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Author

Lu Liu

Bio: Lu Liu is an academic researcher from Peking University. The author has contributed to research in topics: Visible light communication & Grating. The author has an hindex of 13, co-authored 31 publications receiving 483 citations.

Papers
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Journal ArticleDOI
Te Chen1, Lu Liu1, Bo Tu1, Zhong Zheng1, Weiwei Hu1 
TL;DR: This letter proposes an imaging receiver scheme for an indoor multiple-input-multiple-output (MIMO) visible light communication (VLC), in which a fisheye lens with ultrawide field-of-view is used for high-quality imaging, so it can realize omnidirectional receiving and provide high-spatial diversity for decoding of the MIMO signals.
Abstract: This letter proposes an imaging receiver scheme for an indoor multiple-input-multiple-output (MIMO) visible light communication (VLC), in which a fisheye lens with ultrawide field-of-view is used for high-quality imaging, so it can realize omnidirectional receiving and provide high-spatial diversity for decoding of the MIMO signals. In addition, the fisheye lens projects planar and small-sized images, which means that the integration with compact planar receiver array is feasible. Using the polynomial projection model, the optical intensity on the receiving plane is obtained, which is in accordance with the experimental result and shows that the images of the light-emitting diodes are clearly separated. The simulation results indicate that low correlations of the channel matrix are achieved, so high spectral efficiency is realized with various receiver positions under indoor circumstance. Consequently, this fisheye-lens-based imaging receiver is a potential candidate for high-performance indoor MIMO VLC applications.

73 citations

Journal ArticleDOI
TL;DR: Numerical and experimental results of an arbitrary-ratio 1×2 MMI power splitter, constructed by simply breaking the symmetry of the multimode region, indicate that the original advantages of MMI devices, such as low excess loss, weak wavelength dependence, and large fabrication tolerance are kept.
Abstract: Free choice of splitting ratio is one of the main properties of a power splitter required in integrated photonics, but conventional multimode interference (MMI) power splitters can only obtain a few discrete ratios. This Letter presents both numerical and experimental results of an arbitrary-ratio 1×2 MMI power splitter, which is constructed by simply breaking the symmetry of the multimode region. In the new device, the power splitting ratio can be adjusted continuously from 100∶0 to 50∶50, while the dimension of the multimode section stays in the range of 1.5×(1.8–2.8) μm. The experimental data also indicate that the proposed arbitrary-ratio splitter keeps the original advantages of MMI devices, such as low excess loss, weak wavelength dependence, and large fabrication tolerance.

70 citations

Journal ArticleDOI
TL;DR: A polarization beam splitter assisted by a subwavelength grating (SWG) enables nearly 20-fold beat length reduction for TE, which makes the high extinction ratio (ER) possible and affects the refractive index of the even mode in the coupling region and broadens the bandwidth of the splitter.
Abstract: A polarization beam splitter assisted by a subwavelength grating (SWG) is proposed The SWG enables nearly 20-fold beat length reduction for TE, which makes the high extinction ratio (ER) possible On the other hand, the embedded SWG preferably affects the refractive index of the even mode in the coupling region and broadens the bandwidth of the splitter As a result, the ER of 287 dB (248 dB) for TE (TM) is obtained, while the insertion loss is only 010 dB (011 dB) at the wavelength of 1550 nm The ER is more than 10 dB in the wavelength range of 1450–1625 nm for TE and 1495–1610 nm for TM

67 citations

Journal ArticleDOI
TL;DR: This Letter presents both numerical and experimental results of a polarization-independent directional coupler based on slot waveguides with a subwavelength grating with a fabrication tolerance of ±20 nm for the grating structure and the coupling efficiencies for the two polarizations are both higher than -0.5” dB.
Abstract: This Letter presents both numerical and experimental results of a polarization-independent directional coupler based on slot waveguides with a subwavelength grating. The measured coupling efficiency is 97.4% for TE and 96.7% for TM polarization at a wavelength of 1550 nm. Further analysis shows that the proposed subwavelength grating directional coupler has a fabrication tolerance of ±20 nm for the grating structure and that the coupling efficiencies for the two polarizations are both higher than −0.5 dB (∼89%), exceeding the entire C-band (1525–1570 nm) experimentally.

51 citations

Journal ArticleDOI
TL;DR: This Letter presents both numerical and experimental results of a strip-slot mode converter based on symmetric multimode interference (MMI), which makes full use of the symmetry of the two-fold image of MMI, and its field distribution similarity with a slot waveguide to convert the mode.
Abstract: Optical mode mismatch makes coupling between strip and slot waveguides a tough issue in integrated photonics. This Letter presents both numerical and experimental results of a strip-slot mode converter based on symmetric multimode interference (MMI). Distinct from previous reported converters which gradually convert the mode through sharp tips, the proposed solution makes full use of the symmetry of the two-fold image of MMI, and its field distribution similarity with a slot waveguide to convert the mode. A converter based on this mechanism is able to convert light from a TE-polarized fundamental mode of a strip waveguide to that of a slot waveguide, and vice versa. Strip-slot waveguide coupling though this mode converter has a measured efficiency of 97% (−0.13 dB), and the dimensions are as small as 1.24×6 μm. Further analysis shows that the proposed converter is highly tolerant to fabrication imperfections, and is wavelength-insensitive.

44 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, the state-of-the-art technologies on photonics-based terahertz communications are compared with competing technologies based on electronics and free-space optical communications.
Abstract: This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

1,238 citations

Journal ArticleDOI
TL;DR: In this article, the authors provide a technology overview and a review on optical wireless technologies, such as visible light communication, light fidelity, optical camera communication, free space optical communication, and light detection and ranging.
Abstract: New high-data-rate multimedia services and applications are evolving continuously and exponentially increasing the demand for wireless capacity of fifth-generation (5G) and beyond. The existing radio frequency (RF) communication spectrum is insufficient to meet the demands of future high-data-rate 5G services. Optical wireless communication (OWC), which uses an ultra-wide range of unregulated spectrum, has emerged as a promising solution to overcome the RF spectrum crisis. It has attracted growing research interest worldwide in the last decade for indoor and outdoor applications. OWC offloads huge data traffic applications from RF networks. A 100 Gb/s data rate has already been demonstrated through OWC. It offers services indoors as well as outdoors, and communication distances range from several nm to more than 10 000 km. This paper provides a technology overview and a review on optical wireless technologies, such as visible light communication, light fidelity, optical camera communication, free space optical communication, and light detection and ranging. We survey the key technologies for understanding OWC and present state-of-the-art criteria in aspects, such as classification, spectrum use, architecture, and applications. The key contribution of this paper is to clarify the differences among different promising optical wireless technologies and between these technologies and their corresponding similar existing RF technologies.

338 citations

Journal ArticleDOI
TL;DR: The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored and advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are outlined.
Abstract: The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications, the availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined.

208 citations

Journal ArticleDOI
03 Aug 2018
TL;DR: A comprehensive review of the state of the art in subwavelength silicon structures, including recent applications including broadband waveguide couplers, high-sensitivity evanescent field sensors, low-loss devices for mid-infrared photonics, polarization management structures, spectral filters, and highly efficient fiber-to-chip coupler.
Abstract: Segmenting silicon waveguides at the subwavelength scale produce an equivalent homogenous material. The geometry of the waveguide segments provides precise control over modal confinement, effective index, dispersion and birefringence, thereby opening up new approaches to design devices with unprecedented performance. Indeed, with ever-improving lithographic technologies offering sub-100-nm patterning resolution in the silicon photonics platform, many practical devices based on subwavelength structures have been demonstrated in recent years. Subwavelength engineering has thus become an integral design tool in silicon photonics, and both fundamental understanding and novel applications are advancing rapidly. Here, we provide a comprehensive review of the state of the art in this field. We first cover the basics of subwavelength structures, and discuss substrate leakage, fabrication jitter, reduced backscatter, and engineering of material anisotropy. We then review recent applications including broadband waveguide couplers, high-sensitivity evanescent field sensors, low-loss devices for mid-infrared photonics, polarization management structures, spectral filters, and highly efficient fiber-to-chip couplers. We finally discuss the future prospects for subwavelength silicon structures and their impact on advanced device design.

156 citations

Journal ArticleDOI
TL;DR: In this paper, the authors provide an insight on the activity of the proposed revision of IEEE 802.15.7r1, which targets communication systems that mainly use either image sensors or cameras, known as the optical camera communications (OCC).
Abstract: Wireless technologies based on radio frequencies (RFs) have always dominated other types of wireless technologies up until now. However, the recent proliferation of media-rich smart devices has pushed the RF spectrum usage to its limit. Therefore RF band expansion towards the optical spectrum is imminent in commercial scale. Indeed, the research on wireless communications using the optical spectrum has gained tremendous ground during the past couple of decades and standardised, respectively, by infrared data association for infrared communication and IEEE 802.15.7 for visible light communication. However, only few shortcomings of the IEEE 802.15.7 standard have led to the development of a revised version, called IEEE 802.15.7r1. This article provides an insight on the activity of the proposed revision of IEEE 802.15.7r1. The proposed revision version targets communication systems that mainly use either image sensors or cameras, known as the optical camera communications (OCC). Leveraging the existing infrastructure, OCC systems will be able to provide ubiquitous coverage in both indoors and outdoors. The authors present their survey focusing on the key technology consideration in IEEE 802.15.7r1, current research status, impairments, enhancements and futuristic application scenarios of the OCC systems.

121 citations