JPH0611351A - Optical fiber revolution sensor - Google Patents

Abstract

PURPOSE: To avoid the error in rotation detection and improve the accuracy of a rotation sensor by phase-modulating at a specific frequency, an amplitude modulation in odd harmonic of photooutput signal induced by phase modulation. CONSTITUTION: A rotation sensor is provided with a light source 10 guiding the light to an optical fiber strand 12 and a part of the strand 12 is wound on a detection loop 14. A phase modulator 38 driven by an AC generator 40 and connected to it modulates the phase of transmitting waves W1 and W2. By using a modulation frequency giving the phase difference of 180 degree against the waves W1 and W2, eliminated is amplitude modulation induced by the modulator 38 at the photooutput measured with a detector 30. This modulation frequency is selected as c/2nL. Where L is length difference of the optical fibers between from the modulator 38 to one end of the loop 16 and from the modulator to the other end of the loop, (c) is the free space velocity of light, (n) is the equivalent refractive index of the optical fiber in the loop. An amplifier 46 detects a signal having a basic frequency of the modulator 38 and outputs a signal proportional to the revolution velocity of the loop 16 to a display 47.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to rotation sensors, such as those used in gyroscopes, and more particularly to fiber optic rotation sensors.

Fiber optic rotation sensors typically comprise a loop of fiber optic material in which light waves are coupled to propagate in opposite directions around the loop. The rotation of this loop is
The amount of the phase difference corresponding to the rotation speed produces a relative phase difference between the waves propagating in opposite directions based on the well-known Sagnac effect. When recombined, the counter-propagating waves constructively or destructively interfere to provide an optical output signal whose intensity varies with the rotational speed of the loop. Usually, rotation detection is performed by detecting this optical output signal.

Prior art devices of this type used large optical components to generate and direct the light entering the detection loop. While these devices have been a significant improvement over other types of rotation sensors, they have certain limitations and other drawbacks. For example, various device components
In order for the devices to work properly, they must be aligned with each other within very small tolerances. It is often difficult to achieve and maintain this delicate alignment, especially if the device is exposed to mechanical vibrations, thermal fluctuations and other physical obstacles. .

A rotation sensor that utilizes the Sagnac effect for rotation detection requires a compensator to provide operational stability. An example of the compensator is (Optics Letters, May 1980, Vol. 5, No. 5) "Fiber Optic Rotation Sensing with Low Drift.
drift) ”in a paper by Ulrich. The method involves modulating a counter-propagating light wave and detecting an optical output wave at this modulation frequency. However, because of the defect in the modulator,
Such modulation is prone to amplitude modulation in the counter-propagating wave and thus the optical output signal. This amplitude modulation may be caused directly by the modulator, or it may be the manifestation of polarization modulation caused by the mechanical action of the modulator on the fiber. In either case, such amplitude modulation distorts the optical output signal,
Therefore, the accuracy of the rotation sensor is reduced, which is inconvenient.

[0005]

SUMMARY OF THE INVENTION The present invention provides all of the detector loops and components for guiding and processing light disposed and configured along a continuous, uninterrupted strand of fiber optic material. By providing an optical fiber rotation sensor, these and other problems of the prior art are overcome. Alignment problems are thereby reduced or overcome, and therefore the rotation sensor of the present invention is significantly more mechanically and less susceptible to mechanical shock than conventional sensors that utilize large optical components. .

The rotation sensor comprises all optical fiber components, such as directional couplers made of optical fibers, which (a) direct light from a light source around a detection loop. Is separated into two waves propagating in the opposite direction, and (b) waves propagating in the opposite direction are combined to give an optical output signal. The correct polarization of the input light, the counter-propagating waves and the optical output signal is established, controlled and maintained by the fiber optic polarizer and fiber optic polarization controller. A second fiber optic coupler is provided for coupling the optical output signal from the continuous strand to a photodetector that outputs an electrical signal proportional to the output signal strength.

The operational stability and sensitivity of the rotation sensor are improved by the phase modulation of the counter-propagating wave and by using a synchronous detection device for measuring the first harmonic of the optical output signal. In the disclosed detection device, the magnitude of this first harmonic is proportional to the rotational speed of the loop, and thus the measurement of such first harmonic directly represents the rotational speed of the loop.

Amplitude modulation in the odd harmonics of the optical output signal caused by the phase modulator (either directly or indirectly by polarization modulation) can be eliminated by performing phase modulation at a particular frequency. all right.
Since the detector used only detects certain odd harmonics (such as the first harmonic), the effect of the phase modulator causing the amplitude modulation can be eliminated by operation at such frequencies. This eliminates sources of error in rotation detection, thereby improving the accuracy of the rotation sensor.

Although the detection system used in this invention greatly improves the accuracy of rotation detection, it has also been found that other sources of error in rotation detection can limit the effectiveness of this detection system. One such source of error is caused by a surrounding magnetic field, such as the magnetic field due to geomagnetism. The magnetic field around these causes a phase difference between the waves propagating in the opposite directions due to the Farade effect, which affects the intensity of the optical output signal. According to the present invention, in order to effectively shield the rotation sensor from the surrounding magnetic field, by placing the rotation sensor inside the housing made of a material having a relatively large magnetic permeability, the influence of the surrounding magnetic field is reduced or eliminated. It

The present invention uses an optical isolation device to prevent the optical output signal from returning to the light source. Preferably, the use of this optical decoupling device eliminates the need for a combiner to combine the optical output signals from the continuous fiber strands. Elimination of this coupler greatly reduces the losses through the device, thereby increasing the intensity of the optical output signal at the detector.

In the disclosed embodiment, the effect of backscattering is a second for modulating the input light into the fiber to reduce the coherence between the backscattered light and the counter-propagating wave. Is reduced by using a phase modulator of. Alternatively, the effect of such backscattering may be reduced by using a light source with a relatively short coherence length.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, a rotation sensor of the present invention comprises a light source 10 for directing light to a continuous length of fiber optic strand 12, a portion of which is a detection loop 14. It is wound around. As used herein, reference numeral 12 designates the entire continuous fiber optic strand while the supplementary letters (A, B,
Reference number 12 with (such as C) indicates a portion of optical fiber 12.

In the illustrated embodiment, the light source 10 comprises 0.8
It includes a gallium arsenide (GaAs) laser that produces light having a wavelength of 2 microns. According to a particular embodiment, the light source 10 comprises a model GO-DIP laser diode, which is a Hardlay Rd., South Plainfield, NJ. 3005 General Optronix Corporation
ics Corp ..). An optical fiber strand, such as strand 12, is preferably a single mode fiber having, for example, an outer diameter of 80 microns and a core diameter of 4 microns. The loop 14 comprises a fiber 12 having multiple turns wrapped around a spool or other suitable support (not shown). In a particular embodiment, this loop 14 has a fiber wound about 1000 turns around a 14 centimeter diameter.

Preferably, the loop 14 is symmetrically wound starting from the center so that the points of symmetry of the loop 14 are close together. In particular, the fiber is wound around the spool so that the winding of the central portion of the loop 14 is located on the innermost side adjacent to the spool and is wound toward the end of the loop. Is the fiber loop 14
Both ends of the loop are symmetrically arranged with respect to the central winding and both ends of the loop are symmetrically arranged with respect to the central winding so that they can move freely outside the loop. The winding part in the direction of is arranged on the outermost side of the spool. This reduces the rotational sensor's sensitivity to the environment, as such symmetry causes temperature and pressure gradients that change over time to have a similar effect on both counter-propagating waves. I understand that

Light from light source 10 is optically coupled to one end of fiber 12 by abutting fiber 12 against light source 10. The various components for guiding and processing this light are continuous strands 12
Are located or formed at various locations along the. In order to describe the relative positions of these components, the continuous fiber 12 will be described divided into seven sections, labeled 12A-12G, respectively. This part 12A to 12E is a loop 1 connected to the light source 10.
4 on one side and portions 12F and 12G on the other side of loop 14.

A polarization controller 24 is provided adjacent the light source 10 between the portions 12A and 12B. Controller 24
A preferred form of polarization controller for use as
It is described in the Electronics Letter (Vol. 16, No. 25) published on September 25, 1980. However,
It will now be appreciated that a description of the polarization controller 24 will be given, which allows the controller 24 to adjust both the state and direction of polarization of the input light.

