Showing papers in "IEEE Journal of Quantum Electronics in 2017"

Diagnostics, Control and Performance Parameters for the BELLA High Repetition Rate Petawatt Class Laser

Abstract: A laser system producing controllable and stable pulses with high power and ultrashort duration at high repetition rate is a key component of a high energy laser-plasma accelerator (LPA) Precise characterization and control of laser properties are essential to understanding laser-plasma interactions required to build a 10-GeV class LPA This paper discusses the diagnostics, control and performance parameters of a 1 Hz, 1 petawatt (PW) class laser at the Berkeley Lab Laser Accelerator (BELLA) facility The BELLA PW laser provided up to 46 J on target with a 1% level energy fluctuation and 13-μrad pointing stability The spatial profile was measured and optimized by using a camera, wavefront sensor, and deformable mirror (ILAO system) The focus waist was measured to be r0 = 53 μm and the fraction of energy within the circular area defined by the first minimum of the diffraction pattern (r = 67 μm) was 075 The temporal profile was controlled via the angle of incidence on a stretcher and a compressor, as well as an acousto-optic programmable dispersive The temporal pulse shape was measured to be about 33 fs in full width at half maximum (WIZZLER and GRENOUILLE diagnostics) In order to accurately evaluate peak intensity, the energy-normalized peak fluence, and energy-normalized peak power were analyzed for the measured spatial and temporal mode profiles, and were found to be 15 kJ/(cm2 J) with 6% fluctuation (standard deviation) and 25 TW/J with 5% fluctuation for 46-J on-target energy, respectively This yielded a peak power of 12 PW and a peak intensity of 17×1018 W/cm2 with 8% fluctuation A method to model the pulse shape for arbitrary compressor grating distance with high accuracy was developed The pulse contrast above the amplified spontaneous emission pedestal was measured by SEQUOIA and found to be better than 109 The first order spatiotemporal couplings (STCs) were measured with GRENOUILLE, and a simulation of the pulse's evolution at the vicinity of the target was presented A maximum pulse front tilt angle of less than 7 mrad was achieved The reduction of the peak power caused by the first order STCs was estimated to be less than 1% The capabilities described in thispaper are essential for generation of high quality electron beams

Photonics-Assisted Millimeter-Wave Wireless Communication

Abstract: High-speed millimeter-wave (mm-wave) wireless transmission at 40 Gb/s or higher will be required in the near future. Due to bottleneck in electrical devices, mm-wave wireless signal at such high bit rates cannot be generated in an all-electrical method. Photonics-assisted mm-wave generation technology has become an effective solution to handle this problem of bandwidth limitation. Recent efforts with a single modulator to generate optical mm-wave signal largely simplify the architecture of the optical transmitter. Heterodyne detection based on advanced digital signal processing can overcome nonlinear effects in optical and electrical devices, and it also can improve the spectral efficiency and receiver sensitivity. Multidimensional multiplexing techniques can reduce the baud rate of each subchannel, and hence it can realize mm-wave signal long distance transmission. In this tutorial, we will describe these key enabling technologies and principle for the realization of ultrahigh speed, large capacity mm-wave signal transmission. These enabling technologies can effectively improve the transmission capacity and distance, as well as reduce the required bandwidth for optical and electrical devices.

A Broadband Terahertz Metamaterial Absorber Based on Two Circular Split Rings

Abstract: We present a broadband terahertz metamaterial absorber, of which the unit cell is made up of two cirular split rings, a dielectric substrate and a metallic ground. The simulation results show that the absorber achieves a broadband absorption from 0.85 to 1.926 THz with the absorptivity beyond 90% at normal incidence, and the bandwidth is 1.076 THz, which is 77.52% with respect to the central frequency. The broadband and the high absorption are mainly resulted from the strong electromagnetic resonance and overlapping of resonant frequencies. Moreover, the proposed absorber has a property of wide angle absorptivity for both TE and TM polarizations, and thus can play an important role in terahertz imaging, detecting, and stealthy technology.

QAM Quantum Noise Stream Cipher Transmission Over 100 km With Continuous Variable Quantum Key Distribution

Abstract: We report the first on-line high-speed and large-capacity secure optical communication system. This was realized by combining a quadrature amplitude modulation/quantum noise stream cipher technology where a coherent multi-level optical signal is hidden in quantum noise and a quantum key distribution technology where secure key delivery is realized with an extremely weak laser light comparable to a single photon. In our security analysis, we adopt a realistic assumption, namely, that an adversary does not have a lossless fiber. To show the advantage of this scheme, we performed a 128 QAM, 70-Gbit/s single-channel transmission over 100 km with a spectral efficiency of as high as 10.3 bits/s/Hz. This is the fastest data rate and highest spectral efficiency yet achieved in quantum cryptography with a realistic assumption.

38-GHz millimeter wave beam steered fiber wireless systems for 5G indoor coverage: architectures, devices, and links

Abstract: Millimeter wave (mm-wave) beam steering is a key technique for the next generation (5G) wireless communication. The 28 and 38-GHz bands are widely considered as the candidates for 5G. In the context of indoor coverage, fiber-wireless systems with multiple simplified remote antenna sites are attractive to avoid the indoor coverage problem caused by the high wall penetration loss of mm-wave signals. To allow enough antenna gain at the mm-wave bands, radio beam steering (and beamforming) is desired. Combining fiber-wireless system with remotely controlled photonic mm-wave beam steering can bring significant advances in terms of energy efficiency and cost. In this paper, we explore two kinds of indoor fiber-wireless network architectures for such mm-wave beam steering. Then, we discuss and investigate the key enabling device, which is an arrayed waveguide grating feedback loop (AWG-loop). Based on the AWG-loop, we further design two fiber-wireless links to accommodate the two network architectures. Both links with bit rates from 50 Mb/s to 8 Gb/s per spatial channel are experimentally demonstrated with a 38-GHz carrier frequency. The advanced reversely modulated optical transmitter and half-cycled 16 quadrature amplitude modulation (QAM-16) are employed to realize a simplified mm-wave beam steered fiber-wireless link with the record-breaking 16-b/s/Hz (4 spatial channels× 4 bits/s/Hz) spatial-spectral efficiency in its kind.

Electrical and Optical Characteristics of Self-Powered Colloidal CdSe Quantum Dot-Based Photodiode

Abstract: In this paper, a novel self-powered Schottky photodiode has been proposed, which uses the low-temperature processing (~250°C) and the implementation of colloidal quantum dots (QDs)-based charge transport layer (ZnO QDs). The self-powered Schottky photodiode has been fabricated on the n-Si substrate, and the colloidal CdSe QDs have been used as an active layer. The Schottky junction has been formed by depositing Gold (Au) on the CdSe QDs by the thermal evaporation method. The CdSe QDs (~30 nm) thin film is assumed to be fully depleted (> 80%) by the Au Schottky contact with the measured effective barrier height of 0.67 eV and built-in potential of 0.34 V. The photoresponse of the self-powered Schottky photodiode has been measured, and the maximum responsivity of 10.23 mA/W and a maximum detectivity of $8.81\times 10^{\mathrm { {9}}}$ cmHz1/2/W at ~522 nm have been achieved. The self-powered Schottky photodiode shows a transient response ( $\text{t}_{\mathrm { {r{}}}}=17.9$ ms and $\text{t}_{\mathrm { {f}}} \,\, =18.0$ ms) under the pulsating white LED light at an on-off period of 1 s.

Phase Resonance Tuning and Multi-Band Absorption Via Graphene-Covered Compound Metallic Gratings

Abstract: 1-D compound metallic grating (CMG) is a periodic structure with more than one slit in each period. When CMG is combined with a graphene sheet as its cover, the incident light is effectively coupled to the plasmons in graphene which in turn can result in strong manipulation of light for both major polarizations. We show that tunable phase resonance and perfect absorption of the incident light are interesting outcomes of this manipulation. In this paper, we demonstrate that fano-like phase resonances which can be observed in CMGs under transverse magnetic polarized incident wave are tuned by changing the Fermi level of graphene. It is shown that while the spectral position of the phase resonances can be shifted up to several gigahertzes, their peak to peak amplitudes are tuned from ~0.9 to ~0.1. On the other hand, we design a graphene-covered CMG, which is able to perfectly absorb both major polarizations of incident wave in two separate bands; hence, providing the opportunity for designing multi-band/wide-band absorbers. We have developed a circuit model for the analysis of the structure. Parameters of the model are derived explicitly and analytically for both major polarizations. Our results are verified through comparison against results of the full-wave simulations.

