Bin Xiangli

A static polarization imaging spectrometer based on a Savart polariscope

Abstract This paper describes the design of an interference imaging spectrometer. A static Polarization Imaging Spectrometer (PIS) based on a single Savart polariscope has been developed. It produces the interferogram and targets image in the spatial domain which are recorded by using a two-dimensional (2D) CCD detector. Imaging lens localizes the interference fringes and targets image coincident with the plane of detector, thereby facilitating an extremely compact design. The spectrum of the input light is reconstructed through the Fourier-transform of the interferogram. The total optics is as small as 20×6 cm Φ in size and the spectral resolution of the prototype system is 97.66 cm −1 between 25,000 and 10,000 cm −1 . The polarization interference imaging device has advantages of ultra-compact size, wide field of view, high throughput and without any moving parts.

Wide-field-of-view polarization interference imaging spectrometer.

A wide-field-of-view polarization interference imaging spectrometer (WPIIS) based on a modified Savart polariscope, without moving parts, and with a narrow slit has been designed. The primary feature of this device is for use with a large angle of incidence, and the target image as well as the interferogram can be obtained at the same time in the spatial domain and are recorded by a two-dimensional CCD camera. Under compensation, the field of view of the WPIIS will extend 3-5 times as large as a common interference imaging spectrometer, and throughput will raise 1-2 orders of magnitude. The developed optics is 20 x 8 cm ø in size. The spectral resolution of the prototype system is 86.8 cm(-1) between 22222.2 and 11111.1 cm(-1). This system has the advantages of being static and ultracompact with wide field of view and a very high throughput. The optics system and especially the wide-field-of-view compensation principle are described, and the experimental result of the interference imaging spectrum is shown.

Permissible deviations of the polarization orientation in the polarization imaging spectrometer

Permissible deviations in polarizer orientation in the polarization imaging spectrometer are analysed and discussed when the given modulation depth is met. It is found that, in order to obtain the optimum modulation depth, the orientations of the polarizer and analyser should be parallel or their deviations should be as small as possible. Furthermore, the polarization orientations of the polarizer and analyser should be symmetric to the ideal direction. The interferogram, the targets image and the changing curved surface of the modulation depth with the orientations of polarizer and analyser are shown. Some experimental tests are given. All of this will provide a theoretical and practical guide for the study, development, modulation, experiment and engineering of the polarization interference imaging spectrometer.

Broad-spectrum and long-lifetime emissions of Nd3+ ions in lead fluorosilicate glass.

A novel Nd(3+)-doped lead fluorosilicate glass (NPS glass) is prepared by a two-step melting process. Based on the absorption spectrum a Judd-Ofelt theory analysis is made. The emission line width of NPS glass is 44.2 nm. The fluorescence decay lifetime of the (4)F(3/2) level is 586+/-20 microsec, and the stimulated emission cross-section is 0.87x10(-20)cm(2)at 1056 nm. A laser oscillation is occurred at 1062 nm when pumped by 808 nm Diode Laser. The slope efficiency is 23.7% with a 415 mJ threshold. It is supposed that NPS glass is a good candidate for using in ultra-short pulse generation and amplification by the broad emission bandwidth and long fluorescence lifetime.

Spectrum Reconstruction Method for Airborne Temporally–Spatially Modulated Fourier Transform Imaging Spectrometers

Temporally-spatially modulated Fourier transform imaging spectrometers (TSMFTISs) have no internal moving parts and have high throughput advantages. In recent years, TSMFTISs have become a research and development focus for spaceborne missions. However, the applications of TSMFTISs for airborne missions are limited because the conventional spectrum reconstruction algorithm requires high stability of the platform during the pushbroom process. To overcome this limitation, we propose a target-tracking-based spectrum reconstruction method for airborne TSMFTISs. The direct georeferencing and motion estimation methods are incorporated for target tracking. The principle of TSMFTIS is presented to discuss the limitation of the traditional spectrum reconstruction method. A simulation based on the proposed scheme was performed to provide preliminary spectrum reconstruction results for the airborne TSMFTIS. The reconstructed spectra have much better agreement with the ideal spectrum than the traditional algorithm does.

Detection of infrared stealth aircraft through their multispectral signatures

Abstract. A concise band selection method employing multispectral signatures of stealth aircraft whose infrared radiation was remarkably reduced was proposed for precise target detection. The key step was to select two or more optimal bands which could clearly signify the radiation difference between the target and its background. The principle of preliminary selection was based on the differences of radiation characteristics for the two main constituents of the aircraft’s plume gas, i.e., CO2 and H2O. Two narrow bands of 2.86 to 3.3 and 4.17 to 4.55  μm were finally selected after detailed analyses on contrast characteristics between the target and background. Also, the stability of the selected bands was tested under varying environments. Further simulations and calculations demonstrated that the multispectral detection method utilizing the two selected narrow bands could markedly improve the essential performances of target detection systems and increase their achievable detection distance. The stability of the aircraft’s multispectral signatures enabled this target detection method to achieve excellent results.

Birefringent laterally sheared beam splitter-Savart polariscope

A laterally sheared beam splitter-savart polariscope was developed based on a single birefringent component. It is a key part in a novel polarization interference imaging spectrometer. The Savart polariscope consists of two paralled plates of calcite cut at an arbitrary angle to the optics axis and can split up a ray of light incident on the surface into an ordinary ray and an extraordinary ray. The output rays are paralled laterally sheared. The Savart polariscope is 25×25×12mm3 in size. The lateral displacement is 1mm and transmissivity is about 90 percent in spectral range of 0.4~2.5μm. A static polarization imaging spectrometer based on a Savart polariscope has advantage of ultra-compact size, wide field of view, high throughput and without any moving parts. The principle and construction of Savart polariscope are described, and the experimental results of the interference imaging spectrum are given.

Spread spectrum phase modulation for coherent X-ray diffraction imaging.

High dynamic range, phase ambiguity and radiation limited resolution are three challenging issues in coherent X-ray diffraction imaging (CXDI), which limit the achievable imaging resolution. This paper proposes a spread spectrum phase modulation (SSPM) method to address the aforementioned problems in a single strobe. The requirements on phase modulator parameters are presented, and a practical implementation of SSPM is discussed via ray optics analysis. Numerical experiments demonstrate the performance of SSPM under the constraint of available X-ray optics fabrication accuracy, showing its potential to real CXDI applications.

Image quality evaluation of light field photography

Light field photography captures 4D radiance information of a scene. Digital refocusing and digital correction of aberrations could be done after the photograph is taken. However, capturing 4D light field is costly and tradeoffs between different image quality metrics should be made and evaluated. This paper explores the effects of light field photography on image quality by quantitatively evaluating some basic criteria for an imaging system. A simulation approach was first developed by ray-tracing a designed light field camera. A standard testing chart followed by ISO 12233 was provided as the input scene. A sequence of light field raw images were acquired and processed by light field rendering methods afterwards. Through-focus visual resolution and MTF were calculated and analyzed. As a comparison, the same tests were taken for the same main lens system as the results of conventional photography. An experimental light field system was built up and its performance was tested. This work helps better understanding the pros and cons of light field photography in contrast with conventional imaging methods and perceiving the way to optimize the joint digital-optical design of the system.

Polarization characteristics of objects in long-wave infrared range.

Research on polarization characteristics of objects has become indispensable in the field of target detection. Though widespread studies on applying polarization to target detection and material identification exist, theoretical descriptions have varied widely in accuracy and completeness. Incomplete descriptions of polarization characteristics invariably result in poor demonstration of changes caused by macroscopic influence factors. For objects that are of finite surface, a comprehensive model is built to analyze the polarization characteristics of their thermal emission. With the Stokes theory and the superposition principle of light waves, the relation between the degree of linear polarization and the spatial geometrical parameters, such as the detection distance and the shape of objects, is discussed in the long-wave infrared range in detail. This model can be applied to analyze the linear polarization characteristics among different materials.