OPTICAL FIBRE CABLE

Abstract: An optical fibre cable of the type comprising a fibre (3) having a central core and a surrounding cladding and made for transmitting optical high-power, specifically power exceeding 1 kW. At least one of the fibre ends is provided with cooling means for optical power loss comprising a cavity with a flowing coolant (2) surrounding the envelope surface of the fibre end. Incident radiation that falls outside the fibre comes into the coolant, preferably a liquid coolant such as water, where it is at least partially absorbed. At least one of the limiting wall surfaces of the cavity is completely or partially non-absorbing for the incident radiation, while the other limiting wall surfaces are arranged to absorb such radiaton that is still present and transmitted through the flowing coolant (2). As these surfaces are in direct contact with the coolant an efficient cooling is obtained.

Description: Optical fibre cable The present invention relates to an optical fibre cable which comprises a fibre having a central core and a surrounding cladding and which fibre cable is made for transmitting high optical power, specifically power exceeding 1 kW. At least one of the contact ends of the fibre has cooling means for optical power loss. Optical fibre cables for transmitting high optical power are frequently used in industrial applications. Specifically they are used in cutting and welding operations by means of high-power laser radiation, but also in other industrial applications such as heating, detection or working operations in high-temperature environments this type of optical fibre cables can be used. One of the main problems in this type of high power applications, however, is the problem how to take care of radiation that falls outside the core of the fibre. The power density is normally very high and specific cooling means are required in order to prevent an uncontrolled heating, especially in case of strong back scattering in for instance welding operations . Different methods to take care of such undesired power radiation are already known. One example is disclosed in DE 4305313, in which the radiation that falls into the cladding of the fibre is spread in a so-called mode stripper and absorbed by a metal surface. This surface can then be cooled from the outside of the component. A similar method is described in SE 83.07140-7. An optical fibre cable of the above-mentioned type is also presented in SE 93.01100-5. In said fibre cable at least one of the end surfaces of the fibre core is provided with a rod having a larger diameter than the core diameter. At this end the fibre is provided with a reflector designed to conduct rays entering outside the fibre towards an area where they can be absorbed without causing any damage. In the illustrated embodiment this area is surrounded by a heat-abducting device with cooling fins, but it is also mentioned that water cooling means may be included in this area for cooling off the generated heat. Also in this case the cooling is provided from the outside of the component. A weakness in all of these methods that now have been de- scribed is the fact that the heat first must be absorbed by a metal surface and then conducted through the metal material to the cooled surface either this surface is cooled by means of air or by water. The object of this invention is to provide an optical fibre cable with an improved cooling capability so that the fibre can be used for transmitting a very high optical power without causing any damage to the fibre itself or to the housing. The invention is based upon the fact that the power (heat) is absorbed directly in a cooling medium instead of being conducted through a metal material. According to the invention at least one of the contact ends of the fibre, comprising the core and the surrounding cladding, are located in a cavity filled with a flowing coolant so that radiation falling outside the fibre is entered into and absorbed at least partially by the coolant. The walls of the cavity comprises at least one non-absorbing surface. The other wall surfaces could be absorbing (metal). Radiation passing through the coolant is absorbed by said surfaces . As these surfaces are directly cooled by the flowing medium (liquid coolant) any uncontrolled heating can be avoided. As the optical radiation is passing through the flowing coolant before reaching the metallic surface, only a minor part of the radiation is absorbed by the surface. According to a preferred embodiment the fibre is directly in contact with the surrounding coolant, for example water. According to an alternative embodiment the fibre is surrounded by a transparent tube which then is directly in contact with the surrounding coolant. In the following the invention will be described more in detail with reference to the accompanying drawings which schematically illustrates some examples of the new optical fibre cable. Figure 1 shows the principles for an optical fibre cable with direct water cooling means of the contact end of the cable, Figure 2 is a detailed view of the interface zone between the end portion of the fibre and the transparent "window" through which the radiation is entered into the cavity with the liquid coolant surrounding the end portion of the fibre, Figure 3 shows a so-called mode stripper arranged around the fibre in the liquid coolant cavity in order to transmit radiation in the cladding of the fibre out to the sur- rounding coolant, Figure 4 is a detailed view of the interface zone between the end portion of the fibre and the transparent window according to an alternative embodiment in which the "win- dow" is made as an optical disc with a central bore which disc is sealed against the circumferential surface of the fibre, and Figure 5 shows three examples how to arrange a transparent