JP2582116B2 - Hikaribaiyairo - Google Patents

Description

Description: (a) Technical field The present invention relates to an optical fiber gyroscope for measuring a rotational angular velocity of a moving body such as an aircraft, an automobile, a ship, and the like.

The rotational angular velocity is conventionally measured by a mechanical gyro. The optical fiber gyro propagates light clockwise and counterclockwise in the optical fiber coil, and obtains a rotational angular velocity from the phase difference.

For this reason, the optical fiber gyroscope has the advantage that it has no moving parts, can be miniaturized, and can be operated in any environment.

(A) Prior art The principle of an optical fiber gyro will be described with reference to FIG.

The light source Q emits monochromatic light. This light enters a single mode optical fiber F having a coupler K and a coil C. There is light passing through the fiber coil C clockwise from the cutler K through the point d, and light passing through the fiber coil C counterclockwise through the point e.

Both the right-handed light and the left-handed light are combined by the cut-off plate K and enter the light receiving element S. When the fiber coil C is stationary,
The light enters the light receiving element S with the same phase. This is because they propagate in the same optical path at the same time.

However, when the fiber coil C is rotating at the angular velocity Ω, there is a difference between the optical path lengths of the left-handed light and the right-handed light. For this reason, the phases of the two lights that interfered are shifted.

Assuming that the phase difference is Δθ, this is proportional to the angular velocity Ω of the fiber coil. The proportional relationship is Given by L is the total fiber length of the fiber coil, a
Is the radius of the fiber coil, c is the speed of light in vacuum, and λ is the wavelength of light in vacuum.

However, the optical paths of right-handed light and left-handed light are not actually the same. For this reason, a phase difference Δθ is generated even at rest (Ω = 0). This is an offset.

Even if there is an offset, this can be compensated if it is a constant value. However, the phase difference Δθ is unstable,
Easy to fluctuate. Drift of the output of the light receiving element is remarkable, and precise measurement cannot be performed.

Thus, the phase difference Δθ between left-handed light and right-handed light is Ω = 0.
It also exists at the time.

This is due to the fact that the optical paths of the two round lights are not the same due to a difference in mode, a difference in polarization, a difference in transmittance of the coupler, a difference in coupling rate, and the like.

Although a single mode fiber is used, there are always two polarization directions in single mode. This is not independent and the plane of polarization may rotate.

If the polarization plane is different, the transmission coupling in the coupler is also different.
For this reason, the optical paths cannot be exactly the same.

Therefore, an optical fiber gyro as shown in FIG. 2 has been proposed.

Light source Q, light receiving element S and both ends d and e of fiber coil C
Is not directly connected with a single coupler.

Light source Q, connecting the light receiving element S in the first Katsupura K _1, the other end h of Katsupura K _1, of the i, i is the end with this.
In the case of h, the mode filter M and the polarizer P are connected.

Ends of Fuaibakoiru C are connected by second Katsupura K _2. This is the end of j of the other ends g and j. g is connected to the polarizer P.

These optical fibers are not simply single-mode optical fibers, but use polarization-maintaining single-mode optical fibers.

The polarization-maintaining optical fiber is one in which an elliptical stress member is inserted or a stress member is inserted in the diameter direction in the clad cross section. This makes the polarization in the x and y directions independent. The polarization in the x direction is preserved. The polarization in the y direction is also preserved. For this reason, the polarization plane is preserved.

If the plane of polarization is intermediate instead of the x and y directions, the plane of polarization is not preserved even if it is linearly polarized light.

Therefore, linearly polarized light must be incident on the fiber such that the plane of polarization coincides with the principal axes of polarization x and y.

For this purpose, a polarizer P is interposed. Thus, the light from the light source Q is converted into linearly polarized light whose polarization plane coincides with the principal axes of polarization x and y.

Then, the light is split into _two by the coupler K2 that has exited the polarizer P and passes through the fiber coil C clockwise and counterclockwise. Since this is a polarization-maintaining optical fiber, the polarization plane is kept constant. Then return to point g.

Accordingly, there is no room for a difference in the optical path to propagate in the fiber coil C clockwise and counterclockwise.

The mode filter M is obtained by winding a polarization-maintaining optical fiber around a cylinder. By bending, the mode in which the plane of polarization is in the bending direction becomes a radiation mode. This will dissipate. As a result, only a mode having a plane of polarization in a direction parallel to the main axis of the cylinder can be transmitted. It is called a mode filter.

The light emitted from the light source Q may be a monochromatic light. Circularly polarized light,
Linearly polarized light or elliptically polarized light may be used. When the light passes through the mode filter M, it becomes light in a mode having a plane of polarization in a certain direction. Further, the light is converted into light polarized in the intrinsic polarization direction of the fiber end g through the polarizer P. After the coupling K ₂ , both ends d and e of the fiber coil C are entered. This propagates as right-handed light and left-handed light in the fiber coil C while maintaining the polarization plane.

