JPS60218020A - Total single-polarization fiber rotary sensor - Google Patents

Abstract

PURPOSE:To reduce noises originating from disturbance due to mechanical oscillation, etc., by using a fiber type polarizer which uses a birefringent fiber instead of a bulk polarizer and using birefringent fibers for the whole part except a light source and a photodetector. CONSTITUTION:Light from a laser 11 is passed through a half-wavelength plate 13 and polarized wave parallel to the main axis of a 3dB optical coupler 15 is obtained from a birefringent fiber. When linear polarized light which is polarized in the main-axis direction of the fiber 23 is incident, linear polarized light whose incidence power is reduced to half appears at projection-side 23a and 24a. There is no problem when the loss that an (x) polarized wave in propagation mode has is <=1dB/m, and the cutoff wavelength lambdac of the fiber is so designed that lambda/1.7<= lambdac<=1.4, where lambda is the wavelength of the light source. Both (x) and (y) polarized waves in use have such wavelength that they propagates in a fiber coil 18. When the sensor is so designed that the (y) polarized wave does not propagate substantially, the influence of mode coupling in the coil is eliminated and a stable rotary sensor is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a highly accurate rotation sensor used for attitude control of a flying object or a robot.

(Prior Art) The configuration of a conventional optical fiber rotation sensor using a birefringent fiber is shown in FIG. In FIG. 3, the light emitted from the laser 1 is collimated by the lens 2, converted into linearly polarized light by the bulk polarizer 4 in the direction of the main axis of the birefringent fiber 7, and then input to the fiber by the lens 5. The light incident on the fiber is
The light is divided into two parts by the 3 dB optical coupler 6 and divided into light propagating clockwise through the fiber coil 7 and light propagating counterclockwise. The light propagated clockwise and counterclockwise through the fiber coil has a 3 dB
After being multiplexed again by the optical coupler 6 and passing through the lens 5 and polarizer 4, it is reflected by the half mirror 3 and enters the photodetector 9.

The photodetector 9 detects the light propagated clockwise in the optical fiber and
Interference with light propagating counterclockwise produces a photocurrent proportional to the intensity of the light. In an optical fiber rotation sensor, a difference occurs in the optical path length of light propagating in both left and right directions within the optical fiber, depending on the rotation of the optical fiber. This phenomenon is known as the Sagnac effect, and E, J, Po5t″8agnac E
ffect”Rev, ofModern Phyi v
ol, 39-2. P, 475-493 (196
7) K is described in detail. A phase difference ψ occurs between the left and right lights corresponding to the optical path difference between the left and right lights.

4πRL ψ=λCΩ (1) Here, λ is the wavelength of the light source in vacuum, C is the speed of light in vacuum, R is the radius of the optical fiber coil, L is the length of the optical fiber coil, and Ω is the rotational angular velocity. . Therefore, the photocurrent output ■ from the photodetector becomes Ioc l +cos ψ (2) due to this phase difference ψ. Therefore, the rotational angular velocity Ω can be detected by reading the change in the photocurrent output. however,
In the case of Equation (2), when the rotation is minute (ψ<1), the curve becomes a nearly constant value and rotation detection becomes impossible. Therefore, as shown in FIG. P
An AC voltage is applied to the ZT resonator to add phase modulation to the light propagating through the fiber. Now, if the angular frequency of the AC voltage applied to the PZT is ω□, and the phase modulation degree is ψm, the output of the photodetector = 1-cosq cos (ψmcos (ωm
t ))-sin9)sin (Icos (ωm t
)) = 1 +cosψ(JO(ψm) + 2J2
(ψrrL)cos(2ωmt)7...)+sinψ(
2J, (ψm) sin (ωmt) + −) (31) However, the divination, Jl, and J2 are Bessel functions.

By synchronously detecting the component of ωm among each frequency component of I(t), the output I, (Ksinψ・2J, (ψm> (41) is obtained. In the case of minute rotation ψ<1, equation (4) is 11″ψ°2J+
(ψm) f51 and an output proportional to the rotational angular velocity can be obtained.

