Josh A. Conway

Plasmonic interconnects versus conventional interconnects: a comparison of latency, crosstalk and energy costs.

The continued scaling of integrated circuits will require advances in intra-chip interconnect technology to minimize delay, density of energy dissipation and cross-talk. We present the first quantitative comparison between the performance of metal wire interconnects, operated in the traditional manner by electric charge and discharge, versus the performance of metal wires operated as surface plasmon waveguides. Surface plasmon wire waveguides have the potential to reduce signal delay, but the high confinement required for low cross-talk amongst high density plasmon wire interconnects significantly increases energy dissipation per transmitted bit, above and beyond that required for electric charge/discharge interconnects at the same density.

Numerical optimization of a grating coupler for the efficient excitation of surface plasmons at an Ag-SiO 2 interface

The efficient generation of surface plasmons from free-space optical waves is still an open problem in the field. Here we present a methodology and optimized design for a grating coupler. The photoexcitation of surface plasmons at an Ag-SiO2 interface is numerically demonstrated to yield a 50% coupling efficiency from a Gaussian beam into surface plasmon voltages and currents.

Photonic Bandwidth Compression Front End for Digital Oscilloscopes

Time-stretch photonic analog-to-digital converter (ADC) technology is used to make an optical front end that compresses radio-frequency (RF) bandwidth before input to a digital oscilloscope. To operate a time-stretch ADC in a continuous-time mode for bandwidth compression, the optical signal on which the RF is modulated must be segmented and demultiplexed. We demonstrate both spectral and temporal methods for overlapping the channels. Using the temporal method, we obtain a compression ratio of 3 with four channels. Mating this optical front end with a state-of-the-art four-channel digital oscilloscope with an input bandwidth of 16 GHz and a sampling rate of 50 GS/s gives a digitizer with 150 GS/s and an input bandwidth of 48 GHz. We digitize RF signals up to 45 GHz and obtain effective number of bits (ENOB) ~ 2.8 with single channels and ~ 2.5 with multiple channels, both measured over the 48-GHz instantaneous bandwidth of our system.

150 GS/s real-time oscilloscope using a photonic front end

We demonstrate an optical front end technology that multiplies the sampling rate of a real-time oscilloscope by a factor of three. Our approach uses an optical pre-processor to compress the signal bandwidth of continuous-time high speed RF waveforms. To operate in continuous-time mode, the optical signal, which carries the RF, must be segmented and demultiplexed into an array of N parallel channels. In prior work, large spectral overlap between channels was needed for calibration and this limited the multiplication factor, M, to values far below the maximum value of N, which is limited by the number of back-end digitizers. In this paper, we demonstrate a novel technique using temporal overlap between channels and achieve higher multiplication. The sampling rate of a four-channel 50 GS/s real-time oscilloscope is increased by a factor of 3, enabling us to digitize a 47 GHz tone at 150 GS/s. To our knowledge, this is a record in continuous time RF digitization.

Distortion Correction in a High-Resolution Time-Stretch ADC Scalable to Continuous Time

Distortions caused by system components and by fundamental physical phenomena can limit the performance of photonic time-stretch ADCs. Here we use a combination of time-stretch linearization & equalization, DC-offset subtraction, and operation in a linear propagation regime to improve the signal-to-noise-and-distortion ratio by 17 dB for a 2-channel time-stretch ADC testbed and therein obtain noise-limited performance of 6-7 ENOB over a 10-GHz RF input bandwidth. Time-stretch linearization & equalization corrects for dispersion mismatches among testbed components by applying time-shifts calculated from component group delays to output ADC samples. DC-offset subtraction removes static errors due to insertion loss imbalances and Mach-Zehnder modulator bias offsets. If optical power levels are too high, nonlinear fiber propagation lowers the frequencies of dispersion-induced nulls in the RF transfer function and causes higher-order signal distortions. The 2-channel testbed can be directly scaled to a practical continuous-time system with the addition of more sub-aperture wavelength channels (total of 13 channels and 42 nm of optical bandwidth for a 90 MHz laser repetition rate). Adaptive online and fixed pre-calibrated stitching methods are demonstrated for joining data from one wavelength channel to the next.

Phase ripple correction: theory and application.

Spectral phase ripple associated with novel dispersive devices can distort broadband optical signals. We present a digital postprocessing algorithm to correct for this distortion by exploiting the static deterministic nature of the ripple. This algorithm is demonstrated with empirical data for several systems employing chirped fiber Bragg gratings (CFBGs). We employ this technique in a photonic time-stretch system incorporating CFBGs, improving the signal fidelity by 9 dB. Simulations and experiments show that this algorithm, which can be reduced to a simple interpolation and matrix multiplication, also mitigates additive noise. We see that the act of distortion correction yields signal fidelity superior to that of an ideal dispersive element.

Stroboscopic Imaging Interferometer for MEMS Performance Measurement

The insertion of microelectromechanical systems (MEMS) components into aerospace systems requires advanced testing to characterize performance in a space environment. Here, we report a novel stroboscopic interferometer test system that measures nanometer-scale displacements of moving MEMS devices. By combining video imagery and phase-shift interferometry with an environmental chamber, rapid visualization of the dynamic device motion under the actual operational conditions can be achieved. The utility of this system is further enhanced by integrating the interferometer onto the chamber window, allowing for robust interferometric testing in a noisy environment without requiring a floating optical table. To demonstrate these unique capabilities, we present the time-resolved images of an electrostatically actuated MEMS cantilevered beam showing the first-order to sixth-order plate modes under vacuum.

4-Channel Continuous-Time 77 GSa/s ADC using Photonic Bandwidth Compression

A four-channel continuous-time implementation of photonic bandwidth compression technique is reported. In our approach, which is based on the transient time-stretched analog-to-digital converter (ADC) technology, continuous high speed RF signals arc segmented in the time domain and multiplexed into an array of parallel channels. The segments in each channel are temporally stretched, or equivalently compressed in bandwidth, before digitization. Benefits of our technique include multiplying the effective sample rate and input bandwidth of an ADC. Segments from all 4-channels arc concatenated to form the continuous signal using an out-of-band real-time calibration tone to independently correct for gain and timing errors. In this paper, a 4-channel continuous-time architecture is demonstrated that increases the bandwidth and sampling rate of a state-of-the-art real-time 50 GSa/s digitizer by 55%. A 4-channel system is of particular interest because it matches the number of input channels available on commercial oscilloscopes. The results indicate a viable path to a 4-channel continuous-time system that would be capable of enhancing existing digitizers by more than 300% and achieving 150 GSa/s over 50 GHz in real-time.

Compensation algorithm for deterministic phase ripple

Phase ripple arising from imperfections in novel dispersive devices can severely distort broadband optical signals. We experimentally and theoretically demonstrate an algorithm that corrects for these distortions while simultaneously reducing the effects of additive noise.