JPH03185304A - Standard device of brillouin frequency shift - Google Patents

Abstract

PURPOSE:To enable highly-precise measurement of a strain independent of a change in an ambient temperature by a method wherein a substance which causes a thermal strain enabling offset of a change in a Brillouin frequency shift of an optical fiber is combined and put in one body with the optical fiber as a standard. CONSTITUTION:The title device is to make it possible to offset a change in a Brillouin frequency shift due to temperature by a change in the Brillouin frequency shift due to a thermal strain, and a device prepared by putting a cover 8 in one body with an optical fiber 7 as a standard of the Brillouin frequency shift is used therefor. The cover 8 needs only to be a material whereby a substance causing the thermal strain enabling offset of the change in the Brillouin shift of the optical fiber 7 can be put in one body, in the form of the cover or the like, with the optical fiber. By determining the material properly, the standard of the Brillouin frequency shift not being affected by a change in temperature can be obtained and thereby highly precise measurement of a strain is enabled.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a Brillouin frequency shift standard device used to measure Brillouin frequency shifts with high precision.

[Prior Art J] Conventionally, methods using Brillouin scattered light have been proposed as methods for measuring strain (for example, Japanese Patent Application No. 63-154
No. 828), which utilizes the fact that the frequency shift of Brillouin scattered light changes in proportion to the strain applied to the material into which the light is incident. That is, when light of frequency f1 is incident on a substance, light of frequency f+-fb is scattered backward due to the interaction between the light and sound waves in the substance. This scattered light is called Brillouin scattered light, ft, is called Brillouin frequency shift. Since fb changes in proportion to the strain applied to the material, it is possible to measure the strain by measuring fb. However, this Brillouin scattered light is extremely weak. Therefore, a probe light having a frequency f2 is also made incident on the substance in opposition to the above incident light (which is used as the pump light). Here, when fz-f, -fb, the Brillouin scattered light intensity increases significantly, so it is possible to measure fb with high precision from the optical frequency difference f, -f2 between the pump light and probe light. (Patent Application No. 154828/1983). However, since this Brillouin frequency shift fb usually has a high frequency of 10 GHz or more, it is quite difficult to accurately measure fb. Therefore, using a reference material (Brillouin frequency shift standard) for which fb has been determined with high precision using some method, we can calculate the Brillouin frequency shift fb of the material to be measured and the Brillouin frequency shift of the reference material. The rb of the substance to be measured is often determined by measuring the difference between the two. This measurement method will be explained below using the measurement of the Brillouin frequency shift of an optical fiber as an example. An example is shown in FIG. 1. In FIG. 1, 1 is a pump light source, 2 is a probe light source, 3 is a reference optical fiber used as a Brillouin frequency shift standard device, 4 is a test optical fiber, 5 is a beam splitter, and 6
is the photoreceiver. Signal light 21 from pump light source 1 is input to reference optical fiber 3 and test optical fiber 4 which are cascade-connected via connection point 7 . Further, the signal light 22 from the probe light source 2 is incident on the test optical fiber 4 so as to propagate through these optical fibers 3 and 4 in the opposite direction to the signal light 21. The optical frequency of the signal light 21 is as shown in FIG.
The frequency is swept in the time axis direction at a frequency sweep speed M. On the other hand, it is assumed that the optical frequency f of the signal light 2 is constant. Now, when f, -f, matches the Brillouin frequency shift fb of the test optical fiber 4 (fb-fr-fz), G
The intensity of the o-th destination light 22 is amplified by the signal light 21 and detected by the light receiver 6 via the beam splitter 5. FIG. 6 shows the temporal change in the intensity of the signal light 22 detected by the light receiver 6 in this manner. When peaks 3′ and 4″ are measured, fr-fx is
They correspond to the Brillouin frequency shifts fbr and fbt of the reference optical fiber 3 and the test optical fiber 4, respectively. Therefore, the signal strength peaks 3' and 4'
The Brillouin frequency shift fbt of the test optical fiber 4 is given by using the time interval Δt between
It can be obtained from (1). [Am that invention tries to solve! 11 However, it is known that the Brillouin frequency shift depends not only on strain but also on temperature. Therefore, in order to perform highly accurate strain measurements, a material that can be used as a Brillouin frequency shift standard is required that has a small Brillouin frequency shift due to temperature changes. However, such a Brillouin frequency shift standard has not been realized to date, and more than zero problems with the prior art will be numerically described below. As an example of the Brillouin frequency shift standard, a standard optical fiber whose core is doped with Gem and whose relative refractive index difference Δ is about 0.3% is taken as an example. The Brillouin frequency shift fb(ε, T) of this fiber is, where T is the reference temperature, fb(ε, T) = fb(0,7-)◆＾t +B
It can be expressed as (T-T-) (2). fb
(ε, T) is the standard Brillouin frequency shift when strain=ε% and temperature T°C. A
is a coefficient representing an increase in Brillouin frequency shift per 1% distortion, and is known to be ^-579MHz/X. In addition, B is a coefficient representing the increase in Brillouin frequency shift per 1°C of temperature, and B-1,18MHz/l
: is known to be. Therefore, the above coefficient A
, B, for example, an ambient temperature change of 40° C. causes a change in the Brillouin frequency shift of 47 MHz, which translates into a strain measurement error of 0.08%. Of course, if the temperature change is accurately known in advance, it is possible to eliminate this strain measurement error, but the temperature change is usually unknown. Therefore, an object of the present invention is to provide a Brillouin frequency shift standard device that can provide a Brillouin frequency shift with high precision (distortion can be measured) regardless of changes in ambient temperature. Means J In order to achieve such an object, the present invention provides, by strain,
