Abstract: The object of the present invention is to provide an optical transmitter which enables the reduction of the strong dependency of the transmission characteristic on the high-speed polarization scrambler driving signal phase. The light outputted from a light source is converted to a RZ pulse by an electro-absorption modulator. The RZ pulse is modulated by a data modulator, and the polarization thereof is scrambled in a high-speed electro-optic polarization scrambler and outputted from the output terminal thereof. Since, in the present invention, the light is once changed to a RZ pulse, the strong dependency of the optical signal transmission characteristic on the high-speed polarization scrambler driving signal phase can be reduced. Further, if the phase of the RZ pulse in the NRZ modulation signal is made to synchronize with the rising edge or the falling edge of the NRZ signal, the dependency on the high-speed polarization scrambler driving signal phase can be further reduced.

Abstract: An optical frequency shifter which has no insertion loss polarization dependency and no polarization mode dispersion is formed by: at least one ultrasonic wave generator for generating ultrasonic waves; at least one electric acousto-optic element forming at least one diffraction grating therein upon being applied with the ultrasonic waves generated by the ultrasonic wave generator; at least one polarization rotator for rotating a polarization plane of a first primary diffracted light produced by the diffraction grating, by 90.degree.

Abstract: An optical submarine branching unit employing an optical circuit structure which is small in the number of optical components used, permits reduction of optical insertion loss and is well compatible with the ADD/DROP function suitable for use in wavelength multiplex transmission. An optical fiber pair provided in an optical submarine cable connecting between first and second points is branched toward a third point, an optical circulator capable of reversing the direction of rotation of an optical input/output is provided as optical switching means by which, in the event of a failure in the branch transmission system having the branched optical fiber pair, the optical fiber pair is connected between the first and second points directly without being branched to the third point.

Abstract: An optically amplifying transmission system available for wider signal bands includes an optical transmission line 42 divided into two sections 42a and 42b, optically amplifying repeaters 48a having 7 m-long erbium-doped optical fibers to repeat transmission optical fibers 46a in the section 42a, and optically amplifying repeaters 48b having 10.8 m-long erbium-doped optical fibers to repeat transmission optical fibers 46b in the section 42b. The transmission optical fibers 46a an 46b are optical fibers absolutely identical in construction and length.

Abstract: A line monitoring apparatus which features a simple configuration is capable of quickly detecting a position where a failure has occurred. A main track signal on an up-stream optical transmission path is set for a different wavelength from that on a down-stream optical transmission path, so that the wavelength of loopback signals fed back from repeaters does not overlap the wavelength of the main track signals transmitted from a transmitting terminal station. This makes it possible to separate the main track signals and the loopback signals by using dividing circuits at receiving terminal stations. In addition, the level of the separated loopback signals can be set higher than that in the past; therefore, a monitoring signal can be quickly detected in the loopback signal.

Abstract: A wavelength division multiplexed (WDM) optical communication method and apparatus uses a pre-emphasis technique to adjust the attenuation or amplification of a particular optical channel at a transmitter terminal to produce identical signal-to-noise ratios for all of the optical channels at a receiver terminal. The pre-emphasis adjustments to the transmitted signals are made on the basis of signal-to-noise ratio measurements performed at the receiver terminal. The signal-to-noise ratio values for each channel are transmitted through a facing line that is also used to transmit data along optical communication lines from the receiver terminal back to the transmitter terminal. The present invention also provides a method and apparatus for monitoring optical transmission paths for the deterioration of optical amplifier repeaters or an optical fiber. The optical signals are then adjusted to account for the location of the particular amplifier or fiber that has deteriorated.

Abstract: An optical communication system of a construction wherein the average wavelength dispersion value of the transmission optical fiber used, the optical output intensity of each optical amplifier repeater inserted in the transmission optical fiber and the widths of return-to-zero optical pulses transmitted over the transmission line are determined so as to compensate for the pulse compression effect by the nonlinear optical effect produced on the optical pulses by the pulse spreading effect by the wavelength dispersion effect. An optical multiplexer in the optical transmitting device time-division multiplexes the return-to-zero optical pulses, and the optical multiplexed signal is provided as an alternating-amplitude optical signal with the amplitudes of the return-to-zero optical pulses alternated.

