Lu Liu

High-Spatial-Diversity Imaging Receiver Using Fisheye Lens for Indoor MIMO VLCs

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.

Strip-slot waveguide mode converter based on symmetric multimode interference.

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% (-0.13u2009u2009dB), and the dimensions are as small as 1.24×6u2009u2009μm. Further analysis shows that the proposed converter is highly tolerant to fabrication imperfections, and is wavelength-insensitive.

Subwavelength-grating-assisted broadband polarization-independent directional coupler.

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 ±20u2009u2009nm for the grating structure and that the coupling efficiencies for the two polarizations are both higher than -0.5u2009u2009dB (∼89%), exceeding the entire C-band (1525-1570 nm) experimentally.

Arbitrary-ratio 1 × 2 power splitter based on asymmetric multimode interference

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)u2009u2009μm. 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.

High precision two-step calibration method for the fish-eye camera

Fish-eye cameras are widely used on many occasions due to their ultrawide field of view (about 180°). In this paper, we present a high-precision two-step calibration method to calibrate fish-eye cameras. The two steps are the global polynomial projection model fitting and local line-fitting calibration optimization. In the first step, we obtain the projection model of the fish-eye camera and apply a quartic polynomial to fit the projection model over the entire image. In the second step, the fish-eye image is partitioned into several sections and line fitting is adopted in each section in order to further reduce the residual error of the first calibration step. Experiments show that the new method is able to correct the distortion of the real scene image well. In addition, its average reprojection error is 0.15 pixel better than 0.40 pixel of the general projection model described. The reason that higher calibration precision is obtained is that this method not only considers the global projection model of the fish-eye camera but also considers the local characteristics, such as small tangential distortion and asymmetry.

Manipulation of beat length and wavelength dependence of a polarization beam splitter using a subwavelength grating.

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 28.7xa0dB (24.8xa0dB) for TE (TM) is obtained, while the insertion loss is only 0.10xa0dB (0.11xa0dB) at the wavelength of 1550xa0nm. The ER is more than 10xa0dB in the wavelength range of 1450-1625xa0nm for TE and 1495-1610xa0nm for TM.

Robust polarization-insensitive strip-slot waveguide mode converter based on symmetric multimode interference

Strip-slot waveguide mode converters for TE0 have been widely investigated. Here we demonstrate a polarization-insensitive converter numerically and experimentally. The polarization-insensitive performance is achieved by matching the optical field distribution of the 2-fold image of the Multimode Interference (MMI) and the TE0 (TM0) mode of a slot waveguide. The working principle for this MMI-based mode converter is thoroughly analyzed with the quantitatively evaluated optical field overlap ratio that is theoretically derived from the orthonormal relation of eigenmodes. Based on the analysis, the MMI-based polarization-insensitive converters are then simulated and fabricated. The simulation and measurement results indicate that the proposed scheme is a robust design since it is not only polarization-insensitive but also wavelength-insensitive and fabrication-tolerant. Moreover, the mode converter is as small as 1.22 μm × 4 μm while the measured conversion efficiencies are 95.9% for TE0 and 96.6% for TM0. All these excellent properties make the proposed mode converter an ideal solution for coupling light between strip and slot waveguides when both TE and TM polarizations are considered.

Aperture-array acquisition scheme for optical links in atmospheric turbulence

Spatial acquisition is essential for the establishment of atmospheric optical links. The detection probability in the acquisition process can be degraded by atmospheric-turbulence-induced scintillation. We present an aperture-array acquisition scheme to suppress this scintillation noise. The aperture array is composed of N receiving elements, each containing an aperture to receive the optical signal, an optical filter to reject the background radiation, and a charge-coupled device (CCD) to detect the optical signal. The mathematical model of the long-term average detection probability (LTADP) for the aperture-array acquisition is derived based on the lognormal distribution in turbulent atmosphere, when the CCD sample time is shorter than scintillation characteristic time. In this case, the average signal count and the detection probability in the CCD sample time are both random variables; therefore, the probability density of the average signal count needs to be considered and the LTADP can be calculated based on this probability density. The simulation results show that this aperture-array acquisition scheme can suppress scintillation effectively and enhance the LTADP when the one-aperture signal-to-noise ratio is fixed.

Acquisition probability analysis of ultra-wide FOV acquisition scheme in optical links under impact of atmospheric turbulence

Reliable data transmission in optical wireless communication is on the premise of the successful establishment of the optical link. In this paper, we propose an ultra-wide field-of-view (FOV) acquisition scheme, which combines the fisheye lens and Voigt anomalous dispersion optical filter (VADOF) to achieve rapid establishment of wireless optical links. Furthermore, the ultra-wide FOV signal-receiving model for this acquisition scheme is presented to analyze the receiving performance. This acquisition scheme utilizes the fisheye lens to obtain the ultra-wide FOV, not only simplifying the system architecture of the spatial acquisition, but also reducing the acquisition time; a VADOF with ultra-narrow-pass bandwidth is adopted to resist the strong background radiation induced by the ultra-wide FOV. For this ultra-wide FOV acquisition scheme, the mathematical model of long-term average acquisition probability (LTAAP) is derived based on the gamma-gamma (GG) distribution. In an atmospheric turbulence environment, the average signal count and the acquisition probability are both random variables; therefore, the probability density of the average signal count needs to be considered and LTAAP can be calculated based on the GG distribution. Comprehensive analysis and numerical results of the key parameters of this ultra-wide FOV acquisition scheme, such as LTAAP, false-alarm probability, signal-to-noise ratio, incident angle of beam, scintillation index, and acquisition threshold, provide an advantageous basis for the actual spatial acquisition system.

Timing jitter measurement of transmitted laser pulse relative to the reference using type II second harmonic generation in two nonlinear crystals

A new method is proposed and analyzed for measuring the timing jitter of the transmitted pulse relative to the reference pulse using two type II phase-matched nonlinear crystals for second harmonic generation (SHG). The polarizations of the two pulses are exchanged in two crystals and the difference between two detected second harmonic signals can reflect the transmitted jitter. This new method provides a high sensitivity and timing resolution compared with the conventional RF (radio frequency) method. Since the overlapping levels in the two crystals are the same, the final output is zero when there is no time delay between the two pulses. Thus no offset is necessary to be subtracted from the final output and no time delay adjustment is required between the two pulses, compared with the previous optical method using one crystal and two dichroic beamsplitters. The jitter measuring performance is studied theoretically using non-stationary nonlinear wave-coupled equations for type II SHG of two pulses. The theoretical computation and analysis show that the sensitivity and the dynamic range of this new method depend on pulse width, crystal pulses and group velocity difference between two fundamental pulses.