JPS6348284B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to an optical fiber/gyro system using a highly directional optical splitter/coupler and a polarization maintaining fiber coil.

(2) Background of the technology Fiber optic laser gyroscopes have become popular because they use a long single-mode optical fiber as the optical path and can detect angular velocity with extremely high sensitivity compared to conventional mechanical gyroscopes or ring gyroscopes. being researched.

FIG. 1 shows the basic configuration of an optical fiber applied laser gyroscope. The light from a laser light source 1 is split into two parts by a beam splitter 2, and each part enters a loop-shaped optical fiber 3 from opposite directions. After passing through the optical fiber 3, these lights are combined again by the beam splitter 2 and enter the light receiving element 4. At this time, when the optical system rotates around the central axis of the optical fiber loop, a phase difference occurs between the lights propagating in opposite directions through the optical fiber due to the Sagnatsk effect.
The interference fringes move from a stationary state. Due to this movement, a sinusoidal output is obtained from the light receiving element 4,
The peak of the output is counted for a certain period of time. The rotation speed can be determined from the counted number, and the rotation angle can be determined from the integral value. However, in the configuration shown in Figure 1, the interval between peaks varies greatly due to fluctuations in the oscillation wavelength of the laser light source 1, so in the basic configuration diagram in Figure 1, the number of counts relative to the amount of movement of the interference fringe varies, and therefore the measurement An error occurs. Also, the light reflected by the relay scattering from the optical fiber and the light reflected by the half mirror are incident on the light receiving element 4, resulting in poor signal to noise.

(3) Purpose of the Invention In view of these drawbacks of the prior art, it is an object of the present invention to provide a highly accurate and highly sensitive optical laser gyroscope system with an excellent signal-to-noise ratio.

(4) Structure of the Invention According to the present invention, the purpose is to guide light from a light source to the first terminal of a four-terminal optical directional coupler, split it into two by a half mirror, and send it to the second and third terminals. The light enters both ends of the connected optical fiber coil, propagates through the optical fiber coil in opposite directions, and is emitted from both ends. The light is combined by the half mirror, enters the photodetector, and is output from the photodetector. In an optical fiber-gyro system that measures the phase difference of light propagating in the opposite direction through the optical fiber coil, the light generation time interval of the light source is set to be longer than the time during which the light passes through the optical fiber coil, and the The modulation timing of the strain element is set to be the same as the light generation time interval of the light source, the modulation time of the electrostrictive element is set to be longer than the modulation time of the light source, and the position of the half mirror is set to λ in the electrostrictive element. /8 movement gives a phase difference to the light incident on both ends of the optical fiber coil, and also measures the phase difference by detecting the phase only when the light synthesized by the half mirror is converted into an electrical signal. This is achieved by providing an optical fiber gyroscope system characterized by the following.

(5) Embodiments of the invention Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 shows an optical laser gyroscope system according to the present invention, in which 11 is a laser diode, 12 is a Glan-Thompson prism, 13 is an oscillator, 14 is an electrostrictive element, and 15 is a constant polarization fiber. , 16 is a phase plate, 17 is a photodetector, 1
8 is a phase detector. In addition, in the electrostrictive element, 21a and 21b are ball lenses, and 22a and 22b are
is the lens holder, 23 is the half mirror, 24
is a space for applying λ/4 modulation, 25a and 25b are plugs, and F ₁ and F ₂ are optical fibers.

In the system shown in FIG. 2, an optical fiber _F1 and one end of a polarization fiber 15 are inserted into a plug 25a, and a ball lens 21a is held by a lens holder 2a and is provided opposite the end surfaces of both optical fibers. There is. Further, a half mirror 23 is fixed to the lens holder 22a. Plug 25b
The other ends of the optical fiber F ₂ and the constant polarization fiber 15 are inserted into the holder, and a ball lens 21b is held by a lens holder 22b so as to face the end surfaces of both optical fibers. Plugs 25a, 25b
is inserted into the through hole 14a formed in the electrostrictive element 14, and the plug 25a is attached to the electrostrictive element 14 with the adhesive 2.
Fixed by 6.

