The coherent optical transponder is ubiquitous in long-haul optical network. The power imbalance among four tributaries limits the transmission performance of coherent transponder. This impairment is particularly severe for the 400 gigabit per second (Gb/s) signal, which utilizes the advanced modulation format like quadrature amplitude modulation (QAM) and operates at the high baud rate. To investigate the impairment, we constructed 400Gb/s coherent optical transponder with high speed DAC, IQ modulator, and high speed ADC. The power imbalance is introduced in digital domain. We experimentally investigated the impact of power imbalance in various scenarios. For the different baud rates like 45Gbd, 64Gbd and 86Gbd, the higher the baud rate is, the larger the penalty from the power imbalance is. For the different modulation formats like QPSK, 16-QAM, and 64-QAM, the higher the order of the modulation format is, the larger the penalty from the power imbalance is. Two types of power imbalance exist: IQ imbalance between in-phase and quadrature tributaries within one polarization, and XY imbalance between two polarizations. With high optical-to-signal noise ratio (OSNR), the penalty from the IQ power imbalance is larger; with low OSNR, the penalty from the XY power imbalance is larger. Furthermore, we demonstrated a technique to detect and compensate the power imbalance during the initial power-up of the coherent IQ transmitter. The technique is implemented through the adjustment of gain scaling factor of finite impulse filter (FIR). It works over multiple wavelength and the accuracy of internal photo diode is sufficient.
Yang Yue and Jon Anderson, "Experimental investigation of power imbalance for 400G coherent IQ transmitter (Conference Presentation)," Proc. SPIE 10947, Next-Generation Optical Communication: Components, Sub-Systems, and Systems VIII, 109470M (Presented at SPIE OPTO: February 07, 2019; Published: 13 March 2019); https://doi.org/10.1117/12.2510059.6013193602001.
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