The first point where the Brillouin frequency shift changes and light can propagate
and a second substance integrated with the first substance, and the amount of change in the Brillouin frequency shift of the first substance due to a temperature change is determined by the heat generated in the second substance due to the temperature change. It is characterized in that it is offset by the amount of change in the Brillouin frequency shift of the second material due to strain. Here, the first material may be coated with the second material. Alternatively, the first member is wound along the core material, the second member is configured in a tape shape, and the first member and the core material are pressed and wound by the tape-shaped second member. , one of the first member and the core material may be the first material, and the other of the first member and the core material and the second member may be the second material. . [Function] According to the present invention, a substance that causes thermal distortion that can cancel out changes in the Brillouin frequency shift of the optical fiber is used in combination with the optical fiber as a reference, so that the two are integrated. Therefore, changes in the Brillouin frequency shift due to temperature changes can be ignored, and highly accurate strain measurement is possible regardless of the operating temperature. That is, in the present invention, the change in Brillouin frequency shift due to temperature C can be offset by the change in Brillouin frequency shift due to thermal strain, so the present invention is significantly different from the conventional technology in which it was necessary to accurately know the temperature. . [Embodiment 1] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. [Embodiment 1] FIG. 1 is a sectional view showing an embodiment of an optical fiber as a Brillouin frequency shift standard of the present invention, where 7 is an optical fiber, and 8 is a coating integrated with the optical fiber 7. be. We found that the temperature coefficient of the Brillouin frequency shift fb of a silica-based optical fiber coated in this way can be expressed by two terms as follows (Kurashima 1, Kame,
Expenditure, “Temperature dependence of Brillouin frequency shift in silica-based optical fibers”, Institute of Electronics, Information and Communication Engineers Research Group, C58
6-3, 0C589-13, June 1989). dfb/dT =afb/aT +<afbl at
)(at/aT) (3) The first term on the right side of equation (3) is the temperature coefficient of the Brillouin frequency shift, which is determined by the material of the optical fiber 7. The second term on the right side of equation (3) is: This is the temperature coefficient of the Brillouin frequency shift based on thermal strain induced by the difference in linear expansion coefficient between the optical fiber 7 and the target N8. The present invention was made based on the recognition of the above formula (3), and what conditions are necessary in order for the temperature coefficients of the first term and the second term to cancel each other out. By considering the following as described below, and selecting appropriate materials based on these considerations, we were able to obtain a Brillouin frequency shift standard that is unaffected by temperature changes. As already mentioned, δfb/B T =1.18MHz/t
(4) tb/δ(-579MH2/%
(5), and the coefficient of thermal strain is 8678
Ding can be expressed by the following formula. δε/δi−[Ezszlo:, s, ◆EzSz) ]
(] α, -αl (6) Here, E is Young's modulus, S
is the cross-sectional area and α is the coefficient of linear expansion. Subscripts 1 and 2 are numbers corresponding to the optical fiber 7 and the RF receiver 18, respectively. Now, to improve the outlook, E+S+(E
Assuming zSi, equation (6) becomes ah/δT-α2-α1(7) From equations (3) and (7), dfb/dT-0
Therefore, the condition that the coefficient of linear expansion α of the coating 8 should satisfy is a2sal−(δfb/δr)/(,arb/9
c) Linear expansion coefficient α of 0 quartz glass as (8)
, ・5xlO-', and equations (4) and (5) are transformed into equation (8)
When substituted into , it becomes α2--2xlO-'. Liquid crystal polymer (LCP), Kevlar, and the like are known as materials having such a negative coefficient of linear expansion. In the present invention, as described above, by integrating the optical fiber 7 in combination with a material that causes thermal strain that can cancel out changes in the Brillouin frequency shift of the optical fiber 7,
Construct the Brillouin frequency shift standard. In the above description, two optical fibers 7 made of quartz glass have been described. However, the present invention is not limited only to this embodiment, and the optical fiber 7 may be used in combination with a material that causes thermal strain that can cancel out changes in the Brillouin frequency shift of the optical fiber 7, such as a coating, etc. It goes without saying that optical fibers made of other materials such as fluoride or multi-component glass may also be used as long as they can be integrated with each other.Especially when the optical fiber 7 is made of soda glass, the coefficient (δtb/δ) Since α2 has a negative value, the coating 8 requires a material in which α2 has a positive value. [Example 2] In the Brillouin frequency shift standard of Example 1 described above, only one layer was coated with a material that causes thermal strain that can cancel out changes in the Brillouin frequency shift of the optical fiber 7. However, in order to increase the adhesion between the optical fiber 7 and the coating 8 and to suppress the microbend loss of the optical fiber 7, it may be desirable to provide a multilayer coating. FIG. 2 shows an example of a Brillouin frequency shift standard device having a structure in which an optical fiber 7 is coated with N layers CI to CN. In this case, the thermal strain is δE/δT−ΣE+S+αdΣE,S,
It can be expressed as (9). Here, the optical fiber 7 and each coating layer C
Assign numbers to 1 to CN, respectively, El, SL,
The α axis is the Young's modulus of the material corresponding to the number i, and
It represents the cross-sectional area and the expansion coefficient a. Therefore, equation (2)
, (9), it is possible to design a Brillouin frequency shift standard using a multilayer coated optical fiber in the same manner as in Example 1.