Fiber 12 is then coupled to the second strand of fiber 28 through ports C and D of coupler 26 so that fiber 12 is coupled to fiber portion 12B, 12C.
It goes through the ports A and B of the directional coupler 26 arranged in between, the port C being on the same side of the coupler as port A and the port D being on the same side of the coupler as port B. Exists. The end of fiber 28 extending from port D is
The end of fiber 28, which terminates non-reflective at C (indicating no connection), while extending from port C, is optically coupled to photodetector 30. According to a particular embodiment, the photodetector 30 is a standard reverse biased silicon PIN type photodiode. The preferred coupler for use in this invention is 1
It is described in detail in the Electronics Letter, Vol. 16, No. 7, issued May 27, 980.

The fiber portion 12C extending from port B of coupler 26 passes through a polarizer 32 located between portions 12C and 12D. This polarizer 32 allows light to pass in one polarization mode of the fiber 12 while preventing light from passing in other polarization modes. Preferably, polarization controller 24 is used to adjust the polarization of the input light so that such polarization is substantially the same as that passed by polarizer 32. This reduces the loss of light output as the input light propagates through the polarizer. A preferred type of polarizer for use in this invention is described in the Optics Letter (Vol. 5, No. 11), published November 1980.

After passing through the polarizer 32, the fiber 12 passes through ports A and B of a directional coupler 34 located between the fiber sections 12D and 12E. Preferably, this combiner 34 is of the same type as described above for combiner 26. Fiber 12 is then loop 1
4 and the polarization controller 36 is located between the loop 14 and the fiber portion 12E. This polarization controller 36
May be of the type discussed with respect to controller 24 and is used to adjust the polarization of the waves propagating in the opposite direction through loop 14 and is thus formed by the superposition of these waves. The light output signal has a polarization that is effectively passed by the polarizer 32 while maintaining minimal light output loss. Thus, by using both polarization controllers 24 and 36, the polarization of the light propagating in the fiber 12 is
It can be adjusted for maximum light output.

A modulator 38, driven by an AC generator 40 and connected to it by a line 39, comprises a fiber 1
2 mounted between the loop 14 and the fiber portion 12F. The modulator 38 comprises a PZT cylinder around which the fiber 12 is wound. Fiber 12 is connected to this cylinder so that fiber 12 is stretched as the cylinder radially expands in response to a modulation signal from generator 40. A preferred alternative form of modulator (not shown) for use with the present invention comprises a PZT cylinder that longitudinally extends four sections of fiber 12 coupled to a short capillary tube at the end of the cylinder. Those skilled in the art will recognize that this alternative form of modulator may provide less polarization modulation for the propagating optical signal than modulator 38, but then modulator 38 It will be appreciated that it can be operated at frequencies that eliminate the unwanted effects of modulation. Thus, either type of modulator is preferred for use in the present invention.

Fiber 12 then passes through ports C and D of coupler 34, with fiber portion 12F extending from port D and fiber portion 12G extending from port C. The fiber portion 12G terminates non-reflective at point "NC" (meaning not connected). The output signal from AC generator 40 is provided on line 44 to lock-in amplifier 46, which is also connected by line 48 to receive the output of photodetector 30. The signal to this amplifier 46 provides a reference signal for the amplifier 46 to synchronously detect the detected output signal at the modulation frequency. Therefore, the amplifier 46 provides a bandpass filter at the fundamental frequency (ie, the first harmonic) of the modulator 38 and blocks all other frequencies than this frequency. In the following,
It will be appreciated that the magnitude of the first harmonic component of the detector output signal is proportional to the rotational speed of the loop 14 over the operating range. The amplifier 46 outputs a signal proportional to the first harmonic component, and thus directly displays the rotation speed, which is provided by the line 49 to the display 47 to the display panel 47.
Is displayed visually.

Coupler 26,34 A preferred fiber optic directional coupler for use as coupler 26,34 in the rotation sensor or gyroscope of the present invention is shown in FIG. Each of the couplers has a rectangular parallelepiped base, that is, blocks 53A and 5A.
Two strands 50 of single mode fiber optic material mounted within longitudinally extending arcuate grooves 52A, 52B formed in optically flat opposing surfaces of 3B.
Including A and 50B. Strands 50A mounted in blocks 53A and grooves 52A are the halves 51A of this coupler.
Block 53B and the strands 50B mounted in this groove 52B are referred to as the coupler half 51B.

The arcuate grooves 52A and 52B are formed in the fiber 50.
Has a relatively large radius of curvature compared to the diameter of the fiber and allows it to fit within the path defined by the bottom wall of the groove 52 when mounted therein. It has a large width. This groove 52
The depths of A and 52B are the blocks 53A and 53B, respectively.
From the minimum depth at the center of the block 53A,
Vary up to the maximum depth at the edge of 53B. This means that the optical fiber strands 50A and 50B have grooves 52
When mounted in A, 52B, it allows the fiber 50 to taper toward the center and gradually widen towards the ends of the blocks 53A, 53B, thereby causing output loss through mode variations. Eliminate sharp bends or sharp changes in all directions. In the illustrated embodiment, the groove 52 is rectangular in cross-section, but it will be understood that other suitable cross-sectional shapes compatible with the fiber 50, such as a U-shaped or V-shaped cross-section, could be used instead. Ah

In the embodiment shown, the depth of the groove 52 in which the strand 50 is mounted is smaller than the diameter of the strand 50 in the center of the block 53, while the depth of the groove 52 is equal to the strand 50 at both end edges of the block 53. Is at least equal to the diameter of. Optical fiber material is 50 for each strand
A and 50B are removed by lapping, for example, so as to form an elliptical flat surface, and this plane is flush with the opposing surfaces of the blocks 53A and 53B. These elliptical surfaces from which the fiber optic material has been removed are referred to herein as the "opposing surfaces" of the fibers. Therefore, the amount of the removed optical fiber material is from 0 toward the both ends of the block 53, and
The maximum gradually increases toward the center of 3. This biased removal of the fiber optic material allows the fiber to gradually narrow and widen, which is advantageous to avoid backward reflection and excessive loss of light energy.

In the embodiment shown, the coupler halves 51A, 5
1B are identical and are both assembled by positioning the opposing surfaces of blocks 53A, 53B so that the opposing surfaces of strands 50A, 50B are in opposing relationship.

An index matching material (not shown), such as index matching oil, is provided between the opposing surfaces of block 53. This material has a refractive index that is approximately equal to the refractive index of the cladding and prevents the optically flat surface from being permanently fixed. This oil is also introduced between the blocks 53 by the capillary action.

An interaction region 54 is formed at the juncture of the strands 50, where light is transmitted between the strands by evanescent field coupling. To ensure the evanescent field coupling, the fiber 5
It can be seen that the amount of material removed from zero must be carefully controlled so that the space between the cores of the strands 50 is within a given "critical area". The evanescent field extends into the cladding and decreases rapidly with the distance outside each core. In this way, a sufficient amount of material is removed to allow each core to be substantially positioned within the other evanescent field. If too little material is removed, the core will not be close enough to allow the evanescent field to cause the desired guided mode interaction,
Therefore, insufficient binding results. Conversely, if too much material is removed, the fiber's propagation characteristics will change,
It results in a loss of light energy due to mode variations. However, if the space between the core portions of the strands 50 is within the critical region, each strand will accept a significant portion of the evanescent field energy from the other strands and good coupling will be achieved without significant energy loss. This critical area is the fiber 50
A, 50B, including regions that ensure that they overlap with each other with sufficient strength to provide the evanescent field coupling, that is, that each core is in the other evanescent field. However, as described above, the mode variation occurs when the core portions are too close to each other.
For weak guiding modes, such as the HE ₁₁ mode in single mode optical fibers, such mode variations occur when sufficient amount of material is removed from the fiber 50 to expose the core. . in this way,
The critical region is defined as the region where the evanescent fields overlap with sufficient strength to produce coupling without loss of power caused by significant mode variations.

The area of the critical region for a particular coupler depends on many correlation factors such as the parameters of the fiber itself and the geometry of the coupler. Moreover, for single mode fibers with step index properties, the critical region is extremely narrow. The type of single mode optical fiber shown requires that the center-to-center distance between the strands 50 at the center of the coupler is no more than a few times (e.g., two to three times) the diameter of the core.

Preferably, the strands 50A, 50B
Are (1) identical to each other, and (2) interaction region 54
At the same radius of curvature, and (3) equal to the amount of fiber optic material removed therefrom to form each opposing surface. Thus, the fiber 50 is symmetrical in the plane of its opposing surface via the interaction region 54, so that the opposing surfaces have the same extent when superposed. This means that the two fibers 50A, 50B have identical propagation characteristics in the interaction region 54, thereby avoiding coupling attenuation associated with different propagation characteristics.

