JPH0464005B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Technical Field The present invention relates to an optical fiber gyroscope for measuring the angular velocity of a moving object.

Location of cars, aircraft, robots, ships, etc.
In a moving object that requires attitude control, it is necessary to continuously calculate its own position and direction. For this reason, it is necessary to constantly measure the rotational angular velocity as well as the speed. A fiber optic gyroscope is used to measure the rotational angular velocity.

(a) Prior art and its problems Fiber optic coils pass light clockwise and counterclockwise through an optical fiber loop and detect the phase difference caused by the rotation of the loop to determine the rotational angular velocity. Since the light beams propagate in opposite directions through the same optical fiber loop, there should be no difference in optical path, but in reality, the axis of the optical system may be misaligned, resulting in a difference in optical path. Therefore, a phase difference occurs even when the loop is stationary. This prevents accurate measurement of angular velocity.

FIG. 5 shows a diagram of the basic optical system configuration of an optical fiber gyro.

As the light emitting element 1, a laser such as a laser diode, a superluminescent diode, or a He--Ne gas laser is used. The light from the light emitting element 1 is split into two beams by a beam splitter 2, and the beams enter the A and B ends of the optical fiber loop 5 through coupling lenses 3 and 4. The optical fiber loop 5 is formed by winding an optical fiber into a coil shape many times. When the optical fiber loop 5 is rotating around the central axis at a rotational angular velocity of Ω, a phase difference Δθ appears in the light propagating in the optical fiber loop 5 in the counterclockwise direction CW and counterclockwise direction in the CCW direction.

Δθ=8πAΩ/λc...(1). A is the total area enclosed by the optical fiber, i.e.
It is the product of the cross-sectional area of the loop and the number of turns. λ is the wavelength of light, and c is the speed of light. This is called the Sagnac effect.

The clockwise rotation propagated through the optical fiber loop 5,
The counterclockwise light is emitted from the B end and the A end, passes through coupling lenses 4 and 3, reaches the beam splitter 2, where it is combined, and the light intensity is detected by the light receiving element 6.

The light receiving element 6 is a photodetector such as a pin photodiode or an avalanche photodiode. Let E ₁ and E ₂ be the amplitudes of the electric fields of the clockwise light and counterclockwise light at the position of the light receiving element 6. If there is a phase difference Δθ, the output I of the light receiving element 6 should be I=E ₁ ² +E ₂ ² +2E ₁ E ₂ cosΔθ (2).

Knowing the phase difference Δθ from the output I of the light receiving element, (1)
Calculate the angular velocity Ω from the formula. This is the measurement principle of the optical fiber gyroscope.

If clockwise light and counterclockwise light travel in strictly the same optical path in opposite directions, no optical path difference will occur.

However, in reality, it is difficult to align the axis of the optical system.
The optical paths of both lights do not necessarily match. When the optical paths differ, an optical path difference occurs. The optical path difference divided by the wavelength and multiplied by (2π) is the phase difference. accordingly
Δθ in equation (2) includes a phase difference based on rotation and a phase difference (offset portion) based on the existence of a static optical path difference.

FIG. 6 is an explanatory diagram showing a case where the optical axes of the coupling lenses 3, 4 and the A and B ends of the optical fiber 5 do not match.

The common optical axis of the light emitting element 1, beam splitter 2, and coupling lenses 3 and 4 is defined as _LX . optical fiber 5
Let F _X be the central axis at the end of . Assume that F _X and L _X are misaligned. The incident light is the optical axis of the lens _L
It is rotationally symmetric around and converged to point e by the lens. Point e is the intersection of the fiber end face and _LX .

The emitted light is emitted from the intersection g between the central axis F _X of the optical fiber and the end face of the fiber.

Point g and point e are different. The light emitted from point g is converted into parallel light by the coupling lens, but this is _L
is not parallel to

Therefore, between the lens and the beam splitter, the optical paths of the incident light and the outgoing light are different, resulting in an optical path difference. Therefore, even if Ω=0, a phase difference Δθ (offset) occurs.

Unless the phase difference corresponding to the offset is compensated for, the rotational angular velocity Ω of the moving object cannot be accurately determined from the output I of the light receiving element.

In order to set the offset to 0, it is sufficient to be able to continuously change the effective optical path length of the clockwise light or the counterclockwise light.

However, since light in both directions passes through the same optical system,
It is not possible to compensate the phase of only one light.

Furthermore, since cosΔθ is an even function in equation (2), only the range of 0<Δθ<π can be detected. Therefore, the detection limit of the rotational angular velocity Ω in equation (1) is limited.

(C) Technical problem of the present invention It is only necessary to compensate the phase of either clockwise light or counterclockwise light.

Clockwise and counterclockwise light propagates simultaneously in all optical systems. The beams travel in the same direction between the light emitting element 1 and the beam splitter 2, and between the beam splitter 2 and the light receiving element 6. Between the beam splitter 2, the coupling lenses 3 and 4, and the optical fiber loop 5,
proceed in the opposite direction.

