JPH0758197B2 - Fiber optic gyro - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical fiber gyro that measures a Sagnac phase difference to measure a rotational angular velocity, and particularly relates to an influence of reflected light generated at an end face of an optical waveguide coupler. It is greatly reduced and the measurement accuracy is improved.

“Prior Art” A conventional optical fiber gyro will be described with reference to FIGS. 8 and 9.

For example, both end faces of a crystal having an electro-optical effect such as lithium niobate are cut obliquely by about 10 °, and a width of
An optical waveguide is formed on a rectangular crystal substrate 2 having a length of 5 mm, a length of about 20 mm and a thickness of about 1 mm. The optical waveguide is to form a portion having a slightly higher refractive index than the crystal substrate, and confine the light in the portion to guide the light. A first input / output port P ₁ is formed on one end surface of the substrate 2. A Y-shaped branch 3 is formed following the port P _1, and the first and second optical waveguides l ₁ , l are formed from the branch 3.
₂ extends along the longitudinal direction of the substrate 2 and reaches the _second and third input / output ports P ₂ and P ₃ formed on the other end surface of the substrate 2. Rectangular electrodes 4a and 4b are formed opposite to each other with the first optical waveguide ₁₁ interposed therebetween, and an oscillator 5 is connected between these electrodes. The light passing through the first optical waveguide l ₁ is phase-modulated by the modulation wave f (t) supplied from the oscillator 5.

As described above, since the two end faces of the crystal substrate 2 are cut obliquely by about 10 ° with respect to the direction perpendicular to the longitudinal direction of the substrate 2, the second optical waveguide l ₂ is d more than the first optical waveguide l _1. Only the length is shortened. The two branch pieces 3a and 3b in the branch section 3 have the same length.

The first input / output port P ₁ is connected to the light source 7 via the optical fiber 6, and the second and third input / output ports P ₂ and P ₃ are connected to one end of the optical fiber coil 10 via the optical fibers 8 and 9, respectively. It is connected to each of the other ends. The first branched light E ₁ incident on the optical fiber coil 10 from the second input / output port P ₂ travels through the optical fiber coil 10 and is returned to the second optical waveguide l ₂ from the input / output port P ₃ . Similarly, the second branched light E ₂ incident on the optical fiber coil 10 from the third input / output port P ₃ reversely rotates around the optical fiber coil 10 and is returned to the first optical waveguide l ₁ from the second input / output port P ₂ . .

An optical coupler 12 is inserted in the middle of the optical fiber 6, and one end thereof is connected with a photodetector 13 for detecting the interference light between the _first and second branched lights E ₁ and E ₂ .

The first and second branched lights E ₁ and E ₂ are also referred to as clockwise light and counterclockwise light, respectively. Further, in FIG. 9, the optical fiber coil 10
Although it is not shown as one turn, it is generally wound multiple turns.

It is the critical angle of the reflected light generated at the end face that cuts both end faces of the crystal substrate 2 obliquely by about 10 °.
This is because most of the reflected light can be removed. That is, most of the reflected light is emitted to the substrate without returning to the optical waveguide or the optical fiber of the substrate 2.

The principle of optical fiber gyro is based on the Sagnac effect. When the entire system in Fig. 9 rotates at angular velocity Ω, light
There is apparent difference in optical path length between E ₁ and E ₂ . This is the Sagnac effect. The phase difference (referred to as Sagnac phase difference) S due to the optical path length difference is expressed by the following equation.

Here, A is the area surrounded by the coil, λ is the wavelength of the light source, and C is the speed of light.

FIG. 9 shows a detection method usually called an open loop method. The interference light of the lights E ₁ and E ₂ is guided to the analyzer 14 of the photodetector 13 and photoelectrically converted, and the obtained electrical output is synchronously detected by the lock-in amplifier 15. The Sagnac phase difference S is obtained from the detected output, and the rotational angular velocity Ω is calculated from the equation (1).

