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Claims  |
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I claim as my invention:
1. A data gathering system comprising:
an optical main fiber having a first and a second end and including a core
and cladding material;
means for transmitting into the first end of the main fiber an
interrogating light pulse which propagates in a first direction along the
main fiber;
a modulator formed on said main fiber, said modulator comprising an optical
fiber segment held substantially parallel to a first section of said main
fiber in a manner so that said segment and said main fiber are separated
by a distance which is variable in response to variations in an incident
external signal, said modulator further comprising means for diverting a
return portion of said interrogating light pulse having intensity which
varies in response to the distance between said segment and said main
fiber, so that the return portion propagates in the main fiber in a
direction opposite said first direction; and
means optically coupled to said main fiber for detecting said return
portion.
2. The invention of claim 1 wherein:
said segment comprises a second section of said main fiber; and
said return portion diverting means comprises a third section of said main
fiber having a first end connected to said second section and a second end
connected to said first section, so that said return portion comprises a
portion of the interrogating pulse diverted from said first section to
said second section via evanescent coupling and a portion of the
interrogating pulse diverted from said second section to said first
section via evanescent coupling.
3. The invention of claim 2 wherein said external signal is an acoustic
signal, and said modulator further comprises:
a hollow structure defining a chamber containing air, which structure
encloses said first section and said second section and substantially
prevents liquid intrusion into the chamber, said structure comprising a
first area and a second area, said first area adapted to move relative to
said second area in response to acoustic vibrations incident on said
structure;
compliant cladding material positioned between said first section and said
second section;
a first pin positioned in said chamber in contact with said first area and
said first section; and
a second pin positioned in said chamber in contact with said second area
and said second section so that acoustic vibrations incident on the
structure will displace said first pin relative to said second pin and
vary the distance between said first section and said second section.
4. The invention of claim 2 wherein said external signal is an acoustic
signal, and said modulator further comprises:
a hollow structure defining a chamber containing air, which structure
encloses said first section and said second section and substantially
prevents liquid intrusion into the chamber, said structure comprising a
first area and a second area, said first area adapted to move relative to
said second area in response to acoustic vibrations incident on said
structure;
a layer of gel, having index of refraction substantially the same as that
of the cladding material of the optical main fiber, separating said first
section from said second section;
a first member positioned in said chamber in contact with said first
section and said first area; and
a second member positioned in said chamber in contact with said second
section and said second area so that acoustic vibrations incident on the
structure will displace said first member relative to said second member
and vary the distance between said first section and said second section.
5. The invention of claim 2 wherein said external signal is a voltage
signal, and wherein said return portion diverting means comprises:
a housing attached to said first section; and
a pusher element attached to the housing and to said second section in a
manner so that the pusher element moves said second section relative to
said first section in response to variations in the voltage signal, so as
to vary the distance between said first section and said second section in
response to variations in the voltage signal.
6. The invention of claim 2, wherein said modulator further comprises:
a sensor for producing a voltage signal in such a manner that a
characteristic of said voltage signal will vary in response to variations
in an external signal incident on said sensor;
a housing attached to said first section; and
a pusher element electrically coupled to the sensor, and attached to the
housing and to said second section in a manner so that the pusher element
moves said second section relative to said first section in response to
variations in the voltage signal, so as to vary the distance between said
first section and said second section in response to variations in the
voltage signal.
7. The invention of claim 1 wherein:
said segment is an optical fiber separate from said main fiber, said
optical fiber having a highly reflective, flat first end face oriented
substantially perpendicular to the axis of said optical fiber, and a
highly reflective, flat second end face oriented substantially
perpendicular to the axis of said optical fiber; and
said return portion comprises a portion of said interrogating pulse
diverted from said main fiber into said optical fiber via evanescent
coupling and then diverted via evanescent coupling from said optical fiber
back into said main fiber so as to propagate in said main fiber in the
direction opposite said first direction.
8. The invention of claim 7 wherein said external signal is an acoustic
signal, and said modulator further comprises:
a hollow structure defining a chamber containing air, which structure
encloses said optical fiber and said first section of the optical main
fiber and substantially prevents liquid intrusion into the chamber, said
structure comprising a diaphragm adapted to move relative to said first
section of the optical main fiber in response to acoustic vibrations
incident on said structure; and
a compliant member attached between said diaphragm and said first section,
and including a compliant layer having index of refraction substantially
the same as that of the cladding material of the optical main fiber, which
compliant layer is positioned between said optical fiber and said first
section so that acoustic vibrations incident on the structure will vary
the distance between said optical fiber and said first section.
