Integrated optical ADC

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. [133]-137).An optically-sampled frequency-demultiplexed wideband analog-to-digital converter (ADC) which has potential to exceed the performance of electronic ADCs by orders of magnitude is studied analytically and numerically. The accuracy of the ADC as a function of its parameters is analyzed and impact of various imperfections of ADC components on its operation is evaluated. A universal error compensation algorithm for improving the conversion accuracy is proposed. On the way to implementation of the integrated optical ADC, two of its critical components - ring resonator filter bank and fiber-to-chip coupler -are designed. A novel coupler from a standard single mode fiber to a strongly confining silicon waveguide is proposed. The results of characterization of the filter bank and fiber-to-chip coupler fabricated on the silicon-on-insulator platform are presented and analyzed.by Anatol Khilo.S.M

Integrated photonic analog-to-digital converters

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 161-172).Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits and 52 dBc spur-free dynamic range (SFDR) using a discrete-component photonic ADC. This corresponds to 15 fs jitter, a 4-5 times improvement over the jitter of the best electronic ADCs, and an order of magnitude improvement over the jitter of electronic ADCs operating above 10 GHz. The feasibility of a practical photonic ADC is demonstrated by creating an integrated ADC with a modulator, filters, and photodetectors fabricated on a single silicon chip and using it to sample a 10 GHz signal with 3.5 effective bits and 39 dBc SFDR. In both experiments, a sample rate of 2.1 GSa/s was obtained by interleaving two 1.05 GSa/s channels; higher sample rates can be achieved by increasing the channel count. A key component of a multi-channel ADC - a dual multi-channel high-performance filter bank - is successfully implemented. A concept for broadband linearization of the silicon modulator, which is another critical component of the photonic ADC, is proposed. Nonlinear phenomena in silicon microring filters and their impact on ADC performance are analyzed, and methods to reduce this impact are proposed. The results presented in the thesis suggest that a practical integrated photonic ADC, which successfully overcomes the electronic jitter bottleneck, is possible today.by Anatol Khilo.Ph.D

Triply resonant coupled-cavity electro-optic modulators for RF to optical signal conversion

We propose an on-chip triply resonant electro-optic modulator architecture for RF-to-optical signal conversion and provide a detailed theoretical analysis of the optimal "circuit-level" device geometries and their performance limits. The designs maximize the RF-optical conversion efficiency through simultaneous resonant enhancement of the RF drive signal, a continuous-wave (CW) optical pump, and the generated optical sideband. The optical pump and sideband are resonantly enhanced in respective supermodes of a two-coupled-cavity optical resonator system, while the RF signal can be enhanced in addition by an LC circuit formed by capacitances of the optical resonator active regions and (integrated) matching inductors. We show that such designs can offer 15-50 dB improvement in conversion efficiency over conventional microring modulators. In the proposed configurations, the photon lifetime (resonance linewidth) limits the instantaneous RF bandwidth of the electro-optic response but does not limit its central RF frequency. The latter is set by the coupling strength between the two coupled cavities and is not subject to the photon lifetime constraint inherent to conventional singly resonant microring modulators. This feature enables efficient operation at high RF carrier frequencies without a reduction in efficiency commonly associated with the photon lifetime limit and accounts for 10-30 dB of the total improvement. Two optical configurations of the modulator are proposed: a "basic" configuration with equal Q-factors in both supermodes, most suitable for narrowband RF signals, and a "generalized" configuration with independently tailored supermode Q-factors that supports a wider instantaneous bandwidth. A second significant 5-20 dB gain in modulation efficiency is expected from RF drive signal enhancement by integrated LC resonant matching, leading to the total expected improvement of 15-50 dB. Previously studied triply-resonant modulators, with coupled longitudinal [across the free spectral range (FSR)] modes, have large resonant mode volume for typical RF frequencies, which limits the interaction between the optical and RF fields. In contrast, the proposed modulators support maximally tightly confined resonant modes, with strong coupling between the mode fields, which increases and maintains high device efficiency across a range of RF frequencies. The proposed modulator architecture is compact, efficient, capable of modulation at high RF carrier frequencies and can be applied to any cavity design or modulation mechanism. It is also well suited to moderate Q, including silicon, implementations, and may be enabling for future CMOS RF-electronic-photonic systems on chip.</p

Sub-decibel efficiency, bi-layer, O-band fiber-to-chip grating coupler demonstrated in a 45 nm CMOS foundry platform

We demonstrate a grating coupler with 0.85 dB fiber-to-chip coupling loss in the O-band (1,300 nm), implemented in a 45 nm CMOS foundry platform.Accepted manuscrip

Silicon waveguides and resonators with sub-0.1 dB/cm propagation loss and over 7 million Q in a foundry process

Propagation loss is characterized vs. waveguide width in a 220 nm silicon photonics foundry platform to form a compact model. Test paperclips and racetrack resonators with quality factors up to 7.6 million reveal losses as low as 0.064 dB/cm.Accepted manuscrip

Ultrafast nonlinear optical processes and free-carrier lifetime in silicon nanowaveguides

Abstract: We report self-consistent femtosecond studies of two-photon absorption, optical Kerreffect and free-carrier index and loss in silicon nanowaveguides using heterodyne pump-probe. Free-carrier lifetime was reduced to 33ps with only 8dB/cm added loss using proton bombardment

In-situ photonic circuit field characterization in electronics-photonics CMOS platform via backside flip-chip near-field scanning optical microscopy

We demonstrate device field characterization using NSOM collection and interaction measurement modes via the backside buried-oxide of large scale photonic circuits fabricated in monolithic electronics-photonics CMOS platforms (here a microdisk resonator) post-processed using flip-chip substrate-removal.Accepted manuscrip

Polarization-Insensitive one-dimensional grating coupler demonstrated in a CMOS-photonics foundry platform

We demonstrate a one-dimensional dual polarization fiber-to-chip grating coupler implemented in a CMOS-photonics foundry platform, with a measured 1 dB polarization-dependent loss bandwidth of 70 nm in the O-band.Accepted manuscrip