About

Dr Chao Wang is currently a Senior Lecturer in the School of Engineering and Digital Arts at University of Kent, where he first joined as a Lecturer in 2013. From 2011 to 2012, he was a NSERC Postdoctoral Fellow in the Photonics LaboratoryUniversity of California, Los Angeles (UCLA), USA. He received his B.Eng degree in Opto-electrical Engineering from Tianjin University, China, in 2002, M.Sc degree in Optics from Nankai University, China, in 2005, and Ph.D degree in Electrical and Computer Engineering from the University of Ottawa, Canada, in 2011. 

Dr Wang's research interests lie in inter-disciplinary areas that study the interaction between photonics and other traditional or state-of-the-art technologies in different fields, such as microwave photonics, optical communications and biophotonics, for widespread industrial, communications, biomedical, and defense applications. His research activities have been well funded by EU Marie-Curie Actions, the Royal Society of UK, the University of Kent, and Natural Sciences and Engineering Research Council (NSERC) of Canada. He has authored more than 80 peer-reviewed journal publications and international conference papers (including five invited papers and three Best Student Paper Awards). For a complete list of his publications, please visit this page. The total citations of his work are over1520 and h-index is 22. His research work has been highlighted in major academic media, such as Nature PhotonicsNature BiotechnologyTime MagazineScientific AmericanOptics & Photonics News and Laser Focus World.

He has received several prestigious international and national awards, including SPIE Scholarship in Optical Science and Engineering (2008), Vanier Canada Graduate Scholarship (2009) , IEEE Photonics Society Graduate Student Fellowship (2009), IEEE Microwave Theory and Techniques Society Graduate Fellowship (2010), Natural Science and Engineering Research Council of Canada (NSERC) Postdoctoral Fellowship (2011), and Marie-Curie Career Integration Grant (2014). He also received the Best Paper Awards from APMP 2009 and MWP 2010.

He is a member of IEEE, the IEEE-PS, the IEEE-MTTS, and OSA, and a Fellow of Higher Education Academy. He also serves as a regular reviewer of many leading photonics journals of OSA, IEEE, Nature, AIP, IET, SPIE, Elsevier and Wiley. He has served as the Co-organizer and Chair of Mini-Symposium on Microwave Photonics in PIERS 2014, and the Symposium Co-chair in CLEO-PR/OECC/PGC 2017, and Technical Programme Sub-committeee co-chair of CLEO-PR 2018. He also served as the Guest Editor of a Special Session on Microwave Photonics in SPIE Optical Engineeringand the Guest Editor of a Specital Issue on Microwave Photonics in MDPI Photonics.

In the School of Engineering and Digital Arts, Dr Wang is the Programme Chair of MSc Advanced Communications Engineering (RF Technology and Telecommunications) and MSc Advanced Communications Engineering (Wireless Systems and Networks).

Research interests

Dr Chao Wang's research interests lie in inter-disciplinary areas that study the interaction between photonics and other traditional or state-of-the-art technologies in different fields, such as microwave photonics, biophotonics and fiber optics, for industrial, communications, biomedical, and defense applications. 

  • Microwave Photonics: photonic generation of microwave arbitrary waveforms, microwave photonic signal processing, radio-over-fiber, optical sensors.
  • Biophotonics: ultrafast high-throughput optical imaging for biomedical applicaitons, optical coherence tomography.
  • Ultrafast Optics: coherent optical pulse shaping, characterization of ultrafast optical pulses, ultrafast optical signal processing.
  • Fiber Optics: fiber lasers and amplifiers, fiber Bragg gratings, fiber-optic sensors and ultrafast interrogation systems, photonic crystal fiber devices.

His research activities have been funded by the EU Marie-Curie Actions, the Royal Society of UK, University of Kent, and the Natural Sciences and Engineering Research Council (NSERC) of Canada. His current funded research projects include:

  • 2016-2017 (PI) The Royal Society Research Grants, "Photonic-enabled big data compression in ultrafast imaging".
  • 2014-2017 (Co-I, with Dr N. Gomes (PI), Prof J. Wang and Dr H. Zhu) EU Horizon 2020 EU-Japan Cooperation Project, "Radio Technologies for 5G using Advanced Photonic Infrastructure for Dense User Environment (RAPID)". 
  • 2014-2018 (PI) EU FP-7 Marie-Curie Career Integration Grant (CIG), "Next generation ULTrafast continuously running Imaging System for biomedical applications (NULTIS)".
  • 2014-2016 (PI) The Royal Society International Exchanges Grant, "Integrated Microwave Photonics Signal Processing for High-speed Optical Imaging".
  • 2014-2017 (PI, with Dr P. Young and Prof J. Stiens) University of Kent - VUB Collaboration Project, "Real-time Photonic Terahertz Coherence Tomography".
  • 2014-2017 (Co-I, with Dr N. Gomes(PI)) University of Kent 50th Anniversary PhD Studentship,  "Optical fibre transport of MIMO/virtual-MIMO radio signals for 5G wireless/mobile systems".
  • 2013-2016 (PI) University of Kent Faculty of Sciences Research Funds.

More information about Dr Wang's research can be found here.

Teaching

Undergraduate modules:

Postgraduate modules:

Foundation Year modules:

Supervision

PhD students supervised as the first supervisor:

PhD students supervised as the second supervisor:

  • AE Aighobahi: Radio over fibre
  • U Habib: Radio over Fibre, Millimetre Wave Communication, 5G Mobile/Communication Networks
  • S Noor: Subcarrier multiplexing radio over fibre

Professional

  • Regular Reviewer of more than 20 international Journals from IEEE, OSA, Nature, AIP, IET, SPIE, Elsevier, and Wiley. 
  • Remote Referee of European Research Council (2016-2017)
  • Grant Reviewer and Panelist of British Council Newton Fund (2014-2017)
  • Member of EPSRC Associate Peer Review College (2016- ) 
  • Guest Editor of SPIE Optical Engineering (2015) and MDPI Photonics (2017).
  • External examiner for PhD viva in Aston, VUB and Monash.  