Abstract: A laser light generator includes an adder 12 which adds an optical frequency modulation signal Sm onto a d.c. voltage for driving a laser diode 10 for continuous laser oscillation, and applies the sum signal to the laser diode 10 to drive it. An light output from the laser diode 10 is introduced to an optical intensity controller 14 that is controlled by a control signal prepared by adjusting the optical frequency modulation signal Sm both in phase and in amplitude by a phase adjusting circuit 16 and an amplitude adjusting circuit 18. The optical intensity controller 14 may be an electroabsorption modulator that changes its transmissivity in response to an output voltage of the circuit 18. Quantities of adjustment by the circuits 16 and 18 are determined so that fluctuation in transmissivity of the optical intensity controller 14 suppresses intensity fluctuation of the light output of the laser diode 10.

Abstract: Light beams emitted from an optical fiber are converted into parallel light beams by a lens, and then allowed to pass through a Faraday device, followed by being converged to an incident end of the optical fiber by a lens. The Faraday device is applied with a magnetic field in a direction of the optical axis thereof from a coil. Moreover, the Faraday device is applied with a magnetic field in a direction perpendicular to the optical axis by a pair of magnets. When an electric current flowing in a coil 74 is changed from I to -I, the direction of a synthesized magnetic field can be rotated. Therefore, by arbitrarily changing the electric current flowing in the coil, polarization of transmission light can be controlled.

Abstract: An optical transmitter which reverses the ON-OFF state of the optical intensity of a bright soliton lightwave and generates a dark soliton lightwave having an optical phase shift, an optical receiver for the dark soliton lightwave, and a superfast, high-capacity optical transmission system which is capable of increasing the soliton pulse array density while suppressing timing jitter. The optical transmission system is provided with the optical transmitter which transmits a dark soliton lightwave having digital information, the optical receiver which receives the dark soliton lightwave as a return-to-zero pulse and a transmission optical fiber interconnecting the transmitter and the receiver.

Abstract: This invention provides an optical amplifier which has low noise characteristics for a wide range of input signal electric power. The optical amplifier has a plurality of cascaded optical amplifying units, a signal level detector, and a pump-source controller. The pump-source controller changes controlling objects from a pump-source corresponding to a first one of the optical amplifying units to a pump-source corresponding to a second one of the optical amplifying units according to a detection level of the signal level detector from a low level to a high level. When the detection level is outside of a control object range, the optical amplifier controls an output from the pump-source to stabilize. When the detection level is within the control object range, the optical amplifier controls an output from a pump-source of each of optical amplifying units according to the detection level.

Abstract: An optical transmission system which permits long-distance, ultrafast, high-capacity optical soliton transmission by suppressing timing jitter with simple means. In an optical transmission system which uses a transmission line composed of an optical fiber for transmitting therethrough a lightwave signal having digital information added to a return-to-zero lightwave pulse and a plurality of optical amplifying repeaters for compensating for losses by the optical fiber and in which the pulse compression effect by the nonlinear optical effect on the optical pulse and the pulse expansion effect by the dispersion effect are compensated, at least one optical band limited element which has a flat group delay characteristic near the center frequency is disposed in the transmission line at intervals of a period Z equal to the soliton period Z.sub.sol and the product, aB.sub.sol.spsb.

Abstract: An optical amplifying-repeating transmission system is disclosed which is composed of an optical fiber for transmitting a lightwave signal with digital information added to return-to-zero lightwave pulses and a plurality of optical amplifying repeaters inserted in the optical fiber for transmission use. The mean value of wavelengths at which the wavelength dispersion of the optical fiber is zero is smaller than the wavelength of the lightwave signal which is transmitted over the system. The accumulated wavelength dispersion value of the optical fiber tends to increase with the distance of transmission, from a macroscopic viewpoint. The optical fiber for transmission is divided into a plurality of sections.

