JPH0350964B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Technical Field The present invention relates to an optical fiber gyroscope 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 coils Optical fiber coils are made by winding a single-mode optical fiber many times into a coil to pass light clockwise (CW) and counterclockwise (CCW). When rotates at an angular velocity Ω, a phase difference Δθ occurs between the clockwise and counterclockwise lights,
Ω is detected by measuring Δθ.

The light source may be a He-Ne laser, a semiconductor laser, a superluminescent diode, or the like. This light of a certain wavelength is split into two by a beam splitter,
Inject from both ends of the optical fiber loop. The clockwise and counterclockwise lights that have passed through the loop are combined by a beam splitter and incident on the light receiving element. The phase difference Δθ is known from the detection intensity of the light receiving element, and Ω is found from Δθ=4πLAΩ/Cλ (1). 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 I is of the form I∝{1+cos(Δθ)} (2).

This has the disadvantage that since Δθ is included in cos, the sensitivity is poor when Δθ is close to 0.

Therefore, the phase of either the left-handed or right-handed light is
A mechanism has been proposed in which the waves are shifted by 90 degrees and square law detection is performed. In this case, the output I takes the form I∝{1+sin(Δθ)} (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.

(c) Prior art and its problems Therefore, many optical fiber coils that attempt to detect Δθ using a dynamic mechanism have been proposed. One of them is an optical fiber coil that includes a phase modulator at one end of an optical fiber loop and determines the phase difference Δθ by measuring the magnitude of a modulated signal. This is tentatively called a phase modulation method.

Regarding the phase modulator, for clockwise light and counterclockwise light,
When installed in an asymmetric position, the light emitted from the light source at the same time passes through the optical fiber loop and modulator in clockwise and counterclockwise directions, but since the modulation times are different, the output is square-law detected by the photodetector. When the modulating signal appears at the output. Since Δθ is included in the amplitude of the modulation signal, Δθ can be determined 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 loop is L, the refractive index of the fiber core is n, 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 _n . When the light emitted from the light source at the same time is split into clockwise light and counterclockwise light and undergoes phase modulation, the phase difference φ of the modulation signal is φ=ω _n τ = nLω _n /c = 2πf _n nL/c (5) becomes.

Due to the Sagnac effect, clockwise light and counterclockwise light are ±
It has a phase difference of Δθ/2, but by the phase modulator,
The phase is further modulated. The amplitude of the phase modulator is b
Then, the electric field strength Er of the clockwise light and counterclockwise light is
El is Er=E ₁ sin {ωt+Δθ/2+b sin(ω _n t+φ)}(6
) El=E ₂ sin {ωt−Δθ/2+b sin(ω _n t)} (7). Since there is a phase difference φ of the phase modulator, Δθ
can be related to the amplitude of the modulating signal.

At the beam splitter, the clockwise light and counterclockwise light are combined into one and square-law detected by the light receiving element, so the output S (Δθ, t) of the light receiving element is proportional to the square of the sum of Er and El. .

S (Δθ, t) = {Er + El} ² (8) Calculating this, S (Δθ, t) = E ₁ E ₂ cos {Δθ + 2b sin (φ/2) c
os(ω _n t+φ/2)}+D.C.+{2ω or more}(9). DC means direct current component. {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. Omitting DC, expand S(Δθ, t) into a series using the Betzel function.

S (Δθ, t) = E ₁ E ₂ [cosΔθcos {2b sinφ/2cos
(ω _n t+φ/2)}−sinΔθsin{2b sinφ/2cos(
ω _n t + ω/2)}(10) From the generating function expansion of the Betzell function, It is. Putting t=e ⁱ 〓, e ^{ix sin} 〓= _∞ 〓 ^n=-∞ Jn(x)e ⁿⁱ 〓 (12). From the expansion of the real and imaginary parts of (12), we find that (10)
Obtain the series expansion of the cos and sin parts. S(Δθ,t)
Divide into these parts, S (Δθ, t) = (Sc cos Δθ + Ss sin Δθ) E ₁ E ₂
(13) After converting θ→θ+π/2, using the property J _-o (x) = (-) ⁿ Jn (x) (14) (where n is a positive integer), , ξ=2b sinφ/2 (15) If we write it as Sc=J ₀ (ξ)+2 _∞ 〓 ⁿ⁼⁰ (−) ⁿ J _2o (ξ) cos2nω _n t (16) Ss=2 _∞ 〓 ^{n =0} (−) ⁿ J _2o+1 (ξ) cos(2n+1)ω _n t (17). This is the series sum of the fundamental wave of the modulation signal ω _n and the harmonic signal.

By using an appropriate filter, it is possible to extract the fundamental wave ω _n or a harmonic signal of any order.
No matter which signal is adopted, the magnitude of cosΔθ or sinΔθ can be known.