Nonlinear Pulse Compression to Sub-40 fs at $4.5~\mu$ J Pulse Energy by Multi-Pass-Cell Spectral Broadening

Abstract: We report on the pulse compression of an 18.5 MHz repetition rate pulse train from 230 fs to sub-40 fs by nonlinear spectral broadening in a multi-pass cell and subsequent chirp removal. The compressed pulse energy is $4.5~\mu \text{J}$ , which corresponds to 84 W of average power, with a compression efficiency of 88%. This recently introduced compression scheme is suitable for a large pulse energy range and for high average power. In this paper, we show that it can achieve three times shorter pulses than previously demonstrated.

Modal Linewidth Dependent Transmission Performance of 850-nm VCSELs With Encoding PAM-4 Over 100-m MMF

Abstract: By changing the transverse-mode spectral linewidth of vertical cavity surface emitting lasers (VCSELs) at 850 nm, the directly encoded four-level pulse amplitude modulation data transmission performance over 100-m-long OM4 multimode fiber (MMF) are demonstrated and compared. The multi-mode VCSEL chip with the largest aperture of $11~\mu \text{m}$ reveals the widest spectral linewidth and the highest optical power, but provides the smallest modulation bandwidth to support only 44- and 28-Gb/s data rates for back-to-back (BtB) and 100-m OM4 MMF transmission cases, respectively. By shrinking the aperture size to reduce transverse-mode number, the few-mode VCSEL with the strongest throughput power enables a BtB transmission capacity as high as 52 Gb/s. However, its modal dispersion induced after OM4 MMF transmission inevitably degrade the data rate to 32 Gb/s. In contrast, the single-mode VCSEL with the smallest aperture of $3~\mu \text{m}$ reveals the highest modulation bandwidth and negligible modal dispersion to show competitive BtB transmission capacity with that of the few-mode VCSEL. In particular, the single-mode VCSEL successfully achieves a data rate of 34 Gb/s with a power penalty as low as 1.4 dB, after 100-m OM4 MMF transmission.

Self-Mixing Interferometer With a Laser Diode: Unveiling the FM Channel and Its Advantages Respect to the AM Channel

Abstract: In a self-mixing interferometer based on a diode laser, the measurement of the external reflector displacement $s(t)$ is carried out by looking at the optical-phase signal $\cos 2ks(t)$ , a signal readily detected as an amplitude modulation (AM) of emitted power. In contrast, the other available signal, sin $2ks(t)$ , a frequency modulation (FM) of the emitted field at optical frequency, is never used because difficult to recover. Recently, Contreras et al. used a narrow-band acetylene cell to convert the FM into an amplitude signal, finding it is larger and has a better SNR than the AM. In this paper, we analyze the advantages of the new CFM (converted-FM) signal, calculating both amplitude and SNR, and compare theoretical results to published experimental evidence, finding good agreement. We then present options for realizing the selective filter in different configurations and technology. Finally, we evaluate the improvement offered by CFM in a number of measurements, like sub-wavelength vibrations, digital readout displacement, and diode laser alpha factor.

Ultracompact and Broadband Silicon-Based Polarization Beam Splitter Using an Asymmetrical Directional Coupler

Abstract: An ultracompact and broadband polarization beam splitter (PBS) is proposed by using an asymmetrical directional coupler composed of a hybrid plasmonic waveguide (HPW) and a regular silicon wire (RSW). By properly choosing the structural parameters, transverse-magnetic (TM) mode is cutoff for the RSW and phase-matching condition is only satisfied for transverse-electric (TE) mode. As a result, the input TM-mode in HPW directly passes through, while the input TE-mode in the same HPW is coupled into the RSW. Consequently, the two polarized modes can be effectively separated. Moreover, the present PBS shows fabrication-friendly design and its length can be significantly reduced, since the gap between the HPW and RSW can be shortened to zero. From the results, a ~2.6- $\mu \text{m}$ -long PBS with a coupling length of $0.1~\mu \text{m}$ is achieved and its bandwidth is ~120 nm with a polarization extinction ratio over 14 dB, and an insertion loss below 1 dB for both polarizations. In addition, fabrication tolerances to the key structural parameters are analyzed in detail and field evolution through the designed PBS is also presented.