capillary tube around the fibre in the liquid coolant cavity. Figure 1 illustrates one end of a conventional optical fi- bre 3 having a core, for example of quartz glass, and a cladding, for example made of glass or some polymer having a suitable refractive index. A laser beam 1 is focused on the end surface of the fibre. Preferably a Nd-YAG laser source is used which has a wavelength of 1,06 μ . This wave-length is suitable for optical fibre transmission. Other examples of lasers that can be used is diode lasers, C0 2 -lasers, CO-lasers and other types of Nd-lasers. A liquid coolant 2 is surrounding the envelope surface of the end portion of the fibre. That part 4 of the incident laser radiation that falls outside the core of the fibre is entered into and absorbed, at least partially, by the coolant. Radiation transmitted through the liquid is absorbed by the walls 8,5 enclosing the liquid. These walls are in direct contact with the coolant so that they are cooled directly on the surface. The rear wall 5 has an input pipe 5a as well as an output pipe 5b for the liquid coolant. The absorption in the liquid should not be too high due to the risk of shock boiling of the liquid when it is hit by the radiation. Water is a suitable cooling medium, for simplicity, but also for the reason that the deep of penetration is suitable. For a Nd-YAG laser, for example, the deep of penetration is approximately 50 mm. The surface hit by the incident laser beam must be transparent in order to allow the radiation to pass into the liquid cavity. This surface, the so-called window 7, can either be glass-clear or diffuse, the important thing is that the absorption in this surface is low. According to a preferred embodiment of the invention the end surface 6 of the fibre is in optical contact with the window 7. The optical window then must have a good optical quality as also the original radiation is passing through this window. By means of the optical contact between the window and the end surface of the fibre, basically all reflection losses can be eliminated in the interface zone, see figure 2. Optical contact can be achieved by means of fusing the fibre and the window together like illustrated in the above-mentioned SE 93.01100-5 for the rod fused together with the end surface of the fibre. The window 7 should be comparatively thick in order to allow an anti- reflex coating on the surface. In addition to the radiation which falls completely outside the fibre, also such radiation that has passed into the cladding 9 should be directed out into the surrounding coolant. This can be achieved by means of a so-called mode stripper 10 arranged on the fibre as illustrated in figure 3. The mode-stripper could be a glass-capillary of the type which has been previously illustrated in SE 83.07140- 7 and SE 93.01100-5 and in which the glass-capillary is bonded to the envelope surface of the cladding and thus removing any radiation from the cladding. Alternatively, the mode-stripping could be achieved by means of a roughening of the envelope surface of the fi- bre. Such roughening is previously known per se, see US 4,575,181. Due to the roughening, the radiation in the cladding will be directed out from the cladding into the coolant where it is absorbed. In the embodiment described so far the window and the fibre were fused together in order to obtain a good optical contact. Another way to obtain a good optical contact is to press the fibre and the window against each other. Also in such case the losses in the contact zone are negligi- ble. Instead of applying the end surface of the fibre against a window 7 an optical disc 11 having a central bore can be used, see figure 4. The disc 11 can be made as a transparent, non-absorbing limiting surface (front wall) for the coolant in the same way as the window 7, but with a central opening into which the fibre end has been inserted. The disc is arranged with an appropriate sealing against the envelope surface of the fibre. The disc need not to be of the best optical quality as the major part of the radiation 2 does not pass through the disc but directly into end surface of the fibre. The aperture disc can be either glass-clear or mat finished. In one of the embodiments the fibre 3 is surrounded by a capillary tube 12 made of a transparent material, for example quartz glass, so that the envelope surface of the capillary tube is in contact with the coolant. In figure 5 there are three examples of capillary tube applications. In figure 5a the capillary tube 12 extends up to and is sealed against the inner surface of the window 7. In the example in figure 5b the fibre 3 extends through the front window (disc) 11. Even in this case the capillary tube is sealed against the inner surface of the disc 11. In the third example, which is illustrated in figure 5c, also the capillary tube 12 itself extends through the front wall 13. In this case it is not necessary that the front wall is transparent as the radiation incident outside the end surface 14 of the capillary tube is negligible. The object of the capillary tube is to provide an extra protecting, non-absorbing casing of the fibre. As already mentioned the capillary tube is sealed against both the front and rear walls of the cavity so that the coolant is closed within the annular spacing formed between the envelope surface of the capillary tube and the cylindrical outer wall of the cavity and will not come into contact with the envelope surface of the fibre itself. Contrary to the glass capillary used as a mode stripper this capillary tube need not be fixed by cement on the fibre. In case a capillary tube is used the mode stripping is made by roughening the surface of the cladding. The invention is not limited to the illustrated examples but can be varied within the scope of the accompnying claims.

