Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1985-09-05
1987-02-10
LeRoche, Eugene R.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250227, G02F 101
Patent
active
046424580
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optical fibre sensing apparatus or polarimetric fibre sensor and, more particularly, to such apparatus which utilises the relative change in optical path lengths of the two orthogonal polarisation eigen modes of a highly birefringent optical fibre to monitor variations in physical parameters, such as temperature and strain. The invention is particularly suitable for remotely monitoring large range physical variations resulting in changes in sensing fibre length of the order of 0.003 .mu.m to greater than 100 .mu.m.
The development of polarisation preserving mono-mode optical fibres has enabled the development of coherent optical fibre systems in which co-linearity of mixed optical beams is required. Futhermore, a variety of optical sensors, which utilise the interference between the orthogonally polarised modes of stress-induced highly birefringent fibres, have been proposed. In such applications, both eigen or polarisation modes of the fibre are excited. Physical perturbations of the fibre, for example, temperature change or axial strain, vary its physical parameters, such as, length and/or refractive index and, hence, the level of intrinsic birefringence, which leads to a change in the state of polarisation of the light at the fibre output, which, in turn, can be detected using a polarisation analyser. This forms the basis of a number of simple polarimetric fibre sensors. A major problem with sensors of this type is that all the fibre acts as the sensor, including the input and output signal leads. This limits the application of this type of sensor. Moreover, most systems proposed have been for small-frequency signals and therefore require refinement if they are to operate over large dynamic ranges with an unambiguous output.
An object of the present invention is to provide an optical fibre sensing apparatus in which the change in modal birefringence of a short length of birefringent optical fibre is monitored in order to measure changes in a physical parameter sensed by the fibre. Another object is to provide such an apparatus which utilises both the polarisation-preserving property and the wavelength dependence of the intrinsic birefringence of stress-induced highly birefringent fibre to achieve remote monitoring of large range environmentally induced changes in the birefringence of a short length of sensing fibre.
The present invention consists in optical fibre sensing apparatus characterised by:
(a) an optical fibre lead spliced to a shorter optical sensing fibre,
(b) said lead and sensing fibres comprising birefringent optical fibres having substantially orthogonal eigen modes and being spliced together with their eigen modes mutually rotated by substantially 45.degree.,
(c) a reflective surface disposed at the distal end of the sensing fibre,
(d) light source means for propagating linearly polarised incident light along one of the eigen modes of the lead fibre, whereby light beams are propagated along both eigen modes of the sensing fibre and are reflected from the distal end of the sensing fibre to propagate return light beams along both eigen modes of the lead fibre,
(e) recovery means for recovering one of the returning light beams from the lead fibre and producing an electrical signal corresponding to modal birefringence of the sensing fibre, and
(f) means for processing the electrical signal and producing an output signal corresponding to the polarisation mode delay of the sensing fibre.
With the present invention, physically induced changes in the modal birefringence of the relatively short length of sensing element or fibre are monitored in reflection through the lead fibre, which may be of any required length subject to the transmitted and reflected light beams not being unduly attenuated. Exciting only one of the eigen modes of the lead fibre ensures that the system displays down lead insensitivity to any induced change in modal birefringence in the lead fibre. This lack of sensitivity is due to the effective polarisation isolation of the light, which is
REFERENCES:
patent: 4405198 (1983-09-01), Taylor
patent: 4554449 (1985-11-01), Taniuchi et al.
Corke Michael
Jackson David A.
Kersey Alan D.
Kent Scientific and Industrial Projects Limited
Lee James C.
LeRoche Eugene R.
LandOfFree
Polarimetric fibre sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polarimetric fibre sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarimetric fibre sensor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2364041