Abstract: An optical transmitter which reverses the ON-OFF state of the optical intensity of a bright soliton lightwave and generates a dark soliton lightwave having an optical phase shift, an optical receiver for the dark soliton lightwave, and a superfast, high-capacity optical transmission system which is capable of increasing the soliton pulse array density while suppressing timing jitter. The optical transmission system is provided with the optical transmitter which transmits a dark soliton lightwave having digital information, the optical receiver which receives the dark soliton lightwave as a return-to-zero pulse and a transmission optical fiber interconnecting the transmitter and the receiver.

Abstract: The present invention provides a wavelength division multiplexed optical fiber transmission equipment which can increase the number of channels of the WDM signal by reducing a required bandwidth of optical signals while suppressing an interference between signals caused by four-wave mixing. A WDM optical transmitting terminal comprises five or more optical transmitters and the wavelengths of optical signals from the optical transmitters are set such that the spacing between the wavelengths of two signals is re-used as the spacing between the wavelengths of other two signals separated by the above signals by two or more waves and there is no periodicity in the channel spacing s. For instance, the wavelengths of the optical signals are set as shown in FIG. 1. As the result, deterioration in the transmission characteristics of the optical signals can be effectively reduced and, at the same time, the maximum number of channels of the WDM signal can be increased.

Abstract: An apparatus for measuring optical transmission characteristic, which has an input unit of data signal and of clock pulses, a signal decision unit, a display unit and a control unit. The signal decision unit comprises a plurality of signal decision circuits. A Q value employed as a parameter for the evaluation of the transmission characteristic of an optical communication system can be obtained in real time since bit error rates necessary for the calculation of the Q value can be simultaneously obtained by the plurality of signal decision circuits.

Abstract: An optical amplifying transmission system is disclosed in which a large quantity of optical fiber whose zero dispersion wavelength is longer than the wavelength of the lightwave signal is introduced to provide an RZ lightwave signal transmission system. In this system, the average zero dispersion wavelength by the optical fiber is shorter than the wavelength of the lightwave signal, the zero dispersion wavelength by the optical fiber whose length is in the range of one-thirds to two-thirds of the entire length of the optical fiber is longer than the wavelength of the lightwave signal, and the absolute value of the wavelength dispersion is larger than one-half the absolute value of the average wavelength dispersion value over the entire length of the optical fiber.

Abstract: It is the object of the present invention to optimize the degradation due to the four-wave mixing of wavelength multiplexed signals and the distortion due to the chromatic dispersion, thereby to provide a wavelength division multiplexed optical fiber transmission equipment having good transmission characteristics. A WDM transmitting terminal consists of four transmitters of channels 1 to 4, and a wavelength-division multiplexer. Further, a WDM receiving terminal consists of an equalization optical fiber, a wavelength-division demultiplexer, and four receivers of the channels 1 to 4. The WDM transmitting terminal and the WDM receiving terminal are interconnected by transmission optical fibers and an optical amplifier. The equipment is designed so that the wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4 of the optical signals output from the transmitters all lie on the short-wavelength side with respect to the average zero-dispersion wavelength .lambda..sub.0.

Abstract: At the time of receiving a repeater supervisory signal superimposed on or combined by wavelength multiplexing with a main optical signal in an optical amplifier repeater system, the received optical signal is subjected to a photoelectric conversion by a photodetector and then branched into two electric signals, the one of which is subjected to equalizing amplification, timing extraction and discrimination and regeneration to extract only the main signal component. The main signal component thus extracted and the main signal contained in the other electric signal, which is not regenerated, are respectively subjected to required equalization, delay adjustment and amplitude adjustment so that they match with one another in waveform, phase and amplitude. In addition, the amplitude adjustment of the main signal is automatically controlled. By differentially combining these signals to remove only the main signal component to minimize its residual value, thereby extracting the supervisory signal to be obtained.

Abstract: An optical transmission system is disclosed, in which the transmission of the optical soliton pulses is controlled by controlling the wavelength dispersion of the optical fiber transmission line for each long section including a plurality of optical amplifier repeaters. In this case,the first sections having an average value of the wavelength dispersion larger than the dispersion value meeting with the soliton condition and the second sections having an average value of the wavelength dispersion smaller than the dispersion value meeting with the soliton condition are alternatively allocated in the optical fiber transmission line, so that the average value of the wavelength dispersion of the entire length of the optical fiber transmission line assumes a positive value.