U.S. patent application number 13/441272 was filed with the patent office on 2013-10-10 for fiber optic position sensing system.
This patent application is currently assigned to Teledyne Scientific & Imaging, LLC. The applicant listed for this patent is Milind Mahajan, John E. Mansell, Graham J. Martin. Invention is credited to Milind Mahajan, John E. Mansell, Graham J. Martin.
Application Number | 20130265583 13/441272 |
Document ID | / |
Family ID | 49292067 |
Filed Date | 2013-10-10 |
United States Patent
Application |
20130265583 |
Kind Code |
A1 |
Mansell; John E. ; et
al. |
October 10, 2013 |
FIBER OPTIC POSITION SENSING SYSTEM
Abstract
A fiber optic sensing system for determining the position of an
object requires a light source, an optical fiber, a fiber optic
splitter, a fiber tip lens, an optical detector and signal
processing circuitry. Light emitted by the light source is conveyed
via optical fiber and the splitter to the lens and onto an object,
such that at least a portion of the light is reflected by the
object and conveyed via fiber and the splitter to the detector.
Signal processing circuitry coupled to the detector determines the
position of the object with respect to the lens based on a
characteristic of the reflected light. The system is suitably
employed with a hydraulic accumulator having a piston, the position
of which varies with the volume of fluid in the accumulator, with
the system arranged to determine the position of the piston, from
which the volume can be calculated.
Inventors: |
Mansell; John E.; (Thousand
Oaks, CA) ; Mahajan; Milind; (Thousand Oaks, CA)
; Martin; Graham J.; (Woodland Hills, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mansell; John E.
Mahajan; Milind
Martin; Graham J. |
Thousand Oaks
Thousand Oaks
Woodland Hills |
CA
CA
CA |
US
US
US |
|
|
Assignee: |
Teledyne Scientific & Imaging,
LLC
|
Family ID: |
49292067 |
Appl. No.: |
13/441272 |
Filed: |
April 6, 2012 |
Current U.S.
Class: |
356/482 ;
356/614 |
Current CPC
Class: |
G01S 7/4868 20130101;
G01B 9/02002 20130101; G01S 7/4865 20130101; G01D 5/266 20130101;
G01B 9/0201 20130101; G01S 7/4818 20130101; G01B 2290/35 20130101;
G01D 5/34 20130101; G01B 9/02049 20130101; G01B 11/026 20130101;
G01B 11/14 20130101; G01D 5/268 20130101; G01B 9/02028 20130101;
G01S 17/10 20130101 |
Class at
Publication: |
356/482 ;
356/614 |
International
Class: |
G01B 9/02 20060101
G01B009/02; G01B 11/14 20060101 G01B011/14 |
Claims
1. A fiber optic sensing system for determining the position of an
object, comprising: a light source; optical fiber; a fiber optic
splitter; a fiber tip lens; an optical detector; and signal
processing circuitry; said system arranged such that light emitted
by said light source is conveyed via optical fiber and said
splitter to said fiber tip lens and onto an object, the position of
which is to be determined, and such that at least a portion of said
light conveyed onto said object is reflected by said object and
conveyed via optical fiber and said splitter to said optical
detector; said signal processing circuitry coupled to said optical
detector and arranged to determine the position of said object with
respect to said fiber tip lens based at least in part on a
characteristic of said reflected light.
2. The system of claim 1, wherein said light source is a laser.
3. The system of claim 1, wherein said fiber tip lens is a
collimator.
4. The system of claim 1, wherein said light conveyed onto said
object is reflected by means of specular reflection.
5. The system of claim 1, wherein said light conveyed onto said
object is reflected by means of retroreflection.
6. The system of claim 1, further comprising a retroreflective
surface affixed to said object such that said light conveyed onto
said object impinges on said retroreflective surface and is
reflected by means of retroreflection.
7. The system of claim 1, wherein said object is a piston or a
bladder.
