U.S. patent number RE35,023 [Application Number 08/218,337] was granted by the patent office on 1995-08-22 for fiber optic gyro with a source at a first wavelength and a fiber optic loop designed for single mode operation at a wavelength longer than the first wavelength.
This patent grant is currently assigned to Honeywell Inc.. Invention is credited to Carl M. Ferrar.
United States Patent |
RE35,023 |
Ferrar |
August 22, 1995 |
Fiber optic gyro with a source at a first wavelength and a fiber
optic loop designed for single mode operation at a wavelength
longer than the first wavelength
Abstract
A low cost fiber optic gyro includes a Sagnac interferometer
configured in a minimum reciprocal configuration and modified to
use a 0.8 micron wavelength laser diode as the interferometer light
source and 1.3 micron, single-mode fiber for the sensing coil.
Inventors: |
Ferrar; Carl M. (East Hartford,
CT) |
Assignee: |
Honeywell Inc. (Minneapolis,
MN)
|
Family
ID: |
24540067 |
Appl.
No.: |
08/218,337 |
Filed: |
March 25, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
633545 |
Dec 21, 1990 |
05137360 |
Aug 11, 1992 |
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Current U.S.
Class: |
356/464 |
Current CPC
Class: |
G01C
19/721 (20130101) |
Current International
Class: |
G01C
19/72 (20060101); G01C 019/72 () |
Field of
Search: |
;356/350 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Rotation Detection by Optical Fibre Laser Gyro with Easy
Introduction Phase-Difference Bais", Electronics Letters, Hotate et
al, Dec. 1980, pp. 941-942..
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Primary Examiner: Turner; Samuel A.
Attorney, Agent or Firm: Shudy, Jr.; John G.
Claims
.[.We claim:.]. .Iadd.What is claimed is: .Iaddend.
1. An interferometric rotation sensor, comprising:
optical signal source means, for providing a source optical signal
.Iadd.having a primary source optical signal
wavelength.Iaddend.;
optical fiber sensing loop means, for providing, in .[.the.].
presence of .[.loop.]. rotation .Iadd.thereof.Iaddend., a Sagnac
phase difference between two sensing loop optical signals
propagating in counter circulating paths therethrough;
.[.integrated optic circuit (IOC) means, having a substrate with a
waveguide array formed thereon, said waveguide array including.]. a
bi-directional common path section responsive to said source
optical signal, said .Iadd.bi-directional .Iaddend.common path
.Iadd.section .Iaddend.having a single polarization mode filter and
a single spatial mode filter formed therein to pass .[.select.].
.Iadd.selected .Iaddend.mode optical signals having a .[.desired.].
.Iadd.selected .Iaddend.spatial mode and a .[.desired.].
.Iadd.selected .Iaddend.polarization mode, .[.said waveguide means
further including.]. .Iadd.a .Iaddend.beam splitter/combiner means
for splitting said .[.select.]. .Iadd.selected .Iaddend.mode
optical signal received from said .Iadd.bi-directional
.Iaddend.common path .Iadd.section .Iaddend.into said two sensing
loop optical signals for counter propagation through said sensing
loop means, and for combining sensing loop optical signals received
from said .Iadd.optical fiber sensing .Iaddend.loop .Iadd.means
.Iaddend.into a common interference signal for return through said
.Iadd.bi-directional .Iaddend.common path .Iadd.section.Iaddend.,
said interference signal amplitude being dependent on the magnitude
of said Sagnac phase difference;
detector means, for sensing the amplitude of said interference
signal; and
means for coupling said source optical signal to said .[.IOC
means.]. .Iadd.bi-directional common path section .Iaddend.and for
coupling said interference signal from said .[.IOC means.].
.Iadd.bi-directional common path section .Iaddend.to said detector
means;
as characterized by:
.[.said optical signal source means comprising a laser diode having
a source optical signal wavelength;.].
said .[.sensing loop.]. .Iadd.optical .Iaddend.fiber .Iadd.sensing
loop means .Iaddend.comprising an optical fiber which is designed
for single mode operation at a wavelength which is longer than said
.Iadd.primary optical signal .Iaddend.source wavelength, and
wherein said .[.sensing loop.]. .Iadd.optical .Iaddend.fiber
.Iadd.sensing loop means .Iaddend.embodies a signal mode conversion
characteristic therein, to prevent .[.sensing loop.]. cross
coupling of all optical power from .[.the select.]. .Iadd.said
selected .Iaddend.spatial mode to .[.undesired.]. .Iadd.other
.Iaddend.spatial modes .Iadd.in said optical fiber sensing loop
means. .Iaddend.
