Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
2001-04-06
2002-12-03
Pyo, Kevin (Department: 2878)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C250S227190
Reexamination Certificate
active
06490045
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a fibre optic sensor and in particular to a cyclic or Sagnac type ring interferometer for sensing mechanical or thermal disturbances, for example.
BACKGROUND OF THE INVENTION
The Sagnac or cyclic interferometer is one of several types of well known interferometer. Initially, such interferometers used a plurality of mirrors arranged so as to reflect light beams around a loop. Subsequently, the advent of laser diodes and fibre optics has allowed the development of Sagnac interferometers having a much longer path length. A light source can be injected into a wave guide which is formed by a fibre optic in the shape of a loop.
In recent years, a number of proposals have been put forward to locate mechanical or thermal disturbances occurring at any non-central location in the loop. One such arrangement is shown in “A novel distributed optical fibre sensing system enabling location of disturbances in a Sagnac loop interferometer”, by J. P. Dakin, D. A. Pearce, A. P. Strong, and C. A. Wade in Proceedings SPIE, vol 838 (1987). The technique is based upon the counter-propagating nature of light in the Sagnac interferometer. When a phase perturbation, &phgr;, occurs at a distance z from the sensor loop centre, it phase-modulates the light travelling in one direction before light travelling in the other. This results in a net phase modulation, &Dgr;&phgr;, between the two returning counter-propagating wavetrains which interfere, when combined at the output of the loop. As set out in the above referenced paper by Dakin et al, &Dgr;&phgr; is given by
Δ
φ
(
t
)
≈
2
z
V
g
ⅆ
φ
(
t
)
ⅆ
t
where V
g
is the group velocity of the guided light.
In initial arrangements, the value of d&phgr;/dt was found by interfering a fraction of the light that had travelled in one direction in the Sagnac loop, with light directly from the source. In order to reduce phase noise from frequency fluctuations of the source, the latter was suitably delayed via a fibre loop to form a balanced path fibre Mach-Zehnder arrangement.
More recently, various architectures using twin-Sagnac configurations have been suggested, to avoid the need for accurately balanced paths. Some of these architectures permit the location of disturbances over sensor loop lengths of up to 800 m, although only under laboratory conditions. For example, twin-source (wavelength multiplexed) devices are shown in the papers “Dual wavelength Sagnac-Michelson distributed optical fibre sensor”, Proceedings SPIE pp2838-2834 (1996) and “A distributed dual wavelength Sagnac sensor impact sensor”, Microwave and optical technology letters, vol 17, No 3, pp 170-173, (1998), by S. J. Spammer, A. A. Chtcherbakov, and P. L. Swart. Other intrinsically lossy arrangements with directional 3 dB couplers and twin detectors are shown in, for example, S. J. Spammer, P. L. Swart, A. Boosen, “Interferometric distributed fibre optical sensor”, Applied Optics, Vol 35, No 22, pp 4522-4523, (1996), E. Ronnekleiv, K. Blotekjaer, K. Krankes, Distributed fibre sensor for location of disturbances, Proceedings 9
th
OFS, PD7, (1993) X. Fang, “A variable loop Sagnac interferometer for distributed impact sensing” Optics letters, vol 21, No 6, (1996), and in U.S. Pat. No. 5,046,848.
The minimum theoretical loss of a dual Sagnac system with 3 dB couplers is 24 dB in each Sagnac.
According to the present invention there is provided a Sagnac interferometer for sensing a disturbance, the interferometer comprising: a light source arranged to generate a light signal over a range of wavelengths &lgr;
s
; an optical receiver arranged to receive light signals generated by the light source; a fibre optic loop sensor in optical communication with both the light source and the optical receiver; first optical splitter means, in optical communication with the said source, for spectrally slicing the light signal received from the source into first and second split signal channels, the first split signal channel having a range of wavelengths centred at &lgr;
1
, and the second split signal channel having a range of wavelengths centred at &lgr;
2
, wherein both &lgr;
1
and &lgr;
2
are subsets of &lgr;
s
and wherein the electromagnetic energy contained in the range of wavelengths centred at &lgr;
1
substantially does not overlap with the electromagnetic energy contained in the range of wavelengths centred at &lgr;
2
; first optical combiner means, in optical communication with the optical detector; the first split signal channel being defined along a first optical path between the optical splitter means and the optical combiner means via the loop sensor, and the second split signal channel being defined along a second optical path, different from the first optical path, between the optical splitter means and the optical combiner means via the loop sensor; the first optical path including a first optical phase angle modulating means arranged to modulate the phase angle of light in the first split signal channel at a first frequency, the first optical path having a total optical path length whose centre lies at a first non-central location around the loop sensor; the second optical path including a second optical phase angle modulating means arranged to modulate the phase angle of light in the second split signal channel at a second frequency different from the said first frequency, the second optical path having a total optical path length whose centre lies at a second non-central location different from the said first location around the loop sensor; the first optical combiner means being arranged to combine phase modulated light in the first and second split signal channels into a composite signal; the optical receiver being arranged to receive the composite light signal and to extract therefrom signal variations arising from light traversing the said first and second optical paths, whereby the distance of the disturbance around the loop sensor may be determined on the basis of the said signal variations.
The present invention provides a Sagnac loop interferometer architecture having two separate optical paths but with a single light source and a single detector. The resultant architecture has significantly lower minimum theoretical losses and is also cheaper to produce than previous arrangements which use multiple light sources and/or detectors.
Preferably, the first split signal channel defines the first optical path between the optical splitter means and the optical combiner means by traversing, in order, the first optical phase angle modulating means, the sensor loop and a first length of fibre defining a first delay loop such that the centre of the first optical path is offset relative to the centre of the sensor loop.
In that case, the second split signal channel may define the second optical path between the optical splitter means and the optical combiner means by traversing, in order, a second length of fibre defining a second delay loop, the sensor loop and the second optical phase angle modulating means, such that the centre of the second optical path is offset relative to the centre of the sensor loop in the opposite direction to the direction of offset of the centre of the first optical path.
The optical splitter means may be, for example, a wavelength division multiplexer (WDM). The light source may be a single superluminescent fibre source pumped by a laser diode producing broadband, low coherence length light. The detector may be a single p-type intrinsic n-type (PIN) semiconductor photodiode, and the first and second phase modulators may for example be piezo electric (PZT) devices.
Preferably, the first frequency of the first phase modulator and the second frequency of the second phase modulator are each Eigenfrequencies of first and second interferometer loops defined between the light source and the detector via the first and second optical paths respectively.
The optical receiver may comprise, for example, a photod
Dakin John Philip
Russell Stuart John
Luedeka Neely & Graham P.C.
Pyo Kevin
University of Southhampton
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