Kevin K. Tsia

Serial time-encoded amplified imaging for real-time observation of fast dynamic phenomena

Ultrafast real-time optical imaging is an indispensable tool for studying dynamical events such as shock waves, chemical dynamics in living cells, neural activity, laser surgery and microfluidics. However, conventional CCDs (charge-coupled devices) and their complementary metal–oxide–semiconductor (CMOS) counterparts are incapable of capturing fast dynamical processes with high sensitivity and resolution. This is due in part to a technological limitation—it takes time to read out the data from sensor arrays. Also, there is the fundamental compromise between sensitivity and frame rate; at high frame rates, fewer photons are collected during each frame—a problem that affects nearly all optical imaging systems. Here we report an imaging method that overcomes these limitations and offers frame rates that are at least 1,000 times faster than those of conventional CCDs. Our technique maps a two-dimensional (2D) image into a serial time-domain data stream and simultaneously amplifies the image in the optical domain. We capture an entire 2D image using a single-pixel photodetector and achieve a net image amplification of 25 dB (a factor of 316). This overcomes the compromise between sensitivity and frame rate without resorting to cooling and high-intensity illumination. As a proof of concept, we perform continuous real-time imaging at a frame speed of 163 ns (a frame rate of 6.1 MHz) and a shutter speed of 440 ps. We also demonstrate real-time imaging of microfluidic flow and phase-explosion effects that occur during laser ablation.

Amplified dispersive Fourier-transform imaging for ultrafast displacement sensing and barcode reading

Dispersive Fourier transformation is a powerful technique in which the spectrum of an optical pulse is mapped into a time-domain waveform using chromatic dispersion. It replaces a diffraction grating and detector array with a dispersive fiber and single photodetector. This simplifies the system and, more importantly, enables fast real-time measurements. Here we describe a novel ultrafast barcode reader and displacement sensor that employs internally amplified dispersive Fourier transformation. This technique amplifies and simultaneously maps the spectrally encoded barcode into a temporal waveform. It achieves a record acquisition speed of 25MHz—four orders of magnitude faster than the current state of the art.Dispersive Fourier transformation is a powerful technique in which the spectrum of an optical pulse is mapped into a time-domain waveform using chromatic dispersion. It replaces a diffraction grating and detector array with a dispersive fiber and single photodetector. This simplifies the system and, more importantly, enables fast real-time measurements. Here we describe a novel ultrafast barcode reader and displacement sensor that employs internally amplified dispersive Fourier transformation. This technique amplifies and simultaneously maps the spectrally encoded barcode into a temporal waveform. It achieves a record acquisition speed of 25MHz—four orders of magnitude faster than the current state of the art.

Performance of serial time-encoded amplified microscope

Serial time-encoded amplified microscopy (STEAM) is a new high-sensitivity ultrafast real-time imaging modality. Here we describe an analysis of its spatial resolution, frame rate, and detection sensitivity.

Asymmetric-detection time-stretch optical microscopy (ATOM) for ultrafast high-contrast cellular imaging in flow

Accelerating imaging speed in optical microscopy is often realized at the expense of image contrast, image resolution, and detection sensitivity – a common predicament for advancing high-speed and high-throughput cellular imaging. We here demonstrate a new imaging approach, called asymmetric-detection time-stretch optical microscopy (ATOM), which can deliver ultrafast label-free high-contrast flow imaging with well delineated cellular morphological resolution and in-line optical image amplification to overcome the compromised imaging sensitivity at high speed. We show that ATOM can separately reveal the enhanced phase-gradient and absorption contrast in microfluidic live-cell imaging at a flow speed as high as ~10 m/s, corresponding to an imaging throughput of ~100,000 cells/sec. ATOM could thus be the enabling platform to meet the pressing need for intercalating optical microscopy in cellular assay, e.g. imaging flow cytometry – permitting high-throughput access to the morphological information of the individual cells simultaneously with a multitude of parameters obtained in the standard assay.

Manipulating supercontinuum generation by minute continuous wave

We report a simple triggering mechanism that greatly enhances and stabilizes supercontinuum generation by using an extremely weak cw light (~200,000 times weaker than the pump light). Such an active manipulation scheme can be enabled by a wide range of input conditions and circumvents complex techniques such as precise time delay tuning and dedicated feedback control. It thus offers a handy and versatile approach to control and optimize supercontinuum generation, expanding its range of applications, including ultrafast all-optical signal processing, spectroscopy, and imaging. The utility of the present technique for improving signal integrity in chirped pump optical parametric amplification is also demonstrated.

