A nested Mach-Zehnder (MZ) modulator, which consists of two sub MZ interferometers embedded in a main MZ interferometer, is a multifunctional modulator. It can work as an optical single side-band (SSB) modulator, a differential quadrature phase shift keying (DQPSK) modulator and so on. Despite difficulties due to their complex structure, in order to avoid the operation instability of such modulators the bias condition of each MZ interferometer must be controlled automatically according to the modulation formats. In this paper, we proposed a specific bias condition monitor technique to the nested modulator.
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