Publications

Article

  • He, H. et al. (2018). Arbitrary spectral synthesis and waveform generation with HiBi fiber loop mirrors. IEEE Photonics Technology Letters [Online] 30:943-946. Available at: https://doi.org/10.1109/LPT.2018.2818326.
    An arbitrary spectral synthesis scheme with parallel-connecting high-birefringence fiber loop mirrors (HiBi-FLMs) based on Fourier synthesis theory has been proposed and demonstrated. Three typical spectra of triangle, rectangle and sawtooth shape have been synthesized by implementing only four HiBi-FLMs. The experimental results are in good agreement with theoretical simulations with a goodness of fit of 0.9565. Furthermore, higher precise optical spectrum with narrower bandwidth can be obtained by adopting longer polarization- maintaining fiber and more sections of HiBi-FLMs. Besides, a typical application of arbitrary waveform generation has been implemented. By incorporating with frequency-to- time mapping, triangle- and sawtooth-shaped electrical pulses with repetition rate of 1 GHz and pulse width of ~860 ps have been generated
  • Xing, J. et al. (2018). Modulation fading in temporal Talbot effect. IEEE Photonics Technology Letters [Online] 30:1376-1379. Available at: https://doi.org/10.1109/LPT.2018.2848634.
    Temporal Talbot phenomenon is the time-domain counterpart of the well-known spatial Talbot phenomenon, which can be used to compress/expand input pulses or multiply the pulse repetition rate. It is of great interest to study the temporal Talbot effect with modulation due to its potential applications in information transmission, in which the envelope of the input pulse sequence is modulated by a signal. In this letter, we report the modulation fading phenomenon in the temporal Talbot effect, i. e. the phenomenon that the modulation depth degrades with the increase of the order of the Talbot effect, for the first time to our knowledge. Distinct from the attenuation caused by the fiber loss, the modulation fading relates to the pulse width, the repetition period of the pulse sequence, and the frequency of the modulating signal as well as the order of the Talbot effect. A fully analytical expression of the modulation transfer function is presented to explain this phenomenon. Potential application of the modulation fading in temporal cloaking is discussed.
  • Wang, G. et al. (2018). Highly efficient optical beam steering using an in-fiber diffraction grating for full duplex indoor optical wireless communication. Journal of Lightwave Technology [Online] 36:4618-4625. Available at: https://doi.org/10.1109/JLT.2018.2832200.
    Diffraction gratings have been widely used in wavelength-controlled non-mechanical laser beam steering for high data-rate indoor optical wireless communications (OWC). Existing free-space diffraction gratings suffer from inherent difficulties of limited diffraction efficiency, bulky configuration, high cost and significant coupling loss with optical fiber links. In this work, a new optical approach for highly efficient, compact and fiber compatible laser beam steering using an in-fiber diffraction grating is proposed and experimentally demonstrated for the first time to our best knowledge. In-fiber diffraction is made possible based on a 45° tilted fiber grating (TFG), where wavelength dependent lateral scattering is obtained due to the strongly tilted grating structure. Improved diffraction efficiency of 93.5% has been achieved. In addition, the 45° TFG works perfectly for both light emission and reception, enabling full-duplex optical wireless transmission. Utility of the 45° TFG in all-fiber laser beam steering for multi-user full duplex optical wireless communications has been verified in experiments. 1.4 m free-space full-duplex wireless transmission has been demonstrated with data rate up to 12 Gb/s per beam using 2.4 GHz bandwidth OFDM signals.
  • Yang, C. et al. (2018). Tuning of polarization-dependent whispering gallery modes in grapefruit microstructured optical fibers infiltrated with negative dielectric anisotropy liquid crystals. Journal of Lightwave Technology [Online]. Available at: https://doi.org/10.1109/JLT.2018.2845104.
    An electrically tunable whispering gallery mode (WGM) microresonator based on microstructured optical fibers (MOFs) infiltrated with negative dielectric anisotropy liquid crystals (LCs) is proposed and experimentally demonstrated. Experimental results indicate that the second radial order mode of the MOF microresonator has stronger electric field response than the first radial order mode and the resonance dip for TE polarization mode is more sensitive to the applied electric field intensity in comparison with the TM polarization mode resonance dip. The Freedericksz transition threshold of the proposed MOF microresonator is experimentally found to be about 2.0 V/um. The electrically tunable microresonator integrated with negative dielectric anisotropy LCs is anticipated to find potential applications in optical filtering, all-optical switching, and electrically controlled micro-optics devices.
  • Wang, G. et al. (2018). Improved resolution optical time stretch imaging based on high efficiency in-fiber diffraction. Scientific Reports [Online] 8:600. Available at: https://www.nature.com/articles/s41598-017-18920-8.
    Most overlooked challenges in ultrafast optical time stretch imaging (OTSI) are sacrificed spatial resolution and higher optical loss. These challenges are originated from optical diffraction devices used in OTSI, which encode image into spectra of ultrashort optical pulses. Conventional free-space diffraction gratings, as widely used in existing OTSI systems, suffer from several inherent drawbacks: limited diffraction efficiency in a non-Littrow configuration due to inherent zeroth-order reflection, high coupling loss between free-space gratings and optical fibers, bulky footprint, and more importantly, sacrificed imaging resolution due to non-full-aperture illumination for individual wavelengths. Here we report resolution-improved and diffraction-efficient OTSI using in-fiber diffraction for the first time to our knowledge. The key to overcome the existing challenges is a 45° tilted fiber grating (TFG), which serves as a compact in-fiber diffraction device offering improved diffraction efficiency (up to 97%), inherent compatibility with optical fibers, and improved imaging resolution owning to almost full-aperture illumination for all illumination wavelengths. 50 million frames per second imaging of fast moving object at 46 m/s with improved imaging resolution has been demonstrated. This conceptually new in-fiber diffraction design opens the way towards cost-effective, compact and high-resolution OTSI systems for image-based high-throughput detection and measurement.
  • Wu, W., Liu, X. and Wang, C. (2018). Quantum speed-up capacity in different types of quantum channels for two-qubit open systems. Chinese Physics B [Online] 27:60302. Available at: http://dx.doi.org/10.1088/1674-1056/27/6/060302.
