Aparna Bhatnagar

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.

Transform spectrometer based on measuring the periodicity of Talbot self-images

We demonstrate a compact transform spectrometer based on measuring the periodicity of Talbot self-images. The system has no moving parts; it contains only a tilted absorption grating that is imaged onto a CCD camera. The linear architecture of the system makes it possible to use this design in imaging arrays of spectrometers. Unlike other transform spectrometers, its resolution is independent of wavelength.

Opportunities for optics in integrated circuits applications

Optics potentially addresses two key problems in electronic chips and systems: interconnects and timing. Short optical pulses (e.g., picoseconds or shorter) offer particularly precise timing. Results are shown for optical and electrical four-phase clocking, with <1 ps rms jitter for the optical case.

Pump-probe measurements of CMOS detector rise time in the blue

This paper proposes the use of shorter wavelengths and monolithic integration for chip-to-chip and on-chip optical communication. The promise of monolithic detectors for high-speed interconnection is demonstrated through experimental measurements and matching simulations. Responsivities >0.06 A/W and transit-time-limited response can be expected in the blue from planar p-i-n silicon-on-insulator (SOI) detectors.

Receiverless clocking of a CMOS digital circuit using short optical pulses

We use short optical pulses to clock a digital logic block without using clock receivers. We measure 12 ps rms jitter at the output of a digital PRBS with optical clocking and 30 ps with electrical clocking.

High-impedance high-frequency silicon detector response for precise receiverless optical clock injection

We report the direct injection of precise clock signals into standard CMOS circuits using short optical pulses by a novel receiverless scheme that eliminates the delay, skew and jitter of a typical receiver. To accomplish the optical injection we designed small silicon detectors along-side standard 0.25micrometers CMOS-circuits. Due to the low intrinsic capacitance of the detectors, the photogenerated carriers can directly generate voltage swings that are comparable with CMOS voltage levels if the detectors are loaded with high-impedance circuits. As a first step to implement this scheme we characterized various detectors built in the CMOS process for their high-frequency response. In a test set-up the silicon detectors are sampled with on-chip samplers that only present a small capacitive loading to the detector node. We present the high-frequency high-impedance response measured with this scheme together with capacitance measurements and DC responsivities of various types and sizes of detectors. The characterized long tails typically observed with silicon detectors allowed us to set up a model for the power penalty we have to take into account for precise clock detection. Finally, as a proof-of-principle demonstration we present the first results of this receiverless scheme in which a totem-pole of silicon detectors directly drives an on-chip CMOS inverter.

Receiver-less optical clock distribution using short pulses

We have used short optical pulses to clock digital logic on CMOS, without using clock receivers. We present the benefits of this approach compared to electrical clock distribution by evaluating the resulting timing and power savings in a fan-out-of-four clock tree modeled after modern microprocessors.

Transform spectrometer based on measuring periodicity of Talbot self-images

We demonstrate a spectrometer based on measuring the periodicity of Talbot self-images. The system contains a tilted absorption grating imaged onto a CCD camera and has no moving parts.