Bit Error Rate Monitoring for Automatic Bias Control of Quadrature Amplitude Modulators

The most common quadrature amplitude modulator (QAM) applies two Mach-Zehnder Modulators (MZM) and one phase shifter to generate high order modulation format. The bias of MZM changes over time due to temperature, vibration, and aging factors. The change in the biasing causes distortion to the generated QAM signal which leads to deterioration of bit error rate (BER) performance. Therefore, it is critical to be able to lock MZM’s Q point to the required operating point for good performance. We propose a technique for automatic bias control (ABC) of QAM transmitter using BER measurements and gradient descent optimization algorithm. The proposed technique is attractive because it uses the pertinent metric, BER, which compensates for bias drifting independently from other system variations such as laser source output power. The proposed scheme performance and its operating principles are simulated using OptiSystem simulation software for 4-QAM and 16-QAM transmitters.

[1] Bae Y., et al. "Automatic Bias Control Technique of Dual-Parallel Mach–Zehnder Modulator Based on Simulated Annealing Algorithm for Quadrupled Signal Generation". Photonics Journal, 2021.
[2] Albakay N., and Nguyen L., "Design and performance analysis for a binary-driven QAM transmitter," Journal of lightwave technology, Nov 2018.
[3] Yan. S et al., "Generation of square or hexagonal 16-QAM signals using a dual-drive IQ modulator driven by binary signals, " Opt. Exp, 2012.
[4] Devgan P., "Cancellation of photodiode-induced second harmonic distortion using single side band modulation from a dual parallel Mach-Zehnder, " Opt. Exp, 2012.
[5] Zhu Z., eta al., "Filterless frequency 12-tupling optical millimeter-wave generation using two cascaded dual-parallel Mach-Zehnder modulators, "Appl. Opt, 2015.
[6] Li J., eta al "Third-order intermodulation distortion elimination of microwave photonics link based on integrated dual-drive dual-parallel Mach-Zehnder modulator,” Opt. Lett., 2013.
[7] Li X., et al. "Arbitrary bias point control technique for optical IQ modulator based on dither-correlation detection." Journal of Lightwave Technology, 2018.
[8] Švarný, J. "Analysis of quadrature bias-point drift of Mach-Zehnder electro-optic modulator." 12th Biennial Baltic Electronics Conference. IEEE, 2010.
[9] Li X., et al. "Modulation-format-free and automatic bias control for optical IQ modulators based on dither-correlation detection." Optics express, 2017.
[10] Kawakami H., Eiji Y., and Yutaka M. "Auto bias control technique based on asymmetric bias dithering for optical QPSK modulation." Journal of lightwave technology,2012.
[11] Zhu X., et al. "Coherent detection-based automatic bias control of Mach–Zehnder modulators for various modulation formats." Journal of lightwave technology,2014.