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 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.

A system including an optical engine. In some embodiments, the system includes an integrated circuit in a first-level package, and the system includes the optical engine, in the first-level package, and the optical engine includes an electro-optical chip.

A system including an optical engine. In some embodiments, the system includes an integrated circuit in a first-level package, and the system includes the optical engine, in the first-level package, and the optical engine includes an electro-optical chip.

An apparatus at least includes a processor to, after a first signal passes through a narrowband photodetector, perform frequency mix of the first signal with a first reference signal and a second reference signal respectively and perform noise reduction, to obtain a first detection signal and a second detection signal. The first signal is obtained after a measurement signal passes through a filtering module of an optical transmitting end, the measurement signal being transmitted in a path of multiple branches, signals not transmitted in other paths of the multiple branches, a frame structure of the measurement signal including at least one two-tone signal, two tones in the two-tone signal having a fixed tone interval. The processor is to calculate group delays at multiple frequency bins according to the first detection signal and the second detection signal; and determine a phase response of the filtering module of the optical transmitting end in the path according to the group delays at multiple frequency bins.

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.

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.

An apparatus at least includes a processor to, after a first signal passes through a narrowband photodetector, perform frequency mix of the first signal with a first reference signal and a second reference signal respectively and perform noise reduction, to obtain a first detection signal and a second detection signal. The first signal is obtained after a measurement signal passes through a filtering module of an optical transmitting end, the measurement signal being transmitted in a path of multiple branches, signals not transmitted in other paths of the multiple branches, a frame structure of the measurement signal including at least one two-tone signal, two tones in the two-tone signal having a fixed tone interval. The processor is to calculate group delays at multiple frequency bins according to the first detection signal and the second detection signal; and determine a phase response of the filtering module of the optical transmitting end in the path according to the group delays at multiple frequency bins.

A transmitter optical subassembly may include an optical modulator for modulating output light from the light source. The optical modulator has a characteristic that a current depending on amount of optical absorption has a positive correlation with an applied voltage thereto. The transistor at the second terminal is connected in series to the optical modulator. A drive voltage applied to the optical modulator and the transistor is divided into a first voltage applied to the optical modulator and a second voltage applied to the transistor. A drive current flowing through the optical modulator and the transistor depends on the control signal input to the first terminal. The first voltage is based on the drive current and is subject to the characteristic of the optical modulator. The second voltage fluctuates in response to the first voltage.