Note: Descriptions are shown in the official language in which they were submitted.
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REDUCED MINIMUM CONFIGURATION
FIBER OPTIC CURRENT SENSOR
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of U.S. Application No. 09/459,438,
filed December 13, 1999, which is a continuation of U.S. Application No.
08/835,307,
now abandoned, claiming the benefit of Provisional Application No. 60/014,884,
filed
April 15, 1996. This application also claims the benefit of Provisional
Application No.
60/143,847, filed July 15, 1999.
FIELD OF THE INVENTION
The present invention relates to fiber optic sensors. Specifically, the
invention
relates to fiber optic current sensors and their signal processing
electronics.
BACKGROUND OF THE INVENTION
The interferometric Fiber Optic Sensor (FOS) is an established technology for
accurately measuring angular rotation (interferometric Fiber Optic Gyroscope,
FOG)
and magnetic fields (interferometric or polarimetric fiber optic current
sensor, FOCS). It
will be understood that the FOCS does not directly sense an electric current,
but rather
the effect of a magnetic field produced by that current. Because the FOS is an
optical,
solid state design with no moving parts, it can be used for long life, high
reliability
applications such as vehicle navigation and remote sensing of electric
currents.
The fundamental working principle behind the FOS is the Sagnac effect for
rotation sensors (FOG) and the Faraday effect for current sensors (FOCS). In
the FOG
measuring rotation, two counter-propagating light waves traversing a loop
interferometer acquire a phase difference when the loop is rotated about its
axis. In a
FOCS measuring an electric current flowing in a wire that passes through the
coil plane,
the phase difference is produced by the magnetic field associated with the
current.
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Depending on the configuration of the optical path of the light waves, the FOS
may
incorporate interferometric or polarimetric phase modulation. Accurate
measurement of
the phase difference induced by the rotation or current requires the parasitic
phase
differences, which can vary with the environment, be suppressed. For this
reason, the
principle of optical reciprocity is used to select portions of the counter-
propagating
waves which pass through the interferometer or polarimeter along a common
path. In the
following, "interferometer" is meant to refer to both interferometric or
polarimetric
devices. Variations induced in the system by the environment change the phase
of both
waves equally and no difference in phase delay results; the sensor is
environmentally
stable.
In a conventional interferometric fiber optic gyroscope (FOG), the light
emitted
from a suitable light source passes through a first 3dB coupler where half of
the light is
dissipated, and half is sent into the interferometer through the polarizer. A
second 3dB
coupler splits the light into two approximately equal intensity, counter-
propagating
beams which traverse the coil. The two light beams then recombine at the
second
coupler where they interfere. This combined light beam then passes through the
polarizer a second time in the opposite direction, and half of the light is
directed to the
detector by the first coupler. The first coupler is not part of the optically
reciprocal
Sagnac interferometer. Its sole purpose is to direct some of the returning
light into a
photodetector and to minimize direct coupling of light energy from the source
into the
detector. An optical splitting ratio of 3 dB is selected for the couplers to
maximize the
optical power incident on the detector. This leads to an inherent 6 dB of
system loss
since this coupler is passed twice.
In a conventional interferometric fiber optic current sensor (FOCS), the light
beam also passes through a first directional coupler that isolates the optical
detector, and
then through a polarizer which produces linearly polarized light. The linearly
polarized
light beam is then split in two by the second directional coupler, with one
beam being
directed into one end of a sensing coil comprising loops of non-birefringent
fiber. The
other of the two light beams is directed through a phase modulator into the
other end of
the sensing coil. Before entering the sensing coil, each of the linearly
polarized light
beams passes through a respective quarter wave plate and emerges therefrom as
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circularly polarized light, which is the light entering the sensing coil.
Light emerging
from the two fiber ends of the sensing coil passes again through the quarter
wave plate,
producing linearly polarized return light. The return light is recombined by
the second
directional coupler and the intensity of the recombined light is detected by
an optical
detector.
Another type of conventional FOCS is based on a reflective current sensing
coil
which simplifies the design by eliminating the second directional coupler. The
orthogonally linearly polarized light beams pass through a quarter wave plate
and
emerges therefrom as counter-rotating circularly polarized light prior to
entering the
sensing coil. Upon reflection at the end of the fiber, the sense of rotation
of the two light
waves are reversed and the light travels back through the sensing region,
whereafter the
light is converted back to linearly polarized light. The intensity of the
recombined light
is detected by an optical detector. If a birefringence modulator is used
instead of a phase
modulator, a 45° splice has to be inserted before the light enters the
modulator.