Conventionally, the fiber coil and 3 (IB optical coupler') shown in FIG.
Although there are examples of stabilizing polarization characteristics using birefringent fibers, bulk polarizers were used as polarizers. When using a bulk polarizer, as shown in Fig. 1, part of the light is emitted from the fiber through the lens 5, which tends to cause axis misalignment due to mechanical vibration of the lens or polarizer. There were drawbacks such as output fluctuations becoming noise that prevented detection of minute rotations, and the signal level fluctuating over a long period of time.

(Problems to be solved by the invention) In order to eliminate such drawbacks of the conventional technology, the present invention uses a fiber-type polarizer using birefringent fibers instead of a bulk polarizer, and uses birefringent fibers for all components other than the light source and photodetector. By doing so, noise caused by disturbances such as mechanical vibrations is reduced.The drawings will be described in detail below.

(Structure and operation of the invention) FIG. 1 is a diagram of an embodiment of the invention, in which 11 is a laser;
12 and 14 are lenses, and 13 is a 174 wavelength plate.

The light from the laser is polarized by the 1/2 wavelength plate parallel to the main axis of the 3 dB optical coupler 15 made of birefringent fiber. The 3dB optical coupler has a structure as shown in Fig. 4, and after aligning the principal axes of the birefringent fibers 23 and 24 in parallel,
It is made by heating and stretching a part of the fiber.

When linearly polarized light polarized in the direction of the main axis of the fiber B is input, a portion of the input power is output from the output side 23a, '2Aa while maintaining the linearly polarized state. Although some orthogonal polarization components occur due to the heated and stretched portions, crosstalk is suppressed to less than one additional dB. Further, the excessive loss of light due to heating and stretching is 1 dB or less. 3dB optical coupler 15, 17. FIG. 5 shows the cross-sectional structure of the birefringent fiber used in the fiber coil 18. core 5
Stress applying parts having different thermal expansion coefficients from the cladding 27 are arranged on both sides of the cladding 27. During the cooling process after drawing, different stresses act on the core in the X-axis direction and the y-axis direction due to thermal contraction of the stress-applying part, and the
The propagation constants of axially polarized light will be different. X
Let β be the propagation constants of light polarized in the axial direction and the y-axis direction, respectively.
If X and βy, the mode birefringence B is given by B = (βX - βy)/k (61). However, k = 2π/λ.
If l0, practically sufficient polarization stability is achieved.

Next, the fiber polarizer 16 in FIG. 1 will be explained. FIG. 6 is a cross-sectional view of a birefringent fiber used in a fiber polarizer. Core relative refractive index difference Δ = bite, core diameter 2a = 5.4 μm, fiber outer diameter 2b = 150. am,
Inner radius r1 of stress applying part = 3.1a, boron concentration of stress applying part 15) 7%, TE01 mode cutoff wavelength λc =
It is 0.81 μm. Mode birefringence B=4.1XlO
It is.

FIG. 7 shows loss spectral characteristics measured with this birefringent fiber wound around a bobbin with a radius R=15 cm. As shown in FIG. 7, the loss of the X polarization starts to increase from around the wavelength λ = 1.12 μm = '1384. On the other hand, the loss of X polarization is λ" 1.28μm = 1.58λ. It increases from around 1.58λ. This difference in bending loss of both X and 7 polarizations is due to the following. The refractive index distribution felt by both the x and 7 polarized waves in the birefringent fiber is given by the following equation.

n<Cr, θ)=n(r, θ)−qσx (r, θ)−
q(σy(r+θ)+σz(r, θ)](force n y(r
, θ)=n(r, θ)−qσy(r, θ)−C, (az
(r, θ) + σx (r, θ), ) (8) Here, n
('r, θ) is the refractive index distribution without stress, CI, c2 are photoelastic constants, and σX, σy, and az are the principal stress distributions.

Normalized propagation constant at λ = 1.15 μm calculated from cloth. From Figure 8, the propagation constant βy/ of the X polarized wave is
It can be seen that because the refractive index of the cladding is closer to that of the polarized wave, the bending loss is greater than that of the X polarized wave for the same bending. The relative refractive index difference Δ of the core, the inner radius λ of the stress applying part, and the cutoff wavelength λ. FIG. 9 shows the results of measuring the bending loss characteristics of both XrY polarized waves by bending various birefringent fibers under different conditions such as bending radius R = 2 mm = 20 cm. The area surrounded by diagonal lines is the range in which the measurement results are distributed. From FIG. 9, the conditions under which a birefringent fiber cannot be used as a fiber polarizer are derived as follows.

