KR19980702046A - Optimum coupler configuration of a fiber optic speed gyroscope using a 3 × 3 coupler - Google Patents

Abstract

An optical fiber rotation sensor comprising a 3x3 optical coupler composed of first, second and third optical waveguides is disclosed. The first, second and third optical waveguides are formed such that the ratios of the light coupled to the other two optical waveguides from any one of them are constant regardless of the temperature change. The optical signal source is arranged to provide an input optical signal to the first optical waveguide such that a portion of the input optical signal is coupled into the second and third optical waveguides in the first optical waveguide. The optical fiber in which the sensing loop is formed receives the optical signals forming the backpropagation optical wave in the sensing loop and the ends coupled to the second and third optical waveguides to couple the backpropagation optical waves after they cross the sensing loop Respectively.

Description

Optimum coupler configuration of a fiber optic speed gyroscope using a 3 × 3 coupler

Optical fiber rotation sensors are known in the art. A conventional optical fiber rotation sensor is equipped with a missing field coupler that combines light between two lengths of optical fiber. Then, a fiber optic rotation sensor using 3 × 3 couplers was developed. The main advantage of using a 3x3 coupler on a fiber optic rotation sensor is that it is easier to interface with electronic devices.

U.S. Patent Nos. 4,440,498 and 4,479,715 disclose two types of optical fiber rotation sensors with 3x3 couplers. U.S. Patent No. 4,440,498 relates to an optical fiber rotation sensor having an optical fiber sensing loop and an input optical fiber. The 3 × 3 optocoupler divides the light between the input fiber and the two legs of the fiber optic sensing loop.

U.S. Patent No. 4,479,715 discloses a Sagno-effect rotation sensor having a structure in which ends of an optical fiber sensing loop are coupled to a pair of optical waveguides. The light is input into a central input waveguide located between the optical waveguides coupled to the ends of the sensing loop fiber. And three optical waveguides are arranged so as to form a 3x3 optical coupler. The input light is coupled at the center input waveguide to optical waveguides connected to the optical fiber coil to generate a backpropagation light wave in the fiber optic sensing loop. The light waves are coupled at the coupler across the sensing coil. The combined light waves are detected and the resulting electrical signal is processed to calculate the rotational speed.

U.S. Patent No. 4,944,590 discloses an optical fiber gyroscope that uses a 3x3 coupler to couple an optical signal into and out of an optical fiber sensing loop. This patent discloses a photodetector arranged to detect light input into a 3x3 coupler that is not coupled into the fiber optic sensing loop. An electrical signal obtained by detecting such light is used in a signal processing circuit to control the intensity of input light.

Such a fiber optic rotation sensor can be driven in quadrature, providing maximum sensitivity at a rotational speed of zero. Unfortunately, conventional optical fiber rotation sensors with 3x3 couplers are sensitive to temperature variations. The coupling ratio of the 3 × 3 coupler is sensitive to temperature, and a bias error of 1000 degrees per hour is generally observed. This size of error is unacceptable in most rotating sensor applications.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved optical fiber rotation sensor using a 3x3 coupler with an optimal coupling ratio. The coupling ratio is selected to prevent changes in the amount of light coupled to the two backpropagated light waves caused by temperature changes and other mechanical factors. In addition, the selected coupling ratio maximizes the optical power delivered to the rate photodetector.

The optical fiber rotation sensor according to the present invention comprises a 3x3 optocoupler having first, second and third optical waveguides, wherein the first, second and third optical waveguides have an interaction And the ratio of the light that is coupled to the other two optical waveguides in any one of them is constant regardless of the thermal change of the interaction length. The optical signal source is arranged to provide an input optical signal to the first optical waveguide such that a portion of the input optical signal is coupled into the second and third optical waveguides in the first optical waveguide. The optical fiber in which the sensing loop is formed receives the optical signals forming the backpropagation optical wave in the sensing loop and the ends coupled to the second and third optical waveguides to couple the backpropagation optical waves after they cross the sensing loop Respectively.

The optical coupler is formed such that the light splitting ratio between the first, second and third optical waveguides is 0.4108: 0.1783: 0.4108 so that when the light intensity A ² is input to the first waveguide, The light intensity output to the optical fiber by each of the third waveguides is 0.4108A ² and the light intensity output by the first waveguide is 0.1783A ² .

A better understanding of the objects of the present invention and its construction and method of operation will be obtained by reference to the following description of the preferred embodiments and the accompanying drawings.

