Objective An imaging spectrometer is the fusion of imaging technology and spectral technology that uses multiple channels to detect targets. We can obtain spatial information and spectral information on the target at the same time. According to the different spectral characteristics of different objects, targets can be identified and analyzed in detail to obtain more comprehensive data. An infrared dual-band imaging spectrometer can detect and recognize targets with high precision and resolution in a complex environment. It also has the advantages of high accuracy and low false alarm rate. A middle wave infrared imaging spectrometer is used to detect the spectrum of radiation from high-temperature objects. It is mainly used to detect volcanic activity and red alert for forest fires in civil use. It can be used in the military field to detect high-temperature exhaust gas from aircraft and tanks. A long wave infrared imaging spectrometer is used to detect objects at normal temperatures. It is very useful in mineral resource exploration and atmospheric gas detection. It is also used in the military to identify camouflaged targets with strong stray radiations. Therefore, research on infrared dual-band spectral imaging technology is of great significance to the development of military and civilian fields.
Methods The front telescope objective system, the spectral spectroscopic system, and the secondary imaging relay system of the infrared dual-band common image plane imaging spectrometer were designed using the modular design method. Because the infrared dual-band system had wide spectra and chromatic aberration was difficult to correct, the front telephoto objective adopted an off-axis two-mirror system with few degrees of freedom. The spectroscopic system adopted an Offner convex grating structure with less smile. We designed the diffractive order of the system. The diffractive order of the middle wave band was second order, and the diffractive order of the long wave was first order. The dual diffraction orders could obtain a better spectral resolution, and make more effective use of the detector in the meantime. The blazed wavelength of the convex grating was the center wavelength of the middle wave band, which could ensure a high diffraction efficiency for both bands. To reduce the influence on the imaging results of stray light reaching the detector, we designed the secondary imaging relay system to ensure that the optical system showed 100% cold-stop efficiency. Each part of the system had telecentricity. Through pupil connection and matching, the overall system design was completed. The imaging quality of the system was evaluated and analyzed through the spot diagram, modulation transfer function (MTF) curve, and distortion curve. We analyzed the narcissus phenomenon of the system. Through real ray tracing, the values of YNI and I/IBAR were obtained for each surface of the transmission system. If one of the parameters is greater than 1, no obvious narcissus will occur. If the values of the two parameters are both less than 1, it is necessary to analyze the narcissus phenomenon further through reverse tracing.
Results and Discussions The finally designed infrared dual-band common image plane imaging spectrometer has good imaging quality. The spectral resolution of the imaging spectrometer is high, the spectral resolution of the center wavelength of the middle wavelength band is better than 5 nm, and the spectral resolution of the center wavelength of the long-wavelength band is better than 10 nm (Fig. 16). We design the blaze wavelength of the convex grating to ensure the dual-band diffraction efficiency of the imaging spectrometer is relatively high and the diffraction efficiency of the middle wave band and the long wave band are both higher than 80% (Fig. 10). The narcissus phenomenon of the refrigerated imaging spectrometer was analyzed and the results showed that the system had no obvious narcissus phenomenon (Fig. 18).
Conclusions In this paper, an infrared dual-band common image plane imaging spectrometer was designed for the refrigerated dual-color quantum-well infrared detector. The pixel size of the detector is 25 μm, the array size is 384 pixel×288 pixel , and the working spectral range includes middle wave 4.4--5.4 μm and long wave 7.8--9.2 μm, F nuber is 2.5. Through the modular design method, the front telephoto objective system, Offner spectroscopy system, and relay system are designed separately. The front telephoto objective system is an off-axis two-mirror system with a spatial resolution of 0.1 mrad. The dual-band diffraction efficiency of the Offner spectroscopy system is higher than 80%, and the smile and keystone are small. The relay system achieves 100% cold-stop efficiency. The three systems compose the overall design of the refrigerated infrared dual-band imaging spectrometer through pupil connection and matching. The design results show that, in the full wavelength band and full field of view, the root mean square radius of the spot diagram is smaller than the size of a single pixel of the detector, the modulation transfer function is close to the diffraction limit, the spectral resolution is better than 10 nm, the distortion is less than 1%, and there is no obvious narcissus phenomenon. The imaging quality of the system is good, which meets the design requirements of the infrared detection system.
