A process of estimating a transfer function or an inverse transfer function of the optical transmitter from first data obtained by the optical receiver when a first known signal is transmitted from the transmitter to the receiver, and a temporary transfer function or a temporary inverse transfer function of the optical receiver, is performed for multiple frequency offsets between the optical transmitter and the optical receiver. At this time, the transfer function or the inverse transfer function of the optical transmitter is estimated by comparing the first data obtained by compensating at least one or none of a temporary transfer function of the optical receiver and transmission path characteristics detected in the receiver, with a first known signal before transmission to which what is not compensated for the first data between the temporary transfer function of the optical receiver and the transmission path characteristic is added.

Methods, circuits, and techniques for reflection cancellation. Laser output is tapped. A tapped portion of the laser output is phase shifted to generate a feedback signal, with the feedback signal being out-of-phase with a parasitic reflection of the laser output. The feedback signal is directed towards the laser such that the parasitic reflection and feedback signal are superpositioned before entering the laser. A magnitude and a phase of the feedback signal are such that superposition of the feedback signal and the parasitic reflection results in a resulting signal of lower magnitude than the parasitic reflection alone. During laser operation, a magnitude of the resulting signal is monitored and, as the parasitic reflection varies, the magnitude of the resulting signal is adjusted by adjusting at least one of the magnitude and the phase of the feedback signal in response to the monitoring of the resulting signal.

An optical transceiver including a submount, a Mach-Zehnder Modulator (MZM), bonding wires, and a low pass filter type matching network is provided. The MZM includes an input port and an output port and disposed on the submount. The bonding wires are coupled to the submount and the MZM. The low pass filter type matching network is coupled to the bonding wires and is configured to absorb inductance of the bonding wires at a high frequency.

An optical communication system includes a transmission side system for multi-level pulse amplitude modulation (PAM) and a corresponding receiver side system, where the transmission side comprises a laser source providing an optical beam, a signal source of electrical signals to be modulated onto the optical beam, and a modulator coupled to the laser source and the signal source to modulate the electrical signals onto the optical beam using amplitude modulation and at least four signal levels, wherein the at least four signal levels are non-uniformly distributed. The receiver side includes a corresponding equalizer which is implemented as a filter of the form f.sub.1y+f.sub.2y.sup.2+f.sub.0, where y is the incoming signal and the parameters f.sub.0, f.sub.1 and f.sub.2 are obtained using an adaptive filter.

A method for compensating the distortions introduced by impairments of MZMz implementing an optical transmitter, according to which the level of total amplitude and phase distortions caused by the optical transmitter is measured and all impairments in the constellation domain are compensated by pre-distorting the input signal to be transmitted by symmetrically adding imbalance to the voltage applied to the MZM arms. The imbalance is determined by introducing a phase rotation in either I or in the Q path of the optical transmitter, which compensates the total amplitude distortion, and also introducing a phase rotation to both I and Q paths of the optical transmitter, which compensate the total phase distortion and the phase shift caused by compensating the amplitude distortion, until reaching a desired operating point, which corresponds to the level of pre-distortion.

An optical transport system configured to compensate nonlinear signal distortions using a backward-propagation algorithm in which some effects of polarization mode dispersion on the nonlinear signal distortions are accounted for by employing two or more different approximations of said effects within the bandwidth of the optical communication signal. In an example embodiment, the corresponding digital signal processor (DSP) is configured to switch between different approximations based on a comparison, with a fixed threshold value, of a difference between frequencies of various optical waves contributing to the nonlinear signal distortions, e.g., through four-wave-mixing processes. In different embodiments, the backward-propagation algorithm can be executed by the transmitter's DSP or the receiver's DSP.

An optical transport system configured to compensate nonlinear signal distortions using a backward-propagation algorithm in which some effects of polarization mode dispersion on the nonlinear signal distortions are accounted for by employing two or more different approximations of said effects within the bandwidth of the optical communication signal. In an example embodiment, the corresponding digital signal processor (DSP) is configured to switch between different approximations based on a comparison, with a fixed threshold value, of a difference between frequencies of various optical waves contributing to the nonlinear signal distortions, e.g., through four-wave-mixing processes. In different embodiments, the backward-propagation algorithm can be executed by the transmitter's DSP or the receiver's DSP.

An optical modulator according to embodiments includes a first MZI and a second MZI each including a first optical coupler that splits CW light into two, a second optical coupler that couples the CW light split by the first optical coupler and outputs the CW light, and a bias electrode that adjusts a phase of the CW light split by the first optical coupler, a third optical coupler that couples outputs of the first MZI and the second MZI with at a predetermined ratio and outputs the light, and a bias adjustment circuit that adjusts an output voltage of a bias power supply applied to a bias electrode so that an optical path length difference between the CW light beams split by the first optical coupler is a predetermined times a carrier wavelength under a condition that an output of a differential output amplifier is a zero level, in accordance with an operating mode of the own apparatus.

An optical transceiver including a submount, a Mach-Zehnder Modulator (MZM), bonding wires, and a low pass filter type matching network is provided. The MZM includes an input port and an output port and disposed on the submount. The bonding wires are coupled to the submount and the MZM. The low pass filter type matching network is coupled to the bonding wires and is configured to absorb inductance of the bonding wires at a high frequency.

Embodiments of this disclosure provide a method and apparatus for monitoring a modulation depth of a dither signal and an optical transmitter. The apparatus includes a detector to detect an optical signal output by a Mach-Zehnder modulator to obtain an electrical signal, a dither signal at a determined frequency is superimposed on a direct current bias voltage of the Mach-Zehnder modulator; a frequency-doubling dither signal synchronization detection module to perform synchronization detection on the electrical signal and a frequency-doubling dither signal at a frequency twice the determined frequency, to obtain an amplitude of a signal component contained in the electrical signal at a frequency identical to the frequency of the frequency-doubling dither signal A signal processor is to calculate a modulation depth of the dither signal superimposed on the direct current bias voltage according to the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal.