An apparatus and a method for separating a light of a first wavelength .lambda..sub.1 from a second wavelength .lambda..sub.2, where .lambda..sub.1 <.lambda..sub.2, in a waveguide such as an optical fiber is described. The apparatus includes a core surrounded by a depressed cladding, which itself is surrounded by a secondary cladding. The core cross-section, the depressed cladding cross-section, the secondary cladding cross-section, and the refractive indices of the core, the depressed cladding and the secondary cladding are selected to produce a fundamental mode cutoff wavelength .lambda..sub.c such that .lambda..sub.1.lambda..sub.c <.lambda..sub.2, and produce a high loss at the secondary wavelength .lambda..sub.2. The apparatus can be used as a filter, an amplifier, a laser, or in a nonlinear optical switch.

Μια συσκευή και μια μέθοδος για ένα φως ενός πρώτου μήκους κύματος λαμψδα..σuψ.1 από ένα δεύτερο μήκος κύματος λαμψδα..σuψ.2, όπου λαμψδα..σuψ.1