The lights traveling in opposite directions are not emitted from the light emitting element 1 at the same time. Regarding the intermediate point in the optical fiber loop, the light emitted from the light emitting element 1 at the same time passes through the intermediate point at the same time.
However, at other passing points, clockwise light
The time of exit from the light emitting element 1 for CW and counterclockwise light CCW is different.

Therefore, if a phase compensation element is inserted at a point other than the midpoint of the fiber loop, and this is pulse-driven, and the light-emitting element 1 is also pulse-driven in synchronization with this, it is possible to generate either CW light or CCW light. Only one side can be phase compensated. For example, it is possible to drive the phase compensation element when the CW light passes through, and not drive the phase compensation element when the CCW light passes through.

If the length of the optical fiber loop is L, then the difference in time between the CW and CCW lights passing through the ends A and B of the optical fiber and emitted from the light emitting element 1 is τ=
It is given in nL/c. c is the speed of light and n is the refractive index of the fiber core. If L is about 10 ³ m,
τ is about 10 ⁻⁵ sec. At this speed, it is easy to internally or externally modulate the light from the light emitting element 1. There is no problem in driving the phase compensation element.

τ is called the fiber loop transit time.

(d) Optical Fiber Wear According to the Invention The optical fiber iron according to the present invention newly includes a phase compensation element at either end of the optical fiber loop. The light to be incident on the fiber is internally or externally modulated to produce repetitive pulsed light with a pulse width shorter than the fiber loop transit time τ and a duty of 1/2 or less. The phase compensation element is synchronized with this and is pulse driven with the same repetition rate, and is pulsed with CW light or
Compensates the phase of only CCW light. Further, the amount of phase compensation of the phase compensation element is changed so that the output intensity of the light receiving element is always constant.

The optical fiber coil of the present invention includes: (1) a light emitting element that generates repetitive pulsed light with a pulse width shorter than the fiber loop transit time τ and a duty of 1/2 or less through external modulation or internal modulation, and (2) from the light emitting element. a beam splitter that splits the light beam into two; (3) an optical fiber loop in which the optical fiber is wound many times; (4) a coupling lens that makes the split light enter the end of the fiber loop; 5) a phase compensation element provided at one end of the optical fiber loop; and (6) a drive that drives the phase compensation element only while clockwise light or counterclockwise light passes through it in synchronization with the modulation of the light emitting element. (7) a phase compensation control circuit that controls the phase compensation amount of the phase compensation element so that the output intensity of the light receiving element is always constant.

(e) Embodiment An optical fiber coil according to an embodiment of the present invention will be explained with reference to FIG.

The light from the light emitting element 1 is split into two by a beam splitter 2, and is incident on the end of an optical fiber loop 5 through coupling lenses 3 and 4. A phase compensation element 7 is newly provided at one end of the optical fiber loop 5. The pulse generator 8 includes a voltage control circuit 1 that controls the amount of phase compensation of the light emitting element 1 and the phase compensation element 7.
Drive 1.

FIG. 2a is a diagram of a light emission pulse waveform when the light emitting element is internally modulated. The horizontal axis shows time, and the vertical axis shows the power of light.

The light emission time is P ₁ , the pause time is P ₂ , and (P ₁ +P ₂ ) is one cycle. Duty P ₁ / (P ₁ + P ₂ ) is 0.5
It is as follows. Also, P ₁ <τ.

FIG. 2b is a waveform diagram showing the driving state of the phase compensation element. _P3 is the drive period, and time measurement starts in synchronization with the moment of pulse emission, and _P3 is slightly longer than _P1 .
P ₄ is the rest time. One period (P ₃ +P ₄ ) corresponds to one period (P ₁ +P ₂ ) of light emission modulation.

In this example, since the phase compensation element 7 is located at the entrance of the CCW light, all the CCW light undergoes phase compensation.

However, since the CW light reaches the phase compensation element τ time after entering the optical fiber loop, it does not undergo phase compensation.

During the rest period _P4 of the phase compensation element 7, the CW light is made to pass through it.

This compensated the phase of the light (here CCW) before entering the optical fiber loop. However, conversely, the phase of the light emitted from the optical fiber loop can also be compensated.

In this case, the phase compensation element 7 is driven from the time τ has elapsed since the start of light emission. FIG. 2c is a drive waveform diagram of the phase compensation element 7 in such a case.

The offset phase difference due to the presence of optical path difference is expressed as Δθ′,
Assuming that the phase compensation amounts of the CW light and the CCW light are φ ₁ and φ ₂ , the phase may be compensated so that Δθ′+φ ₁ −φ ₂ =2mπ (3) (m is an integer). Figure 2 b is φ ₁
=0, c is an example of φ ₂ =0.

Furthermore, when the fiber loop is rotating, if there is a phase offset Δθ' and a phase compensation amount φ by the phase compensation element 7, equation (1) becomes I=E ₁ ² +E ₂ ² +2E ₁ E ₂ cos(Δθ+Δθ ′−φ) ………(4). Since Δθ' is a constant, I can always be kept constant by changing φ as Δθ changes. That is, the amount of phase compensation of the phase compensation element 7 is controlled by the voltage applied to the phase compensation element 7 from the voltage control circuit 11 so that the output of the light receiving element 6 becomes a constant value in FIG. If this voltage is used to detect the rotational angular velocity, the detection range of Δθ becomes infinite in principle.

