JPH0135284B2 - - 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 directional optical splitter/coupler and a polarization maintaining fiber coil.

(2) Background of the technology Fiber optic laser gyroscopes have been actively researched 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. has been done.

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 an optical fiber loop 3 from opposite directions. After passing through the optical fiber loop 3 in the opposite direction, these lights are combined again at the beam splitter 2 to form interference fringes on the surface of the light receiving element 4. At this time, when the optical system rotates around the central axis O of the optical fiber loop 3, a phase difference occurs between the lights propagating in the opposite direction through the optical fiber loop 3 due to the sagnac effect, and the interference fringe moves from a stationary state. do. The rotation speed can be determined from this amount of movement, and the rotation angle can be determined from the integral value.

However, in this configuration, the refractive index n of the optical fiber changes due to the influence of geomagnetism, so L
The optical length, expressed by =n·l (l is the length of the optical fiber loop), changes. Therefore, the phase of the light incident on the light-receiving element 4 changes randomly due to the earth's magnetism, resulting in a disadvantage that measurement accuracy deteriorates.

(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 high-precision optical laser gyroscope system at a low cost.

(4) Structure of the Invention According to the present invention, this object consists of a laser oscillator, a prism for converting the laser light from the laser oscillator into linearly polarized light, and means for converting the output from the prism into circularly polarized light. It has a circularly polarized light source, an optical fiber loop constituted by a polarization-maintaining fiber, an optical splitter/combiner having four terminals, and a half mirror, and a photodetector. 2 The plane of polarization is orthogonal due to the half mirror.
The light is split into two linearly polarized waves, emitted from the second and third terminals connected to both ends of the optical fiber loop, and propagated through the optical fiber loop in opposite directions, and then combined by the half mirror. The output light from the fourth terminal is detected by a photodetector, and the phase difference between the lights propagating in opposite directions through the optical fiber loop is detected by the output of the photodetector. This is achieved by providing an optical fiber gyroscope characterized by:

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

FIG. 2 shows one embodiment of the present invention, in which 11 is a laser diode, 12 is a lens, 1
3 is the Glan-Thompson prism, 14 is the lens, 1
5 is a phase plate, 16a, 16b are lenses, 17 is a half mirror, 18 is a monitor, 19 is a λ/4 phase plate, 20 is a lens, 21 is a rotation prism,
22 is a lens, 23 is a loop-shaped polarization maintaining fiber, and 24 is a photodetector.

In the embodiment of FIG. 2, the laser diode 1
The light emitted from the lens 1 passes through the lens 12, the Glan-Thompson prism 13, the lens 14, and the phase plate 15, and enters the lens 16a. Part of the incident light is reflected by the half mirror 17 and enters the monitor 18, and part of the light passes through the half mirror 17, passes through the lens 20, and passes through the rotation prism 21, where they are polarized by 90 degrees to each other. The optical signal becomes an optical signal having a plane angle and is input to the polarization-maintaining fiber 23. This fiber 23 has a circular shape with a radius R, and optical signals input from both ends 23a and 23b propagate clockwise and counterclockwise through the fiber, pass through the rotation prism 21, and are reflected by the half mirror 17. and is output through the phase plate 19.

FIG. 3 shows a specific example of the optical system within the dotted line in FIG. 2.

The rotation prism 21 is constructed by superimposing two prisms 21a and 21b whose crystal axes are shifted by 90 degrees, and the entrance end surface T ₁ and the exit end surface T ₂ have an angle α ₁ . and two ball lenses 20',
22' is provided facing the entrance end surface T ₁ and the exit end surface T ₂ . On the other hand, the end faces 23a, 23 of the optical fiber
b are each shaped to have an angle of θ _p in opposite directions. Also, an optical fiber 25 is connected opposite to the spherical lens 20'.
is provided, and its end face is also shaped to have an angle θ _p . Also, the optical axis of the optical fiber 25 is connected to the ball lens 2.
The optical axes of the optical fibers 23 are offset by h with respect to the optical axis of 0', and the optical axes of the ends of the optical fibers 23 are each offset by x ₂ .

Here, light is emitted from the optical fiber 25 at an angle θ ₀ with respect to the optical axis, and enters the end face T ₁ of the rotation prism 21 at an angle β. and,
From the equivalent light branching point M in the Rochon prism 21, the extraordinary light and the ordinary light are separated and emitted at equal separation angles δ with respect to the mechanical axis 26 (optical axis). The separation angle δ at this time is the rotation prism 1
It is known that when the refractive index of the birefringent material constituting the birefringent material 1 for extraordinary light is _ne and the refractive index for ordinary light is n ₀ , it is determined by the following relationship.

α ₁ = (+n _e /n _e +n ₀ −2)・δ (1) Now, when the optical fiber 25 is placed parallel to the mechanical axis 26, the light emitted from the optical fiber 25 with an emission angle θ ₀ is , is incident on the end face _T1 of the rotion prism 21 at an incident angle β through a ball lens 20' whose center is on the mechanical axis 26, and the relationship between the center O of the ball lens 20' and the point of incidence at this time is If the distance is 1 ₁ , the following relationship holds true.

β=F(tanθ ₀ +h)/1 ₁ (2) where h is the distance between the optical axis of the optical fiber 25 and the mechanical axis 26.

Next, the spherical lens 22' is arranged so that its center O' is at a distance of 1 ₁ from the equivalent light branching point M, and the exit angle β from the end surface T ₂ of the rotion prism 21 is
The extraordinary rays and the ordinary rays emitted with the spherical lens 22' respectively pass through the end face 23a of the optical fiber 23, which is arranged parallel to the mechanical axis 26 and at the same distance x ₂ from the center O' of the spherical lens. 23b at an incident angle θ _r , the following relationship holds true.

Note that since the equivalent light branching point M occurs at a point a certain distance away from the mechanical axis 26, the ball lens 2
The center O' of 2' must also be placed the same distance away from the machine axis 26.

Assuming that the following conditions hold, x ₂ =±h (4) θ ₀ =θ _r (5) From equation (3), the incident angle θ _r is as follows.

θ _r = (1 ₁ −F)h/F ² …(6) Therefore, conversely, θ _r , 1 ₁ ,
By determining F and h, the relationships in equations (4) and (5) are established. This indicates that the input and output optical fibers and both sphere lenses are arranged symmetrically with respect to the Rotion prism.

Further, in this case, the angles θ ₀ and θ _r are determined by the following equation in relation to the inclination angle θ _p .

θ ₀ = θ _r = sin ^-1 M (n _c /n _a・sin θ _p )−θ _p …(7) Here, n _c is the refractive index of the core of the optical fiber, and n _a is the refractive index of air. .

Therefore, the light emitted from the optical fiber 25 with an emission angle θ ₀ is separated into extraordinary light and ordinary light, and the light is emitted from the optical fiber 25 with an incident angle θ _r equal to the emission angle.
It is incident on both end faces of 3.

On the other hand, the lights emitted from the end faces 23a and 23b of the optical fiber 23 at an angle of θ _r are combined by the rotation prism 21 and enter the optical fiber 25 at an angle of θ ₀ .

Here, the polarization state of the signal at each point a, b, c, d, e, and f in FIG. 2 is shown by the corresponding symbol in FIG. 4.

In FIG. 4, a is the light emitted by the laser diode, and b is the light that is deflected by the Glan-Thompson prism 13, which becomes linearly polarized light. The phase plate 15 generates circularly polarized light from the linearly polarized light as shown in FIG. The refractive index of the rotation prism 21 differs depending on the polarization direction, so its output is separated as shown in FIGS.
The light enters from 3a and 23b. The light emitted from the polarization preserving fiber 23 is combined by the rotation prism 21 and reaches the half mirror 17 as shown in FIG. 4d, where it is reflected and incident on the phase plate 19. Note that here, the angle θ is L
is the total length of the constant polarization fiber coil, R is its radius,
If Ω is the rotational angular velocity of the fiber in radians/second, λ is the wavelength of light, and C is the speed of light, then θ=2πLR・Ω/λC.

Then, the signal d having a difference of angle 2θ is transmitted to the phase plate 1.
9 further shifts the phase of the light in one polarization direction by π/2, so the angle 2θ is 2θ as shown in Figure 4e.
+π/2 and enters the photodetector 24. FIG. 4f shows the output signal of the photodetector.

