Ali Fard

University of California, Los Angeles, Los Angeles, CA, United States

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Publications (26)57.86 Total impact

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    ABSTRACT: Real-time wideband digitizers are the key building block in many systems including oscilloscopes, signal intelligence, electronic warfare, and medical diagnostics systems. Continually extending the bandwidth of digitizers has hence become a central challenge in electronics. Fortunately, it has been shown that photonic pre-processing of wideband signals can boost the performance of electronic digitizers. In this article, the underlying principle of the time-stretch analog-to-digital converter (TSADC) that addresses the demands on resolution, bandwidth, and spectral efficiency is reviewed. In the TSADC, amplified dispersive Fourier transform is used to slow down the analog signal in time and hence to compress its bandwidth. Simultaneous signal amplification during the time-stretch process compensates for parasitic losses leading to high signal-to-noise ratio. This powerful concept transforms the analog signal's time scale such that it matches the slower time scale of the digitizer. A summary of time-stretch technology's extension to high-throughput single-shot spectroscopy, a technique that led to the discovery of optical rouge waves, is also presented. Moreover, its application in high-throughput imaging, which has recently led to identification of rogue cancer cells in blood with record sensitivity, is discussed.
    Laser & Photonics Review 03/2013; 7(2). · 7.98 Impact Factor
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    ABSTRACT: We present a digital postprocessing linearization technique to efficiently suppress dynamic distortions added to a wideband signal in an analog optical link. Our technique achieves up to 35 dB suppression of intermodulation distortions over multiple octaves of signal bandwidth. In contrast to conventional linearization methods, it does not require excessive analog bandwidth for performing digital correction. This is made possible by regenerating undesired distortions from the captured output, and subtracting it from the distorted digitized signal. Moreover, we experimentally demonstrate a record spurious-free dynamic range of 120 dB·Hz<sup>2/3</sup> over a 6 GHz electrical signal bandwidth. While our digital broadband linearization technique advances state-of-the-art optical links, it can also be applied to other nonlinear dynamic systems.
    Optics Letters 02/2013; 38(4):446-8. · 3.39 Impact Factor
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    ABSTRACT: We describe a real-time image processor that has enabled a new automated flow through microscope to screen cells in flow at 100,000 cells/s and a record false positive rate of one in a million. This unit is integrated with an ultrafast optical imaging modality known as serial time-encoded amplified microscopy (STEAM) for blur-free imaging of particles in high-speed flow. We show real-time image-based identification and screening of budding yeast cells and rare breast cancer cells in blood. The system generates E-slides (an electronic version of glass slides) on which particles of interest are digitally analyzed.
    Imaging, Manipulation, and Analysis of Biomolecules, Cells, and Tissues XI conference, SPIE BiOS; 01/2013
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    ABSTRACT: Laser scanners are essential for scientific research, manufacturing, defense, and medical practice. Unfortunately, often times the speed of conventional laser scanners (e.g., galvanometric mirrors and acousto-optic deflectors) falls short for many applications, resulting in motion blur and failure to capture fast transient information. Here, we present a novel type of laser scanner that offers roughly three orders of magnitude higher scan rates than conventional methods. Our laser scanner, which we refer to as the hybrid dispersion laser scanner, performs inertia-free laser scanning by dispersing a train of broadband pulses both temporally and spatially. More specifically, each broadband pulse is temporally processed by time stretch dispersive Fourier transform and further dispersed into space by one or more diffractive elements such as prisms and gratings. As a proof-of-principle demonstration, we perform 1D line scans at a record high scan rate of 91 MHz and 2D raster scans and 3D volumetric scans at an unprecedented scan rate of 105 kHz. The method holds promise for a broad range of scientific, industrial, and biomedical applications. To show the utility of our method, we demonstrate imaging, nanometer-resolved surface vibrometry, and high-precision flow cytometry with real-time throughput that conventional laser scanners cannot offer due to their low scan rates.
    Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XIII conference, SPIE LASE; 01/2013
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    ABSTRACT: We report high-throughput optical coherence tomography (OCT) that offers 1,000 times higher axial scan rate than conventional OCT in the 800 nm spectral range. This is made possible by employing photonic time-stretch for chirping a pulse train and transforming it into a passive swept source. We demonstrate a record high axial scan rate of 90.9 MHz. To show the utility of our method, we also demonstrate real-time observation of laser ablation dynamics. Our high-throughput OCT is expected to be useful for industrial applications where the speed of conventional OCT falls short.
    Optics Express 08/2012; 20(18):19612-7. · 3.55 Impact Factor
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    ABSTRACT: We propose and demonstrate an all-optical time-stretch digitizer for real-time capture of ultrafast optical signals, beyond the bandwidths achievable by electronics. This approach uniquely combines four-wave mixing and photonic time-stretch technique to slow down and record high-speed optical signals. As a proof-of-concept, real-time recording of 40-Gb/s non-return-to-zero on-off-keying optical data stream is experimentally demonstrated using a stretch factor of 54 and 1.5-GHz back-end electronic bandwidth. We also report on the observation of dispersion penalty and its mitigation via single-sideband conversion enabled by an optical bandpass filter. Our technique may provide a path to real-time capture of ultrahigh-speed optical data streams.
    Applied Physics Letters 08/2012; 101(5). · 3.79 Impact Factor
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    ABSTRACT: Optical microscopy is one of the most widely used diagnostic methods in scientific, industrial, and biomedical applications. However, while useful for detailed examination of a small number (< 10,000) of microscopic entities, conventional optical microscopy is incapable of statistically relevant screening of large populations (> 100,000,000) with high precision due to its low throughput and limited digital memory size. We present an automated flow-through single-particle optical microscope that overcomes this limitation by performing sensitive blur-free image acquisition and nonstop real-time image-recording and classification of microparticles during high-speed flow. This is made possible by integrating ultrafast optical imaging technology, self-focusing microfluidic technology, optoelectronic communication technology, and information technology. To show the system's utility, we demonstrate high-throughput image-based screening of budding yeast and rare breast cancer cells in blood with an unprecedented throughput of 100,000 particles/s and a record false positive rate of one in a million.
    Proceedings of the National Academy of Sciences 07/2012; 109(29):11630-5. · 9.81 Impact Factor
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    ABSTRACT: We propose an all-optical time-stretch oscilloscope, combining four-wave mixing and time-stretch technique for real-time capture of ultrafast optical time-series, beyond the bandwidths achievable by electronics. As a proof-of-concept, we demonstrate capture of 40-Gbits/s optical data.
    06/2012;
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    ABSTRACT: Laser scanning technology is one of the most integral parts of today's scientific research, manufacturing, defense, and biomedicine. In many applications, high-speed scanning capability is essential for scanning a large area in a short time and multi-dimensional sensing of moving objects and dynamical processes with fine temporal resolution. Unfortunately, conventional laser scanners are often too slow, resulting in limited precision and utility. Here we present a new type of laser scanner that offers ∼1,000 times higher scan rates than conventional state-of-the-art scanners. This method employs spatial dispersion of temporally stretched broadband optical pulses onto the target, enabling inertia-free laser scans at unprecedented scan rates of nearly 100 MHz at 800 nm. To show our scanner's broad utility, we use it to demonstrate unique and previously difficult-to-achieve capabilities in imaging, surface vibrometry, and flow cytometry at a record 2D raster scan rate of more than 100 kHz with 27,000 resolvable points.
    Scientific Reports 06/2012; 2:445. · 5.08 Impact Factor
  • D. Lam, A.M. Fard, B. Jalali
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    ABSTRACT: We present a digital post-processing linearization technique to suppress dynamic distortions added to a wideband signal in an analog optical link. We demonstrate record spurious-free dynamic range of 120 dB.Hz2/3 over 6-GHz electrical signal bandwidth.
    Photonics Conference (IPC), 2012 IEEE; 01/2012
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    ABSTRACT: High-speed high-contrast imaging modalities that enable image acquisition of transparent media without the need for chemical staining are essential tools for a broad range of applications; from semiconductor process monitoring to blood screening. Here we introduce a method for contrast-enhanced imaging of unstained transparent objects that is capable of high-throughput imaging. This method combines the Nomarski phase contrast capability with the ultrahigh frame rate and shutter speed of serial time-encoded amplified microscopy. As a proof of concept, we show imaging of a transparent test structure and white blood cells in flow at a shutter speed of 33 ps and a frame rate of 36.1 MHz using a single-pixel photo-detector. This method is expected to be a valuable tool for high-throughput screening of unstained cells.
