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. His research has been funded by Marie-Curie Actions, the Royal Society, University of Kent, and Natural Sciences and Engineering Research Council (NSERC) of Canada.
A list of his grants and funded projects can be viewed here.
The following is a list of his on-going research activities.
High-throughput Spectroscopy and Microscopy for Biomedial Applications
Optical spectroscopy has been one of the most integral tools in scientific research, manufacturing, and medical practice. To investigate rapid transient phenomena such as chemical reactions, phase transitions of thermodynamic systems, and protein dynamics in living cells, fast real-time spectroscopy is highly desired. Unfortunately, conventional spectrometers, which usually rely on optical diffraction devices, such as prism or diffraction gratings, are often too slow to perform single-shot spectroscopic measurements due to the use of a moving component or a line camera with limited refresh rate (typically up to ~10 kHz).
Continuous running ultrafast real-time optical spectroscopy can be implemented based on dispersive Fourier transformation (DFT), which enables pulse-by-pulse spectroscopic measurement by mapping the spectrum of a broadband optical pulse into a time-domain waveform (frequency-to-time conversion) with the help of chromatic dispersion. Update rate of the spectrum measurement is same as the pulse repetition rate, which ranges from tens of MHz to a few GHz, at least four orders of magnitude higher than conventional spectrometers. In addition, by encoding the spatial information (image) of an object into the optical spectrum of an ultrashort pulse using an optical diffraction device (space-to-time conversion), the real-time spectroscopy can be adapted, based on the two-fold conversions, to real-time microscopy for ultrafast and high-throughput optical imaging, which is a very powerful tool in various biomedical applications.
For example, when combined with a microfluidic device, the ultrafast optical microscopy can act as an imaging-based flow-cytometer for ultrafast and high-throughput imaging of individual microparticles . The following video shows the operational principle of the automated flow-through single-cell optical microscope that Dr. Wang and his colleagues developed at UCLA. This technique enables real-time screening of rare metastatic cancer cells in blood, hence holding great promise for noninvasive, low-cost, real-time diagnosis of cancer.
While dispersive Fourier transform-based fast real-time spectroscopy and microscopy have been well investigated in the fiber-optic communications band at ~1550 nm owing to the mature optical transmission and amplification techniques for telecommunications, its practical utility in biological and biomedical applications is very limited due to the strong water absorption and poor image resolution in the 1550 nm band. Recently, dispersive Fourier transform based ultrafast imaging and high-throughput imaging flow cytometry have been demonstrated in the ~800 nm spectral range , to take advantages of larger penetration depths in tissues, a considerable reduction in autofluorescence background noise and better imaging resolution. Also, powerful broadband (femtosecond) Ti:Sapphire lasers are available in this spectral band. One challenge of implementing dispersive Fourier transfor at ~800 nm band is the lack of good dispersive elements with large dispersion-to-loss ratio. Recently, the first implementation of dispersive Fourier transform at 800-nm using a chirped fiber Bragg grating (CFBG) has been demonstrated  (left Figure). Featuring low loss, low cost, and compact footprint, the CFBG is an effective dispersive element for DFT in the industrially and biomedically important spectral range of ~800 nm.
- K. Goda, A. Ayazi, D. R. Gossett, J. Sadasivam, C. K. Lonappan, E. Sollier, A. Fard, S. C. Hur, J. Adam, C. Murray, C. Wang, N. Brackbill, D. Di Carlo and B. Jalali, “High-throughput single-microparticle imaging flow analyzer," Proceedings of the National Academy of Sciences (PNAS) , (2012). link, pdf
- K. Goda, A. Mahjoubfar, C. Wang, A. Fard, J. Adam, D. R. Gossett, A. Ayazi, E. Sollier, O. Malik, E. Chen, Y. Liu, R. Brown, N. Sarkhosh, D. Di Carlo and B. Jalali, “Hybrid dispersion laser scanner,”Scientific Reports, 2, 445 (2012). link, pdf
- C. Wang, K. Goda, M. Ibsen, and B. Jalali, "Dispersive Fourier transformation in the 800 nm spectral range," 2012 Conference on Lasers and Electro-Optics (CLEO). link, pdf
Photonic Microwave Arbitrary Waveform Generation
Microwave photonics is an interdisciplinary area that studies the interaction between microwaves and optical signals. As one important topic in the field of microwave photonics, photonic microwave arbitrary generation has been intensively investigated for numerous scientific and industrial applications, such as in ultrawide-band (UWB), multiple-access communication systems, electronic countermeasures and pulsed radar systems. Microwave arbitrary waveform generation, which is an important topic within the field of microwave photonics, can be usually implemented based on optical pulse shaping using active free-space optical devices, such as a spatial light modulator (SLM), with key advantage of high reconfigurability. These techniques, however, suffer from the difficulties such as complex alignment, high cost and high coupling loss. On the other hand, microwave waveforms can also be generated using pure fiber-optics devices, such as a fiber Bragg grating (FBG), offering the advantages such as simpler structure, lower cost, lower loss, better stability and higher potential for integration . A FBG can be designed to have an arbitrary spectral response in both magnitude and phase, which is essential for microwave arbitrary waveform generation.
