Richard D. Younger

Optical down-sampling of wide-band microwave signals

Phase-encoded optical sampling allows radio-frequency and microwave signals to be directly down-converted and digitized with high linearity and greater than 60-dB (10-effective-bit) signal-to-noise ratio. Wide-band electrical signals can be processed using relatively low optical sampling rates provided that the instantaneous signal bandwidth is less than the Nyquist sampling bandwidth. We demonstrate the capabilities of this technique by using a 60-MS/s system to down-sample two different FM chirp signals: 1) a baseband (0-250 MHz) linear-chirp waveform and 2) a nonlinear-chirp waveform having a 10-GHz center frequency and a frequency excursion of 1 GHz. We characterize the frequency response of the technique and quantify the analog bandwidth limitation due to the optical pulse width. The 3-dB bandwidth imposed by a 30-ps sampling pulse is shown to be 10.4 GHz. We also investigate the impact of the pulse width on the linearity of the phase-encoded optical sampling technique when it is used to sample high-frequency signals.

Crosstalk Analysis of Integrated Geiger-mode Avalanche Photodiode Focal Plane Arrays

Arrays of photon-counting Geiger-mode avalanche photodiodes (APDs) sensitive to 1.06 and 1.55 μm wavelengths and as large as 256 x 64 elements on 50 μm pitch have been fabricated for defense applications. As array size, and element density increase, optical crosstalk becomes an increasingly limiting source of spurious counts. We characterize the crosstalk by measurement of emitted light, and by extracting the spatial and temporal focal plane array (FPA) response to the light from FPA dark count statistics. We discuss the physical and geometrical causes of FPA crosstalk, suggest metrics useful to system designers, then present measured crosstalk metrics for large FPAs as a function of their operating parameters. We then present FPA designs that suppress crosstalk effects and show more than 40 times reduction in crosstalk.

Precision calibration of an optically sampled analog-to-digital converter

The general configuration of the proposed high-speed ADC was described. A train of optical pulses samples the analog signal applied to the optical modulator. A dual-output Mach-Zehnder electro-optic modulator is used to implement phase-encoded optical sampling. Output circuits detect and integrate the modulated optical pulses. The integrated level is held until electronic digitizer converts the signal into digital form. The circuit then resets and waits for the next input pulse. The output circuits include amplifiers before the digitizers.

Optical sampling for high-speed, high-resolution analog-to-digital converters

The combination of phase-encoded optical sampling and optical time-division demultiplexing has been demonstrated to extend the performance of electronic analog-to-digital converters (ADCs). We report a 10-effective-bit, 505-MS/s ADC with a spur-free dynamic range in excess of 70 dB.

Large-Format Geiger-Mode Avalanche Photodiode Arrays and Readout Circuits

Over the past 20 years, we have developed arrays of custom-fabricated silicon and InP Geiger-mode avalanche photodiode arrays, CMOS readout circuits to digitally count or time stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image sensors for lidar, optical communications, and passive imaging. Starting with 4 × 4 arrays, we have recently demonstrated 256 × 256 arrays, and are working to scale to megapixel-class imagers. In this paper, we review this progress and discuss key technical challenges to scaling to large format.

Arrays of 128x32 InP-Based Geiger-Mode Avalanche Photodiodes

Arrays of InP-based avalanche photodiodes operating at 1.06-μm wavelength in the Geiger mode have been fabricated in the 128x32 format. The arrays have been hermetically packaged with precision-aligned lenslet arrays, bump-bonded read-out integrated circuits, and thermoelectric coolers. With the array cooled to -20C and voltage biased so that optical cross-talk is small, the median photon detection efficiency is 23-25% and the median dark count rate is 2 kHz. With slightly higher voltage overbias, optical cross-talk increases but the photon detection efficiency increases to almost 30%. These values of photon detection efficiency include the optical coupling losses of the microlens array and package window.

Timing jitter in short-distance pulsed-analog optical fiber links

Investigation of the timing jitter generated by propagating high-power optical pulses through short lengths of optical fiber is presented. It is measured using the phase-encoded optical sampling technique. Comparisons are made between measured and simulated results.

Optical down-sampling of an X-band 1-GHz-bandwidth nonlinear chirp signal

We have demonstrated a new technique to characterize wideband nonlinear waveforms at microwave carrier frequencies. The analog sampling bandwidth can be increased with wider bandwidth sampling modulators and shorter optical pulses. The instantaneous Nyquist bandwidth can be extended using time-interleaving techniques.

An optically sampled wideband digital receiver

Phase-encoded optical sampling allows the processing of RF and microwave signals with high linearity and >60-dB signal-to-noise ratio in an optically sampled digital receiver. The wide analog-input bandwidth (>1 GHz) enables novel processing capabilities as demonstrated for a 250-MHz-bandwidth chirped signal undersampled at 60 MS/s.

Large-format image sensors based on custom Geiger-mode avalanche photodiode arrays

Over the past 20 years, we have developed arrays of custom-fabricated silicon and InP Geiger-mode avalanche photodiode arrays, CMOS readout circuits to digitally count or time stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image sensors for lidar, optical communications, and passive imaging. Starting with 4 × 4 arrays, we have recently demonstrated 256 × 256 arrays, and are working to scale to megapixel-class imagers. In this paper, we review this progress and discuss key technical challenges to scaling to large format.