Optical fibers are commonly used for delivery of optical beams produced by laser sources such as laser diodes to a work surface or other target. Optical fibers offer many advantages over conventional optical systems used for beam delivery as optical fiber based systems tend to be small, inexpensive, and permit convenient beam delivery even to difficult locations. Optical fibers of many sizes and designs are commercially available.
Conventional optical fibers are typically cylindrical dielectric waveguides that transmit optical flux (e.g., light) along an axis using total internal reflection. Optical fibers typically consist of a core surrounded by a cladding layer, both of which are made of dielectric materials. The core is provided for optical flux propagation, while the cladding serves to confine the optical flux within the core. Further, the cladding can be surrounded by a buffer layer or coating (e.g., a polymeric coating) that can provide mechanical protection to the components of the optical fiber. To confine the optical beam in the core, the refractive index of the core is greater than that of the cladding. Confinement can be provided by an abrupt refractive index change at the core/cladding boundary (a step-index fiber) or a gradual variation in core refractive index (a graded-index fiber).
In coupling fibers to light sources, the fiber typically is cleaved or polished such that the ends of the core and cladding are substantially co-planar. An optical flux (e.g., a beam from a laser diode) is focused into the fiber at the planar end surface.
In order for the optical flux to propagate in the fiber, the optical flux must enter the fiber at an angle within the fiber numerical aperture. Optical flux that enters the fiber within the numerical aperture is confined in the core by the core/cladding boundary while optical flux at greater angles tends to enter the fiber cladding, producing a cladding mode flux. Light that misses the fiber core can also be coupled into the cladding. Many practical applications require a laser flux to be confined strictly to the core of the fiber. Optical flux not confined to the core can produce a halo effect at the end of the fiber and can result in substantial power loss and/or damage, especially in irradiation systems based on high power laser diodes. Optical flux coupled to the cladding must generally be removed, such as by using a mode stripper. However, use of a mode stripper can be an undesirable complication. Thus, there is a need for improved methods and apparatus for coupling optical power into optical fibers.