JPH0715393B2 - Optical fiber sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Technical Field The present invention aims to stabilize the amount of incident light in an optical fiber sensor that constitutes a Matsuhatsu Ender interferometer with two optical fibers. Regarding improvement.

Here, the optical fiber is a single mode optical fiber. Two optical fibers are used. At least one side is provided with a sensing coil for measuring some physical quantity. The light that passes through this optical fiber is called signal light. This fiber is called a signal optical fiber.

The other optical fiber may be provided with a coil equivalent to the sensing coil, but such a coil may not be provided. This optical fiber is called a reference optical fiber.

A light source that emits coherent monochromatic light is used. For example,
Examples include He-Ne lasers and semiconductor lasers. For this light, the fiber is in single mode.

The light from the light source is split into two beams, which are made incident on the signal optical fiber and the reference optical fiber. Then, the emitted lights are united to cause interference.

(A) Prior art Fig. 2 shows a conventional example of Matsuhatsu Ender using optical fiber.
It is a block diagram of a Mach Zehnder type sensor.

The light source 1 is a coherent light source that emits monochromatic light. This is an optical fiber for signal 2 which is a single mode optical fiber.
Then, the light enters the reference light fiber 3 at the same time. Therefore, the beam splitter 10 splits the light.

The signal light passes through the sensing coil 4. The sensing coil is affected by temperature and pressure. Therefore, it can be used as a thermometer or a pressure gauge. As an example, a hydrophone (Hydrophon) for detecting the intensity of underwater sound waves
There is a use as e). Hikari fiber hydrophone.

It can be used as a sensor for various physical vibrations as well as underwater sound waves. Of course it can also be used as a thermometer.

Since the refractive index and the length of the optical fiber of the sensing coil change depending on the temperature and the pressure, the optical path length of the optical fiber substantially changes. Therefore, the phase of light changes here.

The signal light and the reference light 3 are integrated by a beam splitter (for multiplexing) 13 and are incident on the photodetector 5. Two lights interfere at the light receiving surface. The interference light intensity is measured by the photodetector 5. This is amplified to obtain an output signal.

Let S be the electric field strength of the signal light on the light receiving surface of the photodetector 5. The amplitude of this is S ₀ . Let ω be the angular frequency of light.

The change in phase in the sensing coil is a _sin Ωt.
Here, a is the strength of pressure, and Ω is the angular frequency of pressure change. In the case of the optical fiber hydrophone, a is the strength of the underwater sound wave, and Ω is the angular frequency of the underwater sound wave.

The electric field strength S of the signal light is given by S = S _{0 sin} (ωt + a _sin Ωt) (1). The electric field intensity R of the reference light is represented by R = R _{0 sin} (ωt + Φ) (2). Φ is the phase difference caused by the difference in the length of the two optical fibers. This varies with temperature. (−Φ) may be attached to S, but in this case, R is added as (+ Φ). Same thing.

In the photodetector 5, since these interfere, the output is Becomes Vibration part sin (2ωt) that is twice the angular frequency of light
Is a photodetector, so it falls.

The first item is the DC component and does not become a signal. The second item is the signal. This is because it has a parameter a proportional to the strength of pressure such as sound waves.

Since the phase shift Φ changes greatly with temperature, it is necessary to devise to keep Φ constant when measuring the parameter a.

In the case of a thermometer, the phase shift Φ (T) can be obtained by applying the phase modulation a _sin Ωt having a constant strength in advance.

In any case, the output includes the amplitude S _{0 of the} signal light and the amplitude R _{0 of the} reference light.

Bucaro was the first to propose the optical fiber hydrophone.
Etc.

JABucaro, HDDardy and EFCarome, “Optical fibe
r acoustic sensor, "Appl.Opt. 16 , 1761-1762 (197
7). And JA Bucaro and HDDardy, “Fiber-optichydrophon
e, "J. Acoust. Soc. Am. 62 , 1302-1304 (1977).

It is necessary to extract the phase shift Φ from the intensity of the interference light and perform temperature compensation. To extract the phase shift Φ, Jackson et al. Have proposed a detection system using a bottle skipper. This is to combine the reference light and the signal light not by a half mirror but by a bottle cutter that performs evanescent coupling. The two outputs of the bottle skipper are 1/1 of the wavelength of the light when viewed from the signal optical fiber and the reference optical fiber.
It is adjusted so that there is an optical path difference of 4. Then, while the can _{(I 0 + I 1 cos Φ} ), the other is (I ₀ -I _{1 cos} Φ) is outputted. By differentially amplifying this, the phase shift Φ
It means that

DAJackson, R. Priest, A. Dandridge and AB, Tveten,
“Elimination of drift in a single−mode optical f
iber interferometerusing a piezoelectrically stret
ched coil fiber, "Appl. Opt. 19 ,, 2926-2929 (1980). Furthermore, as a Levy of optical fiber hydrophone written in Japanese, Takahashi, Kikuchi" Underwater sound wave detection method using optical fiber I ""The Acoustical Society of Japan, Vol. 40, No. 2, p. 101-106 (1984) is available.

In order to solve the problem of fluctuation of the phase shift Φ due to temperature, the applicant of the present invention has invented an optical fiber hydrophone capable of removing the influence of the fluctuation of Φ by taking out a 2Ω vibration part (Japanese Patent Application No. 61- 119989, S61.5.24 application).

The Applicant has further added a phase modulator to
We have invented a method that can detect the phase shift Φ more easily (Japanese Patent Application No. 61-237405, S61.10.6 application).

Furthermore, there is an optical fiber gyro as a tachometer using an optical fiber. This has a relativistic effect Sagnac
It uses the effect.

When the optical fiber core is rotated, the speeds of left-handed light and right-handed light are different. Therefore, when these two lights are made to interfere with each other, the rotation speed can be known from the phase difference Φ.

Normally, one optical fiber with one sensor coil is used to pass light to the left and right. The emitted lights are combined by a beam splitter and interfered by a photodetector.

If the phase difference Φ due to rotation Ω is Δθ, then Sa
By the gnac effect, The phase difference of appears. Ω is the rotational speed of the coil, r is the radius of the coil, L is the length of the coil, k is the wave number of light, and C is the speed of light.

(C) Problems to be solved by the invention From the equation (3), the strength of the signal component is multiplied by S ₀ and R ₀ . If S ₀ and R ₀ change, the strength of the output signal becomes unstable.

The AC component of equation (3) Then, Then, by the expansion of the Bessel function, Becomes

Includes all harmonics from the fundamental wave of Ωt.

Since it is not possible to investigate all harmonics, we choose a specific order. Since J ₀ (φ) is a DC component, it is difficult to detect, and when φ is close to 0, it is 1 so that the sensitivity is poor.

Therefore, J ₁ (φ) is often used. Other than this will be discarded. Using a bandpass filter, sin (Ω
Only the frequency of t) should be passed. This output
If Y ₁ is written, then Y ₁ = 2 S ₀ R ₀ J ₁ (φ) sinΩt (7) as a part of equation (4). Thus, as the amplitude of sin (Ωt), J
₁ (φ) is obtained. When φ is close to 0, it is ~ φ / 2, so it is obtained as a function form with good linearity.

However, the amplitudes S ₀ and R _{0 of} the light may change. In this case, it cannot be distinguished from the case where φ changes.

If there is a second-order term, it is possible to distinguish the fluctuation of φ from the fluctuation of S ₀ R ₀ . If φ << 1, Y ₁ and
The quadratic term Y ₂ is Y ₁ S ₀ R ₀ φ (8) Becomes (9) is the amplitude component of the term of cos (2Ωt). By doing so, the fluctuation of the amplitude can be excluded by setting Y ₂ / Y ₁ . That is, It is because

However, this requires two sets of bandpass filters, amplifiers, and the like. A division circuit is also required. There is a drawback that the circuit configuration becomes complicated.

(D) Object It is an object of the present invention to provide an optical fiber sensor which can eliminate the influence of amplitude fluctuations of signal light and reference light without complicating the configuration of an electric circuit.

(E) Configuration In the present invention, the intensity of the reference light and the signal light is monitored, and further, a light quantity adjusting mechanism is provided at the axis alignment portion of the optical fiber entrance end to check the degree of coupling between the light source and the optical fiber. The intensity of the reference light and the signal light was adjusted to be constant.

An electrostrictive element or the like can be used as the light amount adjustment mechanism.

It will be described with reference to the drawings. FIG. 1 is a block diagram of an optical fiber sensor of the present invention.

The light source 1, the signal optical fiber 2, the reference optical fiber 3, the photodetector 5, and the beam splitters 10 and 13 form a Matsuhatsu Ender interferometer. This point is the same as that of FIG.

The light source 1 is a coherent light source that produces monochromatic light. For example, a He-Ne laser or a semiconductor laser is used.

This monochromatic light is split into two light beams by the beam splitter 10 and is incident on the signal and reference light fibers 2 and 3.

These optical fibers are single mode fibers with respect to the wavelength of incident light. Since the core diameter is extremely small,
A condensing optical system such as a lens is provided in the axis alignment unit. Illustration is omitted here. In the present invention, the light amount adjusting mechanism 6,15 for adjusting the degree of coupling between the light source and the optical fiber,
It is provided at the incident ends of the signal and reference optical fibers 2 and 3.

This displaces the optical fiber axis in a direction perpendicular to the optical fiber axis. Any combination such as an electrostrictive element, a combination of a servo motor and a gear reducer is possible.

Further, the method of changing the optical path is not limited to the method of mechanically displacing, and the method of changing the optical path by causing birefringence by an electro-optical element or the like.

In the middle of the signal optical fiber 2, there is a sensing coil 4 that senses pressure, temperature and the like.

When the length of the sensing coil is L and the refractive index is n, the phase change with respect to the pressure change P Becomes λ is the wavelength in vacuum.

Is the phase shift per unit length per unit pressure.

μ is sometimes called a coupling coefficient. A in equation (1) is Will be given by.

In the case of temperature change, since it is not an oscillation term, the phase shift Φ
Change. Similarly, if the increment due to this is indicated by δΦ, It means that. The temperature can be measured from δΦ.

Now, the signal light S and the reference light R propagate through the optical fibers 2 and 3 and are emitted from the other end. Beam splitters 9 and 11 for extracting monitor light are provided near the emission end.

A part of the signal light S is reflected by the beam splitter 9 to become monitor light U, which is incident on the photodetector 8. Here, the intensity of the monitor light is known.

A part of the reference light R is reflected by the beam splitter 11, becomes a monitor light W, and enters the photodetector 7. Here, the intensity of the monitor light is known.

(F) Action The intensity signals of the monitor lights U and W are amplified and given to the light amount adjusting mechanisms 6 and 15 through the signal lines 16 and 17.

When the monitor light becomes larger than the normal value, the light amount adjusting mechanism deviates the axis of the fiber to reduce the incident light amount.

When the monitor light becomes smaller than the normal value, the light quantity adjusting mechanism moves the axis of the fiber toward the center of the light beam to increase the incident light quantity.

In this way, S ₀ and R ₀ can be kept constant. Further, the beam splitter 13 integrates the signal light S and the reference light R. This enters the photodetector 5 and interferes with it. The intensity of the interference light is amplified by the amplifier 14 and becomes an output signal.

The photodetectors 7 and 8 and the beam splitters 9 and 11 monitor the intensities of the reference light R and the signal light S. Then, according to this intensity signal, the degree of axial coupling of the light amount adjusting mechanisms 6 and 15 is changed, and R ₀ and S ₀ are returned to constant values.

Thus, if R ₀ and S ₀ are constant, the primary component of the output Y ₁
Alternatively, the pressure amplitude a can be correctly obtained from the secondary component Y ₂ .

The light amount adjustment mechanism may use an electrostrictive element whose strain amount changes depending on the voltage. The core diameter of the single mode fiber is about 5 μm to 10 μm, though it depends on the wavelength of light. It suffices if the fiber incident end can be displaced in the direction perpendicular to the axis by a distance about half the core diameter.

Alternatively, a servo motor and a multi-stage gear reducer are provided so that the fiber entrance end is moved in the direction perpendicular to the axis. If you put a worm or worm gear in the gear reduction system, you can kill the backlash and make fine adjustments.

The one shown in FIG. 1 is used as a pressure and temperature sensor.

The present invention can also be applied to an optical fiber gyro. In this case, there is only one optical fiber, and the input and output are the same. As shown in FIG. 3, the beam splitters 10 and 13 are integrated, and the beam splitters 11 and 9 are arranged in front of the beam splitter 10. The action is similar.

(G) Effect The light intensities of the reference light R and the signal light S entering the photodetector 5 are stable. Therefore, the amplitude part of the output of the interference light becomes constant.

If only one of the first-order component Y ₁ or the second-order component Y _{2 of the} interference light is taken out as the output signal,
If S ₀ and R ₀ change, the exact value of the intensity signal φ cannot be known. In the present invention, since S ₀ and R ₀ can be made constant, the intensity φ can be accurately obtained from the output.

Also in the case of the optical fiber gyroscope, the intensity of the clockwise light and the counterclockwise light can be stabilized, so that the phase difference Δθ can be accurately obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of the present invention when applied to an optical fiber hydrophone. FIG. 2 is a block diagram of an optical fiber hydrophone according to a conventional example. FIG. 3 is a block diagram of the present invention when applied to an optical fiber gyro. 1 ...... Light source 2 ...... Signal optical fiber 3 ...... Reference optical fiber 4 ...... Sensing coil 5 ...... Photodetector 6, 15 ...... Light intensity adjustment mechanism 7, 8 ...... Photodetector 9, 10, 11 , 13 …… Beam splitter 14 …… Amplifier

Claims (1)

[Claims] 1. A signal single mode optical fiber having a sensing coil 4 for sensing a physical quantity such as temperature, pressure and rotation speed, or a signal single mode optical fiber having a sensing coil 4 and a reference. The light generated from the coherent light source 1 that generates monochromatic light is branched and made incident on each start end of the optical fiber, and the light that propagates through the optical fiber and is emitted from the other end is caused to interfere with each other. Then, in the optical fiber sensor which is designed to measure the physical quantity by detecting the intensity of the interference light, a light quantity adjusting mechanism for adjusting the coupling degree between the optical fiber core and the light source is provided at the incident end of the optical fiber, and the optical fiber sensor is provided. Beam splitters 9 and 11 are provided at the exit end of the beam splitter to split the outgoing light and detect the
The optical fiber sensor is characterized in that the intensity of emitted light is detected by means of 8 and the position of the incident end of the optical fiber is adjusted by means of a light quantity adjusting mechanism so as to make the amount of emitted light constant.