Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1991-12-16
1994-03-01
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250226, G02B 514
Patent
active
052910156
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optical fibre sensor system in which a parameter is monitored by a sensor providing a measurement signal output in the form of an optical signal the intensity of which is a function of the monitored parameter.
Dual wavelength intensity modulation sensors are known. One wavelength is used for the transmission of a measurement signal and the other wavelength is used for the transmission of a reference signal. The measurement signal intensity is modulated in accordance with the monitored parameter whereas the reference signal intensity is independent of the monitored parameter. Ideally such a system should be configured such that any extraneous intensity effects proportionally affect both the measurement and the reference signal. If this is the case a ratio of the two optical signals provides a fully referenced output independent of such extraneous intensity effects
In one known system (K. Kyuma, S. Tai, T. Sawabe and M. Nunoshita, "Fiber optic instrument for temperature measurement", IEEE J. Quantum Electron., Vol. QE-18, pp. 676-679, 1982) the optical absorption of a thin semiconductor material is temperature dependent at one wavelength and almost transparent at another. Optical emissions from two LEDs operating at these two wavelengths were coupled together and transmitted through the system. The output signal of the temperature dependent light wave normalised by that of the reference light and the ratio was therefore dependent on the absorption spectrum of the semiconductor material.
A somewhat similar referencing technique again using two LEDs emitting at different wavelengths has been proposed (P. R. Wallace, E. S. R. Sikora and A. J. Walkden, "Engineering optical fibre sensors for process control", GEC J. of Research Vol. 2, pp. 129-134, 1984). In this system the individual wavelength optical signals are reflected back both before and after a fibre microband sensor. The optical signal reflected before the sensor functions as the reference signal whilst the optical signal modulated by the microbending sensor provides the measurement signal.
A problem with all of the devices referred to above is that they employ two separate LED sources to produce the reference and measurement signals. Consequently unequal spectral shift, or output power fluctuations, in the LED sources can create problems within the sensor system and hence invalidate the referencing. In particular, it has been found that the normalised output power of LEDs is highly temperature dependent and thus unless compensation for optical source temperature is provided the true relationship between the measurement and reference signals cannot be predicted and thus sensor system calibration is inaccurate.
It is an object of the present invention to provide an optical fibre sensor system which obviates or mitigates the problems outlined above.
According to the present invention there is provided an optical fibre sensor system comprising a transmitter, a receiver and a sensor interconnected by an optical link, wherein the transmitter comprises an optical source arranged to transmit to the sensor via the optical link two optical signals at different wavelengths, the sensor is arranged to transmit to the receiver via the optical link measurement and reference signals, the measurement signal being derived by modulating the intensity of one of the two optical signals in dependence upon a parameter monitored by the sensor and the reference signal having an intensity which has a predetermined relationship with the intensity of the other optical signal, and the receiver is arranged to compare the intensities of the measurement and reference signals to derive a measure of the monitored parameter, characterised in that the two wavelengths are selected such that changes in the intensities of the optical signals resulting from variations in the temperature of the optical source are minimised or are of substantially equal magnitude for both the optical signals.
The transmitter may comprise a single LED, in which case filters a
REFERENCES:
patent: 4249076 (1981-02-01), Bergstrom et al.
patent: 4270050 (1981-05-01), Brogardh
patent: 4281245 (1981-07-01), Brogardh et al.
patent: 4417140 (1983-11-01), Adolfsson et al.
patent: 4514860 (1985-04-01), Adolfsson et al.
patent: 4529875 (1985-07-01), Brogardh et al.
patent: 4560868 (1985-12-01), Brogardh et al.
patent: 4743119 (1988-05-01), Ida
Engineering Optical Fibre Sensors for Process Control, Wallace et al., published 1984, pp. 129-134.
Fiber-optic Instrument for Temperature Measurement, Kyuma et al., published Apr. 1982, pp. 676-679.
Murtaza Ghulam
Senior John M.
Allen Stephone B.
Nelms David C.
LandOfFree
Optical fibre sensor system where variations in temperature of o does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical fibre sensor system where variations in temperature of o, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical fibre sensor system where variations in temperature of o will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-579717