JPS59128409A - Optical fiber gyroscope - Google Patents

Abstract

PURPOSE:To decrease the number of parts and to ensure easy adjustment and the like, by using lenses of one axis crystal as an optical coupler in an optical fiber gyroscope. CONSTITUTION:An optical coupler in an optical fiber gyroscope is formed by spherical lenses L3 and L4 made of one axis crystal such as sapphire and a half mirror HF. Propagated light from an optical fiber F1 is reflected by the mirror HF through the lens L3 and becomes counterclockwise light toward an optical fiber coil through the lens L3 and a fiber F3, and also becomes clockwise light toward the lens L4, an optical fiber F4, and the optical fiber coil through the mirror HF. The phase difference between both light beams is adjusted by the rotary angles of the planes of polarization of the fibers F1 and F2 based on respective twistings. The phase difference is made to be pi/2 without using an optical modulator, a polarizing plate, and depolarizer. When an interval F' between the lens and the fiber and an interval L between the lens and the mirror are changed, the coupling efficiency of four optical fibers becomes the maximum value. In this way, the number of parts can be decreased, and the easy adjustment can be ensured.

Description

[Detailed description of the invention] Technical field of invention The present invention is applicable to aircraft, space vehicles, etc.

This relates to an optical fiber gyroscope used to determine posture.

Prior Art and Problems As a conventional gyroscope, an optical fiber gyroscope utilizing the Sagnac effect as shown in FIG. 1 has been proposed.

That is, - the laser light from the laser diode LD is incident on the polarizing plate POL via the beam splitter BS, where only light having a specific polarization plane is emitted and is incident on the optical fiber directional coupler DCO.

The optical fiber directional coupler DCO divides the incident laser beam into 3 dB units and outputs the divided beams to two points a and b. The laser beam emitted to point a is returned to a non-polarized state by the depolarizer DPOL, and is sent to the optical fiber coil FC at the end face FL.
'The incident occurs from '. On the other hand, the emitted light is modulated by the optical modulator PM, phased by π/4, and enters the optical fiber loop FL.

The laser beams traveling in opposite directions through the optical fiber coil FC enter the optical fiber directional coupler DLO again, are combined there, and enter the polarizing plate POL.

Here, only the laser light with a specific polarization plane is emitted again, enters the beam splinter BS, is reflected there, and enters the photodetector DET.

The electrical signal from the photodetector DET is sent to the lock-in amplifier LA.
Here, the gate is opened and amplification is performed only when the light from the optical fiber coil FC returns.

This improves the S/N ratio.

Here, the phase of the laser light traveling clockwise and the laser light traveling counterclockwise through the optical fiber coil FC is shifted by the Sagnac effect, and cos(wt+θ) and cos
It is expressed as (wt-θ).

These two laser beams are combined, but the five combined laser beams are.

cos(wt+θ)+cos(wt-θ)=2sin
wt-cosθ.

Therefore, by detecting the value of cos θ, the rotational angular velocity can be detected.

However, when the value of cos θ is used, the sign is the same around "0" regardless of the direction of one rotation, and there is a drawback that the direction of one rotation cannot be determined.

Therefore, the phase of the light traveling in the counterclockwise direction is shifted by π by the optical modulator PM.

The destination moving counterclockwise from this point is cos(wt-θ+
π) and the combined light is cos(wt+θ)+c
os(wt-θ+π)=2CO3wt-sinθ.

Since the sign of the value represented by sin θ is reversed around "0", the direction of rotation can be detected.

Furthermore, since the degree of coupling in the optical fiber directional coupler largely depends on the plane of polarization, it is necessary to use a polarizing plate POL to obtain the best degree of coupling.

Furthermore, the plane of polarization of the light returned from the optical fiber coil FC may be rotated by the earth's magnetism, etc., and the optical fiber directional coupler may not be able to obtain the best degree of coupling. I have to.

In this way, the conventional fiber optic gyroscope.

A polarizer, depolarizer, and optical modulator are required.

It had the disadvantage of having a large number of parts.

Purpose and Structure of the Invention The present invention aims to eliminate such drawbacks, and the purpose is to provide a laser diode, a photodetector, and a first and second lens arranged on both sides of a half mirror. and the first
．． having a coiled optical fiber disposed so as to face either end of the second lens, the light from the laser diode being incident on the half mirror via the first lens;
The light reflected by the half mirror is passed through the first lens.

