Low, drive voltage gaas integrated electrooptical phase modulator for the 10, 6 microns wavelength

In this paper we describe a low drive voltage phase modulator for the 10, 6 microns wavelength. We present also the experimental results that have been obtained with such a component and the related theoretical model. The wave- guiding basic structure is realized by the vapour phase epitaxy of a slightly N doped GaAs layer on a heavily N doped (100) substrate of the same material. An ohmic contact under the substrate and a planar Schottky electrode on the weakly doped medium permit the application of an electric field in the guiding region via a reverse polarization of the device. The experimental results we present show that the phase modulation we have mesured on the TE_o mode of the guide for different applied voltages proceeds from two effects. The first one is of course electrooptical and is due to the electric field which only exists in the depletion region lying under the Schottky electrode. The second one, which is not negligible because of the long working wavelength, comes from the refractive index increase due to the free electron density lowering in that depletion region. In the particular case of our experiments this last phenomenon leads to a drive voltage decrease: 80 volts for a phase-shift of π on the TE_o mode for a 16 mm long interaction instead of 100 volts without this depletion effect. The proposed theoretical model which takes into account these two contributions is in a good agreement with the experimental results and give the real possibility to predict the performances of such a device. In addition to these results related to the active aspect, we present a low loss wave- guiding planar configuration using a strongly N doped InP substrate where losses below 1 dB/cm on the TE_o mode have been measured at the 10, 6 microns wavelength.