JPH0676911B2 - Optical fiber hydrophone - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a homodyne optical fiber hydrophone that does not require phase compensation.

The optical fiber hydrophone is a device that detects the magnitude of underwater sound using the optical fiber.

A combination of a signal optical fiber having a sensing coil and a reference optical fiber, a monochromatic light source and a beam splitter,
A Matsuhatsu ender type interferometer is made using a photodetector.
The sensing coil is submerged in water to sense the pressure fluctuation of sound waves. The refractive index and the length of the optical fiber change due to the pressure fluctuation. Therefore, the phase of light passing through the sensing coil changes. Light that passes through the reference light fiber,
When the light that has passed through the signal optical fiber is interfered, a phase fluctuation is included in the intensity of the interference light.

By detecting the magnitude of this phase fluctuation, the strength of the sound wave can be detected.

However, the refractive index and the length of both the signal fiber and the reference light fiber change due to the temperature change. For this reason,
The phase difference Ψ of the signal light and the reference light fluctuates depending on the temperature. This is quite large. Phase difference Ψ (τ) is detected, phase compensation Φ (τ) is performed, and Ψ (τ) −Φ
The conventional method is to keep (τ) = constant.

That is, in the conventional homodyne optical fiber hydrophone, the drift of the phase difference Ψ due to temperature is the most serious problem, and it has been necessary to compensate for this.

INDUSTRIAL APPLICABILITY The present invention can provide an optical fiber hydrophone capable of accurately detecting the intensity of a sound wave without performing phase compensation.

(B) Conventional technology Bucaro was the first to propose the optical fiber hydrophone.
Etc. JABucaro, HDDardy and EFCarome,
"Optical fiber acoustic sensor," Appl.Opt. 16 , 17
61-1762 (1977).

JABucaro and HDDardy, “Fiber−Optic hydroph
one, "J.acoust.Soc.Am. 62 .1302-1304 (1977).

The early ones are explained in.

Since then, many proposals have been made regarding the method of phase compensation. In many cases, the piezoelectric element is used to change the length of the optical fiber to perform phase compensation. Each of these has a narrow range of phase compensation.

Therefore, Jackson et al. Have proposed, as a phase compensation mechanism, an optical fiber wound around a thin-walled piezoelectric element. Electrodes are formed on the inner surface and the outer surface of the thin-walled cylinder, and a voltage is applied between the outer surface and the inner surface of the cylinder. Thus, the diameter of the cylinder changes in proportion to the voltage. When the diameter changes, the optical path length of the optical fiber also changes.

DAJackson, R.priest, A.Dandridge and ABTveten,
"Elimination of drift in a single-mode op
tical fiber interfero-meter using a piezoel
ectrically stretched coil fiber, "Appl.Opt. 19 , 29
26-2929 (1980). Among them, Jackson uses a bottle cutler to obtain the phase difference Ψ. It is a bottle cap that holds a signal fiber and a reference fiber close to each other in the order of wavelength. The output of the cutler is detected by two photodetectors. Since both fibers are close to each other in the Katsupura, they are joined by an evanescent wave.

The coupling causes the electric field of one optical fiber to seep out and enter the other optical fiber. Since the optical path is lengthened by λ / 4 when entering, it is delayed by this much. Then, the output of _one photodetector becomes (I ₀ + I ₁ cos Ψ). The output of the other photodetector is (I ₀ + I ₁ cos Ψ).

When this is differentially amplified, cos Ψ is obtained. The highest sensitivity is to keep Ψ = π / 2. Then we know the deviation of Ψ from π / 2. Knowing this, to the phase compensation mechanism (Ψ−π / 2)
A voltage proportional to is applied. In this way, cos Ψ = 0.

However, the variation of the phase difference Ψ with temperature may be about 10,000 radians. The conventional phase compensation mechanism can only compensate about 10 radians. Jackson wrote that a thin-walled piezoelectric cylindrical element can compensate about 6000 radians.

This is because about 2π can be compensated per 1 volt and about 1000 V of the piezoelectric element can be applied.

However, it is inconvenient to use a high voltage of 1000V. If the power supply is ± 15V at most, the compensation range is about 90 radians.

Of course, since it is only necessary to compensate the phase by 2π, it is possible to use 10 radians or 90 radians. However, when the upper limit of phase compensation is reached, the lower limit must be switched. This switching is done by rounding down the angle by an integral multiple of 2π.

This range switching is an extremely complicated operation. For example, in the case of a phase compensation mechanism having a dynamic range of 90 radians, if a phase change of 9000 radians occurs, the range must be switched 100 times.

As a review of the optical fiber hydrophone written in Japanese, Takahashi and Kikuchi “Underwater sound wave detection method using optical fiber I”,
Journal of the Acoustical Society of Japan, Vol. 40, No. 2, p. 101-106 (1984). Takahashi, Kikuchi "Hikari fiber hydrophone", Electronic Ceramics '84 Spring issue, p.51-56 (1984).

The applicant of the present invention has invented an optical fiber hydrophone for detecting by taking out a 2Ω oscillating portion in order to avoid the problem of the phase variation of the homodyne system (Japanese Patent Laid-Open No. 62-276419).

The Applicant has also invented a device for detecting the phase shift Ψ by a method different from the Jackson method. (JP-A-63-91525).

(C) Problems to be solved by the invention In the optical fiber hydrofon of the homodyne system, in order to perform phase compensation, the phase difference Ψ must be detected and the phase Φ must be given to the phase compensation mechanism so that it can be canceled. I have to. Moreover, since the range of phase compensation is narrow, the range of the phase compensation mechanism must be switched frequently.

The accuracy of phase compensation must also be high.

It is difficult to satisfy all of these conditions.

(D) Purpose It is an object of the present invention to provide a homodyne type optical fiber hydrophone without any phase compensation.

Since the phase compensation is unnecessary, the detector for the phase difference Ψ is not necessary and the phase compensation mechanism is also unnecessary.

(E) Configuration A signal of sin (Γt) is generated by the oscillator. By using this, a sawtooth-like phase variation that gives a phase of 2π / mΓ and continuously increases the phase from 0 to 2mπ is given. Phase modulator is Γ
The phase is increased at a rate of t and reset at t = 2π / mΓ to return the phase to zero. The component of Γt enters the interference light. This is synchronously detected by the signals of sin Γt and cos Γt. Squared values obtained by synchronous detection are summed, and the square root is calculated. This is the output.

Hereinafter, description will be given with reference to the drawings.

FIG. 1 is a block diagram of an optical fiber hydrophone of the present invention.

The coherent light source 1 that emits monochromatic light is, for example, a He-Ne laser, a semiconductor laser, or the like. The light generated from the light source 1 is split into two light beams by the beam splitter 7.

These light rays enter the signal light fiber 2 and the reference light fiber 3 by using a condensing optical system (not shown).

In the middle of the signal optical fiber 2, the underwater sound wave φsin (Ω
A sensor coil 4 that senses t) is provided. The refractive index and the length of this change due to the change in water pressure. Then, the phase of the signal light propagating through this changes with amplitude φ and angular frequency Ω.

Ω is equal to the angular frequency of the sound wave. φ is proportional to the strength of the sound wave, the length of the optical fiber, the refractive index of the optical fiber, the ratio of the length change to the pressure, and the like. Therefore, φ is proportional to the strength of the sound wave, which may be considered as the strength of the sound wave. Finding φ is the purpose of the optical fiber hydrophone.

The reference optical fiber 3 is also provided with a reference coil 5. This is a signal optical fiber 2 and a reference optical fiber 3.
This is to make the lengths of the two equal and reduce the phase difference variation due to temperature.

However, in the present invention, since there is no problem even if there is a phase difference variation, there is no strong demand for equalizing the lengths of both optical fibers. Therefore, the reference coil 5 can be omitted.

A phase modulator 11 is provided in either one or both of the signal optical fiber 2 and the reference optical fiber 3. In this example, it is provided in the middle of the reference optical fiber 3.

The signal light S is subjected to phase modulation by a sound wave by the sensor coil 4. The reference light R (or the signal light S) is subjected to a fixed phase modulation by the phase modulator 11.

It should be noted that it is phase modulation, not phase compensation.

The signal light S and the reference light R are combined by the beam splitter 8 for combining and enter the light receiving element 6. Since these lights are monochromatic lights emitted from the same light source, they interfere at the light receiving element surface. The intensity of the interference light is measured by the light receiving element.
This is amplified by the amplifier 9. This output is I.

The output I contains a fundamental wave of Ωt and a harmonic component of mΩt. The bandpass filter 10 extracts only the component of the fundamental wave Ωt. This is J.

The signal of the phase modulator 11 is generated by converting the signal of the oscillator 13 into a sawtooth wave and giving it to the phase modulator. Oscillator 13
The vibration of can be expressed by a sin Γt. The sawtooth wave generation circuit 12 produces a sawtooth wave from this.

The synchronous detection circuit 15 uses the signal sin Γt of the oscillator 13,
The output J is synchronously detected. Synchronous detection is a detection method in which a signal component having the same frequency and phase as sin Γt is extracted from a certain waveform. Specifically, sin
Γt and its waveform may be multiplied and averaged.

The 90 ° phase shifter 14 shifts the phase of the signal of the oscillator 13 by 90 °. This can be configured by PLL. A signal of sin Γt is obtained by shifting the phase by 90 °.

The synchronous detection circuit 16 synchronously detects the output J by using cos Γt.

The outputs K and L of the synchronous detection circuits 15 and 16 are squared by the squaring circuits 17 and 18. This is done by inputting the same signal to the two inputs of the multiplication circuit.

The outputs M and N of the squaring circuits 17 and 18 are added in the adding circuit 19.

The addition output O becomes the output P through the square root calculation circuit 20.

(F) Action The electric field intensity of the signal light S on the light receiving surface of the light receiving element 6 is represented by S = Gsin {ωt + φsin (Ωt)} (1). G is the amplitude, ω is the previous angular frequency, and φsin
(Ωt) is the amount of phase change due to sound waves.

Since the reference light R is phase-modulated, R = Hsin (ωt + Γt + Ψ) (2). The reason why Γt is included in sin due to the phase modulation will be described later. Ψ is the phase difference due to the difference in the length of both optical fibers. The fact that this changes with temperature was a serious problem.

The output I of the light receiving element 6 and the amplifier 9 is the square-law detection of the sum of S and R. Excluding the photoelectric conversion coefficient, Becomes A band pass filter extracts only the fundamental component of (Ωt). As a result, the direct current component and the harmonic component of mΩt drop.

We use a population function expansion of the Bethel function.

Is. In the following description, when φΩ >> Γ, the bandpass filter passes only the sin (Ωt) component. In reality, the frequency becomes Ω ± (Γ / φ). The bandpass filter may have a center frequency of Ω and a band of 2Γ / φ or more.

From (3), (4) and (5), the output J of the bandpass filter is J = 2GHJ ₁ (φ) sin (Γt + Ψ) sin (Ωt) (6).

The oscillation of the oscillator 13 is sin Γt. When J is synchronously detected by this, the following multiplication is performed.

The integration range is 0 to 2π / Γ.

Furthermore, since cos Γt which is phase shifted by 90 ° and J are synchronously detected, After all, the coherent detection outputs K and L are K = GHJ ₁ (φ) cos Ψsin (Ωt) (9) L = GHJ ₁ (φ) sin Ψsin (Ωt) (10) Note that the phase difference Ψ
Is entered in the form of sin and cos.

The squaring circuits 17 and 18 square, and the adding circuit 19 adds them.

M = K ² (11) N = L ² (12) O = M + N (13) Square root calculation circuit 20 Therefore, P = 2GHJ ₁ (φ) sin (Ωt) (15), and a signal with no phase difference Ψ is obtained.

The synchronous detection circuit, the square circuit, and the square root calculation circuit can be configured by using an analog multiplier.

From P, the size of φ can be known. If φ << ₁ , J ₁ (φ)
Since it is φ / 2, if the values of G and H are stable, the output P will be proportional to the magnitude of the sound wave.

(G) How to give a sawtooth wave In the phase modulator, the maximum modulation phase is set to 2mπ. m is an integer. As shown in FIG. 2, the phase-modulated signal is provided in a sawtooth shape, but the sawtooth period is not 2π / Γ,
This is m times 2mπ / Γ.

That is, the phase modulator is reset every T = 2mπ / Γ.

This gives a gradient that is the phase modulation divided by time.
Then, Γt becomes a variable in sin. The domain of Γt is 0
It is ~ 2mπ, but since it is in sin, it means that the phase modulation of 0-2π is actually repeated m times.

However, since the oscillator 13 generates a signal of sin Γt, it is necessary to divide this by 1 / m.

The sawtooth generation circuit 12 includes a 1 / m frequency dividing circuit.

Given a signal with a period of 2mπ / Γ, it is easy to make it a sawtooth wave.

A simple integrating circuit and a reset circuit may be combined.

The phase modulator 11 can be made of a Pockels element or the like.

Alternatively, a fiber may be wound around the piezoelectric element and a voltage may be applied to change the phase.

Now, since the phase modulator also has temperature fluctuations, t =
It is also possible that the phase does not become 2mπ in 2mπ / Γ. The time axis is accurate, but the phase modulation is subject to temperature changes. But this is a trivial matter. Even if the maximum phase deviates from 2 mπ, this difference is only added to the phase offset Ψ.

In the present invention, since the phase offset Ψ disappears, Ψ may vary.

(H) Effect In the homodyne optical fiber hydrophone, the phase compensating circuit for compensating for the fluctuation of Ψ due to the temperature fluctuation becomes unnecessary. As a result, it is possible to construct an optical fiber hydrophone with stable performance regardless of temperature changes.

[Brief description of drawings]

 FIG. 1 is a block diagram of the optical fiber hydrophone of the present invention. FIG. 2 is a waveform diagram of a sawtooth wave. 1 ... Light source 2 ... Signal optical fiber 3 ... Reference optical fiber 4 ... Sensor coil 5 ... Reference coil 6 ... Receiving element 7,8 ... Beam splitter 9 ... Amplifier 10 ... Band Pass filter 11 …… Phase modulator 12 …… Sawtooth wave generation circuit 13 …… Oscillator 14 …… 90 ° phase shifter 15,16 …… Synchronous detection circuit 17,18 …… Squaring circuit 19 …… Adding circuit 20… … Square root arithmetic circuit

Claims (1)

[Claims] 1. A signal optical fiber 2 which is a single mode fiber for branching light generated from a light source 1 which generates a monochromatic light.
And the light emitted from the respective fibers 2 and 3 to be incident on the reference optical fiber 3 and the intensity of the interference light is detected by the light receiving element 6, and the sensor coil 4 provided in the middle of the signal optical fiber 2 is detected. In the optical fiber hydrophone, which is determined to obtain the phase change due to the underwater sound wave that is sensed by the optical fiber from the intensity of the interference light, a signal of sin (Γt) is generated by the oscillator 13 and the reference optical fiber 3 or the signal optical fiber is generated. By the phase modulator 11 provided on one or both of the two, sawtooth phase modulation with a maximum phase shift of 2πm and a period of 2π / Γm is added, and the intensity of the interference light is converted into an electric signal by the light receiving element 6. After changing, only the component of the internal frequency of the signal of angular frequency Ω is taken out by the band pass filter 10, and this signal is phase-shifted by 90 ° with the signal sin (Γt) of the oscillator 13. (Γ
t) is used for synchronous detection, the synchronously detected outputs are each squared, and these are added together to obtain the square root by the square root calculation circuit 20 to obtain the intensity of the sound wave. Huon.