The block or base 53 can be made of any suitable rigid material. In one preferred embodiment, the base 53 is approximately 1 inch (2.54 centimeters) long and 1 inch (2.54 centimeters) wide.
And a substantially rectangular parallelepiped block made of fused silica glass having a thickness of 0.4 inch (1.16 cm). In this example, the fiber optic strand 50 comprises:
It is fixed in the groove 52 by a suitable adhesive such as an epoxy adhesive. One effect of fused silica block 53 is to have a coefficient of thermal expansion similar to that of glass fiber, which effect is very important if block 53 and fiber 50 are subjected to any heat treatment during the manufacturing process. is there. Another suitable material for block 53 is silicon, which also has excellent thermal properties for this application.

This coupler has four ports A, B, C and D as shown in FIG. When viewed from the perspective view of FIG. 2, the ports A and C respectively corresponding to the strands 50A and 50B are on the left side of the coupler, and the ports B and D respectively corresponding to the other strands 50A and 50B are on the right side of the coupler. Exists. For discussion purposes, assume that input light is provided at port A. This light passes through the coupler and is output at port B and / or port D, depending on the amount of output coupled between the strands 50. In this regard, the term "standardized cohesive strength" is defined as the ratio of combined output to total output. In the example above,
The standardized combined output is equal to the ratio of the output at port D to the sum of the outputs at port B and port D. This ratio is also referred to as "coupling efficiency" and is usually expressed as a percentage when used as such. Thus, when the term "standardized combined power" is used in this specification, it can be seen that the corresponding coupling efficiency is 100 times the standardized combined power. In this regard, experiments have shown that a coupler of the type shown in FIG.
It was shown to have a coupling efficiency of up to a percentage. However, the coupler can be tuned to adjust the coupling efficiency to any desired value between 0 and the maximum by offsetting the opposing surfaces of the block 53. Such tuning may preferably be achieved by sliding the blocks 53 laterally relative to each other.

The combiner has a high degree of directionality so that substantially all output applied to one side of the combiner is transferred to the other side of the combiner. That is, substantially all of the light provided to input port A will not couple in the opposite direction to port C, and will be output port B,
Informed to D. Similarly, substantially all light provided to input port C is provided to output ports B and D.
Furthermore, this directionality is symmetrical. In this way, the light input to either the input port B or the input port D is transmitted to the output ports A and C. Moreover, this combiner has little selectivity with respect to polarization, and thus preserves the polarization of the combined light. Thus, for example, when a light beam having a vertical polarization is input to port A, the light coupled from port A to port D as well as the light linearly passing from port A to port B is vertically polarized. Will maintain.

As is apparent from the above description, this coupler has two counter-propagating waves W1 and W2.
It can be seen that it functions to separate (see FIG. 1). In addition, the coupler also functions to recombine counter-propagating waves as they traverse loop 14 (FIG. 1).

In the illustrated embodiment, each coupler 26, 34 has a coupling efficiency of 50 percent because the selection of this coupling efficiency provides the maximum light output at photodetector 30 (FIG. 1). As used herein, the term "coupling efficiency" is defined as the ratio of combined output to total output, which is expressed as a percentage.
For example, referring to FIG. 2, when light is input to port A, the coupling efficiency will be equal to the ratio of the output at port D to the sum of the outputs at port B and port D. Furthermore, the coupling efficiency for the coupler 34 is 50
If it is a percentage, waves W1 and W2 propagating in opposite directions
Are guaranteed to be quantitatively equal.

Polarizer 32 A preferred polarizer for use in the rotation sensor of FIG. 1 is shown in FIG. The polarizer includes a birefringent crystal 60, which is positioned within the evanescent field of light transmitted by the fiber 12. The fiber 12 is mounted in a slot 62, which is a substantially rectangular parallelepiped quartz block 64.
Has an opening on the upper surface 63 thereof. The slot 62 has an arcuate curved bottom wall, and the fiber is mounted in the slot 62 along the bottom wall. The upper surface 63 of block 64 is wrapped to remove some cladding from fiber 12 in region 67. The crystal 60 is mounted within a block 64 and the lower surface 68 of the crystal is positioned in the block 64 to position the crystal 60 within the evanescent field of the fiber 12.
Facing the upper surface 63 of the.

The relative index of refraction of the fiber 12 and the birefringent material 60 is such that the wave velocity of the desired polarization mode is greater in the birefringent crystal 60 than in the fiber 12, while the wave velocity of the undesired polarization mode is greater. It is chosen to be larger in fiber 12 than in birefringent crystal 60. Light in the undesired polarization mode is coupled from the fiber 12 to the birefringent crystal 60, while light in the preferred polarization mode continues to be guided by the core of the fiber 12. In this way, the polarizer 32 operates such that light is prevented from passing in one polarization mode and in the other polarization mode. As indicated above, polarization controllers 24, 36 (FIG. 1) are used to adjust the polarization of the input light and the optical output signal, so output loss through the polarizer is minimized. .

Polarization Controllers 24,36 An example of a preferred polarization controller for use with the rotation sensor of FIG. 1 is shown in FIG. The controller includes a base 70 having a plurality of upright blocks 72A-72D mounted therein. Between parts adjacent to one side of the block 72, spools 74A to 74C are tangentially attached to the shafts 76A to 76C, respectively. The shafts 76 are axially aligned with each other,
Moreover, it is rotatably mounted between the blocks 72. The spool 74 has a generally cylindrical shape, and is positioned tangentially to the shaft 76 by the axis of the spool 74 which is perpendicular to the axis of the shaft 76. Strands 12 extend through axial holes in shaft 76 and are wound around each spool 74 to form three coils 78A-78C. The radii of the coils 78 are chosen such that the fiber 12 is drawn within each coil 78 to form a birefringent medium. The three coils 78A to 78C are
To adjust the birefringence of the fiber 12, they are each independently rotatable about the axis of the shafts 74-74C and thus control the polarization of the light passing through the fiber 12.

The diameter and number of windings in the coil 78 are such that the outer coils 78A and 78C provide a quarter wavelength spatial delay, while the center coil 78D provides a half wavelength spatial delay. Is supposed to give. Quarter-wave coil 78
A and 78C control the ellipticity of polarization, and the half-wave coil 78B controls the direction of polarization. This allows polarization adjustment of the entire range of light passing through the fiber 12. However, since the direction of polarization (as opposed to being provided by the center coil 78B) can be indirectly controlled via the correct adjustment of the ellipticity of the polarization by the two quarter wave coils 78A, 78C, It will be appreciated that the polarization controller may be modified to include only two quarter wave coils 78A, 78C. Therefore,
The polarization controllers 24 and 36 are quarter-wave coils 78A,
It is shown in FIG. 1 to include only 78C. This arrangement reduces the overall size of the controllers 24, 36 and may be advantageous in certain size limited applications of the invention. Thus, the polarization controllers 24, 36 provide a means for establishing, maintaining and controlling the polarization of both the input light and the counter-propagating waves.

To better understand the function and importance of operating polarizer 32 (FIG. 1) and phase modulator 38 without phase modulation or polarization control, the operation of the rotation sensor must first be such that these components It is described as being removed from this device. Accordingly, FIG. 5 schematically illustrates the rotation sensor of FIG. 1 with the modulator 38, polarizer 32, and associated components removed.

Light is coupled from laser source 10 to fiber 12 for propagation through the fiber. Light is a coupler 2
6 enters port A, where some light is lost by port D. The remaining light propagates from port A to port B of combiner 34 where it is separated into two oppositely propagating waves W1 and W2.
Wave W1 propagates around port 14 in the counterclockwise direction from port B, while wave W2 propagates around port 14 in the counterclockwise direction from port D. After the waves W1, W2 have traversed the loop 14, these waves are recombined by a combiner 34 to form an optical output signal, which is output from port A of combiner 34 to port B of combiner 26. Propagate to. Some optical output signals are sent to the fiber 2 through the photodetector 30.
8 is coupled from port B to port C of combiner 26 for propagation along 8. The photodetector 30 outputs an electrical signal proportional to the light intensity applied thereon by the optical output signal.