(F) Phase Compensation Element FIG. 3 is a perspective view of a phase compensation element using a piezoelectric element.

An optical fiber 10 is wound around the outer periphery of a cylindrical piezoelectric element 9. A pulsed voltage is repeatedly applied to the piezoelectric element 9 by the drive circuit 8 . Since the piezoelectric element expands and contracts in proportion to the voltage, the optical fiber expands and contracts, and the optical path changes.

FIG. 4 is a perspective view of a phase compensation element using an electro-optic crystal. Optical fiber 1 to electro-optic crystal 12
0 edges are glued, electrodes are attached to the crystal planes, and a voltage is repeatedly applied in the form of pulses by the drive circuit 8.

The piezoelectric element is, for example, PZT (Pb(ZrxTi ₁ −x)
O ₃ ), is used. Electro-optic crystal is BSO
(Bi ₁₂ SiO ₂₀ ) single crystal or NiNbO ₃ crystal can be used.

(g) Effect When the light emitting element 1 is a loser diode or superluminescent diode, it is internally modulated, and when it is a gas laser, it is externally modulated using a modulator.
Create pulsed light as shown in Figure 2a.

The optical path difference offset Δθ' is measured in advance, and the voltage to be applied to the phase compensation element to compensate for this is determined. The phase compensation element is driven in synchronization with the pulsed light according to the pulse waveform shown in FIG. 2b or c.

The phase of CW light or CCW light is compensated, and Ω=
When it is 0, offset adjustment is performed so that the phase difference Δθ becomes 0.

Although the output I of the light-receiving element 6 is a pulse-like output, Δθ can be obtained from equation (2) and the angular velocity Ω can be calculated.

In addition, by controlling the pulse voltage that drives the phase compensation element 7 to cancel out the phase difference Δθ caused by the Sagnac effect and keeping the output of the light receiving element constant, the rotational angular velocity can be detected from the necessary pulse voltage. .

(h) Effect It is possible to compensate for the random phase difference Δθ′ caused by the optical path difference between the CW light and the CCW light. This makes it possible to accurately measure the angular velocity Ω.

Further, by adopting a method of detecting angular velocity from the voltage applied to the phase compensation element, the detection range of angular velocity can be infinite.

(k) Applications Used to measure and calculate the rotation and direction of moving objects such as automobiles, aircraft, robots, and ships.

[Brief explanation of drawings]

FIG. 1 is an optical system configuration diagram of an optical fiber gyro according to an embodiment of the present invention. Figure 2 shows the driving waveforms of the light emission and phase compensation elements, where a is the optical output of the light emission modulation element, and b
is the drive waveform of the phase compensation element, and c is also the drive waveform of the phase compensation element. The phase of b is compensated first, and the phase of c is compensated later. FIG. 3 is a perspective view of a phase compensation element using a piezoelectric element. FIG. 4 is a perspective view of a phase compensation element using an electro-optic crystal. FIG. 5 is an optical system configuration diagram showing the principle of an optical fiber gyro. FIG. 6 is an explanatory diagram for explaining poor coupling between the coupling lens and the end of the optical fiber. 1...Light emitting element, 2...Beam splitter,
3, 4... Coupling lens, 5... Optical fiber loop, 6... Light receiving element, 7... Phase compensation element, 8...
...Pulse generation circuit, 9...Piezoelectric element, 10...Optical fiber, 11...Voltage control circuit, 12...Electro-optic crystal, CW...Clockwise light, CCW...Counterclockwise light.

Claims (1)

[Claims] 1. A light emitting element that generates repetitive pulsed light with a pulse width shorter than the fiber loop transit time τ and a duty of 1/2 or less through external modulation or internal modulation;
A beam splitter 2 that splits the light source from the light emitting element into two, an optical fiber loop 5 in which the optical fiber is wound many times, and coupling lenses 3 and 4 that make the split light enter the end of the fiber loop. , a phase compensation element 7 provided at one end of the optical fiber loop 5, and a phase compensation element 7 provided in synchronization with the modulation of the light emitting element.
A drive circuit that drives the optical fiber only while either clockwise light or counterclockwise light is passing through the optical fiber loop, and a light receiver that combines the light that has propagated clockwise and counterclockwise through the optical fiber loop and detects the intensity of the interference light. and a phase compensation control circuit that controls the amount of phase compensation of the phase compensation element so that the output intensity of the light receiving element is constant,
An optical fiber gyro characterized in that a phase change due to rotation is canceled out by a phase compensation element, and a rotational angular velocity is detected based on the compensation amount of the phase compensation element. 2. The phase compensation element 7 is formed by winding an optical fiber 10 around a cylindrical piezoelectric element 9, and applies a pulse voltage to the piezoelectric element 9 to change the optical path. Optical fiber gyroscope described in section. 3. The phase compensation element 7 is configured to change the refractive index by bonding the end of the optical fiber 10 to the electro-optic crystal 12 and applying a pulse voltage to the electro-optic crystal 12. The optical fiber gyro described.