A Mach-Zehder modulator is not perfectly balanced. Moreover it is subject to drift caused by thermal changes, thermal inhomogeneity, aging, photo refractive effects, static electrical charge accumulation… This drift causes the transfer function to move in the horizontal direction ; the modulation signal is then applied to a changing operating point, that can modify strongly the obtained modulation.

 The bias voltage applied to the DC electrodes aims to :

- selecting the desired operating point of the modulator

- compensating for the possible modulator drift and locking the device operating point so as to keep stable operation conditions

 The bias voltage can be supplied by a simple voltage source and manually adjusted so as the desired operating point is reached. In such conditions, the voltage will have to be re-adjusted manually in case of drift of the modulator. This may be workable in laboratory with low drift modulators and stable environmental conditions

However, for a long term operation and especially in all systems that have to operate over changing temperature conditions, an automatic bias control circuit is necessary so as to permanently supply the right DC voltage and to lock the selected operating point.

Back to lexicon