The strength of this output signal is such that when the waves W1, W2 are recombined or superposed in the combiner 34,
It will vary depending on the amount and type (ie constructive or destructive) of the interference between the waves W1, W2. At this time, if the influence of the birefringence of the fiber is ignored, the waves W1, W
2 pass the same optical path around the loop 14.
Thus, assuming loop 14 is stationary,
When the waves W1 and W2 are recombined in the combiner 34, the wave W
1, W2 will be strengthened without generating a phase difference, and the optical output signal strength will be maximum. However, loop 1
When W4 is rotated, waves W1 and W2 propagating in opposite directions
Will shift in phase due to the Sagnac effect, and will thus interfere to weaken the optical output signals when combined in combiner 34. Such a Sagnac phase difference between the waves W1 and W2 caused by the rotation of the loop 14 is defined by the following relational expression.

[0042]

[Equation 1]

Where A is the area coupled by the loop 14 of the optical fiber, N is the number of turns of the optical fiber around the area A, Ω is the angular velocity of the loop about an axis perpendicular to the plane of the loop, And λ and c respectively indicate the wavelength and angular velocity of the light provided to the loop.

The intensity of the optical output signal ( _IT ) is determined by the waves W1, W
It is a function of the Sagnac phase difference (φ _WS ) between the two and is defined by the following equation.

[0045]

[Equation 2]

Where I ₁ and I ₂ are the waves W1,
The individual intensities of W2 are shown.

From equations (1) and (2), it can be seen that the optical output signal strength is a function of the rotation speed (Ω). Thus, this numerical value of the rotational speed can be obtained by measuring the optical output signal intensity using the detector 30.

FIG. 6 shows a curve 80 which shows the intensity of the optical output signal (I _T ) and the waves W1, W2 propagating in the half direction.
Represents the relationship between the Sagnac phase difference (φ _WS ). This curve 80 shows a cosine curve, and the optical output signal strength is Sa
It becomes maximum when the gnac phase difference (φ _WS ) is zero.

Assuming that the only source of phase difference between the counter-propagating waves W1 and W2 is the rotation of the loop 14, this curve 80 will change symmetrically about the vertical. However, in reality, the waves W1 and W propagating in opposite directions
The phase difference between the two can be caused not only by the rotation of the loop 14 but also by the residual birefringence of the optical fiber 12.
Since the birefringence of the fiber causes light to propagate at different speeds in each of the two polarization modes of the single mode fiber 12, a phase difference induced by the birefringence occurs. this is,
The phase difference induced irreversibly and non-rotatably by the wave W1,
It occurs between W2 and this is, for example, the curve 8 shown by an imaginary line.
As represented by 2, the waves W1, W2 are interfered to distort or shift the wave 80 of FIG. The non-reciprocal retardation induced by such birefringence is indistinguishable from the rotationally induced Sagnac retardation and is an environment that changes the birefringence of the fiber such as temperature and pressure. Depends on the element. Therefore, fiber birefringence is a major source of error in fiber optic rotation sensors.

The problem of non-reciprocal operation due to the birefringence of the working fiber by the polarizer 32 is due to the invention of the polarizer 32 (of FIG. 1) as described above, which allows the use of only a single polarization mode. It is solved by the rotation sensor of. Thus, when the polarizer 32 is introduced into the device, at the point indicated by reference numeral 84 in FIG.
Light input through the polarizer 32 propagates in the loop 14 in the desired polarization mode. Further, when the counter-propagating waves are recombined to form an optical output signal, the optical output signal is polarized as it propagates from port A of coupler 34 to port B of coupler 26. Since it also passes through the child 32, all light of a different polarization than the light entering the loop is prevented from reaching the photodetector 30. Thus, when the light output signal reaches the detector 30, it will have exactly the same polarization as the light entering the loop. Therefore, by passing the input light and the output optical signal through the same polarizer 32, only one optical path is used, thereby eliminating the problem of birefringence-induced phase difference. You can In addition, the polarization controllers 24, 36 (FIG. 1) respectively adjust the polarization of the input light and the optical output signal to maximize the signal strength at the detector 30 in order to reduce the optical output loss at the polarizer 32. It can be used to adjust

Operation of Phase Modulator 38 Referring again to FIG. 6, since the curve 80 is a function of cosine, the optical output signal has a small phase difference (φ) between the waves W1, W2.
_It can be seen that it is nonlinear with respect to _WS ). Moreover, the optical output signal strength is relatively insensitive to changes in phase difference for small values of φ _WS . This non-linearity and non-responsiveness is due to the light intensity ( _IT ) measured by the detector 30.
Makes it difficult to convert the rotation speed of the loop 14 into a signal display Ω (Equation 1).

Further, as described above, by using the polarizer 32, the phase difference induced by the birefringence between the waves W1 and W2 is eliminated, but between the polarization modes caused by the birefringence of the fiber. Cross-coupling prevents such cross-coupled light from reaching the detector 30 by the polarizer 32, thus reducing the optical intensity of the optical output signal. Thus, changes in fiber birefringence cause the magnitude of curve 80 in FIG. 6 to change, as shown, for example, by curve 84. It will be appreciated that the curves 80, 82, 84 of FIG. 6 are not drawn to scale.

The above-mentioned problems are solved in the rotation sensor of the present invention by the synchronization detecting device utilizing the phase modulator 38, the signal generator 40 and the lock-in amplifier 46 shown in FIG. Referring to FIG. 7, the phase modulator 38 includes waves W1 and W2 propagating at a certain frequency of the signal generator 40.
Modulate each phase of. However, as is apparent from FIG. 1, the phase modulator 38 is arranged at one end of the loop 14. Thus, the modulation of the wave W1 does not necessarily have to be in phase with the modulation of the wave W2. In fact, the modulation of the waves W1, W2 is not in phase,
This is necessary for the correct operation of this sync detector. Referring to FIG. 7, the modulation of wave W1 represented by curve 90 is preferably 180 degrees out of phase with the modulation of wave W2 represented by curve 92. Wave W1 Wave W2
Using a modulation frequency that provides such a 180 degree phase difference for the modulation of 1 is particularly effective in eliminating modulator-induced amplitude modulation in the optical output signal measured by detector 30. This modulation frequency (fm) is calculated by the following formula.

[0054]

[Equation 3] Where L is the difference in fiber length between the coupler 34 and the modulator 38 for the counter-propagating waves W1, W2 (ie, between the modulator 38 and the point of symmetry on the other side of the loop 14). Distance measured along the fiber) and neg
Is the transmitted index of refraction for the single mode fiber 12 and c is the free space velocity of the light entering the loop 14.

At this modulation frequency (fm), the curves 90,
The phase difference between the waves W1, W2 propagating in opposite directions, based on the phase modulation of the waves W1, W2 by 92, is represented by the curve 94 in FIG. This modulation of the phase difference between the waves W1 and W2 is due to such a phase difference φ _Wm being induced by rotation.
Since it is indistinguishable from the Sagnac phase difference φ _WS , it will modulate the optical output signal strength ( _IT ) according to curve 80 in FIG.

The above description can be understood more clearly with reference to FIGS. 8 and 9, which show the effect of the phase modulation φ _Wm defined by the curve 94 of FIG. When, it is representative of the effect of the Sagnac phase difference phi _WS based on the intensity of the optical output signal represented by curve 80 in FIG. 6 (I _T) graphically. However, prior to the discussion of FIGS. 7 and 8, first the modulated optical output signal is wave W.
It should be understood that it is a function of the total phase difference between 1 and W2. Furthermore, such a total phase difference consists of the rotation-induced Sagnac phase difference φ _WS and the time-dependent modulation-induced phase difference φ _Wm . in this way,
The total phase difference φ _W between the waves W1 and W2 can be expressed by the following equation.

[0057]

[Equation 4]

[0058] Thus, the effect of the induced modulation like the phase difference phi _WS induced by rotational phase difference phi _Wm is considered in connection with FIGS. 8 and 9, the horizontal axis for the curve 80 , Φ _W , to show that the total phase difference is taken into account rather than simply the phase difference induced by rotation, as shown in FIG.