The present invention relates to a Sagnac effect rotation sensor, and more particularly, to an optical fiber rotation sensor that guides a light wave propagating backward in a sensing loop and measures a rotation about a sensing axis perpendicular to a sensing loop plane. More specifically, the present invention provides an optical signal to the sensing loop using 3x3 couplers and guides the optical output signal output from the sensing loop to an electrical device that processes the optical output signal and measures the rotational speed To an optical fiber rotation sensor.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view of an optical fiber rotation sensor with a 3x3 loss field optocoupler.

2 is a schematic diagram showing a 3x3 coupler;

3 is a diagram showing a derivative of the combined length of signal strength output by the optical fiber rotation sensor of Fig.

Preferred Embodiment

Referring to FIG. 1, the optical fiber rotation sensor 10 includes a 3 × 3 optical coupler 12 and a long optical fiber 13 arranged to form an optical fiber sensing coil 14. The optical coupler 12 includes optical waveguides 1-3 formed on the substrate 15. The optical fiber 13 has end portions 16, 18. The fiber ends 16,18 are respectively in contact with the ends 20,22 of the corresponding optical waveguides 1,2.

A pair of output optical fibers 30 and 32 are connected to the ends 34 and 36 of the optical waveguides 2 and 3, respectively. The output optical fiber 30 transmits the light beam to the first optical detector 38 which produces an electrical signal S1 representative of the intensity of the optical signal transmitted by the output optical fiber 30. [ Similarly, the output optical fiber 32 transmits the light beam to a second photodetector 40 that generates an electrical signal S2 representative of the intensity of the optical signal transmitted by the output optical fiber 32. [

The optical waveguide 1 is formed on the substrate 15 between the optical waveguides 2 and 3. The optical waveguides 1-3 are arranged to form a 3x3 optocoupler 12. The 3x3 optocoupler 12 is preferably an evanescent field coupler.

The input optical fiber 46 has an end 48 for receiving light from the light source 50. The other end of the input optical fiber 46 is in contact with the end portion 54 of the optical waveguide 1. The output optical fiber 56 has an end 58 that is in contact with the end 60 of the central light pipe 3. The other end 62 of the output optical fiber 56 transmits the light beam to a third detector 64 which produces an electrical signal S3 representative of the intensity of the optical signal transmitted by the output optical fiber 56. The 3x3 optocoupler 12 is preferably a lossy field coupler that combines optical signals between the optical waveguide 1 and the optical waveguides 2,3. A part of the light input to the 3x3 optical coupler 12 remains in the optical waveguide 1.

Referring to FIG. 3, the 3x3 coupler has a coupling length L in which the loss fields of the optical waves guided by the waveguides 1-3 cooperate so that light is coupled between the waveguides. The intensity of the light coupled between the waveguides 1-3 is a function of the coupling length L. Since the temperature of the optical coupler 12 fluctuates, the light intensity coupled between the waveguides 1-3 also fluctuates.

In an ideal environment, the three coupling ratios of the 3x3 optocoupler 12 are all equal to one third. When all the combining ratios are 1/3, the light intensity output from the three light waveguides 1-3 is the same. However, due to the temperature sensitivity described above, when the coupling ratios of the 3x3 coupler 12 are all 1/3 at the desired specific operating temperature, unacceptable large errors occur due to unavoidable temperature fluctuations that change the coupling ratio. The present invention includes a fiber optic rotation sensor that uses a 3x3 coupler that has a coupling ratio selected to minimize bias error from about 10 degrees to about 100 degrees per hour and is not sensitive to temperature variations. It has been verified that there exists a set of coupling ratios in which the ratio of light coupled into arbitrarily selected one of the optical waveguides 1-3 remains constant when the coupling length L changes in response to a temperature change.

The optimum configuration of the 3x3 coupler 12 is not sensitive to temperature variations and provides greater speed discrimination than can be achieved with conventional configurations. Referring to Figures 2 and 3, the 3x3 optocoupler 12 will be described by linear differential equations.

Here, j = 1, 2, 3,

j = j + 3,

a _j is the optical amplitude at waveguide j,

k is the coupling ratio between any two of the three optical fibers 1-3.

For example, k ₁₂ is the coupling ratio between the waveguides 1 and 2, k ₂₃ is the coupling ratio between the waveguides 2 and 3, and k ₃₁ is the coupling ratio between the waveguides 3 and 1. The optical coupler 12 is preferably formed such that the coupling ratios are k ₁₂ = k ₂₃ = k ₃₁ = k, so that the solution to equation (1) is given by the following equation (2).

a _j (z) = c _j e ^ikz + de- ^2z

Here, Equation (2) , And c and d are constants. 3 × 3 when the input to the fiber optic rotation sensor 10 by the input optical fiber 46 to the optical coupler 12, power of A ^2, the light amplitude in the wave guides in the z = 0 the input end is given by:

a ₁ (0) = A

a ₂ (0) = a ₃ (0) = 0

Information that can be used to find the expressions for the constants c and d is obtained by A, which can easily confirm the numerical value using Equation (3) and Equation (4) in Equation (1).