2. The rotation sensor of claim 1, wherein said .Iadd.optical fiber
.Iaddend.sensing loop .Iadd.means .Iaddend.comprises .[.a.].
.Iadd.an optical .Iaddend.fiber coil configuration having a fiber
winding geometry which causes said .Iadd.optical fiber
.Iaddend.sensing loop .Iadd.means .Iaddend.to display inherent mode
conversion characteristics.
3. The rotation sensor of claim 1, wherein said .Iadd.optical fiber
sensing loop .[.coil configuration.]. .Iadd.means
.Iaddend.comprises .Iadd.an optical fiber coil configuration having
.Iaddend.a discrete mode conversion element positioned at one end
.[.of said sensing loop coil.]. .Iadd.thereof.Iaddend..
4. The rotation sensor of claim 3, wherein said discrete mode
conversion element comprises a serpentine segment of fiber bends
positioned at one end of the sensing loop coil.
5. The rotation sensor of claim 1, wherein said .Iadd.primary
.Iaddend.source .Iadd.optical signal .Iaddend.wavelength is on the
order of 0.8 microns and the .Iadd.optical fiber .Iaddend.sensing
loop .Iadd.means .Iaddend.optical fiber is designed to operate as a
wavelength on the order of 1.3 microns.
6. The rotation sensor of claim 1, further comprising
depolarization means, at least one, located between the sensing
loop coil and said .[.IOC combiner/splitter.]. .Iadd.beam
splitter/combiner means.Iaddend., to prevent sensing loop cross
coupling of all optical power from .[.the desired.]. .Iadd.said
selected .Iaddend.polarization mode to .[.an undesired.].
.Iadd.another polarization .Iaddend.mode.
7. The rotation sensor of claim 6, wherein said depolarization
means comprises a length of single mode fiber having high
birefringence.
8. The rotation sensor of claim 1, further comprising:
serrodyne modulation means, for applying a linear ramped phase
modulation of said sensing loop optical signals, to provide a phase
bias in opposition to the Sagnac phase difference; and
control circuitry, for continuously changing the value of the
serrodyne modulation frequency in dependence on said interference
signal amplitude so as to cause the phase bias to continuously null
the Sagnac phase difference magnitude, whereby the nulling value of
the serrodyne modulation frequency is proportional to said sensing
loop rotation rate.
9. The rotation sensor of claim .[.6.]. .Iadd.11.Iaddend., further
comprising discrete polarizer means, positioned between said means
for coupling and said IOC .Iadd.bi-directional .Iaddend.common path
.Iadd.section .Iaddend.waveguide segment, to increase the
extinction ratio of said IOC single polarization .Iadd.mode
.Iaddend.filter.
10. The rotation sensor of claim 5, wherein said .Iadd.primary
.Iaddend.source .Iadd.optical signal .Iaddend.wavelength may range
from 750 to 900 nanometers and the design wavelength of the sensing
loop fiber may range from 1200 to 1600 nanometers. .Iadd.11. The
rotation sensor of claim 1 further comprising said bi-directional
common path section and said beam splitter/combiner means provided
in a waveguide array formed on a substrate of an integrated optics
chip (IOC) means. .Iaddend. .Iadd.12. The rotation sensor of claim
1 wherein said optical signal source means comprises a laser diode.
.Iaddend.
Description
DESCRIPTION
1. Technical Field
This invention relates to fiber optic gyros, and more particularly
to optical interferometer type rotation sensors.
2. Background Art
Measurement of rotation rate is required in applications ranging
from robotic and ballistic missile control, to aircraft and
spacecraft navigation. Performance accuracy ranges from 0.001 to
0.01 degrees/hour for Inertial Grade spacecraft/aircraft navigation
systems (10.sup.-3 to 10.sup.-4 of earth's 15 degrees/hour rotation
rate), through Moderate Grade sensing accuracies of 0.02 to 1.0
degrees/second. Intermediate Grade performance is in the 0.1 to 10
degrees/hour range.
