High frequency electro-optic measurement of strained silicon racetrack resonators

The observation of the electro-optic effect in strained silicon waveguides has been considered as a direct manifestation of an induced $\chi^{(2)}$ non-linearity in the material. In this work, we perform high frequency measurements on strained silicon racetrack resonators. Strain is controlled by a mechanical deformation of the waveguide. It is shown that any optical modulation vanishes independently of the applied strain when the applied voltage varies much faster than the carrier effective lifetime, and that the DC modulation is also largely independent of the applied strain. This demonstrates that plasma carrier dispersion is responsible for the observed electro-optic effect. After normalizing out free carrier effects, our results set an upper limit of $8\,pm/V$ to the induced high-speed $\chi^{(2)}_{eff,zzz}$ tensor element at an applied stress of $-0.5\,GPa$. This upper limit is about one order of magnitude lower than the previously reported values for static electro-optic measurements

Fabrication tolerant high-speed SiP ring modulators and optical add-drop multiplexers for WDM applications

Silicon ring resonator modulators (RRMs) have great potential to reduce footprint and power consumption and to increase modulation speeds in wavelength division multiplexed (WDM) transmitters. However, the optical properties of RRMs are highly sensitive to fabrication variations, which makes them challenging to design for volume production or a large number of WDM-channels. In this work, we present an RRM design that was specifically designed and experimentally validated to have reduced sensitivity to fabrication variations. This includes a sensitivity analysis of resist over- and under-exposure (±30 nm lateral dimension deviation) and of etch depth variability (±10 nm depth variation) within the coupling section. For our design, the deviation from the targeted coupling strength is improved twofold. The proposed devices are fabricated on SOI wafers using a standard CMOS-compatible process. We demonstrate RRMs with an extinction ratio above 5 dB, an OMA better that -7 dB (at 2 Vpp) and a 29 GHz electro-optical bandwidth, showing open eye diagrams at 32 Gb/s limited only by our measurement setup. The measured coupling coefficients are in good agreement with the simulated values. Furthermore, we applied the same design modifications to realize low-doped RRMs as well as ring based adddrop-multiplexers (OADMs). The agreement between the simulated and the measured coupling coefficients (that we identified as the main source of device performance variability) further confirms the effectiveness of our design modifications. These results suggest that the proposed design can be exploited to enable reliable fabrication of resonantbased devices on a large scale, especially in WDM systems

Power-efficient lumped-element meandered silicon Mach-Zehnder modulators

Driving electro-optic modulators in lumped-element (LE) configuration allows for small footprint, reduced power consumption, and improved high-speed performance. The main shortcoming of conventional rectilinear LE modulators are the required high drive-voltages, resulting from their shortened phase-shifters. To address this, we introduce a Mach-Zehnder modulator with meandered phase shifters (M-MZM), which can be driven in LE configuration, while keeping the optical phase shifter length in the same order as traveling-wave modulators (TW-MZMs). A design limitation that needs to be taken into account consists in the optical transit time of the device, that limits the overall electro-optic bandwidth. First, we review the overall power consumption improvement as well as the bandwidth enhancement in LE modulators compared to TW-MZMs, also taking the driver output impedance and parasitics from wire- or bump-bonds into account. Then, we report on the design, implementation, and experimental characterization of carrier-depletion based M-MZMs fabricated on silicon-on-insulator (SOI) wafers using standard CMOS-compatible processes. The fabricated M-MZMs, provided with low (W1), moderately (W2) and highly (W3) doped junctions, require 9.2 Vpp, 5.5 Vpp, and 3.7 Vpp for full extinction, with optical insertion losses of 5 dB, 6.3 dB and 9.1 dB. For all three M-MZMs, open eye diagrams are recorded at 25 Gb/s using a 50Ω driver and termination. For unterminated M-MZMs, higher data rates could be achieved, provided that a low output impedance driver be wire- or bump-bonded to the modulators. Finally, we compare the power consumption of the M-MZMs with TW-MZMs and show that the M-MZMs feature a 4X reduced power consumption at 25 Gb/s

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

Mode-locked laser timing jitter limitation in optically enabled, spectrally sliced ADCs

Novel analog-to-digital converter (ADC) architectures are motivated by the demand for rising sampling rates and effective number of bits (ENOB). The main limitation on ENOB in purely electrical ADCs lies in the relatively high jitter of oscillators, in the order of a few tens of fs for state-of-the-art components. When compared to the extremely low jitter obtained with best-in-class Ti:sapphire mode-locked lasers (MLL), in the attosecond range, it is apparent that a mixed electrical-optical architecture could significantly improve the converters' ENOB. We model and analyze the ENOB limitations arising from optical sources in optically enabled, spectrally sliced ADCs, after discussing the system architecture and implementation details. The phase noise of the optical carrier, serving for electro-optic signal transduction, is shown not to propagate to the reconstructed digitized signal and therefore not to represent a fundamental limit. The optical phase noise of the MLL used to generate reference tones for individual slices also does not fundamentally impact the converted signal, so long as it remains correlated among all the comb lines. On the other hand, the timing jitter of the MLL, as also reflected in its RF linewidth, is fundamentally limiting the ADC performance, since it is directly mapped as jitter to the converted signal. The hybrid nature of a photonically enabled, spectrally sliced ADC implies the utilization of a number of reduced bandwidth electrical ADCs to convert parallel slices, resulting in the propagation of jitter from the electrical oscillator supplying their clock. Due to the reduced sampling rate of the electrical ADCs, as compared to the overall system, the overall noise performance of the presented architecture is substantially improved with respect to a fully electrical ADC

GeSn lasers for CMOS integration

In search of a suitable CMOS compatible light source many routes and materials are under investigation. Si-based group IV (Si)GeSn alloys offer a tunable bandgap from indirect to direct, making them ideal candidates for on-chip photonics and nano-electronics. An overview of recent achievements in material growth and device developments will be given. Optically pumped waveguide and microdisk structures with different strain and various Sn concentrations provide direct evidence of gain in these alloys and the width of the emission wavelength range that can be covered. Towards the aim of electrically pumped lasers, a set of different homojunction light emitting diodes and more complex heterostructure SiGeSn/GeSn LEDs is presented. Detailed investigation of electroluminescence spectra indicate that GeSn/SiGeSn heterostructures will be advantageous for future laser fabrication

Optical Arbitrary Waveform Measurement (OAWM) on the Silicon Photonic Platform

We demonstrate optical arbitrary waveform measurement (OAWM) using a silicon pho-tonic spectral slicer. Exploiting maximal-ratio combining (MRC), we demonstrate the viability of the scheme by reconstructing 100-GBd 64QAM signals with high quality