[Problems to be Solved by the Invention] In the conventional optical fiber gyro, the other end face of the optical waveguide coupler 1 is added to the interference light between the clockwise light E ₁ and the counterclockwise light E ₂ to be detected by the lock-in amplifier 14. It is difficult to measure the interference light amount accurately because the interference lights of the reflected lights of the lights E ₁ and E ₂ superposed on each other are overlapped. Therefore, an error occurs in the Sagnac phase difference S obtained from the interference light amount. Therefore, there is a problem that an error occurs in the rotational angular velocity Ω obtained from the Sagnac phase difference S. As described above, in order to reduce the reflected waves generated on the end faces of the conventional optical waveguide coupler 1, both end faces are obliquely cut by about 10 ° or an antireflection film is vapor-deposited on the end faces. However, the influence of reflection could not be sufficiently removed, and it was an overflow path for realizing a highly accurate gyro.

An object of the present invention is to significantly reduce the influence of reflection at the end face of the optical waveguide coupler, and to accurately measure the interference light amount between the clockwise light E ₁ and the counterclockwise light E ₂ with high accuracy. It is intended to realize a gyro.

"Means for Solving the Problems" (1) A first input / output port is provided on one of the facing end faces of a substantially rectangular crystal substrate having an electro-optic effect, and a second is provided on the other end face.
A third input / output port is formed, and a branch portion for branching the light incident on the first input / output port into two is formed following the first input / output port. First and second optical waveguides reaching the three input / output ports are formed along the longitudinal direction of the substrate to form an optical waveguide coupler, and the first input / output port of the optical waveguide coupler is an optical fiber. To the light source, the second and third input / output ports are respectively connected to one end and the other end of the optical fiber coil via an optical fiber, and the first branch proceeds from one end of the optical fiber coil to the other end. A photodetector for detecting an interference light between the light and the second branched light traveling from one end to the other end is coupled to the optical fiber connecting the light source and the first input / output port, and the photodetector Based on the detection output of Measuring a click phase difference in fiber optic gyro which measures the rotational angular velocity than the Sagnac phase difference, in this invention, the second of the branch point of the branch portion of the optical waveguide coupler, the third
The difference in the optical distance to the input / output port is set to nλ / 4 (λ is the light source wavelength, n is a positive integer).

(2) In the above item (1), the other end face of the optical waveguide coupler on which the second and third input / output ports are formed is
It is desirable to obtain the above optical distance difference by selecting an angle formed with the longitudinal direction of the crystal substrate.

(3) In the above item (1), the other end face of the optical waveguide coupler on which the second and third input / output ports are formed is
The angle formed with the longitudinal direction of the crystal substrate, the first, second
By selecting the distance between the optical waveguides, the above optical distance difference can be obtained.

(4) In the above item (1), it is possible to obtain a difference in optical distance by providing a difference in length between the first and second optical waveguides of the optical waveguide coupler.

(5) In the above item (1), one of the first and second optical waveguides of the optical waveguide coupler is loaded with a dielectric film to change the refractive index of the optical waveguide to obtain the optical distance. You can also get the difference.

(6) In the above item (1), a pair of electrodes are formed to face each other with one of the first and second optical waveguides of the optical waveguide coupler sandwiched therebetween, and the magnitude of the DC voltage applied between the electrodes. Thus, the refractive index of one of the optical waveguides can be changed to obtain the above optical distance difference.

[Embodiment] First, the influence of reflection at the end face of the optical waveguide coupler of the conventional optical fiber gyro will be devised in detail with reference to FIG.

The modulated wave supplied from the oscillator 5 is represented by f (t) = a sin (ω _P t) (2). Further, when the light emitted from the light source 7 is E ₀ e ^iωot , the clockwise light E ₁ and the counterclockwise light E ₂ are represented by the following equations, respectively.

In the above equation, τ is the coil propagation time, and α is the total loss of light received by the optical waveguide coupler 1, the optical fibers 8 and 9, and the optical fiber coil 10. Interfering light amount I of light E ₁ and E ₂ received by analyzer 14
Is the conjugate complex number of E ₁ and E ₂ expressed as ▲ E ^* ₁ ▼, ▲ E ^* ₂ ▼, In the above equation, J is the Bessel function, If the sin ω _P t component of the interference light amount I of the lights E ₁ and E ₂ is ▲ I ⁰ _P ▼, from equation (5), ▲ I ⁰ _P ▼ = -α ▲ E ² ₀ ▼ sin (S) sin θ ・J ₁ (b)
(9) is obtained. This component is detected by the lock-in amplifier 15,
The Sagnac phase difference S can be calculated from the detected value, and the rotational angular velocity Ω can be obtained from the equation (1).