Now referring to FIG. 8, the output signal (curve 80).
The effect of the phase modulation φ _Wm (curve 94) based on the intensity I _T of will be discussed. In FIG. 8, it is assumed that the loop 14 is stationary, so the optical signal is unaffected by the Sagnac effect. In particular, the phase difference curve 94 induced by the modulation causes the optical output signal according to the curve 80 to change symmetrically about its vertical axis, so that the optical intensity measured by the detector 30 is of the modulation frequency as shown by the curve 96. It can be seen that the frequency changes periodically at a frequency equal to the second harmonic.
As discussed above, the lock-in amplifier 46 causes the modulator 3
The detector output signal is shown by curve 96, as activated by the signal generator 40 (FIG. 1) to synchronously detect the detector output signal at a modulation frequency of 8 (ie, the first harmonic). Since it is the second harmonic of the modulation frequency, the output signal of the amplifier will be zero and the display 47 will indicate that the rotational speed is zero. Even if birefringence-induced amplitude variations occur in the optical output signal, as discussed with respect to curve 84 of FIG. 6, curve 96 of FIG. 8 will continue to maintain the second harmonic frequency. It should be understood. Thus, the amplitude variation induced by this birefringence will not affect the output signal of amplifier 46. Hence,
The detection device of the present invention provides a substantially stable operating point and is not subject to changes in birefringence, especially when the loop 14 is stationary.

When the loop 14 is rotated, the counter-propagating waves W1 and W2 are Sagnac, as discussed above.
It is shifted in phase according to the effect. In addition to the phase difference φ _Wm produced by the modulator 38, the Sagnac effect has a phase difference φ
_WS , so that the entire curve 94 is displaced in phase from the position shown in FIG. 8 by an amount equal to the position shown in FIG.
This causes the optical output signal to change asymmetrically by the device 80, thereby causing this signal to be harmonically distorted, as shown by curve 96 in FIG. As shown, it contains a component at the fundamental (ie first harmonic) frequency of modulator 38. This curve 9
8 RMS value induced by rotation Sagnac phase difference φ _WS
You will find that it is proportional to the sign of. Amplifier 46
Detects a signal having the fundamental frequency of the modulator 38, the amplifier 46 will output a signal to the display 47 which is proportional to the RMS value of the curve 98 for displaying the rotational speed of the loop.

FIG. 9 represents the intensity of the optical output signal for one direction of rotation of the loop 14 (eg clockwise). However, if the loop 14 is rotated in the opposite direction at the same rate (eg, counterclockwise), the intensity waveform 96 of the optical output signal is such that the curve 98 is shifted 180 degrees from the position shown in FIG. It will be appreciated that it is almost identical to that shown in FIG. 9 except that it has been moved to. The lock-in amplifier 46 detects the 180 degree phase difference for the curve 98 by comparing the phase of the reference signal from the signal generator 40 with this 180 degree phase difference for the curve 98 to rotate the loop. Is clockwise or counterclockwise. Depending on the direction of rotation, amplifier 46 outputs either a positive or negative signal to display 47. However, regardless of the direction of rotation, the magnitude of the signal is equal to the rotational speed of loop 14.

The waveform of the output signal of the amplifier is shown in FIG. 10 as curve 100. It can be seen that this curve 100 is tortuous and that the rotational speed varies from zero to positive or negative depending on whether the rotation of the loop 14 is clockwise or counterclockwise. Furthermore, the curve 100
Has a substantially linear portion 102 that varies symmetrically with respect to the origin and provides a relatively wide range of motion for measuring rotation. Also, the slope of curve 100 provides excellent sensitivity over its entire linear operating range 102.

Thus, by using a synchronism detector, the problems of non-linearity, insensitivity and birefringence-induced amplitude fluctuations are reduced or eliminated.

Yet another advantage of this detector is related to the fact that the state of an artificial phase modulator, such as modulator 38, causes amplitude modulation in the optical output signal, either directly or indirectly through polarization modulation. . However, as recalled from the discussion regarding Equation 3, by operating at a particular frequency where the phase difference between the modulation of waves W1 and W2 is 180 degrees, the waves are superposed to form an optical output signal. At this time, the odd-numbered harmonic frequency components of the amplitude modulation induced by the waves W1 and W2 propagating in the opposite directions by the modulator 38 cancel each other out. As described above, since the above-described detection device detects only the even harmonics (that is, the fundamental frequencies) of the optical output signal, the influence of the amplitude modulation is removed. Therefore, by operating at a particular frequency defined by Equation 3, as well as detecting only the even harmonics of the optical output signal,
The rotation sensor of the present invention can operate independently of the modulator-induced amplitude polarization modulation.

A further advantage of operating at a particular frequency is that the modulators 38 in each of these counter-propagating waves W1, W2 when the waves W1, W2 are superposed to form the optical output signal. The even harmonics of the induced phase modulation are in cancellation. This elimination improves the accuracy of rotation detection, because these even harmonics produce spurious even harmonics in the optical output signal that can be detected by the detector.

Phase modulator 38 at the frequency defined by equation 3
It is preferable to adjust the phase modulation amount to maximize the detected first-order harmonic amount of the optical output signal intensity in addition to the above operation. This is because this can improve the rotation detection sensitivity and accuracy. Waves W1, W represented by the magnitude indicated by Z in FIGS. 7, 8 and 9
It can be seen that the first harmonic of the optical output signal strength is maximum when the amount of phase difference induced by the modulator between the two is 1.84 radians. This can be better understood with respect to the following formula for the phase difference φ _W between the _two superposed waves and the total intensity ( _IT ) of the superposed waves with independent intensities I ₁ , I _2. Will.

[0067]

[Equation 5]

In the equation, J _n (Z) is the n-th order Bessel function of Z, and Z represents the magnitude of the peak of the phase difference induced by the modulator between the waves W1 and W2. Therefore, the first of I _T
If you detect only the next harmonic,

[0069]

[Equation 6]

As described above, the first-order harmonic of the optical output signal intensity depends on the value of the first-order Bessel function J ₁ (Z). Z
Since J ₁ (Z) is maximum when is equal to 1.84 radians, the magnitude of the phase modulation is preferably such that the waves W1, W
The magnitude of the phase difference (Z) induced by the modulator between the two is chosen to be 1.84 radians.

Reducing the Effect of Backscattering As is well known, the state of the art in optical fiber technology is not optically perfect and has the drawback of causing a small amount of light scattering. This reduction is well known as Rayleigh scattering. Although such scattering causes some of the light to be lost from the fiber, the amount of such loss is relatively small and therefore not a major issue. The main problem with Rayleigh scattering is not the lost scattered light, but rather the fact that the light is reflected to charge the fiber in its original direction of propagation and in the half direction. This is commonly known as backscattered light. Since such backscattered light is coherent with the light having the waves W1 and W2 propagating in the opposite directions, it causes constructive interference or destructive interference with the propagating wave, thereby generating noise in the device. That is, it causes spurious variations in the optical output signal strength as measured by detector 30.

A portion of the backscattered light from a wave that is coherent with the counterpropagating wave will be scattered within the coherent length of the center of the loop 14. Thus, by reducing the coherence length of the light source, the coherence between the return scattered light and the counter-propagating wave is reduced. The rest of the backscattered light is not coherent with the counterpropagating waves, so the interference between them varies randomly and is averaged. Therefore, the non-coherent portion of this return scattered light has a substantially constant intensity and, as a result, does not cause large variations in the optical output signal intensity. Therefore, in the present invention, the effect of the backscattered light is reduced by using as the light source 10 a laser having a relatively short coherence length, for example within 1 meter or outside. According to a particular embodiment, the light source 10
Is a model GO-DIP as commercially available from General Optronics Corporation as described above.
It may be a laser diode.

An alternative method of preventing constructive or destructive interference between the backscattered light and the counter-propagating wave is to add an additional phase modulator (not shown) to the device at the center of loop 14. ) Is provided. This phase modulator is not synchronized with modulator 38.

The propagating wave will pass through this additional phase modulator only once as it passes around the loop.
For the backscatter generated from the propagating wave before the wave reaches the additional phase modulator, the return scatter is because neither the wave propagating from the source nor the backscattered light passes through the additional modulator. It would not have the phase modulated by this additional modulator.

On the other hand, regarding the backscatter generated from the propagating wave after the wave has passed through this additional phase modulator, when the propagating wave passes through the additional phase modulator, the return scattered light is It will effectively be phase modulated twice, once it has passed through the additional phase modulator.

Thus, if the additional phase modulator introduces a phase shift φ (t), the backscattered waves originating from all points except the center of loop 14 will be 0 or 2φ.
Having any phase shift of (t), any of which is the time varying with respect to the φ (t) phase shift for the propagating wave. The time to change this interference is averaged to effectively eliminate the effect of the backscattered light.

Another alternative method of preventing constructive or destructive interference from the backscattered light is modulator 38.
An additional phase modulator not synchronized with can be introduced at the output of the light source 10.

In this case, the backscatter generated at all points except the center of the loop 14 has various optical path lengths from the light source 10 to the detector 30 rather than the propagating wave generated by the backscattered light. Will.