8. The system of claim 1, wherein said light source is arranged to
emit pulses of light, said signal processing circuitry arranged to
measure the time required for a given pulse to travel from said
light source to said object and back to said optical detector, said
time varying with the distance of said object from said fiber tip
lens.
9. The system of claim 1, wherein said signal processing circuitry
is arranged to compare the intensity of said light emitted by said
light source with the intensity of said light reflected by said
object, the difference between said intensities varying with the
distance of said object from said fiber tip lens.
10. The system of claim 1, wherein said system components form a
Michelson interferometer in which the light emitted by said light
source is split by said fiber optic splitter into a component that
is conveyed to said object and a component which is conveyed via an
optical fiber to a reflective surface, and such that light
reflected by said object and light reflected by said reflective
surface are recombined such that the resulting interference pattern
varies with the distance of said object from said fiber tip
lens.
11. The system of claim 10, wherein said light source is a laser
and one of the arms of the interferometer has a fixed length.
12. The system of claim 11, further comprising a phase modulator
arranged to modulate the phase of said light emitted by said laser,
said signal processing circuitry coupled to said phase modulator
and arranged to adjust the frequency and/or the amplitude of said
phase modulation such that the position of said object with respect
to said fiber tip lens can be determined based on the interference
pattern registered on said optical detector.
13. The system of claim 12, wherein said signal processing
circuitry is arranged to determine the amplitude of the first
harmonic component .omega. present in the output of said optical
detector, and to adjust the frequency and/or the amplitude of said
phase modulation such that the amplitude of said first harmonic
component is made equal to zero.
14. The system of claim 10, wherein said light emitted by said
light source is spectrally broadband with a short coherence length,
and the length of one of the arms of the interferometer is arranged
to be adjustable.
15. The system of claim 1, further comprising a time-multiplexing
means such that the positions of a plurality of objects can be
determined using a single light source, single fiber optic
splitter, single optical detector, and single signal processing
circuit.
16. The system of claim 15, wherein said time-multiplexing means is
an optical splitter/coupler or a sequential optical switch.
17. The system of claim 15, wherein said fiber optic splitter is an
optical circulator.
18. The system of claim 1, wherein said light source is a
continuous laser, said system further comprising: an
electro-optical modulator connected between said fiber optic
splitter and said fiber tip lens and arranged to allow pulses of
laser light to propagate through said modulator toward said object
and reflected pulses of laser light to propagate from said object
toward said splitter at a frequency determined by said signal
processing circuitry; said signal processing circuitry coupled to
said optical detector and arranged to adjust said frequency so as
to maximize the light measured by said detector, said frequency
varying with the distance of said object from said fiber tip
lens.
19. The system of claim 18, further comprising a semiconductor
optical amplifier (SOA) arranged to amplify said reflected pulses
of laser light prior to their being conveyed to said optical
detector.
20. The system of claim 19, further comprising a mirror arranged to
reflect light from said SOA back through the SOA such that said
reflected pulses of laser light are amplified twice before being
conveyed to said optical detector.
21. The system of claim 20, wherein said electro-optical modulator,
said signal processing circuitry, said SOA and said mirror form an
electro-optical servo loop which automatically adjusts the pulsing
rate of said electro-optical modulator to maximize the signal on
said detector and so that light can pass in both directions through
said modulator.
22. The system of claim 18, wherein said SOA is arranged to operate
in continuous mode.
23. The system of claim 18, wherein said SOA is arranged to operate
in a pulsed mode.
24. The system of claim 18, wherein said signal processing
circuitry includes a servoed driver circuit which pulses said
modulator at a rate commensurate with the return time for the light
going to the object and back to the modulator.
25. The system of claim 1, wherein said object is within an
accumulator and has an associated position which varies with the
volume of fluid contained within said accumulator.
26. The system of claim 25, wherein said object is a piston or a
bladder having an associated surface which varies with the volume
of fluid contained within said accumulator, said system arranged
such that said light conveyed onto said object is conveyed onto
said surface.
27. A fiber optic sensing system for determining the position of an
object, comprising: a laser; a phase modulator arranged to modulate
the phase of said light emitted by said laser; optical fiber; a
fiber optic splitter; a fiber tip lens; an optical detector; and
signal processing circuitry coupled to said optical detector and to
said phase modulator; said system components arranged to form a
Michelson interferometer in which the phase-modulated light is
split by said fiber optic splitter into a component that is
conveyed to said object and a component which is conveyed via an
optical fiber to a reflective surface, and such that light
reflected by said object and light reflected by said reflective
surface are recombined such that the resulting interference pattern
varies with the distance of said object from said fiber tip lens;
and said signal processing circuitry arranged to adjust the
frequency and/or the amplitude of said phase modulation such that
the position of said object with respect to said fiber tip lens can
be determined based on the interference pattern registered on said
optical detector.
28. The system of claim 27, wherein said signal processing
circuitry is arranged to determine the frequency of the first
harmonic component .omega. present in the output of said optical
detector, and to adjust the frequency and/or the amplitude of said
phase modulation such that the amplitude of said first harmonic
component is made equal to zero.
29. The system of claim 27, further comprising an isolator between
said laser and said phase modulator.
30. The system of claim 27, wherein said laser has a coherence
length compatible with the differential leg length of said
Michelson interferometer.
31. A sensor for determining the volume of an accumulator,
comprising: a laser; optical fiber; a fiber optic splitter; a fiber
tip lens positioned such that laser light emanating from said lens
impinges on a surface, the position of which varies with the volume
of an accumulator; an optical detector; and signal processing
circuitry; said components arranged such that laser light emitted
by said laser is conveyed via optical fiber and said splitter to
said fiber tip lens and onto said surface, and such that at least a
portion of said light conveyed onto said surface is reflected by
said surface and conveyed via optical fiber and said splitter to
said optical detector; said signal processing circuitry coupled to
said optical detector and arranged to determine the position of
said surface with respect to said fiber tip lens based at least in
part on a characteristic of said reflected light.
32. The sensor of claim 31, wherein said surface is a surface of a
piston or a bladder.
33. The sensor of claim 31, wherein said accumulator is a hydraulic
accumulator.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to position sensing systems
and, more particularly, to position sensing systems employing fiber
optics.
[0003] 2. Description of the Related Art
[0004] It is often necessary to determine the position of a system
component which moves when in operation. For example, the position
of a piston within a hydraulic accumulator can be used to determine
the volume of fluid within the accumulator.
[0005] There are many ways in which position can be determined. For
example, in a hydraulic accumulator system as described above, an
electrical limit switch within the accumulator might be triggered
by an end cap on the moving piston. Alternatively, the position
might be determined using a camera trained on the piston, or by
using ultrasonic or magnetic means. However, measurement devices
installed within the accumulator in this way may prove to be
unreliable in an environment that may corrosive or subject to
excessive vibration.
SUMMARY OF THE INVENTION
[0006] A fiber optic sensing system for determining the position of
an object is presented, which overcomes the problems noted
above.
[0007] The present system requires a light source, an optical
fiber, a fiber optic splitter, a fiber tip lens, an optical
detector and signal processing circuitry. These components are
arranged such that light emitted by the light source is conveyed
via optical fiber and the fiber optic splitter to the fiber tip
lens and onto an object, the position of which is to be determined,
such that at least a portion of the light conveyed onto the object
is reflected by the object and conveyed via optical fiber and the
splitter to the optical detector. Signal processing circuitry
coupled to the optical detector determines the position of the
object with respect to the fiber tip lens, based at least in part
on a characteristic of the reflected light.
