D. Pascal

High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide

Design, fabrication and characterization of germanium on silicon photodetector integrated in SOI waveguide are reported. A responsivity of 1 A/W and a -3 dB bandwidth of 25 GHz under 6 V bias have been obtained at lambda=1.55 mum.

Light injection in SOI microwaveguides using high-efficiency grating couplers

An experimental characterization of the grating couplers for sub-micrometer silicon-on-insulator (SOI) waveguides is presented. The grating couplers have been designed, realized, and characterized for the +1 diffraction order at an operating wavelength of 1.31 mum for TE polarization. At the resonant angle, a coupling efficiency higher than 55% has been measured. The angular coupling range and the wavelength tolerance have been evaluated to 3deg and 20 nm, respectively. The grating coupler is followed by a taper, and about 50% of the input power at 1.31 mum is coupled into sub-micrometer rib and strip SOI waveguides. The ration between light power decoupled toward the cladding and light power decoupled toward the substrate is about three

Optical modulation by carrier depletion in a silicon PIN diode

Experimental results for refractive index variation induced by depletion in a silicon structure integrated in a PIN diode are reported. Thermal effect has been dissociated from the electrical contribution due to carrier density variation induced by a reverse bias voltage. A figure of merit V(pi)L(pi) of 3.1 V.cm has been obtained at 1.55mum. Numerical simulations show a good agreement between experimental and theoretical index variations.

Ultrahigh speed germanium-on-silicon-on-insulator photodetectors for 1.31 and 1.55μm operation

We report the fabrication and the characterization of interdigited metal-germanium on silicon metal photodetectors (metal-semiconductor-metal or MSM) for operation at both optical telecommunication wavelengths: 1.31 and 1.55μm. Femtosecond impulse and frequency experiments have been carried out to characterize those MSM Ge photodetectors. For both wavelengths, the measured 3dB bandwidth under 2V bias are close to 10, 18, 20, and 35GHz for electrode spacings equal to 2000, 1000, 700, and 500nm, respectively.

Integration of germanium waveguide photodetectors for intrachip optical interconnects

The main characteristics of germanium photodetectors integrated in silicon-on-insulator optical waveguides for intrachip optical interconnects are presented. The epitaxial Ge layers are grown on Si(001) by reduced-pressure chemical vapor deposition. The optical absorption of Ge layers is recorded from 1.2 to 1.7 µm and linked to the layer strain. The responsivity of an interdigitated metal-semiconductor-metal Ge photodetector has been measured. Light coupling from a slightly etched submicron rib silicon-on-insulator waveguide to a Ge photodetector is studied for two configurations: butt coupling and vertical coupling.

Experimental demonstration of a low-loss optical H-tree distributionusing silicon-on-insulator microwaveguides

The experimental demonstration of an optical H-tree distribution from one input to sixteen outputs compatible with the size of a microelectronic chip is reported. It is based on low-loss silicon-on-insulator rib submicron waveguides. Each branch is 1cm long and includes four ultracompact T splitters and two mirrors allowing 90° direction changes. The mean value of the global losses has been measured and is found equal to 26dB, which corresponds to a power of 2.6μW at each output, for a 3mW laser diode power at the input. This demonstration is an important step for on-chip optical clock distribution in complementary metal–oxide–Semiconductor integrated circuits.The experimental demonstration of an optical H-tree distribution from one input to sixteen outputs compatible with the size of a microelectronic chip is reported. It is based on low-loss silicon-on-insulator rib submicron waveguides. Each branch is 1cm long and includes four ultracompact T splitters and two mirrors allowing 90° direction changes. The mean value of the global losses has been measured and is found equal to 26dB, which corresponds to a power of 2.6μW at each output, for a 3mW laser diode power at the input. This demonstration is an important step for on-chip optical clock distribution in complementary metal–oxide–Semiconductor integrated circuits.

Low-loss optical waveguide on standard SOI/SIMOX substrate

Propagation loss measurements are reported for silicon waveguides formed by the thin silicon film of standard SIMOX substrate. Values very close to the theoretical limit associated with leakage towards the substrate are obtained when a high quality interface between the silicon film and the passivation layer is achieved.

Modelling of two-dimensional grating couplers on silicon-on-insulator waveguides using beam propagation method

We show how the parameters of a 3-D laser beam can be taken into account in the calculation of the coupling efficiency into a waveguide via a diffraction grating. Two well known formalisms are used: (i) differential analysis of the stratified guiding structure supporting the grating, and (ii) beam propagation method used to model the propagation in the waveguide. Both are linked by the Helmholtz equation in which a source term has been added corresponding to the field diffracted by the grating. Some results are given.

Design of a modulation-doped SiGe∕Si optical modulator integrated in a submicrometer silicon-on-insulator waveguide

A SiGe/Si optical modulator based on the free-carrier plasma dispersion effect is presented. A SiGe/Si multilayer structure is included in a p-i-n diode and integrated in a submicrometric silicon-on-insulator (SOI) rib waveguide. A P+ Si layer inserted in the Si barriers provide holes that are confined in the SiGe wells at the equilibrium and can be depleted by applying a reverse bias. This structure offers high-speed phase modulation properties. Numerical simulations are used to design the optical modulator. An optimal structure is defined comprising three 10-nm-thick SiGe layers and four 5-nm-thick P+ Si layers. The predicted refractive index variation is 1.7×10–4 under a –6-V bias voltage, and the associated absorption variation is 3 dB/cm. Intensity modulation is obtained by embedding the active region inside a microinterferometric structure. Using resonant cavities a few hundred micrometers long, a modulation depth larger than 60% is achieved with insertion losses of 12 dB, whereas Mach-Zehnder interferometers of a few millimeters length lead to modulation depths larger than 95% with insertion losses lower than 8 dB.

Guided wave spatial transients under a limited grating coupler in a silicon-on-sapphire waveguide

The limited size of a grating coupler on a waveguide is responsible for the nonuniform guided wave intensity. Spatial transients have been recorded and a characteristic length of the order of 30 μm has been determined by fitting a simple model to the experimental results. This value is consistent with spatial transient measurements performed under nonlinear operation conditions which reveal a 100 μm long dead zone in which switching contrast is very low.