Abstract: A communication system has a first communication system for performing wavelength division multiplexing communications, and a second communication system independent of the first communication system. In the communication system, a wavelength control unit detects the wavelength arrangement in the first communication system, generates a wavelength control signal on the basis of the detected wavelength arrangement, and transmits the wavelength control signal to the respective communication nodes via the second communication system. Each communication node controls its transmission wavelength on the basis of the wavelength control signal. As the wavelength control unit, there are disclosed an arrangement for arranging a wavelength control node independently of the communication nodes, an arrangement using a communication node in a non-receiving state, and an arrangement for assigning the function of the wavelength control unit to a center node.

Abstract: Disclosed in this application is a method for correcting the relation between a control signal for controlling a selecting wavelength of an optical filter and the selecting wavelength of the optical filter. In the present invention, the selecting wavelength of the optical filter is swept in a wavelength range comprising wavelengths of light of first and second wavelengths generated by optical frequency modulation, and the above correction is carried out based on a difference between values of the control signal when the light of the first and second wavelengths is selected. A wavelength difference is normally controlled with high accuracy between light of two wavelengths generated by optical frequency modulation, and the present invention thus involves performing the above correction based on the wavelength gap controlled with high accuracy.

Abstract: In a light source apparatus, the polarization mode of oscillation light from a semiconductor laser is switchable between two different polarization modes when a modulation current is injected into a portion of a light waveguide of the semiconductor laser. Light in one polarization mode and light in the other polarization mode are separately obtained from the oscillation light from the semiconductor laser. At least the light in one of the two different polarization modes is converted to an electric signal. Current injected into the semiconductor laser is controlled based on the electric signal such that a modulation state of light from the semiconductor laser is stabilized. The light in the other polarization mode, or light in one polarization mode emitted from the other emission side of the semiconductor laser may be used for optical transmission.

Abstract: An optical semiconductor apparatus having no polarization dependency and an optical communication system using the semiconductor apparatus are provided. In an optical communication system wherein a signal is demultiplexed by using a semiconductor waveguide type filter disposed on a receiver side, two semiconductor lasers disposed on a transmitter side modulate two light signals having different wavelengths using a common modulation signal and transmit the modulated light signals. One of the different wavelengths is accorded, for example, to the TE mode transmission wavelength of the filter, and the other is accorded, for example, to the TM mode transmission wavelength of the filter. On the receiver side, lights at different wavelengths may be branched into two light portions and the branched light portions may be respectively detected by two photodetectors.

Abstract: In an optical receiver of the present invention, an optical signal is split into at least two light portions, and the light portions are respectively received by at least two tunable optical filters. The tunable optical filters are respectively capable of changing their transmission wavelengths. Transmitted lights through the tunable optical filters are received by at least two light receiving elements. The light receiving elements respectively supply outputs. A differential amplifier produces a difference signal from the outputs of the light receiving elements. In an optical FDM-FSK transmission system using the optical receiver, an optical FSK signal is received by utilizing the difference signal. The tracking operation of the tunable optical filters, whose object includes, for example, at least a center optical frequency of the tunable filters, can be readily performed.

Abstract: A distributed feedback semiconductor laser selectively performs one of two oscillations in different polarization modes having different polarization planes. The laser includes a substrate, a light waveguide formed on the substrate, which at least partially includes an active layer, a diffraction grating formed around the waveguide, and an injecting unit for injecting a modulation current into a portion of the waveguide. The waveguide and the grating are constructed such that oscillation wavelengths of the polarization modes are different from each other and minimum values of threshold gain in the polarization modes near the Bragg wavelength are approximately equal to each other. The semiconductor laser can be driven by a minute modulation current and prevents degradation of response characteristics due to dynamic wavelength fluctuation in a high-frequency range when used in optical communication systems in which the switching of polarization mode is performed.

Abstract: A tunable-filter control method or apparatus tracks a central wavelength in a transmission spectrum of a tunable-filter to a wavelength of light to be received. The central wavelength in the transmission spectrum of the tunable-filter is periodically and minutely modulated. An error signal is generated for use in tracking the central wavelength to the wavelength of the light to be received, by using a tunable-filter output which exhibits the effect of the minute modulation of the central wavelength. A tracking condition, which corresponds to one of a draw-in state and a stable state, is monitored to control the modulation of the central wavelength based on a result of the monitoring, so that a modulation amplitude of the central wavelength in the minute modulation is increased at the time of the draw-in state of the tracking condition and is decreased at the time of the stable state of the tracking condition.

Abstract: An optical communication network includes at least one optical transmission path, a plurality of terminals connected to the optical transmission path, a plurality of optical nodes for receiving and transmitting an optical signal between the optical transmission path and the terminals, and a controller for controlling the intensity of a signal output from each optical node so as equalize its intensity with the intensity of a signal input to each optical node from the optical transmission path. In this optical communication network, the loss of a signal's intensity is determined solely by the total extension length of the optical transmission path, so that the optical modes can be freely re-connected in the network.

