Steven J. Spector

Photonic ADC: overcoming the bottleneck of electronic jitter

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 for many years 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 using a photonic ADC built from discrete components. This accuracy corresponds to a timing jitter of 15 fs - a 4-5 times improvement over the performance of the best electronic ADCs which exist today. On the way towards an integrated photonic ADC, a silicon photonic chip with core photonic components was fabricated and used to digitize a 10 GHz signal with 3.5 effective bits. In these experiments, two wavelength channels were implemented, providing the overall sampling rate of 2.1 GSa/s. To show that photonic ADCs with larger channel counts are possible, a dual 20-channel silicon filter bank has been demonstrated.

CMOS-Compatible All-Si High-Speed Waveguide Photodiodes With High Responsivity in Near-Infrared Communication Band

Submicrometer silicon photodiode waveguides, fabricated on silicon-on-insulator substrates, have photoresponse from <1270 to 1740 nm (0.8 AW-1 at 1550 nm) and a 3-dB bandwidth of 10 to 20 GHz. The p-i-n photodiode waveguide consists of an intrinsic waveguide 500times250 nm where the optical mode is confined and two thin, 50-nm-thick, doped Si wings that extend 5 mum out from either side of the waveguide. The Si wings, which are doped one p-type and the other n-type, make electric contact to the waveguide with minimal effect on the optical mode. The edges of the wings are metalized to increase electrical conductivity. Ion implantation of Si+ 1times10 13 cm-2 at 190 keV into the waveguide increases the optical absorption from 2-3 dBmiddotcm-1 to 200-100 dBmiddotcm-1 and causes the generation of a photocurrent when the waveguide is illuminated with subbandgap radiation. The diodes are not damaged by annealing to 450 degC for 15 s or 300 degC for 15 min. The photoresponse and thermal stability is believed due to an oxygen stabilized divacancy complex formed during ion implantation

Silicon waveguide infrared photodiodes with >35 GHz bandwidth and phototransistors with 50 AW -1 response

SOI CMOS compatible Si waveguide photodetectors are made responsive from 1100 to 1750 nm by Si+ implantation and annealing. Photodiodes have a bandwidth of >35 GHz, an internal quantum efficiency of 0.5 to 10 AW-1, and leakage currents of 0.5 nA to 0.5 microA. Phototransistors have an optical response of 50 AW-1 with a bandwidth of 0.2 GHz. These properties are related to carrier mobilities in the implanted Si waveguide. These detectors exhibit low optical absorption requiring lengths from <0.3 mm to 3 mm to absorb 50% of the incoming light. However, the high bandwidth, high quantum efficiency, low leakage current, and potentially high fabrication yields, make these devices very competitive when compared to other detector technologies.

CMOS-compatible dual-output silicon modulator for analog signal processing.

A broadband, Mach-Zehnder-interferometer based silicon optical modulator is demonstrated, with an electrical bandwidth of 26 GHz and V(pi)L of 4 V.cm. The design of this modulator does not require epitaxial overgrowth and is therefore simpler to fabricate than previous devices with similar performance.

All silicon infrared photodiodes: photo response and effects of processing temperature

CMOS compatible infrared waveguide Si photodiodes are made responsive from 1100 to 1750 nm by Si(+) implantation and annealing. This article compares diodes fabricated using two annealing temperatures, 300 and 475 degrees C. 0.25-mm-long diodes annealed to 300 degrees C have a response to 1539 nm radiation of 0.1 A W-(-1) at a reverse bias of 5 V and 1.2 A W(-1) at 20 V. 3-mm-long diodes processed to 475 degrees C exhibited two states, L1 and L2, with photo responses of 0.3 +/-0.1 A W(-1) at 5 V and 0.7 +/-0.2 A W(-1) at 20 V for the L1 state and 0.5 +/-0.2 A W(-1) at 5 V and 4 to 20 A W(-1)-1 at 20 V for the L2 state. The diodes can be switched between L1 and L2. The bandwidths vary from 10 to 20 GHz. These diodes will generate electrical power from the incident radiation with efficiencies from 4 to 10 %.

Effect of carrier lifetime on forward-biased silicon Mach-Zehnder modulators

We present a systematic study of Mach-Zehnder silicon optical modulators based on carrier-injection. Detailed comparisons between modeling and measurement results are made with good agreement obtained for both DC and AC characteristics. A figure of merit, static VpiL, as low as 0.24Vmm is achieved. The effect of carrier lifetime variation with doping concentration is explored and found to be important for the modulator characteristics.

Photonic Analog-to-Digital Conversion with Electronic-Photonic Integrated Circuits

Photonic Analog-to-Digital Conversion (ADC) has a long history. The premise is that the superior noise performance of femtosecond lasers working at optical frequencies enables us to overcome the bottleneck set by jitter and bandwidth of electronic systems and components. We discuss and demonstrate strategies and devices that enable the implementation of photonic ADC systems with emerging electronic-photonic integrated circuits based on silicon photonics. Devices include 2-GHz repetition rate low noise femtosecond fiber lasers, Si-Modulators with up to 20 GHz modulation speed, 20 channel SiN-filter banks, and Ge-photodetectors. Results towards a 40GSa/sec sampling system with 8bits resolution are presented.

High Speed Analog-to-Digital Conversion with Silicon Photonics

Sampling rates of high-performance electronic analog-to-digital converters (ADC) are fundamentally limited by the timing jitter of the electronic clock. This limit is overcome in photonic ADCs by taking advantage of the ultra-low timing jitter of femtosecond lasers. We have developed designs and strategies for a photonic ADC that is capable of 40 GSa/s at a resolution of 8 bits. This system requires a femtosecond laser with a repetition rate of 2 GHz and timing jitter less than 20 fs. In addition to a femtosecond laser this system calls for the integration of a number of photonic components including: a broadband modulator, optical filter banks, and photodetectors. Using silicon-on-insulator (SOI) as the platform we have fabricated these individual components. The silicon optical modulator is based on a Mach-Zehnder interferometer architecture and achieves a VπL of 2 Vcm. The filter banks comprise 40 second-order microring-resonator filters with a channel spacing of 80 GHz. For the photodetectors we are exploring ion-bombarded silicon waveguide detectors and germanium films epitaxially grown on silicon utilizing a process that minimizes the defect density.

High-speed silicon electro-optical modulator that can be operated in carrier depletion or carrier injection mode

A high-speed silicon optical modulator has been demonstrated which can operate either in forward bias using carrier injection, or in reverse bias using carrier depletion. In forward bias, the device requires less than 10 mW of drive power, but has a low bandwidth of 100 MHz. In reverse bias, the device has a nearly flat response to 18 GHz, but requires 700 mW for large modulation depths.

Polycrystalline anatase titanium dioxide microring resonators with negative thermo-optic coefficient

We fabricate polycrystalline anatase TiO2 microring resonators with loaded quality factors as high as 25,000 and average losses of 0.58  dB/mm in the telecommunications band. Additionally, we measure a negative thermo-optic coefficient dn/dT of −4.9±0.5×10−5  K−1. The presented fabrication uses CMOS-compatible lithographic techniques that take advantage of substrate-independent, non-epitaxial growth. These properties make polycrystalline anatase a promising candidate for the implementation of athermal, vertically integrated, CMOS-compatible nanophotonic devices for nonlinear applications.