In this way, the propagating wave passes through loop 14 by 1
The backscattered wave and the propagating wave it originates from will pass through a portion of loop 14 twice. If this part is half of the loop, the path length is different.

Due to the different path lengths, the propagating wave reaching the detector 30 must be generated at the light source 10 a different number of times than the returning scattered wave reaching the detector 30 at the same time.

The phase difference introduced by the additional phase modulator in the light source 10 is the phase difference φ with respect to the propagating wave.
Introducing (t), but injecting φ (t + k) for scattered light, where K is the time difference between the waves passing through the loop 14. As φ (t + k) changes with time with respect to φ (t), the backscatter interference is averaged over time, effectively eliminating the effect of backscatter.

Reduction of Influence of Surrounding Magnetic Field A surrounding magnetic field such as a magnetic field due to geomagnetism introduces a phase difference between waves W1 and W2 propagating in opposite directions, thereby limiting the rotation detection accuracy of the present invention. I know that. Such a magnetic field is generated by waves W1 and W2 propagating in opposite directions.
Of one of the two are shifted in opposite directions relative to each other, leading one and delaying the other.

This phase shift of the waves W1, W2 is
1, based on the components of the surrounding magnetic field having a B field parallel to the propagation direction of W2. This magnetic field component causes a phenomenon known as the Faraday effect, which rotates the polarization direction for each wave. This is referred to herein as the Faraday rotation. Expressing the polarization of a light wave as the composition of two counter-rotating circularly polarized components propagating in each polarization mode, this magnetic field reduces the propagation velocity of light in one polarization mode, while in the other. By increasing the light propagation speed of
It can be considered that this leads to the Faraday effect.

The polarization of a light wave can be characterized by its degree of ellipse. If the ellipticity is zero, then the polarization is usually shown as a "straight line". In this state, there is an equal amount of light in each mode. Similarly, if the ellipticity is 1, this polarization will be shown as circularly polarized and all light will be in a single mode. Furthermore, when the ellipticity lies between 0 and 1, this polarization is shown as elliptically polarized and this mode is not equal to the amount of light.

When the wave polarization is linear, the change in the difference in the propagation velocity of the polarization modes due to the Faraday effect is
It has no effect on the phase of the waves. However, if the polarization of the wave is other than linear (ie elliptically or circularly polarized), the change in the difference in propagation velocity causes a shift in the phase of the wave, such an amount of phase shift depending on the ellipticity of the polarization. When the polarization is circularly or elliptically polarized, there is an unequal amount of light in each of the two polarization modes, as described above, so if the propagation velocity of one mode increases, the other decreases. , The total effect increases or decreases the propagation velocity of the wave, thereby shifting the phase of the wave. For example, if it is assumed that the Faraday effect increases the propagation velocity for the first-order mode and decreases the velocity for the second-order mode, then the light of the first-order mode has more spread than the light of the second-order mode. This Farade effect would result in a phase shift. On the other hand, assuming the light in the second-order mode is more divergent than in the first-order mode, the Faraday effect will result in a delay layer.

As indicated above, the phase shift resulting from the Faraday effect is eliminated by maintaining linear polarization for the waves W1 and W2 as the waves W1 and W2 propagate backwards through the loop 14. Can be done. Unfortunately, however, this is difficult to achieve because currently available optical fibers have residual birefringence that changes the states of polarization as they pass through the waves W1, W2. For example, the waves W1 and W2 are loops 14
If it is linearly polarized when introduced into
As it passes through 4, such residual birefringence will change this polarization, for example elliptical. Therefore, each wave W1, W2 exhibits a phase shift due to the Farade effect as it exits the loop 14. Moreover, in the embodiment of FIG. 1, these phase shifts are in the half direction, thus creating a phase difference between the waves W1, W2.

The above description may be better understood with reference to the examples. To simplify this discussion, the loop portion of the rotation sensor of FIG. 1, shown as being a single-turn loop rather than a multi-turn loop, is shown in FIG. Further, the residual birefringence of the fiber is believed to be concentrated at the center of loop 14, at the point indicated by reference numeral 117. In addition, the geomagnetic field (B
Field) is believed to lie in the plane of the loop in the direction indicated by arrow 118, so this B field is approximately parallel to the fibers present at the top and bottom of the loop, as observed in FIG. As will be recalled from the discussion of FIG. 1, the waves W1, W2 are linearly polarized as they enter the loop 14 and the polarization controller 36 is tuned to compensate for the birefringence of this fiber, so that the wave W
When W1 and W2 leave the loop 14, waves W1 and W2
The polarization of is also linear. Furthermore, as long as the fiber birefringence is symmetrically distributed around the loop 14, FIG.
, The polarization of the waves W1, W2 with the controller 36 so adjusted will be similar to that of any point of the loop 14. In this example, loop 117
It is believed that the birefringence at the center of changes the phase of each wave by a quarter wavelength, so that curvilinearly polarized light is converted to circularly polarized light and vice versa. Thus, by shifting the phase by an equal amount, or a quarter wavelength, the controller 36 is adjusted to compensate for this phase change.

When the linearly polarized wave W1 begins to traverse the loop 14, the birefringence of the controller 36 transforms its polarization state into, for example, right-handed circular polarization. When this wave W1 passes through the upper part of the loop 14, its phase will be shifted due to the presence of the field 118 due to the Faraday effect.
When the residual birefringence is reached at the center 117 of the loop 14, the circular polarization of the wave W1 will be converted to linear polarization.
The field 118 will have no effect on the wave W1 as the polarization of the wave W1 remains straight throughout the bottom portion of the loop 14. Similarly, the wave W2 that first crosses the lower part of the loop is not affected by the field 118 in this lower part. Because its polarization will continue to be linear until it reaches the birefringence at point 117. At this point 117, the polarization of the wave W2 is converted to, for example, right-handed circular polarization, so that when the wave W2 crosses the upper part of the loop, its phase will be shifted due to the Faraday effect due to the presence of the field 118. However, the wave W
1 and W2 propagate in the opposite direction with the same polarization in the upper part of the loop 14, but the field 118 induced waves W1 and W2 have opposite phase shifts because the field 118 remains in the same direction. Will. Thus, the wave W1,
When W2 reaches the combiner 34 due to the Faraday effect, there will be a phase difference between the waves W1, W2. Therefore, it can be seen that the surrounding magnetic field causes the non-reciprocal behavior in the fiber optic rotation sensor.

In order to eliminate the rotation detection error caused by the surrounding magnetic field due to the Farade effect, the present invention provides
12 includes a housing 110 for shielding or isolating the rotation sensor, and in particular for shielding or isolating the loop 14 and coupler 34 from such ambient magnetic fields. In the embodiment shown, the housing 1
10 comprises a cylindrical tube made of mu-metal having sufficient permeability to effectively shield the rotation sensor from the surrounding magnetic environment. The size of the housing 110 is
It is chosen to match the structural size of this rotation sensor with the hostility of the surrounding magnetic field. According to a particular embodiment. μ metal shield has a diameter of 7 inches and a length of 1
8 inches and a thickness of 1/16 inch. In any case, the size and material used should preferably reduce the magnetic field adversely affecting the fiber by an amount equal to the detection accuracy of the rotation sensor. That is, the reduction in magnetic field strength should be such that rotation detection accuracy is not limited by the Faraday effect caused by such magnetic fields. Assuming that the surrounding magnetic field is all based on the magnetic field due to geomagnetism (ie, a magnetic field of about 0.5 Gauss), the shield example above would have a magnetic field of about 100.
To a factor of 0.005, which is necessary to achieve long term stability of about 0.1 degree / hour.

The fiber optic components of the rotation sensor, including the loop 14, may be mounted on a base plate 112 mounted within the housing 110. Both ends of the housing are closed by a μ metal cap 114, one of which has suitable openings 116 for passage of the amplifier line 48 and the modulator line 39 (FIG. 1).

Separation of Light Source 10 from Optical Output Signal As recalled from the discussion with respect to FIG. 1, a portion of the input light from light source 10 is coupled by coupler 26 into fiber 28, here "NC". Lost at non-reflective ends marked with. Furthermore, when the waves W1, W2 return from loop 14 and are combined to form an optical output signal,
Some of this signal is lost through port C of combiner 34. The remaining portion of the output signal propagates back towards the light source 10, where a portion of the optical output signal is coupled from fiber 12 to fiber 28 for propagation by coupler 26 to photodetector 30. It The remaining uncoupled portion of the optical output signal propagates to the laser source 10 via fiber 12 but is lost. 50% of the couplers 26 and 34
Assuming that they have a coupling efficiency of, the loss of this device from the couplers 26, 34 is 87.5%. Combiner 2
With 6 alone, this loss in light output is 67.5%.

