Jeremy D. Schaub

Integrated transversal equalizers in high-speed fiber-optic systems

Intersymbol interference (ISI) caused by intermodal dispersion in multimode fibers is the major limiting factor in the achievable data rate or transmission distance in high-speed multimode fiber-optic links for local area networks applications. Compared with optical-domain and other electrical-domain dispersion compensation methods, equalization with transversal filters based on distributed circuit techniques presents a cost-effective and low-power solution. The design of integrated distributed transversal equalizers is described in detail with focus on delay lines and gain stages. This seven-tap distributed transversal equalizer prototype has been implemented in a commercial 0.18-/spl mu/m SiGe BiCMOS process for 10-Gb/s multimode fiber-optic links. A seven-tap distributed transversal equalizer reduces the ISI of a 10-Gb/s signal after 800 m of 50-/spl mu/m multimode fiber from 5 to 1.38 dB, and improves the bit-error rate from about 10/sup -5/ to less than 10/sup -12/.

High-speed Germanium-on-SOI lateral PIN photodiodes

We report the fabrication and characterization of high-speed germanium on silicon-on-insulator lateral PIN photodetectors. At an incident wavelength of 850 nm, 10 /spl times/10-/spl mu/m detectors with finger spacing S of 0.4 /spl mu/m (0.6 /spl mu/m) produced a -3-dB bandwidth of 29 GHz (27 GHz) at a bias voltage of -1 V. The detectors with S=0.6 /spl mu/m had external quantum efficiency of 34% at 850 nm and 46% at 900 nm and dark current of 0.02 /spl mu/A at -1-V bias.

Germanium-on-SOI Infrared Detectors for Integrated Photonic Applications

An overview of recent results on high-speed germanium-on-silicon-on-insulator (Ge-on-SOI) photodetectors and their prospects for integrated optical interconnect applications are presented. The optical properties of Ge and SiGe alloys are described and a review of previous research on SOI and SiGe detectors is provided as a motivation for the Ge-on-SOI detector approach. The photodetector design is described, which consists of lateral alternating p- and n-type surface contacts on an epitaxial Ge absorbing layer grown on an ultrathin-SOI substrate. When operated at a bias voltage of -0.5 V, 10mumtimes10 mum devices have dark current Idark, of only ~10 nA, a value that is nearly independent of finger spacing S, between S=0.3mum and 1.3mum. Detectors with S=1.3mum have external quantum efficiencies eta, of 52% (38%) at lambda=895 nm (850 nm) with corresponding responsivities of 0.38 A/W (0.26 A/W). The wavelength-dependence of eta agrees fairly well with expectations, except at longer wavelengths, where Si up-diffusion into the Ge absorbing layer reduces the efficiency. Detectors with 10 mumtimes10 mum area and S=0.6mum have -3-dB bandwidths as high as 29 GHz, and can simultaneously achieve a bandwidth of 27 GHz with Idark=24 nA, at a bias of only -1 V, while maintaining high efficiency of eta=46%(33%), at lambda=895 nm (850 nm). Analysis of the finger spacing and area-dependence of the device speed indicates that the performance at large finger spacing is transit-time-limited, while at small finger spacing, RC delays limit the bandwidth. Methods to improve the device performance are presented, and it is shown that significant improvement in the speed and efficiency both at lambda=850 and 1300 nm can be expected by optimizing the layer structure design

Chip-to-chip optical interconnects

Terabus is based on a silicon-carrier interposer on an organic card containing 48 polymer waveguides. We have demonstrated 4times12 arrays of low power optical transmitters and receivers, operating up to 20 Gb/s and 14 Gb/s per channel respectively

120-Gb/s VCSEL-based parallel-optical interconnect and custom 120-Gb/s testing station

A 120-Gb/s optical link (12 channels at 10 Gb/s/ch for both a transmitter and a receiver) has been demonstrated. The link operated at a bit-error rate of less than 10/sup -12/ with all channels operating and with a total fiber length of 316 m, which comprises 300 m of next-generation (OM-3) multimode fiber (MMF) plus 16 m of standard-grade MMF. This is the first time that a parallel link with this bandwidth at this per-channel rate has ever been demonstrated. For the transmitter, an SiGe laser driver was combined with a GaAs vertical-cavity surface-emitting laser (VCSEL) array. For the receiver, the signal from a GaAs photodiode array was amplified by a 12-channel SiGe receiver integrated circuit. Key to the demonstration were several custom testing tools, most notably a 12-channel pattern generator. The package is very similar to the commercial parallel modules that are available today, but the per-channel bit rate is three times higher than that for the commercial modules. The new modules demonstrate the possibility of extending the parallel-optical module technology that is available today into a distance-bandwidth product regime that is unattainable for copper cables.