Analysis of the Public Channel of Quantum Key Distribution Link

Abstract: Quantum key distribution (QKD) relies on the laws of physics to establish a symmetric binary key between remote parties. A QKD link involves the realization of a quantum channel for the transmission of quantum key material encoded in certain photon properties, as well as a public channel for verification of the exchanged key material. This paper deals with the mutual dependence of these channels and analyzes the impact of performance of both channels on the overall key material establishment process. This paper presents measurement data obtained under laboratory conditions as well as the results obtained by establishing a virtual QKD link. Despite the common beliefs that increased quantum bit error rate implies a larger amount of traffic on the public channel, our measurements prove the opposite. The obtained data clearly show that the public channel has a major impact on the overall performance of the QKD link.

Multi-Mode VCSEL Chip with High-Indium-Density InGaAs/AlGaAs Quantum-Well Pairs for QAM-OFDM in Multi-Mode Fiber

Abstract: An 850-nm multi-mode vertical cavity surface emitting laser (VCSEL) bare chip with high-indium-density InGaAs/AlGaAs quantum-well pairs is demonstrated for directly encoded QAM-OFDM transmission in multi-mode fiber (MMF) By directly encoding the 850-nm VCSEL bare chip with a pre-leveled 14-GHz 16-QAM OFDM data, >50-Gb/s transmission over 100-m-long OM4 MMF can be realized without using data recovery circuit Increasing the bias current of the VCSEL beyond 75Ith improves the signal-to-noise ratio (SNR) and the bit error ratio (BER) of received QAM-OFDM data to 155 dB and $29\times 10^{-3}$ , respectively The 100-m-long OM4 MMF transmission degrades the SNR with its covered bandwidth reducing to 13 GHz The OFDM subcarrier pre-leveling technique with a slope of 02 dB/GHz ensures the 16-QAM-OFDM data transmission with an error vector magnitude of 171% and a BER of $34\times 10^{-3}$

Quenching of $3.4~\mu \text{m}$ Dual-Wavelength Pumped Erbium Doped Fiber Lasers

Abstract: We report on a quenching behavior of the $3.4~\mu \text{m}$ dual-wavelength pumped fiber laser that can be accounted by the adverse contribution from a new excited state absorption transition in Er3+:ZrF4 glasses. The transition occurs between levels 4F9/2 and 4F7/2 and partially overlaps with the 1976 nm pump wavelength. We show that this upconversion process causes strong quenching of the lasing power at $3.4~\mu \text{m}$ under insufficient 974 nm pumping. Intracavity fluorescence at 550 nm from levels (2H11/2, 4S3/2) is also monitored as a witness of this excited state absorption mechanism. Through numerical modeling, we estimate the cross section value of the process and show that it is essential to reproduce both power and fluorescence curves. This paper also emphasizes the critical role played by the pump wavelengths.

6-mW Single-Mode High-Speed 1550-nm Wafer-Fused VCSELs for DWDM Application

Abstract: This paper presents data on wafer-fused 1550-nm vertical-cavity surface-emitting lasers (VCSELs) based on the active region and distributed Bragg reflectors (DBRs) grown by molecular beam epitaxy. VCSELs with a tunnel junction aperture diameter of 8 μm show lasing at a threshold current density j th <; 3 kA/cm 2 , an output optical power of ~4 mW, and a -3 dB bandwidth of approximately 7 GHz at a 10-mA bias current. The devices demonstrate single-mode continuous wave operation with the transverse side-mode suppression ratio (SMSR) varying in the range of 40-45 dB up to roll-over currents. The increase in mirror losses due to the etching of the top DBR makes the output optical power increase to 6 mW and causes the wallplug efficiency value to reach 20%, but SMSR remains in the range of 40-45 dB. This also makes it possible to reduce both the photon lifetime and, as a result, the effect of damping and increase the modulation bandwidth to 9 GHz. The observed open and clear eye diagrams indicate that non-return-to-zero operation is possible at bit rates of up to 30 Gbps without equalization or forward error correction. The high-output optical power and modulation performance pave the way for the dense wavelength division multiplexing application of wafer-fused 1550-nm VCSELs.