Although spacecraft navigation usually relies on spinning wheel
gyros, advances in laser technology have allowed dual laser beam
gyros ("laser gyros") to be used in high performance applications
such as aircraft navigation systems. The laser gyro offers fast
startup, small size, lower cost, and most importantly the absence
of moving mechanical parts. An outgrowth of the laser gyro is the
fiber optic gyro (or "FOG"), which is an alternative type of
interferometric rotation sensor.
The FOG can be smaller, more rugged, and less costly than the laser
gyro, making it ideally suited for lower performance (Moderate and
Intermediate Grade) applications in the field of advanced
projectiles. Projectile applications for which the FOG is
particularly well suited are roll attitude determination, body rate
sensing, and seeker stabilization.
The FOG uses a Sagnac interferometer to measure rotation based on
the principle that the transit time of an optical signal
propagating through a fiber optic loop rotating about an axis
perpendicular to its plane, varies with the loop rotation rate. The
transit delay for two optical signals traversing the loop in
opposite directions creates a Sagnac phase differential that is
proportional to loop rotation rate: ##EQU1## where: S is the Sagnac
phase difference in radians, L is the length of the fiber loop, d
is the loop diameter, .lambda. is the optical signal wavelength, c
is the speed of light, and .OMEGA. is the loop rotation rate in
radians/sec.
Phase detection sensitivity may be uncreased by modulating both
optical signals with a sinusoidal phase modulator positioned at one
end of the loop. The optical transit time delay causes the
modulator to act on the counter circulating light beams at
different times, dithering the phase difference magnitude and
permitting use of sensitive AC processing to detect
rotation-induced phase differences.
When counter propagating signals of unit intensity are combined
interferometrically, the intensity (I) is:
where P is the total phase difference (Sagnac and phase
modulation).
The intensity I versus Sagnac phase difference relationship is a
cosine function. At zero rotation the phase difference is zero and
the signals interfere constructively to produce a maximum
intensity. Loop rotation creates a phase differential, causing the
signals to destructively interfere and reduce the intensity.
Bessel expansion of the intensity expression at the modulation
frequency (f) produces the rotational velocity component:
and the term 2A* sin (.pi.*f*T) is the dithered phase difference
modulation of amplitude (A) and modulation frequency f. The coil
transit time is T and, if (A) is fixed, F is maximized when f=1/2T;
the coil eigenfrequency.
The analog value of F can be measured directly as an indication of
rotation, or the signal amplitude can be continuously nulled by a
closed loop serrodyne modulator which adds an optical phase bias in
opposition to the Sagnac phase difference. This is a repetitive
linearly ramped phase modulator positioned at one end of the fiber
coil. A peak ramp amplitude of 2.pi. radians produces an
effectively constant phase difference bias between the oppositely
directed beams. The ramp repetition frequency, which is
proportional to the phase bias amplitude, provides a measurable
representation of the loop rotation rate.
The rotation sensing accuracy critically depends on the counter
propagating signals travelling identical ("reciprocal") optical
paths at zero rotation rate (and zero applied bias). The necessary
reciprocity can be assured by arranging the optic elements in a
"minimum reciprocal configuration" which requires the optical
signals to pass through a common single-spatial-mode filter and a
single-polarization filter when propagating from the source to the
sensing coil and from the coil to the detector. This ensures that
the counter propagating optical signals received by the detector
will travel identical paths, associated with a single spatial mode
and a single polarization, even if multiple spatial modes and
polarizations exist in the optical path due, for example, to fiber
birefringence effects and scattering and cross-coupling between
spatial modes.
In practice, when the filtering is imperfect, the FOG offset errors
associated with residual polarization and spatial-mode-related
non-reciprocity (as well as several other types of errors) may be
reduced through use of a broadband, short coherence length optical
source and high birefringence optic fiber in the sensing coil. This
makes the selected mode counter propagating waves incoherent with
certain cross coupled waves.
The FOG offers the potential for good performance and low cost.
Components required for FOG fabrication are readily available at
wavelengths near 0.8 and 1.3 microns. The longer wavelength offers
the advantage of: generally lower light loss, easier coupling of
components, and greatly reduced photorefractive effects in LiNb03
integrated optic devices.
