Jacky C. K. Chan

Time–bandwidth engineering

We describe compression and expansion of the time–bandwidth product of signals and present tools to design optical data compression and expansion systems that solve bottlenecks in the real-time capture and generation of wideband data. Applications of this analog photonic transformation include more efficient ways to sample, digitize, and store optical data. Time–bandwidth engineering is enabled by the recently introduced Stretched Modulation (SM) Distribution function, a mathematical tool that describes the bandwidth and temporal duration of signals after arbitrary phase and amplitude transformations. We demonstrate design of time–bandwidth engineering systems in both near-field and far-field regimes that employ engineered group delay (GD), and we derive closed-form mathematical equations governing the operation of such systems. These equations identify an important criterion for the maximum curvature of warped GD that must be met to achieve time–bandwidth compression. We also show application of the SM Distribution to benchmark different GD profiles and to the analysis of tolerance to system nonidealities, such as GD ripples.

Ultrafast dark-field surface inspection with hybrid-dispersion laser scanning

High-speed surface inspection plays an important role in industrial manufacturing, safety monitoring, and quality control. It is desirable to go beyond the speed limitation of current technologies for reducing manufacturing costs and opening a new window onto a class of applications that require high-throughput sensing. Here, we report a high-speed dark-field surface inspector for detection of micrometer-sized surface defects that can travel at a record high speed as high as a few kilometers per second. This method is based on a modified time-stretch microscope that illuminates temporally and spatially dispersed laser pulses on the surface of a fast-moving object and detects scattered light from defects on the surface with a sensitive photodetector in a dark-field configuration. The inspectors ability to perform ultrafast dark-field surface inspection enables real-time identification of difficult-to-detect features on weakly reflecting surfaces and hence renders the method much more practical than in the...

Reconstruction in time-bandwidth compression systems

Recently it has been shown that the intensity time-bandwidth product of optical signals can be engineered to match that of the data acquisition instrument. In particular, it is possible to slow down an ultrafast signal, resulting in compressed RF bandwidth - a similar benefit to that offered by the Time-Stretch Dispersive Fourier Transform (TS-DFT) - but with reduced temporal record length leading to time-bandwidth compression. The compression is implemented using a warped group delay dispersion leading to non-uniform time stretching of the signals intensity envelope. Decoding requires optical phase retrieval and reconstruction of the input temporal profile, for the case where information of interest is resides in the complex field. In this paper, we present results on the general behavior of the reconstruction process and its dependence on the signal-to-noise ratio. We also discuss the role of chirp in the input signal.

Sparsity and self-adaptivity in anamorphic stretch transform

Warped stretch dispersive Fourier transform offers nonuniform spectrum sampling. Signal Sparsity and self-adaptive properties of the transform are explained. The loss in the optical transformation and reconstruction are also discussed.

Context-Aware Image Compression.

We describe a physics-based data compression method inspired by the photonic time stretch wherein information-rich portions of the data are dilated in a process that emulates the effect of group velocity dispersion on temporal signals. With this coding operation, the data can be downsampled at a lower rate than without it. In contrast to previous implementation of the warped stretch compression, here the decoding can be performed without the need of phase recovery. We present rate-distortion analysis and show improvement in PSNR compared to compression via uniform downsampling.

Digitally synthesized beat frequency-multiplexed fluorescence lifetime spectroscopy

Frequency domain fluorescence lifetime imaging is a powerful technique that enables the observation of subtle changes in the molecular environment of a fluorescent probe. This technique works by measuring the phase delay between the optical emission and excitation of fluorophores as a function of modulation frequency. However, high-resolution measurements are time consuming, as the excitation modulation frequency must be swept, and faster low-resolution measurements at a single frequency are prone to large errors. Here, we present a low cost optical system for applications in real-time confocal lifetime imaging, which measures the phase vs. frequency spectrum without sweeping. Deemed Lifetime Imaging using Frequency-multiplexed Excitation (LIFE), this technique uses a digitally-synthesized radio frequency comb to drive an acousto-optic deflector, operated in a cats-eye configuration, to produce a single laser excitation beam modulated at multiple beat frequencies. We demonstrate simultaneous fluorescence lifetime measurements at 10 frequencies over a bandwidth of 48 MHz, enabling high speed frequency domain lifetime analysis of single- and multi-component sample mixtures.

Dynamic chromo-angular-modal excitation of multimode waveguides via acousto-optic deflection (Conference Presentation)

Photonic time stretch has been a successful imaging and spectroscopy technique that provides single-shot and real-time performance by employing highly dispersive optical elements to slow down the modulated optical signals and overcome the speed bottleneck present at analog-to-digital conversion. Photonic time stretch has also been generalized to warped stretch with tailored chromatic dispersion profiles, enabling non-uniform (foveated) sampling. However, such tailored profiles are currently only available with fixed elements which cannot be reconfigured, lowering the robustness of such systems and limiting their applicability to static samples and environments that have relatively constant sample statistics. To address this limitation, we demonstrate an arbitrarily programmable source of chromatic dispersion which has a 1-ns wide delay range, and inherently devoid of ghost artifacts common in spectral phase-based optical devices. Extending the concept of chromo-modal dispersion (CMD), we use an acousto-optic deflector to create a digitally synthesized and modulated angular spread of optical wavelengths, which are then coupled into the chosen waveguide mode of a multimode fiber to create the desired modal dispersion. The range and tunability enables us to on-the-fly reconfigure and correct optical dispersion. As proof-of-concept, we demonstrate real-time channel noise correction via optical feedback for warped stretch spectroscopy at 36.6 MHz.

Optics-inspired context-aware image compression using warped stretch transform

Image compression by the warped stretch transform is introduced, where the input image is reshaped by a signal-dependent mapping i.e. designed “warp kernel”. The warped image can be downsampled at a lower uniform rate for the same PSNR, effectively achieving reversible and context-aware non-uniform sampling.

Analog gearbox: A photonic hardware accelerator

Inspired by mechanical gearboxes, we introduce an optical analog implementation of the gearbox as a means to overcome the mismatch between the ultrahigh speed of optical data and the much slower sampling rate of electronic digitizers and processors.