The phase modulator can be, for example, a piezo-electric transducer (PZT).
Other methods of modulating the phase difference, for example, electro-optic
material
such as lithium niobate can be used. If an integrated optics assembly (IOC) is
used for
modulation, then a Y junction power sputter may be included instead of the
directional
coupler connected to the sensing coil. This phase modulation serves two
purposes. One
is to dynamically bias the interferometer to a more sensitive operating point
and also
allow the determination of rotation sense. The other is to move the detected
signal from
direct current (DC) to alternating current (AC) in order to improve the
accuracy of the
electrical signal processing. With sinusoidal phase modulation in an open-loop
signal
processing configuration, the interferometer output signal is an infinite
series of sine and
cosine waveforms whose maximum amplitudes are Bessel functions related to the
phase
modulation amplitude. The maximum amplitudes of the Bessel functions are
proportional to the sine (odd harnlonics) and cosine (even harmonics) of the
measured
quantity. The fundamental signal is located at the applied modulation
frequency with
subsequent even and odd harmonic signals. Many signal processing approaches
have
been proposed which use ratios of the fundamental and the three lowest order
harmonic
signals amplitudes to detect rotation rate and/or magnetic field, while at the
same time
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maintaining a stable, linear output scale factor. However, implementation of
these
approaches in analog and/or digital electronic hardware is complex and
expensive. A
much simpler signal processing design, which is not affected by an error in
the relative
amplitude of the sensor harmonic signals is therefore desired.
Scale factor linearity (i.e. measured current or magnetic field versus the
applied
current or magnetic field) is maintained due to the intrinsic linearity of the
Sagnac and
Faraday effect for small measured current or magnetic field values. At higher
rate or
currents, during environment changes (i.e. temperature, vibration, etc.) and
over the life
time of the sensor, the linearity can be maintained using conventional signal
processing
techniques.
It would therefore be desirable to provide a fiber optic current sensor with a
smaller number of optical components which can be produced more easily and
less
expensively. It would also be desirable to provide an electronic system for
processing
the current sensor output signal in order to maintain a constant scale factor
during
environment changes.
SUMMARY OF THE INVENTION
The present invention is directed to a reduced minimum configuration
(RMC) fiberoptic current sensor (FOCS) system for measuring a magnetic field,
in
particular a magnetic field induced by an electric current. According to one
aspect of the
invention, the RMC FOCS system includes a fiber sensing coil; a light source
having a
front output and a back output and emitting light with an associated light
source
intensity; an optical coupler, which may also include a polarizer, disposed
between the
front output and the coil and receiving the light from the light source, the
coupler
creating two linearly polarized light beams of substantially equal intensity;
a first
quarter wave plate disposed proximate to a first end of the fiber sensing coil
and
receiving a first of the two linearly polarized light beams and converting the
first linearly
polarized light beam into first circularly polarized light propagate through
the sensing
coil in a first direction; a second quarter wave plate disposed proximate to a
second end
of the fiber sensing coil and receiving a second of the two linearly polarized
light beams
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and converting the second linearly polarized light beam into second circularly
polarized
light propagate through the sensing coil in a second direction opposite the
first direction,
the first and second circularly polarized light passing though the sensing
coil
experiencing a differential phase shift caused by a magnetic field or current
flowing in a
conductor proximate to the sensing coil; the fiber sensing coil supplying
phase-shifted
circularly polarized return light to the first and second quarter wave plates,
the first and
second quarter wave plates converting the phase-shifted circularly polarized
return light
back to linearly polarized return light, the coupler combining and interfering
the linearly
polarized return light into a combined light beam; and a light detector
operatively
coupled to the back output of the light source, the light detector detecting
and providing
an output signal in response to the combined light beam transmitted through
the light
source.