    A potential acceleration of a quantum open system is of fundamental interest in quantum computation, quantum communication, and quantum metrology. In this paper, we investigate the “quantum speed-up capacity” which reveals the potential ability of a quantum system to be accelerated. We explore the evolutions of the speed-up capacity in different quantum channels for two-qubit states. We find that although the dynamics of the capacity is varying in different kinds of channels, it is positive in most situations which are considered in the context except one case in the amplitude-damping channel. We give the reasons for the different features of the dynamics. Anyway, the speed-up capacity can be improved by the memory effect. We find two ways which may be used to control the capacity in an experiment:selecting an appropriate coefficient of an initial state or changing the memory degree of environments.
  • Zhao, W. et al. (2018). High-speed cell recognition algorithm for ultra-fast flow cytometer imaging system. Journal of Biomedical Optics [Online] 23. Available at: https://doi.org/10.1117/1.JBO.23.4.046001.
    An optical time-stretch flow imaging system enables high-throughput examination of cells/particles with unprecedented high speed and resolution. A significant amount of raw image data is produced. A high-speed cell recognition algorithm is, therefore, highly demanded to analyze large amounts of data efficiently. A high-speed cell recognition algorithm consisting of two-stage cascaded detection and Gaussian mixture model (GMM) classification is proposed. The first stage of detection extracts cell regions. The second stage integrates distance transform and the watershed algorithm to separate clustered cells. Finally, the cells detected are classified by GMM. We compared the performance of our algorithm with support vector machine. Results show that our algorithm increases the running speed by over 150% without sacrificing the recognition accuracy. This algorithm provides a promising solution for high-throughput and automated cell imaging and classification in the ultrafast flow cytometer imaging platform.
  • Xie, N. et al. (2018). In-line microfiber-assisted Mach-Zehnder interferometer for microfluidic highly sensitive measurement of salinity. IEEE Sensors Journal [Online]. Available at: https://doi.org/10.1109/JSEN.2018.2869273.
    We present a microfluidic U-shaped micro-cavity sensor by splicing a segment of microfiber of a few hundred micrometers in length tapered from a single-mode fiber (SMF) between two SMFs with predesigned lateral offset for highly sensitive salinity measurement. The proposed sensing probe serves as an in-line microfiber-assisted Mach-Zehnder interferometer (MAMZI) with an ultra-high refractive index sensitivity of 104 nm/RIU. Three Mach-Zehnder interferometer structures with different cavity lengths of 351.82 μm, 242.56 μm and 181.31 μm are fabricated, by which microfluidic sensing systems are established for in-line measurement of sodium chloride (NaCl) solution. Experimental results indicate that the detection limit of NaCl solution is as low as 4×10-3 wt% and the response time is less than 15 s, which would make the MAMZI-based microfluidic measuring system play an important role in label-free biological and chemical detection applications.
  • Zhang, X. et al. (2018). A magnetic field sensor based on a dual S-tapered multimode fiber interferometer. Measurement Science & Technology [Online] 29. Available at: https://doi.org/10.1088/1361-6501/aac00e.
    A multimode interferometer (MMI) for the measurement of a magnetic field based on concatenated S-tapered fibers is proposed and experimentally demonstrated. Spectrally interrogated magnetic field sensing is achieved by integrating the proposed MMI with magnetic fluids. The magnetic sensitivity of the MMI reaches 0.011 dB Oe−1. Owing to its desirable advantages such as compactness, low cost, fast response and flexible structure, the proposed MMI is anticipated to find potential applications in in situ measurements of the magnetic field.
  • Xie, H. et al. (2017). Design of an omnidirectional single-point photodetector for large-scale spatial coordinate measurement. Optical Engineering [Online] 56:104105. Available at: http://dx.doi.org/10.1117/1.OE.56.10.104105.
    In high precision and large-scale coordinate measurement, one commonly used approach to determine the coordinate of a target point is to utilize the spatial trigonometric relationships between multiple laser transmitting stations and the target point. A light receiving device at target point is the key element in large-scale coordinate measurement systems. To ensure high-resolution and highly-sensitive spatial coordinate measurement, a high-performance and miniaturized omnidirectional single-point photodetector (OSPD) is highly desired. Here we report one design of OSPD using aspheric lens, which achieves enhanced reception angle of -5 to 45 degree in vertical and 360 degree in horizontal. As the heart of our OSPD, the aspheric lens is designed in geometric model and optimized by LightTools Software, which enables reflecting wide-angle incident light beam into the single-point photodiode. The performance of home-made OSPD is characterized with working distances from 1 m to 13 m and further analyzed utilizing established geometric model. The experimental and analytic results verify that our new device is highly suitable for large-scale coordinate metrology. The developed device also holds great potential in various applications such as omnidirectional vision sensor, indoor global positioning system, optical wireless communication systems.
  • Han, X. et al. (2017). RF self-interference cancellation using phase modulation and optical sideband filtering. IEEE Photonics Technology Letters [Online] 29:917-920. Available at: http://dx.doi.org/10.1109/LPT.2017.2693690.
    A novel optical approach to implement RF self-interference cancellation for full-duplex communication using phase modulation and optical sideband filtering is proposed and demonstrated experimentally. Based on the inherent out-of-phase property between the left and right sidebands of phase-modulated signal and optical sideband filtering, the RF self-interference cancellation is achieved by tuning the delay time and amplitude in the optical domain. RF self-interference cancellation for single frequency and microwave with various bandwidth of 1MHz, 5MHz and 10 MHz is experimentally demonstrated to verify the proposed technique.
  • Xie, H. et al. (2017). Aspheric optical surface profiling based on laser scanning and auto-collimation. Review of Scientific Instruments [Online]. Available at: https://dx.doi.org/10.1063/1.4995685.
    Nowadays the utilization of aspheric lens has become more and more popular, enabling highly increased degree of freedom for optical design and simultaneously improving the performance of optical system. Hence this also entails a stringent requirement for fast and accurate measuring the shape of these aspheric components. In this paper, the instrument is greatly developed to satisfy the growing need to test axially symmetric aspheric surface, which is implemented by converting the pose of reflective mirror in optical path to the coordinate of reflection point on the surface when laser rapidly scans . At each movement position managed by grating-rule sensor, the rotating angle of reflective mirror is defined using position sensitive detector based on the laser auto-collimating and beam center-fitting principle. Testing a convex and a concave surfaces with highly reproducible results, including coefficient of determination better than 0.999 and RMSE less than =10, validates the feasibility of this method. In comparison to the conventional computer-generated hologram tester or interferometer, the present instrument—essentially builds on the pure geometrical optics technology—is a powerful tool to measure the aspheric surface quickly and accurately with simple structure and algorithm.