FIG. 5 shows a second embodiment of the optical laser gyroscope of the present invention, in which 31 is a laser diode, 32 is a Glan-Thompson prism, 33 is a λ/4 phase plate, 34 is an electrostrictive element, 35
36 is a polarization fiber, 36 is a phase plate, 37 is a photodetector, 38 is a phase detector, 39 is an oscillator, and 40 is a phase modulator. Note that in the adapter 34,
41a and 41b are spherical lenses, 42 is a half mirror, and F ₁ and F ₂ are optical fibers.

In the system shown in FIG. 5, the light emitted from the laser diode 31 is converted into linearly polarized light by the Glan-Thompson prism 32, and is converted into linearly polarized light by the λ/4 phase plate 3.
2, it becomes circularly polarized light. The purpose of using the Glan-Thompson prism is to obtain complete circularly polarized light when converted into circularly polarized light by the λ/4 phase plate.
The circularly polarized light from the λ/4 phase plate 33 is transmitted through an optical fiber.
F ₁ causes the light to enter the ball lens 41a, where it is converged, and the half mirror 42 separates the light into lights with mutually orthogonal polarization planes. The signal becomes a direction signal and returns to the spherical lens 41a through the optical fiber loop with radius R, while the other signal is reflected by the half mirror 43 and reaches the polarization fixed fiber 15 as a counterclockwise signal. The clockwise and counterclockwise signals are then guided to the λ/4 phase plate 36 via the optical fiber F ₂ , where they are phase-shifted and detected by the photodetector 37 and then detected by the phase detector 38 .
It is output after passing through.

The deflection state of the signal at each point a, b, c, d, e, f in FIG. 5 can also be explained with reference to FIG.

That is, in FIG. 4, a is the light emitted by the laser diode, b is the light polarized by the prism 32 and the light traveling clockwise through the constant polarization fiber, and c is light traveling counterclockwise through the constant polarization fiber. , d correspond to the output light of the adapter 34. 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 in radians/sec, λ is the wavelength of light, and C is the speed of light, then θ=2πLR・Ω/ λC.

Then, the signal d having a difference of angle 2θ is passed through the phase plate 3
6, the light in one traveling direction is further phased by λ/2, and the angle 2θ is 2θ+ as shown in Figure 4e.
It becomes large to λ/2 and is input to the photodetector 37. FIG. 4f corresponds to the output signal of the photodetector 37, and this signal is phase-detected by the phase detector 38 and output.

FIG. 6 shows a third embodiment of the invention.
The difference in FIG. 5 from FIG. 4 is that the output of the laser diode 31 is converted into circularly polarized light by a λ/8 or λ/4 phase modulator 44, and a ball lens 4
1a, and the other circuits and operations are exactly the same as those in FIG. 4.

In FIGS. 4 and 5, ball lenses 41a, 4
1b, optical fibers F ₁ , F ₂ , constant polarization fiber 35
The optical input/output end portion of the device is constructed as shown in FIG. In the figure, 43a and 43b are cores, 4
4a and 44b are lens holders, and 45 is a support member.

As shown in the figure, optical fiber F ₁ and polarization constant fiber 35
The end portion 35b of the optical fiber F ₂ and the constant polarization fiber 3 are inserted into the hole 43H' provided in the center of the cylindrical core 43a.
The end portion 35b of the cylindrical core 43b is installed in the hole 43H' provided in the center of the cylindrical core 43b. Each core 43a, 4
3b are inserted into the sleeve-like adapter 34 so that their center positions are aligned with each other, and are fixed by a fixing means (not shown) (such as a hook nut). Ball lenses 41a and 41b are mounted on a support member 44 in front of the optical fibers exposed at the center of the end face of each center 43a and 43b.
a, 44b. Ball lens 41
A half mirror 43 (made of a dielectric multilayer film) is supported by a support member 45 and is provided between a and 41b. Then, the light P ₁ transmitted through the optical fiber F ₁ is separated into lights P ₂ and P ₃ by the half mirror 43.

The lights P ₂ and P ₃ are transmitted in opposite directions through the constant polarization optical fiber 35, and are combined again by the half mirror 43 to become light P ₄ , which enters the optical fiber F ₂ .

