In order to avoid damage of the optical fiber contact in case of incident radiation outside the core of the fiber, or radiation reflected back to the fiber contacts, specific methods have been developed to take care of such power loss. When transmitting low optical power a more conventional type of fiber contact might be used, but in case of high optical power some kind of cooling is required, for instance means for absorbing incident power loss radiation at least partially in a flowing coolant.
Optical fiber cables for transmitting optical power in the range of some mW up to several kW are frequently used in industrial applications. For such applications specific type of contacts have been developed to transmit the radiation between different units. For low optical power it is previously known a contact type called SMA, based on a ferrule which is centering the fiber. The ferrule can be made of a metal, or a ceramic material in order to sustain more high optical power. Such a contact is small and has a low manufacturing cost, but has a limited use because of low cooling capacity so that the contact might be damaged under heavy heating. For that reason there are other types of fiber contacts for high optical power on the market which are based on the fact that a certain length of the fiber is detached. However, only a limited increase of capacity can be achieved by this type of contact design. The contact might transmit high optical power, but if the radiation for some reason comes outside the core of the fiber the contact might be destroyed. For that reason this type of contact should be used only for optical power up to some tenths of W.
The main reason for a damage in the optical system is the heating. Normally, an optical fiber has an inner core made of glass and one or more surrounding layers having a refractive index which exceeds the refractive index of the core in order to “lock” the radiation into the core without any power loss. Such a surrounding layer or layers are called the cladding of the fiber. Outside the cladding there are also one or more protecting layers to stabilise the fiber mechanically. These layers are called buffers or jackets, and they are optimized for a high mechanical capacity but they do not have the necessary optical capacity to take care of high optical power. In case of radiation entering into the cladding such radiation is propagating through the cladding up to the area in which the surrounding protecting layers are connected to the cladding. This is a critical region of the fiber, and therefore a damage might be expected here.
Another critical region of the fiber, which is exposed to damages, is the part of the fiber which is in a mechanical contact with a supporting element. Radiation transmitted in the cladding might leak out in such contacting points and cause a damage of the fiber. As soon as the fiber is built into a contact, the fiber must be maintained and centered in a correct position in the contact body and therefore some mechanical component is required to hold the fiber and which then also comes into a mechanical contact with the fiber.
Different methods to take care of the unwanted power radiation and protect the fiber components against damages are already known. In fiber contacts for high optical power unwanted radiation can be removed from the cladding by means of so-called mode stripping, see for instance EP 0619508. In this case the cladding is provided with an additional layer which can be roughened for diverging unwanted radiation, which is entered into the cladding, into the surroundings. By making the fiber contact big enough, and thereby the outer surface exposed to the surroundings big enough, unwanted heating can be reduced, see for instance DE 4305313. There are also other types of cooling methods, specifically by means of inner water cooling in the contact so that unwanted power radiation is spread away, see EP 0910810.
In SE 0600263-8 it is described another example of an optical fiber contact for high optical power. In this case the end surface of the fiber is in optical contact with a body of a transparent material, for instance a rod or any other type of a solid body made of quartz, which body in connection with the optical fiber end has a surface area exceeding the contact surface area of the fiber end and has a conical design. By such a design of the surface of the transparent body a more efficient flowing geometry is provided around the fiber end. Furthermore, such a surface provides an increased area for incident power loss radiation as well as deflection of such incident radiation towards the optical axis of the fiber contact.
The cooling methods which have been described now can be used also for very high optical power. However, such optical fiber contacts are complicated, expensive and voluminous. There is a need for an optical fiber contact to be used for moderate-magnitude power radiation, typically 50-500 W, and which are not too complicated and voluminous.
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