The selection of component, however, affects the cost/performance
tradeoff. If low cost is a primary objective, it may be difficult
to choose between operation at the 1.3 micron wavelength for which
communication grade fiber is readily available at low cost, but the
most appropriate light sources are very expensive, or operation at
the 0.8 micron wavelength at which suitable inexpensive laser
sources are readily available but the fiber is expensive.
DISCLOSURE OF INVENTION
The object of the present invention is to provide a low cost
rotation sensor design for use in moderate and intermediate grade
fiber optic gyros.
According to the present invention, a Sagnac interferometer is
provided in a known minimum reciprocal configuration, including a
single polarization filter and a single spatial mode filter located
in the common light path between the source/detector and the
sensing loop, but which further includes spatial mode conversion in
the sensing loop to permit use of sensing-loop fiber which may be
multi-moded at the wavelength of the interferometer light source
(e.g. a 1.3 micron single-mode fiber sensing coil with a 0.8 micron
wavelength laser diode light source) to retrieve at least a minimum
level of optical power from undesired spatial modes into the
desired spatial mode.
In further accord with the present invention, the sensing loop
single mode fiber is non-polarization maintaining fiber, and the
interferometer further comprises a depolarizer located in the
sensing loop to prevent signal fading.
In the prior art the use of multi-mode fiber for the sensing coil
fiber is usually considered inappropriate. The conventional
multi-mode fiber supports a large number of modes and the high
degree of singlemode filtering then required to ensure reciprocity
becomes impractical because it extracts and sends to the detector
only a very small fraction of the available light. I have found,
however, that the required filtering can be incorporated without
unacceptably degrading the detected light levels if the sensing
loop fiber supports only a small number of modes, and if
mode-conversion means in the sensing loop ensures that a reasonable
fraction of the light in any undesired modes is returned to the
desired mode for detection.
These and other objects, features, and advantages of the present
invention will become more apparent in light of the following
detailed description of a best mode embodiment thereof, as
illustrated in the accompanying Drawing.
BRIEF DESCRIPTION OF DRAWING
The sole Drawing FIGURE is a system block diagram of a rotation
sensor according to the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
As known, the minimum reciprocal configuration Sagnac
interferometers yield enhanced FOG performance by minimizing output
errors associated with undesired polarizations and spatial modes.
Prior art high performance FOG systems have typically required, in
addition, the use of low coherence superluminescent diode (SLD)
light sources and highly birefringent (polarization preserving)
sensing loop fiber to further reduce these errors. The prices,
however, of the SLD and polarization preserving fiber are a
significant portion of the overall sensor cost.
The present invention comprises a minimum reciprocal configuration
Sagnac interferometer, modified to permit use of a low cost laser
diode optical source (e.g., at 0.8 micron wavelength) and low cost
non-polarization-preserving sensing coil fiber (e.g.. fiber which
is single-moded at 1.3 microns but may be moderately multi-moded at
the source wavelength). The minimum reciprocal configuration in
combination with added components described hereinafter, produces a
rotation sensor suitable for use in intermediate grade FOGs but at
a significantly lower cost than the prior art devices.
Referring to FIG. 1, an interferometer according to the present
invention 10, includes an integrated optic chip (IOC) 12, a light
source 14, light detection circuitry 16, an output tap 18 (which
may be a directional coupler or a beam splitter), a fiber optic
sensing coil 20, and control circuitry 22. As described in detail
hereinafter, the present interferometer further includes a
depolarizer 23 and a mode converter 24 connected between the IOC 12
and the sensing loop 20. An optional polarizer 25 (shown in
phantom) may be located between the light source and the IOC.
The light source 14 comprises a multi-mode low coherence (wide
bandwidth) 0.8 micron wavelength laser diode. It is desirable in an
interferometric FOG to use an optical source with a wide line width
(low coherence) and high power coupling into an optical fiber.
Superluminescent diodes (SLDs) have been used extensively in high
performance FOG systems because they offer a good compromise
between power coupling and line width. SLDs, however, are very
expensive due to low volume production. Alternatively, multi-mode
laser diodes are available with 2-3 nanometer bandwidths (about
one-fifth that of SLDs but sufficient to ensure reasonably low
coherence) and with coupled optical output powers approaching one
milliwatt, but at less than one tenth the cost of the SLD. This
type of laser diode has been selected as the best cost to
performance trade-off for the interferometer light source.
