition, applications of up-conversion imaging under coherent/incoherent radiation illumination conditions are reported. The optical edge enhancement imaging is realized for the objects illuminated by mid-infrared coherent light (Fig.8). Real-time video frame rate imaging of incoherent illuminated objects is realized, and its temperature characteristics can be analyzed (Fig.9).

Conclusions In the experiment, chirped polarized crystal is used to realize the up-conversion imaging detection of the mid-infrared receiving bandwidth of 597 nm and the field angle of view of 16.59°. By comparing with traditional mercury cadmium telluride mid-infrared detector, the up-conversion imaging technique has obvious advantages in improving the signal-to-noise ratio and sensitivity of imaging, and the low-light imaging of mid-infrared is realized by using the photon flux of 1.05×10⁵ Hz. The paper further shows the application of the up-conversion imaging system to the objects illuminated by correlation light and incoherent light. This work has conducted a comprehensive study on the up-conversion based infrared imaging system, which will provide a basis for the design of various application scenarios and improve the system design.