Coalesce both around light and Katsupura K _2, passing through the polarizer P in the opposite direction. Since the polarization directions match, the energy loss at this time is relatively small.

Furthermore, through the mode filter M, so as not occur that rotation of the polarization plane, the first through Katsupura K ₁ leading to the light receiving element S.

If the phase difference is Δθ, the output I of the light receiving element is Becomes E ₀ is the amplitude of the electric field of the clockwise or counterclockwise light.

Since the output I is a function of Δθ, Δθ can be known from I.

(C) Problems to be solved by the invention The configuration shown in FIG. 2 has the following disadvantages.

To the same experience clockwise light and the counterclockwise light, the light emitted from the light source Q through the first Katsupura K ₁ has to be 2 minutes of light.

The light that came out at the i end of the split light was discarded as unnecessary. This means that half of the light energy of the light source is discarded. It used only half of the total light energy, and was inefficient in terms of S / N ratio.

Using the full energy of the light source should provide a stronger signal and improve the signal-to-noise ratio.

(D) Configuration In the present invention, two sets of a detection system including a mode filter, a polarizer, a coupler, and a fiber coil are used. This is connected to both ends h and i of the first coupler. Then, the signal strength is doubled. FIG. 1 is a configuration diagram of an optical fiber gyro according to the present invention.

First Fuaibakoiru C ₁ across d, e second Katsupura K ₂
Are connected by This ends the other end j.

The other end g is One connected to the first polarizer P _1. 1st polarizer
The first mode filter M ₁ to the other end of the P ₁ is One connected. The first mode filter M ₁ is continuous to the first branch h of Katsupura K _1.

The optical fiber connecting these is a polarization-maintaining single-mode optical fiber. Mode filter M ₁ is, as already mentioned, the cylinder to which was wound the optical fiber, removing the light having a polarization plane in curved direction as the radiation mode.

This is the first detection system, but a second detection system equivalent to this is further provided.

It consists of a second mode filter M ₂ , a second polarizer P ₂ ,
Katsupura K _3, a second detection system consisting Fuaibakoiru C _2.

Both ends u and w of the second fiber coil C ₂ are equal to the third cut-off K _3.
Are connected by The other end x of K ₃ is the end at which at the free end.

Other end n is One connected to the second polarizer P _2. The second end of the polarizer P ₂ is connected to the second mode filter M _2. Second mode filter M ₂ is continuous with the first branch i of Katsupura K _1.

The first fiber coil C ₁ and the second fiber coil C ₂
It is an equivalent coil having the same number of turns and the same diameter. As shown in FIG. 4, it is provided so that the axis coincides with the rotation detection axis.

The two coils rotate at the same angular velocity Ω. Number of turns,
Since the diameters are the same, the same phase difference Δθ according to the equation (1)
Will be given.

In the present invention, two equivalent detection systems are used.
In each of the detection systems, light intensity as a function of the phase difference Δθ proportional to the angular velocity Ω can be obtained.

Since the signal received by the light receiving element S is a superposition of these signals, the signal intensity is approximately doubled.

However, the sources of light entering the first and second detection systems are the same. These lights must not interfere on the light receiving element S.

When the lights of the first and second detection systems interfere with each other, a phase difference based on the angular velocity Ω appears, and the intensity of the interference light fluctuates.

Therefore, the effective total optical path lengths Z ₁ and Z _{2 of} the first detection system and the second detection system are set so that this difference is larger than the coherence length Lc of the light source.

The total optical path length is obtained by multiplying the optical path followed by the light by the refractive index n. That is, the total length of the fiber coil and the first
From Katsupura K _1, 2 times the optical path to the Katsupura K _2, K _{3 (2}
And the refractive index is multiplied by the refractive index.

That is, | Z ₁ −Z ₂ |> Lc (3). For example, a necessary difference can be given in the mode filters M ₁ and M ₂ .

The light source must have a finite coherence length Lc. A gas laser having a long coherence length is not suitable. A super luminescent diode or a semiconductor laser is used as a light source.

In the case of a semiconductor laser, the coherence length is expressed by Δ
Assuming that λ and oscillation wavelength are λ, Given by

(E) Function The light source Q emits monochromatic light. This divided into two by the first Katsupura K _1. Each light is guided by a polarization-maintaining single-mode fiber.

Because it is a single mode, it can only go through the basic mode. However, since the plane of polarization is in two directions, the mode filter
M ₁ and M ₂ make the direction of the polarization plane one. More polarizer
At P ₁ and P ₂ , linearly polarized light is combined with the intrinsic polarization direction of the polarization-maintaining optical fiber.