9. The invention of claim 1 wherein said return signal detecting means
comprises a first transducer for converting at least part of the return
portion into a first electrical signal.
10. The invention of claim 9 further comprising:
a directional coupler optically coupled to said main fiber between said
first end of said main fiber and said modulator;
a first optical branch fiber optically coupled to the directional coupler
so that light propagating in said main fiber toward said modulator is
diverted by said directional coupler into said first optical branch fiber;
monitor means optically coupled to the first optical branch fiber for
detecting light diverted into said first optical branch fiber by said
directional coupler; and
a second optical branch fiber optically coupled to the directional coupler
and to the return signal detecting means so that the return portion
propagating in said main fiber is diverted by said directional coupler
through said second optical branch fiber to said return signal detecting
means.
11. The invention of claim 1 further comprising means attached to the
modulator for reducing the effect on the intensity of the return portion
due to change in the incident external signal which change has a rate of
change below a selected minimum rate.
12. A data gathering system employing time-division multiplexing,
comprising:
an optical main fiber having a first end and a second end;
means for transmitting into the first end of the main fiber, at a first
instant, an interrogating light pulse which propagates in a first
direction along the main fiber;
at least two modulators formed on said main fiber, each of which said
modulators comprises an optical fiber segment held substantially parallel
to a first section of said main fiber in a manner so that said segment and
said main fiber are separated by a distance which is variable in response
to variations in an incident external signal, each of said modulators
further comprising means for diverting a return portion of said
interrogating light pulse having intensity which varies in response to the
distance between said segment and said main fiber so that the return
portion propagates in the main fiber in a direction opposite said first
direction; and
means optically coupled to said main fiber for detecting the return portion
from each of said modulators.
13. The invention of claim 12 wherein:
said segment comprises a second section of said main fiber; and
said return portion diverting means comprises a third section of said main
fiber having a first end connected to said second section and a second end
connected to said first section, so that said return portion comprises a
portion of the interrogating pulse diverted from said first section to
said second section via evanescent coupling and a portion of the
interrogating pulse diverted from said second section to said first
section via evanescent coupling.
14. The invention of claim 12 wherein:
said segment is an optical fiber separate from said main fiber, said
optical fiber section having a highly reflective, flat first end face
oriented substantially perpendicular to the axis of said optical fiber,
and a highly reflective, flat second end face oriented substantially
perpendicular to the axis of said optical fiber; and
said return portion comprises a portion of said interrogating pulse
diverted from said main fiber into said optical fiber via evanescent
coupling and then diverted via evanescent coupling from said optical fiber
back into said main fiber so as to propagate in said main fiber in the
direction opposite said first direction.
15. The invention of claim 12 wherein said return portion detecting means
comprises a first transducer generating a first electric signal having
instantaneous amplitude which varies in response to the instantaneous
amplitude of the return portions detected by the return portion detecting
means.
16. The invention of claim 15 wherein said first electrical signal has
instantaneous amplitude substantially proportional to the instantaneous
amplitude of the return portions detected by the return portion detecting
means.
17. The invention of claim 15 further comprising:
a directional coupler optically coupled to said main fiber and positioned
so that the interrogating pulse will reach the directional coupler prior
to reaching any of the modulators;
a first optical branch fiber optically coupled to the directional coupler
so that a portion of the interrogating pulse propagating in said main
fiber toward said modulators is diverted by said directional coupler into
said first optical branch fiber;
monitor means optically coupled to the first optical branch fiber for
detecting the portion of the interrogating pulse diverted into said first
optical branch fiber by said directional coupler; and
a second optical branch fiber optically coupled to the directional coupler
and to the return portion detecting means so that the return portion
propagating in said main fiber is diverted by said directional coupler
through the second optical branch fiber to said return portion detecting
means.
18. The invention of claim 17 wherein the monitor means comprises a second
transducer for converting at least part of the portion of the interrogated
pulse diverted into said first optical branch fiber into a second
electrical signal, and further comprising:
signal processing means electrically coupled to the first transducer and to
the second transducer for converting said first electric signal and second
electrical signal into an array signal representing the total power of the
return portions returned from a selected subset of said at least two
modulators.
