JPH05322590A - Optical fiber gyroscope - Google Patents

Abstract

PURPOSE:To achieve easy manufacturing at a low cost. CONSTITUTION:In an optical fiber coil 16 for detecting angular velocities, polarization-plane preserving optical fiber coils 16b and 16c are connected to both ends of a single-mode optical fiber coil 16a. Thus the optical fiber coil 16 is constituted. The lengths 11 and 12 of the polarization-plane preserving optical fiber coils 16b and 16c are selected so as to satisfy the following expression: 11, 12>LPB {LC/lambda)+(L/LSB)}. In the expression, LPB is the beat lengths of the polarization-plane preserving optical fibers 16b and 16c, lambda is the wavelength of a light source 11, LC is the coherent length of the light source 11, L is the coil length of the single-mode optical fiber coil 16a and LSB is the beat length of the single-mode optical fiber coil 16a.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention propagates clockwise light and counterclockwise light in an optical fiber coil, detects the phase difference between these clockwise light and counterclockwise light, and applies them to the optical fiber coil. The present invention relates to an optical fiber gyro that detects an angular velocity around a center.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional optical fiber gyro. The light from the light source 11 passes through an optical branching device 12 such as an optical fiber coupler, and further passes through a polarizer 13 to extract only a component in a predetermined polarization direction. The light from the polarizer 13 is a light such as an optical fiber coupler. The light is split into two by the splitter 14, one light of which is incident on one end of the single-mode optical fiber coil 16 as a clockwise light through the depolarizer (depolarizer) 15, and the other light passes through the optical phase modulator 17. And enters the other end of the optical fiber coil 16 as counterclockwise light.

The right-handed light and the left-handed light propagating through the optical fiber coil 16 return to the optical branching device 14 to be combined and interfere with each other, and the interfering light is extracted by the polarizer 13 only in the component of a predetermined polarization direction. The light that has passed through the polarizer 13 is split by the optical splitter 12 and is incident on the photodetector 18, and is converted into an electrical signal corresponding to the intensity of the light. Modulation signal generator 1
The optical phase modulator 17 is driven by the periodic function from 9, for example, a sine wave signal, and the light passing therethrough is phase-modulated. The output of the photodetector 18 is synchronously detected by the synchronous detection circuit 21 by the reference signal from the modulation signal generator 19, and the detected output is output to the output terminal 22.

When the angular velocity around the axis of the optical fiber coil 16 is not applied, the phase difference between the clockwise light propagating in the optical fiber coil 16 and the counterclockwise light is zero, and the synchronous detection circuit. The output of 21 is also zero, but when an angular velocity around the axis is applied to the optical fiber coil 16, a phase difference occurs between the clockwise light and the counterclockwise light in response to this, and the synchronous detection circuit 21 The output of the polarity and level is generated according to the direction and the magnitude of the applied angular velocity, and the applied angular velocity can be detected.

As described above, the optical fiber gyroscope detects the phase difference between the clockwise light and the counterclockwise light. However, the polarization state changes during propagation through the optical fiber coil 16, and the polarization direction is right angle. When such a component is generated, since the optical fiber coil 16 has a small amount of birefringence, the two lights in the polarization directions orthogonal to each other have different propagation velocities in the optical fiber coil 16 and are thus combined by the optical branching device 14. If one polarization component of the clockwise light and the other polarization component of the counterclockwise light interfere with each other, the phase difference between the clockwise light and the counterclockwise light cannot be correctly detected.

From this point, in the conventional case, the depolarizer 15 is inserted, the intensity of one polarization component is equal to that of the other polarization component orthogonal thereto, and the phase difference is large, so that there is no correlation, that is, The coherent state (non-polarized state) is set so that one polarization component of the clockwise light and the other polarization component of the counterclockwise light do not interfere with each other. As the depolarizer 15, a LYOT type fiber depolarizer is generally used. This depolarizer is formed by connecting two constant polarization optical fibers (polarization maintaining optical fibers, that is, birefringent optical fibers) with their principal axes inclined at 45 °.

There is also one in which a polarization maintaining optical fiber is used as the optical fiber coil 16 and the depolarizer 15 is omitted.

[0008]

Optical fiber coil 16
Since the polarization-maintaining optical fiber is used as the optical fiber coil 16 because the polarization-maintaining optical fiber is expensive,
There was a drawback that the price was about half of the total price of the optical fiber gyro. If a single mode optical fiber is used as the optical fiber coil 16, the price will be low, but since the depolarizer 15 is used, it takes time to manufacture. That is, the depolarizer 15 fusion splices the two polarization-maintaining optical fibers by shifting the main axis by 45 ° as described above, but the required accuracy for the angular deviation of the main axes is extremely high. In addition, SPIE Vol. 412, P.
It is effective to use the Quadrapole winding described in No. 268 to 271 (1983). In the configuration in which the debolizer is arranged at one end of the coil, there is a problem that the effect of disturbance is asymmetrical due to the presence of the debolizer, so that the drift reducing effect cannot be sufficiently obtained.

