BACKGROUND

Optical communication systems are known in which light output from a laser is modulated by an external modulator to provide a modulated optical signal carrying data in accordance with an analog or drive signal applied to the modulator. In certain optical communication systems, the modulated optical signal may include a plurality of subcarriers, each of which carrying different data. In addition, each subcarrier may be modulated to have both in-phase (I) and quadrature (Q) components, if coherent detection is employed. Digital signal processors (DSPs) may be employed the generate appropriate digital signals on the transmit side of the optical communication system and such transmit digital signals corresponding to the I and Q components are converted to corresponding analog signal by digital-to-analog (D/A) circuits. These analog signals may be further processed in analog circuitry, such as a driver circuit or digital-to-analog circuits, to provide the drive signals to the modulators.

On the receive end of the system, the modulated optical signals may be converted to corresponding electrical signals by one or more photodiodes after mixing with local oscillator light in an optical hybrid, for example. These electrical signals may be further processed by analog circuitry, such as a transimpedance amplifier. The processed analog signals may then be provided to other analog circuits, such as analog-to-digital (ND) circuits that supply corresponding digital signals to a receive-end DSP for further processing.

At high data or baud rates, e.g., rates greater than 16 Gbaud, any distortion, such as an impedance mismatch, variations, or differences in the paths traversed by electrical signals associated with the I and Q components in the A/D, D/A, transmit and/or receive analog circuits may result in noise observed in the optical subcarriers output from the modulators. Moreover, path differences between the I and Q optical components in the modulator and the optical hybrids may also result in noise in corresponding electrical signals at the receive end of the optical link. The path variations may be caused by process-related imperfections in the integrated circuits used in the A/D and D/A circuits, for example, as well as process-related variations in modulators and optical hybrids. The resulting optical subcarrier noise, therefore, may differ from one system to the next and is difficult to predict. Accordingly, conventional approaches, including complex adders or multipliers, may not effectively correct for such optical subcarrier noise.

SUMMARY

Consistent with the present disclosure, an apparatus is provided that comprises analog circuitry including first and second paths for carrying electrical signals associated with in-phase and quadrature components of an optical signal. In addition, a digital signal processor is provided that includes digital circuitry that corrects for impairments that would occur in the optical signal in the absence of the digital circuitry. The impairments are associated with at least a difference in a characteristic between the first and second paths.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