From one end of the coiled optical fiber, the light that has passed through the half mirror enters the other end of the coiled optical fiber via the second lens, and propagates through the coiled optical fiber in opposite directions, In an optical fiber gyroscope, the light emitted from the coiled optical fiber is synthesized by the half mirror and then enters the photodetector, and the rotational angular velocity is detected from the output of the photodetector. An optical fiber gyroscope characterized by using a lens made using a crystal of This is achieved by a fiber optic gyroscope, which is characterized by fine adjustment of the plane of polarization.

Examples of the invention The present invention will be described in detail below based on examples.

FIG. 2 shows an embodiment of the invention, and FIG. 3 shows an optical fiber directional coupler used in the invention.

In the figure, PI and P2 are polarizers, jc1' and PO2 are polarization plane control units, and Pdl and Pd2 are polarization separation prisms.

DETI, DET2 are photodetectors, L, , L,
is a ball lens, DCOI is a fiber optic directional coupler, FC
1 is an optical fiber coil, HF is a half mirror, and the same members as in FIG. 1 are given the same symbols.

In the present invention, lenses I4 and L in the optical fiber directional coupler DCOI are used to shift the phase of the light traveling in two counterclockwise directions CCW by "π" without using an optical modulator.
I+ was made from a uniaxial crystal such as sapphire.

In addition, in order to finely adjust this amount of phase shift,
I, PC, 2 are provided, and the plane of polarization is rotated here.

Specifically, the polarization plane control units PCI and PC2 will be explained.

In Fig. 2, four single mode fibers are connected to two ball lenses LJ as an optical fiber directional coupler DC01.
, L4, and the lens L3.

A configuration in which a half mirror HF is inserted between Lφ is used.

And the single polarized light from the laser diode LD (single mode) is transferred to the optical fiber directional coupler DCO half-miff HF.
The incident light is split into nine reflected lights and transmitted lights. The reflected light passes through the L ball lenses again and passes through the optical fiber coil PC.
to terminal F3 of I.

The transmitted light passes through the ball lens LQ and the optical fiber coil FCI.
are incident on terminals F, respectively.

Rotate the optical fiber coil PCI clockwise CW and counterclockwise C.
The light propagated to the CW is connected to the optical fiber coil FC1, respectively.
They each receive a phase change ±φ proportional to the rotational angular velocity Ω. Here, the phase change φ is expressed by equation (1) using Sagnac's equation (3agnaC), where R is the coil radius -a is the fiber length, and C is the speed of light in vacuum.

±φ−±I2πRβ・Ω/(λc) l −−−−
--'+1) Here, λ is the wavelength of a light wave in a vacuum. + and - in the equation above correspond to the phase shift of counterclockwise (CCW) light and clockwise (CW) light, respectively.

These two lights are combined again at the half mirror HF,
Thereafter, it is branched into two as described above.

Then, one side returns to the optical fiber F1 and enters the photodetector DETI via the beam splinter BS.

The other one enters the optical fiber Fz and is guided to the photodetector DETl.

Here, in the optical fiber directional coupler, optical phase noise is generated when the first reflected light leaks in or when the returned light is coupled back into the optical fiber.

For this reason, the optical axis LBA and mechanical axis MA are separated.

The configuration is such that they are tilted to each other.

Here, the optical fiber end face is relative to the mechanical axis.

It has an inclination angle θp.

In this optical fiber directional coupler DCOI, the optical coupling efficiency between the four fibers can be adjusted to the best value by changing the distance F' between the lens and the fiber and the distance between the lens and the half mirror. for example.

If the structural parameters of the four fibers are the same.

The best coupling efficiency is obtained when the following equations (11, (2)) are satisfied, and adjustments can be made to satisfy these conditions.

θ0 = θz −(L−F) /h/F, --
---121x=-ri, -
・−−１−−−・(3) Here, F is the focal length of the lens,
θ is the exit angle, θλ is the incidence angle 2, and h is the half value of the distance between the fiber core centers.

The light waves that propagate bidirectionally within the optical fiber directional coupler DCO are almost limited to those that satisfy the conditions of (2) and (31), so the optical path within the DCO and the light wave incident position on the half mirror are as follows: It becomes constant, and optical path deviation between bidirectional light waves is less likely to occur.

In the plan view, SUB is a substrate and DM is a dielectric multilayer film.

Now, regarding the fiber gyro in Figure 2.

The conditions under which a π/2 optical phase bias is applied between ccw and cw light waves and the amount of optical phase noise at that time will be clarified.