[0009]

According to the present invention, an optical fiber coil is constructed by connecting polarization maintaining optical fibers to both ends of a single mode optical fiber coil, and the lengths of both polarization maintaining optical fibers are l. ₁ , l ₂ , beat length L _PB , single mode optical fiber coil beat length L
Let _{SB be} L, the coherent length of the light from the light source be L _C , and the wavelength be λ, so that l ₁ −l ₂ > L _PB {(L _C / λ) + (L / _{LS B} )} is satisfied. The length l ₁ of both polarization-maintaining optical fibers,
l ₂ has been selected.

[0010]

FIG. 1 shows an embodiment of the present invention, in which parts corresponding to those in FIG. 2 are designated by the same reference numerals. In the present invention, the optical fiber coil 16 comprises a single mode optical fiber coil 16a and polarization-maintaining optical fibers 16b and 16c connected to both ends thereof. The polarization-maintaining optical fibers 16b and 16c each have a length of l ₁ and l ₂ , each beat length of L _PB , and a single-mode optical fiber coil 16a.
The beat length (in the state wound around the coil) to L _SB, the coil length L, and the wavelength of the light source 11 lambda, when the coherence length and _{L C l 1 -l 2> L} PB {( The lengths l ₁ and l ₂ of the polarization-maintaining optical fibers 16b and 16c are selected so as to satisfy L _C / λ) + (L / L _SB )} (1).

Since a single-mode optical fiber generally has no birefringence, it has no polarization stability and no phase difference between polarizations, so that interference between polarizations occurs. However, when a single-mode optical fiber is wound around a coil, some birefringence occurs, so that the stability of the plane of polarization can be obtained. However, when the single mode optical fiber coil 16a is directly connected to the optical branching device 14 and the optical phase modulator 17, since both ends are not wound in a coil shape, birefringence does not occur. However, in the present invention, the single mode optical fiber coil 16
The polarization-maintaining optical fibers 16b and 16c are connected to both ends of a to ensure the stability of the polarization plane. Moreover, since birefringence occurs in the entire optical fiber coil 16, a phase difference between polarized waves is generated, and interference between polarized waves can be suppressed.

However, two polarization-maintaining optical fibers 16 are used.
Depending on the lengths of b and 16c, two polarization maintaining optical fibers 16b and 16c and a single mode optical fiber 16 may be used.
It is very troublesome because it is necessary to connect the main axis with a and connect them. However, in the present invention, such a trouble does not occur because the lengths of the polarization-maintaining optical fibers 16b and 16c are selected so as to satisfy the expression (1).

That is, the single mode optical fiber coil 1
Phase difference phi _S polarizations occurring 6a is given by 2πL / L _SB. On the other hand, the phase difference caused by the birefringence of the polarization-maintaining optical fibers 16b and 16c is such that the light from one polarization-maintaining optical fiber is incident on the other polarization-maintaining optical fiber with the polarization direction twisted by 90 °. In the worst case, the phase between both polarized waves is the smallest, which is the worst case. At this time, the phase difference φ _P between the polarized waves generated by the polarization maintaining optical fibers 16b and 16c is 2π (l ₁ −l ₂ ) / _LPB .

Therefore, the phase difference generated in the entire optical fiber coil 16, that is, the difference between φ _P and φ _S may be a phase difference that does not interfere in consideration of the coherent length L _C of the light of the used light source 11. In other words, if l ₁ and l ₂ satisfying 2πL _C / λ <φ _P −φ _S (φ _P > φ _S ), that is, satisfying the equation (1),
The single-mode optical fiber coil 16a and the polarization-maintaining optical fibers 16b and 16c can be randomly connected without matching the polarization directions (main axes), and the manufacturing time can be shortened. Polarization maintaining optical fibers 16b, 16
It is preferable that part of c is wound as a coil.
Also, the optical branchers 12 and 14, the polarizer, and the optical phase modulator 17 may be configured by polarization-maintaining optical fibers. Optical splitter 14
May be configured by an optical IC. The present invention can be applied not only to the open loop type optical fiber gyro but also to the closed loop type optical fiber gyro.

[0015]

As described above, according to the present invention, instead of the optical fiber coil 16 using only the polarization maintaining optical fiber, two short polarization maintaining optical fibers are connected to both ends of the single mode optical fiber coil. Moreover, since the formula (1) is satisfied, it is possible to obtain an optical fiber gyro which can be constructed at low cost, is easy to manufacture, and has high accuracy.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional optical fiber gyro.

Claims (1)

[Claims] 1. The light from the light source is distributed by the light branching means, is incident as left-handed light and right-handed light from both ends of the optical fiber coil, and the light that has propagated through the optical fiber coil is In an optical fiber gyro that causes interference by the optical branching means, converts the intensity of the interference light into an electric signal, and detects an angular velocity applied to the optical fiber coil around its center from the electric signal, the optical fiber coil is A polarization-maintaining optical fiber is connected to both ends of a single-mode optical fiber coil, and the lengths of the polarization-maintaining optical fibers at both ends are l ₁ , l ₂ ,
The coherent length L _C of the light from the light source, the beat length of the polarization-maintaining optical fiber L _PB , the beat length of the single mode optical fiber coil L _SB , the length of the single mode optical fiber coil L, and the wavelength When the the _{λ, l 1 -l 2> L} PB {(L C / λ) + (L / L SB)} length of both polarization maintaining optical fibers so as to satisfy l
An optical fiber gyro characterized in that ₁ and ₁₂ are selected.