Definitions

• the present invention relates to fiber optic sensors. Specifically, the invention relates to fiber optic current sensors and their signal processing electronics.
• the interferometric Fiber Optic Sensor 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).
• FOG the Sagnac effect for rotation sensors
• FOCS Faraday effect for current sensors
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• each of the linearly polarized light beams Before entering the sensing coil, each of the linearly polarized light beams passes through a respective quarter wave plate and emerges therefrom as 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.
• the sense of rotation of the two light waves 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 splitter 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.
• DC direct current
• AC alternating current
• 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 harmonics) 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 maintaining a stable, linear output scale factor.
• 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
• Scale factor linearity is maintained due to the intrinsic linearity of the Sagnac and Faraday effect for small measured current or magnetic field values.
• the linearity can be maintained using conventional signal processing techniques.
• 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.
• 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 and
• 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.
• 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.
• 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.
• 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;
• FIG. 4 is a detailed diagram of an electronic control circuit of the second embodiment of the RMC FOCS system of FIG. 2.
• the invention is directed to a "Reduced Minimum Configuration" (RMC) fiberoptic current sensor (FOCS).
• RMC Reduced Minimum Configuration
• FOCS fiberoptic current sensor
• 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 la emits light from a front output facet lb which is polarized by polarizer 2.
• 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 la 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.
• 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.
• 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.
• 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.
• the light source la emits light from a front output facet lb which is polarized by polarizer 2.
• a birefringent modulator 9 modulates the polarization direction of the polarized light.
• a 45° splice 11 is inserted between the polarizer 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.
• 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.
• 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 ⁇ /4 wave plate.
• the current can be directly determined from the amplitude of the fundamental signal (Fl). Since the phase and frequency of the fundamental signal are known, an effective way to determine the amplitude is by synchronous demodulation.
• 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 (Fl).
• 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.
• 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 (Fl).
• 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 (Fl) 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.
• 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.
• 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. 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.
• 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 la 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.
• 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.
• maintaining a sine wave drive having merely a fixed frequency and amplitude will not guarantee a fixed depth of phase modulation.
• the resonant frequency (Fr) of the PZT modulator 9 may drift.
• the mechanical-to-optical phase shift conversion scale factor (Q m ) of the PZT modulator 9 may change.
• 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.
• 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.
• 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.

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