Showing papers in "IEEE Photonics Technology Letters in 2016"

Non-Orthogonal Multiple Access for Visible Light Communications

Abstract: The main limitation of visible light communication (VLC) is the narrow modulation bandwidth, which reduces the achievable data rates. In this letter, we apply the non-orthogonal multiple access (NOMA) scheme to enhance the achievable throughput in high-rate VLC downlink networks. We first propose a novel gain ratio power allocation (GRPA) strategy that considers the users’ channel conditions to ensure efficient and fair power allocation. Our results indicate that the GRPA significantly enhances the system performance compared with the static power allocation. We also study the effect of tuning the transmission angles of the light emitting diodes and the field of views of the receivers, and demonstrate that these parameters can offer new degrees of freedom to boost the NOMA performance. The simulation results reveal that NOMA is a promising multiple access scheme for the downlink of VLC networks.

High Bandwidth GaN-Based Micro-LEDs for Multi-Gb/s Visible Light Communications

Abstract: Gallium-nitride (GaN)-based light-emitting diodes (LEDs) are highly efficient sources for general purpose illumination. Visible light communications (VLC) uses these sources to supplement existing wireless communications by offering a large, licence-free region of optical spectrum. Here, we report on progress in the development of micro-scale GaN LEDs (micro-LEDs), optimized for VLC. These blue-emitting micro-LEDs are shown to have very high electrical-to-optical modulation bandwidths, exceeding 800 MHz. The data transmission capabilities of the micro-LEDs are illustrated by demonstrations using ON–OFF-keying, pulse-amplitude modulation, and orthogonal frequency division multiplexing modulation schemes to transmit data over free space at the rates of 1.7, 3.4, and 5 Gb/s, respectively.

Modulation Format Identification in Coherent Receivers Using Deep Machine Learning

Abstract: We propose a novel technique for modulation format identification (MFI) in digital coherent receivers by applying deep neural network (DNN) based pattern recognition on signals’ amplitude histograms obtained after constant modulus algorithm (CMA) equalization. Experimental results for three commonly-used modulation formats demonstrate MFI with an accuracy of 100% over a wide optical signal-to-noise ratio (OSNR) range. The effects of fiber nonlinearity on the performance of MFI technique are also investigated. The proposed technique is non-data-aided (NDA) and avoids any additional hardware on top of standard digital coherent receiver. Therefore, it is ideal for simple and cost-effective MFI in future heterogeneous optical networks.

Graphene Mode-Locked Fiber Laser at 2.8 $\mu \text{m}$

Abstract: Mid-infrared erbium (Er3+)-doped ZrF4–BaF2–LaF3–AlF3–NaF fiber laser mode-locked by multilayer graphene saturable absorber was demonstrated. Mode-locked pulses at 2.8 $\mu \text{m}$ with an average output power of 18 mW at a repetition rate of 25.4 MHz, corresponding to a pulse energy of 0.7 nJ, were obtained. The pulsewidth was measured to be $\sim 42$ ps by a home-made autocorrelator. Our experiment has validated the mode-locking capability of graphene in the 3- $\mu \text{m}$ wavelength region.

A New Type Circular Photonic Crystal Fiber for Orbital Angular Momentum Mode Transmission

Abstract: We proposed a new circular photonic crystal fiber (C-PCF), which can support 14 orbital angular momentum (OAM) modes transmission, with the good features of wide bandwidth, low confinement loss, and all OAM modes at the same size. At 1.55 μm, the designed C-PCF has a very low confinement loss of 3.434 × 10 -9 dB/m for HE41 mode and a relatively low nonlinear coefficient of 3.979 W -1 km -1 for EH 31 mode. The common bandwidth for the four orders of OAM modes is as large as 560 nm (about 1.25 μm-1.81 μm), which does cover all bands of optical fiber communication. Flat dispersion (a total dispersion variation of <;46.38 ps nm -1 km -1 over a 750-nm bandwidth from 1.25 μm to 2 μm for TE 01 mode) is another feature. With all these good features, the proposed C-PCF could be a well-promising OAM fiber for mode division multiplexing in high capacity fiber communication systems.

