Wing-Ki Liu

Bio: Wing-Ki Liu is an academic researcher from University of Waterloo. The author has contributed to research in topics: Ionization & Laser. The author has an hindex of 23, co-authored 122 publications receiving 2058 citations. Previous affiliations of Wing-Ki Liu include The Chinese University of Hong Kong & University of Hong Kong.

Tunable Passively $Q$ -switched Erbium-Doped Fiber Laser With Carbon Nanotubes as a Saturable Absorber

Abstract: An all-fiber tunable passively Q-switched erbium-doped fiber (EDF) laser is presented. Saturable absorbers are constructed by optically driven deposition of single-wall carbon nanotubes on fiber connectors. A low pump threshold of 11.1 mW is achieved. Self-mode-locking effect is also observed, and it could be suppressed by splicing an extra unpumped EDF into the laser ring cavity. The laser can be tuned by applying axial strain on the fiber Bragg grating which serves as a narrowband external mirror of the cavity.

Simultaneous measurement for strain and temperature using fiber Bragg gratings and multimode fibers

Abstract: An all-fiber sensor capable of simultaneous measurement of temperature and strain is newly presented. The sensing head is formed by a fiber Bragg grating combined with a section of multimode fiber that acts as a Mach-Zehnder interferometer for temperature and strain discrimination. The strain and temperature coefficients of multimode fibers vary with the core sizes and materials. This feature can be used to improve the strain and temperature resolution by suitably choosing the multimode fiber. For a 10 pm wavelength resolution, a resolution of 9.21 mu epsilon in strain and 0.26 degrees C in temperature can be achieved.

Energies and widths of quasibound levels (orbiting resonances) for spherical potentials

Abstract: Various methods for calculating the energies and widths of quasibound levels (orbiting or shape resonances) for spherical potentials are critically compared. A derivation for the previously‐proposed Airy function boundary condition method is presented, and a Weber function boundary condition method for locating resonances which lie above the potential barrier maximum is derived, tested, and found wanting. It is shown that the Weyl m‐function method of Hehenberger et al. [J. Chem. Phys. 65, 4559 (1976)] yields results in essentially exact agreement with the time‐delay maximum method of Le Roy and Bernstein [J. Chem. Phys. 54, 5114 (1971)]. An improved semiclassical method of calculating these resonance widths, suggested by M.S.Child, is presented and shown to be reliable even for levels lying right at a potential barrier maximum.

On the theory of the relaxation matrix and its application to microwave transient phenomena

Abstract: The phenomenological relaxation times T1 and T2 measured in microwave transient experiments are expressed in terms of specific molecular relaxation matrix elements. Conditions for this simple T1, T2 description are also given. The system is described in terms of a kinetic equation for the density matrix, which provides a unified treatment of the microwave transient behavior, spectral line shapes, and double resonance phenomena. Spatial degeneracy of the states and m dependence of T1 and T2 are discussed in detail. The recently developed semiclassical theory of molecular collisions is then applied to obtain expressions for the relevant relaxation times.

Temperature- and phase-independent lateral force sensor based on a core-offset multi-mode fiber interferometer.

Abstract: A novel lateral force sensor based on a core-offset multi-mode fiber (MMF) interferometer is reported. High extinction ratio can be obtained by misaligning a fused cross section between the single-mode fiber (SMF) and MMF. With the variation of the lateral force applied to a short section of the MMF, the extinction ratio changes while the interference phase remains almost constant. The change of the extinction ratio is independent of temperature variations. The proposed force sensor has the advantages of temperature- and phase-independency, high extinction ratio sensitivity, good repeatability, low cost, and simple structure. Moreover, the core-offset MMF interferometer is expected to have applications in fiber filters and tunable phase-independent attenuators.

Graphene photonics, plasmonics, and broadband optoelectronic devices.

Abstract: Graphene has been hailed as a wonderful material in electronics, and recently, it is the rising star in photonics, as well. The wonderful optical properties of graphene afford multiple functions of signal emitting, transmitting, modulating, and detection to be realized in one material. In this paper, the latest progress in graphene photonics, plasmonics, and broadband optoelectronic devices is reviewed. Particular emphasis is placed on the ability to integrate graphene photonics onto the silicon platform to afford broadband operation in light routing and amplification, which involves components like polarizer, modulator, and photodetector. Other functions like saturable absorber and optical limiter are also reviewed.

IEEE transactions on pattern analysis and machine intelligence

Abstract: In the real world, a realistic setting for computer vision or multimedia recognition problems is that we have some classes containing lots of training data and many classes contain a small amount of training data. Therefore, how to use frequent classes to help learning rare classes for which it is harder to collect the training data is an open question. Learning with Shared Information is an emerging topic in machine learning, computer vision and multimedia analysis. There are different level of components that can be shared during concept modeling and machine learning stages, such as sharing generic object parts, sharing attributes, sharing transformations, sharing regularization parameters and sharing training examples, etc. Regarding the specific methods, multi-task learning, transfer learning and deep learning can be seen as using different strategies to share information. These learning with shared information methods are very effective in solving real-world large-scale problems. This special issue aims at gathering the recent advances in learning with shared information methods and their applications in computer vision and multimedia analysis. Both state-of-the-art works, as well as literature reviews, are welcome for submission. Papers addressing interesting real-world computer vision and multimedia applications are especially encouraged. Topics of interest include, but are not limited to: • Multi-task learning or transfer learning for large-scale computer vision and multimedia analysis • Deep learning for large-scale computer vision and multimedia analysis • Multi-modal approach for large-scale computer vision and multimedia analysis • Different sharing strategies, e.g., sharing generic object parts, sharing attributes, sharing transformations, sharing regularization parameters and sharing training examples, • Real-world computer vision and multimedia applications based on learning with shared information, e.g., event detection, object recognition, object detection, action recognition, human head pose estimation, object tracking, location-based services, semantic indexing. • New datasets and metrics to evaluate the benefit of the proposed sharing ability for the specific computer vision or multimedia problem. • Survey papers regarding the topic of learning with shared information. Authors who are unsure whether their planned submission is in scope may contact the guest editors prior to the submission deadline with an abstract, in order to receive feedback.

Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation

Abstract: Black phosphorus (BP), an emerging narrow direct band-gap two-dimensional (2D) layered material that can fill the gap between the semi-metallic graphene and the wide-bandgap transition metal dichalcogenides (TMDs), had been experimentally found to exhibit the saturation of optical absorption if under strong light illumination. By taking advantage of this saturable absorption property, we could fabricate a new type of optical saturable absorber (SA) based on mechanically exfoliated BPs, and further demonstrate the applications for ultra-fast laser photonics. Based on the balanced synchronous twin-detector measurement method, we have characterized the saturable absorption property of the fabricated BP-SAs at the telecommunication band. By incorporating the BP-based SAs device into the all-fiber Erbium-doped fiber laser cavities, we are able to obtain either the passive Q-switching (with maximum pulse energy of 94.3 nJ) or the passive mode-locking operation (with pulse duration down to 946 fs). Our results show that BP could also be developed as an effective SA for pulsed fiber or solid-state lasers.

LEVEL: A computer program for solving the radial Schrödinger equation for bound and quasibound levels

Abstract: This paper describes program LEVEL, which can solve the radial or one-dimensional Schrodinger equation and automatically locate either all of, or a selected number of, the bound and/or quasibound levels of any smooth single- or double-minimum potential, and calculate inertial rotation and centrifugal distortion constants and various expectation values for those levels. It can also calculate Franck–Condon factors and other off-diagonal matrix elements, either between levels of a single potential or between levels of two different potentials. The potential energy function may be defined by any one of a number of analytic functions, or by a set of input potential function values which the code will interpolate over and extrapolate beyond to span the desired range.

Graphene-based passively Q-switched dual-wavelength erbium-doped fiber laser.

Abstract: We demonstrate a compact Q-switched dual-wavelength erbium-doped fiber (EDF) laser based on graphene as a saturable absorber (SA). By optically driven deposition of graphene on a fiber core, the SA is constructed and inserted into a diode-pumped EDF laser cavity. Also benefiting from the strong third-order optical nonlinearity of graphene to suppress the mode competition of EDF, a stable dual-wavelength Q-switching operation has been achieved using a two-reflection peak fiber Bragg grating as the external cavity mirror. The Q-switched EDF laser has a low pump threshold of 6.5mW at 974nm and a wide range of pulse-repetition rate from 3.3 to 65.9kHz. The pulse duration and the pulse energy have been characterized. This is, to the best of our knowledge, the first demonstration of a graphene-based Q-switched laser.