JP2675165B2 - Optical fiber gyro signal processing circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a phase modulation type optical fiber gyro. An optical fiber gyro is a device that measures the angular velocity of a moving body. A single-mode optical fiber is wound in a coil shape many times, and monochromatic light is introduced into both ends of the optical fiber and propagates counterclockwise and clockwise in the optical fiber so that the lights emitted from both ends interfere with each other. If the optical fiber coil rotates around the axis, a phase difference appears between the counterclockwise light and the clockwise light. Since this phase difference is proportional to the rotation angular velocity, if the phase difference is known, the rotation angular velocity can be obtained. When the phase difference is Δθ and the angular velocity is Ω _f , the following relationships are established. Here, L is the fiber length of the sensor coil, a is the diameter of the coil, c is the speed of light in vacuum, and λ is the wavelength of light in vacuum. This is called the Sagnac effect and is known per se. As the optical fiber gyro, methods such as frequency modulation, phase modulation, and phase shift optical fiber gyro have been proposed. The present invention relates to a phase modulation type optical fiber gyro.

[Prior art]

About the basic form of phase modulation optical fiber gyro,
This will be described with reference to FIG. This is one in which an optical fiber at one end of an optical fiber sensor coil is wound around a piezoelectric element to apply phase modulation. Taking the first-order term of the modulated wave, the phase difference is si
It is calculated in the form of nΔθ. The coherent light emitted from the light emitting element 1 is reflected by the beam splitter 2
Is divided into two rays. One is focused by the coupling lens 4 and is incident on the B end of the optical fiber 5. This propagates counterclockwise in the sensor coil 6. The other light beam is focused by the coupling lens 3, enters the optical fiber 5 from the A end, and propagates clockwise in the sensor coil 6. Most of the optical fiber 5 is the sensor coil 6, but a portion near the A end is wound around a piezoelectric element or the like to form a phase modulation element 7. Since the excitation AC power supply 10 gives an oscillating voltage to the piezoelectric element, the piezoelectric element expands and contracts. Since the phase modulator 8 of the optical fiber is wound around the piezoelectric element, it expands and contracts together with the piezoelectric element, and the optical signal contains the modulation component. The clockwise light and the counterclockwise light pass through the phase modulator 8 of the phase modulator 7 and the sensor coil 6, and are emitted from the other end. These are combined by the beam splitter 2 and are incident on the light receiving element 9. The light receiving element 9 square-detects the interference light. Since the phase modulation element 7 is provided at an asymmetrical position when viewed from the entire optical fiber 5, the counterclockwise light and the clockwise light have different timings of receiving the phase modulation. The optical fiber length of the sensor coil 9 is L, and the refractive index of the optical fiber core is n. The time τ required for light to pass through the sensor coil 6 is Given by When the phase modulation element 7 is provided near the A end, the clockwise light is first phase-modulated and then enters the sensor coil 6. The counterclockwise light passes through the sensor coil 6 and then enters the phase modulation element 7. The angular frequency of the modulation signal is Ω. Since the difference between the time when the phase modulation element 7 receives the phase modulation and the time when it enters the light receiving element 9 is τ, the phase difference φ of the modulation signal included in the interference light is φ = Ωτ (3). As described above, due to the Sagnac effect, the clockwise light and the counterclockwise light have a phase difference of Δθ, but the phase modulation part further has a phase difference of φ due to the phase modulation. The amplitude due to the action of the phase modulation element 7 is b. If the electric field strengths of the counterclockwise light and the clockwise light are E _L and E _R , Becomes Left-handed light and right-handed light having such an electric field intensity are square-law detected by the light receiving element 9. The output of the light receiving element 9 is S (Δ
θ, t) is Becomes Here, DC means a direct current component. ω is
In the frequency of light, 2ω means a frequency component that is twice this frequency. Such a fast signal is 0 because the light receiving element 9 cannot detect it. Since the phase modulation φ is included in the signal thus obtained, the phase difference Δθ can be obtained in association with the amplitude of the modulation signal. If the direct current component is removed and S (Δθ, t) is rewritten in the form of sum, Becomes These are expanded by the Bessel function. From the generating function expansion of the Bessel function, It is. Putting t = exp (iθ), Becomes From the expansion of the real part and imaginary part of this equation, S (Δθ,
Obtain the series expansion of the Ss and Sc parts of sin and cos of t). S (Δθ, t) = (S _c cos Δθ + S _s sin Δθ) E ₀ ² (10) is defined. Using the well-known property of the Bessel function, J _-n (x) = (-1) ⁿ J _n (x) (11) (where n is a positive integer) , And put Becomes Rewriting using these equations, the signal S (Δ
θ, t) is Becomes This is a development due to harmonics of the modulation frequency Ω. By performing synchronous detection, an arbitrary harmonic component can be obtained. Of these, the first-order term is the fundamental wave component P and the second-order term is the second harmonic component Q. P (t) = 2E ₀ ² J ₁ (ξ) cosΩt sin Δθ (16) Q (t) = 2E ₀ ² J ₂ (ξ) cos 2Ωt cos Δθ (17) In many cases, the fundamental wave P is detected and Δθ is obtained. Maximize J ₁ (ξ) so that the sensitivity of P is maximized. Therefore, the modulation factor is set so that ξ = 1.8.
Then J ₀ (ξ) is about 0.3. The above is the basic configuration of the phase modulation optical fiber gyro. Since this system also has problems such as light amount fluctuation, reflected light, and phase modulation degree fluctuation, various proposals have been made. For example, there is a method for determining Δθ from the fundamental wave P while keeping the DC component constant (Japanese Patent Laid-Open No. 61-147106). In addition, JP-A-60-13
The 5816 divides the fundamental wave P by the DC component D and tries to solve the problem of light quantity variation. These methods cannot completely solve the problems such as reflected light, light quantity fluctuation, and phase modulation degree fluctuation. Therefore, the fundamental wave P is divided by the fourth harmonic T, and the quotient
There is an invention of a method for obtaining the phase difference Δθ from P / T in the form of tan Δθ (Japanese Patent Application No. 1-57637, H1.3.8). The fundamental wave P excludes the modulation frequency component of (16) and E ₀ ² is
Rewriting as E ₁ and E ₂ , P = 2E ₁ E ₂ J ₁ (ξ) sin Δθ (18). The 4th harmonic T becomes T = 2E ₁ E ₂ J ₄ (ξ) cos Δθ (19), and when the fundamental wave P is divided by the 4th harmonic T, Becomes In this way, the P / T can provide an output that is not affected by the amount of reflected light or the output variation of the light emitting element. Output and Δθ
The relationship is stable and easy.