The linearly polarized light passes through the fiber coils C ₁ and C ₂ clockwise and counterclockwise. If the fiber coil is rotating, a phase difference Δθ proportional to the rotation is generated. Since the fiber coils C ₁ and C ₂ are equivalent, Δθ is common.

In the first detection system, the electric field of the right-handed light and the left-handed light at the light receiving element S is represented by D (t) and E (t). Can be written. E ₁ is the amplitude.

In the second detection system, if the electric fields of the right-handed light and the left-handed light at the light receiving element S are U (t) and W (t), the amplitude is E ₂ and Can be written. Where Γ is a phase difference based on the difference in fiber optical path length between the first and second detection systems. Can be sought.

Assuming that the output from the light receiving element S is S (t), S (t) = | D (t) + E (t) + U (t) + W (t) | ² (10) Substituting Equations (5) to (8) into Equation (10) and expanding the terms Term disappears because Γ is a phase difference due to a distance longer than the coherence length. Also, ^sin wt and its harmonics disappear because the light receiving element does not respond.

After all Becomes This means that the sum of the signals of the two detection systems can be obtained.

Assume that the two detection systems are equivalent and E ₁ E ₂ E ₀ .

Becomes In other words, a signal output twice as large as that in FIG. 2 can be obtained.

(F) Other applications The present invention is to obtain twice the sensitivity by effectively utilizing light emitted from both ends of a first coupler in an optical fiber gyro having a mode filter and a polarizer.

FIG. 1 shows a basic type in which a modulator is not provided in a fiber. However, it is easy to apply phase modulation, frequency modulation, the null method, etc. to the optical fiber gyro of the present invention.

By doing so, an output can be obtained in the form of ^sin Δθ instead of ^cos Δθ, or in a form linear to Δθ.

(G) Effect The intensity of the signal light reaching the light receiving element is doubled. For this reason, the sensitivity is doubled.

Therefore, the level of the minimum detectable angular velocity Ωmin becomes about half. That is, the performance as a gyro is improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber gyro according to the present invention. FIG. 2 is a configuration diagram of an optical fiber gyro according to a conventional example using a mode filter and a polarizer. FIG. 3 is a principle diagram of the optical fiber gyro. FIG. 4 is a perspective view showing a positional relationship between a first fiber coil and a second fiber coil. Q ...... source S ...... receiving element M _1, M ₂ ...... mode filter C _1, C ₂ ...... Fuaibakoiru P _1, P ₂ ...... polarizer _{K 1, K 2, K 3} ...... Katsupura

Claims (1)

(57) [Claims] 1. A light source Q for generating monochromatic light, a light receiving element S for receiving light and converting the light into an electric signal, and the light source Q and the light receiving element S are connected to one side and have branches i and h. The first coupler K _{1 which} divides the light emitted from Q into two and outputs it to branches h and i, and combines the light coming from branches h and i and inputs the combined light to the light receiving element S.
And a first mode filter M ₁ connected to one branch h of the first coupler K _1, a first polarizer P ₁ following the first mode filter M ₁ , and a polarization-maintaining single mode optical fiber wound in a coil shape many times. first and Fuaibakoiru C _1, second Katsupura connecting first Fuaibakoiru C ₁ ends d, the first polarizer P ₁ combines e
And K _2, winding the second mode filter M ₂ connected to the first Katsupura K ₁ in the other branch i, a subsequent second polarizer P ₂ to the polarization-maintaining single-mode optical fiber into multiple coil a second Fuaibakoiru C ₂ coupled coaxially first Fuaibakoiru C ₁ equivalent Kaiwashi, second Fuaibakoiru C ₂ ends u, third connected to the second polarizer P ₂ combine w Cutlery
K ₃ and more becomes, 2 light emitted from the light source Q is at a first Katsupura K ₁
Book light, one of which is a first mode filter M ₁ ,
The first polarizer P _1, propagate second Katsupura the first Fuaibakoiru C ₁ via the second Katsupura K ₂ as clockwise light and the counterclockwise light
The first polarizer P ₁ taken together with K _2, the first mode filter M _1, the intensity of the incident interference light is detected first Katsupura K ₁ in passing connexion light receiving element S, the first Katsupura K ₁ The other light split is a second mode filter M ₂ , a second polarizer P ₂ , and a third coupler K ₃
Second Fuaibakoiru C ₂ taken together with the propagation and third Katsupura K ₃ as clockwise light and the counterclockwise light second polarizer P ₂ through the second mode filter M _2, the first Katsupura K ₁ through connexion and summer so that the intensity of incident on the light receiving element S interference light is detected, the coherence length of light difference of the total optical path length of two light divided by the first Katsupura K ₁ is traced occurs a light source Q An optical fiber gyroscope characterized by being larger than L _C.