JPS60135816A - Optical fiber gyro - Google Patents

Abstract

PURPOSE:To remove the variation in a scale factor due to the variation in quantity of light, by detecting the magnitude of the DC component contained in the output of a light receiving element by a DC component detection circuit and dividing the magnitude of the modulation frequency component in the output of the light receiving element on the basis of the detection value. CONSTITUTION:The output beam of a light emitting element 1 is divided into two beams by a beam splitter 2 and two beams are incident to terminal points A, B of an optical fiber loop 5 wound a large number of times in a coil form as right-turn beam and left-turn beam while modulation voltage is applied to a phase modulator 6 to change the light path length of the optical fiber and the phase difference DELTAtheta generated in the right-turn and left-turn beams is measured to detect a rotary angle speed. The variation in left-turn and right-turn amplitudes E1, E2 of beams entering a light receiving element 7 through the beam splitter 2 at this time is removed by dividing the magnitude of the modulation frequency component detected by a detector circuit 8 by that of the DC component detected by the DC component detection circuit 10.

Description

DETAILED DESCRIPTION OF THE INVENTION (g) Technical Field The present invention relates to an optical fiber gyro for measuring the rotational angular velocity of a moving object such as an aircraft, automobile, or ship.

In particular, the purpose is to suppress fluctuations in the so-called scale factor, in which the output of the optical fiber gyro changes even for inputs of the same angular velocity.

(a) Principle of optical fiber gyro An optical fiber gyro uses an optical fiber lube, which is a 111 single-mode optical fiber wound in a coil, in a clockwise direction (
CW, ) and counterclockwise (CCW), when the optical fiber loop rotates at an angular velocity Ω, a phase difference Δθ occurs between the clockwise light and the counterclockwise light, and Ω is detected by measuring Δθ. It is something.

The light source is a He-Ne laser, a semiconductor laser, a superluminescent diode, or the like. This light of a certain wavelength is split into two beams by a beam splitter, and the beams are made to enter from both ends of the optical fiber. The right touch passed through the loop,
The counterclockwise light is combined by a beam splitter and enters a light receiving element. The phase difference Δθ is known from the detection intensity of the light receiving element, and Ω is calculated from Cλ. C is the speed of light, λ is the wavelength of light, L is the length of the optical fiber loop, and a is the radius of the optical fiber loop. This is called the Sagnac effect.

There are various detection mechanisms for Δθ, and various mechanisms have been proposed.

Most simply, when the sum of the left-handed light and the right-handed light is square-law detected using a light receiving element, the output is of the form σ(1,+C03(Δθ)) (2).

This has the disadvantage that, since there is Δθ inside the fish, the sensitivity is poor when Δθ is close to 0.

Therefore, a mechanism has been proposed in which the phase of either the left-handed or right-handed light is shifted by 90° and square-law detection is performed.

In this case, the output crab has the form Io: (l-1-sin (Δθ) l (3), so the sensitivity is good when Δθ is close to 0.

However, in order to separate one of the lights, three new beam splitters are required to separate the optical paths. Furthermore, the lengths of the separated optical paths must always be made equal.

Static detection mechanisms have such drawbacks.

(1) Prior art and its problems Therefore, many optical fiber gyros that attempt to detect Δθ using a dynamic mechanism have been proposed. One of them is an optical fiber gyro that includes a phase modulator at one end of an optical fiber loop and measures the magnitude of a variable signal to determine the phase difference Δθ. This is tentatively called a phase modulation method.

If the phase modulator is installed in an asymmetric position for clockwise and counterclockwise light, the light that exits the light source at the same time will be divided into clockwise and counterclockwise and pass through the optical fiber loop and modulator, but the light will not be modulated. Since the times are different, when the output is square-law detected by the light receiving element, a modulated signal appears in the output. Since Δθ is included in the amplitude of the modulation signal, Δθ can be calculated by knowing the magnitude of the modulation signal.

For example, assume that a phase modulator is provided near the input end of counterclockwise light.

When the length of the optical fiber loose is L1, the refractive index of the fiber core is n1, and the speed of light is C, the time τ required for light to pass through the fiber loop is τ = nL/c (4).

Assume that the modulation signal is a sine wave with an angular frequency ω.