Assume a linearly polarized light wave as a light source.

In this case, the electric field uses the complex vector representation of light.
−−−−−−・−−−−−−−−−−−(41 here.

Ox: amplitude of light W: angular velocity of light wave ψX: phase of light wave By the way, the optical fiber directional coupler DC shown in Fig. 3
O is made of uniaxial crystals such as sapphire,
When a lens is used, a phase shift 2θK of light waves occurs between the ordinary light and the extraordinary light. If the angle between the axis of the incident light wave and the crystal axis is α-, then the light wave passing through the lens undergoes a phase shift change (T) expressed by the following equation.

Da (Tr) − −−−−−−−−・−・−−−−−−−−−− The value.

K=2 is the value between F4 and 10M for lens L4.

And =3 indicates the value between F3 and HF for lens L3.

By the way, 1.In a normal single-mode fiber coil, the plane of polarization of a light wave not only rotates as it propagates, but also becomes elliptically polarized, but by twisting the fiber,
It can be returned to linearly polarized light.

Considering this, the change in the light wave after propagation in the same coil (T
) is expressed by a linear equation using the rotation α of the plane of polarization F
You can F.

−・・−−m−−−−・・−・・−−−+61 Furthermore, (
Equation 6) becomes α 5O in the case of a polarization maintaining fiber. But from fiber coil FC.

If the light input/output end (F3 or F4 in FIG. 2) of the lens L3 or L4 is rotated by α, equation (6) will hold true.

First, consider the case without polarizer pH (see Figure 2). The light wave from the fiber coil FC is
The light enters the photodetector DETI via the half mirror HF. CCW light and CW light waves are transmitted.

+E = CW x −−−−・・−・・−・・−−−・−−−−−(71E
= W x −−−−−−−−−−−−−−−−−−−−−(8) o
x ox −・・−・−・・−・−・−−−−−−−− The rotation angle of the plane of polarization caused by rotation or twisting was defined as β.

Also, from equation (5), (T)=(T,) −1・−m−−・−(10
) ni = 1. 2. 3, and the electric field obtained by combining the CCW light and the CW light wave becomes (11).

This E can be expressed by a simple quadratic equation using equations (1) and equations (4) to (11).

! ! However, f and fλ in the above equation are given by --------- (13). Furthermore, from equation (13), it can be shown that the following equation holds true.

* Every −fl, −−−−−−−−−−
1・(14)* f3−− f2−−−−−−−−−−−−−(15) Electrical output I obtained when this E enters the photodetector DETI
, when the X-polarized light wave is taken out, using equation (12), = - (lf, cos β + f, sin β11 fl
It is given by cosβ−f2sin12.

Here, +f/+f are each a function of α.

Its complex amplitude varies with α. Similarly, even if β is changed, (f7cosβ+fzs inβ) in equation (16)
The complex amplitude of each element can be changed. For this reason, the phase difference component ΔψX can be adjusted by adjusting α and β.

From equation (12), the X polarization direction is −arg(f, cosβ−
f) sin β) π. This is the optimal optical phase bias condition.

Furthermore, from the equation (12>), the electric output It'/ obtained at detl by the light wave polarized in the y direction can also be expressed using the same equation as the equation (16).

Furthermore, similarly to equation (17), the phase difference component Δψy can be optimally adjusted by adjusting α and β for the light wave in the X polarization direction.

△ψy=a r g (f, sinβ-f)c
o sβ)-arg(f7sinβ+f2c o sβ
)~π −・−−−−−−−−−−(18) The absolute values of each term are equal. This holds true in exactly the same way in the case of IIy. Therefore, the following relational expressions hold true for the fiber/gyro/sensing outputs I, x, and ItV, respectively.

1/X ot ± 5in2φ ・----(1
9) Or.

1, y cx +5in2φ -----(20'
) Here, as can be seen from equations (17) and (18), insertion is not possible.

Instead, by inserting a polarization separation prism Pd immediately before the photodetector]) ETI, I/X, It"l
can be extracted as detection outputs.

By using 1, x and l, y simultaneously, common mode noise can be removed. Further, leakage from imperfections in the polarization separation prism PtL causes a decrease in electrical output, but does not cause an increase in 5-phase noise.

Furthermore, when using a simpler optical system, it is also possible to insert a polarizer P2 and take out either I, x or It''l.

It is reported that in this case, the optical phase noise of a fiber gyro having a general optical system can be minimized. However, as is clear from equations (16) to (20), the optical phase noise of the fiber gyro proposed here can theoretically be zero.