[Problems to be solved by the invention]

In this way, by dividing the odd-order harmonic component (including the fundamental wave) by the even-order harmonic component, an expression relating to Δθ in which the fluctuation of the light quantity is eliminated can be obtained. Alternatively, the fundamental wave is divided by the DC component to obtain an expression regarding Δθ without the term of the light amount variation. Bessel functions with ξ as a parameter are included in these expressions. It is. b is the amplitude of phase modulation, n is the refractive index of the optical fiber core, and L is the length of the optical fiber forming the optical fiber coil. As the phase modulation element, for example, an optical fiber wound around a cylinder made of a piezoelectric element is used. In this case, b can be increased or decreased by the drive voltage of the excitation AC power supply. If the drive voltage is constant, the phase modulation degree b seems to be constant, but this is not the case. The phase modulation element has a considerable temperature characteristic. The phase modulation degree b changes due to temperature fluctuations. Of course, ξ also changes. Since the expression for Δθ above includes the Bessel function,
These values must be known in advance.
For this reason, the parameter ξ of the Bessel function must take a certain constant value. In order for ξ to be constant, the value of the Bessel function of an appropriate degree may be kept constant. As described above, the nth-order odd-order or even-order harmonic component contained in the light-receiving element output is J _n (ξ) and Δθ si
Contains n or cos components. Therefore, if the nth harmonic component is set to 0, J _n (ξ) = 0 can be set. However, the odd harmonics include sin Δθ, and sin Δθ = 0 when the gyro is stationary.
However, it cannot be distinguished whether J _n (ξ) = 0 or Δθ = 0. The even harmonics include Δθ in the form of cos Δθ, which is 0
do not become. If the even harmonic components are 0, then J
_2n (ξ) = 0, and ξ is defined as the zero point of the 2n-th order Bessel function. There are, of course, innumerable zeros of the 2n-order Bessel function, but ξ is determined so that it becomes one of them. And J _2n (ξ)
Ξ can be held constant by increasing or decreasing the voltage applied to the piezoelectric element so that = 0. For example, the quadratic Bessel function J ₂ (ξ) is set to 0. If we decide to take the first zero, then ξ = 5.1. To do this, the second harmonic Q should be set to zero. Alternatively, the fourth-order Bessel function J ₄ (ξ) is set to 0. If we take the first zero, we keep ξ = 7.6. To do so, the fourth harmonic component T may be set to zero. Although any even harmonic component may be selected, if the quotient obtained by dividing the fundamental wave P by the fourth harmonic T is used to measure the angular velocity, the second harmonic component Q is set to 0 and ξ is set to a constant value. Good to keep. Alternatively, also in the case of obtaining Δθ from the fundamental wave P and the DC component D, the double harmonic component Q may be set to 0 and ξ may be set to a constant value (for example, ξ = 5.1). When it is desired to obtain Δθ from the fundamental wave P and the second harmonic wave Q, the fourth harmonic wave T may be set to zero. It is an object of the present invention to provide an optical fiber gyro signal processing circuit that can maintain the phase modulation degree b or ξ at a constant value even if the temperature changes.