Optical spectral shaping of a transform-limited optical pulse followed by the frequency-to-time mapping in a dispersive element has become a promising technique to achieve microwave arbitrary waveform generation , as shown in the above figure(a). By properly designing the response of the optical spectral filters, such as specially designed FBGs, a temporal pulse with the shape identical to the shaped-spectrum is obtained after the mapping process. For example, an ultra-wide band (UWB) pulse can be generated based on this technique . As can be seen from the above figure(b), an optical spectral shaper that consists of an FBG and a tunable optical bandpass filter (TOF) has a spectral response corresponding to a UWB monocycle or doublet pulse. After frequency-to-time mapping in a dispersive element, a UWB monocycle or doublet pulse with a shape that is a scaled version of the shaped spectrum is generated. Various FBG-based optical spectral filters for microwave AWG have been proposed and demonstrated based on this concept [3-4]. In addition, a dispersive element with higher-order dispersion, for example, a nonlinear chirped fiber Bragg grating (NL-CFBG), has also been employed to perform nonlinear frequency-to-time mapping for the generation of chirped microwave pulses .
To further simplify the system configuration, a properly designed linearly chirped fiber Bragg grating (LCFBG) integrating the functionalities of both spectral shaping and wavelength-to-time mapping, has been demonstrated to generate arbitrary-waveform microwave pulses . Most recently, an approach using a spatially-discrete chirped fiber Bragg grating (SD-CFBG) to achieve microwave AWG based on optical pulse shaping was proposed. Compared to the LCFBG-based technique, the SD-CFBG provides one extra feature: the mapped temporal waveform can be further time shifted by the same FBG. Large time-bandwidth product arbitrary microwave waveforms have been generated based on simultaneous spectral slicing, frequency-to-time mapping, and temporal shifting of the input optical pulse in the single SD-CFBG [7-8].
- C. Wang and J. P. Yao, "Advanced fiber Bragg gratings for photonic generation and processing of arbitrary microwave waveforms," 2010 IEEE International Topical Meeting on Microwave Photonics. link, pdf
- C. Wang, F. Zeng, and J. P. Yao, "All-fiber ultrawideband pulse generation based on spectral shaping and dispersion-induced frequency-to-time conversion," IEEE Photonics Technology Letters, 3, 137 (2007). link, pdf
- C. Wang and J. P. Yao, “Photonic generation of chirped microwave pulses using superimposed chirped fiber Bragg gratings,” IEEE Photonics Technology Letters, 11, 882 (2008). link, pdf
- C. Wang and J. P. Yao, “Chirped microwave pulse generation based on optical spectral shaping and wavelength-to-time mapping using a Sagnac loop mirror incorporating a chirped fiber Bragg grating,” Journal of Lightwave Technology, 12, 3336 (2009). link, pdf
- C. Wang and J. P. Yao, “Photonic generation of chirped millimeter-wave pulses based on nonlinear frequency-to-time mapping in a nonlinearly chirped fiber Bragg grating,” IEEE Transactions on Microwave Theory and Techniques, 2, 542 (2008). link, pdf
- C. Wang and J. P. Yao, “Simultaneous optical spectral shaping and wavelength-to-time mapping for photonic microwave arbitrary waveform generation,” IEEE Photonics Technology Letters, 12, 793 (2009). link, pdf
- C. Wang and J. P. Yao, "Large time-bandwidth product microwave arbitrary waveform generation using a spatially discrete chirped fiber Bragg grating," Journal of Lightwave Technology, 11, 1652 (2010). link, pdf
- C. Wang and J. P. Yao, “Phase-coded millimeter-wave waveform generation using a spatially discrete chirped fiber Bragg grating,” IEEE Photonics Technology Letters, 17, 1493 (2012). link, pdf
Photonic Processing of Microwave Signals
Photonic techniques can generate microwave waveforms and signals with very high frequency and large bandwidth. Processing of such microwave signals becomes another difficulty for conventional electronic techniques. It is therefore desirable that the generated high frequency microwave waveforms can be processed in the optical domain as well. A photonic microwave filter is usually applied to process the microwave waveforms. Two different photonic microwave filters have been explored, with one based on a nonuniformly spaced photonic microwave multi-tap delay-line filter using a single spatially-discrete chirped fiber Bragg grating (SD-CFBG)  and the other based on optical filter response to microwave filter response conversion . As an interesting application, compression of frequency-chirped microwave pulses using the developed photonic microwave filters has been demonstrated.