A = c ₁ e ^ik0 + de- ²

A = c ₁ + d

c ₁ = Name

0 = c ₂ + e ^ik0 de ^-2k0

c ₂ = -d

0 = c ₃ e ^ik0 + de ^{-2 k0}

c ₃ = -d

From Equations (7), (9) and (11):

c ₁ + c ₂ + c ₃ = 0

A-d-d-d = 0

Thus, the constants c ₁ , c ₂ , c ₃ and d are given by:

At the other end of the coupler with distance z = L, the solution for the optical waveguide 1 is:

To obtain the light intensity at the light pipe 1, the square of the amplitude gives the following equation: < RTI ID = 0.0 >

At the other end of the coupler with distance d = L, the equation for the solution for light pipe 2 is given by:

Since the distance z = L and a ₂ = a ₃ , the solution for the optical waveguide 3 is:

The light intensity at the optical waveguides 2,3 is the light intensity input to the optical fiber sensing coil 14 at the ends 16,18 of the optical fiber 12. [ Thus, the input to the legs of the fiber optic gyroscope is given by Equations 29 and 30 | (A ₂ (L) | ² And | (A ₃ (L) | ² to be. The inputs to such an optical fiber sensing coil have a limited phase relationship. After traversing the optical fiber sensing coil 14, there is a phase shift (?) Between the back propagating optical waves. The return inputs to the optical coupler 12 after traversing the optical fiber sensing coil 14 are given by:

a ₁ (0) = φ

The loop loss of the optical fiber 12 is ignored. The angle? Is a sagnosphase shift angle between the rays generated by the rotation of the sensing loop to the angular velocity? With respect to the sensing axis perpendicular to the surface of the optical fiber sensing coil 14. [ The phase angle? And the angular velocity? Are given by the Sagnac equation as follows:

Where l is the fiber length of the sensing coil 14, D is the diameter of the sensing coil 14,? Is the wavelength of the optical signals, and c is the speed of light.

After passing the coupler length L through the optical coupler 12, the optical signals output from the output of the optical fiber sensing coil are given by:

We define the quantity S ₁ by the following equation:

S ₂ and S ₃ are defined by the following equations:

Referring to Equation 39, S ₂ and S ₃ are given by:

In Equation 42, the - sign is applied to S ₂ , and the + sign is applied to S ₃ .

The signal output of the optical fiber rotation sensor 10 can be expressed using S ₁ , S ₂ and S ₃ as follows:

It is required to find the maximum value and the minimum value in the relation of the equation (44) with respect to the argument 3kL of the trigonometric functions in the equation (44) in order to judge whether or not there exists an optimum set of coupling ratio that does not change even when the coupler length L changes . Thus, taking the derivative of (44) with respect to 3kL, the following equation is given: < RTI ID = 0.0 >

Equation 45 has a slope of zero at maximum and minimum values. Thus, near the maximum and minimum values of equation (45), the optical coupler 12 has a minimum sensitivity to temperature variations. In order to find the maximum and minimum values in the equation (45), the derivative is set to 0 and the following equation is given:

0 = -4cos ³ 3kL-10cos ² 3kL + 5cos3kL + 9

Solving equation 46 for 3kL gives:

3 kL = 148.061 rad

Thus, the quantity kL is:

kL = 49.354 rad

Referring to the graph of FIG. 2, it can be seen that the output has a maximum value when 3kL = 148.061 rad. Using the kL value obtained from the equation (48) in equation (44), the signal output of the optical fiber rotation sensor 10 is given by:

Referring to equations (22), (29) and (30), the following equation can be obtained:

Therefore, the coupler coupling ratio is 0.4108: 0.1783: 0.4108. Referring to Figure 34, the solution for the optical fiber rotation sensor is:

For this particular set of coupling ratios, the optical coupler 12 is generally not sensitive to changes in coupling length that occur with time and temperature. The signals on the output legs are higher in the combining ratio set than in the other combining ratio.

The structures and methods disclosed herein illustrate the principles of the present invention. The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments are to be considered in all respects as illustrative and descriptive, and not as restrictive. Accordingly, the invention is defined by the appended claims rather than the foregoing description. Modifications of the above-described embodiments that are within the meaning and range of equivalency of the claims are intended to be within the scope of the present invention.

