Interferometric sensor

Details Interferometric sensor Interferometric sensor

1986-02-27

1988-02-16

Willis, Davis L.

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

250227, G01B 902

Patent

active

047251433

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to an interferometric sensor and, more particularly, to such a sensor which combines the features of two different configurations of interferometer in order to provide a measuring instrument which has relatively high resolution over an improved unambiguous dynamic range.
A conventional interferometric sensor, for example, embodying a Michelson, Fabry-Perot or Mach-Zehnder configuration, in which a measurand is detected by the change in optical path length induced in a sensing element, has been shown to be applicable to a wide variety of measurands, such as, temperature, acceleration, magnetic field, fluid flow and acoustic pressure, and extremely high resolution has been achieved. However, because the interferometer output is periodic, the value of the measurand derived is ambiguous and the inherent high resolution results in a very limited unambiguous dynamic range. Signal processing schemes, including active phase compensation and heterodyne schemes, have been developed which overcome problems of signal fading and extend the dynamic range. However, these schemes do not provide the absolute value of the measurand when the system is initialised.
More recently, optical fibre polarimetric sensors, which utilise the measurand-induced change in modal birefringence of highly birefringent optical fibre, have been proposed. These polarimetric sensors have a much reduced resolution with a corresponding increase in the period of the interferometer transfer function and they therefore provide an increased unambiguous dynamic range at the expense of reduced resolution. Generally, the ratio of sensitivities for conventional and polarimetric configurations is different for different measurands.


SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel configuration of interferometric sensor which combines the features of conventional and polarimetric interferometers to produce a measuring instrument having the wider unambiguous dynamic range of a polarimetric interferometer and the higher resolution of a conventional interferometer.
To this end, the invention consists in an interferometric sensor characterised by a sensing element adapted to detect a required measurand and comprising a polarimetric interferometer device, means for illuminating the polarimetric sensing element with an input beam of polarised, e.g. linearly polarised, light, means for deriving a reference light beam from the input light beam, and means for selectively mixing the output light beams of the polarimetric sensing element and the reference light beam to produce interference signals corresponding to the phase differences of the mixed beams.
Preferably, the means for selectively mixing the light beams is arranged to mix the two output light beams of the polarimetric sensing element to produce one interference signal and mix a selected one of the output beams of the sensing element with the reference light beam to produce the other interference signal.
Hence, by utilising one of the output beams of the polarimetric sensing element in conjunction with the reference light beam, the present invention effectively combines the features of a conventional interferometer, for example, a Michelson or Fabry-Perot interferometer, with the polarimetric interferometer forming the sensing element and thereby enables the construction of a measuring instrument which has the high resolution of a conventional interferometer over a wider unambiguous dynamic range than hitherto. The invention also enables the simultaneous determination of more than one measurand.
The means for selectively mixing the light beams may comprise linear or plane polarising elements. The interference signals provided by the selectively mixed beams may be sensed by photodetectors which produce electrical signals corresponding to the interference signals, and signal processing means may be connected to the photodetector outputs for producing an output signal representing the measurand. In one preferred embo

REFERENCES:
patent: 4442350 (1984-04-01), Rashleigh

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