19. The invention of claim 18 wherein the signal processing means further
comprises:
means for generating a third electrical signal having amplitude at any
second instant after said first instant substantially proportional to the
time integral of said first electrical signal, integrated from said first
instant to said second instant; and
means for applying said third electrical signal to said first electric
signal so as to reduce the effect on said first electric signal due to
upstream modulator losses on the return portion from each of the
modulators in said selected subset, and to reduce the effect on said array
signal due to said upstream modulator losses.
20. The invention of claim 18 wherein the transmitting means is adapted to
transmit successively a plurality of substantially identical interrogating
pulses into the first end of the main fiber, and wherein the signal
processing means further comprises:
means for generating a fourth electrical signal having amplitude following
the peak amplitude of said second electrical signal; and
means for applying said fourth electrical signal to said first electrical
signal so as to reduce the effect on said first electrical signal due to
differences between the interrogating pulses and to reduce the effect on
said array signal due to differences between the interrogating pulses.
21. The invention of claim 20 wherein the signal processing means further
comprises:
means for sampling the first electric signal prior to application of said
third electric signal to said first electric signal during selected
periods when the data gathering system is in a quiet state;
means for holding the sampled signal; and
means, including a differential amplifier, for applying the held signal to
said first electric signal in a manner so as to reduce the effect on the
intensity of said first electric signal due to changes in the average
external conditions affecting the modulators between successive
interrogating pulse transmissions.
22. The invention of claim 12 further comprising means attached to each of
the modulators for reducing the effect on the intensity of the return
portion due to change in the external signal incident on the modulator
which change has a rate of change below a selected minimum rate. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates generally to systems employing time division
multiplexing for gathering data from two or more sensors and to sensors
and modulators used in such systems. Throughout this specification, the
term "sensor" will be used to denote a device for detecting a physical
phenomenon under test and directly converting the detected signal to a
modulated sensor output signal. Throughout this specification, the term
"modulator" will be used to denote the broad class of devices which
includes both "sensors" and devices (used in conjunction with "sensors")
that do not directly detect a physical phenomenon under test, but instead
receive the output signal of a sensor and convert such sensor output
signal into another type of modulated signal suitable for transmission.
More particularly, the present invention relates to modulators (and
sensors) capable of modulating the intensity of an interrogating light
signal in response to variations in an incident signal, and to time
division multiplexing data gathering systems comprising one or more arrays
of such modulators (or sensors).
The invention utilizes the effect known as "evanescent field coupling,"
whereby a portion of the electromagnetic energy injected into an optical
fiber segment is coupled to an adjacent optical fiber segment, with the
intensity of the coupled portion depending on the separation between the
two fiber segments. Signals incident on the modulator (and sensor)
disclosed herein cause displacement of a fiber segment through which an
interrogating light pulse propagates relative to another fiber segment to
produce a coupled return signal in the latter segment whose intensity
depends on the separation between the two segments at the instant the
interrogating pulse passes through the former segment.
In gathering data from a large number of sensors, two general types of
methods have been used. In the first, a wire pair is run from each sensor
to a data recording unit. In the second, some form of multiplexing is used
so that data from many sensors is impressed on a data bus consisting of a
single wire pair, coaxial cable, or optical cable. In practicing the
second type of method, a saving in wire (or other data transmission
material) and space for cable runs is realized. However, in practicing
conventional embodiments of such type of method, a significant amount of
electronic equipment has generally been required to digitize and encode
information from each sensor input location. In practicing the method of
the present invention, the advantages of multiplexing are obtained, and
the amount of electronic equipment required at each sensor-data bus
interface is reduced.
One important application for the present invention is in the field of
marine seismology. In marine seismology the most commonly employed
technique for obtaining geophysical data is the reflection seismograph
technique which typically requires the use of a large number of hydrophone
arrays connected to form what is known as a "marine streamer." The marine
streamer is towed behind a seismic vessel. The individual hydrophones may
be made up of a piezoelectric element which converts acoustic signals to
electrical signals. Marine streamers typically use electrical cables to
transmit such electrical signals from the submerged hydrophones to
instruments which display or record these signals on board the seismic
vessel.
