JPH0129547Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) The present invention relates to an improvement in the structure of a low temperature detection optical cable.

(Prior art and its problems) Low-temperature detection optical cables have explosion-proof properties for fuel in fuel storage facilities, etc., and can detect low temperatures when low-temperature media flows out in the longitudinal direction, allowing rapid and accurate low-temperature detection. It is attracting attention as an optical cable with excellent safety.

As a method for detecting low temperatures using optical fibers, a method of measuring the backscatter of light propagating through an optical fiber, known as the BS (backscatter) method, was considered to measure changes in transmission loss due to changes in the temperature of the optical fiber. A common type of optical fiber whose transmission loss changes according to temperature changes using this method is plastic-clad optical fiber (hereinafter abbreviated as PCF).
It has been known. PCF has a structure with a glass core and a silicone resin cladding. This principle is based on the fact that, as shown in the refractive index temperature characteristics in Figure 1, the refractive index of silica glass is stable with respect to temperature as shown by curve A, and the refractive index of silicone resin changes as shown by curve B. As a result, PCF exhibits a phenomenon in which the refractive index difference between the core and cladding decreases or becomes negative as the temperature decreases, and the amount of light propagation decreases as the refractive index difference decreases as the temperature decreases. will decrease.

However, PCF has a temperature at which transmission is impossible, that is, a temperature at which the refractive index difference between the core and the cladding is reversed and light no longer propagates, and in order to lower this temperature, it is necessary to use a silicone resin that has a low refractive index at room temperature. figure,
This naturally results in a fiber with a large difference in refractive index. In such an optical fiber, Rayleigh scattering increases and transmission loss increases.

(Structure and Examples of the Invention) The present invention is an optical fiber cable for low temperature detection that eliminates the above-mentioned drawbacks, and its feature is that it can detect temperatures close to -100°C.

An embodiment of the present invention will be described with reference to FIG.
An optical fiber 1 whose core and cladding are made of glass is housed in a groove 2 of an aluminum spacer (hereinafter abbreviated as Al spacer) 3 having several spiral grooves 2 on its outer periphery, and is simultaneously filled with silicone resin 4. Covered with aluminum pipe 5. Silicone resin has a small Young's modulus at room temperature and has a cushioning effect, but as shown in Figure 3, which shows the temperature characteristics of the Young's modulus of silicone resin, silicone resin undergoes a glass transition at low temperatures and its Young's modulus increases, causing microscopic damage to optical fibers. Give a bend. Therefore, the transmission loss of the optical fiber increases when the temperature becomes low, and this can be used to detect low temperatures. In this way, a low-temperature sensing optical fiber can be made by coating it with a resin that undergoes a glass transition at low temperatures, such as silicone resin.

Optical fibers that produce microbends include:
A common structure is an optical fiber whose core and cladding are made of glass (glass optical fiber), coated with silicone resin, and then coated with nylon, as shown in the temperature characteristics of optical fiber transmission loss in Figure 4. Furthermore, as shown by curve C, the optical fiber has a lower limit of its usable range as a sensor up to -60°C. On the other hand, the glass fiber coated with silicone resin has a 10 dB/10 dB gain even at -100°C, as shown by curve D.
Km, the transmission loss increases, and the required performance of a normal sensor △α (transmission loss change) = 1 to 20 dB/Km is satisfied, and the present invention is designed to enable it to be used as a sensor even at -100℃. In addition, low-temperature detection is made possible by utilizing the low-temperature properties of silicone resin to create microbends in optical fibers.

Figure 5 shows the 20m middle section of the cable according to the present invention.
This shows the temperature characteristics of low temperature position detection when the temperature is cooled down. Although no change in transmission loss was observed in the temperature range of +20°C to -60°C, a change in transmission loss of about 0.3 dB could be detected at -100°C, indicating that it is also effective in detecting temperatures as low as -100°C. I understand. Furthermore, unlike PCF, optical fiber does not require a large difference in refractive index between the core and cladding in order to lower the temperature detection range, and glass optical fibers with standard structures can be used.

(Effect of the invention) The silicone resin-coated fiber is vulnerable to lateral pressure and needs to be protected, but this problem can be solved by housing the optical fiber in the groove of the core having a plurality of grooves on the outer periphery, as in the present invention. Furthermore, the decrease in the low temperature detection ability of the optical fiber due to the shrinkage of the core at low temperatures has been solved by using a metal such as aluminum or copper, which has a smaller coefficient of linear expansion than silicone resin, for the core. Furthermore, by making the core body of metal, the core body has excellent thermal conductivity and the sensitivity of low temperature detection is improved.

The above are the effects of the present invention.

[Brief explanation of the drawing]

Figure 1 is a temperature characteristic diagram of the refractive index of silicone resin.
Fig. 2 is a cross-sectional view of an embodiment of the low temperature detection optical cable of the present invention, Fig. 3 is a temperature characteristic diagram of the Young's modulus of silicone resin, Fig. 4 is a temperature detection characteristic diagram of various optical fibers, and Fig. 5 is a diagram of the temperature detection characteristic of the present invention. FIG. 3 is a position detection characteristic diagram of an optical cable. 1... Optical fiber, 2... Spiral groove, 3... Spacer, 4... Silicone resin, 5... Aluminum.

Claims (1)

[Scope of utility model registration request] In an optical cable core in which an optical fiber is housed in the groove of a core body having a plurality of spiral grooves on the outer periphery, both the core and the cladding are made of glass, and the core body has a low coefficient of linear expansion and a high thermal conductivity. 1. A low-temperature detection optical cable characterized in that it is made of a metal such as aluminum or copper that has excellent properties, and the groove is filled with a resin that undergoes a glass transition at low temperatures, such as silicone resin.