Offner imaging spectrometer consists of a convex grating and two concave mirrors. The concentric characteristics of the optical structure make it have the advantages of large relative aperture, small distortion and compact structure. In order to reduce the alignment difficulty of the Offner imaging spectrometer and improve the efficiency, a fast alignment method of Offner imaging spectrometer is presented in this paper based on the concentric characteristic and spherical autostigmatic method. Firstly, the spherical autostigmatic device is built, which can generate point source, and when the point source is located at the spherical mirror’s center of curvature (CoC), its reflection image point and the point source coincide. By measuring the distance between the reflection image point and the point source, the position deviation of the spherical mirror’s CoC can be determined. The Offner imaging spectrometer is completed with this device by locating the CoC of its primary mirror, convex grating and tertiary mirrors. The results show that location error of the two off-axis concave mirrors’s CoC can be controlled within 10μm, and the imaging performance of the imaging spectral system is satisfied. Compared with the existing method, this method has the advantages such as easy to operate, low cost and fast alignment.
Due to the restriction of slit width, the SNR of onboard slit-based pushbroom imaging spectrometers with high spatial resolution is difficult to make a breakthrough. In order to achieve higher SNR, Hadamard transform imaging spectroscopy is used to design the imaging spectrometer with high spatial resolution by replacing the slit with coding mask while its concept will be introduced in this paper. The SNR performance of coding imaging spectrometer used this technology will be simulated with main specifications which include orbital height of 500km, spatial resolution of 1m and spectral resolution of 5nm. As a result of the simulation, its SNR performance is 5 times superior than slit-based design in the same specifications. Moreover, other advantages of using coding imaging spectroscopy will be introduced in this paper. In addition to solve the problem that lights from other orders can’t be excluded when we use grating elements, Fery prisms are introduced to the Offner relay system to design the imaging spectrometer. The result shows that the imaging quality is close to the diffraction limit, and the prisms’ dispersive nonlinearity come out to be well corrected. So coding imaging spectroscopy turns out to be a good solution for high spatial resolution design with high SNR.
The uncooled thermal infrared imaging spectrometer has advantages of small size, low cost and so on, can monitor high temperature events in the extreme thermal environment. However，the difficulties in detecting objects with low signal energy, serious thermal infrared background radiation and other problems limit its development and application. Restraining self-radiation stray light is the key to successfully overcome these difficulties. A long-wave infrared imaging spectrometer is designed in this paper, with 7 to 14μm wavelength range and F number of 2.7. By establishing its optomechamical model, two-step method of suppressing self-radiation are studied. The first step is using a mechanical structure with a gold film. Its reflectance is more than 98% in the thermal infrared band, thereby lowering the emissivity of the lens surface. Compared to the blackened mechanical parts, the polished one can reduce the self-radiation by one order of magnitude. However, it is still two orders of magnitude higher than the irradiance of the 500K target, and the spectral image signal is submerged. The next measure is to add a shutter device at the spectrometer entrance slit, so that signals reaching on the image plane when the shutter is opened and closed can be detected consequently. The effective spectral image signal is extracted by the background noise removal algorithm. The measures effectively solve the problem that the self-radiation stray light affects the imaging quality of the uncooled thermal infrared imaging spectrometer.
Many applications like laser manufacturing, homogeneous illumination or laser-induced fluorescence spectroscopy require a uniform intensity distribution and variable size of laser beam. Conventional laser beam shapers have a homogeneous but fixed-size laser spot. In this paper, a continuous zoom beam shaper based on microlens array is designed. It is essentially a multi-channel Kohler illumination system consisting of two identical microlens arrays and a zoom lens group, which transforms a Guassian or other complex spacial intensity distributions to a uniform square distribution of variable size on the target plane. The continuous zoom beam shaper adopts mechanical compensated optical configuration. Cam curve of the continuous zoom beam shaper is smooth enough and avoids inflection point. Compared with conventional laser beam shapers, the continuous zoom beam shaper has high intensity uniformity, variable size of uniform distribution and low cost. The design method and optimum result of continuous zoom beam shaper are presented. As an example, a continuous zoom beam shaper with a zoom ratio of 3× and variable size of uniform square distribution from 4.12×4.12 mm2 to 12.36×12.36 mm2 , is designed. The zoom lens group consists of the front fixed group, zoom group, compensation group and the rear fixed group. Intensity uniformity of output beam is greater than 90% in different zoom stages. It satisfies the needs of laser applications.