| 摘要 |
<p>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 LP<Sub>01</Sub> to an appropriate tunneling leaky (TL) mode, e.g., LP<Sub>11</Sub>. 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 ^(z) is chosen such that</p><p>O<Sub>o</Sub> = (2π/<Image id="ia01" he="3" wi="2" file="IMGA0001.TIF" imgContent="character" imgFormat="TIFF" inline="yes" />) > β<Sub>01</Sub>-2πn<Sub>cl</Sub>^<Sub>o</Sub>, where w is the average grating spacing, β<Sub>01</Sub>, is the propagation constant of the LP<Sub>01</Sub> mode, n<Sub>cl</Sub> is the refractive index of the fiber cladding, and <Sub>o </Sub>is the wavelength of the radiation to be coupled from or into the fiber. Furthermore, ^(z) is to be chosen such that</p><p>Oo β01, - βrs, where βrs is the propagation constant of the selected TL mode. In order for the coupling to be quasi-resonant, it is necessary that αrs, the attenuation constant of the selected TL mode, be relatively small, typically < 1 dB/cm. By appropriately choosing ^(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.</p> |
