Pengwei Zhou

Development of a dual-path system for band-to-band registration of an acousto-optic tunable filter-based imaging spectrometer

Accurate band-to-band registration is crucial when an acousto-optic tunable filter (AOTF)-based imaging spectrometer is used to acquire spectral data on an unstable platform. However, it is difficult to registrate hyperspectral images accurately using traditional image registration algorithms, which will seriously affect the accurate acquisition of spectra and reduce the classification accuracy. Here we demonstrate an optical configuration that acquires the diffracted and undiffracted beam of the AOTF simultaneously. These two beams have identical geometrical optical characteristic and suffer from the same image motion caused by the platform jitter. Compared with the diffracted beam, however, the intensity of the undiffracted beam is changeless when tuning the AOTF. Therefore, the use of undiffracted beams allows accurate measurement of image motion in different bands. The experiment shows that this technique greatly improves the registration accuracy of spectral images with poor correlation and low SNR.

Combined laser and broadband light for measuring the tuning relation of an acousto-optic tunable filter.

To improve the tuning relation accuracy of an acousto-optic tunable filter, the conventional separated laser and broadband light methods were combined. The single laser test measured one accurate point and corrected the large amounts of data obtained from the broadband light method. The final tuning relation was fitted by the corrected data. A simulation and an experiment for several methods were conducted for comparison. The relative error was reduced from 0.2% to 0.05% in the 430~785 nm range. The equivalent wavelength accuracy improved from 1 to 0.2 nm. This method solved the problems associated with the use of a single laser source with few test data values and a single broadband light source with poor collimation.

Accurate optical design of an acousto-optic tunable filter imaging spectrometer

When designing an acousto-optic tunable filter (AOTF) imaging spectrometer, the traditional method used a refractive grating to simulate the effects of the AOTF. We studied the acousto-optic interaction in AOTF with exact analysis and found that simulating AOTF with grating was inaccurate due to angular dependence of grating spacing. We proposed a new method to simulate the diffraction using user defined surface (UDS) function in ZEMAX based on accurate acousto-optic interaction. Optimization and analysis of the optical design for an AOTF imaging spectrometer was carried out with the UDS. The obtained geometric MTF and spectral image indicates that the proposed method is efficient, accurate and feasible for routine design of the AOTF imaging spectrometer.

Crystal geometry measurement of an acousto-optic tunable filter using the tested tuning curves

A simple method of measuring the actual crystal geometry of an acousto-optic tunable filter (AOTF) was proposed. The relations between the tuning curves and the geometry were exactly analyzed for both incident polarizations. Through two optimization algorithms, the acoustic wave phase propagation angle and the incident facet angle were computed from the tested tuning curves successively. The exit wedge angle was measured similarly by swapping the incident and exit facets. A well fabricated 430 ~ 785 nm AOTF with known designed transducer cut angle 9.5° was chosen for experiment. The actual three angles were successfully measured. To verify this method, another experiment of measuring the non-diffracted light deflection angle was conducted. Its result highly approached the theoretical value calculated by the measured crystal geometry. This method is useful to establish the AOTF ray tracing model for optic design and to evaluate the AOTF performance by comparing with the designed geometry.