ULTIMATE SENSITIVITY OF HETERODYNE SPECTROMETERS

Low noise heterodyne receivers are now used in a large variety of inst ruments such as radiotelescopes, far infrared laser side bands spectro meters, supersonic nozzle beams Fourier transform spectrometers, lidar s or plasmas diagnostics devices: Numerous papers have been devoted to the analysis of heterodyne receivers in the attempt to reach ultimate performances. These previous papers usually retain the restrictive hy pothesis of vanishingly small signal powers. Such a condition occurs o nly in a restrictive number of applications, radioastronomic systems b eing probably among the most representative examples. Spectroscopic in struments operate usually in a different regime. The signal to be dete cted consists of a small variation in the source power. The absolute v alue of this source power largely depends on the available technology in the spectral range of interest. The purpose of this paper is to ana lyze the behavior of heterodyne detection in the general case. The res ults apply to a large category of systems including spectroscopic inst ruments. The influence of crucial parameters such as source power and predetection bandwidth is analysed in details. The theory is experimen tally tested on-an example of ultra low noise receiver designed for fa r infrared laser side bands spectroscopy. Some numerical calculations are proposed, illustrating methods for an optimal sizing of receiver's predetection bandwidth and. the estimation of ultimate sensitivity of spectrometers.