Abstract: In optical communications, an optical receiver or semiconductor apparatus and an optical communication system using the semiconductor apparatus of the present invention solve the problem of polarization dependency. The optical receiver or optical semiconductor apparatus includes a pair of optical filters arranged such that tuned wavelengths for two different modes, e.g., tuned wavelengths for TE and TM modes, are set as light components having the same wavelength, and a beam splitting path for directing signal light to the pair of optical filters. Alternatively, the optical receiver or optical semiconductor apparatus includes a filter waveguide or waveguides capable of setting the tuned wavelength of one mode (e.g., TE) to be the same as the tuned wavelength of the other mode (e.g., TM) of two modes having different orders of transverse modes (e.g., two orthogonal modes), and an input waveguide. The input waveguide and filter waveguide are arranged to equally couple a signal light to those two modes.

Abstract: A semiconductor optical amplifying apparatus includes: a substrate; an active layer having a quantum well structure formed on the substrate, the active layer guiding first waveguide mode light and second waveguide mode light having a polarization direction perpendicular to that of the first waveguide mode light and amplifying the first waveguide mode light and the second waveguide mode light, the active layer having quantum wells such that projected quantization axes obtained by projecting quantization axes of the quantum wells thereof on a plane perpendicular to a light waveguide direction are inclined at 45.degree. with respect to vibration directions of electric field vectors of the first waveguide mode light and the second waveguide mode light; and an electrode for supplying a current to the active layer.

Abstract: An optical communication system comprises: a plurality of optical communication apparatus for communicating signals by using a plurality of communicating lights having different wavelengths; an optical transmitter for transmitting a controlling light having a different wavelength than any of the plurality of wavelengths; an optical transmission line for interconnecting the optical transmitter and the optical communication apparatus to transmit the communicating lights and the controlling light; an optical amplifier for amplifying the communicating lights and the controlling light transmitted over the optical transmission line; and control circuit for discriminating the controlling light amplified by the optical amplifier from the communicating lights and controlling a gain of the optical amplifier to keep a light intensity of the controlling light at a constant level.

Abstract: A node for a loop type optical LAN using wavelength division multiplexing includes a light branching device for partially branching light from a loop of the optical LAN and a tunable filter for filtering a signal having a desired wavelength from the light branched by the branching device. The node may further include a light source for transmitting a signal to a loop of the optical LAN, an optical demultiplexer for selecting from the loop light having the same wavelength as the signal transmitted by the light source and an absorbing device for absorbing the entire amount of the signal transmitted by the light source after the signal once circulates the loop and is selected from the loop by the optical demultiplexer.

Abstract: The present invention discloses an optical fiber communication system for an optical network in which a plurality-of types of communication terminal equipments are connected to one another via optical fibers, the optical fiber communication system, being consisting of the steps: conducting wavelength multiplexing using at least three wavelengths, allotting a first optical wavelength to a packet switched communication of a distribution control system, allotting a second optical wavelength to a line switched communication of a time division multiplexing system, allotting the residual optical wavelength to a line switched communication by an optical, wavelength allotment system, and conducting a control of the line switched communication of the time division multiplexing system using the second optical wavelength and the line switched communication by the optical wavelength allotment using the residual optical wavelength, this control being conducted by using the packet switched communication using the first opt

Abstract: A semiconductor optical amplifying apparatus includes: a substrate; an active layer having a quantum well structure formed on the substrate, the active layer guiding first waveguide mode light and second waveguide mode light having a polarization direction perpendicular to that of the first waveguide mode light and amplifying the first waveguide mode light and the second waveguide mode light, the active layer having quantum wells such that projected quantization axes obtained by projecting quantization axes of the quantum wells thereof on a plane perpendicular to a light waveguide direction are inclined at 45.degree. with respect to vibration directions of electric field vectors of the first waveguide mode light and the second waveguide mode light; and an electrode for supplying a current to the active layer.

Abstract: In an optical amplifying apparatus of this invention, there are provided an optical amplifier part for giving a gain to an input light from outside and a selective loss part for giving a light loss of a prescribed ratio for different polarization modes in order to compensate for a polarization dependency of the amplification factor between the input and output lights. Thus, the difference of amplification factors for the different polarization modes is compensated for, as a whole of the apparatus. This polarization non-dependent type optical amplifying apparatus has solved the polarization dependency without degrading other characteristics, so that this can be preferably used in a optical communication system or network. Further, this amplifying apparatus does not require so many external optical components, so this is suitable to the fabrication of a module and is preferably contained in an integrated optical node.

Abstract: The present invention discloses an optical fiber communication system for an optical network in which a plurality of types of communication terminal equipments are connected to one another via optical fibers, the optical fiber communication system being consisting of the steps: conducting wavelength multiplexing using at least three wavelengths, allotting a first optical wavelength to a packet switched communication of a distribution control system, allotting a second optical wavelength to a line switched communication of a time division multiplexing system, allotting the residual optical wavelength to a line switched communication by an optical wavelength allotment system, and conducting a control of the line switched communication of the time division multiplexing system using the second optical wavelength and the line switched communication by the optical wavelength allotment using the residual optical wavelength, this control being conducted by using the packet switched communication using the first optic