    Biomedical Optics Express 12/2011; 2(12):3387-92. · 3.18 Impact Factor
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    ABSTRACT: Optical performance monitoring of high-capacity networks is one of the enabling technologies of future reconfigurable optical switch networks. In such networks, rapid performance evaluation of data streams becomes challenging due to the use of advanced modulation formats and high data rates. The time-stretch enhanced recording oscilloscope offers a potential solution to monitoring high-rate data in a practical time scale. Here we demonstrate an architecture with a differential detection front end for simultaneous I/Q data monitoring of a 100 gigabits/s return-to-zero differential quadrature phase-shift keying signal. This demonstration shows the potential of this technology for rapid performance monitoring of high-rate optical data streams that employ advanced modulation formats.
    Optics Letters 10/2011; 36(19):3804-6. · 3.39 Impact Factor
  • A. Fard, B. Buckley, B. Jalali
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    ABSTRACT: Dual-polarization photonic time-stretch technique, which exploits polarization multiplexing to improve the spectral efficiency of the conventional photonic time-stretch technique, is proposed. This technique reduces the demand on optical bandwidth for large record length of the photonic time-stretch analog-to-digital converter. It is shown that this technique can capture high-bandwidth radio-frequency signals (>;10-GHz instantaneous bandwidth). Experimentally, 12.5-Gb/s data eye-diagram measurement using this preprocessor operating in equivalent-time mode is demonstrated.
    IEEE Photonics Technology Letters 08/2011; · 2.04 Impact Factor
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    ABSTRACT: Wideband real-time analog-to-digital converters are the central tools in waveform analyzers, communication systems, and radar technology. Photonic time-stretch analog-to-digital converters (TSADCs) utilize a broadband optical source and an optical link to extend the capabilities of real-time digitizers, allowing acquisition of wideband radio frequency (RF) signals with high resolution. In the TSADC, it is desirable to improve the signal-to-noise-and-distortion ratio and effective number of bits by increasing the optical power. Here, we numerically evaluate the impact of optical nonlinearity on TSADC performance. It is demonstrated that the optical nonlinearity can impose an upper limit on the effective number of bits and that the RF bandwidth limitation due to dispersion penalty depends on optical power. The trends presented here can also be applied to other optical links in which optical nonlinearity and dispersion are significant.
    Journal of Lightwave Technology 07/2011; 29(13). · 2.56 Impact Factor
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    ABSTRACT: We show how the ability to slow down, amplify, and capture fast transient events can produce high-throughput real-time instruments ranging from digitizers to imaging flow cytometers for detection of rare diseased cells in blood.
    06/2011;
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    ABSTRACT: We report a new method of high-contrast imaging of unstained and transparent objects at ~1000 times higher frame rates than conventional methods. As a proof-of-concept, we demonstrate enhanced image-contrast in 2D imaging of a transmission grating.
    04/2011;
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    ABSTRACT: We report simultaneous I/Q-data monitoring of 100-Gb/s RZ-DQPSK signal using a two-channel time-stretch enhanced recording (TiSER) oscilloscope. TiSER offers a solution to monitoring of high bit rate data.
    04/2011;
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    ABSTRACT: We report a new type of laser vibrometer that performs high-speed imaging-based surface vibration measurements with ~1 nm axial resolution at a record scan rate of 36.7 MHz without the need for beam scanning.
    04/2011;
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    ABSTRACT: Suppression of distortion induced by nonlinearity in a dynamical system (such as an analog optical link) is very challenging, particularly for a wide-bandwidth signal. Conventional compensation techniques are computationally intensive, significantly limiting their realization in real-time applications. Here, we propose and demonstrate an efficient digital postprocessing technique to suppress distortions added to a wideband signal by a nonlinear system with memory effect. Experimentally, digital broadband linearization of the photonic time-stretch analog-to-digital converter (TSADC) is demonstrated. In case of TSADC, a dynamic range improvement of >15 dB compared to conventional memory-less correction method is achieved.
    Optics Letters 04/2011; 36(7):1077-9. · 3.39 Impact Factor
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    ABSTRACT: Conventional laser vibrometers are incapable of performing multidimensional vibrometry at high speeds because they build on single-point measurements and rely on beam scanning, significantly limiting their utility and precision. Here we introduce a laser vibrometer that performs high-speed multidimensional imaging-based vibration and velocity measurements with nanometer-scale axial resolution without the need for beam scanning. As a proof-of-concept, we demonstrate real-time microscopic imaging of acoustic vibrations with 1 nm axial resolution, 1200 image pixels, and 30 ps dwell time at 36.7 MHz scan rate.
    Applied Physics Letters 03/2011; 98(10):101107-101107-3. · 3.79 Impact Factor