IEEE Photonics Technology Letters

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-em... View full abstract»

An intelligent eye-diagram analyzer is proposed to implement both modulation format recognition (MFR) and optical signal-to-noise rate (OSNR) estimation by using a convolution neural network (CNN)-based deep learning technique. With the ability of feature extraction and self-learning, CNN can process eye diagram in its raw form (pixel values of an image) from the perspective of image processing, w... View full abstract»

This letter presents a visible light communication (VLC) system based on a single 50- μm gallium nitride light emitting diode (LED). A device of this size exhibits a 3-dB modulation bandwidth of at least 60 MHz - significantly higher than commercially available white lighting LEDs. Orthogonal frequency division multiplexing is employed as a modulation scheme. This enables the limited modula... View full abstract»

Millimeter-wave communications may be harnessed to help meet future 5G network bandwidth requirements, but the generation of these frequencies in the electronic domain can be difficult and costly. Photonic techniques, such as optical heterodyning, can be exploited to generate millimeter-wave frequencies in a manner which is efficient and compatible with fiber transport infrastructure. In this lett... View full abstract»

We report on an optical chip-to-chip interconnect solution, thereby demonstrating plasmonics as a solution for ultra-dense, high-speed short-reach communications. The interconnect comprises a densely integrated plasmonic Mach-Zehnder modulator array that is packaged with standard driving electronics. On the receiver side, a germanium photodetector array is integrated with trans-impedance amplifier... View full abstract»

The coupling of optical waveguides to ring resonators holds the promise of a new generation of switches (modulators) which employ orders of magnitude smaller switching (modulation) voltages (or control intensities). This requires a means for voltage (or intensity) control of the coupling between the waveguide and the resonator. Schemes for achieving such control are discussed. View full abstract»

We report high-power 1180-nm GaInNAs distributed Bragg reflector laser diodes with and without a tapered amplifying section. The untapered and tapered components reached room temperature output powers of 655 mW and 4.04 W, respectively. The diodes exhibited narrow linewidth emission with side-mode suppression ratios in the range of 50 dB for a broad range of operating current, extending up to 2 A ... View full abstract»

We demonstrated a simple single-exposure method of fabricating phase-shifted fiber Bragg gratings (PSFBGs), in which a non-photosensitive SMF-28 fiber was irradiated by a femtosecond laser beam passing through a uniform period phase mask. The phase-shifted structure in the FBG was formed by shielding a small region of the incident femtosecond laser beam during its irradiation. The fabricated PSFBG... View full abstract»

Non-invasive integrated detectors, named contactless integrated photonic probe (CLIPP), are employed to demonstrate on-chip noise-independent power monitoring of optical channels and in-band optical signal to noise ratio (OSNR) measurement. The proposed technique is based on a two-step lock-in demodulation of optical signals that are suitably labeled with low-modulation-index labels. We demonstrat... View full abstract»

In this letter, a chaos-based nonlinear encryption scheme is presented to enhance the physical layer security and reduce the peak-to-average power ratio (PAPR) in an orthogonal frequency division multiplexing passive optical network (OFDM-PON). A four-dimensional (4D) hyper chaotic system is utilized to generate the chaotic binary sequences and construct the nonlinear substitution boxes (S-boxes).... View full abstract»

Visible light communication (VLC) uses solid-state lightings to transmit information; therefore, it is necessary that modulation schemes for VLC provide dimming control. In this letter, we propose a multiple pulse position modulation (MPPM) to offer both functions of modulating data-stream and controlling the brightness at the same time. According to the dimming level, we control the number of pul... View full abstract»

A robust graphene-coated in-fiber Mach-Zehnder interferometer (MZI) formed with a pair of 3-dB long-period fiber gratings is proposed and demonstrated for ammonia (NH3) gas sensing. The sensor operates on the principle of changing the phase of the cladding mode of a fiber through changing the conductivity of the graphene coating by adsorbed NH3 molecules, which gives rise to ... View full abstract»

A dual-frequency optoelectronic oscillator (OEO) for high sensitivity pressure sensing is proposed and experimentally demonstrated. The oscillation frequency of the OEO is determined by a dual-passband microwave photonic filter in the OEO loop, which is implemented using phase shifted fiber Bragg gratings (PSFBGs) written in side-hole fiber to perform phase-modulation to intensity-modulation conve... View full abstract»

