Photoinduced absorption by VIS radiation in a-Si:H has been studied in-guide, in order to realise a novel all-optical waveguide micromodulator for application at 1.3 and 1.55 μm fiber communication wavelengths. In a-Si:H the photoinduced effects and the NIR absorption both involve dangling bonds states. The density of these states, deep in the gap, can be varied with doping. Therefore three waveguide prototypes have been fabricated by Plasma Enhanced Chemical Vapour Deposition on a silicon wafer. Their structure consist of a a-Si:H/SiO2 stack where the a-Si:H cores have different doping. The upper cladding is air. Optical measures on the core materials and signal transmission analysis in-guide at bit rates up to 200 kBit/s have been carried out. The excitation source of the VIS pump system for in-guide analysis consisted of simple, low cost AlInGaP LED’s controlled by a pulse generator. The pump and probe measures have been performed with different pump wavelengths and by varying the illumination intensity. LED’s with wavelengths of 644, 612, 590 and 571 nm have been alternatively used. For each pump wavelength, the light intensity was varied between 0,15 and 0,85 mW/mm2.
The results confirms that the optical modulation of the NIR signal enhances at high doping levels and for longer wavelengths. The modulation speed is probably limited by recombination phenomena.
Based on the total internal reflection (TIR) phenomenon and the thermo-optic effect in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si), a symmetric rib optical waveguide integrated switch is proposed and theoretically discussed. The device exploits the similar refractive index coupled to the different thermo-optic coefficient in the two materials. The possibility of alloying and doping for the band-gap engineering of a-Si:H, by means of the gas phase composition during the modern plasma enhanced chemical vapour deposition process, which takes place at temperatures as low as 220 degrees C, makes this semiconductor ideal for this type of application. In particular the refractive index at room temperature of the amorphous film can be properly tailored to match that of c-Si in order to achieve the light switching when the device experiences a given temperature change. TIR may be achieved however at the interface by acting on the temperature, because the two materials have different thermo-optic coefficient. The integrated single-mode rib waveguide is 4 μm wide and 3 μm high. The substrate is a SOI wafer with an oxide thickness of 500 nm. The switch has a quite short operation length of about 280 μm. The device performance is analyzed at the wavelength of 1.55 μm. It shows that the output crosstalk and insertion loss are less than -26.9 dB and 3.5 dB, respectively.