[Means for Solving the Problems]

In the present invention, the phase modulation degree b is kept constant by bringing the appropriate even-order harmonic component X included in the output of the light receiving element close to zero. Therefore, the output G of the phase modulation frequency oscillator that generates the phase modulation frequency signal having a constant amplitude is multiplied by the multiplier M by the multiplier to obtain the phase modulation element drive voltage H. Here, the multiplier M is given as a value proportional to the sum of the reference voltage V ₀ of the reference voltage generator and the value V ₂ obtained by integrating the even harmonic component X by the integrating circuit. The reference voltage is a reference temperature T ₀ (for example, 30 ° C)
In, the voltage is such that the even harmonic component becomes 0 when the multiplier M includes only V ₀ . However, the even harmonic component X has its zero point J
_{At 2n} (ξ ₀ ) = 0, if the derivative J _2n ′ (ξ ₀ )> 0, then the integral is given a negative sign, and J _2n ′
If (ξ ₀ ) <0, the integral is given a positive sign. That is, the optical fiber gyro signal processing circuit of the present invention includes the following components in addition to the angular velocity measurement type.
FIG. 1 shows a schematic configuration diagram of the present invention. A. Even harmonic component detection unit This is a circuit that amplifies the electrical signal output from the light receiving element and then performs synchronous detection to find the 2n-th even harmonic component. It is _zero at some reference temperature T ₀ . B. Integrator circuit A circuit that integrates even harmonic components X with time. Reference temperature T ₀
And the Bessel function J _2n (ξ) of this order is zero. If the derivative J _2n ′ (ξ ₀ ) is negative at this zero point, the integral is given a positive sign, and if the derivative J _2n ′ (ξ ₀ ) is positive,
A negative sign is added to the integration. The output of this integrating circuit is V ₂ . C. Reference voltage generator Generates a certain reference voltage V ₀ . D. Adder The reference voltage V ₀ and the output V _{2 of the} integrating circuit are added and the sum (V ₀
+ V ₂ ), and a value proportional to this is taken as the multiplier M. E. Phase modulation element frequency oscillator Generates a phase modulation signal with constant amplitude and angular frequency Ω. F. Multiplier The modulated signal G having a constant amplitude is multiplied by the multiplier M which is the output of the adder. This signal is current-amplified and the phase modulation element 7
Is applied. Although the present invention has such a structure,
V ₀ is an even harmonic component X at a certain reference temperature T _0.
It is desirable to determine so that The whole structure is as follows. The optical fiber gyro signal processing circuit of the present invention is an optical fiber having a portion forming a sensor coil and a portion provided with a phase modulation element, a light emitting element that generates coherent light, and light from the light emitting element, or An optical branching device that splits light from the light emitting element through an optical fiber and supplies the split light to both ends of the optical fiber, and a light branching device that propagates the optical fiber clockwise and counterclockwise and outputs from both ends thereof. And a light receiving element that receives light by combining with the light receiving element, and receives the output of the light receiving element to detect the odd harmonic component (including the fundamental wave) P _{2m + 1} and the even harmonic component T _2n of the phase modulation frequency. In the optical fiber gyro, the odd harmonic component P _{2m + 1} is divided by the even harmonic component T _2n , and the phase difference Δθ is obtained from the quotient thereof. An even other than the one used to ask for An even-order harmonic component detection unit that detects a component of a number-order harmonic component whose phase modulation parameter ξ is set to be the zero point of the Bessel function of that order, and the even-order harmonic component Integrated and positive when the differentiation of the Bessel function at the zero point is negative, and a negative sign when the differentiation is positive, a reference voltage generator that generates a reference voltage V ₀ , and an integral An adder for adding the output V _{2 of the} circuit and the reference voltage V ₀ to generate a multiplier M proportional to the sum thereof, a phase modulator frequency oscillator for generating a phase modulation signal G having a constant amplitude, and a multiplier M for the phase modulation signal G. It is characterized in that it is composed of a multiplier for driving the phase modulation element by the signal H obtained by multiplying by the signal H to bring the even harmonic component X close to zero.