We demonstrate a platform based on etched facet silicon inverse tapers for waveguide-lensed fiber coupling with a loss as low as 0.7 dB/facet. This platform can be fabricated on a wafer scale enabling mass-production of silicon photonic devices with broadband, high-efficiency couplers. View full abstract»

Single-mode optical coupling between fiber and photonic integrated circuit (PIC) requires precision alignment and bonding, and significantly adds to the cost of photonic packaging. This article describes how a pair of micro-lens arrays-one on the Si-PIC and the other on the fiber-array- can be used to achieve fiber-to-PIC grating-coupling with an insertion-loss of 1.7 dB (i.e., a coupling efficien... View full abstract»

We describe a shallow-etched diffractive waveguide grating coupler which achieves efficient coupling between single-mode fiber and a silicon-on-insulator optical waveguide. By the appropriate choice of waveguide/grating thicknesses and varying the coupling strength of the grating coupler via tailoring its fill factor to optimize the mode matching, a coupling loss of only 1.2 dB was obtained for ea... View full abstract»

We propose and demonstrate a refractive index (RI) sensor based on a dual-hole elliptical core fiber (DHECF), which is available for microfluidic chips. By using the heating-twisting technology, we fabricate a helical long period grating (HLPG) structure in the DHECF. This sensor is convenient for microfluidic chips because of the dual-hole structure in the fiber, which is the natural microfluidic... View full abstract»

This letter reports an experimental demonstration of indoor wireless visible-light communication transmission at 1 Gb/s. The system consists of a four-channel multiple-input multiple-output link that uses white LED sources, each transmitting signals at 250 Mb/s using orthogonal frequency division multiplexing modulation. A nine-channel imaging diversity receiver is used to detect the signals, and ... View full abstract»

We investigate the risk of overestimating the performance gain when applying neural network-based receivers in systems with pseudorandom bit sequences or with limited memory depths, resulting in repeated short patterns. We show that with such sequences, a large artificial gain can be obtained, which comes from pattern prediction rather than predicting or compensating the studied channel/phenomena. View full abstract»

By incorporating a phase-shifted fiber Bragg grating (PS-FBG) inscribed in few-mode fiber (FMF) named FM-PS-FBG for the first time, a novel frequency-switchable optoelectronic oscillator (OEO) to perform simultaneous strain and temperature measurement is proposed and experimentally demonstrated. In the OEO loop, two switchable oscillation frequencies are determined by the reflected bands of FM-PS-... View full abstract»

This letter describes a high-speed visible light communications link that uses a white-light light-emitting diode (LED). Such devices have bandwidths of few megahertz, severely limiting the data rates of any communication system. Here, we demonstrate that by detecting only the blue component of the LED, and using a simple first-order analogue equalizer, a data rate of 100 Mb/s can be achieved usin... View full abstract»

We demonstrate single channel 448 Gb/s (56 GBaud) and wavelength division multiplexed (WDM) 1.792-Tb/s transmission of Nyquist polarization division multiplexed 16-ary quadrature amplitude modulation (16-QAM) signal by employing heterodyne detection with single-ended photodiodes. The signal-signal beat interference induced by square-law detection of the single-ended photodiodes is overcome by the ... View full abstract»

We study the spatiotemporal instability generated by a universal unstable attractor in normal dispersion graded-index multimode fiber for femtosecond pulses for the first time. Experimentally observed spatiotemporal instability sidebands are 91-THz detuned from the pump wavelength of 800 nm. Detailed analysis carried out numerically by employing coupled-mode pulse propagation model. Numerically ob... View full abstract»

We investigate the hydrostatic pressure dependence of the Bragg wavelength of a fiber Bragg grating (FBG) inscribed in a perfluorinated graded-index (PFGI-) polymer optical fiber (POF) at 1550 nm. At 0.5 MPa, the Bragg wavelength increased with time and became almost constant ~150 min later. Such a long time constant probably originates from the unique structure of the PFGI-POF, which has a... View full abstract»

A tunable planarized metamaterial absorber with VO2/W multilayer structure is designed to realize the biggest tuning absorption range of 77.10% at the central wavelength λ = 3.00 μm by the reversible phase transition of VO2 between metallic and dielectric state. The finite difference time domain method is used to simulate the effects of main parameters, differen... View full abstract»

