1.  Field of the Invention
The present invention generally relates to optical fibers and, more specifically to double cladding and multi-mode fibers that more efficiently couple an incident power beam, as well as improved methods of treating such fibers for improving coupling efficiency. 
2.  Description of Related Art
Multi-mode fibers, including double cladding fibers, are often designed with high numerical apertures (i.e., greater than about 0.3) in an attempt to achieve the maximum use of a multi-mode incident power beam.  Such fibers often use synthetic materials in their claddings, as those offer small refractive indexes, thus maximizing the numerical aperture of the fiber.  When high-power incident beams (i.e., on the order of about 1 W and more) are used, even small power leakages outside of the core (or inner cladding) frequently damage the fibers beyond use. 
Past efforts to minimize or eliminate fiber damage from the above type of power leakage have included elaborate optical imaging schemes.  For example, an incident beam is passed through a first objective to focus the beam through an aperture.  The aperture is sized to be larger than the cross section of the fiber core, yet small enough to remove wings from the incident beam.  The incident beam then passes through a second objective that images the aperture onto the front end of the fiber core.  It can be seen, however, that such a design scheme adds bulk and complexity to the overall system, as compared to a scheme which does not require the first objective and aperture. 
Given the bulkiness of the above design scheme, efforts have been made to eliminate the first objective and aperture by stripping the cladding and then cleaving or polishing the core (or inner cladding).  That procedure provides a rugged end which is less susceptible to burning when a launched beam extends beyond the core (or inner cladding).  That lessened susceptibility is due to the fact that the non-coupled portion of the launched beam is unable to impinge the cladding, which would otherwise be disposed immediately adjacent the core (or inner cladding) and launched beam interface.  Instead, the non-coupled portion of the launched beam is diffracted away from the fiber because the cladding has been stripped from the end of the fiber. 
But the process of stripping and cleaving/polishing has its disadvantages.  High losses result for incident rays propagating at relatively large angles with respect to the fiber axis.  These losses occur at the edge of the stripped outer cladding, perhaps due to microcracks introduced to the outer cladding in the course of stripping.  Consequently, the effective numerical aperture is reduced, thus reducing the overall coupling efficiency.  Moreover, the power leakage at the edge of the stripped outer cladding may cause fiber damage at high levels of beam power.  To avoid those coupling losses from high angle rays, the stripped end has been coated with a polymer having an index of refraction which is about the same as the index of refraction of the cladding.  In so doing, a polymer coating is formed over the edge of remaining outer cladding and the stripped portion of the core. 
Still, even though the coupling losses are minimized with the polymer coating, disadvantages are present.  When a portion of the high power launched beam misses the core (or inner cladding), the fiber can easily burn since the beam reaches the polymer coating.  That disadvantage has sought to be addressed by the use of the multiple objectives and aperture described above.  Therefore, the disadvantage of bulk again arises, as do issues of being labor intensive to set up and maintain the system.  Also, such system design is sensitive to temperature variances and environmental vibration, both of which can cause shifting of the system components.  Misalignments from such shifting can eventually lead to fiber damage. 
As can be seen, there is a need for improved methods of treating high numerical aperture fibers to minimize potential damage from the wings of a launched beam which miss the core (or inner cladding).  There is also a need for such methods that are easy, inexpensive, and quick, yet still effective in increasing coupling efficiency.  Additionally, there is a need to omit the requirement of multiple components to launch a beam into a fiber while still minimizing the potential for fiber damage.  A need also exits for improved fibers, both multi-mode and single mode double cladded, which are less susceptible to damage from the non-coupled portion of a launched beam.