- C. Wang and J. P. Yao, "A nonuniformly spaced microwave photonic filter using a spatially discrete chirped FBG," IEEE Photonics Technology Letters, 25, 1889 (2013). link, pdf
- C. Wang and J. P. Yao, “Chirped microwave pulse compression using a photonic microwave filter with a nonlinear phase response,” IEEE Transactions on Microwave Theory and Techniques,2, 496 (2009). link, pdf
Instantaneous Microwave Frequency Ientification
Instantaneous microwave frequency identification (IMFI) is of critical importance for demanding scientific, industrial and defense applications, such as cosmology, wireless communications, radar and electronic warfare. The key requirements for instantaneous microwave frequency identification include high speed, wide bandwidth, and high measurement resolution. The capability of measuring microwave signals with multiple frequencies is also an important requirement for practical applications. While conventional electronic solutions can achieve a high measurement resolution and large dynamic range, the frequency measurement range is very limited and the measurement speed is low owing to the electronic bottleneck.
On the other hand, microwave frequency measurement based on photonic techniques has been considered a promising solution that can provide instantaneous microwave frequency identification with broader bandwidth and higher speed. One of the most commonly used techniques based on photonics is to measure the microwave frequency by power monitoring based on frequency-to-intensity mapping using an optical filter or a dispersive element where a unique relationship between the microwave frequency and the optical or microwave power is established. A fiber Bragg grating (FBG) based optical filter has been employed to measure instantaneous microwave frequency based on power monitoring .
While this power monitoring based technique offers good frequency measurement resolution, usually smaller than 200 MHz, it falls short in measuring a microwave signal with multiple frequency components. To achieve multiple microwave frequency measurement, an optical channelizer is usually used to map different frequency components into different spaces (channels). The key limitation of this technique is the poor measurement resolution due to large channel spacing of an optical channelizer, usually greater than 1 GHz. Recently, a temporal channelization method has been demonstrated for microwave frequency measurement, which offers 500 times higher spectral resolution than the channel spacing . 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.
- Z. Li, C. Wang, M. Li, H. Chi, X. Zhang and J. P. Yao, “Instantaneous microwave frequency measurement using a special fiber Bragg grating,” IEEE Microwave and Wireless Components Letters, 1, 52 (2011). link, pdf
- C. Wang and J. P. Yao, "Ultrahigh-resolution photonic-assisted microwave frequency identification based on temporal channelization," IEEE Transactions on Microwave Theory and Techniques, 61, 4275 (2013). link, pdf
Advanced Photonic Signal Processing based on Fiber Bragg Gratings (FBGs)
Fiber Bragg gratings (FBGs) have been widely employed in microwave photonics subsystems . Since an FBG can be designed to have an arbitrary spectral response, an FBG can also be employed for advanced microwave photonic signal processing, such as ultrafast photonic temporal integrator. A single apodized uniform FBG has been designed and fabricated to implement a high-order temporal integrator . Accurate and efficient first- and second-order temporal integrations of ultrafast complex-field optical signals (with temporal features as fast as ~2.5 ps) were successfully demonstrated using the fabricated FBG devices.
This photonic integrator is designed by synthesizing its spectral response as opposed to the time domain response. Major limitations of this design include that strict wavelength matching between the input optical signal and the spectral response of the integrators is required and the operational optical bandwidth is typically small. In many of these applications, integration of wide optical bandwidth signals is required. Recently, a passive all-optical intensity integrator whose operation is independent of the optical signal wavelength and bandwidth was reported . The integrator is implemented based on modal dispersion in a multimode waveguide. By controlling the launch conditions of the input beam, the device produces a rectangular temporal impulse response. Consequently, a temporal intensity integration of an arbitrary optical waveform input is performed within the rectangular time window.
