Abstract: There is a need to prevent two receivers from converging on a state of receiving the same polarization state, fast start receivers, and ensure highly reliable operations. A polarization-multiplexed transmitter previously applies frequency shifts of frequencies +?f and ??f to X-polarization and Y-polarization digital information signals to be transmitted. Optical field modulators modulate and polarization-multiplex the signals. As a result, a frequency difference of 2?f is supplied to X-polarization and Y-polarization components. A polarization diversity coherent optical receiver 215 receives the signal. A frequency estimation portion in a digital signal processing circuit detects a frequency difference signal in both polarization components. This signal is used to a polarization splitting circuit in the digital signal processing circuit.

Abstract: In order to reduce the circuit size for a chromatic dispersion pre-equalization operation, an optical multilevel signal pre-equalization circuit is provided with: (1) a plurality of look-up tables in which a string of middle codes utilizing a symmetry of a signal constellation of a multilevel code or a string of middle codes represented by polar coordinates is stored in association with a waveform response component; and (2) one or more operation circuits to which the waveform response component corresponding to a multilevel signal to be transmitted is inputted from the plurality of look-up tables, and which outputs a pre-equalized output waveform corresponding to the multilevel signal by operating the waveform response components outputted from different look-up tables.

Abstract: In an optical multilevel transmitter (210), a polar representation of an optical multilevel signal (r, ?) is generated by a polar coordinate multilevel signal generation circuit (212), input to an optical amplitude modulator (211) and a polar coordinate type optical phase modulator (201), and output as an optical multilevel modulated signal (213). The polar coordinate type optical phase modulator (201) generates an optical phase rotation proportional to an input voltage, so the modulation distortion of the electric signal is transferred in a linear form to the optical phases of the optical multilevel modulated signal (213). In an optical multilevel receiver (219), a received signal is input to two sets of optical delay detectors (133) and balance receivers (134) and directly demodulated, and a differential phase ?? for the received signal is calculated by arctangent computation from the output signal.

Abstract: Provided is an optical multilevel transmission system, comprising at least one optical multilevel transmitter for transmitting an optical multilevel signal obtained and an optical multilevel receiver for receiving the optical multilevel signal. The received optical multilevel signal has a larger noise in an angular direction than in a radial direction. The optical multilevel receiver sets, in a symbol decision of the received optical multilevel signal demodulated on the complex plane, for positions of all or some of ideal signal points, a width in the angular direction of a decision area, to which each of the ideal signal points belongs and which is measured along a circumference of a circle centered at an origin and passing through a center of the each of the ideal signal points, larger than a width in the angular direction of a decision area defined based on a Euclidean distance.

Abstract: The optical signal to noise ratio is improved when the polarization multilevel signal light is demodulated. A optical polarization multilevel signal receiving apparatus is provided with a polarization multilevel receiver configured to generate at least one estimation symbol A1 to AN estimating a state of the symbol A by utilizing a polarization state of at least one polarization multilevel symbol received in a past before the symbol A, a past value of a decision variable, and a decision result, average the estimation symbols A1 to AN and the symbol A to calculate a reference symbol Ar, and use the calculated reference symbol Ar in place of the symbol A to calculate a decision variable corresponding to a polarization state change of the received symbol R, where R is a received symbol and A is at least one past symbol used as a reference for a change.

Abstract: Bending of an optical fiber where a heat may be generated by a high output power can be detected without using a dedicated light source. An optical communication module that outputs a continuous wave light generated by at least one light source to an optical fiber transmission line, includes: (1) a loss measurement unit that measures a loss of an amplified spontaneous emission generated by allowing the continuous wave light output from the light source to create stimulated Raman scattering in the optical fiber transmission line; (2) a fiber abnormality analyzer that detects the abnormal state of the optical fiber transmission line on the basis of loss information on the ASE measured by the loss measurement unit; and (3) a light source controller that controls a supply state of the continuous wave light from the light source on the basis of the detection of the fiber abnormality analyzer.

Abstract: A pre-equalized optical transmitter includes, a laser source; a duo-binary pre-coder circuit; a pre-equalization circuit for applying an inverse function of chromatic dispersion; at least two D/A converters; and an optical field modulator comprising at least two input terminals for an electric signal. The pre-equalized optical transmitter: converts, by the duo-binary pre-coder circuit, a digital information signal of a predetermined symbol time to be transmitted into a digital complex signal including one sampling point per symbol; equalizes, by the pre-equalization circuit, degradation in transmission of the digital complex signal; converts, by the D/A converters, the equalized digital complex signal into an analog signal; suppresses an analog signal leaking outside a Nyquist bandwidth by at least 23 dB; modulates, by the optical field modulator, light output from the laser source with the analog signal to generate a modulated optical field signal; and transmits the modulated optical field signal.