A typical marine streamer may have 200 hydrophone arrays. Each array may be
15 meters long and may be made up of 17 hydrophones in parallel. Such a
marine streamer would be three kilometers long, would have 3400
hydrophones, and would require at least 400 wires running the length of
the electrical cable to connect each array with the vessel. In addition,
other wires would be needed for depth measurement, control, and other
purposes. The cable diameter necessary for accommodating such a large
number of wires would be about 3 inches.
Longer marine streamers are desirable, but extension of the apparatus
commonly used in the art would be awkward because of the need for
increased cable diameter to accommodate such increased length. Another
approach that has been taken utilizes a digital streamer. In this type of
system, the data from each array is digitized, multiplexed, and then
transmitted down a data bus to instruments on board the seismic vessel.
This digital streamer approach, although allowing smaller diameter
streamers, results in a more expensive system in the water, and usually
requires relatively large diameter electronics packages positioned at
various locations along the streamer which act as noise sources as the
streamer is dragged through the water.
Systems have been proposed which employ optical transducers for converting
acoustic vibrations incident on a device such as a hydrophone or geophone
into optical signals, and then into electrical signals. Such systems would
replace the conventional piezoelectric transducers with generally more
complex fiber optic transducers. The problem of transmitting many such
signals down the streamer remains the same.
One method of alleviating the problem of increased cable diameter is
through the use of optical fibers in place of the electrical wiring. Fiber
optic systems have been proposed which convert incident acoustic
vibrations into optical signals and maintain such optical signals in
optical form for transmission. Such previously proposed systems employ
couplers and lossy sensors which severely limit the number of signals
which practically can be handled.
U.S. Pat. No. 4,071,753, issued Jan. 31, 1978 to Fulenwider et al.
discloses several embodiments of an optical transducers which comprises a
source of optical power connected to one end of an input optical fiber,
means for varying the portion of optical power coupled between the other
end of the input optical fiber and one end of an output optical fiber in
response to oscillatory mechanical motion indicative of incident acoustic
vibrations. One embodiment of the Fulenwider et al. transducer, discussed
at column 6, lines 28 through 58, utilizes the effect known in the art as
"microbending" by applying a varying degree of bending to an optical fiber
to cause light propagated through the fiber to radiate away from the fiber
in the vicinity of the bend, thus decreasing the amount of optical power
transmitted through the bend as a function of its radius of curvature.
Fulenwider et al., however, neither discusses the effect of evanescent
field coupling between cores of adjacent optical fibers nor discloses any
optical transducer utilizing such effect.
Another type of fiber optic transducer mechanism relies on phase modulation
in a single mode fiber immersed in a fluid. The phase modulation in such a
system is due to changes in the optical length of the fiber induced by
sound waves propagating in the fluid. See, for example, J. A. Bucaro, H.
D. Dardy, and E. F. Carone, "Fiber-optic hydrophone", Journal Acoustic
Society of America, Vol. 62, No. 5, pp. 1302-1304, 1977.
A related optical transducer system is disclosed in U.S. Pat. No. 4,313,185
issued Jan. 26, 1982 to Chovan. Chovan discloses a hydrophone system
comprising a first and a second single mode optical fiber and means for
coupling light from the first fiber to the second fiber and from the
second fiber to the first fiber. The optical length of the optical
coupling path between the two fibers is modulated in response to acoustic
vibrations incident on the fibers. The phase and frequency of light
traversing the optical coupling path will vary with the optical length of
the path and the rate of change thereof, respectively. Chovan neither
discusses the effect of evanescent field coupling between cores of
adjacent optical fibers nor discloses any optical transducer utilizing
such effect.
U.S. Pat. No. 4,295,738, issued Oct. 20, 1981 to Meltz et al., discloses a
fiber optic strain sensor comprising a single mode optical fiber having
two or more cores positioned in a common cladding. At one end of the
fiber, one of the cores is illuminated, and as the light propagates down
the fiber, some light is coupled to adjacent cores due to crosstalk.
Detector means are provided at the other end of the fiber for measuring
the intensity of light emerging from each core. A pressure change or
strain acting on the fiber causes a change in the indices of refraction of
the cores and cladding and in the dimensions of the fiber. This results in
a change in the crosstalk between the cores and thus in a change in the
intensity of light emerging from the cores.
The Meltz et al. apparatus has limited sensitivity due to the placement of
several cores within the relatively rigid structure of a single fiber.
This structure de-emphasizes the effect of possible changes in core
separation which may result from the application of strain or pressure to
the fiber. Also the Meltz et al. apparatus is limited in that it requires
a single mode optical fiber, and could not be used with a multi-mode
optical fiber.
A different type of optical transducer system, which may be suitable in a
hydrophone for some applications, is disclosed in U.S. Pat. No. 4,268,116,
issued May 19, 1981 to Schmadel et al. The Schmadel et al. method and
apparatus produces a modulated light signal in a single mode clad optical
fiber by varying the frequency and/or phase of a narrow band of light
reflected back to its source by an optical grating, by sliding the optical
grating relative to the fiber near its core. The Schmadel et al. apparatus
depends on the phenomenon of Bragg reflection by the optical grating. The
present invention, however, requires no such optical grating and does not
utilize the Bragg reflection phenomenon.
The effect of "evanescent field coupling," whereby a portion of the
electromagnetic energy in an optical fiber is coupled to an adjacent
optical fiber, is well understood. The coupling effect occurs between
multi-mode fibers as well as between single-mode fibers. It has been
recognized that the magnitude of power so coupled between two fibers
depends on the separation between them. It also has been recognized that
the effect could, in principle, be utilized in a transducer to produce an
intensity-modulated signal in response to a variation in the separation
between two optical fibers. See, for example, S. K. Sheem and J. H. Cole,
"Acoustic Sensitivity of Single-Mode Optical Power Dividers", Optics
Letters, Vol. 4, No. 10, p. 322 (1979). The apparatus of the present
invention, however, utilizes the evanescent field coupling effect in a
manner not previously suggested in the art.
SUMMARY OF THE INVENTION
The apparatus of the present invention comprises an optical fiber having
one or more detector sections, means for launching a narrow interrogating
light pulse into the fiber, means for producing an optical return signal
at each of the detector sections, the intensity of which return signal
varies in response to variations in an external signal incident on the
detector section, means for launching the return signals into the fiber in
the direction opposite to the direction of the interrogating pulse, and
means for detecting and processing the return signals.
In the preferred embodiment, each detector section is formed by looping a
section of the fiber back on itself, bringing the adjacent fiber cores in
close proximity in a coupling region wherein the adjacent cores are
separated by compliant material having an index of refraction near that of
the fiber cladding. The interrogating light pulse is partially coupled
from the segment of fiber core first reached by the interrogating pulse to
the adjacent fiber core due to the effect of evanescent field coupling.
After traversing the loop, the coupled energy, whose intensity is
proportional to the core separation and therefore depends on any incident
signal varying the core separation, travels back along the fiber in a
direction opposite to that of the interrogating pulse. Light is coupled on
both passes through the coupling region, thus doubling the power of the
return signal. One or more arrays, each comprising several such detector
sections may be formed out of a single fiber. The return signals from the
detector sections making up each array may be fed into a gated integrator
or boxcar averager whose output is a unique return signal representing
each such array.
In an alternative embodiment each detector section comprises a small fiber
section, identical to a small section of the main fiber, positioned
parallel to and separated by a small distance from the main fiber. The
separation between the main fiber and small fiber section is variable in
response to external signals incident on the apparatus. Due to the
evanescent field coupling effect, a portion of the interrogating light
pulse is coupled into such small fiber section. The ends of each small
fiber section are finished flat and substantially perpendicular to the
axis of the fiber section, and a high reflectivity coating is applied
thereto. Since the evanescent process also couples light from the small
fiber section to the main fiber, much of the captured light pulse is
injected back into the main fiber, half in the same direction as the
interrogating pulse; half in the opposite direction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified cross-sectional view of a fiber optic data
multiplexer illustrating the preferred embodiment of the present
invention.
FIG. 2 is a cross-sectional view of a single optical sensor of the type
employed in the system of FIG. 1.
FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2, showing
the preferred mechanical configuration of the sensor coupling region.
FIG. 4 is a cross-sectional view of a sensor of the type employed in the
system of FIG. 1, taken on a plane normal to the optical fiber axis in the
coupling region showing an alternative mechanical configuration for the
coupling region.