Tunable X-Band Optoelectronic Oscillators Based on External-Cavity Semiconductor Lasers

Abstract: Laser diodes with optical feedback can exhibit periodic intensity oscillations at or near the relaxation-oscillation frequency. We demonstrate optoelectronic oscillators based on external-cavity semiconductor lasers in a periodic dynamical regime tunable over the entire $X$ -band. Moreover, unlike standard optoelectronic oscillators, we need not employ the time-dependent optical intensity incident on a photodiode to generate the microwave signal, but rather have the option of generating the electrical microwave signal directly as a voltage $V(t)$ at the laser-diode injection terminals under constant current operation; no photodiode need be involved, thus circumventing optical-to-electrical conversion. We achieve a timing jitter of $\lesssim 10$ ps and a quality factor of $\gtrsim 2\times 10^{5}$ across the entire $X$ -band, that ranges from 6.79 to 11.48 GHz. Tuning is achieved by varying the injection current $J$ .

Design and Analysis of Light Trapping in Thin Film GaAs Solar Cells Using 2-D Photonic Crystal Structures at Front Surface

Abstract: This paper proposes a design using a 2-D photonic crystal (PhC) structure-based thin film heterojunction GaAs solar cell with a periodic pattern extending from top transparent conducting oxide to inside the p-AlGaAs window layer placed just above the active layer of GaAs material. This paper presents the theoretical optical study and optimization of all the required parameters. This paper presents the optical optimization study for thin active layer cell in the generalized manner for 50-nm active layer, and then the performance of the proposed structure is compared with Lambertian limits and the planar cell, taken as reference. It has been predicted that these wavelength scale periodic structures greatly enhances the performance of the cell. The improvement in the performance is basically accounted for the better diffraction capability, impedance matching, trapping of high wavelength photons, and reduction in reflections from the top due to the PhC structure. The parameters have been optimized and calculated by means of rigorous coupled wave analysis.

What Limits the Efficiency of High-Power InGaN/GaN Lasers?

Abstract: Blue light emitting InGaN/GaN lasers currently exhibit less than 40% wall-plug efficiency, while infrared InGaAs/GaAs laser diodes exceed 70%. This paper explores the reasons behind the efficiency limitation by the numerical analysis of measured InGaN/GaN laser characteristics. The study reveals a strong increase of the threshold current due to self-heating. The influence of Auger recombination, electron leakage, free-carrier absorption, and series resistance changes with rising injection current.

High-Performance Mid-Wavelength InAs/GaSb Superlattice Infrared Detectors Grown by Production-Scale Metalorganic Chemical Vapor Deposition

Abstract: We demonstrate high-performance mid-wavelength $p$ - $i$ - $n$ infrared detectors based on InAs/GaSb type-II superlattices (SLs) grown by a production-scale metalorganic chemical vapor deposition system. Through a specially-designed gas switching sequence and the use of GaAs-type of interfacial layers, strain-balanced SLs with excellent material quality were obtained, as evidenced by atomic force microscopy, photoluminescence, X-ray diffraction, and transmission electron microscopy. The processed devices, with a cutoff wavelength around 4.8 $\mu \text{m}$ , exhibited a R0A of $2.3\times 10^{4}~\Omega $ cm2 and a peak responsivity of 1.4 A/W at 77 K. A specific detectivity of $7.2\times 10^{12}$ cmHz $^{1/2}$ /W was achieved at $3.7~\mu \text{m}$ . These values are comparable with those reported for InAs/GaSb SL detectors with similar cutoff wavelengths grown by molecular beam epitaxy.