[Operation]

Let X be the output of the even harmonic component detection unit A. this is
2E ₁ E ₂ J _2n (ξ) A value proportional to cos Δθ. Since it is after synchronous detection, it is DC. Of course there are positive and negative distinctions. This is integrated by the integrating circuit. This sign is J _2n ′ (ξ
₀ ) is positive if J _2n ′ (ξ ₀ ) is negative and positive. If such a sign is represented by ε, the output of the integrator circuit
V ₂ is Becomes CR is the time constant of the integrating circuit. Although the reference voltage V ₀ is constant, V ₀ and V ₂ are added by the adder D, so that the output multiplier M is M = k (V ₀ + V ₂ ) (23). However, k is a proportional constant. Since the phase modulation element frequency oscillator E produces a modulation signal G having a constant amplitude, this can be written as G = G ₀ sin (Ωt) (24) where G ₀ is the amplitude. In the multiplier E, the constant amplitude signal G of (24) is multiplied by the multiplier M of (23), so that the signal of MG is generated. Further, this is current-amplified or voltage-amplified to drive the phase modulation element drive voltage
And When this voltage amplification factor is a, the drive voltage is H = ka (V ₀ + V ₂ ) sin (Ωt) (25). If the drive voltage is large, the phase modulation degree b is also directly proportional to this. Of course, the parameter ξ proportional to b also increases or decreases in proportion to H. The relationship between b and ξ is determined by (21). At the reference temperature T ₀ , only the reference voltage V ₀ is added to the adder D, and at this time, ξ = ξ ₀ (26) X = 0 (27). However, ξ ₀ is a specific zero point of the even harmonic generation, and J _2n (ξ ₀ ) = 0. The meaning of (26) is that at the temperature T = T ₀ , a modulation voltage of H ₀ = kaV ₀ sin (Ωt) (28) is given to the phase modulation element, but the modulation parameter ξ at this time becomes equal to ξ _0. That's what it means. If ξ = ξ ₀ , this even harmonic generation X including J _2n (ξ) is 0. (26) and (27) express the equivalent. Now, it is assumed that the modulation degree b and the modulation parameter ξ are increased due to the temperature variation even though the driving voltage H ₀ is the same. FIG. 3 shows the quadratic Bessel function J ₂ (ξ) near the first zero point (ξ ₀ = 5.1). When the temperature is T = T ₀ , there is an operating point at the U point. It is assumed that ξ increases due to the temperature change and the operating point moves to W. J ₂ ′ (ξ ₀ ) is negative here, so ε = +
It is one. Since X becomes negative, the integral V ₂ becomes negative. (2
From 2), V ₂ continues to decrease as a negative value as long as X is negative. Then, the multiplier M decreases from (23). For this reason (25)
The drive voltage H of is decreased. As a result, the phase modulation degree b and the phase modulation parameter ξ decrease. That is, in FIG. 4, the operating point approaches W to U. And when U point is reached, ξ
= Ξ ₀ and X = 0. That is, this harmonic component becomes 0, and the output V _{2 of the} integrating circuit does not increase any more.
The multiplier M has a constant value, and the amplitude of the drive voltage H also has a constant value. The above has explained that ξ returns to ξ ₀ when ξ becomes larger than ξ ₀ . The same is true for the opposite case, and in any case, the recovery motion in which ξ returns to ξ ₀ is performed. This recovery exercise will be described in more detail. Substituting (22) for (25) gives Becomes Since H is a drive voltage for phase modulation oscillating with Ω, the phase modulation degree ξ should be proportional to the amplitude of H. If this proportional constant is w, Becomes Since ξ is set to be ξ ₀ when the input of the adder is only V ₀ , ξ ₀ = wkaG ₀ V ₀ (31). The even harmonic generation X in question is given by X = 2E ₁ E ₂ J _2n (ξ) cos Δθ (32), but since it is a story near the zero point, J _2n (ξ) = (ξ− It can be written as ξ ₀ ) J ' _2n (ξ ₀ ) (33). That is, X becomes X = u (ξ−ξ ₀ ) J ′ _2n (ξ ₀ ) (34). u is a positive constant (including cos Δθ, which is approximately 1). Substituting (31) and (34) into (30) gives Becomes εJ ′ _2n (ξ ₀ ) is negative. If ξ deviates from ξ ₀ and becomes ξ ₁ . The motion after this is ξ−ξ ₀ = (ξ ₁ −ξ ₀ ) exp−t / τ ₀ (36) Becomes τ ₀ gives the time constant of the recovery movement and is a positive constant. The recovery movement from ξ to ξ ₀ becomes slower when the time constant CR of the integrating circuit is large. However, the time constant of this integrator is not the time constant of the recovery movement. If the performance w of the phase modulation element, the coefficient k that gives the multiplier, the voltage amplification factor a, u that gives the amount of light, the amplitude G _{0 of} the oscillator, etc. are large, the recovery movement can be performed quickly. The output V _{2 of the} integrator circuit is Becomes

