Acousto-optic Spectrometers Research Articles

New modulation method in acousto-optic spectrometers

A new method is proposed for modulating radiation in collinear acousto-optic (AO) spectrometers, which is based on the sequential diffraction of incident polarised optical radiation using two sound packets with the same frequencies and initial phases. A theory of diffraction of optical radiation on two successive packets of sound waves propagating in a crystal is constructed, and the instrumental function of the AO spectrometer is found. It is established that the instrumental function of such a spectrometer becomes time-dependent, and the modulation frequency of this dependence, in turn, depends on the intensity of the sound wave and on the detuning from the phase-matching conditions. The frequency dependence of the measured photocurrent for some finite number of sound packets is obtained explicitly, and it is shown that its measurement allows the spectral composition of the incident optical radiation to be determined with greater accuracy.

Construction of adaptive spectral analyzers on the basis of acousto-optic spectrometers

This paper discusses the problem of selecting the working spectral points in the method of selective spectral recording, used to detect contaminants in air by means of differential optical absorption spectroscopy on acousto-optic spectrometers with random spectral addressing. Reducing the number of recorded points of the spectrum, which allows the measurement time to be reduced by an order of magnitude, can simultaneously decrease the selectivity of the analysis, and this presents a problem if the composition of the impurity substances is unknown. A partial solution of the problem is proposed that consists of a preliminary study of the variations of the spectra of the gas mixture, and this makes it possible to statistically distinguish the main variable components in the composition of the gaseous mixture and to identify them with definite gases. Versions are proposed for the construction of adaptive systems based on acousto-optic spectrometers and using the technique developed here.

The Herschel-Heterodyne Instrument for the Far-Infrared (HIFI)

This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) to be launched onboard of ESA's Herschel Space Observatory, by 2008. The instrument is designed to be electronically tunable over a wide and continuous frequency range in the Far Infrared, with velocity resolutions better than 0.1 km s-1 and a high sensitivity. This will allow detailed investigations of a wide variety of astronomical sources, ranging from solar system objects, star formation regions to nuclei of galaxies. The instrument comprises 5 frequency bands covering 480–1150 GHz with SIS mixers and a sixth dual frequency band, for the 1410–1910 GHz range, with Hot Electron Bolometer Mixers (HEB). The Local Oscillator (LO) subsystem consists of a Ka-band synthesizer followed by 14 chains of frequency multipliers, 2 chains for each frequency band. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarization front-ends to provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 kHz to 1 MHz), better than <0.1 km s-1. After a successful qualification program, the flight instrument entered the testing phase. We will also report on the first pre-flight test and calibration results together with the expected in-flight performance.

Collinear acousto-optic diffraction using two nearby sound frequencies

Collinear acousto-optic spectrometers with sound consisting of one frequency have a spectral transfer function with many good characteristics, such as a narrow bandwidth, … the side lobes, however, are rather high. To reduce these side lobes, one can try to apodize the sound in the acousto-optic cell by using a sound wave consisting out of two frequencies. At certain times, this produces a more smoothened sound field in the cell than in the one-frequency case. The resulting transfer function will contain lower side lobes. Also, the influence of all the sound field parameters — the amplitude, frequency difference, and time — on the transmission function is studied. Out of these results, better-performing collinear spectrometers can be designed.

A high-resolution chirp transform spectrometer for microwave measurements

A microwave heterodyne spectrometer consists of a radiometer front-end and a real-time spectrum analyser back-end. Common spectrometer types are filterbanks, autocorrelators and acousto-optic spectrometers (AOS). A high-resolution chirp transform spectrometer using reflective array compressor (RAC) filters with a time bandwidth product of 6400 is presented. The spectrometer has 1600 channels within a 40 MHz input bandwidth and a linear dynamic range of 63 dB for a CW input. It seems to be very promising for future space applications.