Optics: measuring and testing – Range or remote distance finding – With photodetection
Patent
1991-12-18
1993-11-09
Hellner, Mark
Optics: measuring and testing
Range or remote distance finding
With photodetection
356 45, 356 285, 356 28, G01C 308
Patent
active
052607623
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a device for non-contact measurement of speed, displacement travel and/or distance of a reflecting measurement object, comprising a laser, in particular a semiconductor laser, as a light source, in which the level of light intensity in the laser resonator influences the optical length thereof, a transmitting and receiving optical system for directing laser light from the semiconductor laser onto the measurement object and for feeding back a part of the laser light reflected by the measurement object into the laser resonator, and a detector means for detecting and analysing the laser light which is emitted by the semiconductor laser and which is modulated in dependency on the laser light feedback from the measurement object into the laser resonator.
It is already known to use a semiconductor laser (laser diode) as a receiver or as a "converter" for the light which returns from the measurement object, which converts individual photons which are fed back from the object into the laser resonator, into changes in the laser emission parameters (for example intensity). By virtue of that conversion procedure, it is possible, even when dealing with very weak signals from the measurement object, to obtain the measurement information from relatively high levels of optical power (fractions of the laser output power), which makes only extremely low levels of requirement in regard to the detection system to be used. The article "Ranging and velocimetry signal generation in a backscatter-modulated laser diode" by Peter J. de Groot, Gregg M. Gallatin and Steven H. Macomber in Appl. Opt. 27, No 21, 1988, pages 4475-4480, describes such a device for non-contact distance measurement of a diffusely scattering measurement object. The transmitting and receiving optical system used in that arrangement can be of a very simple design configuration and does not require an expensive adjustment operation, in contrast to conventional interferometers for length measurement purposes. The known device makes use of the fact that the light which is fed back from the measurement object into the laser resonator results in amplitude modulation of the light which is emitted by the laser diode. For distance measurement purposes, the laser diode current and therewith the optical phase in modulated, and the procedure involves looking for amplitude modulation of the output signal of the laser diode at the frequency of that phase modulation. A serious disadvantage of the known device however is the fact that measurement of speed and/or distance is possible only up to a relatively short distance of the measurement object from the laser diode (a few meters when using typical laser diodes).
OBJECT OF THE INVENTION
The object of the invention is to provide a device for non-contact measurement of speed, displacement travel and/or distance, of the kind set forth in the opening part of this specification, which is simple to set and relatively inexpensive and which has an increased level of measurement sensitivity and which thus permits measurement in respect of distance of weakly reflecting and in particular diffusely scattering measurement objects in a wide measurement range (preferably between fractions of millimeters and 50 m and more).
SUMMARY OF THE INVENTION
In accordance with the invention that is attained in that the detector means has at least one means for detecting the modulation frequency of the frequency-modulated laser light which is emitted by the semiconductor laser.
The invention is based on the realisation that semiconductor lasers (laser diodes), in response to a feedback of laser light into the laser resonator, which has a frequency which is shifted with respect to the emission frequency and a level of intensity (typically 10.sup.-4 and below) which is low in comparison with the output laser power, results in frequency modulation of the laser light emitted by the semiconductor laser. That effect generally occurs in the case of lasers, the optical resonator length of which depe
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Hellner Mark
Tabarelli Werner
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