An optical modulation unit included in an optical transmitter includes two optical modulators that modulate each of two light beams based on applied bias voltages and input modulation signals, and an optical phase regulator that is connected to either of the two optical modulators, and regulates a phase of the light beam incident on the optical modulator. In a state where no modulation signal is input to the two optical modulators, while keeping bias voltages to be applied to one optical modulator and the optical phase regulator constant, the controller determines a first initial bias voltage such that an output light beam from the other optical modulator becomes zero. Thereafter, the controller determines a second initial bias voltage such that an output light beam from the one optical modulator becomes zero, while applying the first initial bias voltage to the other optical modulator.

A system comprises: a polarization state aligner (PSA) comprising: an input port; a first polarization beam splitter (PBS) coupled to the input port; a first phase shifter (PS) coupled to the first PBS; a first polarization rotator (PR) coupled to the first PBS; a first beam splitter (BS) coupled to the first PS and the first PR; a first output port coupled to the first BS; and a second output port coupled to the first BS.

Systems and methods for detecting and correcting bias errors in optical coherent transponders are disclosed. An “outer” modulator in a transponder may, when properly biased, produce a phase offset of π/2 radians between in-phase and quadrature components of the optical signals transmitted in optical modulation formats by the transponder. The method may include providing input to a transponder to produce a periodic (and generally sinusoidal) output signal, measuring (using an optical power meter) the optical power of positive and negative harmonics of the signal while varying the amount of skew introduced by a de-skewing filter in the transponder, and determining that a curve representing the measurements performed on the positive harmonics and a curve representing the measurements performed on the negative harmonics are not orthogonal. The method may include adjusting the bias voltage of the modulator to make the two curves orthogonal, thus eliminating the bias error.

Disclosed herein is a modulator (50) for polarization-division multiplexing (PDM) transmission. The modulator (50) comprises first and second DP-MZMs (12, 28) associated with first and second polarizations, each DP-MZM (12, 28) having an input for an in-phase and a quadrature driving signal for modulating the in-phase and quadrature components of an optical signal according to respective transfer functions, and a detector (58) suitable for detecting light comprising at least a portion of the light outputted by the first DP-MZM (12) and a portion of the light outputted by the second DP-MZM (28). The modulator (50) is adapted to superimpose a first pilot signal on one of the in-phase and quadrature driving signals of the first DP-MZM (12) and on one of the in-phase and quadrature driving signals of the second DP-MZM (28), and a second pilot signal on the respective other of the in-phase and quadrature driving signals of the first and second DP-MZMs (12, 28). Further, the first and second pilot signals are chosen such that the signal detected by said detector (58) is indicative as to whether the slopes of the transfer functions are different for the in-phase and quadrature components of one of the first and second DP-MZMs (12, 28) and identical for the other of the first and second DP-MZMs (12, 28).

An optical communication device includes an optical modulator of a Mach-Zehnder type, a low frequency superimposing circuit configured to superimpose a low frequency signal on a substrate bias voltage applied to the optical modulator, a monitor configured to monitor a modulated light output from the optical modulator, and a substrate bias controller configured to control the substrate bias voltage based upon a low frequency component contained in a monitor signal output from the monitor.

A control processor performs, in a startup sequence of an IQ optical modulator using a nested MZI, a first-stage process of controlling, so that a signal quality of an optical QAM signal output from a monitor port of the IQ optical modulator approaches a target quality, a voltage applied by a Bias_I voltage generator to I-component MZ optical modulator, a voltage applied by a Bias_Q voltage generator to a Q-component MZ optical modulator, and a voltage applied by a Bias_Ph voltage generator to a Bias_Ph phase adjusting means for controlling an optical path length of a parent MZI. After a completion of the first-stage process, the control processor changes a voltage output from the Bias_Ph voltage generator by a predetermined amount.

In order to provide a configuration for suppressing deterioration in the transmission quality of a signal light due to a nonlinear phenomenon in an optical fiber, a polarization dispersion adder is provided with: a polarization rotation unit which, with respect to each pulse of signal light generated by modulating a light carrier, rotates and outputs the polarization of the pulse during a period from a pulse rise start time (T0) to a pulse fall completion time (T1); and a delay addition unit which adds a delay of an amount corresponding to the rotation amount of the polarization added by the polarization rotation unit to the pulse outputted from the polarization rotation unit.

A control processor performs, in a startup sequence of an IQ optical modulator using a nested MZI, a first-stage process of controlling, so that a signal quality of an optical QAM signal output from a monitor port of the IQ optical modulator approaches a target quality, a voltage applied by a Bias_I voltage generator to I-component MZ optical modulator, a voltage applied by a Bias_Q voltage generator to a Q-component MZ optical modulator, and a voltage applied by a Bias_Ph voltage generator to a Bias_Ph phase adjusting means for controlling an optical path length of a parent MZI. After a completion of the first-stage process, the control processor changes a voltage output from the Bias_Ph voltage generator by a predetermined amount.

Methods and systems for a distributed optical transmitter with local domain splitting are disclosed and may include, in an optical modulator integrated in a silicon photonics chip: receiving electrical signals, communicating the electrical signals to domain splitters along a length of waveguides of the optical modulator via one or more delay lines, and generating electrical signals in voltage domains utilizing the domain splitters for modulating the optical signals in the waveguides of the optical modulator by driving diodes with the electrical signals generated in the voltage domains. The delay lines may comprise one delay element per domain splitter, or may comprise a delay element per domain splitter for a first subset of the domain splitters and more than one delay element per domain splitter for a second subset of the domain splitters.

An optical device. In some embodiments, the device comprises: an input waveguide, configured to receive light; a first electro-absorption modulator, coupled to receive light from the input waveguide, and operable to produce a first output or a second output, wherein the second output has a lower amplitude than the first output; a second electro-absorption modulator, coupled to receive light from the input waveguide, and operable to produce a third output or a fourth output, wherein the fourth output has a lower amplitude than the third output; and an output waveguide, coupled to receive light from the first electro-absorption modulator and the second electro-absorption modulator, and output a combined signal comprising an output of the first electro-absorption modulator and an output of the second electro-absorption modulator.