JPH0340363B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a mechanical light modulation device that modulates light passing through an optical fiber or reflected light using an external mechanical force.

Mechanical optical modulation methods for light within an optical fiber include a method of changing the distance between the end faces of optical fibers in a straight line with their end faces facing each other, a method of moving a shutter in and out between the end faces, and a method of adjusting the centering between the end faces. A method of causing this to occur, a method of changing the angle between the end faces, etc. have been proposed. These methods are possible in the laboratory, but in industrial applications in harsh environments, problems such as difficulty in fiber alignment, misalignment over time, adhesion of dust to the end face, and dew condensation may occur. , not practical.
If used underwater, it will become unusable in a short period of time due to water scale formation, air bubbles adhering to the end surface, etc.

In order to eliminate the above-mentioned drawbacks inherent in the prior art, the present inventor conducted research, the contents of which are disclosed in Japanese Patent Application No. 154814/1983 (mechanical light modulation device (hereinafter referred to as This is referred to as a conventional example)). The present invention relates to an improvement on this conventional example.

The basic principle of the conventional example described in the specification of Japanese Patent Application No. 55-154814 is as shown in FIG. 1, in which 1 is a capillary made of plastic or metal, 2, 3 are input and output optical fibers inserted into the capillary tube 1 so that their end faces face each other. The end faces of both fibers are separated by a distance L and form a modulation space 8. capillary tube 1
The inner wall is roughened and blackened to prevent light reflection. When the input optical fiber 2 is fixed and the output optical fiber 3 is bent by θ as shown by the dotted line, the amount of light output from the optical fiber 3 decreases. This allows the light to be mechanically modulated.

As a result of experiments, it was found that the conventional system shown in FIG. 1 still had some unsatisfactory points in terms of performance. The main points that need improvement are as follows.

(1) The change in the amount of light passing with respect to the bending angle θ, that is, the bending sensitivity is insufficient. Another cause is insufficient roughening and blackening of the inner wall of the capillary tube.

(2) Therefore, when the bending angle θ is increased, the stress on the tube wall of the metal capillary tube increases, and there remains a slight problem in the fatigue life of the metal capillary tube in applications where the tube wall is subjected to repeated bending.

(3) Increasing the modulation space width L definitely improves the bending sensitivity, but the absolute value of the amount of transmitted light decreases, making the photometry system expensive.

The present invention has been made in view of the above-mentioned conventional circumstances, and therefore, an object of the present invention is to eliminate the need for troublesome operations such as alignment by improving the above-mentioned items (1) to (3). It is an object of the present invention to provide a novel mechanical light modulation device that is highly reliable, compact, and inexpensively constructed, which does not deteriorate over time even under adverse environments.

The above object of the present invention is to form a modulation space in an elastic body, make one or both of the two optical fibers introduced into the elastic body protrude into the modulation space, and make the two optical fibers protrude into the modulation space. This is achieved by a mechanical light modulation device characterized in that each end face of the optical fiber is closely opposed to each other, and the amount of light from the optical fiber is modulated by elastic deformation of the modulation space.

Next, a preferred embodiment of the present invention will be specifically described with reference to the drawings.

In FIG. 2 showing the first invention, reference number 1
2 represents a capillary tube, 2 represents an input optical fiber, and 3 represents an output optical fiber. Optical fibers 2 and 3 are supported on the central axis of capillary tube 1 by spacers 4 and 5. A modulation space 8 having a width L is formed between both end surfaces of the spacers 4 and 5. The end face 6 of the input optical fiber 2 is held flush with the end face of the spacer 4 . The end face 7 of the output optical fiber 3 projects into the modulation space 8 and faces the input optical fiber end face 6 at a small distance.

In an actual light modulation device, the modulation space width L is about 10 to 100 times larger than the inner diameter D of the capillary tube 1. For the sake of clarity, the inner diameter of the capillary tube is exaggerated and shown in a large size in FIG.

If the input optical fiber 2 is fixed and the output optical fiber side is bent at an angle of θ as shown by the dotted line, the modulation space 8
is bent with radius of curvature R. In this case, the output optical fiber 3 protruding into the modulation space 8 does not bend, so it appears as a dotted line. Therefore, the light emitted from the input optical fiber end face 6 does not enter the end face 7 of the output optical fiber 3, is scattered and lost within the modulation space, and the output light from the output optical fiber 3 becomes almost zero.