Claims (4)

An optical fiber rotation sensor for detecting rotation of the sensing loop with respect to a sensing axis perpendicular to a sensing loop plane of an optical fiber, Second, and third waveguides formed on the substrate, wherein the first, second, and third waveguides are configured such that the ratios of light coupled from any one of the first, second, and third waveguides to the other two optical waveguides are constant A 3x3 optocoupler arranged to have coupling ratios that result in a < RTI ID = 0.0 > An optical signal source arranged to provide an input optical signal, a portion of which is coupled into the second and third optical waveguides in the first optical waveguide, to the first optical waveguide; And Wherein the sensing loop is configured to receive the optical signals forming the back propagating optical waves in the sensing loop and to couple the back propagating optical waves after the optical signals traverse the sensing loop, The optical fiber having ends coupled to optical waveguides The optical fiber rotation sensor comprising: An optical fiber rotation sensor for detecting rotation of the sensing loop with respect to a sensing axis perpendicular to a sensing loop plane of an optical fiber, Second, and third optical waveguides formed on the substrate, wherein the ratio of the light that is coupled to the other two optical waveguides from any one of the first, second, and third optical waveguides is constant A 3x3 optocoupler arranged to have coupling ratios that result in a < RTI ID = 0.0 > An optical signal source arranged to provide an input optical signal, the portion of which is coupled into the second and third optical waveguides in the first optical waveguide, to the first optical waveguide; And Wherein the sensing loop is configured to receive the optical signals forming the back propagating optical waves in the sensing loop and to couple the back propagating optical waves after the optical signals traverse the sensing loop, Said optical fiber having ends coupled to optical waveguides, The first, second and third optical waveguides are formed such that the optical dispersion ratios are 0.4108: 0.1783: 0.4108, so that when the optical intensity A ² is input to the first optical waveguide, the second and third optical waveguides Wherein the light intensity output from the optical fiber by each of the optical fibers is 0.4108A ² , and the optical intensity output by the first optical waveguide is 0.1783A ² . A method for measuring the number of revolutions with an optical fiber rotation sensor that senses rotation of the sensing loop with respect to a sensing axis perpendicular to a sensing loop surface of an optical fiber, Forming a 3x3 optocoupler on the substrate including first, second and third optical waveguides; Arranging the first, second and third optical waveguides so as to have an interaction length in which optical coupling is made between the first, second and third optical waveguides; Second, and third optical waveguides so that the ratio of the light coupled to the other two optical waveguides from any one of the first, second, and third optical waveguides is constant regardless of the temperature change in the 3x3 optical coupler. Forming third optical waveguides; Arranging an optical signal source to provide an input optical signal to the first optical waveguide, the input optical signal being partly coupled by the 3x3 optocoupler into the second and third optical waveguides in the first optical waveguide; And Receiving end portions of the optical fiber formed with the optical fiber sensing loop to receive the optical signals forming the back propagating optical waves in the sensing loop and to combine the back propagating optical waves after the optical signals traverse the sensing loop, And third optical waveguides Wherein the rotational speed of the motor is measured. A method for measuring the number of revolutions with an optical fiber rotation sensor that senses rotation of the sensing loop with respect to a sensing axis perpendicular to a sensing loop surface of an optical fiber, Forming a 3x3 optocoupler on the substrate including first, second and third optical waveguides; Arranging the first, second and third optical waveguides so as to have an interaction length in which optical coupling is made between the first, second and third optical waveguides; Second, and third optical waveguides so that the ratio of the light coupled to the other two optical waveguides from any one of the first, second, and third optical waveguides is constant regardless of the temperature change in the 3x3 optical coupler. Forming third optical waveguides; Arranging an optical signal source to provide an input optical signal to the first optical waveguide, the input optical signal being part of which is coupled into the second and third optical waveguides in the first optical waveguide; Receiving end portions of the optical fiber formed with the optical fiber sensing loop to receive the optical signals forming the back propagating optical waves in the sensing loop and to combine the back propagating optical waves after the optical signals traverse the sensing loop, And third optical waveguides; And When the light intensity A ² is input to the first optical waveguide so that the optical dispersion ratios are 0.4108: 0.1783: 0.4108, the second optical waveguide is output to the optical fiber by the second optical waveguide and the third optical waveguide, light intensity for forming is 0.4108A ^2, forming the first, second and third optical waveguide light intensity output by the first optical waveguide is to be 0.1783A ² Wherein the rotational speed of the motor is measured.