  • Mididoddi, C. and Wang, C. (2017). Adaptive non-uniform photonic time stretch for blind RF signal detection with compressed time-bandwidth product. Optics Communications [Online] 396:221-227. Available at: http://dx.doi.org/10.1016/j.optcom.2017.03.052.
    Photonic time stretch significantly extends the effective bandwidth of existing analog-to-digital convertors by slowing down the input high-speed RF signals. Non-uniform photonic time stretch further enables time bandwidth product reduction in RF signal detection by selectively stretching high-frequency features more. However, it requires the prior knowledge of spectral-temporal distribution of the input RF signal and has to reconfigure the time stretch filter for different RF input signals. Here we propose for the first time an adaptive non-uniform photonic time stretch method based on microwave photonics pre-stretching that achieves blind detection of high-speed RF signals with reduced time bandwidth product. Non-uniform photonic time stretch using both quadratic and cubic group delay response has been demonstrated and time bandwidth product compression ratios of 72% and 56% have been achieved respectively.
  • Liu, X., Wu, W. and Wang, C. (2017). Modified quantum-speed-limit bounds for open quantum dynamics in quantum channels. Physical Review A: Atomic, Molecular and Optical Physics [Online] 95:52118. Available at: http://dx.doi.org/10.1103/PhysRevA.95.052118.
    The minimal evolution time between two distinguishable states is of fundamental interest in quantum physics. Very recently Mirkin et al. argue that some most common quantum-speed-limit (QSL) bounds which depend on the actual evolution time do not cleave to the essence of the QSL theory as they grow indefinitely but the final state is reached at a finite time in a damped Jaynes-Cummings (JC) model. In this paper, we thoroughly study this puzzling phenomenon. We find the inconsistent estimates will happen if and only if the limit of resolution of a calculation program is achieved, through which we propose that the nature of the inconsistency is not a violation to the essence of the QSL theory but an illusion caused by the finite precision in numerical simulations. We also present a generic method to overcome the inconsistent estimates and confirm its effectiveness in both amplitude-damping and phase-damping channels. Additionally, we show special cases which may restrict the QSL bound defined by “quantumness”.
  • Xie, H. et al. (2017). Design of high-efficiency diffractive optical elements towards ultrafast mid-infrared time-stretched imaging and spectroscopy. Journal of Modern Optics [Online]:1-7. Available at: http://dx.doi.org/10.1080/09500340.2017.1386332.
  • Mididoddi, C. et al. (2017). High throughput photonic time stretch optical coherence tomography with data compression. IEEE Photonics Journal [Online]. Available at: https://doi.org/10.1109/JPHOT.2017.2716179.
    Photonic time stretch enables real time high throughput optical coherence tomography (OCT), but with massive data volume being a real challenge. In this paper, data compression in high throughput optical time stretch OCT has been explored and experimentally demonstrated. This is made possible by exploiting spectral sparsity of encoded optical pulse spectrum using compressive sensing (CS) approach. Both randomization and integration have been implemented in the optical domain avoiding an electronic bottleneck. A data compression ratio of 66% has been achieved in high throughput OCT measurements with 1.51 MHz axial scan rate using greatly reduced data sampling rate of 50 MS/s. Potential to improve compression ratio has been exploited. In addition, using a dual pulse integration method, capability of improving frequency measurement resolution in the proposed system has been demonstrated. A number of optimization algorithms for the reconstruction of the frequency-domain OCT signals have been compared in terms of reconstruction accuracy and efficiency. Our results show that the L1 Magic implementation of the primal-dual interior point method offers the best compromise between accuracy and reconstruction time of the time-stretch OCT signal tested.
  • Wang, G. et al. (2016). Highly efficient spectrally encoded imaging using a 45° tilted fiber grating. Optics Letters [Online] 41:2398-2401. Available at: http://dx.doi.org/10.1364/OL.41.002398.
    A novel highly efficient, fiber-compatible spectrally encoded imaging (SEI) system using a 45° tilted fiber grating (TFG) is proposed and experimentally demonstrated for the first time. The TFG serves as an in-fiber lateral diffraction element, eliminating the need for bulky and lossy free-space diffraction gratings in conventional SEI systems. Under proper polarization control, due to the strong tilted reflection, the 45° TFG offers a diffraction efficiency as high as 93.5%. Our new design significantly reduces the volume of the SEI system, improves energy efficiency and system stability. As a proof-of-principle experiment, spectrally encoded imaging of a customer-designed sample (9.6 mm × 3.0 mm) using the TFG-based system is demonstrated. The lateral resolution of the SEI system is measured to be 42 μm in our experiment.
  • Wang, W. et al. (2016). Background-Free Microwave Signal Generation Based on Unbalanced Temporal Pulse Shaping. IEEE Photonics Technology Letters [Online] 28:903-906. Available at: http://dx.doi.org/10.1109/LPT.2016.2517181.
    We propose a new method to generate background-free high-frequency pulsed microwave signal based on an unbalanced temporal optical pulse shaping (TPS) system and balanced photodetection. The proposed system consists of a polarization modulator and an unbalanced TPS system realized by two conjugate dispersion elements. The carrier frequency of the pulsed microwave signal could be tuned by changing the residual dispersion of the TPS. The proposed method is theoretically analyzed and experimentally demonstrated. The experimental results show that the carrier frequency of the generated microwave pulse could be tuned over a broad frequency range. Moreover, the generated microwave pulse signal is background-free by suppressing the baseband frequency components using balanced photodetection.
  • Ahmad, E. et al. (2016). High temporal and spatial resolution distributed fiber Bragg grating sensors using time-stretch frequency-domain reflectometry. IEEE/OSA Journal of Lightwave Technology [Online] 35:3289-3295. Available at: http://dx.doi.org/10.1109/JLT.2016.2605401.
    A novel interrogation technique for fully distributed linearly chirped fiber Bragg grating (LCFBG) strain sensors with simultaneous high temporal and spatial resolution based on optical time-stretch frequency-domain reflectometry (OTS-FDR) is proposed and experimentally demonstrated. LCFBGs is a promising candidate for fully distributed sensors thanks to its longer grating length and broader reflection bandwidth compared to normal uniform FBGs. In the proposed system, two identical LCFBGs are employed in a Michelson interferometer setup with one grating serving as the reference grating whereas the other serving as the sensing element. Broadband spectral interferogram is formed and the strain information is encoded into the wavelength-dependent free spectral range (FSR). Ultrafast interrogation is achieved based on dispersion-induced time stretch such that the target spectral interferogram is mapped to a temporal interference waveform that can be captured in real-time using a single-pixel photodector. The distributed strain along the sensing grating can be reconstructed from the instantaneous RF frequency of the captured waveform. High-spatial resolution is also obtained due to high-speed data acquisition. In a proof-of-concept experiment, ultrafast real-time interrogation of fully-distributed grating sensors with various strain distributions is experimentally demonstrated. An ultrarapid measurement speed of 50 MHz with a high spatial resolution of 31.5 µm over a gauge length of 25 mm and a strain resolution of 9.1 µε have been achieved.
  • Zou, X., Szep, A. and Wang, C. (2015). Microwave Photonics: Deep Interactions between Microwaves and Lightwaves. Optical Engineering [Online] 55:1-2. Available at: http://dx.doi.org/10.1117/1.OE.55.3.031101.
  • Wang, C. (2014). Dispersive Fourier Transformation for Versatile Microwave Photonics Applications. Photonics [Online] 1:586-612. Available at: http://dx.doi.org/10.3390/photonics1040586.
    Abstract: Dispersive Fourier transformation (DFT) maps the broadband spectrum of an ultrashort optical pulse into a time stretched waveform with its intensity profile mirroring the spectrum using chromatic dispersion. Owing to its capability of continuous pulse-by-pulse spectroscopic measurement and manipulation, DFT has become an emerging technique for ultrafast signal generation and processing, and high-throughput real-time measurements, where the speed of traditional optical instruments falls short. In this paper, the principle and implementation methods of DFT are first introduced and the recent development in employing DFT technique for widespread microwave photonics applications are presented, with emphasis on real-time spectroscopy, microwave arbitrary waveform generation, and microwave spectrum sensing. Finally, possible future research directions for DFT-based microwave photonics techniques are discussed as well.
  • Wang, C. (2014). Microwave photonic signal processing using fiber Bragg gratings. Journal of Data Acquisition & Processing:859-873.
    Microwave photonics(MWP)is an emerging issue concering the interaction between microwaves and optical waves.A variety of photonic technologies are investigated to enable and enhance functionalities in microwave systems which are usually very challenging when using pure electrical solutions.A fiber Bragg grating(FBG)is one of the key photonic devices in microwave photonics signal processing due to its unique features such as flexible spectral characteristics,low loss,light weight,compact footprint,and inherent compatibility with other fiber-optic devices.The recent development in FBG-based microwave photonics signal processing applications is discussed,with an emphasis on microwave photonic filters,microwave arbitrary waveform generation,microwave spectrum sensing and real-time FBG sensor interrogation.The limitations and potential solutions of FBG-based MWP system are also discussed.
  • Chen, H. et al. (2013). Ultrafast web inspection with hybrid dispersion laser scanner. Applied Optics [Online] 52:4072-4076. Available at: http://dx.doi.org/10.1364/AO.52.004072.
    We report an ultrafast web inspector that operates at a 1000 times higher scan rate than conventional methods. This system is based on a hybrid dispersion laser scanner that performs line scans at nearly 100 MHz. Specifically, we demonstrate web inspection with detectable resolution of 48.6  μm/pixel (scan direction) ×  23  μm (web flow direction) within a width of view of 6 mm at a record high scan rate of 90.9 MHz. We demonstrate the identification and evaluation of particles on silicon wafers. This method holds great promise for speeding up quality control and hence reducing manufacturing costs.
  • Wang, C. and Yao, J. (2013). Fiber Bragg gratings for microwave photonics subsystems. Optics Express [Online] 21:22868-22884. Available at: http://dx.doi.org/10.1364/OE.21.022868.
    Microwave photonics (MWP) is an emerging filed in which photonic technologies are employed to enable and enhance functionalities in microwave systems which are usually very challenging to fulfill directly in the microwave domain. Various photonic devices have been used to achieve the functions. A fiber Bragg grating (FBG) is one of the key components in microwave photonics systems due to its unique features such as flexible spectral characteristics, low loss, light weight, compact footprint, and inherent compatibility with other fiber-optic devices. In this paper, we discuss the recent development in employing FBGs for various microwave photonics subsystems, with an emphasis on subsystems for microwave photonic signal processing and microwave arbitrary waveform generation. The limitations and potential solutions are also discussed.
  • Wang, C. and Yao, J. (2013). Ultrahigh-Resolution Photonic-Assisted Microwave Frequency Identification Based on Temporal Channelization. IEEE Transactions on Microwave Theory and Techniques [Online] 61:4275-4282. Available at: http://dx.doi.org/10.1109/TMTT.2013.2285094.
    Real-time ultrahigh-resolution microwave frequency identification is paramount for widespread applications, such as communications, radar, and electronic warfare. Photonics-assisted microwave frequency identification can be achieved using an optical channelizer. While this technique enables simultaneous measurement of multiple frequencies, it has poor measurement resolution due to the large channel spacing, which is usually greater than 1 GHz. Here, we introduce a new channelizer-based microwave frequency measurement technique that offers nearly 500 times higher spectral resolution. This method employs largely dispersed broadband optical pulses to encode the time-domain characteristics of the modulating signal to the optical spectral domain. An optical channelizer is employed to slice the spectrum, which is equivalent to performing temporal sampling of the time-domain waveform. The unknown microwave signal is then reconstructed and its spectral distribution is analyzed by a digital processor. To evaluate the proposed technique, frequency measurements of a single-tone, a multiple-tone, and a frequency-hopping microwave signal are demonstrated. A measurement resolution as high as 55 MHz is achieved using an optical channelizer with a channel spacing of 25 GHz.