Optical time-stretch confocal microscopy at 1 μm

We demonstrate optical time-stretch confocal microscopy in the 1 μm spectral window for high-speed and high-resolution cellular imaging. In contrast to the prior demonstrations of time-stretch imaging, which all operated in the telecommunication band, the present work extends the utility of this imaging modality to a wavelength regime (~1 μm), which is well known to be the optimal diagnostic window in biophotonics. This imaging technique enables us to image the nasopharyngeal epithelial cells with cellular resolution (<2 μm), at a line scan rate of 10 MHz, and with a field of view as wide as ~0.44 mm × 0.1 mm. We also theoretically and experimentally characterized the system performance. As the low-loss dispersive fibers for the time-stretch process as well as other essential optical components for enhancing the imaging sensitivity are commonly available at 1 μm, time-stretch confocal microscopy in this wavelength range could usher in realizing high-speed cell imaging with an unprecedented throughput.

Energy harvesting in silicon wavelength converters

Nonlinear loss is the central problem in silicon devices that operate using nonlinear optical effects. Wavelength converters are one example of such devices, wherein high optical intensities required for nonlinear interactions cause two-photon absorption and severe free-carrier absorption. In this paper, we report the first demonstration of nonlinear photovoltaic effect in silicon wavelength converters. This useful phenomenon allows us to eliminate the nonlinear loss caused by free-carrier absorption, while harvesting the optical power that is normally consumed by two-photon absorption.

Serial time-encoded amplified microscopy (STEAM) based on a stabilized picosecond supercontinuum source

Temporal stability of the broadband source, such as supercontinuum (SC), is the key enabling factor for realizing high performance ultrafast serial time-encoded amplified microscopy (STEAM). Owing to that the long-pulse SC (picosecond to nanosecond) generation generally results in an ultrabroadband spectrum with significant pulse-to-pulse fluctuation, only the ultrashort-pulse (femtosecond) SC sources, which offer better temporal stability, have been employed in STEAM so far. Here we report a simple approach to achieve active control of picosecond SC stability and to help extend the applicability of SC in STEAM from the femtosecond to the picosecond or even nanosecond regimes. We experimentally demonstrate stable single-shot STEAM imaging at a frame rate of 4.9 MHz using the CW-triggered picosecond SC source. Such CW-stabilized SC can greatly reduce the shot-to-shot fluctuation, and thus improves the STEAM image quality significantly.

Megahertz all-optical swept-source optical coherence tomography based on broadband amplified optical time-stretch

We demonstrate all-optical ultrahigh-speed swept-source optical coherence tomography (OCT) based on amplified optical time-stretch (AOT). Such an inertia-free wavelength-swept mechanism, via group velocity dispersion, enables us to realize OCT with an A-scan rate well above MHz. More importantly, the key significance of AOT-OCT is its simultaneous broadband Raman amplification during the time-stretch process-greatly enhancing the detection sensitivity compared with prior attempts to apply optical time-stretch to OCT. Here, we report on an AOT-OCT system which is operated at an A-scan rate of 7.14 MHz, a superior roll-off performance (>2 mm/dB), a record-high sensitivity of time-stretch-based OCT (>80 dB) with a broadband gain bandwidth of 80 nm, which results in an axial resolution of ∼15 μm. Our AOT-OCT system is thus able to, for the first time to the best of our knowledge, perform time-stretch-based OCT of biological tissue in vivo. It represents a major step forward in utilizing AOT as an alternative for achieving practical MHz OCT, without any long-term mechanical stability concerns as in typical swept-source OCT or bypassing the speed limitation of the image sensor employed in spectral-domain OCT.

Interferometric time-stretch microscopy for ultrafast quantitative cellular and tissue imaging at 1 μm

Abstract. Quantitative phase imaging (QPI) has been proven to be a powerful tool for label-free characterization of biological specimens. However, the imaging speed, largely limited by the image sensor technology, impedes its utility in applications where high-throughput screening and efficient big-data analysis are mandated. We here demonstrate interferometric time-stretch (iTS) microscopy for delivering ultrafast quantitative phase cellular and tissue imaging at an imaging line-scan rate >20  MHz—orders-of-magnitude faster than conventional QPI. Enabling an efficient time-stretch operation in the 1-μm wavelength window, we present an iTS microscope system for practical ultrafast QPI of fixed cells and tissue sections, as well as ultrafast flowing cells (at a flow speed of up to 8  m/s). To the best of our knowledge, this is the first time that time-stretch imaging could reveal quantitative morphological information of cells and tissues with nanometer precision. As many parameters can be further extracted from the phase and can serve as the intrinsic biomarkers for disease diagnosis, iTS microscopy could find its niche in high-throughput and high-content cellular assays (e.g., imaging flow cytometry) as well as tissue refractometric imaging (e.g., whole-slide imaging for digital pathology).