The reflected lights ▲ E ^A ₁ ▼ and ▲ E ^A ₂ ▼ which are split at the branch point P _b and reflected at the ports P ₂ and P ₃ Is represented. In the above formula, R _A is the reflectance of light at the ports P ₂ and P ₃ , φ _d is the phase difference of the distance of d, and α _A is P ₃ and P ₄
It is the total loss received by the reflected light ▲ E ^A ₁ ▼, ▲ E ^A ₂ ▼ at the point.

By performing the same calculation as that for obtaining the interference light ▲ I ⁰ _P ▼, sin ω of the interference light due to the reflected light ▲ E ^A ₁ ▼ and ▲ E ^A ₂ ▼
_{The P} t component I _P ^A is expressed as ▲ I ^A _P ▼ = -α _{A RA} ▲ E ² ₀ ▼ sin (2φ _d ) J ₁ (2a) (12). Reflected light ▲ E generated by the lights E ₁ and E _{2 which} are reflected by the ports P ₃ and P ₂ around the fiber coil 10 respectively.
^B ₁ ▼ and ▲ E ^B ₂ ▼ are It is represented by. In the above equation, R _B is the light E ₁ and E ₂ is P
Is the reflectance when reflected at points ₃ and P ₂ , where α _B is P ₂ ,
It is the total loss received by the reflected lights ▲ E ^B ₂ ▼ and ▲ E ^B ₁ ▼ at P ₃ .

Interfering light ▲ I ⁰ _P ▼ Reflected light ▲
Due to E ^B ₁ ▼ and ▲ E ^B ₂ ▼, the Sin ω _P t component I _P ^B of the interference light is as follows: ▲ I ^B _P ▼ = −α _B R _B ▲ E ² ₀ ▼ sin (2φ _d ) cos (2
θ) · J ₁ (b _B ) (15) In the above equation, b _B = 2a cos (2 θ) (16) Therefore, all the interference light detected by the lock-in amplifier 15
The sin ω _P t component I _P is I _P = ▲ I ⁰ _P ▼ + ▲ I ^A _P ▼ + ▲ I ^B _P ▼ (17), and the second and third terms are bias errors (offset error or zero point error). It is understood that it appears.

As is clear from the above description, although most of the reflected waves can be removed by cutting both end faces of the optical waveguide coupler 1 obliquely by about 10 °, the input / output ports P ₂ and P ₃ cannot be removed. The influence of the reflected wave from the end face on one side cannot be ignored. But,
The effect of reflection on the end face on the side of the input / output port P ₁ is negligible as long as it is cut obliquely by about 10 ° as compared with the case on the other end face described above, and can be ignored.

The present invention determines the optical distance difference between _two optical waveguides l ₁ and l ₂ branched in an optical waveguide coupler used in an optical fiber gyro. By doing so, the bias error due to the end face reflected light included in the interference light to be detected by the lock-in amplifier 15 is to be removed in principle.

Thus, the phase shift φ _d caused by giving an optical distance difference to l ₁ and l ₂ is as follows from the relation of λ: 2π = Δ: φ _d (19) From equations (12) and (15), ▲ I ^A _P ▼ = ▲ I ^B _P ▼ = 0 (22) Therefore, the influence of end face reflection can be removed in principle.

FIG. 1 shows an optical waveguide coupler used in the optical fiber gyro of the present invention. The parts corresponding to those in FIG. 8 are designated by the same reference numerals, and the duplicated description will be omitted. In this optical waveguide coupler, for example, lithium niobate is X-cut, and a Y-branch optical waveguide is formed on it. As a well-known forming method, there is a method of changing the refractive index of the waveguide by, for example, thermal diffusion of titanium. The inclination angle β of the end face of the crystal substrate 2 with respect to the direction perpendicular to the longitudinal direction of the substrate is given by the following equation. d is the difference between the lengths of the optical paths l ₁ and l ₂ . The optical distance difference Δ between the waveguides of the optical paths l ₁ and l ₂ is Δ = n _e · d (n _e = 2.17) (24) if TE mode light is incident. Substituting the above equation into equation (23) When the distance l between the optical paths l ₁ and l ₂ is l = 80 μm and the light source wavelength is λ = 0.83 μm, β = 10 ° (A) Δ = 37λ; β = 10.032 ° (b) Δ = (37 + 1/4) λ; θ = 10.098 ° (c) Δ = (37 + 2/4) λ; β = 10.165 ° By accurately cutting the end face of the optical waveguide coupler 1 to β, the cause of bias error (12),
The components of equation (15) can be completely removed.

