Abstract: An apparatus and method for controlling the gain in an erbium-doped fiber amplifier (EDFA) incorporated into a multi-wavelength communication system so as to amplify each of the wavelength signals. The amplifier operates near to saturation so that, if one or more of the multi-wavelength signals is removed from the transmission, the remaining channels are increasingly amplified, leading to problems with other components in the system which depend upon intensity. According to the invention, an optical signal at a wavelength that is not within any of the transmission channels is selectively fed back around the amplifier and caused to lase in a wavelength-filtered ring-laser configuration. The lasing signal governs the saturation of the amplifier such that any gain shed by a disappearing data signal is predominately used by the lasing signal, not by the remaining data signals. Thereby, the data signals do not experience gain variations dependent upon the number of data signals being amplified.

Abstract: A method and apparatus for compensating for temporally varying effects, such as temperature, in a liquid-crystal cell, particularly a wedge-shaped liquid-crystal cell. The temperature of the liquid crystal is measured, either directly with a thermocouple or the like, or by optical means which depend upon the temperature of the liquid crystal. For example, an intensity detector monitors the polarization rotation of a probe beam of light having a substantially different wavelength than that of the beam modulated by the liquid crystal, and a feedback circuit keeps the intensity at a predetermined value corresponding to the optimum path length in the liquid crystal of the modulated beam. The feedback signal from the temperature measurement is used to effectively change the gap filled with liquid crystal. For a wedge-shaped cell, the gap size is changed by moving the cell in the wedge direction.

Abstract: An optical switch particularly usable as a wavelength-division add/drop multiplexer (WADM) in a multi-wavelength communication system. Four multi-wavelength beams optically coupled to the input, output, add, and drop channels are arranged in parallel in a rectangular array and are incident upon a diffraction grating. The grating operating in one direction disperses the beams into their wavelength components and operating in the other direction recombines the wavelength components into a multi-wavelength beam. A lens focuses the components of the beams having a particular wavelength upon one of an array of tiltable micro-mirrors integrated on a silicon substrate. In one position, the mirror for a particular wavelength reflects that wavelength component from the input beam back to the output beam. In a second position, that mirror reflects that wavelength component from the input beam back to the drop beam and reflects that same wavelength component from the add beam back to the output beam.

Abstract: An integrated multi-wavelength transmitter, particularly useful in a wavelength-division multiplexed optical communications system. An arrayed waveguide grating has feedback channels between two coupling regions and is constructed such that, at each coupling region, one port carries all the optical signals of differing wavelengths while a set of other ports carry wavelength-separated signals. The multi-wavelength signal from a first coupling region is reflected from a mirrored surface back into the first coupling region and the arrayed waveguide grating, the mirrored surface thus forming one end of an optical cavity. Similar elements are arranged between corresponding single-wavelength ports of the two regions. The channel from the second coupling region is split into two parts. One part is passed through an optical gain region and is reflected back from the mirrored surface through the power splitter, now acting as a combiner.

Abstract: A liquid-crystal cell having plates defining a gap between them which is filled with a liquid crystal, especially a twisted nematic. The plates are made non-parallel to a predetermined degree so that the liquid crystal assumes a wedge shape, producing different effective thicknesses of the liquid crystal depending on the lateral position within the cell. In use, a narrow beam irradiates a portion of the cell, and the cell is positioned along the wedge direction so as to optimize the cell performance. The invention is particularly useful with a liquid-crystal multi-wavelength switch, which requires extinction ratios between the two states of the cell.

Abstract: A wavelength-interchanging cross-connect for use as an optical switch within a wavelength-division multiplexing (WDM) communications network in which multiple optical carrier signals at different optical wavelengths are switched in an all-optical process according to their wavelength. The cross-connect of the invention further allows the optical wavelength of the signal being switched to be changed to another WDM wavelength. The cross-connect includes a series of multi-wavelength 2.times.2 spatial optical switches. One serial path is directly connected between the spatial switches. The other serial path leads through intermediate wavelength-interchanging modules that interchange the carrier wavelengths of at least one pair of signals. Preferably, the wavelength interchanging is performed by difference frequency generation relying on second-order non-linear susceptibilities and a pump signal providing parametric gain.

Abstract: An opto-electronic integrated circuit including an active ridge waveguide (60), for example, a semiconductor laser diode, and a passive buried heterostructure semiconductor waveguide (64). The two types of waveguides are chosen for their respective tasks so as to minimize the lasing wavelength dependencies arising from fabricational variations and to simultaneously reduce the allowable bending radius, thus reducing the chip size. The two waveguides are coupled by a transition structure (62) including a laterally undefined slab waveguide. A fabricational method is described that self-aligns the ridge and buried heterostructure waveguides so that the transition loss is negligible. The method can be integrated with the fabrication of a window facet (118', 118") between an end of the ridge waveguide and the chip edge, which prevents unintended back reflections from the chip edge.