High power injection of laser light into optical fibers is a sensitive point for medical environment. Alignment, focal adjustment or poor spot quality can quickly destroy the fiber or the connector. So careful fiber selection and protection is mandatory. Metallic structures protections are particularly adapted to power delivery systems for security issues. Such structures can avoid the risk of outgoing laser emission in case of fiber breakage and also limit too sharp bending radius.
For power delivery applications, a crucial point is to select the proper fiber considering the wavelength, the numerical aperture, the core size as well as the material of the core, cladding, coating and protection. The quality of the laser delivery system will heavily depend on all these parameters.
Most of the time the fibers for medical power deliveries will be in silica / silica construction. A polyimide coating will increase temperature resistance and consequently their survival to “injection accident”.
Wavelength : Optical fibers materials are optimized for their transmission of a given wavelength range. As an example, multimode step index all silica fibers usually have a pure silica core and doped silica cladding. The OH ions content will determine the wavelength range transmitted. “High OH” fibers will better transmit UV and visible while a “Low OH” fiber will be selected for transmission of visible and near IR.
The maximum power that can be injected into an optical fiber is highly dependent of many parameters and particularly of the mechanical conditions. Fiber preparation, optical end face quality, stripping and connector termination procedure have a direct impact on performances. Fiber retention obtained by gluing (to be avoided) or poor crimping can induce fatal stress into the silica of the fiber.
Perfect optical end faces mandatory for most fiber applications can be obtained by polishing or cleaving. With high power applications results are better with cleaved end faces because even with careful polishing abrasion scratches can reduce transparency. Light absorption will increase temperature with a risk of destruction.
High transparency end faces can be obtained by cleaving.
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