The reason for choosing β = 10 ° is that when the refractive index difference between the optical waveguide and the crystal substrate is Δn = 5 × 10 ^-3 , this angle β10 ° is the critical angle of the reflected light, and most of the reflected light is This is because it can be removed.

The inclination of the end face on the I / O port P ₁ side may be selected to be about 10 ° as in the conventional case. (The same applies to other examples described below.) In the above, the inclination angle β of the end face of the optical waveguide coupler 1 is set to set the optical distance difference Δ between the optical paths l ₁ and l ₂ to nλ / 4. It was selected, but the inclination angle β is fixed (for example, β = 10 °), and (24)
From the formula, May be selected. For example, n _e = 2.17, λ =
With 0.83 μm and θ = 10 °, the distance l between the branch waveguides is (d) Δ = 37λ; l = 80.26 (μm) (e) Δ = (37 + 1/4) λ; l = 80.80 (μm) (f) It is also possible to select Δ = (37 + 2/4) λ; l = 81.34 (μm).

After cutting both end faces of the optical waveguide coupler 1 obliquely by about 10 ° as in the conventional case, the difference Δ = n in the optical distance between the optical paths l ₁ and l ₂
One optical path may be modified in order to add λ / 4 (n = 1, 2, ...). For example, the physical distance of the optical path l ₂ is made larger than the optical path l ₁ by d = nλ / 4n _e (27). The example shown in FIG. 2 is a case where the arcuate optical path 21 is inserted in the middle of the optical path l ₂ to make it longer.
The shape change to increase the optical path length may be arbitrary.

Alternatively, both end faces of the optical waveguide coupler 1 are obliquely cut by about 10 °, and then a dielectric film 22 of, for example, SiO ₂ is shown in FIG. 3 on a region including a part of the optical path l ₁ or l ₂ . By vapor deposition as described above, a difference Δn _e is provided in the equivalent refractive index of the optical paths l ₁ and l ₂ ,
An optical distance difference Δ = nλ / 4 may be given between the optical paths l ₁ and l ₂ . For simplification, let the lengths of the optical paths l ₁ and l _{2 be} equal,
In terms of Δn _e L = nλ / 4 (n = 1,2, ...) (28) Δn _e = nλ / 4L (29) L is the length of the optical paths l ₁ and l ₂ .

As shown in FIG. 4, a direct current power supply 23 is connected in series with the oscillator 5 and the electro-optic effect of the crystal is used to change the refractive index of the portion of the optical path to which the electrode is added according to the magnitude of the direct current voltage. Similarly to the above, the optical distance of the optical paths l ₁ and l ₂ is Δ = nλ /
You can also add a difference of 4. In this case, the optical distance difference can be set to a predetermined value with high accuracy by finely adjusting the magnitude of the applied DC voltage.
As will be described later, when the modulation is not performed by the optical waveguide coupler 1 but is performed outside the coupler 1, the oscillator 5 is connected to another portion, and therefore only the DC power supply 23 is connected between the electrodes 4a and 4b. To be done.

Although the Y branch is used as the branch unit 3 in the above description, the present invention is not limited to this case, and as shown in FIG. 5, the X branch is used and the input / output ports P ₁ and P ₁ ′ are connected. One may be the open end. Alternatively, as shown in FIG. 6, a directional coupler may be provided instead of the X branch.

In the above description, in order to modulate one of the branched lights branched by the branching unit 3, the electrodes 4a and 4b are loaded in the optical path l ₁ and the modulated wave f (t) is applied. The present invention is not limited to this case. For example, as shown in FIG. 7, by inserting a modulation element 24 made of PZT or the like in the middle of the optical fiber 8,
A modulated wave may be applied to this. Such a modulation method is well known.

