Noah C. Helman

The benefits of ultrashort optical pulses in optically interconnected systems

Many properties of an optically interconnected system can be improved through the use of a modelocked laser. The short pulse duration, high peak power, wide spectral bandwidth, and low timing jitter of such a laser lead to these benefits. Timing advantages include simplified synchronization across large chip areas, receiver latency reduction, and data resynchronization. Lower power dissipation may be achieved through improved receiver sensitivity. Additional applications of short optical pulses include time-division multiplexing, single-source wavelength-division multiplexing, and precise time-domain testing of circuits. Several of these concepts were investigated using a high-speed chip-to-chip optical interconnect demonstration link. The link employs a modelocked laser and surface-normal optoelectronic modulators that were flip-chip bonded to silicon CMOS circuits. This paper outlines experiments that were performed on or simulated for the link, and discusses the important benefits of ultrashort optical pulses for optical interconnection.

Receiver-less optical clock injection for clock distribution networks

We present a new technique of injecting clocks optically onto CMOS chips without the use of a receiver amplifier. We discuss the benefits of such a direct approach and present proof-of-principle experiments of the technique. We analytically compare a receiver-less optical clock distribution and an electrical clock distribution in a fan-out-of-four clock tree to evaluate the timing and power benefits of the optical approach for present microprocessors. We also compare receiver-less direct injection of optical clocks to trans-impedance receiver based injection within the same distribution framework.

Misalignment-tolerant surface-normal low-voltage modulator for optical interconnects

We present a surface-normal modulator architecture for optical interconnects that offers misalignment tolerance as well as high contrast ratio over a wide wavelength range for a small drive voltage. A contrast ratio greater than 3 dB was achieved for only 0.8-V drive across a 16-nm wavelength range from 1498 to 1514 nm. The misalignment tolerance between this device, and the input optical beam was measured to be 30 /spl mu/m.

Wavelength division multiplexed optical interconnect using short pulses

We demonstrate operation of a wavelength division multiplexed chip-to-chip optical interconnect using surface-normal electroabsorption modulators, and a modelocked laser as a single broadband source. The link was successfully operated at 80 Mb/s. While this rate was limited by the repetition rate of the modelocked source, individual CMOS circuits and optoelectronic devices have been shown to work at data rates approaching 1 Gb/s.

An Optical Interconnect Transceiver at 1550 nm Using Low-Voltage Electroabsorption Modulators Directly Integrated to CMOS

A low-voltage, 90-nm CMOS optical interconnect transceiver operating at 1550-nm optical wavelength is presented. This is the first demonstration of a novel optoelectronic modulator architecture (the quasi-waveguide angled-facet electroabsorption modulator) in a system. It features a simple electronic packaging via flip-chip bonding to silicon. Devices have a broad optical bandwidth, are arrayed two dimensionally, and feature surface normal, spatially separated, and misalignment-tolerant optical ports. The modulators are driven with a novel pulsed-cascode driver capable of supplying an output-voltage swing of 2 V (twice the nominal 1-V CMOS supply) without overstressing thin-oxide core CMOS devices. At the receiver side, a sensitivity of -15.2 dBm is obtained with an integrating/double-sampling front end. The transceiver includes clock generation and recovery circuitry that enables a data serialization factor of five. At a maximum data rate of 1.8 Gb/s, the optical transmitter, receiver, and clocking circuitry consume 12.6, 4.5, and 6.5 mW, respectively, for a total link electrical power dissipation of 23.6 mW. To the best of our knowledge, this is the first demonstration of an interconnect transceiver operating at 1550 nm with a III-V output device directly integrated to the CMOS.

Latency reduction in optical interconnects using short optical pulses

We present a new method of latency reduction in optical interconnects: using very low duty cycle return-to-zero encoding (i.e., subpicosecond pulses). An analytical comparison of three different receiver architectures, including transimpedance, integrating, and totem-pole diode pair, is presented. For all three receivers, we demonstrate that using short pulses instead of nonreturn-to-zero (NRZ) shortens the circuit delay. We also experimentally demonstrate a /spl sim/65% reduction in latency of a transimpedance receiver by using short optical pulses. Finally, we show that the latency of optical interconnects can be comparable to or even less than electrical interconnects for global on-chip communication.

Optical pump-probe measurements of the latency of silicon CMOS optical interconnects

We present the first measurements of optical-electrical-optical conversion latency in a hybridly-integrated optoelectronic/silicon complementary metal-oxide-semiconductor (CMOS) chip designed for optical interconnection. Using an optical pump-probe technique, we perform precise measurements with picosecond resolution that closely match our simulations. Our findings suggest that optical interconnects have the potential to provide equal or lower latency than on-chip global wires in future CMOS microelectronics.

Differential optical remoting of ultrafast charge packets using self-linearized modulation

We demonstrate conversion of ultrafast input analog charge packets into a differential optical signal, using quantum-well self-electrooptic effect devices in a novel self-linearized mode of operation.

Performance enhancement of an optical interconnect using short pulses from a modelocked diode laser

Summary form only given. We have demonstrated a short-pulse optical interconnect that uses a practical, high-repetition-rate modelocked source. BER measurements show that operation with short pulses improves system performance by providing a receiver sensitivity enhancement of 3.3 dB. The link has additional benefits related to timing issues and latency reduction.

1550nm Optical Interconnect Transceiver with Low Voltage Electroabsorption Modulators Flip-Chip Bonded to 90nm CMOS

A low-voltage 90 nm CMOS optical interconnect transceiver operating at 1550 nm is presented. This is the first system demonstrated using the recent quasi-waveguide angled facet electroabsorption modulator (QWAFEM), featuring simple electronic and optical packaging.