$\chi ^{(2)}$ Modulator With 40-GHz Modulation Utilizing BaTiO3 Photonic Crystal Waveguides

Abstract: Future telecommunication and data center networks as well as quantum optical communication systems will require optical modulators with wide bandwidths, large extinction, low operating voltage, and small size. We report the first quantitative demonstration of slow light enhancement of the electro-optic (EO) coefficient in a χ (2) ferroelectric waveguide at microwave modulation frequencies. This is demonstrated in a compact (1 mm) photonic crystal (PC) device with a voltage-length product (Vπ·L) of 0.66 V-cm at 10 GHz and measured EO modulation out to 40 GHz. A local enhancement factor of 12 and effective EO coefficient of 900 pm/V is measured in the PC region at a wavelength of 1530 nm. By further optimizing the PC structure, devices with EO 3-dB bandwidths greater than 40 GHz and voltage-length product of 0.16 V-cm are predicted with 100-μm interaction length.

High Figure of Merit Graphene Modulator Based on Long-Range Hybrid Plasmonic Slot Waveguide

Abstract: By exploiting the electro-absorption effect of graphene, we present a graphene-based long-range hybrid plasmonic slot (LRHPS) waveguide modulator. The waveguide structure consists of a silica substrate, two high-index silicon strips, two low-index slots, two graphene layers, and a metal. The modes are confined within the two horizontal slot regions, which make graphene layer close to the maximum of tightly confined electric field. The designed LRHPS waveguide takes advantages of both the traditional long-range surface plasmon polariton waveguide and the hybrid plasmonic waveguide. The combined effects of long-range surface plasmon polaritons and discontinuity of electric field at the interface between two dielectrics with high-contrast refractive index enable low insertion loss together with high modulation depth for potential high-density nanophotonic integration. The modulator performance is comprehensively studied in terms of attenuation, insertion loss, modulation depth, and figure of merit (FoM). The designed graphene-based LRHPS waveguide modulator shows a 3-dB modulation depth with only ~363-nm-long waveguide, a 3-dB optical bandwidth of 15 THz with a center wavelength of 1550 nm, a low energy per bit consumption of 0.008 fJ/b, and a high FoM of 218 improved by more than one order of magnitude. Moreover, gate-variable broadband operation is also available covering the entire $O$ , $E$ , $S$ , $C$ , $L$ , and $U$ telecommunication bands.

Impact of Interface Roughness Scattering on the Performance of GaAs/Al x Ga 1–x As Terahertz Quantum Cascade Lasers

Abstract: We investigate the impact of interface roughness (IFR) scattering on the performance of a series of the state-of-the-art GaAs/AlxGa1–xAs terahertz quantum cascade lasers (THz-QCLs) through a calculation of the induced inhomogeneous broadening and intersubband scattering rates. Our analysis includes two GaAs/Al0.15Ga0.85As THz-QCL devices with measured maximum operating temperatures at $T _{\mathrm {max}} =177$ K and 175 K, two GaAs/Al0.30Ga0.70As devices with $T _{\mathrm {max}} = 150$ K and 89 K, a GaAs/AlAs-Al0.15Ga0.85As device with $T _{\mathrm {max}} = 181$ K, and a GaAs/AlAs device that did not lase. The investigated QCL wafers were grown at the same solid-source molecular beam epitaxy facility and at fixed parameters, so that we expect a constant interface roughness quality for all devices. We find negligible impact of IFR scattering on the performance of devices that use $x =0.15$ barriers as well as for devices that use $x =0.30$ barriers with wide > 40 monolayers (ML) wells to support upper and lower laser levels. Fixing the barrier height to $x =0.30$ , we calculate a drastic increase of the IFR-induced linewidth broadening from ~0.66 meV to ~2 meV when the quantum wells thicknesses reduce from ~40 ML to ~30 ML and relate this effect to the observed reduction of $T _{\mathrm {max}}$ from 150 to 89 K. Furthermore, we calculate a large (~2 meV) IFR linewidth broadening and short (~1 ps) IFR intersubband scattering times for the device with pure AlAs layers and relate the consequent reduction of optical gain to the nonlasing of this device.