Here, the light emitted from the laser diode 11 passes through the Glan-Thompson prism 12, becomes linearly polarized light, is converged by a ball lens 21a, and is split into two by a half mirror 23, one of which passes through this and passes through the ball lens 21b. The clockwise light CW reaches the constant polarization fiber 15 and returns to the spherical lens 21a through an optical fiber loop with radius R, while the other light is reflected by the half mirror 23 and travels counterclockwise to the constant polarization fiber 15. light
Becomes CCW. The clockwise and counterclockwise signals are then phase-shifted via a phase plate 16 to the halo detector 1.
7, the signal is outputted via a phase detector 18.

The deflection states of the signals at points a, b, c, d, e, and f in FIG. 2 are indicated by corresponding symbols in FIG. 3, respectively.

In Figure 3, a is light emitted by a laser diode, b is linearly polarized light in the Glan-Thompson prism 12 and light traveling clockwise through a constant polarization fiber, and c is light traveling counterclockwise through a constant polarization fiber. The position of the half mirror when it is first reflected by the half mirror and enters the optical fiber 15 by 1/4λ modulation, and the position of the half mirror when it is emitted from the optical fiber 15 and reflected by the half mirror are λ. Since the half mirror is shifted by /8, the difference in round trip λ/4 is different from the clockwise light and the λ/4 phase compared to when the half mirror does not move. Therefore, waveform c is shifted from waveform b by λ/4. d is the sum of this waveform, and the optical fiber 15 is
Since the phase difference θ due to the angular velocity when rotating acts in opposite polarities in the clockwise and counterclockwise directions, one becomes λ/4+θ and the other becomes -θ. Furthermore, the waveform is shifted by λ/4 using a phase plate, resulting in λ/2+θ and −θ' as shown in waveform e. Note that these phases are shown as relative phases.
Note that Z indicates the length direction of the fiber or the traveling direction of light. Here, the angle θ is as follows: L is the total length of the constant polarization fiber coil, R is its radius, Ω is the rotational angular velocity of the fiber (radian/second), λ is the wavelength of light, and C is the speed of light, then θ = 2πLR・Ω/λC. The waveform f has a different coordinate system and θ represents the phase difference due to angular velocity, and when θ = 0, the synthesis of clockwise and counterclockwise light differs by λ/2, so it is 0.
It is strengthened or weakened at other phases, and eventually becomes an amplitude of sin θ depending on the phase θ.

The signal d having a difference of angle 2θ+λ/4 is then phase-shifted by the phase plate 16, and as shown in e, the signal d has a difference of λ/4.
The signal is further amplified by 4 and input to the photodetector 17. FIG. 3f shows the output signal of the photodetector 17,
This signal is phase-detected by a phase detector 18 and output.

Here, the timing relationship between the output light of the laser diode 11, the modulated output of the electrostrictive element 14, and the clockwise and counterclockwise optical signals propagating in the constant polarization fiber will be explained using FIGS. 4 and 5. .

The laser diode 11 is driven by the output of the modulator 13 at the timing shown in FIG. 4a. That is, the light generation time interval t _ppc is longer than the time τ _c during which the light passes through the optical fiber loop 15 . In the example shown, t _ppc is twice τ _c . Here, τ _c is expressed by the following formula.

τ _c =L·n/C where L is the total length of the optical fiber coil, n is the refractive index of the core, and C is the speed of light. On the other hand, the electrostrictive element 14 is modulated at the timing shown _{in FIG} .

As the plug 25a moves due to this modulation of the electrostrictive element 14, the half mirror moves from the position shown by the solid line to the position shown by the dotted line as shown in FIG. The moving distance is λ/8. Due to this movement, the reflection point at the half mirror 23 is shifted by λ/8 for the signal CCW traveling in the polarization constant fiber 15 in the counterclockwise direction.

On the other hand, light traveling clockwise through the constant polarization fiber 15
There is no impact on CW.

