S. Sharif Azadeh

On the measurement of the Pockels effect in strained silicon

We measure the voltage-dependent phase shift in silicon waveguides strained by a silicon nitride layer and show that, in our measurements, the phase shift is due to free carrier accumulation inside the waveguides. Nonetheless, inverting the applied voltage also inverts the applied phase shift-an effect due to a quasi-static surface charge in the silicon nitride. Since the measured effect is on the same order as recently published second-order nonlinearities attributed to the Pockels effect, inclusion of these carrier-based effects in the analysis of experimental data is of paramount importance.

Optical Peaking Enhancement in High-Speed Ring Modulators

Ring resonator modulators (RRM) combine extreme compactness, low power consumption and wavelength division multiplexing functionality, making them a frontrunner for addressing the scalability requirements of short distance optical links. To extend data rates beyond the classically assumed bandwidth capability, we derive and experimentally verify closed form equations of the electro-optic response and asymmetric side band generation resulting from inherent transient time dynamics and leverage these to significantly improve device performance. An equivalent circuit description with a commonly used peaking amplifier model allows straightforward assessment of the effect on existing communication system architectures. A small signal analytical expression of peaking in the electro-optic response of RRMs is derived and used to extend the electro-optic bandwidth of the device above 40 GHz as well as to open eye diagrams penalized by intersymbol interference at 32, 40 and 44 Gbps. Predicted peaking and asymmetric side band generation are in excellent agreement with experiments.

Advances in silicon photonics segmented electrode Mach-Zehnder modulators and peaking enhanced resonant devices

We report recent progress made in our laboratory on travelling wave Mach-Zehnder Interferometer based Silicon Photonics modulators with segmented transmission lines, as well as on resonant ring modulators and add-drop multiplexers with peaking enhanced bandwidth extended beyond the photon lifetime limit. In our segmented transmission lines, microstructuring of the electrodes results in radio-frequency modes significantly deviating from the transverse electromagnetic (TEM) condition and allows for additional design freedom to jointly achieve phase matching, impedance matching and minimizing resistive losses. This technique was found to be particularly useful to achieve the aforementioned objectives in simple back-end processes with one or two metallization layers. Peaking results from intrinsic time dynamics in ring resonator based modulators and add-drop multiplexers and allows extending the bandwidth of the devices beyond the limit predicted from the photon lifetime. Simple closed form expressions allow incorporating peaking into system level modeling.

Epitaxially grown vertical junction phase shifters for improved modulation efficiency in silicon depletion-type modulators

High-speed silicon modulators based on the plasma effect in reverse-biased p(i)n junction phase shifters have been extensively investigated. The main challenge for such modulators is to maximize their modulation efficiency without compromising high-speed performance and insertion losses. Here, we propose a highly efficient silicon modulator based on a Mach-Zehnder Interferometer in which the doping profile of a vertical pin junction is precisely controlled by means of in-situ doping during silicon epitaxial growth. The precise level of control afforded by this fabrication procedure allows separately optimizing doping concentrations in the immediate vicinity of the junction and in surrounding electrical transport layers at the nanometric scale, enabling high performance levels. Free carrier absorption losses are minimized by implementing high carrier densities only in the waveguide regions where they benefit the most, i.e., in the immediate vicinity of the junction. Since these devices rely entirely on single crystal silicon, performance degradation caused by poor transport and high optical losses in poly- or amorphous silicon (as utilized in similar vertical phase shifter geometries such as semiconductor-insulator-semiconductor capacitive phase shifters) is avoided. Furthermore, unlike conventional plasma effect silicon phase shifters, the bandwidth of the proposed phase shifters is largely independent of the applied reverse voltage and the phase shift versus applied voltage is linearized, making them more suitable for complex modulation formats. The efficiency of the single ended phase shifters is expected to reach a VπL of 0.56 V•cm and absorption losses of α=4.5 dB/mm, a good performance metric for depletion-type modulators. Lumped element Mach-Zehnder Modulators as well as travelling-wave modulators with phase matching based on meandered waveguides have been designed and their RF characteristics simulated and optimized with Ansoft HFSS. First experiments have validated the growth of the epitaxial stack and complete devices are currently being fabricated.