Book section

  • Wang, C. and Yao, J. (2013). Fiber Bragg Gratings for Microwave Photonics Applications. in: Lee, C. H. ed. Microwave Photonics, second edition. CRC Press, pp. 125-174. Available at: https://www.crcpress.com/Microwave-Photonics-Second-Edition/Lee/p/book/9781466502864.

Conference or workshop item

  • Mididoddi, C. and Wang, C. (2018). Photonic compressive sensing enabled data efficient time stretch optical coherence tomography. in: Podoleanu, A. G. H. and Bang, O. eds. Second Canterbury Conference on Optical Coherence Tomography, 2017, Canterbury, United Kingdom. SPIE. Available at: https://doi.org/10.1117/12.2283035.
    Photonic time stretch (PTS) has enabled real time spectral domain optical coherence tomography (OCT). However, this method generates a torrent of massive data at GHz stream rate, which requires capturing as per Nyquist principle. If the OCT interferogram signal is sparse in Fourier domain, which is always true for samples with limited number of layers, it can be captured at lower (sub-Nyquist) acquisition rate as per compressive sensing method. In this work we report a data compressed PTS-OCT system based on photonic compressive sensing with 66% compression with low acquisition rate of 50MHz and measurement speed of 1.51MHz per depth profile. A new method has also been proposed to improve the system with all-optical random pattern generation, which completely avoids electronic bottleneck in traditional binary pseudorandom binary sequence (PRBS) generators.
    © (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
  • Wang, G. et al. (2017). Wavelength-controlled beam steering for optical wireless transmission using an in-fiber diffraction grating. in: 2017 Conference on Lasers and Electro-Optics.. Available at: http://dx.doi.org/10.1364/CLEO_SI.2017.SF1L.5.
    Passive beam steering for optical wireless transmission based on wavelength tuning using a novel in-fiber diffraction grating featuring compactness, high diffraction efficiency and inherent fiber-compatibility, is proposed and experimentally demonstrated for the first time.
  • Mididoddi, C., Ahmad, E. and Wang, C. (2017). All-optical random sequence generation for compressive sensing detection of RF signals. in: International Topical Meeting on Microwave Photonics (MWP), 2017. IEEE, pp. 1-4. Available at: https://doi.org/10.1109/MWP.2017.8168639.
    Photonic compressive sensing is a promising data compression method and has been successfully applied in high-speed RF signal detection with greatly reduced requirement for receiver bandwidth. A key challenge is due to the electronic bottleneck in high-speed random sequence generation and mixing. In this work, we propose and experimentally demonstrated for the first time all-optical random sequence generation and mixing for compressive sensing detection of RF signals. The technique is based on photonic time stretch involving cascaded Mach-Zehnder Interferometers (MZIs) for spectral domain random mixing. In a proof-of-concept experiment, successful detection of 1 GHz RF signal with 25% compression ratio using only 50 MHz detection bandwidth has been demonstrated。
  • Wang, G. et al. (2017). In-fibre diffraction grating based beam steering for full duplex optical wireless communication. in: International Topical Meeting on Microwave Photonics (MWP), 2017.. Available at: https://doi.org/10.1109/MWP.2017.8168644.
    A novel approach to achieve wavelength controlled optical beam steering using a 45° tilted fiber grating (TFG) for full-duplex indoor optical wireless transmission is proposed and experimentally demonstrated for the first time. The 45° TFG functions as an in-fiber passive diffraction device for wavelength steered light emission and reception, which enables full-duplex optical wireless transmission. The unique advantages of using an in-fiber TFG device for beam steering include high diffraction efficiency, low cost, compactness and inherent compatibility with existing fiber links. In a proof-of-concept experiment, free-space full-duplex transmission over 1.4 m with data rate of 9.6 Gb/s per beam has been demonstrated using 2.4 GHz bandwidth signals.
  • Qin, H. et al. (2017). Theoretical analysis of diffraction grating based on 45°-tilted fiber gratings. in: 2017 Opto-Electronics and Communications Conference (OECC).. Available at: https://doi.org/10.1109/OECC.2017.8114931.
  • Mididoddi, C., Ahmad, E. and Wang, C. (2017). Data-efficient high-throughput fiber Bragg grating sensors using photonic time-stretch compressive sensing. in: European Conferences on Laser and Opto-electronics.. Available at: https://doi.org/10.1109/CLEOE-EQEC.2017.8086896.
    In this paper, we demonstrate the first application of photonic compressive sensing technique in a data-efficient interrogation system for high-throughput distributed FBG sensors. In particular, reconstruction of a wide bandwidth chirped temporal waveform has been achieved using compressive sensing with optical integration. This enables data-compressed high-throughput interrogation of FBG sensors for dynamic non-uniform strain sensing
  • Mididoddi, C. et al. (2017). Photonic time-stretch optical coherence tomography with data compression and improved resolution. in: 2017 Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR). IEEE. Available at: http://dx.doi.org/10.1109/CLEOPR.2017.8118840.
    In this paper, we investigate the reconstruction of non-harmonic tones in data-compressed PTS-OCT based on multiple pulse integration. To the best of our knowledge, this is the first time that frequency reconstruction resolution less than the pulse repetition rate has been demonstrated in PTS-OCT through all-optical compressive sensing, leading to improved depth resolution in OCT measurement.
