The Mach-Zehnder electro-optical modulator is an important device in optical communication systems. The basic operation of a Mach-Zehnder interferometric intensity modulator is described in U.S. Pat. No. 5,074,631, Hamano et al. (1991). Their patent discloses a technique of using separate electrodes and drive units for the two optical waveguides of the interferometer to eliminate the effects of temperature fluctuations and to reduce the drive voltages.
U.S. Pat. No. 6,046,838, Kou et al. (2000) discloses an automatic bias control for electro-optic modulators, which uses one or two pilot tones (a dither signal with a single frequency or a combination of two different frequencies) for controlling the DC bias point to eliminate the time-varying effects of the physical process in the modulator, such as those caused by temperature fluctuations. U.S. Pat. No. 6,392,779, Iannelli et al. (2002), discloses another automatic bias control system that uses two pilot tones that are swept in frequency over a frequency range to allow the amplitude of the pilot tones to be increased, so that the signal level for the control circuit is increased.
U.S. Pat. No. 7,034,977, Harel et al. (2006), discloses an automatic DC bias control for duobinary modulation with a Mach-Zehnder electro-optical modulator. The automatic DC bias control uses a dither, and the feedback signal is demodulated synchronously with the spread-spectrum source that generates the dither. The control circuit adjusts the dither signal to maintain a constant value of a component of the optical output signal that is synchronous with the dither. However, the disclosed approach for automatic DC bias control applies only to the duobinary modulation format utilizing low-pass electrical filtering of binary electrical signal. Furthermore, there is no discussion about the design and specifications of the spread-spectrum source for achieving automatic DC bias control.
Despite the aforementioned concepts for maintaining the proper bias condition of a Mach-Zehnder modulator, today a typical bias controller still employs the conventional approach (illustrated in FIG. 1) of using a low-frequency dither signal of approximately 1 KHz, with the bias controller minimizing the second harmonic of the feedback signal.
Needs exist for improved bias controllers for optical modulators.