An optical fiber is described which provides for the delivery of laser energy with a high power density from non-circular laser sources. It includes a transparent ceramic core having a generally rectangular cross section covered by a transparent cladding having a generally elliptical outer surface. The rectangular cross section has a width-to-thickness ratio substantially equal to a length-to-width ratio of a laser source output such as, for example, a laser diode. The equality of dimensional ratios permits efficient coupling of laser radiation to the transparent ceramic core. The rectangular cross section, thus, minimizes reduction in energy density at the output end of the fiber core, as occurs when a rectangular output of a solid state laser is coupled to a fiber core having a circular cross section. Embodiments are disclosed having dimensional ratios of from about 4:1 to 20:1 and beyond. The improved dimensional matching permits delivery of radiation from laser diodes at energy densities particularly useful for medical treatments.

The present invention is directed to an optical fiber comprising a core and a cladding layer wherein at least one of the constitutional parameters thereof changes along the longitudinal direction of the optical fiber. In order to present a useful optical fiber wherein the occurrence of a Brillouin scattering is prevented, an optical fiber having altered constitutional parameters is found to be effective. The constitutional parameters mean the parameters which determine the constitution of the optical fiber and is capable of influencing the condition of electromagnetic wave transmitting therethrough such as light or acoustic wave. The constitutional parameters include diameter of the core, index of refraction of the core, diameter of the optical fiber, composition of the glass, residual stress of the core. Some examples are disclosed about their manufacturing process and the test results. Much improvement was measured, especially in the use for a single mode optical fiber.

A clad optical fiber having a non-circular section is provided by apparatus which continuously extrudes a flowable jacketing material around a circular fiber preform and then draws the jacketed fiber through a circular sizing die to provide the desired cross-­sectional shape. A minicomputer with inputs from a winding bobbin position sensor and pressure transducer not only controls feeding of the circular fiber preform and the flow of jacketing material to the extruder, but also the supply of inert gas to the apparatus. A particularly favorable clad fiber cross-sectional shape includes tangentially alternating circular segments and radially extending "flaps" positioned to fall in the interstices formed when the fiber is wound on a bobbin with abutting and overlapping turns.

A low loss fiber optic coupler is fabricated by forming a coupler preform (10) having a plurality of spaced glass cores (12,13) extending longitudinally through a matrix of glass having a refractive index lower than that of the cores. The preform is heated and stretched to form a glass rod (20) which is then severed into a plurality of units (21). Heat is applied to the central region (23) of each unit while the ends of the unit are pulled apart to elongate the taper inwardly the heated central region. The unit is then provided with a plurality of optical fibers (69, 70, 75, 76°, 82, 83; 89), one of which extends from each of the cores at the endfaces of the unit. A preferred method of providing the optical fibers involves forming the coupler preform of a matrix glass (46) that is easily dissolves in a solvent. Each of the fiber cores within the matrix is surrounded by a layer of cladding glass (54) that is relatively resistant to disolving by the solvent.