Abstract: An optical signal of an optical transmission part is brought into a high-speed polarization scrambling state by a polarization scrambling part, and transmitted to en optical fiber transmission line as the optical signal from the optical transmitter. The optical signal passing through the optical fiber transmission line is inputted to an optical receiver. The optical signal inputted to the optical receiver is converted into an electric signal by a polarization dependent photoelectric detection part. The converted electric signal is inputted to a digital signal processing part having a polarization scrambling cancel part of canceling the polarization scrambling state by a digital signal processing operation. At the digital signal processing part, the polarization scrambling state of the electric signal is canceled, and a data signal is outputted.

Abstract: It is provided an optical field transmitter comprising a light source, one or more DA converters, an optical field modulator, a complex information multilevel signal generator circuit, and a phase pre-integration circuit. The optical field modulator modulates light output from the light source into a optical field signal by using the analog signal converted from a complex multilevel information signal including phase pre-integration complex information by the one or more DA converters. A phase angle of the complex multilevel information signal at a complex signal point is any one of values of integral multiples obtained by dividing 360 degrees by a positive integer N. An amplitude value of the complex multilevel information signal at the complex signal point is any one of values of a positive integer M. A total number of the complex signal points which may be taken is lower than a product of N and M.

Abstract: Optical receiver 300 uses two optical delay detectors 223 (set such that the delay times T are equal to symbol time and the phase differences are zero and 90 degrees) to receive an optical multilevel signal 215 and the output signals are A/D converted, thereafter subjected to retiming processes, and then subjected to a differential phase detection, thereby detecting a differential phase at a symbol center time point. In the receiver, the detected differential phase is integrated for each symbol and thereafter combined with an amplitude component obtained from a separately disposed optical intensity receiver, thereby reproducing an optical electric field. Thereafter, a wavelength dispersion compensation circuit (231) of a time period T is used to compensate for the wavelength dispersion of the transmission path. Moreover, an electric or optical Nyquist filter may be inserted to perform a band limitation, thereby enhancing the wavelength dispersion compensation effect.

Abstract: A polarization multiplexing transmitter which generates polarization-multiplexed signals which are arbitrarily polarization-scrambled at high speed, without adding a polarization modulator and a polarization scrambler. In the transmitter, an orthogonally polarized signal generator includes two optical modulators which modulate the electric fields of optical signals and generate two optical signals with mutually orthogonal polarized waves. The transmitter includes electric field mappers which convert two data strings into electric field signals, polarization mappers which give different polarized waves to the two signals, polarization rotators which rotate the polarized waves of the signals uniformly, a polarization multiplexer which multiplexes the two polarization-rotated signals, a polarization demultiplexer which demultiplexes the multiplexed signal into polarized wave components of optical signals generated by the orthogonally polarized signal generator, and a driver.

Abstract: An optical balanced receiver including an optical coupler for combining input optical information signal and optical reference signal and outputting two optical interfering signals whose phases are opposite to each other, two photodetectors for receiving the two optical interfering signals and outputting detection signals as electrical signals having the amplitudes corresponding to the interference intensities of the received optical interfering signals, a balance compensation type difference device for compensating an intensity fluctuation component added to a difference signal of the two detection signals due to the difference in amplitude and/or delay between the detection signals output from the two photodetectors in accordance with an input control signal, and outputting the compensated difference signal of the two detection signals, and a control circuit for outputting the control signal to the balance compensation type difference device.

Abstract: In an optical multilevel transmitter (210), a polar representation of an optical multilevel signal (r, ?) is generated by a polar coordinate multilevel signal generation circuit (212), input to an optical amplitude modulator (211) and a polar coordinate type optical phase modulator (201), and output as an optical multilevel modulated signal (213). The polar coordinate type optical phase modulator (201) generates an optical phase rotation proportional to an input voltage, so the modulation distortion of the electric signal is transferred in a linear form to the optical phases of the optical multilevel modulated signal (213). In an optical multilevel receiver (219), a received signal is input to two sets of optical delay detectors (133) and balance receivers (134) and directly demodulated, and a differential phase ?? for the received signal is calculated by arctangent computation from the output signal.

