Jason Chou

Adaptive RF-photonic arbitrary waveform generator

Optical and radio-frequency waveforms with wide-band arbitrary modulation are generated using spectral shaping of a supercontinuum source followed by wavelength-to-time mapping. Adaptive computer control is used to mitigate the nonideal features inherent in the optical source and in the spectrum modulation process.

Femtosecond real-time single-shot digitizer

We demonstrate a single-shot digitizer with a 10 Tsample/s sampling rate. This feat is accomplished with a photonic time stretch preprocessor that slows down the electrical waveform by an unprecedented factor of 250 before it is captured by a commercial electronic digitizer. To achieve such a large stretch factor, distributed Raman optical amplification is realized inside the dispersive element that performs the time stretch.

Time-wavelength spectroscopy for chemical sensing

Time-wavelength spectroscopy, using a linearly chirped broad-band pulse, eliminates the need for an optical spectrometer and enables real-time analysis. Utilizing digital filtering and matched detection, we demonstrate identification of gas signatures that are entirely masked by highly nonuniform source spectra.

Real-time spectroscopy with subgigahertz resolution using amplified dispersive Fourier transformation

Dispersive Fourier transformation is a powerful technique in which spectral information is mapped into the time domain using chromatic dispersion. It replaces a spectrometer with an electronic digitizer and enables real-time spectroscopy. The fundamental problem in this technique is the trade-off between the detection sensitivity and spectral resolution, a limitation set by the digitizer’s bandwidth. This predicament is caused by the power loss associated with optical dispersion. We overcome this limitation using Raman-amplified spectrum-to-time transformation. An extraordinary lossless −11.76ns∕nm dispersive device is used to demonstrate single-shot gas absorption spectroscopy with a 950MHz resolution—a record in real-time spectroscopy.

Giant tunable optical dispersion using chromo-modal excitation of a multimode waveguide

The ability to control chromatic dispersion is paramount in applications where the optical pulsewidth is critical, such as chirped pulse amplification and fiber optic communications. Typically, devices used to generate large amounts (>100 ps/nm) of chromatic dispersion are based on diffraction gratings, chirped fiber Bragg gratings, or dispersion compensating fiber. Unfortunately, these dispersive elements suffer from one or more of the following restrictions: (i) limited operational bandwidth, (ii) limited total dispersion, (iii) low peak power handling, or (iv) large spatial footprint. Here, we introduce a new type of tunable dispersive device, which overcomes these limitations by leveraging the large modal dispersion of a multimode waveguide in combination with the angular dispersion of diffraction gratings to create chromatic dispersion. We characterize the devices dispersion, and demonstrate its ability to stretch a sub-picosecond optical pulse to nearly 2 nanoseconds in 20 meters of multimode optical fiber. Using this device, we also demonstrate single-shot, time-wavelength atomic absorption spectroscopy at a repetition rate of 90.8 MHz.

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.

Predistortion technique for RF-photonic generation of high-power ultrawideband arbitrary waveforms

By employing an RF-photonic arbitrary waveform generator, the authors experimentally obtained complex high-power RF waveforms with spectra spanning more than a decade and voltage amplitudes of more than 13 V in a system limited by a gain ripple of up to 5 dB and phase distortion of over 200/spl deg/. A spectral predistortion technique tailored to photonically assisted waveform generators that can overcome substantial RF amplifier gain ripple and phase distortion in order to generate high-power (> 30 dBm) ultrawideband arbitrary RF waveforms is presented.

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.