- C. Wang and J. P. Yao, "Fiber Bragg gratings for microwave photonics subsystems", Optics Express, 21, 22868 (2013). (Invited Paper) link, pdf
- M. H. Asghari, C. Wang, J. P. Yao and J. Azaña, “High-order passive photonic temporal integrators,” Optics Letters, 8, 1191 (2010). link, pdf
- Z. Tan, C. Wang, E. D. Diebold, N. K. Hon and B. Jalali, "Real-time wavelength and bandwidth-independent optical integrator based on modal dispersion," Optics Express, 13, 14109 (2012). link, pdf
Fourier Transform Optical Pulse Shaping
Fourier synthesis, also called Fourier transform pulse shaping, is the most commonly used technique for coherent ultrashort optical pulse shaping. Fourier transform pulse shaping can be implemented in the frequency domain using an optical spectral filter. In the pulse shaping system, the optical spectral filter is usually located between two complementary dispersive/diffractive devices. A simplified frequency-domain Fourier transform optical pulse shaping system in which a single LCFBG was employed has been demonstrated . The LCFBG in the system was functioning as a spectrum shaper and at the same time as a conjugate dispersive element pair to perform pulse stretching and pulse compression. The use of a single LCFBG guarantees an exact cancellation of the dispersion, making the pulse shaping system have a better pulse shaping accuracy with a simplified structure.Fourier transform pulse shaping can also be implemented in the time domain using a temporal pulse shaping (TPS) system. A conventional TPS system usually consists a pair of dispersive elements with opposite dispersion and an electro-optic modulator placed between the two dispersive elements. At the output of the system, a temporal waveform that is the Fourier transform of the modulation signal applied to the modulator is obtained. Recently, an unbalanced Fourier transform TPS system having a pair of dispersive elements with opposite sign but non-identical in magnitude has been proposed . The entire system can be considered as a conventional balanced TPS system for real-time Fourier transformation followed by a residual dispersive element to achieve a second real-time Fourier transformation. Therefore, high-frequency microwave waveforms can be generated based on continuously tunable frequency multiplication . In addition, if the second dispersive element has higher-roder dispersion (for example, a nonlinearly chirped fiber Bragg grating), a frequency-tunable chirped microwave waveform can be generated using the unbalanced Fourier transform TPS system .
- C. Wang and J. P. Yao, "Fourier transform ultrashort optical pulse shaping using a single chirped fiber Bragg grating," IEEE Photonics Technology Letters,19, 1375 (2009). link, pdf
- C. Wang, M. Li and J. P. Yao, “Continuously tunable photonic microwave frequency multiplication by use of an unbalanced temporal pulse shaping system,” IEEE Photonics Technology Letters, 17, 1285 (2010). link, pdf
- M. Li, C. Wang, W. Li and J. P. Yao, “An unbalanced temporal pulse shaping system for chirped microwave waveform generation,” IEEE Transactions on Microwave Theory and Techniques, 11, 2968 (2010). link, pdf
Complete Characterizaiton of Ultrashort Optical Pulses
Ultrafast optical pulses have found wide applications in various scientific and engineering fields, such as optical communications, medical diagnostics, and direct observation of ultrafast dynamics. Fast and precise characterization of an optical ultrashort pulse is essential to evaluate and improve the performance of an optical system based on ultrafast optics. Linear self-referencing time-domain interferometric technique enables simple (without a known reference pulse) and direct (noniterative) pulse characterization with higher sensitivity. However, since an optical interferometer is usually used to generate two time-delayed interfering optical pulses, the performance of pulse characterization methods is greatly affected by the poor stability of the interferometer due to its inherent high sensitivity to environmental perturbations. Recently, a simple technique for the complete characterization of an ultrashort (subpicosecond) optical pulse based on temporal interferometry without using a discrete optical interferometer has been proposed . An unbalanced temporal pulse shaping (UB-TPS) system was employed to simultaneously generate and temporally stretch two time-delayed replicas of the input optical pulse to be characterized. The method eliminating the need for an optical interferometer, hence featuring better stability, higher adaptability, and single-shot measurement capability.