The source provides the light beam on output fiber 26 to the tap
18. The tap rejects a portion of the light (e.g., 50%), which may
be used for other purposes, and transmits the remainder through
waveguide 28 to the IOC 12. The waveguide 28 is an optical fiber
which is single-moded at the source wavelength. The IOC, which also
operates single-moded at 0.8 microns, is formed using a two step
proton exchange technique described in a commonly owned, copending
application entitled Single-Polarization, Inteqrated Optical
Components for Optical Gyroscopes, S/N 329,121, filed Mar. 27, 1989
by Suchoski et al.
The IOC includes a single polarization filter 30 and a single
spatial mode filter 32 formed in a waveguide section 33. The
polarization filter extinction ratio is on the order of 60 dB. The
waveguide 33 is the "common path" for propagating the source
optical signal to the sensing coil 20 and for guiding return
propagation of the interference signal from the coil to the
detection cicuitry 16. The spatial-mode filter ensures that only
selected spatial mode light enters the sensing coil and only
selected mode optical power is coupled back from the loop to the
detector.
The filtered optical signal approaching the sensing loop is
presented to a beam splitter/combiner 34, e.g. either a Y-junction
or a 3 dB directional coupler, which divides the source optical
signal into two equal intensity optical signals presented on
waveguide sections 35, 36. In the best mode embodiment a phase
("dither") modulator 37 and a serrodyne modulator 38 (each
described hereinafter) are connected to the guide sections 35, 36,
respectively. The modulated optical signals at IOC connections 39,
40 are presented through the depolarizer 23 and mode conversion
means 24 to the opposite ends 42, 44 of the sensing loop.
After circulating through the sensing loop, light returns toward
the source, being combined at splitter/combiner 34 into an
interference signal which then proceeds back along the common path
guide 33, through the mode filter and polarizer, to tap 18 which
extracts a portion (e.g., 50%) of the signal and couples it through
path 44 to detection circuitry 16, the remainder of this signal
being guided toward the source and effectively lost. The path 46
may be an optical fiber which may be single moded at the source
wavelength. The detection circuitry may include a known PIN-diode
transimpedance amplifier detection system.
In the best mode embodiment, a closed loop serrodyne modulation
technique is used to measure the Sagnac phase difference. This
makes the measurement insensitive to intensity variations resulting
from fluctuations of the light source or from multi-mode
transmission of the light through the sensing loop. The serrodyne
modulator 38 applies a linear ramped phase modulation to each of
the counter circulating light beams. If the ramp peak amplitude is
2.pi. radians and the flyback is essentially instantaneous, the
serrodyne modulation, acting on the two signals at different times
due to optical delay in the coil, adds an effectively constant bias
to the differential phase. The bias can be controlled by a servo
loop within the control circuitry 22 to continuously oppose, and
null, the Sagnac phase difference. The serrodyne frequency then
constitutes a gyro output proportional to the loop rotation
rate.
The phase modulation of the beams is provided by the dither
modulator 37, which causes the interference signal amplitude to
dither. This allows for AC detection of the differential phase. The
dither amplitude is at a maximum when the modulation frequency is
equal to the eigenfrequency of the fiber sensing coil. This
modulation frequency also offers other known advantages in reducing
certain types of FOG measurement errors.
In the best mode embodiment the sensing coil fiber is a 1.3 micron,
non-polarization preserving, single mode fiber. The fiber was
selected because of its commercial availability and low cost. The
cost per meter is approximately one-fifth that of non-polarization
preserving single mode fiber designed to operate at 0.8 microns,
and less than 1/20th the cost per meter of 1.3 micron polarization
preserving (high birefringence) fiber.
Multiple modes (typically two to five) may exist in the 1.3 micron
fiber when operating with 0.8 micron optical signals. In this case
it is conceivable that substantially all of the selected mode
optical power might become converted to an undesired mode during
propagation through the loop, leaving no selected mode light for
return to the detector. To prevent this it is necessary to
functionally incorporate mode converter (or "mode-scrambler") means
at one or both ends of the loop. This ensures that some light from
any existing mode will couple into the desired mode before leaving
the loop. The light so coupled will pass through the selected-mode
filter 32 to the detector 16.
