Single-pixel optical modulation analyzer based on phase retrieval for dual-polarization IQ modulators.

In-service monitoring and adaptive digital compensation of analog imperfections in optical transponders are vital in the next-generation optical coherent transmission systems employing extremely high-order, high-speed modulation formats. A notable example of such analog impairments is the imbalance of amplitude, phase, and/or timing between the in-phase (I) and quadrature (Q) tributaries in an optical IQ modulator, namely the IQ imbalance. Recently, an IQ-imbalance estimation technique based on phase retrieval without using a coherent receiver, the so-called single-pixel optical modulation analyzer (SP-OMA), has been proposed as an affordable in-service monitoring solution for the frequency-dependent IQ imbalance in a (single-polarization) IQ modulator. In this work, we extend the concept of the SP-OMA to dual-polarization IQ modulators. A novel phase retrieval algorithm with an alternating minimization procedure is proposed for identifying the frequency-dependent IQ imbalances on both polarization channels simultaneously from a single photodetector output. The validity and feasibility of the proposed SP-OMA for a dual-polarization IQ modulator are demonstrated numerically and experimentally with a 63.25-Gbaud DP-16QAM signal.

Single-pixel optical modulation analyzer: a low-complexity frequency-dependent IQ imbalance monitor based on direct detection with phase retrieval.

Tiny mismatches in timing, phase, and/or amplitude between in-phase (I) and quadrature (Q) tributaries in an electro-optic IQ modulator, namely IQ imbalance, can severely affect high baud-rate and/or high modulation-order signals in modern coherent optical communications systems. To maintain such analog impairment within the tight penalty limit over wavelength and temperature during the product lifetime, in-service in-field monitoring and calibration of the IQ imbalance, including its frequency dependence, become increasingly important. In this study, we propose a low-complexity IQ monitoring technique based on direct detection with phase retrieval called a single-pixel optical modulation analyzer (SP-OMA). By reconstructing the optical phase information lost during the detection process computationally via phase retrieval, SP-OMA facilitates the in-service in-field monitoring of the frequency-dependent imbalance profile without sending dedicated pilot tones and regardless of any receiver/monitor-side IQ imbalance. The feasibility of SP-OMA is demonstrated both numerically and experimentally with a 63.25-Gbaud 16QAM signal.