FIG. 5 is a cross-sectional view of a sensor of the type employed in the
system of FIG. 1, taken on a plane normal to the optical fiber axis in the
coupling region showing another alternative mechanical configuration for
the coupling region.
FIG. 6 is a simplified cross-sectional view of a fiber optic data
multiplexer illustrating an alternate embodiment of the present invention.
FIG. 7 is a cross-sectional view of a single fiber optic sensor (optical
hydrophone) of the type employed in the system of FIG. 6.
FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 7.
FIG. 9 is a block diagram of a fiber optic data multiplexer according to
the present invention showing means for detecting and processing the
return signals from the individual sensors or groups of sensors of the
system.
FIG. 10 is a set of seven graphs representing three external signals
incident on three different groups of sensors of a fiber optical data
multiplexer according to the present invention, an interrogating light
pulse for interrogating the sensor array of the system, and return
signals, generated in response to the interrogating light pulse, before
and after processing by the signal processing means of the system.
FIG. 11 is a simplified cross-sectional view of a fiber optic data
multiplexer illustrating semi-schematically another preferred embodiment
of the present invention.
FIG. 12 is a cross-sectional view of a single optical modulator of the type
employed in the system of FIG. 11.
FIG. 13 is a cross-sectional view of the coupling region of another
embodiment of an optical modulator of the type employed in the system of
FIG. 11.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a simplified cross-sectional view of a fiber optic data gathering
system (also referred to herein as a "fiber optic data multiplexer")
illustrating the preferred embodiment of the present invention. A section
of optical fiber 1 is looped back on itself to form detector section 10.
Similarly, other sections of fiber 1 are looped to form identical detector
sections 11 and 26. The system may contain other detector sections, but
only three detector sections are shown in FIG. 1 in order to simplify
explanation of the invention. A preferred mechanical configuration of
detector sections 10, 11, and 26 will be discussed in detail below with
reference to FIGS. 2 and 3. It should be understood that any number of
detector sections or arrays of detector sections may be formed on fiber 1.
Fiber 1 may be a single-mode fiber or a multi-mode fiber. A suitable
multi-mode fiber may be fabricated in a manner well known in the art by
choosing fiber dimensions and materials of fabrication so that more than
one mode of electromagnetic radiation can propagate as a guided wave in
the fiber. A suitable single-mode fiber may be fabricated in a manner well
known in the art by choosing fiber dimensions and materials of fabrication
so that only the lowest order mode (the propagating mode having lowest
frequency) will propagate as a guided wave in the fiber.
Transmitter 2, capable of launching a narrow interrogating light pulse into
fiber 1, is positioned at one end of fiber 1. Transmitter 2 may be a laser
diode or any other suitable light source selected from those types well
known in the art. Directional coupler 3 diverts a portion of the
interrogating pulse to monitor photodetector 6 via optical fiber 4. The
remainder of the interrogating pulse propagates through directional
coupler 3 and along fiber 1 to detector sections 10, 11, and 26.
Due to the effect of evanescent field coupling, a first portion of the
interrogating pulse is coupled from segment 32 of fiber 1 into segment 33
of fiber 1. Such first portion will propagate back along fiber 1 toward
directional coupler 3. The remainder of the interrogating pulse will
traverse the loop of detector section 10 and enter into segment 33 as it
continues to propagate away from directional coupler 3. Due to evanescent
coupling, a second portion of the interrogating pulse will be coupled from
segment 33 into segment 32. This second portion will propagate back along
fiber 1 toward directional coupler 3, along with the first portion.
Directional coupler 3 will divert part of the return signal from detector
section 10 (which return signal comprises the first and second portions)
to photodetector 7 via optical fiber 5. If the data gathering system
includes several detector sections, a series of such return signals or
pulses is received at photodetector 7, each successive return pulse
produced by the next detector section along the fiber. Each return pulse
amplitude is modulated by the signal of interest (which may be an acoustic
signal) present at the relevant detector section at the instant the
interrogating light pulse passes.
A suitable photodetector may be selected from those well known in the art.
For example, photodetector Model MDA 7708, manufactured by Meret, Inc.,
has been found satisfactory. The return signal from detector section 10,
and similarly ge | | |
