Time-Resolved Multispectral Imaging Spectrometer

Abstract

A new multispectral imaging spectrometer with millisecond resolution has been developed. This instrument is based on the use of an acousto-optic tunable filter (AOTF) for spectral tuning and a simple progressive scan camera capable of snapshot operation for recording. The fast multispectral imaging can be performed in two configurations: recording images as a function of time or as a function of wavelength. In the first configuration, multiple images are recorded, grabbed, and stored per one wavelength. Upon completion, the AOTF is scanned to a new recording wavelength and a new set of images are recorded. It was found that, in this configuration, the imaging spectrometer is capable of recording, grabbing, and storing up to 33 images per s (i.e., 30 ms per image). Because an external signal is used to start the event and the recording of images, and the period between the start of the event and the recording and grabbing of images can be appropriately adjusted by a delay line, the time resolution of the spectrometer is not limited to 30 ms but rather can be adjusted to a shorter or longer time scale. In the second configuration, the recording wavelength is rapidly scanned (by the AOTF) and only one image is rapidly recorded, grabbed, and stored for each wavelength. Because additional time is needed to scan the AOTF, the maximum number of images that can be grabbed in this case is 16 frames per s. Preliminary applications of the imaging spectrometer include measurements of photo-induced changes in temperature-sensitive cholesteric liquid crystals as a function of time and wavelength. It was found that irradiating with a near-infrared (NIR) diode laser of 805 nm led to changes in the liquid crystal. The changes were found to vary with time and wavelength, namely, at about 360 ms after the NIR laser pulse the liquid crystal underwent changes in the visible region around 570 nm. The changes shifted toward longer wavelength concomitantly with time; i.e., maximum change at about 600 ms shifted to 718 nm.