On the other hand, in the case shown by the solid line without bending, the light loss is only the light loss between the opposing end faces 6 and 7 plus the Fresnel reflection loss at the end faces 6 and 7, and the output optical fiber 3 The output light amount is only about half that of the previous one.

Assuming that only the modulation space width L is curved with a constant radius of curvature R, the condition for the passing light to be completely cut off as shown by the dotted line in Figure 2 is based on the geometric relationship where the outer diameter of the optical fiber is d. , the following equation (1) holds.

d≦Lθ/2 (1) The coefficient 1/2 is included on the right side because the end face of the spacer 5 is shifted in the opposite direction to the end face 7. Assuming that the modulation space width L is constant, the smaller the outer diameter d is, the more preferable the light is cut by a slight bending angle θ. Although the outer diameter d has been taken as the outer diameter of the optical fiber, since cladding mode light can be easily cut off as described later, the core diameter may be used as d in the above equation (1). A commercially available standard multimode optical fiber has a clad outer diameter of 125 μm and a core diameter of 50 μm, so L=
In the case of 10 mm, θ≧0.01 (radian) = 0.573°.
A bend of less than 1° will sufficiently cut out the light.

On the other hand, in the conventional example shown in Figure 1, L=10mm
When θ=5°, the light is cut off. This is a calculated value based on the assumption that the inner wall of the capillary is completely blackened, and since this is difficult to achieve, in reality, a complete light cut cannot be obtained unless the capillary is bent by about 10 degrees. As a result, the sensitivity of the present invention is 10 to 20 times better than the conventional example shown in FIG. If a stainless steel pipe with a wall thickness of 25 μm is used as the capillary tube in FIG. 1, when L=10 mm, a stress of about 10 Kg/mm ² will be generated in the stainless steel pipe when bent by 5°. This value is close to the allowable fatigue stress for applications that involve repeated bending.

In the present invention, the bending angle may be approximately 1/10. instead of,
If optical fibers of the same diameter are used, the diameter of the capillary tube will be about three times as large, so the stress in the capillary tube will be reduced to about 1/3, resulting in favorable results in terms of fatigue. If a plastic capillary tube is used instead of a metal capillary tube, the allowable strain can be increased by an order of magnitude, so there is no problem in terms of fatigue. The resonant frequency of the output optical fiber protruding into the modulation space is 635 Hz when L=10 mm and d=125 μm.
For high-frequency applications where the bending frequency is higher than this, it is necessary to increase the resonance frequency by reinforcing it.

FIG. 3 shows an embodiment of the second invention.
In the figure, 1 is a capillary tube, 2a is an optical fiber cladding, and 2b is a core.The cladding 2a and the core 2b are held on the central axis by a spacer 4. A tapered beam 12 is held on the central axis of the spacer 5 . The left end surface 13 of the beam 12 is mirror-finished and faces the end surface 6 of the core 2b. core 2
The input light 14 of b is reflected by the beam end face 13 and becomes the return light indicated by the dotted line 15. When the modulation space width L is bent by θ as shown in FIG. 2, the reflecting mirror 13 separates from the end face 6 of the core, and the amount of returning light becomes almost zero. In order to increase the bending sensitivity, it is best to turn off the cladding mode light. For this purpose, in FIG. 3, a part of the cladding is removed by etching. There are various methods for extinguishing the cladding mode light, such as making the refractive index of the spacer 4 slightly larger than that of the cladding. The beam 12 and the spacer 5 may be integrated as necessary.

It is desirable that the beam 12 be tapered as shown in FIG. 3, since this increases the mechanical resonance frequency. It is undesirable for the tip of the beam 12 to come into contact with the inner wall of the capillary tube 1 when the modulation space 8 is bent. By making a portion of the capillary tube as thick as 16, the above-mentioned mechanical contact can be avoided. Although not shown, the critical angle θc
It is desirable to install a stopper to prevent bending beyond this point.

If silicone oil or the like having an appropriate viscosity is sealed in the modulation space 8, the amplitude of the beam 12 near the resonance frequency is limited and the upper limit of the usable frequency is raised.

As shown in FIG. 4a, the reflector at the beam end face 13 vibrates in the direction of arrow 10. Beam end face 13
The bending sensitivity increases when the thickness t in the vibration direction is equal to or smaller than the core diameter. On the other hand, the width b is preferably larger than the core diameter. Irregular vibration in a direction perpendicular to the normal vibration direction 10 and manufacturing eccentricity are unavoidable. If the reflecting mirror is rectangular as shown in the figure, it is possible to prevent the reflecting mirror from coming off to the left or right from the core 2b.