At the same time, the light emitted from the light source is divided into clockwise light and counterclockwise light,
The phase difference φ between the modulated signals when receiving phase modulation is as follows: φ=ω τ =nLω/C = 2 yr f nL/c (5).

Due to the Sagnac effect, clockwise light and counterclockwise light are ±Δ
Although it has a phase difference of θ/2, the phase is further modulated by a phase modulator. Assuming that the amplitude of the phase modulator is b), the electric field strengths Er and El of the right-handed light and the left-handed light are as follows.

Because of the phase difference φ of the phase modulator, Δθ can be related to the amplitude of the modulation signal.

The right-handed light and left-handed light are combined into one by the beam splitter and square-law detected by the light-receiving element, so the output S (Δθ, 1) of the light-receiving element is proportional to the square of the sum of Er and Ed.

S (Δθ, t) - (Er + F612 (8) Calculating this gives the following. D, C, mean DC components. (2ω or more)
means a term with a frequency twice the angular frequency of light, and since it is not applied to the detector, it is 0. D.C.
is omitted, and S(Δθ, 1) is defined as Bethselbesse/1. −
From the base function expansion of the function, Uθ. If we set t=e, then From the expansion of the real and imaginary parts of (12), (10
) to obtain the series expansion of the cos and sin parts. S(Δθ
tt) into these parts, S(Δθ, t) = (ScCO3Δθ+5ssinΔθ
) ``1 ''2 (13), after exchanging θ→θ1π/2, ty, n (X) −()'' Jn (
x ) (14) (where n is a positive integer) and write it as Sc = Jo (ξ) + 2Σ (-) J2n(ξ
) cos2nw, t (16) n=1 Ss −= 2Y ()” J2n+i (ξ) C0
5(2n+1.) ω, t (17)n =Q. This is the sum of the series of the fundamental wave of the modulation signal ω and the harmonic signal.

By using a suitable filter, it is possible to extract the fundamental wave ω or a harmonic signal of any order. No matter which signal is adopted, the magnitude of O:lSΔθ or sinΔθ can be known.

In that case, the amplitude b1 of the modulator, the modulation angular frequency ω, and the loop passage time τ should be set so that the value of the Bessel function Jn(ξ) of that order becomes large.

The highest sensitivity can be expected from the first-order term (
n=0). This is the fundamental wave component.

If the fundamental wave component is P(Δθ, t), then P(809 t
) =2E1 E2 Jl (ξ)cosωI,lt
sinΔθ (18). The output can be changed in proportion to sinΔθ. By calculating the amplitude of the fundamental wave component, Δθ can be determined.

Maximizing Jl(ξ) improves sensitivity, so ξ−1
,8. At this time, the DC component J. (ξ) is almost 0
It is.

The above is the basic configuration of a phase modulation type optical fiber gyro.

As can be seen from equation (+8), the amplitudes E1 and E2 of the clockwise light and counterclockwise light are included in the amplitude of the fundamental wave component p of the light receiving element output. There is also a coefficient called Jl(ξ).

In order to be able to accurately determine Δθ from such an output, the values of El, E2, J](ξ) must be stable.

However, these values fluctuate.

In particular, the light amplitude E1, "2" tends to fluctuate.

El and E2 easily change due to fluctuations in the output of the light source, fluctuations in the amount of light passing through a polarizer inserted to eliminate the optical path difference, positional deviation of the optical system, etc.

Even if the polarization direction is determined when the light enters the fiber, the polarization state changes due to the effects of temperature, pressure, strain, etc. while propagating through the fiber, and the polarization of the emitted light is not constant.

As a result, the amplitudes E1 and E2 of the light reaching the light receiving element vary.

It is sufficient to monitor the amplitudes E1 and E2.

For this purpose, there is also an optical fiber gyro that inserts a beam splitter into the optical path of the clockwise and counterclockwise lights, extracts each light for monitoring, measures the power, and learns E□ and E2. Proposed.

However, such a configuration has disadvantages in that the number of parts increases, the amount of light reaching the light receiving element decreases, and the S/N ratio decreases.

Naka) Composition of the invention The present invention will be explained in detail with reference to FIG.

The light emitting element 1 emits light, and can use a gas laser, a semiconductor laser, a superluminescent diode, or the like.

