Stabilization of the bias point of an electro-optic modulator based on polymer material

Electro-optic modulators based on polymer material are very promising devices because of the expected very high modulation rate and low cost fabrication process. These modulators are mainly based on Mach-Zehnder structure, but phase modulators associated to polarizer and analyzer can also be used. Until now there are no real devices commercially available. One of the problems concerns the temperature stability. Because of the optical power of the laser beam and the absorption of the polymer material used for the optical waveguide there is a slight temperature evolution of the modulator leading to a change in the bias point of the modulator and then to a slow drift of the bias point leading to a dissymmetry of the optical modulated signal. This evolution can be compensated by adding a small DC value to the voltage applied to the electrodes. A control loop has been designed and tested in order to stabilize the bias point of the modulator. This loop acts as a synchronous detection with a low frequency modulation at 500 Hz and a practical detection at 1 kHz. By this way it is in fact the first order derivative of the signal which is stabilized leading to the signal symmetry control. This low frequency signal can be added without any problem to the informative modulating signal. By using this control loop the modulator can be used for a very longer time than without it. Of course a temperature control of the modulator by a Peltier effect module should also be implemented for a better and complete stabilization.