In the case of FIG. 3, the bending angle θ can be determined by measuring the amount of reflected light 15. The vibration frequency of bending can be determined from the frequency of light intensity change, and the flow rate of the Karman vortex flowmeter can be determined. In the case of FIG. 2, this information can be obtained from the amount of passing light.

It goes without saying that in the embodiment of the first invention shown in FIG. 2, if the end face 7 is mirror-finished, it is possible to construct a device equivalent to a reflective modulation device as shown in FIG. 3.

In the embodiment, for convenience of explanation, 1 is a capillary tube, but it does not need to be a capillary tube with a circular cross section. It may also be a tube with a square cross section. The cross-sectional shape of the tube is determined by ease of manufacture and availability. The wall thickness of the tube does not necessarily have to be as thin as shown. If it is made of an elastic material such as rubber or plastic, it is easy to bend the modulation space even if the wall is thick.

Furthermore, although the explanation has been made so that the entire modulation space is bent, it is clear that the light will be modulated in the same way even if an external force causes misalignment.

In FIG. 2, the output optical fiber is projected into the modulation space, but the effect is exactly the same even if the input optical fiber is projected instead of the output optical fiber.

Since scattered light in the modulation space causes noise, it is preferable to roughen the inner wall of the capillary tube and make it black at the same time. The cylindrical outer surface of the optical fiber protrusion is preferably shielded from light by an appropriate method.

Remove the cladding near the optical fiber end face,
If unnecessary cladding mode light is prevented by fixing a suitable light shielding material to the removed portion, the bending sensitivity will be improved.

If the end face of the fiber is crushed and flattened as shown in FIG. 4c, this is equivalent to reducing d in equation (1), and the bending sensitivity is improved.

A similar effect can be obtained by masking the end face of the optical fiber with a light absorber 11 as shown in FIG. 4b and restricting the light entering and exiting from the end face to within an elongated slit. Even if the core diameter is lowered by stretching only the vicinity of the end face to be particularly thin, the bending sensitivity is improved.

If a single mode optical fiber with a core diameter of 7 μm is used instead of a multimode optical fiber with a core diameter of 50 μm, the bending sensitivity increases by a factor of 50/7. However, single mode optical fiber is difficult to center for optical connectors and the like.

Even if the core diameter is constant, bending sensitivity can be increased by increasing the modulation space width L. However, since the resonant frequency of the protruding portion is lowered, the application is limited.

The various bending-sensitive pair cables described above tend to make it difficult to align the opposing surfaces. Fortunately, a certain degree of misalignment is tolerated when measuring the frequency of the Karman vortex flowmeter.

In the modulation space in Figure 2 or Figure 3, θ=
Assume that a bending vibration of Asin2πt is applied. Here, A is a constant and is a frequency. Since the end faces face each other twice for each cycle of vibration, the fundamental wave of the alternating current component of the modulated light is 2. The higher the bending sensitivity is, the sharper the pulsed light with fundamental wave 2 can be obtained. The light amount changes in an ON-OFF manner, which facilitates signal processing in the subsequent stage.

By creating a shift between the end faces or bending the modulation space in advance, a response with the fundamental wave can be obtained. This may be more convenient when used as an optical switch rather than for vibration measurement.

The above is a case where all bending sensitivity is improved, but there are also cases where it is desired to intentionally worsen bending sensitivity. This corresponds to a case where it is desired to obtain an analog signal that corresponds to a large angle change. In this case, if the reflecting mirror at the end of the beam is made triangular as shown in FIG. 4c, it will become less sensitive to bending. The frequency of the output fundamental wave is also no longer 2. In this case, it does not necessarily have to be a triangle. Any shape may be used as long as the area increases uniformly in the vibration direction. Second
In the figure, the optical fiber 2 is removed and a reflecting mirror is installed on its end face, which is almost equivalent to the case shown in FIG. 3. In this case, the optical fiber 3 serves as an input/output optical fiber. As in the case of FIG. 3, a rectangular reflecting surface is desirable.

The dotted line 9 in FIG. 2 is a reinforcing material, and its presence makes it difficult for the left half of the modulation space to bend. The amount of relative deviation between the end faces for the same bending angle θ is larger than when there is no reinforcing material. Although the form of the expression is complicated, the expression
The coefficient 1/2 in (1) approaches 1, improving bending sensitivity.
A similar effect can be obtained by increasing the thickness of a portion of the capillary tube without adding any reinforcing material.

