DEVICE FOR DETECTING FRACTURES IN POWER TRANSMISSION FIBERS

Abstract: A device for detecting fracture faults in optical power transmission fibers in which a plastic fiber is laid parallel to and closely adjacent the power transmission fiber. The plastic fiber has a melting point lower than a temperature reached by a portion of the power transmission fiber containing a fracture fault when power is applied to the power transmission fiber. The portion of the plastic fiber adjacent the fault melts, thereby interrupting the path of a reference beam through the plastic fiber from a reference beam source to a detector. The absence of a returned reference beam at a detector is taken as an indication that the power transmission fiber has a fracture fault.

Description: DEVICE FOR DETECTING FRACTURES IN POWER TRANSMISSION FIBERS BACKGROUND OF THE INVENTION The present invention relates to optical power transmission fibers for use in laser knives and laser machining devices. A fracture (crack) in an optical power transmission fiber presents a serious problem in transmitting power through the optical fiber. Various conventional methods are known for detecting fractures in optical fibers. One such method is disclosed in Japanese Published Patent Application No. 12602/81 and Japanese Utility Model Application No. 69211/82. Both of these methods, however, use an electrical circuit that extends to the terminating end of the power transmission line. If the transmission line is used to supply power to medical equipment such as a laser knife or a machining tool that requires a high degree of electrical insulation, special provisions must be taken to insulate the terminating end of the transmission line. However, this cannot be effected without detracting from the inherently high insulating property of the optical fiber. SUMMARY OF THE INVENTION The present invention provides a device for detecting fractures in optical fibers that is free from the above-described defects of the prior art. In accordance with the present invention at least one plastic fiber is disposed parallel to and close to the power transmission fiber to be tested A reference light beam is carried by the plastic fiber , If a fracture is present in the power transmission fiber, heat at a high temperature is generated in the area around the fracture site of the fiber. The plastic fiber disposed parallel to the power transmission fiber is melted by the heat in that region. By detecting an amount of the reference light beam passing through the plastic fiber a fracture of the power transmission fiber can be detected. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A diagramatically shows a detection system according to the present invention; Fig. 1B shows how the system shown in Fig. 1A detects a fracture occurring in a power transmission fiber; Fig. 2 is a diagram showing a preferred embodiment of a detection system of the present invention; Fig. 3A diagrammatically shows a portion of the detection system according to the present invention; Fig. 3B is a diagram showing how the system shown in Fig. 3A detects a fracture occurring in a power transmission fiber; Fig. 4 shows another embodiment of the present invention; Fig. 5A diagrammatically shows a detection system according to the present invention; Fig. 5B shows how the system shown in Fig. 5A detects a fracture in a power transmission ìber; and Fig. 6 shows yet another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following are three embodiments according to the present invention. Figs. IA and 1B show the basic construction of the detecting system of the first embodiment. In Fig. lA, a power transmission fiber 1 and a plastic fiber 2 are laid side by side. A reference light beam 3 is introduced into the plastic fiber 2. The receiving or terminating end of the plastic fiber is provided with a reflecting coating 4 capable of efficient reflection of light at a wavelength equal to that of the reference light beam. The reference beam, upon being reflected from the terminating end, travels back to the transmitting end where it is branched at a right angle by a half-silvered mirror 9 and received by a photodetector 5. If a fracture is present in the power transmission fiber as shown in Fig. 1B, the area around the fracture site 8 becomes hot, as hot as several hundred degrees Celsius, due to the presence of the fracture. If the plastic fiber has a sufficiently low melting point, for instance, about 100 degress Celsius, it will melt. As a result, the reference light beam will not be reflected and no light will be received by the detector. Therefore, by sensing the output of the detector, fractures in the power transmission fiber can readily be detected. Modes for carrying out the present embodiment are as follows: (1) Any kind of light can be used for the reference light beam, but visible light1 such as produced by a He-Ne laser, is convenient and easy to handle. The reasons for this are: i) The spectrum of the beam produced by a CO2 or YAG laser is in the infrared region hence and invisible to the eye. Such a beam need not be supplemented by a visible guide beam for alignment purposes. ii) The visibility of the He-Ne laser light permits easy adjustment and maintenance. (2) If the reflective coating applied to the terminating end of the plastic fiber is designed to provide a reflectance of 50% and a transmittance of 50%, the light passing through the plastic fiber can be directly used as the guide beam. This presents an advantage in that the plastic fiber and associated components can also be used as part of the power transmission system. However, the reference light reflected by the half-silvered mirror before it enters the plastic fiber is wasted, and the reduced reflectance at the terminating end tends to impair the signal-to-noise ratio. (3) Instead of applying a reflective coating 4, the terminating end of the plastic fiber may be provided with a mirror 41 that faces the terminating end through a part of matching oil 42 (such as matching oil OF37A produced by Shinetsu Chemical Co. of Japan) , This arrangement is somewhat complex, but is easy to fabricate and ensures high reliability. (4) Aside from a consideration of the absorption of light, the most efficient transmittance of the halfsilvered mirror is 50% (i.e. 50% reflectance). (5) The reference beam can be efficiently introduced into the plastic fiber by means of a lens that focuses the light into a small spot. The transmittance of this lens can be increased to almost 100% by applying a nonreflecting coating 21 to the lens 20. The transmitting end of the plastic fiber may also be provided with a nonreflecting coating 31 that is capable of eliminating the noise component due to reflection from the lens and at the transmitting end. (6) Many kinds of detector devices and associated circuits can be used. However, the system should be so designed that when a fracture is indicated by a change in the output of the detector, the laser power source is turned off and other necessary actions are taken immediately. A block diagram of a detection system that embodies all the features described above, except (2), is shown in Fig. 2. A signal received by the detector 5 is fed to a comparator circuit 15. The detected signal 16, the active state of which indicates the occurrence of a fracture, is fed back to the laser power source 14 through a control circuit 18. Also, the signal 16 is supplied to a warning circuit 18 that actuates a buzzer 19, lamp 20 or other suitable warning devices 21. Example 2 Figs. 3A and 3B show the basic construction of a detecting system of a second embodiment. In Fig. 3A, a fiber 1 transmitting a power beam 3 and two plastic fibers 2a and 2b are laid side by side. A reference beam 3 is introduced into the plastic fiber 2a. The receiving or terminating end of the plastic fiber is provided with a reflecting surface 5 capable of reflecting the reference beam 3. The reflected light 6 passes through the plastic fiber 2b and is received at a detector 7. If a fracture is present in the power transmission fiber, as shown in Fig. 3B, the area around the fracture site 8 becomes hot, as hot as several hundred degrees Celsius. Assuming that the plastic fiber has a low melting point of about 100 degrees Celsius, the fiber will melt. As a result, the reference light beam 3 will not arrive at the end and be reflected and no light will be received by the detector 5. Therefore, by sensing the output of the detector 5, open-circuit failure in the power transmission fiber 2 can be detected. Each of the two plastic fibers 2a and 2b may be made of a single fiber or a bundle of fibers. The modes for carrying out this embodiment of the invention are the same as in the embodiment described previously. Also, the detector system is the same. Figs. 5A and 5B show the basic construction of the detection system of a third embodiment. In Fig. 5A, a fiber 1 transmitting a power beam 7 and a plastic fiber 2 folded back at a point close to the receiving end of the power transmission fiber are laid side by side. A reference beam 3 is introduced into one end of the plastic fiber 2. The reflected beam 6 from the folded point of the plastic fiber is received by a photodetector 5. If a fracture occurs in the power transmission fiber shown in Fig. 5B, the area around the site 8 becomes as hot as several hundred degrees Celsius, whereupon the low-melting-point plastic fiber is melted. As a result, the reference light beam is not reflected, and no light is received by the detector 5. Therefore, by sensing the output of the detector 5, the fracture in the power transmission fiber 2 can be detected A preferred implementation of this embodiment is shown in Fig. 6. The same modes discussed above are applicable to this embodiment, as well as the same detector system. According to the present invention, an opencircuit failur-e in a power transmission fiber is detected by a change in the amount of the light reflected through the plastic fiber. This provides positive protection against hazards resulting from open-circuit failure

