Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1987-11-12
1989-09-19
McGraw, Vincent P.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356346, G01B 902
Patent
active
048675652
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an opto-electronic detection device for remotely detecting a physical magnitude, the device including a source of incoherent light connected via an optical cable to a spectrum modulation encoding sensor which is itself connected via an optical fiber to spectrum analyzer means for analyzing the spectrum of the light coming from the sensor.
In order to measure a displacement by optical means with high accuracy, it is known to use a Michelson interferometer illuminated with monochromatic light (a helium-neon laser) together with a fringe-counting electronic system. This apparatus suffers from the drawback of providing displacement measurements only, rather than position measurements. In order to know the position of an object, it is therefore necessary to keep track of all of its displacements from its starting point to its finishing point. Any interruption in the light beam therefore leads to a complete loss of the origin, and thus makes the measurement valueless.
It has been shown that replacing the coherent monochromatic light source with an incoherent polychromatic light source makes it possible to remedy this drawback (article by Bosselmann, Ulrich and Arditty entitled "Capteur de deplacement a interferometres couples par fibres multimodales" (i.e. displacement sensor using interferometers which are coupled by multimode fibers) published in Opto 85, 21/23 May 1985. For detection purposes, the device described in this article uses a second Michelson interferometer whose path length difference is made variable by displacing a mirror. Measurement thus consists in setting the detection interferometer about zero path length difference (zero order correlation peak), and then in varying the position of the moving mirror until the path length difference in the detector is comparable in absolute value to that recorded by the sensor (correlation peaks of order plus or minus one).
If the full accuracy of the method is to be retained, it is necessary to measure the position of the moving mirror to very high accuracy. In the article mentioned, this is done by using a helium-neon laser at the detector operating as a fringe-counting device. This method suffers from the drawback of returning at the detector to the limitations related to using a monochromatic source (i.e. to the relative nature of the measurements).
Further, interferometer devices using space division of the wavefront, such as a Michelson interferometer, are difficult to make, in particular for use with polychromatic light. They require the use of very severe manufacturing tolerances, stabilities, and optical quality.
The use of such a system as the sensor portion of the device can only be justified in the special case of displacement measurements. This system is not appropriate for measuring other physical magnitudes such as temperature, magnetic field, or electrical voltage, since it is difficult to escape from interfering effects related to the structure of the sensor or to cross dependencies between the mangitudes (as applicable to temperature and magnetic fields, for example).
It is also known, from French patent No. 2 436 976, to measure temperature remotely by means of an opto-electronic device including an incoherent light source connected by an optical fiber to a sensor which includes a birefringent element which is sensitive to the physical magnitude to be measured, together with means for analyzing the light signal coming from the sensor and serving to transform said signal into a light signal whose intensity is a sinusoidal function of time having peaks which are separated by time intervals which are a function of the detected temperature. This light signal is transformed by a photodiode into an electrical signal from which the DC component is removed and which is transformed into a squarewave whose period, representative of the detected temperature, is then measured. In order to obtain proper accuracy, two counters must be used in order to perform the measurement
REFERENCES:
patent: 4320973 (1982-03-01), Fortunato et al.
Optics Letters, vol. 11, No. 8, Aug. 1986, "Fiber-Optic Angular Sensor with Interleaved Channel Spectra".
Bertin & Cie
McGraw Vincent P.
Turner S. A.
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