Abstract: A variable gain optical amplifier, in which homogeneous gain broadening is dominant, has first and second fixed gain rare-earth doped optical waveguide amplifiers (21, 23) optically in series together with an intervening variable attenuation optical attenuator (22). This arrangement circumvents the problem of gain tilt encountered when operating such amplifiers under variable gain conditions. An alternative form of the module has variable gain waveguide amplifiers, but these are co-regulated so that the aggregate of their gain at a wavelength within the gain spectrum is maintained constant. A further alternative form of module is employed in a concatenation of such modules. In such a concatenation, the gain of individual modules is allowed to vary, but the aggregate of the gain, at a wavelength within the gain spectrum, of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.

Abstract: To support interleaving of optical channels and to provide sufficient isolation between such interleaved channels, a circulator (60-62) receives, on an optical fiber (16-18), a broadband signal containing a plurality of adjacent channels. The circulator selectively routes the broadband signal through a filter (64, 66) to isolate, on an individual basis, at least one desired channel. A filtered signal, corresponding to the at least one desired channel, is then output from the filter (64-66) and reflected back into the filter by a mirror (68-70) such that the at least one desired channel is subjected to a second filtering process that further improves isolation of the at least one desired channel. Following this second filtering process, the at least one desired channel is applied to the circulator (60-62) for re-inserting into the optical fiber (16-18), as illustrated in FIG. 6.

Abstract: A variable gain optical amplifier, in which homogeneous gain broadening is dominant, has first and second fixed gain rare-earth doped optical waveguide amplifiers (21, 23) optically in series together with an intervening variable attenuation optical attenuator (22). This arrangement circumvents the problem of gain tilt encountered when operating such amplifiers under variable gain conditions. An alternative form of the module has variable gain waveguide amplifiers, but these are co-regulated so that the aggregate of their gain at a wavelength within the gain spectrum is maintained constant. A further alternative form of module is employed in a concatenation of such modules. In such a concatenation, the gain of individual modules is allowed to vary, but the aggregate of the gain, at a wavelength within the gain spectrum, of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.

Abstract: A variable gain optical amplifier, in which homogeneous gain broadening is dominant, has first and second fixed gain rare-earth doped optical waveguide amplifiers (21, 23) optically in series together with an intervening variable attenuation optical attenuator (22). This arrangement circumvents the problem of gain tilt encountered when operating such amplifiers under variable gain conditions. An alternative form of the module has variable gain waveguide amplifiers, but these are co-regulated so that the aggregate of their gain at a wavelength within the gain spectrum is maintained constant. A further alternative form of module is employed in a concatenation of such modules. In such a concatenation, the gain of individual modules is allowed to vary, but the aggregate of the gain, at wavelength within the gain spectrum, of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.

Abstract: In an optical transmission system employing optical amplifiers a method of regulating the gain of such an amplifier uses a non-linear control system whose non-linearity of operation is provided at least in part by the enabling/disabling of a portion of the control system by the operation of a transient magnitude threshold sensor.

Abstract: An optically amplifying waveguide guides both the LP.sub.01 and the LP.sub.02 mode. Its erbium distribution profile is preferentially matched with the modal field distribution of the LP.sub.02 mode. The waveguide is provided with long period perturbations of a pitch and strength that resonantly couples signal power of different wavelengths from propagation in the LP.sub.01 mode to propagation in the LP.sub.02 mode and back again to propagation in the LP.sub.01 mode whereby the longitudinal distribution of gain along the waveguide at one wavelength is longitudinally displaced with respect to that at least one other wavelength.