Claims: CLAIMS 1. A device for detecting a fracture fault and in an optical power transmission fiber, comprising: a plastic fiber laid parallel to and closely adjacent said power transmission fiber, said plastic fiber having a melting point lower than a temperature generated at a fracture fault within said power transmission fiber; means for introducing a reference light beam at a first end of said plastic fiber; and means for sensing the presence/absence of said reference light beam at the other end of said plastic fiber. 2. The device of claim 1, wherein said plastic fiber comprises first and second portions laid parallel to one another along the length of said power transmission fiber. 3. The device of claim 2, further comprising means disposed adjacent an outward end of said power transmission fiber for transferring light between said first and second portions of said plastic fiber. 4. The device of claim 2, wherein said light transferring means comprises a mirror and a pool of matching oil disposed between said first and second portions and said mirror. 5. The device of claim 2, wherein said light transferring means comprises a bent segment of said plastic fiber. 6. The device of claim 1, wherein said plastic fiber has a reflecting coating on an end thereof adjacent an outward end of said power transmission fiber. 7. The device of claim 6, wherein said detecting means comprises a beam splitter disposed in a path of said reference beam, and means for producing an electrical signal in response to a return beam reflected by said halfslivered mirror.

Inventors: KATSUYOSHI, SUNAGO C/O OSAKA WORKS OF SUMITOMO, SHINYA, TAKENAKA C/O OSAKA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1983
Main classification: G01M11
IPC codes: G01M11