According to another aspect of the invention, the reduced minimum
configuration (RMC) fiber optic current sensor (FOCS) system includes a fiber
optic
sensing region with an optical fiber; a light source having a front output and
a back
output and emitting light with an associated light source intensity; and an
optical path
operatively connecting the front output of the light source with the fiber
optic sensing
region for transmitting two linearly polarized light beams from the light
source along the
optical path. At least one quarter wave plate is disposed between the optical
path and
the sensing region for converting the two linearly polarized light beams into
two
opposing circularly polarized light beams propagating through the sensing
region, with
the two opposing circularly polarized light beams which propagate though the
sensing
region experiencing a differential phase shift caused by a magnetic field or
current
flowing in a conductor proximate to the sensing coil. The fiber sensing region
may
include a reflector located at an end portion of the fiber sensing region that
reflects the
circularly polarized light beams, supplying phase-shifted circularly polarized
return light
to the at least one quarter wave plate which converts the phase- shifted
circularly
polarized return light back to interfering linearly polarized return light
beams. A light
detector which is operatively coupled to the back output of the light source
detects the
interfered return light beams transmitted through the light source and
provides an output
signal in response to the interfered return light beams transmitted through
the light
source.
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Embodiments of the RMC FOCS system may include one or more of the
following features. The RMC FOCS may include a polarizer disposed between the
front
output and the sensor coil, with the polarizer polarizing the light emitted
from the light
source and the return light beam. In a reflective RMC FOCS, a 45° twist
may be inserted
between the polarizer and the sensor coil. An optical phase or birefringence
modulator
may be coupled to the sensor coil; an oscillator, which controls the
modulation
amplitude, may be coupled to the modulator. An electrical amplifier which
receives the
output signal may be coupled to the detector, with a direct current block, a
rectifier, an
integrating comparator, and light source drive means coupled to the amplifier,
for
controlling the associated light source intensity. Electrical signal
processing means may
also be coupled to the amplifier for processing the output signal and
providing an output
value correlated with the magnetic field or current.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a first embodiment of a RMC FOCS system
according to the invention;
FIG. 2 is a schematic diagram of a second embodiment of a RMC FOCS system
according to the invention;
FIG. 3 is a detailed diagram of an electronic control circuit of the first
embodiment of the a RMC FOCS system of FIG. 1; and
FIG. 4 is a detailed diagram of an electronic control circuit of the second
embodiment of the RMC FOCS system of FIG. 2.
DETAILED DESCRIPTION OF CERTAIN ILLUSTRATED EMBODIMENTS
While the invention is susceptible to various modifications and alternative
forms,
specific embodiments thereof are shown by way of example in the drawings and
will
herein be described in detail. It should be understood, however, that this is
not intended
to limit the invention to the particular forms disclosed, but on the contrary,
the intention
is to cover all modifications, equivalents, and alternatives failing within
the spirit and
scope of the invention as defined below.
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The invention is directed to a "Reduced Minimum Configuration" (RMC) fiber-
optic current sensor (FOCS). Unlike conventional FOCS, the first coupler is
omitted
and the interferometer output is read out by a detector positioned at a back
facet output
of the light source. Referring first to FIG. 1, a light source 1 a emits light
from a front
output facet 1b which is polarized by polarizer 2. Several types of light
sources can be
used, including a laser diode (LD), a superluminescent diode (SLD), and light
emitting
diode (LED), or a superradiant fiber amplifier. The polarizer 2 can be, for
example, a
fiber polarizer, a lithium niobate polarizer or a polymer waveguide polarizer.
Coupler 3
splits the light into two counter-propagating beams of an approximately equal
intensity.
Quarter wave plates 14, 16 are inserted in the optical path at both ends of
the sensing
coil. The quarter wave plates 14, 16 convert the linearly polarized light
produced by
polarizer 2 into circularly polarized light which counter-propagates in the
sensing coil 4.
A magnetic field introduces a phase shift (Faraday rotation) of the counter-
propagating
circularly polarized light beams, wherein the direction of the phase shift
depends on the
propagation direction of the circularly polarized light beams with respect to
the direction
of the magnetic field. The quarter wave plates 14, 16 convert the respective
returned
circularly polarized light beams, which have passed through the sensing coil,
back into
linearly polarized light beams which are then recombined in the coupler 3. The
coupler
can be a directional coupler formed from optical fiber or integrated optics
components.