  • Ahmad, E., Feng, D. and Wang, C. (2017). Analysis on spatial and temporal resolution in photonic time stretch frequency domain reflectometry based fully distributed fiber Bragg grating sensors. in: The 15th International Conference on Optical Communications and Networks (ICOCN 2016).. Available at: http://ieeexplore.ieee.org/document/7875829/.
    We propose and experimentally demonstrate fully distributed strain sensing along the length of a linearly chirped fibre Bragg grating with simultaneously high spatial and temporal resolution based on photonic time stretch frequency domain reflectometry (PTS-FDR). The distributed strain information is reconstructed from the instantaneous RF frequency of a temporal interference waveform via short-time Fourier transform analysis. The utility of the proposed method is characterized by analysing the spatial and temporal resolution obtained. An ultrafast strain measurement at a speed of 50 MHz with a high spatial resolution of 31.5 µm over a gauge length of 25 mm and a strain resolution of 9.1 µε have been achieved.
  • Wang, G. and Wang, C. (2016). Diffraction Limited Optical Time-Stretch Microscopy Using an In-Fibre Diffraction Grating. in: Frontiers in Optics (FiO) 2016.. Available at: http://dx.doi.org/10.1364/FIO.2016.FF2A.5.
    Ultrafast optical time-stretch imaging using a 45° tilted fiber grating as an in-fibre diffraction device is proposed and experimentally demonstrated for the first time featuring high efficiency, complete fiber-compatibility and diffraction-limited lateral resolution.
  • Feng, D., Ahmad, E. and Wang, C. (2016). Ultrafast and High Resolution Crack Detection Using Fully Distributed Chirped Fiber Bragg Grating Sensors. in: The 6th Asia Pacific Optical Sensors Conference (APOS 2016).. Available at: http://dx.doi.org/10.1364/APOS.2016.W4A.47.
    We demonstrate for the first time that photonic time-stretch frequency domain reflectometry (PTS-FDR) enables ultrafast and high spatial-resolution crack detection using fully distributed chirped fiber Bragg grating strain sensors.
  • Wang, G. et al. (2016). A fiber-compatible spectrally encoded imaging system using a 45° tilted fiber grating. in: SPIE Photonics Europe 2016. Spie-Int Soc Optical Engineering. Available at: http://dx.doi.org/10.1117/12.2225140.
    We propose and demonstrate, for the first time to our best knowledge, the use of a 45° tilted fiber grating (TFG) as an infiber lateral diffraction element in an efficient and fiber-compatible spectrally encoded imaging (SEI) system. Under proper polarization control, the TFG has significantly enhanced diffraction efficiency (93.5%) due to strong tilted reflection. Our conceptually new fiber-topics-based design eliminates the need for bulky and lossy free-space diffraction gratings, significantly reduces the volume and cost of the imaging system, improves energy efficiency, and increases system stability. As a proof-of-principle experiment, we use the proposed system to perform an one dimensional (1D) line scan imaging of a customer-designed three-slot sample and the results show that the constructed image matches well with the actual sample. The angular dispersion of the 45° TFG is measured to be 0.054°/nm and the lateral resolution of the SEI system is measured to be 28 μm in our experiment.
  • Habib, U. et al. (2016). Radio over fiber transport of mm-wave 2×2 MIMO for spatial diversity and multiplexing. in: Microwave Photonics (MWP), 2016 International Topical Meeting on. IEEE. Available at: http://dx.doi.org/10.1109/MWP.2016.7791280.
    DWDM-RoF transport and photonic generation of millimeter-wave MIMO signals has been demonstrated. Generation and modulation of independent data streams over different wavelengths provides allocation flexibility and centralization. EVM results show that this low-cost technique provides antenna diversity/multiplexing gain for STBC-Alamouti and Zero-Forcing algorithms based OFDM-MIMO.
  • Mididoddi, C., Wang, G. and Wang, C. (2016). Data Compressed Photonic Time-Stretch Optical Coherence Tomography. in: 2016 IEEE Photonics Conference (IPC).. Available at: http://ieeexplore.ieee.org/document/7830959/.
    Photonic time stretch enables real-time optical coherence tomography, but at the cost of extreme requirement for high-speed signal acquisition and massive data set. This work reports a data compressed real-time Fourier-domain optical coherence tomography based on photonics-assisted compressive sensing. Compression ratio of 66% is achieved.
  • Ahmad, E. et al. (2016). Ultrafast Interrogation of Fully Distributed Chirped Fibre Bragg Grating Strain Sensor. in: 2016 IEEE Photonics Conference (IPC).. Available at: http://ieeexplore.ieee.org/document/7831190/.
    A novel ultrafast and high spatial-resolution interrogation method for fully distributed chirped fibre Bragg grating sensors based on photonic time-stretch frequency-domain reflectometry is presented. Real-time interrogation at measurement speed of 50 MHz with a spatial resolution of 35 µm was experimentally demonstrated.
  • Mididoddi, C., Feng, D. and Wang, C. (2016). All-optical high-speed temporal random pattern generation based on photonic time stretch. in: The 15th International Conference on Optical Communications and Networks (ICOCN 2016).. Available at: http://ieeexplore.ieee.org/abstract/document/7875814/.
    We propose a novel all-optical temporal random pattern generation scheme based on photonic time stretch involving cascaded Mach-Zehnder interferometers (MZIs) with different chirped spectral response. The overall spectral response represents a broadband random spectral pattern. Temporal random patterns can then be generated thanks to photonic time stretch which mirrors spectrum encoding to temporal waveform. Tuning of the generated temporal patterns is achieved using a rapidly tunable optical delay module in one of the MZIs.
