JPS63195529A - optical fiber hydrophon - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (7) Technical field, )'s Hibaku L+ ★Relates to optical fiber hydro 7-on for keeping out eggplants in mountainous areas. In particular, the present invention relates to the simplification of the optical system of an optical fiber hydrophon using only one polarization-maintaining single-mode optical fiber.

Hydro-on is a device that detects underwater acoustics.
It's called a phone. Devices using piezoelectric elements and the like are already in use.

A device that uses an optical fiber as a detection element is called an optical fiber hydro 7-on. Light is passed through an optical fiber, and changes in underwater sound pressure are detected by changes in interference light. A homodyne method and a heterodyne method have been proposed.

The present invention belongs to the category of homodyne systems.

The homodyne system includes a MachZehnder type that uses two single-mode optical fibers, and one that uses one polarization-maintaining single-mode optical fiber. The present invention relates to improvements in the latter optical system.

Homodyne type optical fiber hydrophon has f'ading, which causes the intensity of interference light to fluctuate depending on temperature fluctuations.
There is a phenomenon called. If this happens, the pressure of the sound waves cannot be detected accurately. Eliminating temperature fluctuations was the most important problem in the homodyne system.

Buc was the first to propose the fiber optic hydro 7-on.
aro et al.

J., A., Bucaro, H-D., Dardy an.
d E, F-, Carome, s 0pticalf
'1ber acoustic 5ensor,'A
ppl, Opt, 16.1761-1762 I1 Matsuhatsu Enda Type Optical Fiber Hydrophone The principle of a known Matsuhatsu Enda type optical fiber hydrophone will be explained with reference to FIG.

Coherent monochromatic light emitted from a light source 1 is split by a branch 10 and input into two single mode optical fibers 2.3.

A sensing coil 4 is provided in the middle of the signal fiber 2. This is a coil that is immersed in water to capture underwater sound waves.

The output lights of the signal fiber 2 and the reference fiber 3 are combined by a combiner 13. The light enters the photodetector 5 and interferes there. The intensity of the interference light becomes the output of the photodetector 5.

The strength of the underwater sound waves can be determined by the strength of the interference light.

However, such a simple optical fiber Hydro7on has poor temperature characteristics. It's not very effective in practice. Changes in temperature change the refractive index and length of the optical fiber. Therefore, the effective optical path length changes. This change is much larger than the increase or decrease in optical path length due to underwater acoustics.

Unless temperature compensation is properly performed, such a detection system is completely useless.

Let the angular frequency of the sound wave be Ω. When a sound wave hits the sensing coil 4, the phase of the light when it passes through this coil changes. This is expressed by the total φsinΩt(1). The strength of the sound wave is proportional to φ.

The purpose is to seek φ.

The electric field strength E(t) of the reference light when it reaches the photodetector 5 is E(t) = where G is the amplification and ω is the angular frequency of the light.
It can be written as G 5in (cc+t+φ) (2). φ is a phase difference due to a difference in optical path length between the signal light and the reference light.

The signal light passes through the sensing coil,
It will undergo phase modulation of equation (1). This electric field strength F(t) is F(t) = H5in(ωt+φsinΩt)
It can be written as (3). H is the amplitude.

Output of a photodetector that performs photoelectric conversion! are (2), (8)
This is done for square-law detection of the sum of I =
l E(t) + F(t) l2(4)ωt, 2
Since the term ωt falls, it becomes. I (t) excluding the constant part is I (t)
= GH(m1lJcos(φ51rLit) +
sinφsin (φ5inQt) (6). COS (φ癲Ωt) and 5in (φsinΩt) can be expanded using Bessel functions.

It is. Substituting these into (6), I(t) iΩ
It can be expressed as the sum of the fundamental wave and harmonic components.

All appropriate components are extracted through a filter circuit.

In conventionally proposed optical fiber hydrophons, the fundamental wave component is extracted.

11(t)=2GHsinilJ1(φ)sin (
Ωt ) (9). Since the phase change φ due to underwater sound waves is not very large, if we approximate Jl(φ) = φ/2, we get ■, (t)''; GHφsin sin (
Ωt) (1, 5In(Ωt
) can be used to determine the strength of the sound wave φ.

ri) Difficulties with the Matsuhatsu Enda type However, since there is a term of sinφ, it is not as simple as that. Although it is set to Φ=π/2, this value may deviate due to temperature fluctuations.

The temperatures of the two optical fibers are not the same. Φ changes significantly due to temperature fluctuations. For example, 1♂rad~10ra
It may vary by about d. If Φ=O, the signal disappears, and this is called f'ading.

For example, the temperature change range in which the phase change is π/2 or less is six. This size will be evaluated for a silica glass fiber.

1 m of quartz glass, He-NeL/-the light λ = 68
Let's say 28 people passed through. n=1.4, α=O-4X
10/l, L=1m1. Then ΔT=Q, $℃
It is.

The length of the fiber, including the sensing coil, is several meters at the shortest. Temperature changes range from several tens of degrees to one hundred degrees. It is not possible to control the temperature within the range of 3°C.

The sensing coil is immersed in the target water. It is not practical to control the temperature of water. On the other hand, it must be possible to measure all sound waves in water at any temperature.

For this reason, the optical fiber Hydro 7-on shown in FIG. 2 has not yet been put into practical use.

2) Polarization maintaining fiber type optical fiber HydroFon 2
Since a single-mode fiber is used, the Matsuhatsu Enda type optical fiber hydrophon has extremely significant fluctuations in the phase difference φ due to temperature. It also becomes 10”wIQrad.

To solve this problem, Takahashi et al. proposed the optical fiber Hydro 7-on, which uses one single-mode optical fiber that preserves the plane of polarization.

S, Takahashi, T, Kikuchi, J
, Matsushita & Y., Murakami;
' A Single Fiber Interfe
rometric Hydrophone Using
Polarization-Maintaining
Fiber, “4th International
Conference On Optical Fi
berSensors (OFS 86) p, 39
-p, 42 (1986) Figure 3 shows the entire configuration of this type of optical fiber hydrophon.

Coherent monochromatic light emitted from a light source 1 is converted into linearly polarized light by a polarizer 6. This is made incident on a constant polarization optical fiber 8 having a principal axis x1yl of a plane of polarization that is orthogonal to each other. However, the plane of polarization of linearly polarized light is
The beam is made incident at an angle of 45°kl.

In this way, it can be divided into an X-direction component and a Y-direction component, but
The amplitude becomes the same.

FIG. 4 shows a cross-sectional view of a constant polarization optical fiber.

There is a core 12 at the center and a cladding 15 around the periphery.

The refractive index of the core is slightly larger than the refractive index of the cladding.

Since it is a type of single mode fiber, the core diameter is small. The wavelength of the light sets an upper limit on the core diameter of a single-mode fiber.

In order to achieve rotational symmetry kl<, stress applying part 11.11 2
Place 1. ) 17 with Boo 17 pattern? , Cooking sweet and sour sauce U
lzusu. Therefore, the X-axis direction connecting the stress applying portions 11.11 and the Zy-axis direction perpendicular thereto are not equivalent directions.

Light having a plane of polarization in the X direction and light having a plane of polarization entirely in the X direction propagate independently without mutual energy conversion. Since the effective parts of the refractive index tensors for these lights are different, the propagation constants are different.

The X and Y axes are called the main axes of the fiber. The propagation direction is 2
Become the axis.

The polarization plane 14 of the incident light is made to form an angle of 45° with respect to the X1y axis.

In a constant polarization plane single mode optical fiber, light having a polarization plane in the X direction and light having a polarization plane in the Y direction are separated in terms of energy, so that they are not coupled together. The propagation constants are also different.

In the middle of the constant polarization optical fiber 8, there is provided a sensing coil 4 whose optical path length is effectively changed by changes in water pressure caused by underwater sound waves.

This light is called iT having a plane of polarization in the X direction.

Light having a plane of polarization in the Y direction is called Mi.

These are independent modes due to the anisotropy of the fiber. Since the propagation constants are different and there is no exchange of energy, it is a conservation mode.

The refractive index and length of the sensing coil 4 change in response to changes in water pressure. Therefore, the propagation constant varies. This can also be said to be a change in the phase of the light passing through the coil.

T-light and ternary light have different propagation constants, and these propagation constants change depending on water pressure. This amount of change is different for T light and ternary light. Therefore, a phase difference is caused by a change in water pressure.

In the case of the Matsuhatsu Enda type, the phase difference between the sensor light and the reference light is taken, but in the case of FIG. 3, the phase difference between the T light and the ternary light is taken. Both change in proportion to water pressure. The difference arises because the constants of proportionality are not the same. This difference is small, but it allows us to understand water pressure fluctuations. Naturally, the detection sensitivity is lower than that of the Matsuhatsu Enda type.

However, it has the advantage that phase offset due to temperature changes is extremely small.

A polarization beam splitter 7 is provided at the output end of the constant polarization optical fiber 8 .

This transmits only light with a certain plane of polarization, and reflects light with a plane of polarization perpendicular to this plane. That is, the T light and the ternary light are separated by the polarization beam splitter 7.

For example, assume that the transmitted light is f:T and the reflected light is 3.

Mi, T light is reflected by total reflection mirror 18.18, and synthesizer 1
Unite at 3.

However, for one light, a 172 wavelength plate 16 is inserted to rotate the plane of polarization by 90 degrees.

In order to cause interference, the planes of polarization must match. Therefore, the plane of polarization of any one of T%3 is rotated by 90° to make the planes of polarization coincide.

The light combined by the combiner 13 enters the photodetector 5. The light interferes here, and the intensity of the interference light is extracted as an electrical signal by a photodetector. This is amplified by an amplifier and used as an output.

The principle of such an optical fiber hydrophon is not much different from that of the Matsuhatsu Enda type, and will be explained below.

Assuming the electric field strength t-E(t) of the polarization component of the light that has become linearly polarized by the polarizer 6, E(t) = Asi (ωt)
(11). A is the amplitude. This is xy
Since the plane of polarization is in the direction of the bisector of the axis, the electric field intensity component Fx1Fy in the direction of each principal axis x1y at the input end of the constant polarization optical fiber is as follows. Light Fx and Fy propagate through the fiber.

Electric field strength f of two components at the output end: Gx(t)
, Gy(t). A phase difference of δ occurs between these.

That's it. However, Φ is a constant phase term determined by the fiber length. This is a common part of both. (
12), which is the existing phase delay with respect to (1B).

(1) changes in proportion to the fiber length, but since the propagation constant differs between the T light and the ternary, the portion proportional to the fiber length is not equal for the T light and the ternary. Only the equal parts were designated as (I). Depends on fiber length,
The phase difference that appears due to the difference in propagation constants is included in δ.

Now, assume that a sound wave Po5in (Ωt) is applied to the sensing coil. A phase change synchronized with sin (Ωt) occurs in all three T1 lights. Since the constants are different, this becomes part of the phase difference δ.

δ = δ(1+usjllΩt (16
). u sin Ωt is the change in phase difference due to the sound wave.

U is the difference in the constant of phase fluctuation between the T light having a polarization plane in the X direction and the ternary sound wave having a polarization plane in the X direction.

δ0 is a phase difference due to fiber length, and is caused by a difference in propagation constant.

The Gx and Gy lights (14) and (15) are separated by a polarization beam splitter 7. The plane of polarization of any of the lights is rotated by 90° by the 100-wavelength plate 16.

In this way, the planes of polarization match, so interference can occur. The photodetector 5 performs square-law detection of the interference light.

Except for constants, the output of the photodetector 5 is: I(t) = l GX(t) + Gy(t)
l (17). The frequency of light ω and these two
The frequency part of twice 2ω is dropped by square law detection.

I(t) = −A cosδ + (DC needle)
(18). Since δ is given by (16), I (t) = −A 2cos (δo+
usin(Ωt) ) + (DOJ#) (1
9). If the part of I (t) that varies with time t is '(I-IAC(t)), it is expanded by the Bessel function, and IAqt) = -AC(Jo(u)+
2 Σ J 2 n(u)cos 2nΩt)co
It will be sδ02 days. If only the fundamental wave component is extracted using a noise filter, 11(t) = A J, (u) 5in(Ωt) s
mδO(21). Since Jt(u) includes the strength U of the sound wave, the strength of the sound can be determined from this.

In this case, unlike the Matsuhatsu Enda type, all light propagates through the same fiber, and δ0 is the phase difference of the light, so even if there is a temperature change, δ. The variation in is extremely small.

(a) Problems to be Solved by the Invention The Matsuhatsu Enda type optical fiber buffer 1 shown in FIG. 2 has poor output stability with respect to temperature changes. This is because two optical fibers are used.

The polarization constant optical fiber shown in FIG. 3 requires a 172-wavelength plate 16 in order to rotate the plane of polarization of one light. Furthermore, the optical system is complicated, as it requires two total reflection mirrors. Also, axis alignment is required to combine the two beams.

As described above, there were some difficult points regarding the optical system.

Force Configuration The present invention significantly simplifies the optical system at the output end of an optical fiber hydrophon using a constant polarization optical fiber.

Do not split the emitted light into two beams. Since they are not separated, there is no need to combine them later. This reduces the number of mirrors and eliminates the need for alignment for synthesis.

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

The configuration from the light source 1 to the output end of the constant polarization optical fiber 8 is the same as that shown in FIG.

The light source 1 generates coherent monochromatic light such as a semiconductor laser, a He-Ne laser, or an argon laser.

This light is converted into linearly polarized light by a polarizer 6. ' The polarization constant optical fiber 8 is an optical fiber whose plane of polarization in two orthogonal main axes X1 and Y directions is maintained.

There is anisotropy in dimensions, refractive index, shape, etc. in the x1Y directions, and the plane of polarization is preserved.

The light that has become linearly polarized by the polarizer 6 is made to have a full polarization plane that makes an angle of 45° with the xY principal axis.

When the light enters the optical fiber 8, it is divided into three light beams T1 each having a plane of polarization in the x1 and Y directions, but since the light is made incident at 45°, the amplitudes of these lights are equal.

It does not have to be exactly 45°. It is sufficient that the direction of the polarization plane does not coincide with the X direction or the Y direction.

This light passes through the sensing coil 4, undergoes a phase change due to underwater acoustics, and is emitted from the other end of the fiber.

In the configuration shown in Figure 3, the emitted light is divided into T light and ternary light, but
The present invention does not do such a thing.

A polarizer 17 is installed at the output end. This polarizer extracts only the polarized light components that form an angle of 45 degrees with the two principal axes x1y. It's not a polarizing beam splitter, it's just a polarizer.

A photodetector 5 detects the intensity of the light transmitted through the polarizer. This is amplified through all amplifiers and used as an output.

This is an extremely simple optical system at the output end. However, in this way, the intensity of the interference light of the two lights is detected.

In this way, all of the interference light is detected by simply detecting the intensity of the light that has passed through the polarizer without separating it into two lights.

This point is a novel point of the present invention.

Since some parts are more difficult to understand than the example shown in FIG. 3, the operation will be explained next.

(1) Effect Interference between two lights does not occur unless the planes of polarization of the lights are the same.

In the conventional example shown in Fig. 3, the T light beams with different polarization planes of 90°, three components, are taken out separately, one of the lights is passed through a 172-wave plate, the plane of polarization f is rotated by 90°, and the other light is polarized. It is made to match the wave surface.

Unlike this, in the present invention, only the total soot component at an angle of 45° with the two main axes of the polarization plane of the fiber is extracted from the light propagating along the two main axes of the polarization plane of the fiber and interfered with them.

What is important here is that the light propagated through one constant polarization optical fiber is not divided into two beams, but interference is caused only by inserting the polarizer 17.

The principle is shown below.

Two main axes regarding the polarization plane of a constant polarization optical fiber
y(t), this is given by (14) and (15). It passes through a polarizer 17 and extracts only the components forming an angle of 45° with the x1y axis. This electric field component total Hz (t), H
When y(t), A(23) ==-S11 (ωt+Φ). These are lights with the same plane of polarization.

Therefore, these lights can interfere. Similarly to (4), the output of the photodetector 1 (t) is I (t) = l Hx(t) + Hy(t) l
2 (24). From this, the component of the interference term (cos δ
) is obtained.

Now that we have this term, we can find the size of U from it. This is the same as shown in (19) to (21).

However, when the numerical coefficients are (18) and (25), in the case of the present invention, it is 1/2 when compared. Therefore, (20), (21)
For the expression, 1/2 will be added.

That is, the fundamental wave component I1(t) is I1(t)=-! -A2J, (u) :n (Ω
t) sin δo(26). This is 45°
It comes from the fact that light with a tilted plane of polarization is extracted using a polarizer. When sin (Ωt) is passed through a t-band pass filter, 11(t) is obtained, so Jl(u) can be found. For u<<1, '1('')';;:
Since u/2, 11(t) is approximately linear with the sound wave intensity u.

In this way, as shown in FIG.
T-light and ternary light can be interfered without the need for other components.

The polarizer 17 functions to rotate linearly polarized light having two orthogonal polarization planes by 45°.

Next, it will be explained that if the polarizer 17f is not inserted, there will be no interference. It is clear that even if one with a full electric field in the X direction and one with an electric field in the X direction are vectorially combined, the cross term will be O since xly forms an angle of 90°.

Just to be sure, I will explain it with a formula.

(22) and (23) are the direction T that makes 45° with the x1y axis
This is the electric field component due to 1-mi light. If there is no polarizer, an electric field component in a direction 90° to this axis remains.

If this is Kx(t) and Ky(t), then Kx(t)
= , Acos (-45°) sin (ωt+Φ
10δ)K)'(t) = , -As1n (-45°)
sin (ωt+Φ). The output of the photodetector I (
t), these will be square-law detected separately, so I(t) = l Hz(t) + Hy(t) I + l
Kx(t)+Ky(t) l (29)A2(30
) becomes. The interference term has disappeared. There will be only a DC component. This is natural; the electric field component of the T light in the X direction is Gx(t), and the electric field component of the ternary X direction is G3(t), so I(t) is I(t) = l Gx (t) l” + l Gy(
t) l2(81)” (8
2) and they match.

From these facts, it can be seen that the function of the polarizer 17 is important.

(2) Effects In the polarization-maintaining fiber type optical fin, the conventional one required a polarizing beam splitter, a total reflection mirror, a 172-wavelength plate, and a combiner at the output end. The invention only requires a polarizer.

The entire optical system can be significantly simplified.

Also, since the output light from the single mode optical fin is not divided into two beams, there is no need for axis alignment.

Since axis alignment is a difficult task, it is extremely convenient to set this to zero.

It is possible to provide an optical fiber hydrophone that can reduce the total cost and is easy to operate.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical fiber hydrophon of the present invention using a constant polarization optical fiber. FIG. 2 is a schematic diagram of a known Matsuhatsu Enda type optical fiber hydrophon. FIG. 3 is a schematic configuration diagram of a known polarization-maintaining fiber type optical fiber hydrophon. FIG. 4 is a cross-sectional view of a polarization-maintaining optical fiber. 1...Light source 2...Signal optical fiber 3...Reference optical fiber 4...Sensing coil 5...Photodetector 7... Polarizing beam splitter 8...
... Polarization constant optical fiber 10 ... Brancher 11 ... Stress applying section 12 ... Core 13 ...
Combiner 14 ... Polarization plane of incident wave 15 ... Rirad 16...1
72 wavelength plate 17...Polarizer 18...Total reflection mirror Inventor: Nishikawa
Yozo Mannishiura

Claims (1)

[Claims] A polarization-constant optical fiber 8 is provided with a sensing coil 4 whose optical path length is effectively changed by changes in water pressure caused by underwater sound waves, a light source 1 that generates coherent monochromatic light, and a linearly polarized light source 1 that generates coherent monochromatic light. a polarizer 6 for controlling the polarization, and a photodetector 5 for detecting the intensity of the light emitted from the constant polarization optical fiber 8.
It is an optical system composed of a polarization constant optical fiber 8 and an optical system in which the polarization plane of light from a light source 1 is made to enter the polarization constant optical fiber 8 and the two main axes of the constant polarization optical fiber 8 at an angle of 45°. Between the detector 5, the two principal axes of the polarization-constant optical fiber 8 and 4
An optical fiber hydrophon is equipped with a polarizer 17 that extracts only polarized components forming an angle of 5 degrees.