George A. Sefler

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.

Frequency comb generation by four-wave mixing and the role of fiber dispersion

A fiber-optic frequency comb generator utilizes four-wave mixing (FWM) to expand a modulated continuous wave (CW) lightwave into a comb of discrete, equispaced frequencies. The sign and magnitude of the fiber dispersion are shown to control comb expansion by affecting the interference among different four-wave mixing interactions at particular comb frequencies. Positive dispersion initiates comb expansion while zero and negative dispersions suppress expansion. Dispersion mapping engineers fiber links for maximum comb bandwidths. By managing dispersion in a 35 km link of dispersion-shifted fiber, a frequency comb spanning 1.1 THz is experimentally demonstrated. Promising applications include pulse train synthesis affording low-chirp, arbitrarily profiled, short pulses.

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.

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.

Mitigation of Group-Delay-Ripple Distortions for Use of Chirped Fiber-Bragg Gratings in Photonic Time-Stretch ADCs

Low-loss dispersive components are critical to photonic time-stretch analog-to-digital converters (TS-ADCs) for attaining high resolutions. Chirped fiber-Bragg gratings (CFBGs) provide low losses plus high nonlinear thresholds but introduce time-warp and intensity-modulation distortions from group-delay ripple (GDR). Practical signal processing routines to mitigate GDR distortions are assessed for the two fundamental TS-ADC differential architectures. For differential TS-ADCs that use a common time-stretching element, differential RF extraction together with time-stretch linearization provides effective GDR mitigation. For differential TS-ADCs that use separate time-stretching elements, intensity scale factors proportional to the reciprocal of the GDR-induced intensity modulation are first applied, or alternatively DC-offset subtraction can provide effective mitigation when the separate path losses are sufficiently balanced. Using pulse propagation simulations with actual CFBG GDR data, these routines are projected to mitigate CFBG GDR distortions to levels commensurate with >;11 effective resolution bits for input RF bandwidths of at least 0-to-10 GHz and time-stretch factors >; 10.

Holographic multichannel radio-frequency correlator

We introduce a multichannel radio-frequency (RF) correlator capable of correlating a received RF signal simultaneously against a library of known reference waveforms stored as angularly multiplexed holograms in a photorefractive crystal. The systems operation is inher- ently two-dimensional and may yield a compact, robust, and efficient device through integrated optical techniques, in contrast to previous op- tical multichannel schemes. Operation of a 16-channel system is experi- mentally demonstrated.

Photonic rf processing systems

We introduce a multichannel RF correlator that is capable of simultaneously computing the correlations between a given input signal and a set of stored reference waveforms. The system relies on angularly multiplexed volume holograms to store the reference waveforms and is a new example of an emerging class of RF systems that use photonics to solve complex problems. Preliminary experimental data derived from a demonstration system are presented and discussed.

Application of laser speckle to randomized numerical linear algebra

We propose and simulate integrated optical devices for accelerating numerical linear algebra (NLA) calculations. Data is modulated on chirped optical pulses and these propagate through a multimode waveguide where speckle provides the random projections needed for NLA dimensionality reduction.

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.