Claims: CLAIMS 1. An optical fibre cable of the type comprising a fibre with a central core and a surrounding cladding and which fibre cable is made for for transmitting optical high-power, specifically power exceeding 1 kW, and wherein at least one of the fibre ends has cooling means for optical power loss c h a r a c t e r i s e d i n that said cooling means comprises a cavity with a flowing coolant (2) surrounding the envelope surface of said fibre end so that incident optical radiation falling outside the fibre comes into and is absorbed at least partially by said coolant (2) and that at least one of the limiting wall surfaces of the cavity is completely or partially non-absorbing for the incident radiation while the other limiting wall surfaces are arranged to absorb radiation transmitted through said flowing coolant and are directly cooled by said coolant. 2. Optical fibre cable according to claim 1 c h a r a c t e r i s e d i n that said coolant (2) is a liquid coolant, preferably water. 3. Optical fibre cable according to claim 2 c h a r a c t e r i s e d i n that the fibre (3) is in direct contact with the liquid coolant. 4. Optical fibre cable according to claim 2 c h a r a c - t e r i s e d i n that the fibre ( 3 ) is surrounded by a transparent tube (12) which is in direct contact with the liquid coolant. 5. Optical fibre cable according to claim 2 c h a r a c - t e r i s e d i n that the limiting wall surfaces of the cavity filled with the liquid coolant comprises a forward at least partially non-absorbing limiting wall surface (7,11) through which the radiation is entered into the cavity, one substantially cylindrical, absorbing limiting wall surface (8) extending coaxially in the longitudinal direction of the fibre and a rear, absorbing limiting wall surface through which inlet- and outlet pipes (5a, 5b) are arranged for the liquid coolant. 6. Optical fibre cable according to claim 5 c h a r a c t e r i s e d i n that the forward limiting wall surface comprises a transparent window ( 7 ) . 7. Optical fibre cable according to claim 6 c h a r a c t e r i s e d i n that the end surface (6) of the optical fibre (3) is brought into optical contact with said window ( 7 ) . 8. Optical fibre cable according to claim 7 c h a r a c t e r i s e d i n that the end surface (6) of the optical fibre is fused together with the window (7). 9. Optical fibre cable according to claim 7 c h a r a c t e r i s e d i n that the end surface (6) of the optical fibre has been pressed against the window (7) for a good optical contact between the window and the fibre. 10. Optical fibre cable according to claim 5 c h a r a ct e r i s e d i n that the forward limiting wall surface comprises an optical, transparent disc (11) with a central through bore through which the end of the fibre is extending and wherein the envelope surface of the fibre has been sealed against said optical disc. 11. Optical fibre cable according to claim 1 c a r a ct e r i s e d by a so-called mode-stripper (10) arranged on the fibre (3) in order to divert radiation from the cladding (9) to the surrounding coolant (2). 12. Optical fibre cable according to claim 11 c h a r a c- t e r i s e d by the mode-stripper (10) being made as a roughened portion of the envelope surface of the fibre. 13. Optical fibre cable according to claim 11 c h a r a ct e r i s e d i n that the mode-stripper (10) comprises a glass-capillary in optical contact with the envelope surface of the cladding (9). 14. Optical fibre cable according to claim 11 c h a r a c- t e r i s e d i n that the mode-stripping (10) is achieved by the refractive index of the coolant being larger than or equal to the refractive index of the cladding. AMENDED CLAIMS [received by the International Bureau on 27 November 1997 (27.11.97); original claims 1 and 5-14 replaced by amended claims 1 and 5-7; remaining claims unchanged (2 pages)] CLAIMS 1. An optical fibre cable of the type comprising a fibre (3) with a central core and a surrounding cladding (9) and which fibre cable is made for transmitting optical high- power, specifically power exceeding 1 kW, and wherein at least one of the fibre ends has cooling means for optical power loss comprising a cavity with a flowing, absorbing coolant (2) surrounding the envelope surface of said fibre end so that incident optical radiation falling outside the fibre comes into and is absorbed at least partially by said coolant (2) and that the limiting wall surfaces of the cavity comprises a forward, at least partially non-absorbing surface (7) through which the radiation is entered, while the limiting wall surfaces are arranged to be directly cooled by said flowing coolant in order to avoid any uncontrolled heating of said surfaces due to absorbed radiation c h a r a c t e r i s e d i n that the forward limiting wall surface comprises a transparent window (7) and that the end surface (6) of the optical fibre (3) is brought into optical contact with said window (7). 2. Optical fibre cable according to claim 1 c h a r a c t e r i s e d i n that said coolant (2) is a liquid coolant, preferably water. 3. Optical fibre cable according to claim 2 c h a r a c t e r i s e d i n that the fibre (3) is in direct contact with the liquid coolant. 4. Optical fibre cable according to claim 2 c h a r a c - t e r i s e d i n that the fibre (3) is surrounded by a transparent tube (12) which is in direct contact with the liquid coolant. 5. Optical fibre cable according to claim 2 c h a r a c t e r i s e d i n that the limiting wall surfaces of the cavity filled with the liquid coolant comprises a substantially cylindrical limiting wall surface (8) extending co- axially in the longitudinal direction of the fibre and a rear, limiting wall surface through which inlet- and outlet pipes (5a, 5b) are arranged for the liquid coolant. 6. Optical fibre cable according to claim 1 c h a r a c - t e r i s e d i n that the end surface (6) of the optical fibre -s fused together with the window ( 7 ) . 7. Optical fibre cable according to claim 1 c h a r a c t e r i s e d i n that the end surface (6) of the opti- cal fibre has been pressed against the window (7) for a good optical contact between the window and the fibre.

Inventors: ROOS SVEN-OLOV, SANDSTROEM ULF, TORSTENSSON PER-ARNE, ROOS, SVEN-OLOV, SANDSTROEM, ULF, TORSTENSSON, PER-ARNE
Applicants: PERMANOVA LASERSYSTEM AB, ROOS SVEN OLOV, SANDSTROEM ULF, TORSTENSSON PER ARNE, ROOS, SVEN-OLOV, SANDSTROEM, ULF, TORSTENSSON, PER-ARNE
Application year: 1997
Main classification: G02B6
IPC codes: G02B6