In order to reduce the loss of these devices, the present invention, as shown in FIG.
Including 0. The separator 120 comprises a polarizer 122 and a magnetic rotator or Faraday rotator 124. In this embodiment, the need for coupler 26 (FIG. 1) is
It is eliminated by placing a detector 30 to measure the intensity of the light blocked by 22. This light is focused on the photodetector 30 by the lens 126.

The operation of the optical fiber separator can be understood more clearly with reference to FIGS. Referring first to FIG. 14, the polarization of the light passed by the polarizer 122 is aligned with the light produced by the light source 10 so that all light introduced into the fiber 12 is polarized 12.
2 to reach the magnetic rotor 124. However,
It will be appreciated that this polarization matching can be achieved with a polarization controller of the type discussed above with respect to the controllers 24,36. For the purposes of this discussion, assume that the light generated by the light source 10 is linearly polarized in the vertical direction, and that the polarizer 122 passes polarized light in this direction while blocking polarized light in the other directions. This linearly polarized light is represented by arrow WS in FIG.

As is apparent from FIG. 14, the light source light WS generated by the light source 10 does not change the polarization when passing through the polarizer 122. However, this light
As it passes through 24, its polarization direction is rotated by 45 degrees. Returning to FIG. 13, next, this light WS is transmitted to the polarization controller 2
4 and its polarization is adjusted in this polarization controller 24 to more effectively pass through the polarizer 32, as previously discussed. For example, if the polarizer 32 is designed to pass light having a linear and vertical polarization, the controller 24 will polarize the light by 45 degrees in the opposite direction to that generated by the rotator 124. It is arranged to rotate the direction, so that this light is redirected to vertical again. This light is then separated by coupler 34 into counter-propagating waves W1, W2 for propagation around loop 14. After traversing the loop 14, the waves W1, W2 are recombined by the combiner 34 and the polarizer 32
Forming an optical output signal that propagates back through. It will be recalled that the polarization controller 36 is used to adjust the polarization of the counter-propagating wave, so that the optical output signal effectively passes through the polarizer 32, for example with a linear vertical polarization. . The polarization controller 24, which is a reversible device, then rotates the direction of polarization of the optical output signal by 45 degrees, so that when it exits the controller 24 and with the source light as it enters the controller 24. Have the same polarization. Therefore, as shown in FIG. 15, the optical output signal WO is shown to enter the separator 120 with the same polarization as the polarization of the source light WS (FIG. 14) exiting the separator 120 as seen by the observer. Has been done. As the optical output signal WO passes through the rotator 124, the direction of polarization is rotated an additional 45 degrees. The rotor 124 has the same polarization direction regardless of the light propagation direction.
Is a unique feature of. Thus, the first four of the light from the light source
The 5 degree rotation and the second 45 degree rotation of the optical output signal
When the light output signal WO leaves the rotator 124, it is added to have a horizontal polarization. The light output signal WO is prevented from propagating through the polarizer 122 to the light source 10 because it blocks the polarization perpendicular to the polarization that it passes through. Polarizer 122 couples the light from fiber 12 and
As a result, the signal WO is emitted from the polarizer 122 in the directionally dispersed light. Preferably, the polarizer 122
Is of the same type as discussed above for polarizer 32. The light blocked by this type of polarizer is emitted from it by a linear light beam with a relatively small angle of incidence (eg 20 degrees), so that the photosensitive surface of the detector 30 is specially large or unique. It need not have a shape. In the example shown, this photosensitive surface has a diameter of about 1 mm.

This light is reflected by the lens 126 as described above.
Is projected onto the detector 30 by focusing on the detector 30 with. Alternatively, an optical fiber (not shown) having a core diameter of 500 microns, for example, can be used to direct the light to the detector 30, by positioning one end of this fiber in the immediate vicinity of the polarizer. ,
The light blocked by the polarizer can be introduced into the fiber and the light from the fiber can be incident on the detector 30 by positioning the other end of the fiber. Even if all the light blocked by the polarizer 122 is not projected onto the photosensitive surface of the detector 30, for example due to slight misalignment of the focusing lens or guiding fiber, the removal of the combiner 26 (FIG. 1). It can be seen that this is more than compensated for by the increase in the intensity of the optical output signal caused by.

Furthermore, when light is blocked by this polarizer, the direction of the above-mentioned linear light emitted from the polarizer 122 will be different from one propagation direction than another. That is, for propagation in one direction,
Light is emitted on one side of the polarizer 122 and, for propagation in the other direction, light is emitted on the other side of the polarizer.
Therefore, even if the polarization of the source light is not exactly the same as the light passed through the polarizer, the source light blocked by all the polarizers will not be directed towards the detector and thus the optical output signal strength. Does not interfere with the measurement of.

As shown in FIG. 16, the magneto-optical rotator is
A fiber 132 wound around an axis 170 is provided to provide a series of fiber loops having a curved portion and a straight portion. The shaft 170 is made of a non-magnetic material such as aluminum and has a central bar portion 170 with a square cross section. 1
A pair of cylindrical portions 174, 176 are formed at opposite ends of the central portion 172 and are perpendicular to them. The cylindrical portions 174, 176 are oriented at right angles to each other. As can be seen in FIG. 16, the cylindrical portion 174 has a right end 175 (a) and a left end 175 (b) that project from each parallel side of the central portion 172. Similarly, the cylindrical portion 176 has a central portion 172.
Has an upper end portion 177 (a) and a lower end portion 177 (b) protruding from each of the parallel side surfaces. Further, the cylindrical portion 17
4, 176 have a diameter equal to or larger than the side surface of the central portion 172.

The fiber 132 is first wound around the upper end 177 (a) of the cylindrical portion 176 and joined to the two straight portions 180, 182 with a curved fiber portion 178.
To form. Next, this fiber is
A curved portion 183 that is wound around the left end portion 175 (b) of 74 and connects the straight portion 184 and the straight portion 182.
To form. This winding changes the curved portion 184 into the straight portion 1
Followed by forming another curved portion 186 around the lower end 177 (b) of the vertical cylindrical portion 176 to connect to 88. Finally, another curved section 190 connects the fiber 132 around the right end 175 (b) of the horizontal section 174 to connect the straight section 188 with the straight section 192.
Is formed by winding. It can be seen that this winding is such that the straight sections 180, 182, 184, 188, 192 are parallel to each other. further,
By winding the fiber as described above, curved portions 178, 186 are present in the horizontal plane and curved portion 1
83 and 190 lie in the vertical plane.

For clarity of explanation, only four turns (curved portions) are provided in the fiber 132 of FIG. 16, but the fiber 132 is identical to provide more turns. It may be wound by the method of. Further, as can be seen from the description below, the curved portion is 2
Although shown as having one-half turn, these are N
It provides +1/2 rotations (n is an integer) and further allows the linear portion to be positioned as shown in FIG.

The shaft 170 is connected to the magnet 200 as shown in FIG.
By positioning between the poles of the
B field of this magnet is therefore given to the fiber 132
Is parallel to the straight line part of. The magnet 200 may have any form or shape. For example, it may be either an electromagnet or a permanent magnet. further,
This magnet may have a toroidal shape, for example,
Alternatively, it may have a horseshoe shape.

The light is linear portions 180, 182, 184, 1
As it propagates through 88,192, its polarization direction is rotated by the magnetic field due to the Faraday effect. When the fiber 132 is wound as shown in FIG.
The light propagating through reverses the direction of its polarization as it passes from one straight section to another. Thus, light propagates in the same direction as the B field, eg, via straight sections 180, 184, 192, while, for example, straight sections 182, 18
Propagation in half direction from the B field via 8. Therefore, the magnetic field rotates its polarization by the Farade effect,
Thus, from the perspective of a fixed (ie, stationary) observer, any two adjacent straight line portions 180, 18
The Faraday rotations at 2,184,188,192 would appear to be in the same direction. However,
Even if these rotations are in the same direction, they usually do not add to each other. because,
Propagation around the curved sections 178, 183, 186, 190 reverses the direction of polarization (as seen by a stationary observer) in a linear section, mirroring the light of the adjacent linear section, Therefore, the rotations due to the Faraday effect in any two straight line portions cancel each other out to give a total rotation of 0. The magneto-optic rotator of FIG. 16 forms the curved sections 178, 183, 186, 190 (ie, by selecting the diameter of the cylindrical sections 174, 176 and the number of turns of the fiber 132).
Each of these curved sections eliminates this problem, so that each of them produces a linear birefringence sufficient to provide spatial separation between the half-wavelength polarization modes, i.e., 180 degree phase difference light. This is more advantageous in that the polarization direction seen by the observer is in each of the linear portions 180, 182, 184, 18
8 and 192, so that the Farade rotations add to each other rather than cancel each other out. Therefore, by providing a series of straight line portions, a large Farade rotation can be obtained even if the Farade rotation for one straight line portion is relatively small.