These matters are related to the output light wave of the photodetector DETI, but the output of the photodetector DET2 and the output of the photodetector DET2 are
You can find 12. In this case as well, almost the same tendency is obtained for Ik x, h3'.

However, optical phase noise does not become zero.

Next, FIG. 4 shows the results of an experiment using the optical fiber gyro shown in FIG. 2. In the configuration shown in Figure 2, insert the polarizer P1 and read the voltage of the photodetector DETI with a voltmeter
The results recorded with a recorder are shown in FIG. 4A. In this case, polarizer P2 is removed. Further, in FIG. 2, the output voltage waveforms of Pd (X, I and Iy) are shown in FIG. 4B.

In this case, the experiment was carried out using polarizer P2.

In the figure, the horizontal axis shows time T, the vertical axis shows output voltage V,
CW is when clockwise rotation is detected.

CCW indicates the case where counterclockwise rotation is detected.

From the above, it is clear that equations (11) and (20) hold almost simultaneously. At this time.

By making adjustments such as twisting the fibers in PCI and PC2 in Figure 2, β and α were changed, and the phase bias of the light wave was able to be adjusted to the optimum value of ±π/2.

Therefore, it has become clear that equations (17) and (18) hold simultaneously. Generally, it is thought that a fiber gyro can be realized if either equation (19) or equation (20) holds, so this result is considered to be sufficient.

Next, FIG. 6 shows the measurement results obtained by pulse-modulating the laser diode LD and performing synchronous detection using the optical system shown in FIG. 2 and the circuit shown in FIG. 5A.

It is shown in FIGS. 7 and 8.

In FIG. 5A, FGS is shown in FIG. 2, an optical fiber gyro system, PC a pulse generator, and LA
is a lock-in amplifier, and SW is a switch.

In the figure, the pulse generator PG outputs the clock shown in FIG. 5B (a), and the laser diode LD
The clock shown in (bl) is input to the lock-in amplifier LA, and the clock shown in (C) is input to the lock-in amplifier LA.

The period τ of the clock is defined as τ−β·n/c.

Here β is the length of the fiber coil.

n is the refractive index of the fiber core C is the speed of light in vacuum It is.

What this τ means is that the laser light from the laser diode passes through the fiber coil and is detected by the photodetector DETI or D
This is the time until the light is guided to ET2, and by keeping the gate of the lock-in amplifier closed until the light enters the photodetector, the noise component included in the output 0ut3 of the lock-in amplifier LA is reduced. .

Next, the measurement results will be explained. FIG. 6 shows the results when the optical fiber gyro was placed on a rotary table and a rotational angular velocity of ±500 degrees/hour was applied.

In the figure, ST is the output when rotation is stopped, and RO
is the output when rotating.

FIG. 7 is a diagram showing the linearity of the sensitivity of the optical fiber gyro, where the horizontal axis shows the rotational angular velocity (degrees/hour) and the vertical axis shows the output voltage. Further, the right side from the Y axis shows the case of clockwise rotation CW, and the left side shows the case of counterclockwise rotation CCW.

As is clear from the figure, it is symmetrical with respect to the rotation n in both directions and exhibits more linearity.

FIG. 8 shows the measurement results when the range of one rotation angle is further widened, and it is possible to measure from 5 degrees/hour to 2000 degrees/hour, and the linearity is also good.

Incidentally, FIG. 8A shows the case of clockwise rotation CW, and FIG. 8B shows the case of counterclockwise rotation.

In the above experiment, a semiconductor laser (VSB, 1.3 μm wavelength) was used, and an optical fiber coil with j2=460 m and R=7 cm was used. Here, the experimental results shown in FIGS. 6 to 8 are based on stabilization of the laser diode LD and temperature control 1. This was obtained without using high frequency superposition, optical isolators, PL, P2, Pdl, PD2 polarizers, etc. Furthermore, we also conducted an experiment using a 0.78 μm band BC3-LD as a light source, and confirmed that similar results could be obtained.

Effects of the Invention As described above, in the configuration of the present invention, π
/2 optical phase bias can be applied between the left and right optical waves, simplifying the optical and electrical systems.