  • Ahmad, E., Wang, C. and Stiens, J. (2015). Broadband frequency swept millimeter-wave source based on cascaded temporal optical pulse shaping. in: Microwave Photonics (MWP), 2015 International Topical Meeting on.. Available at: http://dx.doi.org/10.1109/MWP.2015.7356713.
    A new approach to generate high-quality broadband frequency swept millimeter-wave (mm-wave) waveforms based on photonics-assisted frequency up conversion of a low-frequency chirped drive signal has been proposed. This is made possible by using an unbalanced temporal pulse shaping system involving two cascaded Mach-Zehnder modulators both biased at the minimum transmission points. The proposed approach is verified by simulations. A frequency swept mm-wave pulse with its instantaneous frequency spanning from 40 to 100 GHz and a chirp rate of 66.7 GHz/ns is generated thanks to 40 times frequency up conversion. Two-fold improvement on the time-bandwidth product of the generated waveform is also achieved thanks to the cascaded modulation scheme.
  • Wang, C. (2015). Advanced fiber Bragg gratings for microwave photonics applications. in: 14th International Conference on Optical Communications and Networks (ICOCN), 2015. IEEE, pp. 1-3. Available at: http://doi.org/10.1109/ICOCN.2015.7203606.
    Owing to the unique features of flexible spectral characteristics, low loss, light weight, compact footprint, and compatibility with other fiber-optic elements, fiber Bragg gratings (FBGs) have been extensively used in various microwave photonics systems. In this work, the recent development in employing advanced FBGs for various microwave photonics applications are reviewed, with emphasis on microwave arbitrary waveform generation and photonic microwave filtering. An innovative FBG sensor interrogation scheme using microwave photonics techniques is also presented.
  • Ahmad, E. and Wang, C. (2015). Microwave Signal Generation based on Photonics. in: 41st European Conference on Optical Communication (ECOC),.
  • Ahmad, E. and Wang, C. (2015). Photonic microwave frequency up conversion using unbalanced temporal pulse shaping in a cascaded modulation scheme. in: 14th International Conference on Optical Communications and Networks (ICOCN). IEEE, pp. 1-3. Available at: http://doi.org/10.1109/ICOCN.2015.7203730.
  • Wang, C. and Gomes, N. (2014). Photonics-enabled sub-Nyquist radio frequency sensing based on temporal channelization and compressive sensing. in: IEEE Int. Topical Mtg. Microwave Photonics, MWP/APMP 2014. pp. 145-148.
  • Wang, C. and Gomes, N. (2014). Photonics-enabled sub-Nyquist radio frequency sensing based on temporal channelization and compressive sensing. in: 2014 International Topical Meeting on Microwave Photonics (MWP) and the 2014 9th Asia-Pacific Microwave Photonics Conference (APMP),. pp. 335-338. Available at: http://doi.org/10.1109/MWP.2014.6994567.
    A novel approach to sensing broadband radio frequency (RF) spectrum beyond the Nyquist limit based on photonic temporal channelization and compressive sensing is proposed. A spectrally-sparse RF signal with unknown frequencies is modulated onto a highly chirped optical pulse. An optical channelizer slices the modulated pulse spectrum, which is equivalent to temporally sampling the RF waveform thanks to the dispersion-induced wavelength-to-time mapping. This serial-to-parallel conversion avoids the use of a high-speed detector and digitizer. Furthermore, compressive sensing with optical random demodulation is achieved using a spatial light modulator, enabling the system to capture the wideband multi-tone RF signal with a sampling rate far lower than the Nyquist rate. It is demonstrated that the temporal channelization system with a channel spacing of 20 GHz achieves RF spectrum sensing with a high resolution of 196 MHz. With an equivalent sampling rate of only 25 GHz, a 50-GHz broadband two-tone RF signal can be captured and reconstructed by the system thanks to compressive sensing with a compression ratio of 4.
  • Wang, C. (2014). Photonic time stretch channelizer for broadband microwave spectrum sensing. in: 35th Progress in Electromagnetics Research Symposium (PIERS),.
    Microwave spectrum sensing is of critical importance in wireless communications,
    radars and electronic warfare. Measuring high-frequency and broadband RF signals in a high-
    speed and high-resolution while low-cost manner is challenging. Photonically assisted techniques
    have shown superior performance over their electronic counterpart. Particularly, optical spectral
    channelizer based technique has attracted great interest as it enables lost-cost spectrum mea-
    surement for multi-tone microwave signals. One di±culty of this technique is its relatively poor
    measurement resolution, limited by the channel spacing.
    Based on photonic time-stretch technique, which mirrors the spectrum shape of a largely chirped
    optical pulse to its temporal waveform, here we propose to use an optical channelizer to equiva-
    lently sample the modulated optical pulse. Since no high-speed photodetector is required, system
    cost is greatly reduced. More importantly, the method features greatly improved measurement
    resolution, which is more than two orders of magnitude higher than the channel spacing. The
    technique was veriØed by experimental demonstrations. Means to further improve the measure-
    ment resolution and operational bandwidth will be discussed as well
  • Wang, C. (2013). Dispersive Fourier transformation for microwave photonics applications. in: 34th Progress in Electromagnetics Research Symposium (PIERS 2013).
  • Fard, A. et al. (2013). Near-100 MHz Optical Coherence Tomography at 800 nm. in: 1st International Symposium on Optical Coherence Tomography for Non-Destructive Testing (OCT4NDT).