Misalignment tolerant couplers for hybrid integration of semiconductor lasers with silicon photonics parallel transmitters

Hybrid integration of prefabricated III-V laser diodes with sub-micrometric silicon photonic waveguides suffers from a tradeoff between alignment tolerance and coupling efficiency. In this work, we demonstrate integrated coupling devices that substantially alleviate this problem by means of a balanced distribution of the laser power between two on-chip single mode SOI waveguides. With the reported coupling devices, a horizontal misalignment of the laser is converted in a variation of the relative phase of the light coupled into the two waveguides, allowing to satisfy the reciprocity principle while maintaining a high total coupling efficiency and a balanced power splitting. The relaxed alignment tolerances facilitate passive assembly of the lasers with pick-and-place tools. The balanced splitting of the power between waveguides is particularly well suited for optical interconnects with parallel transmitters. Here, the device design is discussed for both edge couplers and grating couplers relying on similar design principles. Furthermore, experimental characterization of edge-coupling structures with a lensed fiber and a Fabry-Pérot laser is presented. These devices have been fabricated with 193nm DUV optical lithography and are compatible with mainstream CMOS technology. The edge couplers with the best horizontal misalignment exhibits an excellent 1 dB loss horizontal misalignment range of 3.8 μm with excess insertion losses below 3.1 dB (in addition to the 3dB splitting). The back-reflection induced by the device has been assessed to be below -20 dB and measured relative intensity noise is better than measured from the same laser coupled to a lensed fiber.

Silicon photonics WDM interconnects based on resonant ring modulators and semiconductor mode locked laser

We demonstrate wavelength domain multiplexed (WDM) data transmission with a data rate of 14 Gbps based on optical carrier generation with a single-section semiconductor mode-locked laser (SS-MLL) and modulation with a Silicon Photonics (SiP) resonant ring modulator (RRM). 18 channels are sequentially measured, whereas the best recorded eye diagrams feature signal quality factors (Q-factors) above 7. While optical re-amplification was necessary to maintain the link budgets and therefore system measurements were performed with an erbium doped fiber amplifier (EDFA), preliminary characterization done with a semiconductor optical amplifier (SOA) indicates compatibility with the latter pending the integration of an additional optical filter to select a subset of carriers and prevent SOA saturation. A systematic analysis of the relative intensity noise (RIN) of isolated comb lines and of signal Q-factors indicates that the link is primarily limited by amplified spontaneous emission (ASE) from the EDFA rather than laser RIN. Measured RIN for single comb components is below -120 dBc/Hz in the range from 7 MHz to 4 GHz and drops to the shot noise level at higher frequencies.

Co-integration of a temperature tolerant low impedance resonantly enhanced silicon photonics modulator

We report on an optically wideband, resonantly enhanced Mach-Zehnder modulator co-integrated with a 4Ω output impedance, 28Gbd driver from Mellanox Technologies. Error free transmission is demonstrated, at 14Gbps (25Gbps), in 4nm (3nm) wide optical wavelength range at a 10mW (20mW) laser output power level.

Design of a high-speed germanium-tin absorption modulator at mid-infrared wavelengths

We propose a high-speed electro-absorption modulator based on a direct bandgap Ge0.875Sn0.125 alloy operating at mid-infrared wavelengths. Enhancement of the Franz-Keldysh-effect by confinement of the applied electric field to GeSn in a reverse-biased junction results in 3.2dB insertion losses, a 35GHz bandwidth and a 6dB extinction ratio for a 2Vpp drive signal.

WDM transceiver with semiconductor mode locked laser

We demonstrate a Silicon Photonics transmitter/receiver pair wire bonded to chip-scale electronics and operated with a single section semiconductor mode locked laser. The compact WDM system supports twelve independent error free (BER<;1e-12) 14 Gbps channels without error correction, preemphasis or equalization.