The recombined light beam then passes through the source 1 a and is received
at a
back facet output 1 c of the light source 1 a by a detector 5. The detector 5
can be a
photodetector coupled to an amplifier 6, for example a transimpedance
amplifier, which
converts the optical input to an output voltage. The output of detector 5 is
passed
through a suitable amplifier 6 providing a voltage gain of, for example, one
million. The
amplifier output is applied to demodulator 7. Demodulator 7 can be a phase
sensitive
detector (PSD) which receives a signal from oscillator 8. For an FOCS, the
output of the
demodulator 7 is a function of the magnetic flux through the sensing coil 4
and is
sinusoidal and can be approximated by a linear function for low to moderate
magnetic
fields or currents. If the phase and frequencies of the two signals entering
demodulator 7
are the same, the output is at a maximum, if they are different, the output is
reduced. The
oscillator 8 and the phase modulator 9, as will be explained in greater detail
below,
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maintain the interferometer depth of the phase modulation. Alternatively,
other phase
modulators can be used such as those constructed from lithium niobate or
another
electro-optic material. Light source controller/driver 10, which regulates the
intensity of
the light source, may include additional elements, such as a filter, a
rectifier and an
integrating comparator, as will be explained in greater detail below.
Referring now to Fig. 2, depicting another embodiment of a RMC FOCS system
according to the present invention, the light source 1 a emits light from a
front output
facet 1b which is polarized by polarizes 2. A birefringent modulator 9
modulates the
polarization direction of the polarized light. A 45° splice 11 is
inserted between the
polarizes 2 and the birefringent modulator 9 to provide orthogonally linearly
polarized
light. A single quarter wave plates 12 disposed between the one end of the
sensing coil
4' and the phase modulator 9 converts the modulated linearly polarized light
into two
opposing circularly polarized light beams propagating through the sensing
region.
Unlike the first embodiment, the light is introduced only at one end of the
sensing coil
4'. The other end of the sensing coil 4' has a reflector 13 which reflects the
circularly
polarized light back into the sensing coil 4' while reversing the polarization
direction.
The reflective sensor coil geometry has the added advantage of being less
sensitive to
mechanical vibrations and sensor coil rotation.
The sensing coil's scale factor can be designed so that this maximum current
range is well within an essentially linear region of the sensor's output
transfer function.
Alternatively, electronic linearization techniques known in the art can also
be used. The
fiber coil length of the sensing coil depends on the current range to be
measured and can
be between 1 m to 1,000 m, preferably between 1 m and 20 m. The coil is wound
in such
a way that a conductor can pass through the coil opening. The separation
between the
first directional coupler and the sensing coil and the ~,/4 wave plate,
respectively, can be
between 20 m and 1,000 m, so that polarization-maintaining fiber is employed
between
the first directional coupler and the sensing coil or 7~/4 wave plate. With
this type of
construction, the current can be directly determined from the amplitude of the
fundamental signal (F1). Since the phase and frequency of the fundamental
signal are
known, an effective way to determine the amplitude is by synchronous
demodulation.
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Referring now to FIG. 3, the broadband signal current detected by detector 5
is
passed through a transimpedance pre-amplifier 26 which converts the detected
photo
current to a voltage and through amplifier 27 which provides voltage signal
gain. The
amplified voltage signal is then applied to a low pass filter (LPF) 28 whose
corner
frequency is at the fundamental frequency (F 1 ) prior to being synchronously
demodulated in a synchronous demodulator 29. The low pass filter 28 attenuates
all
harmonics from the voltage signal leaving only the fundamental frequency (F1).
The
other input of the synchronous demodulator 29 receives a voltage signal from a
self
resonant oscillator and amplifier circuit 8, which, as discussed below, may
use a Colpitts
oscillator and an AGC amplifier to provide a stable amplitude and self
resonant
frequency to the modulator 9. However, as is known in the art, the circuit 8
can be a
crystal oscillator providing a fixed amplitude, a digital synthesizer, fixed
frequency
signal or can be a Colpitts oscillator (only) providing a self resonant signal
of fixed
amplitude to the modulator.
The output of circuit 8 is passed through a phase shifter and low pass filter
32
whose corner frequency is equal to or higher than the fundamental frequency
(F1). The
output of demodulator 29 is at a maximum when the phases and frequencies of
its input
signals are equal, and is proportional to the magnitude of the sensor output
signal at the
fundamental frequency (F1) which is a function of the magnetic field or
current. This
output may be passed through another low pass filter 30 which produces a DC
signal
proportional to the magnetic field or current. Finally, the DC signal is
amplified in DC
amplifier 31 to set the desired sensor scale factor. The demodulation produces
a linear
output over a wide dynamic magnetic field and/or current range. Resolution of
the
magnetic field measurement is determined by the noise figure of the
transimpedance
pre-amplifier 26 and the bandwidth of the measurement.
