A multicore fiber 1 includes: a small diameter portion 33 in which a propagation constant of light of an x.sub.1-th order LP mode of the first core 11 (here, x.sub.1 is an integer of 2 or more and x or less, x is an integer of 2 or more) and a propagation constant of light of a y1-th order LP mode of the second core 12 (here, y.sub.1 is an integer of 1 or more and y or less other than x.sub.1, y is an integer of 1 or more) coincide with each other and a large diameter portion in which a propagation constant of light of each LP mode of the first core 11 and a propagation constant of light of each LP mode of the second core 12 are configured not to coincide with each other are arranged.

An optical device includes a waveguide in which light is guided and that has electro-optical effect and thermo-optical effect, and a heater electrode that is arranged on one of side surfaces of the waveguide and that heats the waveguide. The optical device includes a parallel electrode that is arranged on the other of the side surfaces of the waveguide, that is electrically connected to the heater electrode, and that has high resistance as compared with the heater electrode.

An optical device includes a waveguide in which light is guided and that has electro-optical effect and thermo-optical effect, and a heater electrode that is arranged on one of side surfaces of the waveguide and that heats the waveguide. The optical device includes a parallel electrode that is arranged on the other of the side surfaces of the waveguide, that is electrically connected to the heater electrode, and that has high resistance as compared with the heater electrode.

Multimode beam combiners include at least one gradient-step index optical fiber in which a refractive index difference at a core/cladding interface is selected to provide a numerical aperture so as to provide stable, uniform beam output. One or more such fibers is formed into a tapered bundle than can be shaped to provide a selected illuminated aperture. The fibers in the bundle can be separated by respective tapered claddings so as to be optically coupled or uncoupled. Illumination systems can include a plurality of such fibers coupled to a plurality of laser diodes or other light sources.