JPS63172928A - Optical fiber hydrophone - Google Patents

Abstract

PURPOSE:To eliminate the need for phase compensation by giving a sin (GAMMAt) signal a saw-tooth phase shift, detecting GAMMAt which is a component of its interference light with a sin GAMMAt ana a cos (GAMMAt) signal synchronously, and summing their squares and extracting the square root. CONSTITUTION:The sin GAMMAt generated by an oscillator 13 is shifted in phase in a sine-wave shape by a saw-tooth wave generating circuit 12 so as to increase in phase continuously from 0 to 2mpi while its period is 2pi/(mGAMMA). A phase modulator 11 increases the phase at a rate GAMMAt and resets the phase to 0 when t=2pi/(mGAMMA). Them interference light contains a component GAMMAt. This is detected synchronously by synchronization detecting circuits 15 and 16 with the sin GAMMAand cos GAMMAt signals. Those synchronously detected signals are squared by squaring circuits 17 and 18 and added by an adding circuit 19 and a square root arithmetic circuit 20 extracts the square root.

Description

Detailed Description of the Invention (1) Technical Field The present invention relates to a homodyne optical fiber hydrophone that does not require phase compensation.

An optical fiber hydrophone is a device that uses optical fibers to detect the magnitude of underwater sound.

A signal optical fiber with a sensing coil and a reference optical fiber are combined, a monochromatic light source and a beam splitter,
Build a Matsuhatsu Enda-type interferometer using a photodetector.

The sensing coil is immersed in water and senses pressure fluctuations from sound waves. Pressure fluctuations change the refractive index and length of the optical fiber.

Therefore, the phase of light passing through the sensing coil varies. When the light that has passed through the reference optical fiber and the light that has passed through the signal optical fiber are caused to interfere, phase fluctuations will occur in the intensity of the interference light.

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

However, due to temperature changes, the refractive index and length of both the signal fiber and the reference optical fiber change.

Therefore, the phase difference V between the signal light and the reference light varies depending on the temperature. This is quite large. The phase difference F(τ) is detected, phase compensation Φ(τ) is performed, and F(τ
)−Φ(τ)=constant is the conventional method.

That is, in the conventional homodyne type optical fiber hydrophone, the most serious problem is the drift of the phase difference V due to temperature, and this has to be phase compensated.

The present invention can provide an optical fiber Hydro 7-on that can accurately detect the intensity of sound waves without phase compensation.

(b) Conventional technology Buc was the first to propose an optical fiber hydrophone.
aro et al. J., A., Bucaro, H.
, D., Dardy and E., F.

Carome, “0ptical fiber
acoustic 5ensor,” Appl,
Opt.

A, 1761-1762 (1977).

J, A, Bucaro and H, D, D
ardy, ” Fiber-Optic hydro
phone, 'J, acoust, Soc, A
m, 62-, 1302-1304 (1977).

Early examples are described in .

Since then, many proposals have been made regarding phase compensation methods. Phase compensation is often performed by changing the length of the optical fiber using a piezoelectric element. All of these have a narrow range of phase compensation.

Therefore, Jackson et al. proposed a phase compensation mechanism in which an optical fiber is wound around a thin cylindrical piezoelectric element. Electrodes are formed on the inner and outer surfaces of the thin cylinder, and a voltage is applied between the outer and inner surfaces of the cylinder. Thus, the diameter of the cylinder changes proportionally to the voltage. As the diameter changes, the optical path length of the optical fiber also changes.

D., A. Jackson, R. Priest.
A, Dandridge and A, B, Th
eten.

Elimination of drift in a
single-mode optical fiber
r interfero-meter using a
piezoelectrically 5trec
hedcoil fiber, ”Appl, Opt,
19.2926-2929 (1980).

Jackson uses a bottle coupler in order to obtain the -phase difference V. This is a bottle coupler that holds a signal fiber and a reference fiber close to each other on the order of wavelength. The output of the coupler is detected by two photodetectors. Since both fibers are close to each other in the coupler, they are coupled by an evanescent wave.

The coupling causes the electric field of one optical fiber to bleed into the other optical fiber. Since the optical path is longer by λ/4 when entering, there is a delay of this amount.

Then, the output of one photodetector is (I□+11cosF
)become. The output of the other photodetector is (10+11c
osF).

By differentially amplifying this, ctISF can be found. V=π/
Maintaining it at 2 provides the highest sensitivity. So 1's π/2
Know the deviation from. Knowing this, the phase compensation mechanism (F
-π/2) is applied. Thus C05
This is done so that F=0.

However, the variation in the phase difference V due to temperature is 10,000
It can be as much as radians. Conventional phase compensation mechanisms can compensate only about 10 radians. Jackson writes that compensation of about 6,000 radians can be achieved with a thin-walled piezoelectric cylindrical element.

This is because compensation of about 2π can be achieved per volt, and an voltage of about too v can be applied to the piezoelectric element.

However, it is inconvenient to use a high voltage of 1000v.

If the power supply is at most ±15V, the compensation range will be about 90 radians.

Of course, the phase compensation only needs to be 2'11, so
It may be 10 radians or 90 radians. However, when the upper limit of phase compensation is reached, it is necessary to switch to the lower limit. This switching is done by rounding down the angle by an integer multiple of 2π.

This range switching is an extremely complicated operation.

For example, in the case of a phase compensation mechanism with a dynamic range of 90 radians, if a phase change of 9000 radians occurs, the range must be switched 100 times.

A review of optical fiber hydrophones written in Japanese is Takahashi, Kikuchi “Detection method of underwater sound waves using optical fiber 1”
, Journal of the Acoustical Society of Japan, Vol. 40, No. 2, p. 101-106 (1
984).

Takahashi, Kikuchi "Optical Fiber Hydrophone", Electronic Ceramics '84 Spring Issue, p. 5l-56 (1984)
and so on.

The present applicant has invented an optical fiber hydrophone that extracts and measures the 2Ω vibration part in order to avoid the problem of phase fluctuation in the homodyne system (Japanese Patent Application No. 1199-1981).
89, S61.5.24 application).

The applicant has also invented a device for detecting the phase shift V using a method different from Jackson's method (Japanese Patent Application No. 61-237405, filed on October 6, 1983).
.

(1) Problems to be Solved by the Invention In order to perform phase compensation in a homodyne type optical fiber hydro 7-on, a phase compensation mechanism is required to detect a phase difference 1 and adjust the phase Φ to cancel it. must be given to Moreover, since the range of phase compensation is narrow, the range of the phase compensation mechanism must be changed frequently.

The precision of phase compensation must also be high.

It is difficult to satisfy all of these conditions.

00 Purpose It is an object of the present invention to provide a homodyne optical fiber hydrophone that does not require phase compensation at all.

Since phase compensation is not required, there is no need for a detection section for the phase difference V, and there is no need for a phase compensation mechanism.

(4) A component oscillator generates a signal of sin(Γt). This is used to provide a sawtooth phase variation in which the phase is continuously increased from 0 to 2 mπ with a period of 2717 mr. The phase modulator increases the phase at a rate of rt, t = 2X/
Reset with ml' to return the phase to O. r in the interference light
The component of t is included. This is synchronously detected using a signal of 5 lnpt and cos 7"t. The synchronously detected values are squared and added together to find the square root. This is the output.

This will be explained below with reference to the drawings.

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

The coherent light source 1 that emits monochromatic light is, for example, He-N.
These include e-lasers and semiconductor lasers. The light generated from the light source 1 is split into two beams by the beam splitter 7.

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

In the middle of the signal optical fiber 2, an underwater sound wave φsin(Ω
A sensor coil 4 is provided which senses e). The refractive index and length of this material vary as a result of changes in water pressure. Then, the phase of the signal light propagating through this is the amplitude φ
, which varies with the angular frequency Ω.

Ω is equal to the angular frequency of the sound wave. φ is proportional to the intensity of the sound wave, the length of the optical fiber, the refractive index of the optical fiber, the ratio of change in length to pressure, etc. Therefore, φ is proportional to the strength of the sound wave, and can be considered as the strength of the sound wave. Determining φ is the purpose of the fiber optic hydro7on.

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

However, in the present invention, there is no problem even if there is a variation in phase difference, so there is not a strong requirement that the lengths of both optical fibers be equal. Therefore, the reference coil 5 can also be omitted.

A phase modulator 11 is provided on either 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 predetermined phase modulation by the phase modulator 11 .

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

The signal light S and the reference light R are combined by a beam splitter 8 for multiplexing and enter the light receiving element 6. Since these lights are monochromatic lights emitted from the same light source, they interfere on the surface of the light receiving element. The intensity of the interference light is measured by a light receiving element.

This is amplified by amplifier 9. Let this output be ■.

The output ■ includes a fundamental wave of Ωt and harmonic components on mΩ. The bandpass filter 10 extracts only the component of the fundamental wave Ωt. Let this be J.

The signal of the phase modulator 11 is generated by converting the signal of the oscillator 13 into a sawtooth wave and applying it to the phase modulation element. The vibration of the oscillator 13 can be expressed by asinFt. The sawtooth wave generating circuit 12 generates a sawtooth wave from this.

The synchronous detection circuit 15 uses the signal sinrt of the oscillator 13 to synchronously detect the output J. Synchronous detection is a detection method that extracts a signal component having the same frequency and phase as 5lnrt from a certain waveform. Specifically, 5lnrt and its waveform may be multiplied and averaged.

The 90° phase shifter 14 shifts the phase of the signal from the oscillator 13 by 90°. This can be configured by a PLL. By shifting the phase by 90°, a signal of =nr t is obtained.

The synchronous detection circuit 16 synchronously detects the output J using CQS7'i.

1 L is the square circuit 17 for the output of the synchronous detection circuit 15 and 16
．． squared by 1g. This is done by inputting the same signal to the two inputs of the multiplier circuit.

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

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

(2) Effect The electric field strength of the signal light S on the light receiving surface of the light receiving element 6 is S =
G51n(ωt+φsin(Ωt))
It is expressed by (1). G is the amplitude, ω is the angular frequency of light, and φsin (Ωt) is the phase change due to the sound wave.

Since the reference light R has undergone phase modulation, R= Hsin
(ωt +rt +F) (2). The reason why rt is included in the distortion due to phase modulation will be explained later. V is the phase difference due to the difference in length between the two optical fibers. The major problem was that this value changed depending on the temperature.

The outputs of the light receiving element 6 and the amplifier 9 are obtained by square-law detection of the sum of S and R. If you exclude photoelectric conversion coefficients, etc., it becomes . Only the fundamental wave component of (Ωt) is extracted using a bandpass filter. This reduces the DC component and the harmonic component of mΩ.

Use the generating function expansion of the Bessel function.

It is. The following explanation assumes that when φΩ>>r, the bandpass filter passes only the sin(Ωt) component. In reality, the frequency is Ω±Cr/φ).

The center frequency of the Bandhus filter is Ω, and the band is 2r/
It is sufficient if it is φ or more.

Now, (3L (4L) From (5), the output J of the bandpass filter is J=2GHJ, (φ) sin (7't+F ) sin
(Ωt) (6).

The vibration of the oscillator 13 is sinrt. When J is synchronously detected using this, the following multiplication will be performed.

The integration range is 0 to 2π/r.

Furthermore, since the 90° phase shifted cOsrt and J are synchronously detected, the synchronous detection output L is K=GHJ1(φ) cos y sin (Ωt)
(9) L=GHJ, (φ)S! I'l W
This means sin (Ωt) Qd. What should be noted here is that the phase difference V is sin, co
This means that it is in the form of an s.

Square circuits 17 and 18 square the signals, and an adder circuit 19 adds them.

M = K2QI) N = L2 (I2) 0 = M + N Q3) Since the square root calculation circuit 20 performs the calculation of (14) with P = (, P = 2GHJl (φ) sin (Ωt)
j15), and a signal without phase difference γ can be obtained.

The synchronous detection circuit, squaring circuit, and square root calculation circuit can be constructed using analog multipliers.

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

(e) How to give a sawtooth wave In the phase modulator, the maximum modulation phase is assumed to be 2mπ.

m is an integer. As shown in FIG. 2, the phase modulation signal is provided in a sawtooth pattern, but the period of the sawtooth is not 2π/F but m times this, 2mπ/r.

In other words, the phase modulator is reset every T=2mπ/F.

In this case, the slope of phase modulation divided by time becomes r.

Then, rt becomes a variable in sin. The range of rt is O~2mπ, but since it is in sin, the phase modulation of 0~2π is actually repeated m times.

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

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

If a signal with a period of 2 myr/r is given, it is easy to convert it into a sawtooth wave.

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

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

Alternatively, the phase may be changed by winding a fiber around a piezoelectric element and applying a voltage.

Now, since the phase modulator also has temperature fluctuations, t=
It is also possible that the phase is not 2mπ at 2mπ/r. This is because although the time axis is accurate, phase modulation is subject to temperature changes. However, this is not a problem. Even if the maximum phase deviates from 2m7'l, this difference is only added to the phase offset V.

In the present invention, since the mechanism is such that the phase offset 1 disappears, it does not matter if V fluctuates.

(i) Effects In a homodyne optical fiber hydrophone, a phase compensation circuit that compensates for fluctuations in 1 due to temperature fluctuations is no longer necessary. This makes it possible to construct an optical fiber hydro 7-on with stable performance despite temperature changes.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical fiber hydrophone according to the present invention. Figure 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・・・・・・・・・Photodetector 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...Synchronized detection circuit 17.1
8... Square circuit 19...
...Addition circuit 20...Square root calculation circuit Inventor: Yozo Nishiura Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (1)

[Claims] The light emitted from the light source 1 that produces monochromatic light is branched and input into the signal optical fiber 2 and the reference optical fiber 3, which are single mode fibers, and the lights emitted from the respective fibers 2 and 3 are combined to generate interference light. In an optical fiber hydrophone, the intensity is detected by a light receiving element 6, and the phase change due to the underwater sound wave sensed by the sensor coil 4 provided in the middle of the signal optical fiber 2 is determined from the intensity of the interference light.
The oscillator 13 generates a signal of sin(Γt), and the phase modulator 11 provided on one or both of the reference optical fiber 3 and the signal optical fiber 2 generates a signal with a maximum phase shift of 2πm and a period of 2π/Γm. Add sawtooth phase modulation,
After the intensity of the interference light is converted into an electrical signal by the light receiving element 6, only the component of the internal sound wave angular frequency Ω is extracted from the signal by the bandpass filter 10, and this signal is transmitted to the oscillator 13.
signal sin(Γt) and cos which is phase-shifted by 90°
(Γt), square the outputs of the synchronous detection, and add these together.
An optical fiber hydrophone that is characterized by finding the intensity of sound waves by finding the square root.