"Effect of the Invention" According to the present invention, since the difference in the optical distance between the optical waveguides l ₁ and l ₂ of the optical waveguide coupler 1 is set to nλ / 4, the input / output port P _{2 of the} optical waveguide coupler is set. And reflected waves generated at P ₃ ▲ E ^A ₁ ▼, ▲
E ^A ₂ ▼ Mutual interference light and fundamental wave components ▲ I ^A _P ▼, ▲ I ^B _P ▼ of mutual interference light and ▲ E ^B ₂ ▼, ▲ E ^B ₁ ▼ can be made zero in principle.

The above-mentioned ▲ I ^A _P ▼, ▲ which was always superposed on the fundamental wave component ▲ I ⁰ _P ▼ of the interference light between the branched lights E ₁ and E ₂ detected by the conventional photodetector.
Since the I ^B _P ▼ component disappears, the interference light amount of the branched light ▲ I ⁰ _P ▼
Can be accurately measured, and thus a highly accurate optical fiber gyro can be realized.

[Brief description of drawings]

FIG. 1 is a plan view showing an example of an optical waveguide coupler used in the optical fiber gyro of the present invention, FIG. 2 is a plan view showing a modification of the optical waveguide coupler of FIG. 1, and FIG. FIG. 4 is a plan view showing another modification of the optical waveguide coupler shown in FIG. 4, FIG. 4 is a plan view showing still another modification of the optical waveguide coupler shown in FIG. 1, and FIG. 5 is optical waveguide coupling shown in FIG. FIG. 6 is a plan view in the case where an X branch is used in the branch part of the optical device, FIG. 6 is a plan view showing an example in which a directional coupler is used in the branch part of the optical branching coupler of FIG. 1, and FIG. FIG. 8 is a block diagram of an optical fiber gyro having an optical fiber gyro inserted between an optical waveguide coupler and an optical fiber coil. FIG. 8 is a perspective view showing an example of an optical waveguide gyro used in a conventional optical fiber gyro. FIG. 1 is a block diagram showing an example of a conventional optical fiber gyro.

Claims (6)

[Claims] 1. A first input / output port is formed on one of opposite end faces of a substantially rectangular crystal substrate having an electro-optical effect, and second and third input / output ports are formed on the other end face, and the first input / output port is formed on the first input / output port. Subsequently, a branching portion for branching the light incident on the first input / output port into two is formed, and the first and second optical waveguides extending from the branching portion to the second and third input / output ports are provided on the substrate. Is formed along the longitudinal direction of the optical waveguide coupler, and the first input / output port of the optical waveguide coupler is connected to a light source through an optical fiber, and the second and third input / output ports are connected. Are respectively connected to one end and the other end of the optical fiber coil via an optical fiber, and there are a first branched light traveling from one end of the optical fiber coil to the other end and a second branched light traveling from the other end to one end. A light detection unit for detecting interference light between the light source and the first An optical fiber that is coupled to the optical fiber that connects to the output port, measures the Sagnac phase difference between the first and second branched lights from the detection output of the photodetector, and measures the rotational angular velocity from the Sagnac phase difference. In the gyro, from the branch point of the branch portion of the optical waveguide coupler, the second and third
An optical fiber gyro, wherein the difference in optical distance to the input / output port is set to nλ / 4 (λ is the wavelength of the light source, n is a positive integer). 2. The method according to claim 1, wherein the other end face of the optical waveguide coupler on which the second and third input / output ports are formed is selected from the angle with the longitudinal direction of the crystal substrate. , An optical fiber gyro characterized by obtaining the above optical distance difference. 3. The angle according to claim 1, wherein the other end face of the optical waveguide coupler, on which the second and third input / output ports are formed, forms an angle with the longitudinal direction of the crystal substrate. An optical fiber gyro characterized in that the optical distance difference is obtained by selecting the distance between the second optical waveguides. 4. The optical fiber gyro according to claim 1, wherein the optical distance difference is provided by providing a difference in length between the first and second optical waveguides of the optical waveguide coupler. . 5. The optical system according to claim 1, wherein a dielectric film is loaded on one of the first and second optical waveguides of the optical waveguide coupler to change the refractive index of the optical waveguide. An optical fiber gyro that is characterized by obtaining a dynamic distance difference. 6. A pair of electrodes are formed so as to face each other with one of the first and second optical waveguides of the optical waveguide coupler sandwiched therebetween, and a DC voltage applied between the electrodes is set. Depending on the size, change the refractive index of one optical waveguide,
An optical fiber gyro characterized by obtaining the above optical distance difference.