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

The highest sensitivity can be expected from the first-order term (n=0) in equation (17). This is the fundamental wave component. If the fundamental wave component is P (Δθ, t), then P (Δθ, t) = 2E ₁ E ₂ J ₁ (ξ) cosω _n t sinΔθ
(18). An output proportional to sinΔθ can be obtained. Δθ can be found by finding the amplitude of the fundamental wave component.

Maximizing J ₁ (ξ) improves sensitivity, so
Set ξ=1.8. At this time, the DC component J ₀ (ξ) is approximately 0.

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

As can be seen from equation (18), the amplitudes E ₁ and E ₂ of the clockwise and counterclockwise lights are included in the amplitude of the fundamental wave component P of the light receiving element output. There is also a coefficient called J ₁ (ξ).

In order to be able to accurately determine Δθ from such an output, the values of E ₁ , E ₂ , and J ₁ (ξ) must be stable.

However, these values fluctuate.

In particular, the light amplitudes E ₁ and E ₂ tend to fluctuate.

E ₁ and E ₂ are fluctuations in the output of the light source, fluctuations in the amount of light passing through the polarizer inserted to remove the optical path difference,
It can easily change due to misalignment of the optical system, etc.

Even if the direction of polarization when the light enters the fiber is determined, the state of polarization 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 of the light reaching the light receiving element E ₁ , E ₂
will change.

All that is required is to monitor the amplitudes E ₁ and E ₂ .
For this purpose, a beam splitter is inserted into the optical path of the right-handed light and the left-handed light, and each light is extracted for monitoring and the power is measured _.
An optical fiber gyro based on E ₂ has also been 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.

(d) Structure of the invention The configuration of the present invention will be explained 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 splits the light emitted from the light emitting element 1 into transmitted light and reflected light.

Lenses 3 and 4 allow the separated light to enter 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 one of the end points A and 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 the element. When a modulating voltage is applied, the diameter of the piezoelectric element changes, causing the optical fiber wrapped around it to expand and contract. This changes the optical path length and therefore the phase.

Let the modulation angular frequency of the phase modulator be ω _n and the modulation amplitude be b.

The light split into two beams by the beam splitter 2 propagates through the optical fiber loop 5 as clockwise light (CW) and counterclockwise light (CCW). In this example, the right-handed light undergoes phase modulation first, and the left-handed light undergoes 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 lenses 4 and 3, are combined at 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 _n is detected by the detection circuit 8 and the amplitude is determined.

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 integration circuit, etc.

The division circuit 9 divides the magnitude of the ω _n component output from the detection circuit 8 by the magnitude of the DC component from the DC component detection circuit 10 .

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

(E) Effect The amplitude of the modulated fundamental wave ω _n component of the output of the light receiving element is 2E ₁ E ₂ J ₁ (ξ) sinΔθ (19) from equation (18). The DC component is 1/2 (E ₁ ² + E ₂ ² ) + E ₁ E ₂ J ₀ (ξ) (20). However, in order to maximize J ₁ (ξ), ξ=
It is often set to 1.8. At this time J ₀ (ξ)0
Therefore, the term J ₀ (ξ) drops from the DC component (20).

E ₁ and E ₂ are split by the beam splitter 2, pass through a lens, an optical fiber loop, a beam splitter, a polarizer, etc., and enter 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, E ₁
and E ₂ are proportional. Let k be the constant of proportionality.

When E ₂ =kE ₁ (21), the value obtained by dividing the ω _n component by the DC component is 4k/k ² +1J ₁ (ξ) sinΔθ (22). This no longer includes the light amplitudes E ₁ , E ₂ . It is possible to completely eliminate fluctuations in scale factor due to fluctuations in light amount. Since ξ1.8
J ₁ (ξ) is maximized and the sensitivity is highest.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of the optical system of the optical fiber coil according to the present invention. 1...Light emitting element, 2...Beam splitter,
3, 4... Lens, 5... Optical fiber loop, 6
... Phase modulator, 7 ... Light receiving element, 8 ... Detection circuit, 9 ... Division circuit, 10 ... DC component detection circuit.

Claims (1)

[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, an optical fiber loop 5 in which a single-mode optical fiber is wound into a coil many times, and an optical A phase modulator 6 provided near either end point A or B of the fiber loop 5 and a lens that makes the two light beams separated by the beam splitter 2 enter the end points A or B of the optical fiber loop 5. 3, 4, a light receiving element 7 for detecting the intensity of the light that is combined at the beam splitter 2 after passing through the optical fiber loop 5 and the phase modulator 6 clockwise and counterclockwise; It consists of a detection circuit 8 that detects the modulation frequency component of the output, a DC component detection circuit 10 that detects the DC component, and a division circuit 9 that divides the magnitude of the modulation frequency component by the magnitude of the DC component. , where the modulation angular frequency of the phase modulator is ω _n , the amplitude is b, and the time required for light to pass through the optical fiber is τ, then 2b sin (ω _n τ/2) 1.8, and the division circuit 9 An optical fiber gyroscope characterized in that the phase difference between right-handed light and left-handed light is determined by the output of , and the rotational angular velocity is determined from this.