'(IIY polarization loss is practically sufficient loss 40dB/-
In order to receive the above, λ/λC≧18.4 must be satisfied.

(2) The loss suffered by the X polarization, which is the propagation mode, is 1 dB.
/m or less and the condition is λ/2°≦1.7.

Therefore, when using a birefringent fiber as a fiber-type polarizer, the cutoff wavelength P of the fiber satisfies the condition λ/1.7 < λv and λ/1.4 (9) with respect to the wavelength λ of the light source. must be designed to do so.

FIG. 1O shows the relationship between the extinction ratio and the number of turns N of the fiber polarizer 16 used in the all-single polarization fiber rotation sensor of FIG. The wavelength of the light used is λ = 1.3μ
m, which satisfies the condition of equation (9). In the measurement, circularly polarized light was incident on a fiber polarizer, and the output light was passed through a bulk analyzer to measure the extinction ratio. As shown in FIG. 10, bending radius R=10. In this case, an extinction ratio of 43 dB is obtained when N=6 or so, and the excess loss of the X polarized wave at this time is 0.05 dB or less. In the configuration of the all-single-polarized fiber rotation sensor shown in FIG. 3, the 3 dB optical coupler and the fiber polarizer are connected in FIGS.
B optical coupler 17 is connected. FIG. 11 shows the output when the central axis of the fiber coil is oriented east-west (rotational angular velocity Ω=0).

Fluctuations in the output due to noise were within 0.5°'lhr (time constant T of the lock-in amplifier = cutting seconds), and there was no drift over a long period of ω minutes (a sufficiently stable rotation sensor was realized).

In the above description, the fiber coil 18 is used at a wavelength that allows both x and 7 polarized waves to propagate. However, if the fiber coil is designed in such a way that the X-polarized wave will not actually propagate, a more stable fiber rotation sensor can be realized because the influence of mode coupling in the fiber coil can be eliminated.

Similar to FIG. 9, FIG. 12 shows the results of measuring the bending loss characteristics of x and X polarized waves using various birefringent fibers. Figure 1
2, it can be seen that the following conditions must be satisfied in order to obtain a single-polarized single-mode fiber that extremely minimizes the adverse effects of mode coupling as a fiber coil and has good transmission characteristics.

(The condition for the loss of at

(bl The excess loss suffered by the X polarization, which is the propagation mode, is 1
The condition for being below dB/b is λ/2°≧1.65.

Therefore, as a fiber coil of a fiber rotation sensor, a single-polarized single-mode fiber that minimizes the influence of mode coupling due to external disturbances has a T
& mode cutoff wavelength J2 is λ/1.65<]3)<λ/
1.4 QO) must be designed to satisfy the following conditions.

The difference in design value from the birefringent fiber used in the fiber polarizer depends on the difference in length of the fiber used. Fiber-type polarizers are made of shorter fibers than fiber coils (1).

Based on the condition (2), design values such as equation (9) are used.

FIG. 13 shows another example of this embodiment, which is an all-single-polarized fiber rotation sensor using a single-polarized single-mode fiber in which only the X-polarized wave can actually propagate in the fiber coil path.

The cross-sectional structure of the birefringent fiber used in the fiber coil path is similar to that shown in FIG. Figure 14 shows the wavelength dependence of the crosstalk and extinction ratio measured with the above birefringent fiber wound around a bobbin with a radius R''15 (m).
E. , mode cutoff plate length 12' = 0.86 ImL
It is. Crosstalk is the amount of X-polarized waves generated when linearly polarized light is incident in the X-axis direction, and extinction ratio is the amount of X-polarized waves generated relative to the X-polarized waves on the output side when circularly polarized light is incident. From FIG. 14, λ=1.3 μm (λ/A'c' = 1.5
1) Crosstalk -41.5dB, extinction ratio 42
dB is obtained, and it can be seen that the X polarized wave is not actually propagating. FIG. 15 shows the output when the central axis of the fiber coil is oriented east-west (rotational angular velocity Ω=0). The output fluctuation due to noise is within 081°lhr (time constant 'i:'
c = 4) Sec), and a sufficiently stable rotation sensor with no drift over a long period of 10 hours was realized.