[Embodiment 3] FIG. 3 shows the third embodiment of the present invention. , where 9 is a core material, 10 is an optical fiber wound around the core material 9 or a coated optical fiber, and 11 is an optical fiber 7.
The core material 9 and the optical fiber l
This is a pressure winding tape that presses and winds O. Here, it is assumed that the core material 9 and the tape 11 have different coefficients of thermal expansion from those of the optical fiber IO. The optical fiber or coated optical fiber lO is tightly wound around the core material 9. By adopting such a configuration, even if the optical fiber 7 is coated with a material that is difficult to coat as in Example 1, the thermal strain that can cancel the change in the Brillouin frequency shift of the optical fiber 7 due to temperature change can be reduced. It can be used as a material that causes Note that the substance that causes thermal strain that can offset the change in the Brillouin frequency shift of the optical fiber 7 is the optical fiber 10.
It can be used for its own coating and/or for the tape 11. Alternatively, an optical fiber is placed at the position of the core material 9, and around it is made of a material that causes thermal strain that makes it possible to cancel out changes in the Brillouin frequency shift of the optical fiber 7, and has a coefficient of thermal expansion different from that of this optical fiber. The material may be placed at the location of the optical fiber 10 and at the location of the tape 11 in FIG. In the above embodiments, an optical fiber was used as an example of a material whose Brillouin frequency shift changes due to strain. However, it goes without saying that such a substance may be a bulk material.

【Effect of the invention】

As explained above, according to the present invention, a substance that causes thermal strain that can cancel out changes in the Brillouin frequency shift of the optical fiber is used in combination with the optical fiber as a reference, thereby integrating the two. As a result, changes in the Brillouin frequency shift due to temperature changes can be visualized, making highly accurate strain measurement possible regardless of the operating temperature.

[Brief explanation of drawings]

1 is a sectional view showing a first embodiment of the present invention, FIG. 2 is a sectional view showing a second embodiment of the present invention, and FIG. 3 is a sectional view showing a third embodiment of the present invention.
FIG. 4 is a configuration diagram showing an example of a normal Brillouin frequency shift measurement system, FIG. 5 is an explanatory diagram of the sweep of the frequency f of the pump light, and FIG. 6 is the measurement shown in FIG. FIG. 3 is a diagram showing the relationship between the intensity of a signal measured by the system and time. DESCRIPTION OF SYMBOLS 1... Pump light source, 2... Probe light source, 3... Reference optical fiber (Brillouin frequency shift standard), 4... Test optical fiber, 5... Beam splitter, 6... Light receiver , 7... Optical fiber, 8, CI, C2,..., CN... Coating, 9...
Core material, lO... optical fiber or coated optical fiber, 11
...presser tape.

Claims (1)

[Claims] 1) A first material whose Brillouin frequency shift changes due to strain and through which light can propagate, and a second material integrated with the first material; The amount of change in the Brillouin frequency shift of the first material is offset by the amount of change in the Brillouin frequency shift of the second material due to thermal strain caused in the second material due to the temperature change. Brillouin frequency shift standard device. 2) The Brillouin frequency shift standard device according to claim 1, wherein the first material is coated with the second material. 3) the first member is wound along with the core material, the second member is formed into a tape shape, and the tape-shaped second member presses and winds the first member and the core material; One of the first member and the core material is the first substance, and the other of the first member and the core material and the second member are the second substance. The Brillouin frequency shift standard device of claim 1.