FSO Detection Using Differential Signaling in Outdoor Correlated-Channels Condition

Abstract: In this letter, we introduce a detection technique based on differential signaling scheme for outdoor free space optical communications. This method requires no channel state information (CSI) and does not suffer from the computational load compared with the conventional receivers, where adjusting dynamically the detection threshold level (either based on CSI knowledge, or by using pilots) leads to increased computational time and reduced link throughput. This letter shows that the performance of the proposed technique only depends on the correlation of propagating optical beams. Especially under highly correlated-channels condition the fluctuation of the detection threshold level in the receiver is significantly small. We also show experimentally that under weak turbulence regime the variance of detection threshold level reduces for the correlated channel.

Nonlinearity Mitigation Using a Machine Learning Detector Based on $k$ -Nearest Neighbors

Abstract: A powerful machine learning detector based on the k-nearest neighbors (KNN) algorithm is proposed to overcome system impairments. The zero-dispersion link (ZDL), dispersion managed link (DML), and dispersion unmanaged link (DUL) are considered. Meanwhile, an improved algorithm, the distance-weight KNN, is introduced, which outperforms the conventional maximum likelihood-post compensation approach. The numerical results show that KNN is feasible for overcoming various impairments, especially for non-Gaussian symmetric noise, such as laser phase noise and nonlinear phase noise in the ZDL or DML.

LiNbO 3 Thin-Film Modulators Using Silicon Nitride Surface Ridge Waveguides

Abstract: Lithium niobate (LiNbO3) is an excellent electrooptic material due to its low loss, large electrooptic coefficient, linear modulation response, and large modulation bandwidth. In this letter, we present the first LiNbO3 thin-film Mach–Zehnder optical modulator that employs silicon nitride surface ridge optical waveguides. The modulator contains a 1.2-cm-long push–pull phase modulation section and a pair of multimode interferometric 3-dB couplers. It demonstrated $V_{{\pi }} \cdot {L}$ of $\sim 3\text {V}\cdot \text {cm}$ and a 3-dB bandwidth of $\sim 8$ GHz.

Broadband Tunable Polarization Converter Realized by Graphene-Based Metamaterial

Abstract: We present a broadband tunable polarization converter in terahertz (THz) frequency based on a hybrid metamaterial comprising I-shaped metallic resonators and double layer graphene sheets. The proposed device can dynamically switch its functionalities among linear-to-linear, linear-to-circular, and linear-to-elliptical polarization conversion in broadband by electrically controlling the Fermi energy of the graphene sheets without reoptimizing and refabricating the structures. This device concept offers a further step in developing broadband tunable polarizers and polarization switchers and has potentially applications in THz communications, sensing and spectroscopy.

High Sensitivity Twist Sensor Based on Helical Long-Period Grating Written in Two-Mode Fiber

Abstract: We demonstrate the fabrication of the helical long-period grating (HLPG) in a two-mode fiber (TMF) with CO2 laser. The torsion characteristics of the helical gratings are experimentally investigated. It was found that the resonance wavelength of the TMF-HLPG linearly shifts with a twist sensitivity of 0.47 nm/(rad/m), which is an order magnitude higher than that of the conventional long-period fiber gratings (LPFGs). The temperature sensitivity of the TMF-HPFG was measured to be 23.9 pm/°C, which is one third of that of the conventional LPFGs. The proposed TMF-HLPG-based twist sensor would have a promising application for the twist monitor.

Mid-Infrared Silicon-on-Insulator Fourier-Transform Spectrometer Chip

Abstract: Mid-infrared absorption spectroscopy is highly relevant for a wide range of sensing applications. In this letter, we demonstrate a Fourier-transform spectrometer chip based on the principle of spatial heterodyning implemented in the silicon-on-insulator waveguide platform, and operating near 3.75- $\mu \text{m}$ wavelength. The spectrometer comprises a waveguide splitting tree feeding to an array of 42 Mach–Zehnder interferometers with linearly increasing optical path length differences. A spectral retrieval algorithm based on calibration matrices is applied to the stationary output pattern of the array, compensating for any phase and amplitude errors arising from fabrication imperfections. A spectral resolution below 3 nm is experimentally demonstrated.