The mode conversion feature may be accomplished by control of the
sensing coil geometry, i.e. by controlling the coil diameter and
the fiber winding technique. The range of acceptable diameter
values is not critical. Small diameter values tend to be associated
with high winding stresses and fiber distortions which enhance the
spatial mode cross coupling (scrambling). At the same time, a small
diameter increases the attenuation of high order modes and so may
ultimately lead to single mode transmission through the coil,
obviating the need for scrambling. However too small a diameter
(e.g., less than about 2 cm) may also unacceptably increase the
attenuation for even the desired mode.
Subject to physical packaging limits, a large diameter coil
provides greater FOG sensitivity, However, a large diameter (e.g.,
greater than 8 cm), combined with a smooth winding technique with
controlled fiber cross-overs, may yield little or no spatial mode
selection or conversion. In this case it may be necessary to
incorporate a separate, discrete mode scrambling means comprising
any one of the known scrambler configurations, such as a serpentine
series of small random fiber bends at one end of the fiber
coil.
In laboratory experiments using a 16 cm diameter 180 meter long
random-wound coil of conventional single-mode 1.3 micron
communications fiber it was found unnecessary to include a separate
mode scrambler, suggesting that fiber crossovers in such a coil may
provide adequate mode mixing.
The use of non-polarization-preserving single mode fiber in the
best mode embodiment also increases the chance of environmentally
sensitive polarization mode coupling into undesired polarizations.
To prevent the possibility that all light might couple out of the
desired polarization (polarization fading), the depolarizing means
23 may be included at one or both ends of the sensing loop. When a
low coherence light source is used, a depolarizer for this purpose
may, for example, comprise a short length of high birefringence
(polarization preserving) fiber connected with properly orientated
polarization axes relative to the IOC, using methods known to those
skilled in the art. The anti-fading action of the depolarizer is
analogous to that of the mode scrambler described above. It ensures
that some light will always return to the detector in the desired
polarization. However, it also ensures that a similar amount of
light will return toward the detector in the undesired
polarization, so the extinction ratio of the polarizer needed to
block this light must be higher than if substantially all returning
light were in the desired polarization state.
The extinction ratio of the IOC polarization filter 30 is on the
order of 60 dB. It is not now known whether larger extinction
coefficients can be obtained by increasing the length of the IOC
waveguide 33, or whether there is a performance limit at 60 dB
beyond which a single-substrate IOC polarizing filter fails to
improve with length. Such a limit might result from re-entry of
previously rejected light into the primary waveguide.
Tests have shown, however, that polarization related errors in a
Sagnac interferometer employing a proton exchanged IOC based on a
lithium niobate substrate and having an effective extinction
coefficient of about 60 dB can be substantially reduced by
inserting a supplemental polarizer just ahead of the input to the
IO circuit. Polarization-related errors in the output of our
experimental gyro were typically less than 10 degrees/hour
equivalent rotation rate when only the IOC polarizer was used.
However, when a commercial prism type polarizer specified to have a
60 dB extinction coefficient was inserted ahead of the IO
polarizer, the errors decreased to less than 1 degree/hour.
Depending on the performance accuracy required for a particular
application, it may be preferable to include the supplemental
polarizer 25, shown in series with the IOC 12 in FIG. 1. This
configuration effectively separates the two polarizers and achieves
enhanced polarization extinction. The supplemental polarizer need
not be of the prism type. It may be an additional,
separate-substrate IOC polarizinq element, or any other known type
of fiber polarizer.
Test results have shown that the present rotation sensor exhibits
output noise and drift no larger than a few degrees/hour,
indicating that gyros constructed in accordance with the
above-described ideas may be useful in a variety of applications
where cost is important and moderate performance levels are
required.
It should be understood that the source and sensing fiber
wavelengths need not be limited to 0.8 and 1.3 microns. Any
relatively short wavelength light source may be used in combination
with any relatively long wavelength sensing fiber. For example, the
source wavelength may range from 750 to 900 nanometers and the
design wavelength of the fiber may range from 1200 to 1600
nanometers.
Similarly, although the invention has been shown and described with
respect to a best mode embodiment thereof, it should be understood
by those skilled in the art that the foregoing and various other
changes, omissions, and additions in the form and detail thereof,
may be made therein without departing from the spirit and score of
this invention.
* * * * *