Abstract: Since it is difficult to emit a control a IQ modulator emitting modulated lightwave according to QAM format without signal degradation of said emitted signal due to low frequency dither used for the control of said modulator, a method for controlling an optical transmitter according to an exemplary aspect of the invention includes: generating a multilevel electrical signal by means of combining two or more binary electrical signals, where said multilevel signal is used to drive the modulator of adding a low frequency dither signal on several of the binary electrical signals, wherein the phase of the added dither signal depends on the value of the binary signal to which it is added; tapping a portion of light after the modulator and generating a monitor signal from the tapped light; controlling the modulator according to the monitor signal.

Abstract: A method is provided for setting a timing of phase modulation by a target phase modulator within an optical transmitter which performs 2n order phase shift keying, n being a natural number equal to or more than two. The method is provided with: feeding a specific pattern to the optical transmitter to allow the xPSK transmitter to emit an optical carrier in accordance with the specific pattern; receiving the optical carrier by a delay interferometer; converting a pair of light signals emitted from constructive and destructive outputs of the delay interferometer into an electrical signal; detecting a peak to peak value of the electrical signal; and setting the timing of the phase modulation by the target phase modulator so as to minimize the peak to peak value of the electrical signal.

Abstract: Since it is difficult to emit a stable and reliable modulated lightwave signal by means of IQ modulators used for QAM format, a method for controlling an optical transmitter according to an exemplary aspect of the invention includes the steps of (a) keeping an optical amplitude of a continuous wave light output from the optical transmitter constant, (b) making operating point values in optical modulation converge to predetermined values during step (a), and (c) modulating the continuous wave light with multiple amplitudes and phase levels around the operating point values converged in step (b).

Abstract: In order to appropriately demultiplex the polarization multiplexed BPSK signal without using a training sequence and decreasing the resistance to a frequency offset, an optical receiver includes a coherent optical detection unit receiving an optical signal in which BPSK modulated carrier waves are polarization-multiplexed, performing coherent detection by mixing the received optical signal with local light, and outputting first electrical signals corresponding to the carrier waves; a butterfly FIR filter receiving the first electrical signals and extracting second electrical signals corresponding to each of the carrier waves from the first electrical signals; and a coefficient control unit for calculating a sum of respective phases of the second electrical signals output from the butterfly FIR filter, adaptively controlling tap coefficients of the butterfly FIR filter so that the calculated phase sum may become equal to 0 or ?, and outputting tap coefficients after being controlled to the butterfly FIR filter

Abstract: Since it is difficult to fast, simply monitor impairments of received signals with higher receiver sensitivity, a monitoring method for an optical communication system according to an exemplary aspect of the invention includes the steps of emitting lightwave signals to be modulated according to a data, forming dips at transitions between temporally consecutive groups of n symbols of the lightwave signals, wherein the dips are formed at each of (n?1) first transitions of the group, no dip is formed at the n-th transition on the lightwave signals, receiving the lightwave signals, extracting frequency components characterized by the numerical value n from received lightwave signals, and monitoring the received lightwave signals by using the frequency components.

Abstract: A Pseudo-Return-to-Zero modulator is provided with a narrow pulse clock generator, a modulator driver, and an optical modulator. The narrow pulse clock generator generates a narrow pulse clock of order n, where one of levels occupies half a symbol period and the other level occupies (n?1) plus half a symbol period, n being equal to or more than two. The modulator driver generates an electrical signal in response to binary data and the narrow pulse clock. The optical modulator modulates an optical carrier in response to the electrical signal so that the modulated optical carrier is in a PRZ(n) format.

Abstract: A method of monitoring chromatic dispersion when transmitting an optical signal includes: applying, to an optical signal in which the symbol rate is f, a dip in optical intensity for every n symbols by means of pseudo-RZ modulation where n is an integer equal to or greater than 2, and transmitting the optical signal to which dips have been applied to a transmission path; receiving the optical signal that is transmitted in by the transmission path and detecting the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1; and based on the detected intensity, generating a monitor signal that represents the chromatic dispersion amount.

Abstract: A method is provided for detecting the skew between parallel light signals generated from a serial data stream. The method can be used with polarization multiplexed signal, as well as with wavelength division multiplexed signals, spatial division multiplexed signals, phase modulated signals, or intensity modulated signals. The method can be used with direct detection schemes as well as with coherent detection schemes. The method is provided with: imprinting dips between a fixed number of transmitted symbols of the parallel signals; detecting an electrical signal related to the dips for each parallel signal; and comparing the electrical signals in delay.

Abstract: A laser device includes a cavity and a control portion. The cavity has an optical amplifier, a wavelength selectable portion having a changeable transmission wavelength range, and a mirror. The control portion controls the wavelength selectable portion so that the transmission wavelength range of the wavelength selectable portion is changed to a given range. The control portion controls the wavelength selectable portion so that the cavity outputs a desirable lasing wavelength light and optical intensity of the desirable lasing wavelength light is a given value, after controlling the wavelength selectable portion so that the cavity outputs the desirable lasing wavelength light.

Abstract: A method is provided for setting a timing of phase modulation by a target phase modulator within an optical transmitter which performs 2n order phase shift keying, n being a natural number equal to or more than two. The method is provided with: feeding a specific pattern to the optical transmitter to allow the xPSK transmitter to emit an optical carrier in accordance with the specific pattern; receiving the optical carrier by a delay interferometer; converting a pair of light signals emitted from constructive and destructive outputs of the delay interferometer into an electrical signal; detecting a peak to peak value of the electrical signal; and setting the timing of the phase modulation by the target phase modulator so as to minimize the peak to peak value of the electrical signal.

Abstract: A laser module has an optical amplifier, a first etalon and a wavelength selectable mirror. The first etalon has wavelength peaks at a given wavelength interval in transmission characteristics and transmits a light from the optical amplifier. The wavelength peaks are tunable. The wavelength selectable mirror acts as an external mirror of an external cavity laser including the optical amplifier, and has a relatively high reflection intensity at a part of wavelength range in an effective gain range of the optical amplifier. The effective gain range of the optical amplifier includes more than one wavelength peak of the first etalon. A reflection bandwidth where the wavelength selectable mirror has a relatively high reflection intensity is less than twice of the wavelength interval of the wavelength peaks of the first etalon.

Abstract: A laser device includes a cavity and a control portion. The cavity has an optical amplifier, a wavelength selectable portion having a changeable transmission wavelength range, and a mirror. The control portion controls the wavelength selectable portion so that the transmission wavelength range of the wavelength selectable portion is changed to a given range. The control portion controls the wavelength selectable portion so that the cavity outputs a desirable lasing wavelength light and optical intensity of the desirable lasing wavelength light is a given value, after controlling the wavelength selectable portion so that the cavity outputs the desirable lasing wavelength light.