(6) Effects of the Invention As explained in detail above, according to the present invention, the laser beam can mostly travel through the optical fiber, so it is less susceptible to environmental influences such as air fluctuations, and the polarization preserving coil Before the clockwise and counterclockwise light in the polarization preserving coil enters the polarization preserving coil,
After being emitted, it travels in the opposite direction along the same path, so for example, phase fluctuations occur in the opposite direction.As a result, phase fluctuations can be canceled out, and detection sensitivity close to the theoretical limit can be obtained. I can do it.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional optical fiber coil, Fig. 2 is a block diagram of an embodiment of the optical fiber coil according to the present invention, and Fig. 3 shows the specific structure within the dotted line in Fig. 2. 4 are diagrams showing the polarization state of light in each part in an embodiment of the present invention, FIGS. 5 and 6 are block diagrams of other embodiments of the present invention, and FIG. 5 is a partially detailed diagram of FIG. 5. In the drawing, 11 is a laser diode, 12
is a lens, 13 is a Glan-Thompson prism, 14
is a lens, 15 is a phase plate, 16a, 16b are lenses, 17 is a half mirror, 18 is a monitor, 19 is a λ/4 phase plate, 20 is a lens, 21 is a rotation prism, 22 is a lens, 23 is a polarization preservation device 31 is a laser diode, 32 is a Glan-Thompson prism, 33 is a λ/4 phase plate, 34 is an adapter, 35 is a constant polarization fiber, 36 is a phase plate,
37 is a photodetector, 38 is a phase detector, 39 is an oscillator, 40 is a phase modulator, 41a and 41b are lenses, and 42 is a half mirror.

Claims (1)

[Claims] 1. A circularly polarized light source comprising a laser oscillator, a prism for converting laser light from the laser oscillator into linearly polarized light, and means for converting the output from the prism into circularly polarized light; It has an optical fiber loop composed of a wavefront preserving fiber, an optical branching/combining device having four terminals, a half mirror, and a photodetector.
A second circularly polarized light incident from the terminal of the optical fiber loop is separated into two linearly polarized lights whose planes of polarization are perpendicular to each other by the half mirror, and the two are coupled to both ends of the optical fiber loop.
The light emitted from the third terminal and propagated in opposite directions through the optical fiber loop is combined by the half mirror and emitted from the fourth terminal, and the output light from the fourth terminal is detected by a photodetector. An optical fiber gyroscope characterized in that a phase difference between lights propagating in opposite directions through the optical fiber loop is detected by the output of the photodetector. 2. The optical fiber/gyro according to claim 1, wherein the means for converting into circularly polarized light is a phase plate. 3. The optical fiber gyroscope according to claim 1, wherein the means for converting into circularly polarized light is a phase modulator. 4. A circularly polarized light source consisting of a laser oscillator, a prism that converts the laser light from the laser oscillator into linearly polarized light, and means for converting the output from the prism into circularly polarized light, and a polarization preserving fiber. It has an optical fiber loop, an optical branching combiner having four terminals, a half mirror, and a photodetector, and the first
A second circularly polarized light incident from the terminal of the optical fiber loop is separated into two linearly polarized lights whose planes of polarization are perpendicular to each other by the half mirror, and the two are coupled to both ends of the optical fiber loop.
The light emitted from the third terminal and propagated through the optical fiber loop in mutually opposite directions is combined by the half mirror and emitted from the fourth terminal, and the output light from the fourth terminal is phase-shifted by λ/4. An optical fiber gyroscope characterized by detecting through a plate. 5. The optical fiber/gyro according to claim 4, wherein the means for converting into circularly polarized light is a phase plate. 6. The optical fiber gyroscope according to claim 4, wherein the means for converting into circularly polarized light is a phase modulator. 7 A laser oscillator, a polarization conversion optical system that converts the output of the laser oscillator into linearly polarized light and then into circularly polarized light, an optical fiber loop composed of a polarization preserving fiber, four terminals, and a half mirror. the laser oscillator is phase-modulated by the phase modulator, the output of the polarization plane conversion optical system is input to the first terminal, and the laser oscillator is phase-modulated by the phase modulator; A half mirror separates the light into two linearly polarized lights whose polarization planes are orthogonal, and the light is emitted from the second and third terminals that connect both ends of the optical fiber loop, and the light propagates through the optical fiber loop in opposite directions. The fourth image is synthesized by the half mirror.
The output light from the fourth terminal is detected by a photodetector, the output of the photodetector is input to the phase detector for phase detection, and the optical fiber loops are connected in opposite directions. An optical fiber gyroscope that detects the phase difference of propagating light.