$1.3~\mu $ m Optical Interconnect on Silicon: A Monolithic III-Nitride Nanowire Photonic Integrated Circuit

Abstract: A feasible optical interconnect on a silicon complementary metal-oxide-semiconductor chip demands epitaxial growth and monolithic integration of diode lasers and optical detectors with guided wave components on a (001) Si wafer, with all the components preferably operating in the wavelength range of 1.3-1.55 μm at room temperature. It is also desirable for the fabrication technique to be relatively simple and reproducible. Techniques demonstrated in the past for having optically and electrically pumped GaAs and InP-based lasers on silicon include wafer bonding, selective area epitaxy, epitaxy on tilted substrates, and use of quantum dot or planar buffer layers. Here, we present a novel monolithic optical interconnect on a (001) Si substrate consisting of a III-nitride dot-in-nanowire array edge emitting diode laser and guided wave photodiode, with a planar SiO 2 /Si 3 N 4 dielectric waveguide in between. The active devices are realized with the same nanowire heterostructure by one-step epitaxy. The electronic properties of the InN dot-like nanostructures and mode confinement and propagation in the nanowire waveguides have been modeled. The laser, emitting at the desired wavelength of 1.3 μm, with threshold current ~350 mA for a device of dimension 50 μm × 2 mm, has been characterized in detail. The detector exhibits a responsivity ~0.1 A/W at 1.3 μm. Operation of the entire optical interconnect via the dielectric waveguide is demonstrated.

All-Normal-Dispersion Chalcogenide Waveguides for Ultraflat Supercontinuum Generation in the Mid-Infrared Region

Abstract: We show numerically, how a chalcogenide planar waveguide designed to exhibit normal dispersion over a wide spectral range around the pump wavelength can produce relatively flat supercontinuum in the mid-infrared regime. A 1-cm-long channel waveguide, made using Ge11.5As24Se64.5 glass and pumped at 1.55 $\mu \text{m}$ using short optical pulses with only 25 W peak power, produced a supercontinuum that was nearly 600 nm wide. Employing the same pump source with a peak power of 100 W, the supercontinuum could be extended to beyond 2.2 $\mu \text{m}$ with a bandwidth of 1000 nm. By shifting the pump wavelength to 3.1 $\mu \text{m}$ and using pulses with peak powers of up to 3 kW, the resulting ultraflat supercontinuum extended from 2 to $5.5~\mu \text{m}$ . Even a wider spectral range (1.8-6 $\mu \text{m}$ ) can be realized if MgF2 glass is used for the lower cladding while maintaining power variations below 5 dB over the entire bandwidth.

Traveling Wave Model for Frequency Comb Generation in Single-Section Quantum Well Diode Lasers

Abstract: We present a traveling wave model for a semiconductor diode laser based on quantum wells. The gain model is carefully derived from first principles and implemented with as few phenomenological constants as possible. The transverse energies of the quantum well confined electrons are discretized to automatically capture the effects of spectral and spatial hole burning, the gain asymmetry, and the linewidth enhancement factor. We apply this model to semiconductor optical amplifiers and single-section phase-locked lasers. We are able to reproduce the experimental results. The calculated frequency modulated comb shows potential to be a compact, chip-scale comb source without additional external components.

Deriving Surface Impedance for 2-D Arrays of Graphene Patches Using a Variational Method

Abstract: In this paper, we extract the fundamental resonant mode of a graphene patch using a variational method. We use 2-D eigenvalue problem obtained from the integral equation governing the surface current on graphene patterns under quasi-static approximation. To compute the eigenvalues, we propose three trial eigenfunctions, which meet the boundary conditions. We investigate the accuracy of these eigenfunctions with comparing to the results obtained by full wave simulations. Finally, we analyze square-lattice arrangements of graphene patches using the most accurate proposed eigenfunction and derive a very accurate surface impedance for it. The proposed surface impedance is much more precise than the surface impedance already reported by Padooru et al. for this structure.

