The doppler frequency shift produced in the various diffraction orders of a rotating radial diffraction grating allows such a device to be used as an optical modulator. The theory and performance of such devices is presented.

Optical frequency shifting by means of a rotating diffraction grating

European work in coherent laser radar with 10 /spl mu/m and shorter wavelength lasers is reviewed. Fundamental aspects include heterodyne studies of signal statistics and fluctuations, and detailed experimental and theoretical work on signal amplification and autodyne arrangements with light reinjected into the laser cavity. Progress with lasers, detectors, and modulators has led to the development of several compact robust field systems both continuous-wave and pulsed. Various ground-based programs are described including local wind field measurement and wake vortex investigation at airfields, and study of range, image, and Doppler shift of hard targets. Airborne systems have investigated avionics problems of true airspeed, pressure error, and wind shear warning. Other airborne studies include ground imaging, obstacle warning, terrain following, and a compendium of atmospheric backscattering over the North and South Atlantic. In recent years, the European Space Energy has supported studies and technology development for a space-borne wind lidar in the Atmospheric Laser Doppler Instrument (ALADIN) program.

Coherent laser radar in Europe

Laser Doppler velocimeter for remote measurement of polluted water and aerosols discharges

A particle detector that determines the presence of particles (24) in an enclosed volume (22) includes a laser (10) that directs a coherent optical beam to a beam splitter (14) that produces first and second divergent beams (16, 18). An optical system images the point of origin of the two divergent beams within the beam splitter into the enclosed volume (22), whereby the first and second beams are caused to intersect and interfere at an inspection region within the volume. A detector (32) is positioned adjacent the volume (22) and is responsive to light scattered from one of the beams, as a result of a particle (24) passing through the inspection region, to produce an electrical signal indicative of the intensity of the scattered light. A signal processor (34) analyzes the electrical signals and to determine the presence of the particle (24). An embodiment (14) of the invention includes an acousto-optic modulator to enable one of the beams to be frequency shifted from the other beam so as to enable the presence of a carrier signal on which a particle's Doppler frequency is modulated. Another embodiment relies upon the Doppler modulation of a reflected optical beam to create an interference pattern at a detector.

Optical submicron aerosol particle detector

Wavefront control techniques are provided for the alignment and performance optimization of optical devices. A Shack-Hartmann wavefront sensor can be used to measure the wavefront distortion and a control system generates feedback error signal to optics inside the device to correct the wavefront. The system can be calibrated with a low-average-power probe laser. An optical element is provided to couple the optical device to a diagnostic/control package in a way that optimizes both the output power of the optical device and the coupling of the probe light into the diagnostics.

Dichroic beamsplitter for high energy laser diagnostics

A simple method has been developed to determine the sign of the wind vector in a homodyne cw laser Doppler velocimeter (LDV). In contrast to the usual heterodyne technique—which requires the signal carrier frequency to be large compared to the signal frequency—this method makes use of a small additional frequency change which is superimposed onto either the signal or the local oscillator reference beam. Therefore, the sign of the wind signal Doppler shift can easily be determined and the ambiguity in wind direction—typical for homodyne systems—is removed. Some inconsistencies in earlier publications concerning vertical wind components are now explainable.

Sign of the wind vector: a simple method for its determination with a homodyne cw laser Doppler velocimeter.

In a method in which a heterodyne-laser-Doppler method is used for the demination of the sign of the direction of a particle stream, the beam from a laser source is directed by means of an optical arrangement used as an emitter optical system to a measuring volume in an interferometer device. A frequency difference between the emitted laser beam and the laser beam returning into the optical arrangement is measured through heterodyne reception by means of the same optical arrangement used as a receiving optical system; this returning laser beam is frequency-shifted because of scattering by particles moving with the same speed in the particle stream and because of the Doppler effect. In the method according to the invention, an additional Doppler shift with a definite sign then is alternately impressed on the laser beam in exactly defined areas of beam paths, the additional Doppler shift being preferably considerably smaller than the Doppler shift generated by the particle movement. The exact direction of the particle stream then is determined from the received scattered back laser beam by comparison of this scattered back laser beam with a laser source reference beam. The additional Doppler shift can be impressed on a reference beam or emitted beam split from the laser beam, on the laser beam scattered back by the measuring volume or on the emitted beam and signal beam flowing in opposite directions in the optical arrangement.

Dieter Dipl Phys Dr Hausamann

Papers

Sign of the wind vector: a simple method for its determination with a homodyne cw laser Doppler velocimeter.

Method and apparatus for determining the sign of the direction of a particle stream