The spacers 4, 5 in FIGS. 2 and 3 are only one means for supporting the optical fiber or beam. There are various methods for this purpose, but the simplest method is to locally collapse the capillary tube 1 and support the optical fiber or beam using the capillary wall itself as a support. This is omitted because it complicates the diagram, but this is the most productive. If an optical fiber with a particularly large cladding diameter is used, a spacer is not necessary and the cladding itself can serve as a support. Spacers and optical fibers are explained as being different things, but in reality, the optical fibers are coated with plastic or rubber, and this coating material becomes the spacer. The thus assembled body is inserted into a capillary tube.

As is clear from the above description, according to the present invention,
All the drawbacks of the conventional example shown in FIG. 1 can be solved. Since the bending sensitivity is high, there is no need to make large bends, so the stress level of the capillary tube is low. Since the amount of modulated passing light or reflected light is large, the design of the photometry system is easy and inexpensive parts can be used. In particular, the reflective type shown in FIG. 3 is a bidirectional system in which one optical fiber is shared by both upstream and downstream lights, so the number of optical fibers and optical connectors is reduced by half. Since the amount of return light is large, a commercially available return light distributor can be used as is.

The light modulation device shown in Fig. 1 does not require alignment;
It has the advantage of being able to be used even in adverse environments, and this is inherited in the present invention as well.

The modulated light is ON-OFF, and signal processing is easier, which is superior to the conventional example.

FIG. 5 is a sectional view showing a reference example in which the reflective optical modulator shown in FIG. 3 is applied to a Karman vortex flowmeter. FIG. 6 shows an enlarged cross-sectional view of the light modulation section of FIG. 5 and the relationship between transmitting and receiving light.

In FIG. 5, fluid 21 is flowing inside pipe 20. In FIG. A vortex generator 22 is inserted at right angles to the flow. A Karman vortex street 23 is generated on the downstream side within a certain Reynolds number range. A tail wing 24 is attached to the downstream side of the vortex generator 22 via a constriction 25 . In response to the generation of the Karman vortex street, the tail 24 vibrates in the direction of an arrow 26 with the constriction 25 as the center of rotation.

As shown in FIG. 6, the reflective light modulator shown in FIG. 3 is embedded in the constriction 25. As shown in FIG.

The light from the light emitting element T passes through the distributor 27 as indicated by the solid arrow and enters the reflective light modulator attached to the constriction 25. The reflected light modulated by the Karman vortex oscillation frequency returns within the same optical fiber as shown by the dotted arrow, and is split into two by the distributor 27, one of which enters the light receiving element R and the other of which returns to the light emitting element T side. The light returning to the T side is wasted. The light receiving element R converts the signal into an electrical signal, which in turn converts into a frequency proportional to the vibration frequency of the tail. The flow velocity or flow rate in the pipe can be determined from this frequency.

As the distributor 27, one using a half mirror, one using a self-focus lens, etc. are commercially available.

If the pass-through optical modulator shown in FIG. 2 is used instead of the reflective optical modulator in FIG. 6, two optical fibers, one for upstream and one for downstream, will be required. Instead, the optical distributor 27 becomes unnecessary. Generally, the distance between the modulator and the transmitter/receiver is long, several hundred meters, so at present, reflective optical modulators that use optical fibers bidirectionally are more cost-effective.

Figures 5 and 6 show an example of measuring the flow velocity in a pipe, but as a modification, the wind velocity in the atmosphere can also be measured by forming a long vortex generator into a rod shape and attaching a tail to its tip. . Furthermore, by attaching multiple tail blades to a long vortex generator, it is also possible to measure the flow velocity distribution. Convenient for measuring flow velocity distribution in waterways, etc.

The constriction 25 in FIG. 5 is not essential;
These do not need to be provided.

If you try to expand the measurement range, you will inevitably have to
The vibration amplitude of No. 4 may become excessive. In order to prevent damage during the production process, it is practically useful to install a stopper that prevents excessive vibration amplitude.
This stopper has been omitted in the above description.

In FIG. 6, the left half of the modulation space is filled with vortex generators. Therefore, this part of the modulation space cannot be bent. The sensitivity for the same tail deflection angle θ is expressed by formula (1)
It's getting better than before.

