JPH0238923A - Method for restraining noises of optical fiber sensor - Google Patents

Abstract

PURPOSE:To restrain the occurrence of noise component by holding an optical element constituting an optical fiber with non-adhesive resin or gel substance having the coefficient of thermal expansion approximate to that of material constituting the optical element and having excellent thermal conductivity. CONSTITUTION:A connecting optical fiber 2, the optical element 3 constituting the optical fiber, a reference loop 4, a photocoupler 5, a light power meter 6 and the optical fibers 7 and 8 are housed in a case 1. The case 1 is filled with the non-adhesive resin or gel substance having the coefficient of thermal expansion approximate to that of material constituting a kind of optical element and the excellent thermal conductivity so as to fix the kind of optical element.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for suppressing noise in an optical fiber sensor.

[Conventional technology]

Optical fiber sensors are extremely sensitive sensors, such as 0, 2d+4/Hr for rotation and μPa for pressure sensors.
Temperature sensors detect temperatures of 10-6 to 10-tt degrees, so improving S/N and output stability are extremely important for optical fiber sensors9. There is a method for reinforcing an optical fiber connection part in JP-A No. 61-126506, and an optical fiber core wire fixing device and a method for connecting the same in JP-A-61-32807.

[Problem to be solved by the invention]

Examples of optical fiber sensors include Mannha-Zender interferometers, Fueberry-Bello interferometers, and interferometers that utilize the Sagnac effect. Problems related to the present invention will be explained using a sound pressure sensor using a Matsuhatsu Enda interferometer shown in FIG. 2 as an example of an optical fiber sensor.The configuration of the circuit shown in FIG. 2 is as shown in FIG. is injected into the optical fiber 8. The optical fiber 8 is further connected to a polarizer 9, whereby the light from the laser light source becomes light with a uniform plane of polarization, so that only components effective as light that cause interference occur.

This is the input to one end of the optical coupler 2. The output of the optical coupler is such that the separated light enters the sensing loop 3 and the reference loop 4 at the same time. In a sensing loop, the core of an optical fiber is compressed by sound or pressure, and the change in the average density changes the refractive index of the core and changes the speed of light propagating there. On the other hand, the reference loop is kept free from sound and pressure. Therefore, the optical coupler 5 collects the separated lights, and the lights are combined into a single fiber again, and the opto-electronic converter The interference of two types of light with different phases is measured by This makes it possible to detect sound and pressure. The optical elements that cause particular problems in the optical circuit shown in FIG. 2 are the optical coupler and the polarizer. Optical couplers are made by cutting and polishing the cladding parts of two optical fibers, and aligning the polished surfaces to split the light while maintaining the plane of polarization. If an external force is applied to the part of the optical fiber that is slightly away from the surface, the mating length of the polished surface of the optical fiber and the pressure on the mating surface will change.
The extinction ratio and loss 1 branching ratio change, making it unusable. Furthermore, even if a sudden temperature change is applied, the mating length of the polished surfaces and the pressure will change, making it impractical.

On the other hand, polarizers are constructed by utilizing the rapid attenuation of the light component due to polarization fluctuation due to stress on the core due to bending of the optical fiber, which causes stress to be applied to the optical fiber used as the polarizer. In addition, the amount and direction of bending of the optical fiber leading in and out of the polarizer must be measured while measuring the polarization status (extinction ratio deterioration status) and loss. It is essential to determine the spatial position of the polarizer and the retention of the polarizer. For the optical coupler, it is necessary to hold the coupler and determine the spatial position of the optical fiber lead going in and out in exactly the same way. Next, it is represented as an optical fiber for connection. The optical fibers 7.8 need to be given exactly the same considerations as the optical fiber leads of the optical elements. As for the optical fiber 8, if it is allowed to move freely, for example, by changing the bending R, the amount of rotation of the plane of polarization of the light emitted from the laser will change, allowing only a specific plane of polarization to pass through with low loss. When the light passes through a polarizer, the rotated polarized component is cut, so the power (amount of light) of the light input to 2 changes, which also changes the interference power of 6, which becomes a major noise factor. Also, regarding the optical fiber 7, if there is a change in the bending radius or twist of the optical fiber, a change in the optical power that contributes to interference in the opto-electrical converter 6 will occur.
If we quantitatively calculate the problems listed above 0 that cause an increase in the amount of noise due to the rotational component of the polarization plane and an increase in optical loss and deteriorate the S/N, it will be possible to determine how important the above-mentioned problems are. can.

Interferometer-type optical sensors measure changes in the effective amount of interference light from several tenths of a wavelength (0.1 deg) to tens of thousands of wavelengths (0.1 deg).
, 01deg).

In FIG. 3, the lengths of the optical fibers between the optical couplers are each 10 m including the sensor loop, reference loop, and connecting optical fiber, and the initial values of the equivalent refractive indices of the optical fibers are both 1.4700. Now, the optical fiber circuit containing the reference loop is exposed to wind, which causes a change in the bending of the connecting optical fiber, which leads to a change in density, so that the equivalent refractive index of the reference loop optical fiber is 10-7, i.e. 1.4 against the initial value of 1.4700
7000001, the time difference Δt between the light propagating through the sensing loop and the reference loop (this degree of refractive index change can easily occur) is Δt = (10” x 1.4700) / (3X
10-'m/s) (10w''X 1.470000
01) / (3X IO-'m/s).

This time difference is considered to be that the optical path including the reference loop has apparently changed (shrinked) with respect to the optical path including the sensing loop.

If the amount of shrinkage is Δ2, ΔQ=Δt X 3 X 10"-'m/s"FI
X 10-6m, that is, it has shrunk by about 1μ (micron).

If infrared light with a wavelength of 1.3μ is used as a laser light source, due to a slight change in the bending radius of the connecting optical fiber,
The phase difference Δθ of the interference light is Δθ=1μ/1.3 μX 2 tc (rad)=
It is 2.41 rad: 276 degrees.

This is a few tenths of the wavelength detected by an interferometer.

This is an extremely large value for approximately 0 and 1 degrees = 0 and 05 degrees, and such problems have hindered the practical application of interferometer type optical sensors.

Although the above phenomenon has been explained with respect to a connecting optical fiber, it goes without saying that the exact same phenomenon occurs when a change in stress is applied to an optical fiber in an optical coupler, sensing loop, reference loop, etc.

[Means to solve the problem]

In order to suppress the generation of noise in the optical fiber sensor, it is necessary to minimize the stress applied to the optical element and the optical fiber and to prevent the stress from changing. The means for doing so must be explained using FIG. 3. Now, a case will be described in which the optical element 3 is mounted and the connecting optical fiber 2.3 is wired.

1 is a laser light source, 5 is an analyzer, and 6 is an optical power meter. 3 has no local stress and good heat conduction.

For example, fix it with silicone rubber resin, emit a laser light source, and bend R as shown in 2 and 3.

By appropriately adjusting the twist, the spatial position of the connecting optical fiber that optimizes the loss of the optical circuit can be found mainly using an optical power meter. Moreover, the spatial position of the connecting optical fiber at which the extinction ratio is optimum can also be found mainly using the analyzer. Therefore, the values of optical loss and extinction ratio are respectively appropriately assigned and fixed at the spatial position of the connecting optical fiber. The fixing method is, for example, a method of sealing the optical element, including the optical element, with a silicone rubber resin that is very soft, non-adhesive, and has good thermal conductivity.

FIG. 1 is a sectional view of an optical sensor in which an optical element and a connecting optical fiber are sealed in a cylindrical case and filled with transparent silicone rubber resin.

Case 1 contains optical elements and electrical circuit parts 2 to 8.
After storing the above elements without attaching them to the case, and connecting and wiring the optical elements using optical fibers under the optimum conditions, case 1 is filled with silicone rubber resin, and the optical elements and optical fibers are connected. Fix it.

[Effect]

Sealing an optical element including an optical fiber with a non-adhesive resin has the following advantages.

(1) Since the optical fiber is held by the entire resin, no local force is applied to the optical fiber, and local changes in the refractive index, which is the worst thing for optical fibers, can be avoided.

(2) Since the relative positions of the optical fiber and the optical element are fixed, the optical fiber does not move due to external air flow or vibration, so noise generation can be prevented.

(3) By using a soft or gel-like resin that is non-adhesive to the optical fiber, the stress transmitted from the resin to the optical fiber, etc. is alleviated, and the optical fiber and optical elements expand and contract due to temperature changes. It also relieves stress.

〔Example〕

FIG. 1 is a diagram in which the present invention is applied to a rotation sensor, which is a type of optical sensor that utilizes the Sagnac effect, and shows a cross section.

Optical circuits and electronic circuits have an outer diameter of 70I1
RTV in a (case) container with a length of 90m++ at wIlφ
(silicone rubber) and sealed. None of the built-in parts are rigidly attached to the container; all of them are in contact with the container via silicone rubber. Here, the optical element is a laser light source (laser diode), the photoelectric conversion element is a photodiode, the optical modulator is PZT (electrostrictive element using lead zirconate, optical phase modulator), and the optical coupler is a spliced optical fiber. The optical paths of all these elements are connected by optical fibers using optical couplers, and are further connected to an optical circuit consisting of 466m of optical fiber in an optical fiber loop.The wiring board on which the laser diode is installed, and the installation of the photodiode. The printed wiring boards, like the optical elements, are now floating in a gel-like sea of transparent silicone rubber resin, and the optical fibers that connect them are also floating in the gel-like sea. It is made to look like it is floating.

The hardness of the gel-like resin is exactly the same as that of Tokoroten.

By doing this, the initial position of the optical fiber that connects the optical elements is preserved, eliminating the risk of applying local force to hold the optical fiber, and therefore suppressing noise generation to an extremely small value. can.

〔Effect of the invention〕

According to the present invention, noise components generated in the optical circuit of an optical fiber sensor can be reduced, so a safe and highly sensitive optical sensor can be realized.

[Brief explanation of the drawing]

FIG. 1 is a side view of an optical sensor according to an embodiment of the present invention. FIG. 2 is an optical circuit diagram of the Matsuhatsu Enda interferometer, and FIG. 3 is an optical system circuit diagram illustrating the procedure for applying the present invention. Figure 1 Figure 2

Claims (1)

[Claims] 1. The optical element constituting the optical fiber sensor is held without applying any local force, and the stability of polarization within the optical element is improved;
In order to suppress changes in light loss, the optical element is held with a resin or gel-like substance that has almost no adhesive property and has good thermal conductivity and a coefficient of thermal expansion similar to that of the material constituting the optical element. A method for suppressing noise in an optical fiber sensor, characterized in that: 2. The optical fiber leads coming out of the optical elements constituting the optical fiber sensor and the optical fibers connected to propagate light between the optical elements should be designed to minimize light loss and polarization movement. In order to maintain the shape of the positioned optical fiber in space by applying appropriate bends, twists, etc., the thermal expansion coefficient is close to that of the optical fiber, there is almost no adhesive property, and the thermal conductivity is low. A method for suppressing noise in an optical fiber sensor, characterized in that the spatial position of the optical fiber is fixed using a high-density resin or gel-like substance.