- C. Wang and J. P. Yao, "Complete characterization of an optical pulse based on temporal interferometry using an unbalanced temporal pulse shaping system," Journal of Lightwave Technology, 5, 789 (2011). link, pdf
Ultrafast Interrogation of Fiber Grating Sensors
Real-time diagnostics of fast-vibrating objects, such as a running aircraft engine, relies on high-speed sensor interrogation systems. Most of the fiber grating sensors are functioning based on wavelength modulation, in which the sensed information is directly encoded as the grating wavelength change.To monitor the wavelength shift of an FBG, various FBG sensor interrogation techniques have been developed, with the maximum interrogation speed of tens of kHz. Temporal-spectroscopy technique using a chirped optical pulse to map the optical spectrum to a temporal waveform has been a promising technique for FBG sensor interrogation in the megahertz regime. By applying chirped pulse compression technique in the temporal-spectroscopy-based FBG interrogation system, both spectral resolution and signal-to-noise ratio can be improved [1-2]. Most recently, to overcome the fundamental tradeoff between the interrogation speed and resolution in a temporal-spectroscopy-based FBG interrogation system and that between the measurement resolution and dynamic range in a dual-wavelength heterodyne-based interrogation system, a novel technique to achieve ultrafast and ultrahigh-resolution interrogation of FBG sensors based on interferometric temporal spectroscopy has been proposed and experimentally demonstrated .
- C. Wang and J. P. Yao, "Superimposed oppositely chirped FBGs for ultrafast FBG sensor interrogation with significantly improved resolution," 2010 OSA Bragg Gratings, Photosensitivity, and Poling Topical Meeting (BGPP). link, pdf
- W. Liu, M. Li, C. Wang, and J. P. Yao, "Real-time interrogation of a linearly chirped fiber Bragg grating sensor with improved resolution and signal-to-noise ratio," Journal of Lightwave Technology, 9, 1239 (2011). link, pdf
- C. Wang and J. P. Yao, “Ultrafast and ultrahigh-resolution interrogation of a fiber Bragg grating sensor based on interferometric temporal spectroscopy,” Journal of Lightwave Technology, 19, 2927 (2011). link, pdf
Photonic Crystal Fiber Devices
Photonic crystal fibers (PCF) guide light by corralling it within a periodic array of microscopic air holes that run along the entire fiber length. The limitations of conventional fiber optics could be overcame by this new type of fiber —for example, by permitting low-loss guidance of light in a hollow core. PCFs are providing numerous important technological and scientific applications spanning many disciplines. Unique transmission mechanism of bandgap photonic crystal fibers by filling the holes with nematic liquid crystal has also been investigated [1,2]. Some active PCF-based devices, such as a PCF Raman amplifiers  and passive PCF devices, such as an optical coupler  and a fiber Bragg grating  have also been demonstrated.
- C. Zhang, G. Kai, Z. Wang, T. Sun, C. Wang, Y. Liu, J. Liu, W. Zhang, S. Yuan, and X. Dong, “Design of tunable bandgap guidance in high-index filled microstructure fibers,” Journal of the Optical Society of America B: Optical Physics., 4, 782 (2006). link, pdf
- C. Zhang, G. Kai, Z. Wang, T. Sun, C. Wang, Y. Liu, W. Zhang, J. Liu, S. Yuan, and X. Dong, "Transformation of a transmission mechanism by filling the holes of normal silica-guiding microstructure fibers with nematic liquid crystal," Optics Letters, 18, 2372 (2005). link, pdf
- Y. Liu, C. Wang, T. Sun, Y. Li, Z. Wang, C. Zhang, G. Kai, X. Dong, “Distributed hybrid-fiber Raman amplifiers with a section of nonlinear microstructured optical fiber”, Microwave and Optical Technology Letters, 11, 2267 (2006). link, pdf
- Z. Wang, G. Kai, Y. Liu, J. Liu, C. Zhang, T. Sun, C. Wang, W. Zhang, S. Yuan, and X. Dong, "Coupling and decoupling of dual-core photonic bandgap fibers," Optics Letters, 19, 2542 (2005). link, pdf
- T. Sun, G. Kai, Z. Wang, C. Wang, C. Zhang, Y. Liu, J. Liu, W. Zhang, S. Yuan, X. Dong, “Multi-wavelength erbium-doped fiber laser based on a microstructure fiber Bragg grating,” Microwave and Optical Technology Letters, 2, 162 (2005). link, pdf