Abstract: It is provided an optical field transmitter comprising a light source, a DA converter and an optical field modulator. The optical field transmitter modulates an information signal into an optical field signal. The information signal includes one of multilevel signals arranged irregularly on a complex plane and multilevel signals arranged by combining mutually different numbers of phase values in at least two amplitude values. The optical field transmitter further comprises a phase pre-accumulation circuit for outputting phase pre-accumulation complex information obtained by previously accumulating a phase component of the information signal at predetermined time intervals. The DA converter converts the information signal including the output phase pre-accumulation complex information into an analog signal, and inputs the converted analog signal to the optical field modulator.

Abstract: Bending of an optical fiber where a heat may be generated by a high output power can be detected without using a dedicated light source. An optical communication module that outputs a continuous wave light generated by at least one light source to an optical fiber transmission line, includes: (1) a loss measurement unit that measures a loss of an amplified spontaneous emission generated by allowing the continuous wave light output from the light source to create stimulated Raman scattering in the optical fiber transmission line; (2) a fiber abnormality analyzer that detects the abnormal state of the optical fiber transmission line on the basis of loss information on the ASE measured by the loss measurement unit; and (3) a light source controller that controls a supply state of the continuous wave light from the light source on the basis of the detection of the fiber abnormality analyzer.

Abstract: There is a need to prevent two receivers from converging on a state of receiving the same polarization state, fast start receivers, and ensure highly reliable operations. A polarization-multiplexed transmitter previously applies frequency shifts of frequencies +?f and ??f to X-polarization and Y-polarization digital information signals to be transmitted. Optical field modulators modulate and polarization-multiplex the signals. As a result, a frequency difference of 2?f is supplied to X-polarization and Y-polarization components. A polarization diversity coherent optical receiver 215 receives the signal. A frequency estimation portion in a digital signal processing circuit detects a frequency difference signal in both polarization components. This signal is used to a polarization splitting circuit in the digital signal processing circuit.

Abstract: An optical field receiver comprises an optical brancher that branches a received optical multilevel signal into first and second optical signals; an optical delayed detector that performs delayed detection on the first optical signal by a delayed detector with a delay time of T/2 (where T is equal to a symbol time) and a phase difference of 90 degrees; a balanced optical receiver that converts the optical signal outputted from the first delay detector to an electric signal; and an optical intensity receiver that converts the second optical signal to an electric signal; and an electric field calculating part that generates, from the output signals of the first and second optical receivers, first and second reproduced signals indicative of the phase angle and amplitude value of the received symbol represented by the complex signal in each symbol time T.

Abstract: In a quaternary phase modulator including two phase modulators disposed in parallel and a phase adjuster that adjusts a phase difference when the outputs of the two phase modulators are combined, there are provided a second light source that introduces light propagated in a backward direction, a first controller that controls the bias of the two phase modulators so that the intensity of the backward light is a minimum on the input side of the quaternary phase modulator, and a second controller that controls the bias of the phase adjuster so that a result monitored by a photodiode having a bandwidth not exceeding the bit rate on the output side of the quaternary phase modulator is a minimum, the first controller being implemented after the second controller is implemented.

Abstract: Provided is an optical multilevel transmission system, comprising at least one optical multilevel transmitter for transmitting an optical multilevel signal obtained and an optical multilevel receiver for receiving the optical multilevel signal. The received optical multilevel signal has a larger noise in an angular direction than in a radial direction. The optical multilevel receiver sets, in a symbol decision of the received optical multilevel signal demodulated on the complex plane, for positions of all or some of ideal signal points, a width in the angular direction of a decision area, to which each of the ideal signal points belongs and which is measured along a circumference of a circle centered at an origin and passing through a center of the each of the ideal signal points, larger than a width in the angular direction of a decision area defined based on a Euclidean distance.

Abstract: It is provided an optical field transmitter comprising a light source, one or more DA converters, an optical field modulator, a complex information multilevel signal generator circuit, and a phase pre-integration circuit. The optical field modulator modulates light output from the light source into a optical field signal by using the analog signal converted from a complex multilevel information signal including phase pre-integration complex information by the one or more DA converters. A phase angle of the complex multilevel information signal at a complex signal point is any one of values of integral multiples obtained by dividing 360 degrees by a positive integer N. An amplitude value of the complex multilevel information signal at the complex signal point is any one of values of a positive integer M. A total number of the complex signal points which may be taken is lower than a product of N and M.