Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2002-04-17
2004-07-13
O'Shea, Sandra (Department: 2875)
Optical waveguides
Optical waveguide sensor
C250S227140
Reexamination Certificate
active
06763153
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to fiber-optic sensors, in particular fiber-optic gyroscopes and fiber-optic sensors that measure magnetic fields.
BACKGROUND OF THE INVENTION
Fiber-optic sensors can be used to measure various physical quantities, such as the rotation of an object (fiber-optic gyroscope) or magnetic fields arising from currents in the vicinity of the sensor (fiber-optic current sensor). Such fiber-optic sensors typically include a section of optical fiber that is coupled to a broadband source, such as a laser operated below threshold, a superluminescent diode (SLD) or a fiber superluminescent source, a coupler to couple optical radiation emitted by the broadband source into the fiber, preferably a fiber coil, at least one polarizer and at least one phase modulator, and a detector which detects a phase shift or polarization difference between the optical signals traversing the fiber. The phase or polarization shift between the optical signals may be introduced, for example, by the rotation of the fiber coil or by a magnetic field. The term fiber-optic sensor will be used hereinafter to refer to both fiber-optic gyroscopes and fiber-optic current sensors.
The electrical output signal of a fiber-optic sensor includes noise components arising from various sources. The output noise which tends to manifest itself as angle random walk (ARW), as defined, for example, in IEEE Std-528, Inertial Sensor Terminology (incorporated by reference herein), has a different functional dependence on the optical power at the photodetector input, depending on the origin of the noise component. For example, thermal noise generated in the transimpedance amplifier feedback resistor in the photodetector electronics is independent of the light power. Shot noise related to the quantized nature of the detector current can be shown to be proportional to the square root of optical power, whereas relative intensity noise (RIN), which is inherent in the light of the source due to its finite bandwidth and impinges on the photodetector, and flicker noise (1/f), are proportional to the optical power. Since RIN and flicker noise have the same functional dependence on optical power, the term RIN hereinafter refers to both RIN and flicker noise.
RIN causes the noise-related performance of fiber-optic sensor systems to saturate, rather than continue to improve, as the source power is increased. Unless RIN can be mitigated, there exists a power level beyond which no further practical improvement is possible. For example, EDFS (Erbium doped fiber sources) which, due to their high optical power and wavelength stability, are often considered the natural choice for high performance fiber-optic sensors, tend to have a high RIN. Superluminescent diodes (SLD's), on the other hand, tend to have a lower RIN due to their larger bandwidth, but may suffer from limitations in power and lifetime, limiting their utility.
In one approach described in U.S. Pat. No. 5,331,404 and illustrated in
FIG. 1
, RIN in fiber-optic sensors is reduced by coupling a fraction of the light emitted by a light source
19
into a “dummy” fiber
30
having substantially the same length as the fiber
22
of the fiber-optic sensor
5
. The output signal detected at the end of the “dummy” fiber
30
by detector
34
is then modulated in multiplier
36
by a replica of the signal output of the fiber optic sensor
5
detected by detector
32
and subtracted in subtractor
38
from the output signal of the fiber-optic sensor
5
detected by detector
32
after passing through AC coupled amplifiers
40
,
42
with suitable adjustment of the channel gains. In other words, the “dummy” fiber
30
in this case operates as an analog delay line to match the time delay experienced by the light traversing the fiber-optic sensor
5
. This approach, however, requires a second coil of fiber of approximately the same length as the fiber-optic sensor coil.
In another approach disclosed in U.S. Pat. No. 5,655,035, two fiber-optic sensors can be excited by the same optical source, but with the sensitive axes oriented in diametrically opposed directions. The detected outputs are added, thereby subtracting the RIN, which is common to both channels since it arises in the common source. This approach doubles the entire fiber-optic sensor optical component count (except for the light source), which is expensive and bulky.
In another approach, described in U.S. Pat. No. 6,370,289 issued to Bennett, light from a light source is coupled into an input coupler with a first portion of the light emerging from the input coupler being transmitted to and through a first polarizer and a sensor which contains a fiber coil and other appropriate components. A detector and amplifier is coupled to a return tap of the input coupler and measures a sensor signal after the first portion of the light has transited through the sensor coil and other components. The sensor signal includes, in addition to the desired sensor signal, among others, the RIN noise. A second portion of the light emerging from the unused tap of the input coupler is transmitted through a second polarizer having a polarization axis substantially parallel to that of the first polarizer and is detected by a second detector and amplifier. This detected second portion of the light represents the RIN noise (as well as other incoherent noise sources). The RIN noise sample is delayed in a delay unit whose bandwidth is larger than the detector bandwidth, so that the time delay is essentially constant across the bandwidth where noise cancellation is desired and the sensor signal is present. The delay unit can be either analog delay line or implemented digitally in a shift register or in a computer memory buffer. The RIN noise sample is then multiplied by the waveform of the sensor signal. The sensor signal can also be passed through a DC block to eliminate the DC component of the sensor signal prior to the subtraction. The multiplied RIN noise sample is then subtracted from the sensor signal. The resulting time-dependent waveform having a reduced RIN component can then be processed further. According to another embodiment of the Bennett Patent, the RIN sample signal may be sampled at a rear facet of the light source.
The Bennett Patent approach, while having many advantages over the prior art, requires the use of a delay unit which can be, e.g., either analog delay line or implemented digitally in a shift register or in a computer memory buffer. It may be advantageous, however, to avoid the use of such components and implement the RIN noise reduction using other means to delay the RIN noise signal.
In addition, P. Polynkin, J. deArruda and J. Blake in “All-optical noise-subtraction scheme for a fiber optic gyroscope” in Opt. Lett, Vol. 25, No. 3, pp-147-149, Feb. 1, 2000 describe a method which eliminates the need to delay the source signal, and performs the noise cancellation optically. They have shown that the source noise signal and the noise on the sensor signal have the same form at the optical detector for a closed-loop gyro with square wave modulation(except that they are out of phase), if the gains (losses) in each path are the same and the modulation frequency is the so-called “proper frequency”, which is equal to
f
p
=1/(2&tgr;)
where &tgr; is the transit time of the light though the sensor. The optical signals are imaged on the same detector with the resultant noise cancellation occurring in the detection process.
However, there are some practical difficulties with this technique, as the relative amplitude of the two optical signals is fixed at the time of manufacture, and it is known that the loss in the optical circuit changes with temperature. Also, since there is no provision for multiplying the source signal by the replica of the sensor signal, this method is only appropriate for closed-loop gyros where the output signal is nulled. In addition, the approach only works if the modulation frequency is the proper frequency.
The latter restriction is certainly a limitation for open-loo
Foley & Hoag LLP
KVH Industries, Inc.
Macchiarolo Peter
O'Shea Sandra
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