Polarization Effect in AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

Abstract: The polarization effect in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is investigated, which is critical for the development of DUV LEDs, because the basal material, epitaxial structure, and polarization characteristics are very distinct to those of the well-developed (In)GaN-based near-ultraviolet and visible light emitters. In this paper, the influence of the polarization effect in multi-quantum well active region and p-type layers on the characteristics of DUV LEDs with Ga-face or N-face polarization is explored. Simulation results show that the severe band bending of the p-type layers induced by the polarization field markedly affects the optical and electrical performance. A band-engineered DUV LED structure with compositional grading electron-blocking layer and p-interlayer is proposed to enhance the electron confinement and hole injection with the mitigation of polarization effect. The device performance of the proposed LED structure with Ga-face or N-face polarization is comparable with that of the DUV LED without polarization.

Image-Free Microwave Photonic Down-Conversion Approach for Fiber-Optic Antenna Remoting

Abstract: For commercial and military applications, it is highly desired to acquire broadband radio frequency (RF) signals from remotely located receiving antennas to the central office (CO) for centralized signal processing such as down-conversion In this paper, a photonic approach for image-free microwave frequency down-conversion is proposed for antenna remoting scenarios The RF signal is captured by the phase modulator (PM) at remote antenna unit and transmitted over a long fiber link to the CO for performing frequency down-conversion In analog, with the Hartley architecture in the electronic domain, two in-phase (I) and quadrature (Q) intermediate frequency (IF) components are generated by using an electro-optic polarization modulator (PolM) and two polarizers To achieve large image rejection ratio (IRR), the digital post-processing technique is introduced to accurately compensate the amplitude and phase imbalances between the I and Q IF signals In the experiments, a 2-km single-mode fiber link is inserted between the PM and PolM As the local oscillator signals are set as 35 and 3 GHz, two target sinusoidal signals at 355 and 31 GHz are applied with two image signals, a sinusoidal signal at 345 GHz and a broadband RF signal centered at 29 GHz, respectively Then, both real-time analog and off-line digital processing methods are used to process the generated I and Q IF signals for image rejection, yielding an IRR over 45 and 60 dB respectively, when the sinusoidal image signal is applied The distortions from the broadband 29-GHz image RF signal are also effectively suppressed by using the two processing methods The proposed approach is capable of covering a wide frequency range from 5 to 40 GHz

Dispersion Flattened Porous-Core Honeycomb Lattice Terahertz Fiber for Ultra Low Loss Transmission

Abstract: We propose the design of a novel porous-core honeycomb lattice photonic crystal fiber (PH-PCF) with extremely low loss and flattened dispersion for guiding terahertz (THz) wave. Finite element method is used to investigate in detail the properties of the designed waveguide. The single modeness, effective material loss (EML), confinement loss ( $L_{c}$ ), effective modal area, bending loss, distribution of power in different regions, and the dispersion profile of the designed structure are systematically analyzed. Simulation results of the proposed PH-PCF confirm that the bulk material absorption loss of the host material Topas can be reduced by 88.87% using the proposed design. The obtained EML is as low as 0.02227 cm⁻¹ at operating frequency $f = 1$ THz and the dispersion is ±0.15 ps/THz/cm, flattened over a band of frequency as wide as 1.15 THz. Besides, the reported design also offers negligible $L_{c}$ and relatively smaller bending loss as compared with other contemporary studies.

Analysis of Phase-Locking in Optoelectronic Microwave Oscillators Due to Small RF Signal Injection

Abstract: In this paper, we analyze the phase-locking phenomenon in a single-loop optoelectronic microwave oscillator, when subjected to the influence of small radio-frequency (RF) signal. We derive the differential equations for the amplitude and phase variations in the oscillator. Using quasi-linear approximation, analytical expressions for the lock range and phase-shift after phase-locking are presented. In addition, beat frequency of the unlocked-driven optoelectronic oscillator (OEO) is obtained and the phase-locking dynamics of the driven oscillator is discussed. Also, the spectrum components of the pulled OEO is derived as a function of the frequency detuning, lock range, beat-frequency, and frequency-shift induced by the phase perturbation of the injection signal. It is shown that all the analytical closed-form expressions clearly demonstrate the phase-locking mechanism starting from the fast-beat state through the quasi-locked state to the locked state of the pulled OEO. Finally, the simulation results are given to validate the analytical results.