Therefore, optical CCW and CW are defined as polarization fixed fiber 1.
5, a phase difference of λ/4 occurs.
Figure 4d and e show this state, and figure d shows the optical CCW
Figure e shows optical CW. Then, as shown in FIG. 4b, the timing at which the lights from both ends of the constant polarization fiber 15 are combined by the half mirror 23, enters the photodetector 17, is converted into an electrical signal, and enters the phase detector 18. Then, the gate to the phase detector 18 is opened and phase detection is performed. Here, the light CCW combined by the half mirror 23 is expressed as sin(ωt+λ/4+θ), and the light CW is expressed as sin(ωt−θ). Therefore, the light that combines optical CCW and optical CW is
It is expressed as cosωt・sinθ. Here, the component indicated by cosωt has a very high frequency, and appears to be modulated by the component indicated by sinθ. Accordingly
The angular velocity is determined using the component represented by sinθ.

FIG. 6 shows another embodiment of the invention. In the figure, 27 is an adapter, 28 is a ceramic vibrator, and the same members as in FIG. 2 are given the same symbols. In this embodiment, a ceramic vibrator 28 is attached to the plug 25a, and the ceramic vibrator 28 is vibrated at the timing described above. This causes the plug 25a to move in the direction of the arrow, so that the light
It becomes possible to provide a phase difference of λ/4 between CCW and optical CW.

FIG. 7 shows still another embodiment of the present invention. In the figure, 27 is an adapter, 29 is a ceramic resonator, 30
is a connector, and the same members as in FIG. 2 are given the same reference numerals. In this embodiment, the adapter 27 is provided with a through hole 27a, and the half mirror 23 connected to the ceramic vibrator 29c by a connector 30 is inserted through the through hole 27a. In this configuration, the ceramic vibrator 29 is vibrated. This allows optical CCW and optical CW as described above.
A phase difference of λ/4 is given to

(6) Effects of the invention As explained in detail above, in the present invention, since a signal is input to the phase detector only for a specific time,
The influence of noise due to relay scattering etc. can be reduced. Furthermore, since an electrostrictive element is provided as part of the system and the optical length is temporally modulated by this element, the angular velocity can be measured using the sin θ waveform, and detection sensitivity close to the theoretical limit can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of an optical fiber laser coil, FIG. 2 is a block diagram of an embodiment of the optical fiber laser coil according to the present invention, and FIG. FIG. 4 is a diagram showing the deflection state, FIG. 4 is a time chart for explaining the operation of the block diagram in FIG. 2, FIG. 5 is a diagram showing the movement of the half mirror, and FIGS. FIG. 7 is a diagram showing another embodiment of the invention. In the figure, 11 is a laser diode, 12 is a Glan-Thompson prism, 13 is an oscillator, 14 is an electrostrictive element, 15 is a fixed polarization fiber, 16 is a phase plate, 17 is a photodetector, 18 is a phase detector, 21
a and 21b are lenses, 22a and 22b are lens holders, 23 is a half mirror, and 24 is a space for applying λ/4 modulation, respectively.

Claims (1)

[Scope of Claims] 1. Light from a light source is guided to the first terminal of a four-terminal optical directional coupler, split into two by a half mirror, and connected to the second and third terminals at both ends of an optical fiber coil. The light enters the optical fiber coil, propagates in opposite directions through the optical fiber coil, and is emitted from both ends. The light is combined by the half mirror, enters the photodetector, and travels through the optical fiber coil in the opposite direction from the output of the photodetector. In an optical fiber gyroscope system that measures the phase difference of light propagated to the optical fiber coil, the light generation time interval of the light source is set to be longer than the time for the light to pass through the optical fiber coil, and the modulation timing of the electrostrictive element is set to By making the modulation time of the electrostrictive element equal to the light generation time interval of the light source and larger than the modulation time of the light source, and moving the position of the half mirror by λ/8 by the electrostrictive element, A phase difference is given to the light incident on both ends of the optical fiber coil, and the phase difference is measured by phase detection only when the light synthesized by the half mirror is converted into an electrical signal. Optical fiber gyroscope system. 2 Let τc be the coil delay time determined by the optical fiber coil length L and its propagation speed C in the optical fiber/gyro system, let the light source light generation time interval topc be topc = (τm)τc, and let the light generation time interval topc and light The optical fiber cable according to claim 1, characterized in that the electrical gate opening time intervals of the detectors are the same, and the mutual time difference between the interval TOPC and the interval is equal to the delay time. gyro system.