The beam splitter 2 separates the light emitted from the light emitting element 1 into transmitted light and reflected light.

Lens 3.4 directs the separated light into both ends of single mode optical fiber loop 5. The optical fiber loop 5 is a single mode optical fiber wound in a coil shape many times.

A phase modulator 6 is provided near either end point A or B of the optical fiber loop 5.

A phase modulator periodically changes the phase of light passing through an optical fiber. For example, it can be constructed by winding an optical fiber around a cylindrical piezoelectric element and applying a modulated voltage between the end faces of one element. When a modulating voltage is applied, the diameter of the piezoelectric element changes, causing the optical fiber wound around it to expand and contract. As a result, while the optical path length changes, the phase also changes.

Let the modulation angular frequency of the phase modulator be ω0, and the modulation amplitude be b.

The light split into two beams by beam splitter 2 is
The light propagates through the optical fiber loop 5 as clockwise light (CW) and counterclockwise light (CCW). In this example, the clockwise light is first subjected to phase modulation, and the -1 counterclockwise light is subjected to phase modulation later.

The clockwise light (CW) and the counterclockwise light (CCW) exit the end points A and B of the optical fiber, pass through the lens 4.3, are combined by the beam splitter 2, and enter the light receiving element 7.

The light receiving element 7 generates an electrical signal proportional to the light intensity. Of these, only the component of modulation angular frequency ω is detected by the detection circuit 8,
Adjust the amplitude.

The above configuration is the same as that of a conventional phase modulation type optical fiber gyro.

The present invention further includes a DC component detection circuit 10 that detects the magnitude of the DC component included in the output of the light receiving element 7. This can be constructed by combining an amplifier circuit, an integrating circuit, etc.

The division circuit 9 calculates the magnitude of the output of the ω component of the detection circuit 8 by
Divide by the magnitude of the DC component of the DC component detection circuit 10.

Since the output of the division circuit is proportional to sin Δθ, the phase difference Δθ can be calculated from this.

The amplitude of the modulated fundamental wave ω component of the output of the working photodetector is (1
From equation 8), 2'1E2 Jl(ξ) sinΔθ (19). The DC component is (Et + Ii:2) (2). However, in order to maximize Jl(ξ), ξ=1.
It is often set to 8. At this time, Jo(ξ)γ0, so the DC component (■) is J. The term (ξ) falls.

The beam is split by the El, E21d beam splitter 2, passes through a lens, an optical fiber loop, a beam splitter, a polarizer, etc., and enters the light receiving element 7. This is the amplitude of the right-handed light and the left-handed light.

Since each light passes through the same optical path, El and E2 are proportional. Let k be the constant of proportionality.

E2 = kEl (a) Then, ω. The value obtained by dividing the component by the DC component is:

This no longer includes the light amplitudes E1, E2. It is possible to completely eliminate fluctuations in scale factor due to fluctuations in light amount.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the optical system configuration of the optical fiber gyro of the present invention. 1 ...... Light emitting element 2 ...
・・・・・・ Beam splitter 3.4 ・・・ ・・・
... Syntheses 5 ...... Optical fiber loop 6 ...... Phase modulator

Claims (2)

[Claims] (1) A light emitting element 1, a beam splitter 2 that splits the light emitted from the light emitting element 1 into two beams, and an optical fiber loop 5 in which a single mode optical fiber is wound into a coil shape many times.
The phase modulator 6 provided near either end point A or B of the optical fiber loop 5 and the beam splitter 2 separate the two beams from the end point A or B of the optical fiber loop 5.
A lens 3.4 for making the light incident on B, and a light receiving element 7 for detecting strong changes in the light that has passed through the optical fiber loop 5 and the phase modulator 6 in the clockwise and counterclockwise directions and is then combined at the beam splitter 2. , a detection circuit 8 that detects the modulated frequency component of the output of the light receiving element 7, a DC component detection circuit 10 that detects the DC component, and a division circuit that divides the magnitude of the modulation frequency component by the magnitude of the DC component. An optical fiber gyro characterized by comprising: 9. (2) The optical fiber according to claim (1), where the modulation angular frequency of the phase modulator is ω□, the amplitude is b1, and the time required for the light to pass through the optical fiber loop is τ. gyro.