Highly Sensitive Plasmonic Photonic Crystal Temperature Sensor Filled With Liquid Crystal

Abstract: A novel highly sensitive surface plasmon resonance-based liquid crystal (LC) photonic crystal fiber (PCF) temperature sensor is presented and studied. Through this letter, the coupling characteristics between the core guided mode inside the PCF core infiltrated with nematic LC and surface plasmon mode on the surface of nano gold wire are studied in detail. The structural geometrical parameters of the proposed design, such as number of metal rods, core diameter, and metal rod diameter are optimized to achieve highly temperature sensitivity. The suggested sensor of compact device length of 20 $\mu \text{m}$ proved to surpass the recent sensors with high sensitivity of 10 nm/°C. The results are calculated using a full-vectorial finite-element method with irregular meshing capabilities and perfectly matched layer boundary conditions.

Novel Visible Light Communication Approach Based on Hybrid OOK and ACO-OFDM

Abstract: In this letter, we present a novel hybrid intensity modulation and direct detection communication system, which integrates asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and on-off keying (OOK) modulation schemes. First, the negative ACO-OFDM is proposed based on the conventional ACO-OFDM system to accommodate the on case with direct current in the OOK modulation, while the ACO-OFDM can match the off case in the OOK modulation. Therefore, the novel hybrid system combines the ACO-OFDM and OOK modulation schemes, while the signals can be recovered at the receiver to support the different qualities of service with high spectral efficiency and can be adapted to various receivers with different complexities. Simulation results are reported for the visible light channel and show that both the ACO-OFDM and OOK signals can be well recovered.

On the Performance of Adaptive MIMO-OFDM Indoor Visible Light Communications

Abstract: In this letter, we propose an adaptive indoor multiple input and multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) visible light communication (VLC) system using a receiver module with angular diversity. In order to improve the capacity of indoor MIMO-OFDM VLC systems, tilted receivers are utilized to increase channel diversity, thus reducing channel correlation. With the help of singular value decomposition-based technique, which decomposes the MIMO VLC channels into independent parallel sub-channels, adaptive resource allocation, namely, bit and power loading, is used for these sub-channels to further improve the proposed system’s capacity. Based on a $4\times 4$ indoor MIMO-OFDM VLC system, we investigate bit error rate (BER) performance of the proposed adaptive system with different polar angles in two typical indoor scenarios. Numerical simulation results show that with 50-MHz modulation bandwidth, average BER can be improved from $4.97\times 10^{\mathrm {-3}}$ to $1.66\times 10^{\mathrm {-5}}$ and from $1.90\times 10^{\mathrm {-3}}$ to $1.59\times 10^{\mathrm {-6}}$ for the two scenarios, respectively.

On the Limits of Digital Back-Propagation in Fully Loaded WDM Systems

Abstract: We investigate upper bounds for single-channel and multi-channel digital back-propagation (BP) in fully loaded wavelength-division multiplexed systems. Using the time-domain model for nonlinear interference noise, we expand previous estimates of BP gains to accurately cover a wide range of system configurations, including a variety of modulation formats from quadrature phase-shift keying to 256-ary quadrature amplitude modulation. In typical scenarios, the potential benefit of single-channel BP is limited to $\sim 0.5$ dB in terms of the peak signal-to-noise ratio, and to $\sim 1$ and $\sim 1.2$ dB in the case of joint three- and five-channel BP. The additional gain from increasing the number of jointly back-propagated channels beyond five is limited to $\sim 0.1$ dB per additional back-propagated channel. We also study the role of BP for receivers that separately compensate for nonlinear phase and polarization rotation noise and show that while the additional gain provided by BP does not change significantly in long-haul systems, it holds the promise of being notably higher in short-reach ultra-high-capacity systems.

Dual-Wavelength Soliton Mode-Locked Fiber Laser With a WS 2 -Based Fiber Taper

Abstract: Recently, few-layer WS2, as a novel two-dimensional (2D) material, has been discovered to possess both the saturable absorption effect and the huge nonlinear refractive index. In experiment, by taking advantage of the unique optical properties of 2D WS2, we fabricated a highly nonlinear photonic device using the pulsed laser beam to deposit the WS2 film onto a fiber taper. With the WS2-based fiber taper, we have demonstrated the single- and dual-wavelength soliton pulses in the erbium-doped fiber laser (EDFL) by properly adjusting the pump strength and the polarization state. According to the soliton theory, the pulse width is $\sim 220$ fs for the single-wavelength soliton, and $\sim 585$ and $\sim 605$ fs for the dual-wavelength soliton, respectively. The dual-wavelength soliton fiber laser exhibits the maximum output power of 10.1 mW and the pulse energy of $\sim 1.14$ nJ, when the pump power is increased to $\sim 420$ mW. Our findings suggest that WS2-based fiber taper could operate as both an excellent saturable absorber for obtaining a femtosecond pulse and a promising highly nonlinear photonic material for the multi-wavelength generation.

Sensitivity Gains by Mismatched Probabilistic Shaping for Optical Communication Systems

Abstract: Probabilistic shaping of quadrature amplitude modulation (QAM) is used to enhance the sensitivity of an optical communication system. Sensitivity gains of 0.43 and 0.8 dB are demonstrated in back-to-back experiments by the shaping of 16QAM and 64QAM, respectively. Furthermore, numerical simulations are used to prove the robustness of probabilistic shaping to a mismatch between the constellation used and the signal-to-noise ratio (SNR) of the channel. It is found that, accepting a 0.1-dB SNR penalty, only four shaping distributions are required to support these gains for 64QAM.

A Novel Design of Broadband Terahertz Metamaterial Absorber Based on Nested Circle Rings

Abstract: A terahertz metamaterial absorber (MA) with properties of broadband width, polarization-insensitive, wide angle incidence is presented. Different from the previous methods to broaden the absorption width, this letter proposes a novel combinatorial way which units a nested structure with multiple metal-dielectric layers. We numerically investigate the proposed MA, and the simulation results show that the absorber achieves a broadband absorption over a frequency range of 0.896 THz with the absorptivity greater than 90%. Moreover, the full-width at half maximum of the absorber is up to 1.224 THz which is 61.2% with respect to the central frequency. The mechanism for the broadband absorption originates from the overlapping of longitudinal coupling between layers and coupling of the nested structure. Importantly, the nested structure makes a great contribution to broaden the absorption width. Thus, constructing a nested structure in a multi-layer absorber may be considered as an effective way to design broadband MAs.

A Novel Low-Loss Diamond-Core Porous Fiber for Polarization Maintaining Terahertz Transmission

Abstract: We report on the numerical design optimization of a new kind of relatively simple porous-core photonic crystal fiber (PCF) for terahertz (THz) waveguiding. A novel twist is introduced in the regular hexagonal PCF by including a diamond-shaped porous-core inside the hexagonal cladding. The numerical results obtained from an efficient finite-element method, which confirms a high birefringence of the order $10^{\mathrm {-2}}$ and low effective material loss of 0.07 cm $^{\mathrm {-1}}$ at 0.7-THz operating frequency. The proposed PCF is anticipated to be useful in polarization sensitive THz appliances.

A Novel Low Loss, Highly Birefringent Photonic Crystal Fiber in THz Regime

Abstract: We present a new kind of dual-hole unit-based porous-core hexagonal photonic crystal fiber (H-PCF) with low loss and high birefringence in terahertz regime. The proposed fiber offers simultaneously high birefringence and low effective material loss (EML) in the frequency range of 0.5–0.85 THz with single-mode operation. An air-hole pair is used inside the core instead of elliptical shaped air holes to introduce asymmetry for attaining high birefringence; where the birefringence can be enhanced by rotating the dual-hole unit axis of orientation. The proposed H-PCF provides a birefringence of $\sim 0.033$ and an EML of 0.43 dB/cm at an operating frequency of 0.85 THz.

Design of a Five-Band Terahertz Absorber Based on Three Nested Split-Ring Resonators

Abstract: A novel five-band terahertz metamaterial absorber based on three nested split-ring resonators is proposed. It is found that the structure has five distinctive absorption bands whose peaks are average over 99%. Different from previous reports by combing the resonances of the complex structure to obtain the multi-band responses, the proposed five-band absorber uses the hybrid of the LC resonance and dipolar response of the patterned structure, thus making the proposed structure quite easy to be fabricated. Furthermore, the sensing performance of the absorber is analyzed in terms of the over layer and the surrounding index.

Theoretical and Experimental Analysis of $\Phi $ -OTDR Based on Polarization Diversity Detection

Abstract: Statistical characteristics of location related intensity distribution in phase-sensitive optical time-domain reflectometry ( $\Phi $ -OTDR) are theoretically analyzed and experimentally demonstrated. The polarization diversity scheme is adopted into the $\Phi $ -OTDR with a coherent detection scheme to effectively mitigate the polarization fading effect. A real distributed sensing application for simultaneous multi-events vibration detection can thus be achieved along each position of the sensing fiber with more reliable vibration detection results. The experiment shows that the signal-to-noise ratio of two vibration signals is simultaneously enhanced by 10.9 and 8.65 dB compared with the results obtained by a conventional coherent detection scheme.

Bidirectional Soft Silicone Curvature Sensor Based on Off-Centered Embedded Fiber Bragg Grating

Abstract: A compact bidirectional soft curvature sensor is achieved by embedding a fiber Bragg grating (FBG) off-centered in a silicone sheet. The proposed approach is capable to distinguish both the bending curvatures and bending directions through wavelength shift measurement of the FBG. Based on the pure bending model, the relationship between the FBG embedded position and the sensor sensitivity is studied and verified experimentally. Real-time curvature measurement of both positive and negative bending directions is experimentally achieved. The curvature sensor has a consistent performance and the results are highly repeatable with small variances. A linear relationship between applied curvature and FBG wavelength shift is obtained, with a large measurement range of up to $\pm 80~\text{m}^{-1}$ curvature.

All PM Fiber Laser Mode Locked With a Compact Phase Biased Amplifier Loop Mirror

Abstract: An Yb-doped, all-polarization-maintaining fiber laser oscillator with a nonlinear amplifying loop mirror is presented in this letter. A new compact non-reciprocal phase shifter biased fiber loop mirror was engaged inside the fiber laser, which results in a low threshold and stable mode locking. With a 2.8-nm filter inside the cavity, the mode locked output power is 4.1 mW with the repetition rate of 31.35 MHz at the pump power of 80 mW. The spectral bandwidth of the pulse is 3.1 nm. The pulsewidth of the direct output and the dechirped pulse are 2.13 ps and 538 fs, respectively. The filter bandwidth plays a significant role in the spectrum bandwidth and the direct output pulse width.

Temperature Sensor Based on Fiber Ring Laser With Sagnac Loop

Abstract: In this letter, a temperature sensor based on a fiber ring laser with a reflective Sagnac loop is proposed. The reflective Sagnac loop inserted into the fiber ring laser through a downlead optical fiber is used as the filter and sensing head to supply high temperature sensitivity. As the temperature varies, the transmission spectrum of Sagnac loop changes, leading to the shift of emission wavelength of a fiber ring laser. We obtain a temperature sensor system with a high temperature sensitivity of 1.739 nm/°C, a narrow 3-dB bandwidth less than 0.05 nm, and a high signal-to-noise ratio $\sim 50$ dB. Moreover, it is convenient to achieve the remote sensing by changing the length of the downlead optical fiber.

Terahertz Biosensing Based on a Polarization-Insensitive Metamaterial

Abstract: A polarization-insensitive metamaterial biosensor composed of metallic square ring resonators is proposed and is shown to be especially suitable to work together with the linear polarized femtosecond laser pumped terahertz time-domain spectroscopy. A symmetric slit is distributed on every side of the ring to render the biosensor insensitive to the polarization direction of the incident terahertz wave. The proposed structure is analyzed with the finite-element method and investigated experimentally. The results reveal that a minimal resolution of 17.7 $\mu $ mol/L in the detection of bovine serum albumin is obtained when the biosensor is oriented arbitrarily in the plane containing the electric field vector.

Modified fs-Laser Inscribed FBG Array for Rapid Mode Shape Capture of Free-Free Vibrating Beams

Abstract: We report on the development of a multiplexed sensor array of fiber Bragg gratings (FBGs), inscribed using a femtosecond laser, and its demonstration as a quasi-distributed sensor to capture the mode shapes, through surface displacement sampling, of a free-free vibrating beam. Our method is based on a plane-by-plane inscription approach, whereby a 2-D index change is written across the fiber core, controlling the width and depth of the modified region. This allows for the fast inscription of multiple wavelength FBGs in coated optical fibers, with the advantage of less stringent alignment requirements. The FBGs are multiplexed in the wavelength domain using a high-speed demodulator, using a fast, custom-made computational algorithm. We recover rapid and single-step wavelength- and time-dependent displacement information, extracting the first two mode shapes of a vibrating beam and their respective degrees of freedom resonance frequencies in $ s.

Ultrasensitive Refractive-Index Sensors Based on Tapered Fiber Coupler With Sagnac Loop

Abstract: A kind of compact and highly sensitive refractive-index sensor based on reflective tapered fiber coupler is proposed. The sensing structure is fabricated by fusing and tapering a twisted optical fiber. The achieved maximum sensitivity is 3617 nm/RIU for measuring refractive index ranging from 1.33 to 1.41. Comparing with other similar reflective devices, the transmission loss is relatively low ( $\sim 2$ dB), which is assigned to the employed Sagnac loop mirror to enhance the reflection of the system. Besides the intrinsic advantages of the reflective configuration, the proposed sensing structure has the additional advantages of compactness, low-cost, and ultrahigh sensitivity. It is also promising and cost effective for other all-fiber device applications.

Realization of Ultra-Accurate and Compact All-Optical Photonic Crystal OR Logic Gate

Abstract: By unique combination of single-line defects, Mach-Zehnder interferometer, and ring resonators, we propose a novel, easy-to-fabrication and linear scheme for realizing accurate and compact all-optical OR logic gate based on 2-D photonic crystal structures. To obtain the transmission spectra of the structure, we considered the entire structure as a single cavity and then we applied a fast Fourier transform to the transmitted spectral power densities at the output port and the unexcited input port; this strategy enhances the control on unwanted port-to-port power reflections and hence increasing the distinction between logic levels. Simulating the electromagnetic wave propagation in the proposed device reveals that the accurate all-optical OR logic gate is obtained. The achieved contrast ratio between logic levels is $\sim 7.27$ dB. Moreover, since all ports are located on the same side and the size of the structure is $12.5~\mu \text{m} \times 16~\mu \text{m}$ . It is fully compacted and has the ability of dense integration. The proposed structure is completely easy-to-fabrication and designed for using in future high accuracy photonic integrated circuits.

Design of an Efficient Pumping Scheme for Mid-IR Dy 3+ :Ga 5 Ge 20 Sb 10 S 65 PCF Fiber Laser

Abstract: This letter illustrates the design of a novel medium infrared (Mid-IR) laser based on a photonic crystal fiber made of dysprosium-doped chalcogenide glass, Dy3+:Ga5Ge20Sb10S65. In order to perform a realistic investigation, the simulation is performed by taking into account the spectroscopic parameters measured on the rare earth-doped glass sample. The simulated results show that an optical beam emission close to 4400-nm wavelength can be obtained by employing two pump beams at 2850 nm (pump #1) and 4092 nm (pump #2) wavelengths. The pump beams can be provided by commercial quantum cascade lasers. As example, for the pump powers of 50 mW (pump #1) and 1 W (pump #2), the input mirror reflectivity of 99%, the output mirror reflectivity of 30%, and the optical cavity length of 50 cm, a signal power close to 350 mW at the wavelength of 4384 nm can be generated. This result indicates that the designed source configuration is feasible for high beam quality Mid-IR light generation and it is efficient enough to find applications in optical free propagation links, optical remote sensing, and medicine.