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This paper describes a simple and highly sensitive auto bias control (ABC) technique based on asymmetric bias dithering for optical quadrature phase shift keying (QPSK) modulation and QPSK variants. Using our asymmetric bias dithering proposal, the required bandwidth of the ABC circuit is only dc ~ 2fd, where fd is the frequency of dithering, fd <; 1 kHz. Thus, the circuit can be constructed without using high-speed components, and bias control performance is not limited by the signal bit rate. In this paper, we detail the concept of asymmetric bias dithering. We describe experiments that confirm bias monitoring and ABC operation, using a 20-64 Gbit/s optical QPSK signal. Its typical penalty is 0.1 dB, and Q factor fluctuation is negligible over 6 h of operation.