Conventional devices of this type that utilize Karman vortices include those that detect Karman vortices thermally, those that detect them as differential pressure, those that detect them using ultrasonic waves, those that magnetically detect tail vibration, and those that detect vortices using vortex generators. Some proposals have been made to detect the alternating force acting on the sensor using a piezoelectric element or a strain gauge. Each of these methods has its advantages and disadvantages, but the common drawbacks are that they are generally complex, expensive, have explosion safety issues due to their electrical nature, and are susceptible to external electromagnetic induction. It is weak and is not good at measuring flow rates of small diameters.

The Karman vortex flowmeter shown as an application example of the present invention solves all of the above-mentioned drawbacks. The vortex generator and the tail can be integrally molded from plastic or rubber, and the optical modulator can also be embedded, so it can be constructed at an extremely low cost.

Because it uses optical technology, it is inherently safe against explosions and has no electromagnetic induction. It has the advantage of having few factors that limit the lower limit of the pipe inner diameter, and has great practical effects.

Although each of the inventions has been described above with reference to one preferable embodiment thereof, this is merely an example.
Of course, this does not have a restrictive meaning. Therefore, without departing from the spirit of the present invention, the present invention may be implemented with various modifications other than those described above, but all such modifications and changes are included within the scope of the present invention. It is.

[Brief explanation of drawings]

FIG. 1 is a schematic sectional view of a conventional mechanical light modulation device, FIG. 2 is a schematic sectional view showing an embodiment of the mechanical light modulation device according to the first aspect of the present invention, and FIG. A schematic sectional view showing an embodiment of a mechanical light modulation device according to the second invention, FIG. 4 is an explanatory diagram of an end face of an optical fiber according to the invention, and FIG. 5 is a diagram of a Karman vortex flowmeter to which the second invention is applied. A schematic sectional view showing an example,
FIG. 6 is an enlarged view of the main part of FIG. 5 and an explanatory diagram showing the relationship between transmission and reception of light. DESCRIPTION OF SYMBOLS 1... Capillary tube, 2, 3... Optical fiber, 2a... Clad, 2b... Core, 4, 5... Spacer, 6, 7...
End face, 8...Modulation space, 11...Light absorbing material, 12...Beam, 13...Emission mirror, 14...Input light, 15...Return light, 20...Pipe, 21...Flow, 22...Karman vortex generator, 23...Karman Vortex row, 24... tail,
25...constriction, 27...light distributor.

Claims (1)

[Scope of Claims] 1. A modulation space is formed in an elastic body, one or both of the two optical fibers introduced into the elastic body are made to protrude into the modulation space, and the two optical fibers are made to protrude into the modulation space. The respective end faces of are closely opposed to each other,
A mechanical light modulation device characterized in that the amount of light from an optical fiber is modulated by deforming an elastic body in the modulation space. 2. The mechanical light modulation device according to claim 1, further characterized in that the two optical fibers are supported by a support on the central axis of the elastic body. 3. The mechanical light modulation device according to claim 1 or 2, further characterized in that the cylindrical outer surface of the protruding optical fiber is shielded from light. 4. forming a modulation space in an elastic body, introducing an optical fiber and a beam whose end surface is mirror-finished into the elastic body, and causing the beam to protrude into the modulation space to closely face the end surface of the optical fiber; A mechanical light modulation device characterized in that the amount of light from an optical fiber is modulated by elastic deformation of the modulation space. 5. The mechanical light modulation device according to claim 4, further characterized in that the optical fiber and the beam are supported by a support on the central axis of the elastic body. 6. The mechanical light modulation device according to claim 4 or 5, further characterized in that the cladding near the end face of the optical fiber is removed. 7. The mechanical light modulation device according to claim 4 or 5, further characterized in that the vicinity of the end face of the optical fiber is made thin by crushing or stretching. 8. Claim 4 further characterized in that a light absorbing material is fixed to a part of the end face of the optical fiber.
The mechanical light modulation device according to any one of Items 1 and 5. 9. The mechanical light modulation device according to any one of claims 4 to 8, further characterized in that the end face of the reflecting mirror or the fiber is rectangular or has a shape whose area increases in a certain direction. 10. The mechanical light modulation device according to any one of claims 4 to 9, further characterized in that dipping oil is sealed in the modulation space. 11. The mechanical light modulation device according to any one of claims 4 to 10, further characterized in that the inner diameter of the modulation space is partially increased. 12. The mechanical light modulation device according to any one of claims 4 to 11, further characterized in that a part of the modulation space is made thick.