Resonant-cavity-enhanced high-speed Si photodiode grown by epitaxial lateral overgrowth

We report a resonant cavity Si photodiode grown by merged epitaxial lateral overgrowth. At a reverse bias of 5 V, the dark current was 2.7 pA and the bandwidth exceeded 34 GHz. The peak quantum efficiencies ranged from 42% at 704 nm to 31% at 836 nm. This is the highest speed reported for a Si p-i-n photodiode and the highest bandwidth-efficiency product for any Si-based photodetector.

10-Gb/s all-silicon optical receiver

We report an all-silicon optical receiver operating at 10 Gb/s. The lateral p-i-n photodiodes were fabricated using standard 130-nm complementary metal-oxide-semiconductor technology on silicon-on-insulator substrate. A sensitivity of -6.9dBm (bit error ratio <10/sup -9/) at 10 Gb/s was achieved.

Ge-on-SOI-Detector/Si-CMOS-Amplifier Receivers for High-Performance Optical-Communication Applications

In this paper, an overview and assessment of high-performance receivers based upon Ge-on-silicon-on-insulator (Ge-on-SOI) photodiodes and Si CMOS amplifier ICs is provided. Receivers utilizing Ge-on-SOI lateral p-i-n photodiodes paired with high-gain CMOS amplifiers are shown to operate at 15 Gb/s with a sensitivity of -7.4 dBm (BER=10-12) while utilizing a single supply voltage of only 2.4 V. The 5-Gb/s sensitivity of similar receivers is constant up to 93 degC, and 10-Gb/s operation is demonstrated at 85 degC. Error-free (BER<10-12) operation of receivers combining a Ge-on-SOI photodiode with a single-ended high-speed receiver front end is demonstrated at 19 Gb/s, using a supply voltage of 1.8 V. In addition, receivers utilizing Ge-on-SOI photodiodes integrated with a low-power CMOS IC are shown to operate at 10 Gb/s using a single 1.1-V supply while consuming only 11 mW of power. A perspective on the future technological capabilities and applications of Ge-detector/Si-CMOS receivers is also provided

A high-speed, high-sensitivity silicon lateral trench photodetector

We report a novel silicon lateral trench photodetector that decouples the carrier transit distance from the light absorption depth, enabling both high speed and high responsivity. The photodetector, fabricated with fully VLSI compatible processes, exhibits a 6-dB bandwidth of 1.5 GHz at 3.0 V and an external quantum efficiency of 68% at 845 nm wavelength. A photoreceiver with a wire-bonded lateral trench detector and a BiCMOS transimpedance amplifier demonstrates excellent operation at 2.5 Gb/s data rate and 845 nm wavelength with only a 3.3 V bias.

High-speed monolithic silicon photoreceivers on high resistivity and SOI substrates

We compare monolithic silicon optical receivers fabricated on high resistivity and silicon-on-insulator (SOI) substrates. Each receiver consisted of a lateral p-i-n photodiode and an NMOS transimpedance preamplifier. At a drain voltage (V/sub DD/) of 3.5 V, a photodiode voltage (V/sub PD/) of 30 V, and a wavelength of 850 nm, the high resistivity receiver exhibited sensitivities of -31.9 dBm at 622 Mb/s and -23.2 dBm at the maximum operating speed of 1.0 Gb/s. At V/sub DD/=5 V and V/sub PD/=20 V, the sensitivity of the SOI receiver was -26.1 dBm at 622 Mb/s, -20.2 dBm at 1.0 Gb/s and -12.2 dBm at the maximum speed of 2.0 Gb/s. Single supply operation at 5 V and 3 V was also demonstrated for the SOI receiver. Methods for extending the speed and improving the sensitivity characteristics in more advanced technologies with lower supply voltages are discussed.