A segmented long period fiber grating (LPFG)-based torsion sensor with both high sensitivity and ability to discriminate torsion direction is proposed and experimentally investigated. The sensor is made up of three spatial orthogonal short LPFGs by using a CO2 laser notched method. Experimental results show that the torsion sensitivities can reach −0.54 and 0.33 dB/(rad/m) in dip... View full abstract»

Multicarrier offset quadrature amplitude modulation (MC-OQAM) systems show a high degree of spectral efficiency and flexibility. However, the typical equalization method for MC-OQAM systems, the phase-corrected equalizer, has a large complexity because it uses several equalizers in parallel in order to cope with the inter-carrier interference problem. In this letter, we propose a low-complexity sc... View full abstract»

An $m$ -ary adaptive demodulator based on machine learning for light beams carrying orbital angular momentums (OAMs) over free-space turbulence channels is proposed and demonstrated. Benefiting from natural advantages in the image recognition, convolutional neural network (CNN) is selected to construct the adaptive demodulator. Without extra space light modulators and digital signal processing at ... View full abstract»

A novel Mach-Zehnder interferometer (MZI) in an embedded-core hollow optical fiber (ECOF) was demonstrated, which was composed of a pair of long period gratings (LPGs). In order to form an in-fiber MZI in an ECOF, a high-frequency CO2 laser was used to fabricate two similar LPGs with their peak attenuation of approximate 3 dB. The responses of the MZI to the temperature, the axial strain, and the ... View full abstract»

We demonstrate an active control of near-field coupling between dark and bright eigenmodes in terahertz metamaterials. The metamaterial unit cell consists of two orthogonally twisted split-ring resonators, which are tightly coupled with each other through near field. The monolayer graphene is integrated into the unit cell and the coupling strength of the near field can be actively modulated by the... View full abstract»

We examine the relation between optical signal-to-noise ratio (OSNR), error vector magnitude (EVM), and bit-error ratio (BER). Theoretical results and numerical simulations are compared to measured values of OSNR, EVM, and BER. We conclude that the EVM is an appropriate metric for optical channels limited by additive white Gaussian noise. Results are supported by experiments with six modulation fo... View full abstract»

Research on GaN-based blue and green laser diodes (LDs) has attracted great attention in the past years to meet the demand for laser display application. In this letter, we improve luminescence homogeneity and reduce internal loss of GaN-based blue LD structures grown on c-plane free-standing GaN substrates. The morphologies of homoepitaxial GaN layers are greatly affected by offcut orientation an... View full abstract»

We report recent developments on 980-nm single-transverse-mode ridge-waveguide (RW) lasers with high efficiency and low-vertical divergence. RW lasers fabricated with 7-μm-wide ridge and 3-mm-long cavity achieve 2-W optical power, 59% maximum power conversion efficiency, as well as a beam divergence of 7.2° in lateral and 10.3° in vertical under 1.5-A injection current. The la... View full abstract»

We propose and experimentally demonstrate a compact fiber-optic vibration sensor in which a tilted Bragg grating is inscribed in the core of a depressed cladding fiber. The sensor provides resolution up to $2times 10^{-2} text {m/s}^{2}$ and a linear acceleration response between 0.2 and 6.0 m/s2, together with a f... View full abstract»

We demonstrate an integrated circuit (IC) implemented in 28-nm CMOS technology for Mach-Zehnder modulator (MZM) bias control. Our circuit determines the optimal MZM bias voltage that provides the largest optical modulation amplitude and maintains this bias voltage by monitoring the average modulated power. The controller IC consists of digital-to-analog converter, analog-to-digital converter, tran... View full abstract»

High-speed optical receivers realized in low-cost technology often suffer from unfavorable performance, dictated by the transimpedance limit, the key design constraint of shunt-shunt feedback transimpedance amplifier (TIA). In this letter, we propose a novel TIA architecture to overcome the transimpedance limit, achieving both low noise and high gain that are not realizable in a conventional topol... View full abstract»

This letter presents a miniature fiber-optic pressure sensor fabricated directly on the tip of a fiber. The sensor measures only 125 μm in diameter. A Fabry-Pe/spl acute/rot cavity and a thin silica diaphragm are fabricated by simple techniques involving only fusion splicing, cleaving, and wet chemical etching. Interference pattern of the sensor is analyzed and issues in sensor design are ... View full abstract»

