To provide a modulation control method and an optical transmission device that realize high reliability by more stably performing bias control of an optical modulator using an optical QAM scheme, an optical transmitter according to the present invention comprises a first waveguide and a second waveguide, wherein each of the first waveguide and the second waveguide are provided with an optical modulator that modulates a carrier light with a modulation driving signal that has multiple strength level values, a phase shift unit that provides a predetermined phase difference between a first optical signal outputted from the first waveguide and a second optical signal outputted from the second waveguide, a light detector that detects and photoelectrically converts a portion of a multiple value optical signal obtained by multiplexing the first optical signal and the second optical signal which have been provided with the phase difference and a control circuit that, on the basis of signal amplitude information obtained by a signal amplitude detector from wideband signal components from the light detector, corrects a first voltage provided to the first waveguide and corrects a second voltage provided to the second waveguide.

The present invention relates to telecommunication techniques and integrated circuit (IC) devices. More specifically, embodiments of the present invention provide an off-quadrature modulation system. Once an off-quadrature modulation position is determined, a ratio between DC power transfer amplitude and dither tone amplitude for a modulator is as a control loop target to stabilize off-quadrature modulation. DC power transfer amplitude is obtained by measuring and sampling the output of an optical modulator. Dither tone amplitude is obtained by measuring and sampling the modulator output and performing calculation using the optical modulator output values and corresponding dither tone values. There are other embodiments as well.

A Mach-Zehnder modulator for modulating optical signals, and comprising: a plurality of modulating waveguide sections; at least one bias electrode in electrical communication with at least one modulating waveguide section and configured to apply at least one electrical bias signal to one or more of the modulating waveguide sections; and an output optical combiner comprising a plurality of inputs and a plurality of outputs, wherein the plurality of inputs of the combiner are in optical communication with output sides of the plurality of modulating waveguide sections, and wherein a plurality of the outputs of the combiner are monitor outputs.

A linearized electro-optic modulator includes a substrate comprising a first Mach Zehnder interferometer comprising a first and second optical waveguide and a second Mach Zehnder interferometer comprising a first and a second optical waveguide. A signal electrode is positioned on the substrate to receive a modulation signal. First and second ground electrodes are positioned on the substrate and are electrically connected to ground potential. The signal electrode and the first and second ground electrodes are positioned so that an electric field generated by the signal electrode modulates both the first and second Mach Zehnder interferometers to generate a first and a second linearized modulated optical signal.

An optical transmitter includes a discrete multi-tone (DMT) modulation unit that modulates a carrier signal having a specific frequency with an information signal and a carrier signal having a frequency different from the specific frequency with a monitor signal, to generate a DMT modulation signal that multiplexes the information signal and the monitor signal. The optical transmitter includes a laser diode (LD) unit that optically converts the DMT modulation signal to a corresponding optical DMT modulation signal, a frequency extraction unit that extracts a harmonic distortion component of the monitor signal from the optical DMT modulation signal, and a frequency analysis unit. The optical transmitter includes a bias control unit that controls a bias supply unit that adjusts a bias value to be supplied to the LD unit such that the extracted harmonic distortion component of the monitor signal is reduced.

Methods and devices for IQ time skew and conjugation compensation and calibration of a coherent transmitter or transceiver are described. A pilot tone is combined with a digital data signal such that relative powers of the pilot tone in each of two frequency bands of the transmitted data signal may be detected by a pilot tone detector and used to calculate the time skew between I and Q modulation channels of the transmitter. A transmitter DSP applies IQ time skew bias to the data signal to compensate for any calculated IQ time skew. The pilot tone detector also provides the transmitter DSP with the information necessary to detect phase conjugation of the optical signal, which can be corrected by inverting the polarity of the data signal or changing the phase bias point of the optical modulator.

Aspects of the subject disclosure may include, for example, obtaining two inputs x.sub.I and x.sub.Q based on a digital input signal, and causing a modulator to create two substantially orthogonal output dimensions I and Q based on the two inputs x.sub.I and x.sub.Q, by performing controlled introduction of a correlation between the two inputs x.sub.I and x.sub.Q for the modulator, and detecting a resulting output power of the modulator to facilitate operation of the modulator. Other embodiments are disclosed.

Described herein is a solution to address the intrinsic nonlinearity of analog signals and the restrictions this places on the signals dynamic range. The subject matter described herein produces linear electro-optic modulation over a dramatically wider range of the input signal amplitude. This is accomplished by a distributed multiwavelength design that folds the large dynamic range across multiple linear subranges, with each subrange being addressed using an optical wavelength. As a result, the subrange within the wide dynamic range of the input signal is captured by the linear portion of the transfer function of a single transfer function. Several physical implementations of this subject are presented herein. This innovation enables the efficient use of optical links for the transmission and processing of analog and multilevel signals, overcoming the limitations that were once hindering progress in this field.

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 /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.

An optical modulator uses an optoelectronic phase comparator configured to provide, in the form of an electrical signal, a measure of a phase difference between two optical waves. The phase comparator includes an optical directional coupler having two coupled channels respectively defining two optical inputs for receiving the two optical waves to be compared. Two photodiodes are configured to respectively receive the optical output powers of the two channels of the directional coupler. An electrical circuit is configured to supply, as a measure of the optical phase shift, an electrical signal proportional to the difference between the electrical signals produced by the two photodiodes.