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Research

Chao Wang's research interests lie in the fileds of microwave photonics, ultrafast optics, fiber optics and biomedical photonics for biomedical, defense and communications applications. The following is a list of his past and ongoing research topics.

High-throughput Microscopy

While various novel microscopy techniques that break the diffraction limit of light have been demonstrated, high-throughput microscopy has not received much attention, due to the lack of high-speed and high-quality image sensor. Most recently, a high-throughput real-time imaging technique based on spectral encoding and time-stretch amplifed dispersive Fourier transform has been demonstrated. I am currently working on application of this high-throughput microscopy technique to vairous scientific and medical fields, such as high-throughput imaging, screening and manipulation of rare cells.

 

All-Optical Microwave Arbitrary Waveform Generation (OAWG)

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 [1]. 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 [2-3]. 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 [4]. 

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 [5]. 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. A large time-bandwidth product arbitrary microwave waveform has been generated based on simultaneous spectral slicing, frequency-to-time mapping, and temporal shifting of the input optical pulse in the single SD-CFBG [6].

[1] C. Wang, F. Zeng, and J. P. Yao, "All-fiber ultrawide band pulse generation based on spectral shaping and dispersion-induced frequency-to-time conversion," IEEE Photonics Technology Letters, 3,137 (2007).

[2] 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).

[3] 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,” IEEE/OSA Journal of Lightwave Technology, 12, 3336 (2009).

[4] 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).

[5] 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).

[6] C. Wang and J. P. Yao, "Large time-bandwidth product microwave arbitrary waveform generation using a spatially discrete chirped fiber Bragg grating," IEEE/OSA Journal of Lightwave Technology, 11,1652 (2010).

 

Photonics Processing of Microwave Waveforms and 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) [1] and the other based on optical filter response to microwave filter response conversion [2]. As an interesting application, compression of frequency-chirped microwave pulses using the developed photonic microwave filters has been demonstrated.

[1] C. Wang and J. P. Yao, "Nonuniformly spaced photonic microwave delay-line filter using a spatially discrete chirped fiber Bragg grating," 2011 IEEE International Topical Meeting on Microwave Photonics (MWP2011)

[2] 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).

 

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. The figure below shows a simplified frequency-domain Fourier transform optical pulse shaping system in which a single LCFBG was employed [1]. 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 [2]. 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 [2]. 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 [3].

[1] 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).

[2] 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).

[3] 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, vol. 58, no. 11, pp. 2968-2975, Nov. 2010.

 

Ultrafast Interrogation of Fiber Grating Sensors

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 [3].

[1] 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 (BGPP) Topical Meeting.

[2] 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," IEEE/OSA Journal of Lightwave Technology, 9, 1239 (2011).

[3] C. Wang and J. P. Yao, “Ultrafast and ultrahigh-resolution interrogation of a fiber Bragg grating sensor based on interferometric temporal spectroscopy,” IEEE/OSA Journal of Lightwave Technology, 19, 2927 (2011).

 

Complete Characterization of Ultrafast 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 [1]. 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, which features better stability, higher adaptability, and single-shot measurement capability.

[1] C. Wang and J. P. Yao, "Complete characterization of an optical pulse based on temporal interferometry using an unbalanced temporal pulse shaping system," IEEE/OSA Journal of Lightwave Technology, 5, 789 (2011).

 

Photonic Crystal Fiber (PCF) 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 [3] and passive PCF devices, such as an optical coupler [4] and a fiber Bragg grating [5] have also been demonstrated.

[1] 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 microstructured fibers,” Journal of the Optical Society of America B: Optical Physics., 4, 782 (2006).

[2] 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).

[3] 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).

[4] 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).

[5] 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).