For single-mode fiber coupling element and this transmission system having.

摘要

Optical radiation can be efficiently removed from, or injected into, single mode optical fiber at an intermediate point along the fiber, by causing quasi-resonant coupling of the guided mode LP01 to an appropriate tunneling leaky (TL) mode, e.g., LP11. Such coupling is caused by means of a "grating" in the fiber, with the grating being formed by impressing a periodic (or pseudo-periodic) modulation on the fiber, or by causing a periodic (or pseudo-periodic) variation of the refractive index of the fiber by means of the photoelastic or the photorefractive effect. The nominal grating spacing LAMBDA (z) is chosen such that OMEGA o=(2 pi / LAMBDA ) beta 01-2 pi ncl/ lambda o, where LAMBDA is the average grating spacing, beta 01 is the propagation constant of the LP01 mode, ncl is the refractive index of the fiber cladding, and lambda o is the wavelength of the radiation to be coupled from or into the fiber. Furthermore, LAMBDA (z) is to be chosen such that OMEGA o DIFFERENCE beta 01- beta rs, where beta rs is the propagation constant of the selected TL mode. In order for the coupling to be quasi-resonant, it is necessary that alpha rs, the attenuation constant of the selected TL mode, be relatively small, typically 1 dB/cm. By appropriately choosing LAMBDA (z) and/or the amplitude function of the grating, it is possible to increase the coupling efficiency above what is possible with a constant spacing, constant amplitude grating. Devices according to the invention can be advantageously used not only as radiation couplers but also as filters and, if they are of the photorefractive type, as amplitude modulators.