Furthermore, since this optical phase difference bias can be easily optimized and adjusted, ultra-high precision components are not required in the optical system. Furthermore, according to theory, optical phase noise can be minimized and the difference in optical path length between optical waves in both directions, which causes drift, can be reduced.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a conventional optical fiber gyro. 2 is a diagram showing an optical fiber gyro according to the present invention, FIG. 3 is a diagram showing an optical fiber directional coupler used in the present invention, FIGS. 4A and 4B are diagrams showing the effects of the gyro according to the present invention, Figures 5A and B are diagrams showing an experimental device using an optical fiber gyro according to the present invention and its time chart, Figure 6 is a diagram showing an example of detection of rotational angular velocity, and Figure 7 is a diagram showing linearity and symmetry. The diagrams shown in FIGS. 8A and 8B are diagrams showing the experimental results of the measurement range. In the figure, LD is a laser diode, DCO is an optical fiber directional coupler, and L3. Lφ spherical lens, HF is a half mirror, DETI, DET2 are photodetectors. P d/ , P dλ are polarization separation prisms, F/+F2
is an optical fiber, FCL is an optical fiber coil, PG is a pulse generator, and LA is a lock-in amplifier. Figure 5 B Figure 6

Claims (4)

[Claims] (1) A laser diode, a photodetector, and a first laser diode placed on both sides of the half mirror. It has a coiled optical fiber arranged so that both ends thereof face the second lens,
Light from the laser diode is incident on the half mirror through the first lens, and the light reflected by the half mirror is transmitted from one end of the coiled optical fiber through the first lens to the half mirror. The light is incident from the other end of the coiled optical fiber through the second lens, propagates through the coiled optical fiber in opposite directions, and the light emitted from the coiled optical fiber is In an optical fiber gyroscope in which the optical fibers are synthesized by a half mirror and then incident on the photodetector, and the rotational angular velocity is detected by the output of the photodetector, a lens made using a uniaxial crystal is used as the lens. An optical fiber gyroscope characterized by (2) A laser diode, a photodetector, and a first laser diode placed on both sides of the half mirror. The second lens has a coiled optical fiber arranged so that both ends thereof face each other, and the light from the laser diode is directed to the half mirror through the first lens. The light that is incident and reflected by the half mirror is passed through the first lens, and the light that has passed through the half mirror is sent to the coiled optical fiber from one end of the coiled optical fiber through the second lens. enters the coiled optical fiber from the other end, propagates in opposite directions through the coiled optical fiber, combines the lights emitted from the coiled optical fiber by the half mirror, and then enters the photodetector;
In an optical fiber gyroscope that detects the rotational angular velocity by the output of the photodetector, a lens made using a uniaxial crystal is used as the lens, and a part of the coiled optical fiber is further configured to control the plane of polarization. An optical fiber gyroscope characterized by having a section for fine adjustment of the plane of polarization. (3) A laser diode, a photodetector, and a half mirror were placed on both sides. 1st. a second lens; and the first lens.
A coiled optical fiber is arranged such that both ends thereof face the second lens, and the light from the laser diode is incident on the half mirror via the first lens, and the half mirror is connected to the second lens. The reflected light is passed through the first lens from one end of the coiled optical fiber to the half mirror.
The light that has passed through the coiled optical fiber enters the other end of the coiled optical fiber through the second lens, propagates through the coiled optical fiber in opposite directions, and the light emitted from the coiled optical fiber is transmitted to the half mirror. - In an optical fiber gyroscope, the optical fiber gyroscope is configured such that the optical fiber is synthesized by the photodetector, and then the rotational angular velocity is detected by the output of the photodetector, using a lens made using a uniaxial crystal as the lens, An optical fiber gyroscope further comprising: a polarization plane control section for controlling a polarization plane between the laser diode and the first lens to finely adjust the polarization plane. (4) A laser diode, a photodetector, and a first laser diode placed on both sides of the eighth mirror. the second lens and the first and second lenses;
It has a coiled optical fiber arranged so that both ends face each of the lenses, the light from the laser diode is incident on the half mirror via the first lens, and the light reflected by the half mirror is reflected by the half mirror. The light that has passed through the half mirror enters from one end of the coiled optical fiber through the first lens, and enters from the other end of the coiled optical fiber through the second lens. An optical fiber in which the fibers are propagated in opposite directions, the light emitted from the coiled optical fiber is combined by the half mirror, and then enters the photodetector, and the rotational angular velocity is detected by the output of the photodetector. In the gyroscope, a lens made using a uniaxial crystal is used as the lens, and a polarization plane control section is provided between the laser diode and the first lens and in the coiled optical fiber to finely adjust the polarization plane. An optical fiber gyroscope characterized by the following features: