JPS6287813A - Two-wavelength system optical sensor utilizing optical fiber - Google Patents

Abstract

PURPOSE:To make it possible to detect the change of a physical phenomenon with high accuracy by eliminating the difference in light transmissivity based on wavelengths, by removing a higher-order mode component to obtain only a lower-order mode component. CONSTITUTION:A connector 29 is connected to the end part of a connection optical fiber 28 led out from an optical multiplexer 15 to the side of a photoelectric converter element 17 and a connector 31 is connected to the end part of the connection optical fiber 30 led out from the element 17 to the side of the multiplexer 15. Connectors 33, 34 are connected to both ends of a transmitting optical fiber 32 having an optical sensor element 16 interposed to the intermediate part thereof. One connector 33 is connected to the connector 29 of the connection optical fiber 28 and the other connector 34 is connected to the connector 31 of the optical fiber 30. A loop L is formed to the optical fiber 28 led out from the multiplexer 15 in order to remove a higher-order component by imparting distortion to the core covered with the clad of the optical fiber 28. By this method, the adverse effect of the high-order component from the refraction of the optical fiber is removed to make it possible to perform detection with high accuracy.

Description

[Detailed description of the invention] <Industrial application field> The present invention is a temperature sensor using an optical fiber.

pressure sensor. The present invention relates to optical sensors such as electric field sensors that use optical fibers with two wavelengths.

<Prior art> As an optical sensor using a single wavelength optical fiber, as shown in Fig. 2,
There is something like the one shown in Figure 3.

FIG. 2 shows a transmission type optical fiber-based optical sensor.

In FIG. 2, 1 is an electro-optical conversion element (Elo),
2 is a transmission type optical sensor element, 3 is a photo-electric conversion element (0
/E), 4 is the electro-optical variable element 1a and the optical sensor element 2
An optical fiber 5 connects the optical sensor element 2 and the optical-electric conversion element 3. The optical-electrical conversion element 3 is connected to a measuring instrument. The optical sensor element 2 has a light transmittance that changes depending on changes in physical phenomena such as temperature, pressure, and electromagnetic field.

FIG. 3 shows a reflective optical fiber-based optical sensor.

In FIG. 3, 6 is an electro-optical conversion element (E/○), 7
8 is a reflective optical sensor element, 8 is a photo-electric conversion element (0/
E), 9 is a directional coupler, 10 is an electro-optical conversion element 6
and the directional coupler 9; 11 is an optical fiber that connects the directional coupler 9 and the optical sensor element 7;
Reference numeral 12 denotes an optical fiber that connects the directional coupler 9 and the optical-electric conversion element 8. The optical-electrical conversion element 8 is connected to a measuring instrument. The optical sensor element 7 has a reflective mirror attached to its back surface. The optical sensor element 7 has a light transmittance that changes depending on changes in physical phenomena such as temperature, pressure, and electromagnetic field.

In the case of this optical sensor using optical fiber, the optical sensor element 2
．． The physical quantities are measured by measuring the amount of light that changes in response to physical quantities such as temperature, pressure, and electromagnetic field.

In the case of such an optical sensor using optical fiber, any loss occurring in areas other than the optical sensor element 2.7 causes a total error. Examples of such errors include errors caused by bending of optical fibers and connector connections.

Therefore, in order to eliminate such errors, the two-wavelength method described below is usually adopted.

FIG. 4 is a schematic diagram of a conventional two-wavelength type optical fiber-based optical sensor.

In Fig. 4, 13 and 14 are electrical-to-optical conversion elements such as LEDs (light emitting diodes), 15 is an optical multiplexer, 16 is an optical sensor element, and 17 is an optical-to-electrical converter such as a photodiode or an avalanche photodiode. Conversion element, 18
~21 is an optical fiber, 22~27 is a gradient index lens (
SELFOC lenses). The optical-electrical conversion element 17 is connected to a measuring instrument.

An optical sensor element 1 in which the amount of transmitted light of a specific wavelength λ1 changes when a certain physical phenomenon (temperature, pressure, electromagnetic field) is applied
6, in order to eliminate losses other than losses caused by the optical sensor element 16, for example, losses caused by bending of the optical fiber 20, 21, connector connection, etc., a different wavelength λ is used.
The second light is used as a reference light.

The optical sensor element 16 changes the wavelength λ according to changes in physical phenomena.
The amount of light at wavelength λ2 changes, and the amount of light at wavelength λ2 does not change.

However, the loss due to bending of the optical fiber and connector connection is caused by the same wavelength λ1. It also changes with the light amount of λ2.

This eliminates losses other than the optical sensor element 16.

Expressing this in a formula is as follows. If the detection voltage of the signal light Ls with the wavelength λ is s, and the detection voltage of the reference light Lr with the wavelength λ2 is Vr, then Vs=A, ・αC1・αS
・・・・・・・・・+11Vr=A2・α
C2......(2). However, here, A, , A2 are constants determined by the output of the electro-optical conversion elements 13 and 14 and the sensitivity of the optical-electrical conversion element 17, αcl
+αC, is the light transmittance other than the optical sensor element 16 that occurs during transmission, and αS is the light transmittance at the optical sensor element 16.

The ratio between the detection voltage Vs and the detection voltage Vr is ■. Then, V,=Vs/Vr A, ・αC2 becomes.

If αC1-αC2 can be set,
The voltage ratio v0 is Va = (At / Az > ・Ct 3
---(41. In other words, the light transmittance αcI+αC2 other than the optical sensor element 16 that occurs during transmission is canceled out, and A
Since t/A2 is constant, the voltage ratio is ■. is a function only of the light transmittance αS occurring in the optical sensor element 16, and the voltage ratio ■. By detecting , changes in physical phenomena can be detected.

<Problems to be Solved by the Invention> However, in the case of the conventional example having such a configuration, there are the following problems.

In this optical sensor using two-wavelength optical fibers, the optical fiber 20 connects the electric-to-optical conversion element 13 and the optical sensor element 16, and the optical fiber 20 connects the optical sensor element 16 and the optical-to-electrical conversion element 17. When the optical fiber 21 is bent, the wavelength λ
The light amount of the signal light Ls of 1 and the light amount of the reference light L" of the wavelength λ2 change, but the rate of change, that is, the light transmittance, is different between the signal light Ls and the reference light Lr.

That is, αC1≠αCz.

The reason for this is that the mode distribution within the optical fiber 20.21 is different between wavelength λ1 and wavelength λ2.

For example, when the mode distribution has many high-order modes, and when the mode distribution has only low-order modes, the optical fiber 20
．． For the same bending of the optical fiber 20, 21, the loss in the optical fiber 20, 21 is greater in the former mode distribution.

When αC7≠αCl, in equation (3),
αC1+αC2 cannot be canceled out, and the detected voltage ratio ■. Errors occur, making it impossible to detect changes in physical phenomena with high precision.

The present invention has been made in view of these circumstances, and by removing high-order mode components and reducing only low-order mode components, the present invention eliminates wavelength-related problems that tend to occur at the joints or bends of optical fibers. The purpose is to eliminate differences in light transmittance and enable highly accurate detection of changes in physical phenomena.

Means for Solving the Problems> In order to achieve the above object, the present invention has the following configuration.

That is, the optical sensor using a two-wavelength optical fiber according to the present invention includes an optical multiplexer that multiplexes signal light and reference light that have different wavelengths, and transmits the multiplexed signal light and reference light. It comprises an optical fiber and a loop formed by winding at least a portion of the optical fiber to give a strain of about 083 to 5.0%.

Effect〉 The effects of the configuration of the present invention are as follows.

(i) When an optical fiber is bent, it causes distortion, and this distortion causes loss in optical transmission.

The light transmittance caused by this distortion is generally different between the light transmittance αC8 at a wavelength λ on the short wavelength side and the light transmittance αC2 at a wavelength λ2 on the long wavelength side.

However, as the results obtained in the experimental example explained later,
By winding at least a part of the optical fiber to form a loop and appropriately selecting the strain rate, the light transmittance αC1 for the wavelength λ1 and the light transmittance αCt for the wavelength λ2 can be made to be the same or equal to each other. They can be substantially the same.

As a result of experiments, it was found that such a strain rate is about 0.3 to 5.0%. Also, the distortion rate is 0.8 to 2.0
It has also been found that it is more preferable to make both the light transmittances αC1+αC2 substantially the same in the case of a certain degree.

Even when the strain rate exceeds 5.0%, the difference between the light transmittance αc and the light transmittance αC2 is sufficiently small. It cannot be adopted because there is a risk of

In addition, when the distortion rate is smaller than 0.3%, the light transmittance α
C1 and transmittance αC! Cannot be adopted because the difference is too large.

(ii) Based on the above facts, the present invention specifies the strain rate given to the optical fiber to be approximately 0.3 to 5.0%.

According to the configuration of the present invention, the wavelength λ1 of the short wavelength example
The light transmittance αC4 in the case of λ2 and the light transmittance αC2 in the case of the wavelength λ2 on the long wavelength side are the same or substantially the same. Therefore, the adverse effects of higher-order components caused by bending of the optical fiber are eliminated.

Although the optical fiber becomes longer due to the formation of the loop, since it is a loop, it occupies a relatively small area.

<Example> Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 is a schematic configuration diagram of a two-wavelength type optical fiber-based optical sensor according to an embodiment of the present invention.

In FIG. 1, the same reference numerals as those shown in FIG. 4 according to the conventional example refer to the same parts as the parts 5, etc. indicated by the reference numerals in this embodiment as well.

Further, unless otherwise specified, the present embodiment and the conventional example shown in FIG. 4 have the same configuration regarding the connection relationship.

The configuration of this embodiment differs from the conventional example shown in FIG. 4 as follows.

A connector 29 is connected to the end of the connecting optical fiber 28 led out from the optical multiplexer 15 to the optical-electrical conversion element 17 side, and the connecting light led out from the optical-electrical conversion element 17 to the optical multiplexer 15 side. A connector 31 is connected to the end of the fiber 30. Connectors 33 and 34 are connected to both ends of the transmission optical fiber 32 with the optical sensor element 16 interposed in the middle.
One connector 33 is coupled to the connector 29 of the connecting optical fiber 28, and the other connector 34 is coupled to the connector 31 of the connecting optical fiber 30.

A loop L is formed in the connecting optical fiber 28 led out from the optical multiplexer 15 in order to apply distortion to the core covered by the clad and remove high-order components.

This loop L, the optical multiplexer 15 and the electro-optical conversion element 13,
14 is housed inside the casing 35.

<Experimental example> As the electro-optical conversion element 13 for emitting the signal light Ls with a wavelength λ on the short wavelength side, an A with a wavelength λ = 0.88 μm was used.
It uses fGaAs LEDs.

On the other hand, as the electro-optical conversion element 14 for emitting the reference light Lr with the wavelength λ2 on the long wavelength side, the wavelength λ2-1.30μ
m InC+aAs LEDs are used.

In the relationship between the bending diameter R of the connecting optical fiber 30 on the optical-electrical conversion element 17 side, the diameter r of the loop L, and the strain rate ε, the loss βc+(
dB) and loss βC2 (dB) for wavelength λ2,
Wavelength λ, = 0.88 μm and wavelength λz = +, 30 μm
We conducted an experiment to see how it would change. This experiment was conducted with the loop L being wound twice. The results obtained are shown in Table 1.

(Left below) Table 1 t-1------------------------------------ In Table 1, for example, the strain rate ε=0
．． 3% (loop 1. diameter r = 50 mm) and bending diameter R = 100 mm, loss βC3 = 0.01 dB for wavelength λ1, loss βc for wavelength λ2.
2=0.01 dB, and the difference is OdB. That is, the influence of higher-order components can be completely removed.

In addition, the strain rate ε-0.3% (diameter of loop L = 50 cm)
), and in the case of bending radius R = 25 m + a, the loss for the wavelength island is β C + ” 0.35 d B, and the loss β for the wavelength λ2 (−z = 0.40 d B,
The difference is only 0.05dB. In this case as well, the influence of higher-order components can be substantially eliminated.

In a range where the strain rate ε of the optical fiber 30 is 0.3 to 5.0% (the diameter of the loop L is 3 to 50%), the difference between the loss βC8 and the loss βC2 is smaller than 0.05 dB.

Even when the distortion rate is ε-6.0% (the diameter of the loop L is r-25)...the difference between the loss βC7 and the loss βC2 is smaller than 0.05 dB, but the diameter of the loop L is 25%. Because it is small and the optical fiber 30 may bend too much and break,
Cannot be hired.

In addition, the distortion rate t = 0.2% (diameter of loop L r-75
In the case of Ryu), when the bending diameter R = 35 mm, βC, -β
βC when Cz=O, 11dB, bending radius R=25+n
It cannot be adopted because it becomes quite large, +-βCz=0.21 dB.

The above also applies to the case where the loop L is wound 3 to 5 times.

For the above reasons, the present invention specifies the strain rate ε given to the optical fiber to be approximately 0.3 to 5.0%.

Note that the strain rate ε of the connecting optical fiber 30 is 0.8 to 2.
In the range of 0% (the diameter of the loop L is 7.5 to 19u), the difference between the loss βC8 and the loss βC2 is as small as 0.03 dB or less, so it is more preferable to adopt this range.

<Effects of Example> When the loop L is formed in such a manner that the strain rate ε imparted to the optical fiber is approximately 0.3 to 5.0%, βC, −βC2≦0
．． 05, and in this case, it can be substantially regarded as β01-βC2, so equation (3) can be replaced as equation (4).

Therefore, it is possible to eliminate the adverse effects of higher-order components caused by the bending of the optical fiber 30, thereby enabling highly accurate detection.

Furthermore, connecting the optical fiber 28 having the connector 29 at the end to the optical-to-electrical conversion element 7 means the end of the optical fiber 32 connected to the optical-to-electrical conversion element 17 via the connecting optical fiber 30. Connector 33 to connector 29
This is done by joining to.

That is, the connecting optical fiber 28 is constantly coupled to the optical multiplexer 15, and the connecting optical fiber 28 is connected to the optical multiplexer 1.
Usually I don't do things like leave 5. Therefore,
Changes in connector loss in the amount of light caused by attaching and detaching the connecting optical fiber 28 to and from the optical multiplexer 15 are also prevented.

Furthermore, the connection itself of the central optical fiber 32 is facilitated.

Furthermore, since the connecting optical fiber 28 is housed in the casing 35 in the form of a loop L, it is advantageous in making the casing 35 more compact.

<Effect of the invention> According to the present invention, the following effects are achieved.

Since at least a portion of the optical fiber is formed into a loop with the strain rate applied to the optical fiber being approximately 0.3 to 5.0%, the light transmittance α at wavelength λ1 on the short wavelength side is
C1 and the transmittance αC2 in the case of wavelength λ2 on the long wavelength side,
They can be the same or substantially the same.

Therefore, it is possible to eliminate the adverse effects of higher-order components caused by the bending of the optical fiber, thereby enabling highly accurate detection.

[Brief explanation of drawings]

Fig. 1 is a schematic configuration diagram of a two-wavelength type optical fiber-based optical sensor according to an embodiment of the present invention, Fig. 2 is a conventional transmission-type optical fiber-based optical sensor, and Fig. 3 is a conventional reflective optical sensor. FIG. 4 is a schematic diagram of a conventional two-wavelength type optical fiber-based optical sensor. 13... Electrical-optical conversion element 14... Electrical-optical conversion element 15... Optical multiplexer 16... Optical sensor element 28... Optical fiber... Loop

Claims (3)

[Claims] (1) An optical multiplexer that combines signal light and reference light that have different wavelengths; an optical fiber that transmits the combined signal light and reference light; and at least a portion of this optical fiber. An optical sensor using a two-wavelength optical fiber, which is provided with a loop formed in a state in which a distortion of about 0.3 to 5.0% is applied by turning the loop. (2) The loop is formed to have a diameter of 3 to 50 mm and is wound 2 to 5 times.
) A two-wavelength type optical fiber-based optical sensor described in item 2. (3) A two-wavelength optical fiber-based optical sensor according to claim (1), wherein the loop has a distortion of about 0.8 to 2.0%.