Those skilled in the art will recognize that the output of a current sensor
that
measures an AC magnetic field produced, for example, by the AC current flowing
through an AC power line, instead of the DC magnetic field described above
will
produce a corresponding AC signal including the fundamental frequency of the
AC
power line as well as harmonics thereof.
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Maintaining a constant scale factor during environmental changes requires that
two sensor operating points be accurately maintained. First, the magnitude of
the sensor
signal at the output of the amplifier 6 must be constant. To accomplish this,
one aspect
of the invention takes advantage of the fact that for short coil length
sensors, the
amplitude of the second harmonic signal (F2) is relatively constant over the
entire
rate/magnetic field range. Thus, the broadband sensor signal is high pass
filtered (HPF)
at the second harmonic frequency (F2) by a high pass filter 35, rectified by
rectifier 36,
integrated and compared with a reference by integrating comparator 37, and
applied to
the light source 21 by source driver 10. The resulting DC signal is used to
control the
optical power output of source 1 a by increasing or decreasing the light
source current
and therefore the emitted optical power. High pass filter 35 may be required
to reduce
the influence of the fundamental signal (Fl) on the light source control
circuit accuracy
at high magnetic fields or currents. As is known in the art, the high-pass
filter 35 may be
replaced with a DC-block or a band pass filter and the rectifier 36 may be a
full-wave-
rectifier or a half wave rectifier.
The second sensor operating point which should be maintained, is the
interferometer depth of phase modulation controlled at PZT phase modulator 9.
The
depth of phase modulation is set by the amplitude of the sine wave drive
voltage applied
to the PZT phase modulator 9. However, maintaining a sine wave drive having
merely a
fixed frequency and amplitude will not guarantee a fixed depth of phase
modulation.
Over time and with changes in temperature, the resonant frequency (Fr) of the
PZT
modulator 9 may drift. Also, the mechanical-to-optical phase shift conversion
scale
factor (Qm) of the PZT modulator 9 may change. As discussed above, the
invention uses
the phase modulator 9 as an active part of the oscillator circuit by applying
the output of
the self resonant oscillator and adjustable gain controlled (AGC) amplifier
circuit 8 to
the phase shifter and low pass filter 32. Because the modulator 9 is part of
the active
feedback circuit, any movement in the resonant modulator frequency is tracked.
Changes
in Qm and Fr also change the dynamic impedance of the modulator affecting the
drive
amplitude. The self resonant oscillator and AGC amplifier circuit 8 is used to
maintain a
stable sine wave drive amplitude through changes in the environment, although
other
self resonant oscillators known in the art could also be used.
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Thus, a FOCS system with simplified signal processing electronics is provided
where the fundamental sensor signal amplitude is synchronously demodulated to
determine the magnetic field or the current. The second harmonic sensor signal
(F2) is
used to control the light source intensity. Taking the ratio of these signals
is not required.
The depth of phase modulation is maintained by using a self resonant
oscillator
approach with the phase modulator as part of the active electrical circuit.
This configuration eliminates non-essential optical components and splices
from
the system, allowing for the construction of a lower cost FOCS system. Using
the RMC
FOCS with this simplified signal processing electronics approach produces a
very
attractive cost-effective magnetic field sensor which can be used to measure
electric
currents. The FOCS signal processing system is simple and can be produced at
low cost,
accurately determines the external quantities affecting the sensor coil, and
maintains a
constant scale factor during changes in the environmental conditions.
Referring now to FIG. 4, the same signal processing electronics described
above
with reference to FIG. 3 can be used with the reflective fiber-optic current
sensor of FIG.
2. Using the RMC FOCS with this simplified signal processing electronics
approach
produces a very attractive cost-to-performance magnetic field and current
sensor which
is rather insensitive to mechanical vibrations and coil rotation.
While the invention has been disclosed in connection with the preferred
embodiments shown and described in detail, various modifications and
improvements
thereon will become readily apparent to those skilled in the art. Accordingly,
the spirit
and scope of the present invention is to be limited only by the following
claims.
What is claimed is:
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