[0008] The light source is preferably a laser, and the position of
the object might be determined using any of several different
characteristics of the reflected light. For example, the light
source could be arranged to emit pulses of light, and the signal
processing circuitry arranged to measure the time required for a
given pulse to travel from the light source to the object and back
to the optical detector; the measured time will vary with the
distance of the object from the fiber tip lens. The signal
processing circuitry might also be arranged to compare the
intensity of the light emitted by the light source with the
intensity of the light reflected by the object; here, the
difference between the intensities varies with the distance of the
object from the fiber tip lens.
[0009] Another possible embodiment has the system components
arranged to form a Michelson interferometer in which the light
emitted by the light source is split by the optical splitter into a
component that is conveyed to the object and a component which is
conveyed via an optical fiber of fixed length to a reflective
surface. Light reflected by the object and light reflected by the
reflective surface are then recombined, such that the resulting
interference pattern varies with the distance of the object from
the fiber tip lens.
[0010] The present system is suitably employed with a hydraulic
accumulator having a piston that varies with the volume of
hydraulic fluid contained in the accumulator. The fiber optic
sensing system determines the position of the surface of the
piston, from which the volume can be calculated.
[0011] These and other features, aspects, and advantages of the
present invention will become better understood with reference to
the following drawings, description, and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a block diagram of a fiber optic sensing system in
accordance with the present invention.
[0013] FIG. 2 is a block diagram of an embodiment of a fiber optic
sensing system in accordance with the present invention which
employs a "time of flight" measurement method.
[0014] FIG. 3 is a block diagram of an embodiment of a fiber optic
sensing system in accordance with the present invention which
employs a "loss measurement" measurement method.
[0015] FIG. 4 is a block diagram of an embodiment of a fiber optic
sensing system in accordance with the present invention which
employs an "interferometric" measurement method.
[0016] FIG. 5 is a block diagram of an embodiment of a fiber optic
sensing system in accordance with the present invention which
employs another possible "interferometric" measurement method in
which the laser light is phase modulated.
[0017] FIG. 6 is a block diagram of an embodiment of a fiber optic
sensing system in accordance with the present invention which
employs another possible "interferometric" measurement method.
[0018] FIG. 7 is a block diagram of an embodiment of a fiber optic
sensing system in accordance with the present invention which
includes a multiplexing feature.
[0019] FIG. 8 is a block diagram of another embodiment of a fiber
optic sensing system in accordance with the present invention which
includes a multiplexing feature.
[0020] FIG. 9 is a block diagram of an embodiment of a fiber optic
sensing system in accordance with the present invention which
employs an electro-optical modulator.
DETAILED DESCRIPTION OF THE INVENTION
[0021] FIG. 1 is a block diagram of a fiber optic sensing system
for determining the position of an object in accordance with the
present invention. The system includes a light source 10, optical
fibers 12, 14, 16, 18, a fiber optic splitter 20, an optical
detector 22, and signal processing circuitry 24. The components are
arranged such that light emitted by light source 10 is conveyed via
optical fiber (here, fibers 12 and 14) and optical splitter 20 to a
fiber tip lens 26 and onto an object 28, the position of which is
to be determined. The fiber tip lens is arranged such that at least
a portion of the light conveyed onto the object is reflected by the
object and conveyed via optical fiber (here, fibers 14 and 18) and
splitter 20 to optical detector 22, which detects the reflected
light. Signal processing circuitry 24 is coupled to the output 23
of optical detector 22 and arranged to determine the position of
object 28 with respect to the fiber tip lens, based at least in
part on a characteristic of the reflected light. The unused branch
of splitter 20 (fiber 16) should end at a termination 30.
[0022] The object 28 may be anything that is capable of or can be
made capable of reflecting light, such as a valve component or the
surface of a fluid. The application illustrated in FIG. 1 (and all
subsequent figures) is that of a hydraulic accumulator 31, in which
the position of a piston or bladder 28 (referred to herein as a
piston) varies with the volume of fluid in the accumulator. Here,
light from fiber tip lens 26, which is installed in a feed-through
in the accumulator housing, is reflected off of the top surface 32
of the piston to determine its position with respect to the lens.
Assuming that 1) the fiber tip lens is mounted in a fixed location
on the accumulator, 2) the piston serves as the top of the
accumulator, and 3) the accumulator's other dimensions are known,
determining the position of the piston with respect to the lens
enables the volume of fluid within the accumulator to be
calculated.
[0023] The present system provides a number of advantages over
prior art systems. For example, the system provides a direct
measurement of position passively--i.e., without requiring that any
electronic components be near the object or, as here, within the
accumulator. This also makes this approach corrosion-resistant, and
enables the system to operate despite being subjected to, for
example, electromagnetic or magnetic interference, vibration
including ultrasonic vibration, heat and high pressures.
[0024] Light source 10 is preferably a laser with a long coherence
length, and fiber tip lens 26 is preferably a collimator. Light
conveyed onto the object may be scattered by the object surface on
which it impinges (and/or by contaminants or fluids between the
lens and the object), or reflected by means of specular reflection.
The surface might also be retroreflective, or a retroreflective
surface such as a retroreflective tape (40 in FIG. 2, discussed
below) may be affixed to the object such that light conveyed onto
the object impinges on the retroreflective surface and is reflected
by means of retroreflection; this tends to improve the system's
signal-to-noise ratio. The optical fibers are preferably
bi-directional; this is essential for fiber 14.
[0025] One possible characteristic of the reflected light that can
be used to determine position is the time required for light to
travel from the light source to the detector. As illustrated in
FIG. 2, this "time of flight" measurement method may be
accomplished by having light source 10 arranged to emit pulses of
light 42, with signal processing circuitry 24 arranged to measure
the time required for a given pulse to travel from light source 10
to object 28 and back to optical detector 22. The transit time of
this pulse will vary with the distance of object 28 from fiber tip
lens 26, and thus can be used to determine the object's position
with respect to the lens. Since the transit time will typically
vary only slightly with position, the use of a narrow pulse and
fast measurement equipment, such as that involving electro-optic
intensity modulator devices, which can operate at bandwidths of
several tens of Gigahertz, are recommended when highly accurate
results are needed.
[0026] Another possible characteristic of the reflected light that
might be used to determine position is intensity. As illustrated in
FIG. 3, for this "loss measurement" measurement method, signal
processing circuitry 24 is arranged to compare the intensity of the
light emitted by light source 10 with the intensity of the light
reflected by object 28, with the difference between the intensities
varying with the distance of the object from fiber tip lens 26.
This can be accomplished by, for example, coupling a light
intensity monitor 50 to the end of optical fiber 16, which serves
to measure the intensity of the light emitted by light source 10
and thus provide a reference signal 52 to be compared to the
intensity of the reflected light. The reference signal output 52 of
monitor 50 and the output 23 from detector 22 are fed to signal
processing circuitry 24, so that the comparison of intensities can
be performed and the position of object 28 determined based on the
results.
[0027] The position of object 28 might also be determined by
"interferometric" means, which tend to provide highly accurate
measurements; one possible embodiment is shown in FIG. 4. For this
measurement method, the system components form a Michelson
interferometer in which the light emitted by light source 10--which
must be a laser with a coherence length compatible with the
differential leg length of the Michelson interferometer--is split
by optical splitter 20 into a component that is conveyed to object
28 and a component which is conveyed via an optical fiber (here,
fiber 16) to a reflective surface 60 such as a mirror. Object 28
preferably includes a retro-reflecting surface 40, which may be
necessary to obtain an adequate signal-to-noise ratio. Light
reflected by object 28 and light reflected by reflective surface 60
are recombined such that the resulting interference pattern varies
with the distance of the object from fiber tip lens 26. For this
implementation of the interferometric method, light source 10 is a
laser, one of the arms of the interferometer--here, optical fiber
16--has a fixed length, and the other arm of the
interferometer--consisting of fiber 14, lens 26, and the distance
between lens 26 and object 28, has a length which varies with the
distance between lens 26 and object 28, thereby causing the
interference pattern to vary with the distance of the object from
the fiber tip lens. The signal processing unit 24 preferably has
the capability and bandwidth for counting interference fringes as
they pass over detector 22; the number of fringes counted will
provide a digital readout of the magnitude of any movement of the
piston 28.