Magnetic field applied to fiber 132 (B field)
If the intensity of is approximately 1000 Gauss, a fiber wound according to the properties described below will provide a total Faraday rotation of 45 degrees for light propagating through the fiber in either direction. Thus, if the light wave propagates through fiber 132 in one direction, as well as back in the other direction, the total Faraday rotation is 90.
It will be a degree. This amount of Farade rotation allows using the magneto-optical rotator as an optical separator, as discussed with respect to FIGS. 14 and 15.

For example, in one embodiment, a total of 32 turns are used. The characteristics of this example are as follows.

[Brief description of drawings]

1 is a schematic diagram of a rotation sensor of the present invention showing fiber optic components arranged along a continuous, uninterrupted strand of fiber optic material, and including a signal generator, a photodetector, and a lock-in. Further shown is an amplifier and a display associated with this detection device.

2 is a cross-sectional view of one embodiment of an optical fiber directional coupler for use in the rotation sensor of FIG.

FIG. 3 is a cross-sectional view showing an example of an optical fiber polarizer used in the rotation sensor of FIG.

4 is a perspective view showing an example of an optical fiber polarization controller for use in the rotation sensor of FIG.

5 is a schematic illustration of the rotation sensor of FIG. 1 with a polarizer, a polarization controller and a phase modulator removed therefrom.

FIG. 6 is a graph showing the intensity of the optical output signal as a function of rotation-induced Sagnac phase difference measured by a photodetector, the effect of birefringence-induced phase difference and amplitude variations. Represents

FIG. 7 is a graph of phase difference as a function of time showing the phase modulation of each counter-propagating wave and the phase difference between counter-propagating waves.

FIG. 8 is a schematic diagram showing the effect of phase modulation on the optical output signal strength as measured by the photodetector when the loop is mounted.

FIG. 9 is a schematic diagram showing the effect of phase modulation on the optical output signal strength measured by the detector when the loop is rotating.

10 is a graph showing the output signal of an amplifier as a function of rotation-induced Sagnac phase difference, representing the operating range for the rotation sensor of FIG.

11 is a simplified schematic of the loop portion of the rotation sensor of FIG. 1 to show the effect of geomagnetism on the counter-propagating wave.

FIG. 12 is a schematic diagram showing a housing for surrounding a detection loop to shield the detection loop from the surrounding magnetic field.

13 is a schematic diagram showing the rotation sensor of FIG. 1 with an optical isolation device provided to prevent the light output signal from reaching the light source, and drawing the light output signal from a continuous fiber strand. 2 shows a photodetector arranged to detect the light rejected by this separation device so that no coupler is required.

FIG. 14 is a schematic diagram of a light source and an optical demultiplexer showing the effect of the demultiplexer on the light propagating in the direction of the detection loop from the light source.

FIG. 15 is a perspective view corresponding to FIG. 14 and is a perspective view showing the influence of the optical demultiplexing device on the optical output signal when the optical output signal propagates from the loop toward the return light source.

FIG. 16 is a perspective view showing a preferred method of winding an optical fiber to form a magneto-optical rotator.

FIG. 17 is a schematic diagram showing an example of a magneto-optical rotator for use in the optical separation device shown in FIG.

[Explanation of symbols]

 Number of curved parts: 32 Number of linear parts: 33 Length of one linear part: 12 cm Diameter of axial cylindrical part: 2.5 cm Number of turns of each curved part: 1.5 Fiber outer diameter: 110 μm Wavelength of: 0.633 micron Total length of fiber: 4 meters All are approximate values.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shaw, Herbert John United States, 94305 Stanford, Alberta Row, California, 719 (72) Inventor Refiva, Elbay Sea United States, 94303 Palo Alto, California Moreno Avenue, 992 (72) Inventor Berg, Ralph Allan United States, 94303 Palo Alto, California Moreno Avenue, 992

Claims (13)

[Claims] 1. A light source (10) for generating a light wave; an optical fiber loop (14); an optical fiber (12) between the light source and the loop, the light source being optically coupled to the optical fiber. An optical fiber (12) for transmitting the light wave through the optical fiber to the loop; a first coupler (34) connected between the light source and the loop, An optical output signal that couples lightwaves to the loop to provide a pair of lightwaves propagating in opposite directions through the loop and couples the pair of lightwaves propagating through the loop to indicate the rotational speed of the loop Generating a first evanescent field directional coupler (34); a second evanescent field directional coupler (26) coupling the optical output signal from the optical fiber to a second optical fiber (28). );
A detector (30) arranged to receive the optical output signal from the second optical fiber and detecting the optical output signal to determine the rotational speed of the loop; An optical polarizer (32) between the two directional couplers so that the light wave propagating in the loop and the optical output signal propagating from the loop are linearly polarized in the same predetermined manner. A polarizer (32) having a common input / output path;
A phase modulator (3) in the loop for modulating the phase of the light wave having the predetermined linear polarization at a predetermined modulation frequency.
8); and a detection device (14) for detecting the rotational speed of the loop by measuring only the fundamental frequency component of the modulation frequency, the detection device including a lock-in amplifier (46) synchronized with the phase modulator. And the modulation frequency of the modulator is chosen as c / 2nL, where L is between the phase modulator and one end of the loop and between the phase modulator and the other end of the loop. An optical rotation sensor, where c is the length difference measured along the optical fiber, c is the free space velocity of light, and n is the equivalent refractive index of the optical fiber in the loop. 2. The optical rotation sensor according to claim 1, wherein the optical fiber (12), the loop (14) and the first optical fiber (12).
And an optical rotation sensor in which the second directional coupler (34, 26) and the polarizer are all composed of a single mode optical fiber. 3. The optical rotation sensor of claim 2, wherein the optical fiber comprises a single continuous uninterrupted optical fiber (12A, 12B, 12C, 12D). 4. The optical rotation sensor of claim 3, wherein the single continuous uninterrupted optical fiber is one of the optical fibers at both ends of the loop to form second and third elliptical surfaces. An optical rotation sensor having a part where a part of the clad is removed from the side, and the second and third elliptical surfaces are juxtaposed to form the first directional coupler. 5. The optical rotation sensor according to claim 1, wherein the optical fiber has a portion in which a part of the clad is removed from one side thereof so as to form a first elliptical surface, An optical rotation sensor in which the elliptical surfaces of are aligned with a birefringent material to form the polarizer (32) such that the light propagating in the optical fiber is linearly polarized. 6. The optical rotation sensor of claim 1, wherein a portion of the optical fiber is used to form a deflection controller that controls the polarization of light propagating through the loop. 7. The optical rotation sensor of claim 1, wherein the loop comprises a plurality of turns around a support,
The central part of the winding is located inside the support and the opposite ends of the winding are located outside the support so that the ends of the optical fiber forming the loop are symmetrical with respect to the central part. And an optical rotation sensor configured to be freely accessible from outside the loop. 8. The optical rotation sensor according to claim 1, wherein the odd harmonic components of the amplitude modulation induced by the phase modulator at the selected modulation frequency are canceled. 9. The optical rotation sensor according to claim 1, wherein the lock-in amplifier of the detection device comprises an amplifier (46) providing a narrow band filter characteristic centered on the modulation frequency. 10. The optical rotation sensor according to claim 1, wherein two light waves propagating in the loop in opposite directions have a phase difference of 180 ° from each other. 11. The optical rotation sensor according to claim 1, wherein the shield (110) shields the loop from a surrounding magnetic field to reduce the influence of the magnetic field on a light wave propagating in the loop.
Optical rotation sensor including. 12. The optical rotation sensor of claim 11, wherein the shield comprises a housing for the loop of high permeability material. 13. The optical rotation sensor according to claim 1, wherein a part of the loop is a coil of multiple turns,
An optical rotation sensor forming a polarization controller that controls the polarization of a light wave propagating through the loop by stressing the optical fiber and causing birefringence in the diameter of the coil.