【Example】

A method for keeping the degree of phase modulation constant according to the embodiment of the present invention will be described with reference to FIG. This is to make ξ constant by taking the second harmonic and setting it to zero. The integrating circuit consists of a resistor R ₁ ,
It consists of a capacitor C ₁ and an amplifier 30, and integrates the second harmonic component. The reference voltage generator supplies the Zener diode ZD with the power supply V
It consists of the one tied to +, which generates the reference voltage V ₀ . The adder inputs these voltages to the amplifier 31 through the resistors R ₂ and R ₃ . The reference voltage V ₀ and the integral V ₂ are summed with this. An analog multiplier is used as the multiplier. This is to multiply a modulation signal with a constant amplitude by a multiplier. This output is amplified by the current amplifier 33 and input to the phase modulation element.

【The invention's effect】

Since the degree of modulation is always constant, the value of the Bessel function of each order is fixed in the phase modulation type optical fiber gyro, and the phase difference Δθ can be correctly obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an optical fiber gyro signal processing circuit of the present invention. FIG. 2 is a block diagram of a conventional phase modulation type optical fiber gyro. Fig. 3 is a graph showing the vicinity of the zero point of the 2nth order J _2n (ξ). FIG. 4 is a schematic configuration diagram showing an embodiment of the present invention. A: Even harmonic component detection unit B: Integration circuit C: Reference voltage generation unit D: Adder E: Phase modulation element frequency oscillator F: Multiplier V _0: Reference voltage M: Multiplier 1 ... Light emitting element 2 2 ... Optical branching element 3, 4 ... Lens 5 ... Optical fiber 6 ... Sensor coil 7 ... Phase modulation element 9 ... Light receiving element 10 ... Excitation AC power supply

Claims (1)

(57) [Claims] 1. An optical fiber having a portion forming a sensor coil and a portion provided with a phase modulation element, a light emitting element for generating coherent light, light from the light emitting element, or an optical fiber from the light emitting element. An optical branching device that splits the light passing through the optical fiber and supplies it to both ends of the optical fiber, and light that propagates clockwise and counterclockwise in the optical fiber and is emitted from both ends is coupled and received by the optical branching device. Including a light receiving element,
Receiving the output of the light receiving element, the odd-order harmonic components (including the fundamental wave) P _{2m + 1} and the even-order harmonic components T _{2n of the} phase modulation frequency
Is detected and the odd harmonic component P _{2m + 1} is replaced by the even harmonic component T
_{It is an} optical fiber gyro that divides by _2n and obtains the phase difference Δθ from the quotient, and is an even harmonic component other than that used to obtain Δθ in order to keep the phase modulation degree b constant. The parameter ξ of the phase modulation is set to be the zero point of the Bessel function of that order, and the even harmonic component detection unit that detects the component, and the even harmonic component is temporally integrated, and An integrator circuit that gives a positive sign when the differentiation of the Bessel function is negative and a negative sign when the differentiation is positive, a reference voltage generator that generates the reference voltage V ₀ , the output V _{2 of the} integration circuit, and the reference voltage V ₀ is added to produce a multiplier M proportional to the sum, a phase modulation element frequency oscillator producing a constant amplitude phase modulation signal G, and a signal H obtained by multiplying the phase modulation signal G by a multiplier M are obtained. By this signal H, the phase modulation element It is more a multiplier to drive,
An optical fiber gyro signal processing circuit, characterized in that the even harmonic component X is brought close to zero.