A dual-sideband (DSB) chirped lidar for simultaneous real-time ranging and velocimetry is proposed and demonstrated, in which a Mach–Zehnder modulator (MZM) is applied to generate the required optical DSB frequency-modulated continuous-wave (FMCW) signal. The inherent opposite frequency chirp and contrary wavelength offset to the optical carrier of the two generated sidebands make it possib... View full abstract»

In this letter, we point out that a much simpler ergodic capacity formula for a coherent free-space optical system employing multiple receive apertures over atmospheric turbulence-induced fading channels could be derived, in a more useful and elegant approach to Meijer’s G-function, than the one provided in the above letter. Furthermore, we also derive a concise asymptotic ergodic capacity ... View full abstract»

This letter presents a self-alignment system using a continuous and a proportional-to-position-error correction algorithm. The system is designed to address uninterrupted point-to-point high-speed line-of-sight optical wireless links. The alignment correction employs amplitude detection from a mm2-scale CMOS-compatible 4-PD array connected to a 4-channel receiver and followed by digital... View full abstract»

To lower the cost and to improve the reliability of next generation fronthaul links, we propose and demonstrate an analog 60-GHz radio-over-fiber fronthaul link employing two wavelength-tunable integrated microwave photonics filters for the first time. An integrated notch filter is cascaded with an integrated bandpass filter to offer the optical filtering function, resulting in a dual-passband fil... View full abstract»

With the continuously increasing demand of cost effective, broadband wireless access, radio-over-fiber (RoF) starts to gain more and more momentum. Various techniques already exist, using analog (ARoF) or digitized (DRoF) radio signals over fiber; each with their own advantages and disadvantages. By transmitting a sigma delta modulated signal over fiber (SDoF), a similar immunity to impairments as... View full abstract»

A new approach to coherent detection is demonstrated which achieves the same high sensitivity as homodyne detection but without the need to phase lock the local oscillator laser. In addition, 1470 ps/nm of chromatic dispersion is compensated with zero net penalty by electronic domain equalization, a result which has not been achieved before because zero-penalty equalization is not possible after d... View full abstract»

We describe a new method for extracting the intrinsic response of a laser diode from S-parameters measured using a calibrated vector network analyzer. The experimental results obtained using the new method are compared with those obtained using the optical modulation method and the frequency response subtraction method. Good agreement has been obtained, confirming the new method validity and accur... View full abstract»

A joint frame and carrier frequency synchronization algorithm for coherent optical systems, based on the digital computation of the fractional Fourier transform (FRFT), is proposed. The algorithm utilizes the characteristics of energy centralization of chirp signals in the FRFT domain, together with the time and phase shift properties of the FRFT. Chirp signals are used to construct a training seq... View full abstract»

We demonstrate a wavelength tunable, Q-switched Er-doped fiber laser using a mixture of gold nanorods and nanospheres as a saturable absorber. The laser has a very low Q-switched threshold pump power of 22 mW, and at a pump power of 47.86 mW, a pulse with a maximum pulse energy of 36.01 nJ, repetition rate of 37.50 kHz, and pulse width of $2.74~mu text{s... View full abstract»

With the assistance of subwavelength grating (SWG), an ultra-compact multimode interference (MMI)based diplexer is proposed. A certain number of SWG pitches are implemented in the middle of the MMI section, which can engineer the effective refractive index for the modes and, therefore, tune the beat length. With proper tailoring of the grating parameters, the beat lengths of the two wavelengths ca... View full abstract»

This letter presents a novel method for the analysis of tilted fiber Bragg grating (TFBG) transmission spectra. The processing method is based on the transmission spectrum contour length, which is calculated in the wavelength range where cladding modes appear. The proposed algorithm is based on a modified analysis method that is used in spectrometric surveys of gas spectra in cases with unknown or... View full abstract»

We report on the experimental observation of bound states of solitons in an erbium-doped mode-locked fiber laser using a microfiber-based WS2 saturable absorber (SA). By carefully adjusting the polarization state of the laser cavity, the bound states of two solitons with various pulse separations from 5.40 to 75.14 ps and three solitons with a pulse separation of 23.88 ps are obtained. ... View full abstract»

We report on InGaN/GaN distributed feedback laser diodes with high order gratings emitting at a single wavelength around 428 nm. The 39th order notched gratings have the advantage of a simplified fabrication route with no need for overgrowth. The laser ridge and grating were formed by electron beam lithography followed by ICP etching. The as-cleaved lasers emitted in the pulsed regime with a peak ... View full abstract»