Systems and methods described herein include methods and systems for controlling bias voltage provided to an optical modulating device. The optical modulating device is biased at a bias point that is different from a null point of the device such that an offset to the received optical power due to limited extinction ratio is reduced.

An optical transmitter includes: an optical modulator including an MZ interferometer, a drive signal input electrode, and a phase difference adjustment bias electrode; a drive amplifier; a phase difference adjustment bias voltage generator; a dithering unit that applies dithering of a predetermined frequency to an amplitude of a drive signal or to a half-wave voltage of the MZ interferometer; a controller unit that changes a phase difference adjustment bias voltage based on a modulation component of the frequency that is superimposed onto modulated light that is output from the optical modulator, to thereby bias the MZ interferometer to a null point; and a synchronous detection circuit that synchronously detects the modulation component of the frequency that is superimposed onto the modulated light. The controller unit changes the phase difference adjustment bias voltage such that a result of synchronous detection by the synchronous detection circuit becomes maximized or minimized. Whether the controller unit maximizes or minimizes the result of synchronous detection is determined by a difference between a phase of a referenced clock signal and a phase of the dithering, and does not depend on the amplitude of the drive signal.

A system is configured to determine a first power level of a first signal output from a first modulator, and determine a second power level of a second signal output from a second modulator. The first signal may include a first optical signal associated with a particular polarization orientation, and the second signal may include a second optical signal associated with the particular polarization orientation. The system is configured to determine a relationship between the first power level and the second power level, and to set, based on the relationship between the first power level and the second power level, a reverse bias voltage associated with the first modulator, where the reverse bias voltage may be used to control the first power level of the first signal.

To carry out automatic bias control to an optical modulator without a dither signal with reduced cost, an optical transmitter comprises a light source configured to output a light, an optical first modulator configured to modulate the light and output a first modulated optical signal, a DC component detector configured to detect a DC component of the first modulated optical signal and a controller configured to apply a first specific bias voltage identified based on the DC component to the optical first modulator.

An optical signal transmission apparatus generates a multi-level optical signal from a multi-level electric signal. The optical signal transmission apparatus detects, based on a supervisory signal generated from an optical signal, an electric-to-optical (E/O) conversion characteristic of an E/O converter configured to convert an electric signal into an optical signal. For example, when the E/O converter generates a multi-level optical signal from a multi-level electric signal based on a bias signal, the optical signal transmission apparatus determines a correspondence relationship between the bias signal and the optical signal. The optical signal transmission apparatus adjusts a use range of intensities of the bias signal based on the determined correspondence relationship so that the E/O converter may linearly operate.

An optical transmitter includes an optical modulator including a first modulator, a second modulator, and a phase shifter that provides a predetermined optical phase difference between the first modulator and the second modulator, a light source that makes light enter the optical modulator, and a voltage controller that detects a distortion of light power characteristics from output light of the optical modulator in a state where no data signal is input to the optical modulator to determine a bias voltage to be set in the optical modulator while reducing the distortion.

Embodiments of the present disclosure are directed toward techniques and configurations for an apparatus comprising an electro-optical modulation device with a bias control and adjustment. In some embodiments, the apparatus may comprise an electro-optical modulator having first and second arms, to modulate light passing through the first and second arms in response to an input data signal, and output a corresponding optical data signal. The apparatus may further comprise a control module coupled with the electro-optical modulator, to differentially adjust respective phases of first or second light portions passing through the first and second arms, to achieve a bias point for the optical data signal. The bias point may define a desired power output of the apparatus that corresponds to the optical data signal. Other embodiments may be described and/or claimed.

A light modulation element constituted by a substrate type optical waveguide has a Mach-Zehnder interferometer; and a traveling wave electrode having a signal electrode arranged at least between a first phase modulator and a second phase modulator and electrically connected to both of the first phase modulator and the second phase modulator. A polarity of a semiconductor region of the first phase modulator connected to the signal electrode and a plurality of a semiconductor region of the second phase modulator connected to the signal electrode are different from each other.

A method for generating a tone signal (TS) having a tone frequency, f, wherein the method comprises the following steps: supplying (S1) a binary bit stream (BBS) having a mark pattern with a supply bit rate, BR, to a signal filter unit; and filtering (S2) the supplied binary bit stream (BBS) by said signal filter unit to generate the tone signal (TS), wherein the mark pattern of the binary bit stream (BBS) supplied to said signal filter unit is adapted to minimize a ratio of the supply bit rate, BR, to the tone frequency, f, of the generated tone signal (TS).

An electro-optic device may comprise a Mach-Zehnder modulator (MZM) and one or more components. The one or more components may apply a child DC bias with dither to arms of a first branch of the MZM and to arms of a second branch of the MZM, and determine a second harmonic of a first return signal. The one or more components may apply a child DC bias with phase-shifted dither to the arms of the first branch or to the arms of the second branch, and determine a second harmonic of a second return signal. The one or more components may determine, based on the second harmonics, whether the first branch and the second branch are operating at quadrature, and may selectively adjust parent DC biases, applied to the first branch and the second branch, based on whether the first branch and the second branch are operating at quadrature.