[0028] Another possible implementation that uses an interferometric
measurement method is shown in FIG. 5. Here, a phase modulator 70
is inserted between laser 10 (and preferably an isolator 84) and
optical splitter 20 and arranged to sinusoidally modulate the phase
of the light emitted by the laser, by means of a fiber stretcher or
lithium niobate crystal, for example. One branch (14) of splitter
20 is routed to the object 28 via lens 26, where it is preferably
reflected with retro-reflecting tape 40; this light path forms one
leg of a Michelson interferometer. The other branch (16) is routed
to a reflective surface 60 such as a Faraday mirror, and serves as
the second leg of the Michelson interferometer. Light reflected
from the object is conveyed to detector 22 via splitter 20 and
fiber 18. Here, the signal processing circuitry includes a lock-in
amplifier 72 and an "A/.omega. servo control" circuit 74, which can
servo the amplitude A of the modulation or the frequency to of the
modulation (or both).
[0029] The return light is split and interferes with the light from
the second leg of the interferometer, resulting in an interference
signal which is registered on detector 22. The output of detector
22 is provided to lock-in amplifier 72, which extracts the
amplitude of the frequency component .omega. (first harmonic) of
the detected interference signal and sends it to A/.omega. servo
control circuit 74.
[0030] The amplitude of the first harmonic (at .omega.) of the
detector output signal is proportional to the first order Bessel
function J.sub.1(.beta.), where the modulation index .beta. is
given by:
.beta. = n A .omega. .DELTA. L c ##EQU00001##
where n=the refractive index of the glass fiber, A=the amplitude of
the phase modulation, .omega.=the frequency of the phase
modulation, .DELTA.L=the interferometer differential path length,
and c=the velocity of light in vacuo. Thus, the first-harmonic
amplitude depends linearly on the interferometer leg length (which
varies with the position of object 28), the modulation amplitude
and the modulation frequency. The modulation depth .beta. is
suitably chosen to be such that this first-harmonic amplitude is
zero (which occurs for .beta.=3.832). Then, if the interferometer
leg length changes, the values of the modulation frequency or
modulation amplitude or both can be changed to compensate.
[0031] When so arranged, servo circuit 74, which preferably
controls both the amplitude A and the frequency .omega. of the
phase modulation, adjusts either the modulation frequency or
amplitude (or both) to make the first harmonic component equal to
zero in amplitude. It is preferred to adjust the modulation
frequency, which is easier to monitor digitally with a counter. The
value of this modulation frequency (or modulation amplitude, if
that is varied) represents the position of the object.
[0032] Another possible implementation that uses an interferometric
measurement method is shown in FIG. 6. This is similar to the
configuration shown in FIG. 4, except that the length of one of the
arms of the interferometer--here, optical fiber 16--is arranged to
be adjustable as needed to obtain a desired interference pattern.
For this case, the length of fiber 16 needed to obtain the desired
interference pattern varies with the distance of the object from
fiber tip lens 26. For this implementation, the light source is
preferably spectrally broadband, with a short coherence length. The
adjustable length fiber might be provided by any number of means.
For example, fibers having different fixed lengths might be
provided as shown in FIG. 6, each of which can be selectively
switched into fiber 16 to vary the fiber length. Another
possibility would be to affix fiber 16 around a PZT cylinder such
that a voltage applied to the cylinder causes the length of fiber
16 to vary, or for longer length variations, to incorporate a
collimated free-space section that could be varied in length.
[0033] The present fiber optic sensing system might also include a
time-multiplexing means such that the positions of a plurality of
objects can be determined using a single light source, a single
fiber optic splitter, a single optical detector, and a single
signal processing circuit. One possible implementation is shown in
FIG. 7. This configuration is similar to that shown in FIG. 1,
except that optical fiber 14 is coupled to a multiplexing circuit
80 such as a 1.times.X splitter/coupler or a 1.times.X sequential
optical switch; X is 7 in FIG. 7 (and in FIG. 8), but could be any
desired number. Then, the ports 82 of multiplexing circuit 80 could
be coupled to respective fiber lens tips to measure respective
positions one at a time. The system might also include an isolator
84, to prevent light reflecting back into the laser cavity and
causing instabilities.
[0034] One problem with a system as shown in FIG. 7 is that,
between the light that ends up at termination 30 and the reflected
light that ends up directed back towards laser 10 instead of
detector 22, perhaps 50% of the light provided by laser 10 is
wasted. The embodiment shown in FIG. 8 helps to overcome these
problems. Here, the optical splitter is implemented with an optical
circulator 90 as shown. With this arrangement, there is nominally
no wasted light. One possible application of a multiplexed system
as discussed above might be, for example, an oil rig that has
multiple blowout preventers, each of which includes a piston that
needs to be monitored. A single multiplexed system as discussed
herein could be used to monitor the positions of each piston, in a
recurring sequence.
[0035] Another possible embodiment, which employs an
electro-optical modulator to facilitate a "time of flight"
measurement approach, is shown in FIG. 9. Laser 10, preferably a
continuous laser, emits light that is routed via fiber 12 and
optical splitter 20 to an electro-optical modulator 100, and from
there via an optical fiber 102 to object 28; the system might also
include an isolator 103. The modulator is pulsed by a servoed
driver circuit 104 at a rate commensurate with the return time for
the light going to the object and back to the modulator. If the
timing is right, the returning light pulses will again pass through
modulator 100, and then preferably to a semiconductor optical
amplifier (SOA) 106 via one leg of a second optical splitter 108,
the other leg of which is terminated. A mirror 110 reflects the
light from the SOA back through the SOA; this double-pass through
the SOA amplifies the light, which then passes through splitter 108
to a detector 112. This arrangement creates an electro-optical
servo loop which automatically adjusts the pulsing rate for the
electro-optical intensity modulator 100 to maximize the signal on
detector 112 so that light can pass in both directions through the
modulator 100. In this manner the frequency of the modulator
pulsing, which can be acquired digitally at position 114 by pulse
counting, indicates the time-of-flight of the light and gives a
readout of the piston 28 position.
[0036] The length of optical fiber 102 can be adjusted to make the
light return time convenient for the electronic circuitry. If the
fiber length is, for example, 100 meters, then the nominal
frequency for the pulsing will be around 1 MHz. Then, assuming that
the maximum travel distance for object 28 is 60 cm, a 1 mm position
change for the object will correspond to a change in the modulator
frequency of about 1.7 kHz, which should be relatively easy to
detect. The frequency of the pulsing can be used to establish a
time window during which the system looks for a reflection, which
can be used to exclude unwanted returns to within the pulse length
governed by the bandwidth of modulator 100.
[0037] The ability to detect the maximum signal at the correct
frequency is improved as the length of the light pulse passing
through modulator 100 is made smaller. A pulse length value of 40
picoseconds is suitable for currently available electro-optical
modulators (30 GHz bandwidth). The SOA 106 can be operated in
continuous mode, or, if noise is a problem (possibly from leakage
through modulator 100), the SOA can be pulsed on and off.
[0038] The embodiments of the invention described herein are
exemplary and numerous modifications, variations and rearrangements
can be readily envisioned to achieve substantially equivalent
results, all of which are intended to be embraced within the spirit
and scope of the invention as defined in the appended claims.
* * * * *