(Effects of the Invention) As explained above, in the present invention, everything except the light source and photodetector uses birefringent fibers, so it is stable and highly sensitive for a long time without being affected by disturbances such as mechanical vibrations. This has the advantage that a rotation sensor can be realized.

[Brief explanation of drawings]

FIG. 1 shows an example of an all-single polarization fiber rotation sensor according to the present invention.
2 is a diagram showing the output of a photodetector calculated by equation (2), FIG. 3 is a block diagram of a fiber rotation sensor using a conventional birefringent fiber, and FIG. 4 is a diagram showing a configuration of a fiber rotation sensor using a conventional birefringent fiber. Figure 5 is a cross-sectional view of the birefringent fiber used in the 3 d'B optical coupler and optical fiber coil, and Figure 6 is the birefringent fiber used in the fiber polarizer. A cross-sectional view of the fiber, Fig. 7 shows the loss wavelength characteristics due to bending of both x and 7 polarizations of the fiber shown in Fig. 6, and Fig. 8 shows the loss wavelength characteristics of the fiber shown in Fig. 6.
Figure 9 shows the refractive index distribution of the fiber in consideration of the photoelastic effect shown in Figure 6. Figure 9 shows various birefringent fibers with different core refractive index differences, stress applying part positions, and cutoff wavelengths of the fiber structure shown in Figure 6. Figure 10 shows the number of bends and extinction ratio or bending loss characteristics when the fiber shown in Figure 6 is bent to a bending radius R = toffi. FIG. 11 shows the sensitivity characteristics of the all-single-polarization fiber rotation sensor shown in FIG. 3, and FIG. 12 shows the refractive index difference of the core of the fiber structure shown in FIG. 6, the position of the stress applying part, and the cutoff wavelength. Bending radius R = 10cm-20crrL of various birefringent fibers with different
FIG. 13 is an example of the all-single polarization fiber rotation sensor of the present invention, and FIG. 14 is a diagram showing the crosstalk and extinction ratio of the birefringent fiber shown in FIG. 6. FIG. 15, a diagram showing wavelength characteristics, is a diagram showing sensitivity characteristics of the all-single polarization fiber rotation sensor shown in FIG. 13. 1...Laser light source, '2.5.1.0...Lens, 3...Half mirror-16...3dB optical coupler, 7
... Birefringent fiber coil, 8... PZT resonator, 9... Photodetector, 11...
- Laser light source, 12, 14, 20, 21. ... Lens, 13...L wave plate, 15.17... Birefringent fiber 3dB optical coupler, 16
...Fiber type polarizer, 18...Polarization maintaining fiber coil, 19...PZT resonator, 22... Photodetector, 23
．． 24. ...Birefringent fiber, 5...core, 26.
...Stress applying part, n...Clad, four...Core,
29...Stress applying part, (to)...Clad, Seki...Single polarization single mode fiber coil. Patent applicant Nippon Telegraph and Telephone Public Corporation Patent agent Megumi Yamamoto - Fig. 7 Fig. 8 00rl, 4ilr Fig. 9 Fig. 11 υ Ig Ri V Ri Haruma t (bone) Fig. 12 Fig. 9 Figure 13 × Field (421 to υ Figure 14 shallowest Om) 8/~

Claims (2)

[Claims] (1) Effective optical path length of light propagating in the same direction as the fiber rotation direction and light propagating in the opposite direction to the fiber rotation direction in an optical fiber placed in a plane perpendicular to the rotation axis In a fiber rotation sensor that detects the rotational angular velocity of a system in which an optical fiber is placed using the fact that the difference is proportional to the rotational angular velocity of the optical fiber, an optical fiber coil that detects rotation, a 3dB optical coupler, and a polarizer are used. The cutoff wavelength λT of the TEo1 mode of the birefringent fiber used as the polarizer is 7 times longer than the wavelength λ of the light source. ) < λ/1.4. (2) The condition that the TEoI mode cutoff wavelength source of the birefringent fiber used as the optical fiber coil for detecting rotation is λ/1.65 and λ/1.4 with respect to the wavelength λ of the light source. Claim 1 characterized in that it satisfies
All single-polarized fiber rotation sensor as described in Section.