Showing papers in "IEEE Journal of Selected Topics in Quantum Electronics in 2013"

Hybrid Silicon Photonic Integrated Circuit Technology

Abstract: In this paper, we review the current status of the hybrid silicon photonic integration platform with emphasis on its prospects for increased integration complexity. The hybrid silicon platform is maturing fast as increasingly complex circuits are reported with tens of integrated components including on-chip lasers. It is shown that this platform is well positioned and holds great potential to address future needs for medium-scale photonic integrated circuits.

Flat Optics: Controlling Wavefronts With Optical Antenna Metasurfaces

Abstract: Conventional optical components rely on the propagation effect to control the phase and polarization of light beams. One can instead exploit abrupt phase and polarization changes associated with scattered light from optical resonators to control light propagation. In this paper, we discuss the optical responses of anisotropic plasmonic antennas and a new class of planar optical components (“metasurfaces”) based on arrays of these antennas. To demonstrate the versatility of metasurfaces, we show the design and experimental realization of a number of flat optical components: 1) metasurfaces with a constant interfacial phase gradient that deflect light into arbitrary directions; 2) metasurfaces with anisotropic optical responses that create light beams of arbitrary polarization over a wide wavelength range; 3) planar lenses and axicons that generate spherical wavefronts and nondiffracting Bessel beams, respectively; and 4) metasurfaces with spiral phase distributions that create optical vortex beams of well-defined orbital angular momentum.

A Leaky Integrate-and-Fire Laser Neuron for Ultrafast Cognitive Computing

Abstract: We propose an original design for a neuron-inspired photonic computational primitive for a large-scale, ultrafast cognitive computing platform. The laser exhibits excitability and behaves analogously to a leaky integrate-and-fire (LIF) neuron. This model is both fast and scalable, operating up to a billion times faster than a biological equivalent and is realizable in a compact, vertical-cavity surface-emitting laser (VCSEL). We show that-under a certain set of conditions-the rate equations governing a laser with an embedded saturable absorber reduces to the behavior of LIF neurons. We simulate the laser using realistic rate equations governing a VCSEL cavity, and show behavior representative of cortical spiking algorithms simulated in small circuits of excitable lasers. Pairing this technology with ultrafast, neural learning algorithms would open up a new domain of processing.

High-Power Broadly Tunable Electrooptic Frequency Comb Generator

Abstract: Broadband traveling-wave electrooptic modulators made of lithium niobate have reached a high level of technological maturity. They can provide simultaneously low Vπ, sustain high power (both optical and RF) and yet provide low propagation loss. By combining together these features, we present a high-power handling, broadly tunable, electrooptic frequency comb generator. The device produces between 60 and 75 lines within -10 dB bandwidth over its full tuning range-from 6 to 18 GHz-and can handle up to 1 W of optical input power. This optical frequency comb platform is very well suited for applications in RF photonics and optical communications that require independent RF and optical tuning as well as high-repetition rates but moderate bandwidth.

Efficient High-Power Laser Diodes

Abstract: High-power broad-area diode lasers are the most efficient light sources, with 90-μm stripe GaAs-based 940-980 nm single emitters delivering > 10 W optical output at a power conversion efficiency ηE(10 W) > 65%. A review of efforts to increase ηE is presented here and we show that for well-optimized structures, the residual losses are dominated by the p -side waveguide and nonideal internal quantum efficiency ηi . The challenge in measuring efficiency to sufficient precision is also discussed. We show that ηE can most directly be improved using low heat sink temperature THS with ηE(10 W) reaching > 70% at THS = -50 °C. In contrast, increases in ηE at THS = 25 °C require improvements in both material quality and design, with growth studies targeting increased ηi and reduced threshold current and design studies seeking to mitigate the impact of the p-side waveguide. “Extreme, double asymmetric” (EDAS) designs are shown to substantially reduce p-side losses, at the penalty of increased threshold current. The benefit of EDAS designs is shown here using diode lasers with 30-μm stripes, (in development as high beam quality sources for material processing). Efficiency increases of ~ 10% relative to conventional designs are demonstrated at high powers.

Ring Resonator Modulators in Silicon for Interchip Photonic Links

Abstract: Silicon photonics is the most promising pathway to achieve >10 Tb/s off-chip I/O bandwidth required by next-generation high-performance computing and switching systems. Ring resonator modulators offer the advantages of small footprint, low power, high efficiency, low loss, high speed, and CMOS compatibility for silicon photonic links. This paper presents an in-depth discussion of practical microring modulators in silicon, covering their performance metrics, design tradeoffs, optimization, p-n junction geometries, complex ring configurations, and tuning solutions. Various demonstrated Si ring modulators are reviewed and potential future developments are briefly discussed.

LIONS: An AWGR-Based Low-Latency Optical Switch for High-Performance Computing and Data Centers

Abstract: This paper discusses the architecture of an arrayed waveguide grating router (AWGR)-based low-latency interconnect optical network switch called LIONS, and its different loopback buffering schemes. A proof of concept is demonstrated with a 4 × 4 experimental testbed. A simulator was developed to model the LIONS architecture and was validated by comparing experimentally obtained statistics such as average end-to-end latency with the results produced by the simulator. Considering the complexity and cost in implementing loopback buffers in LIONS, we propose an all-optical negative acknowledgement (AO-NACK) architecture in order to remove the need for loopback buffers. Simulation results for LIONS with AO-NACK architecture and distributed loopback buffer architecture are compared with the performance of the flattened butterfly electrical switching network.

Silicon-Organic Hybrid Electro-Optical Devices

Abstract: Organic materials combined with strongly guiding silicon waveguides open the route to highly efficient electro-optical devices. Modulators based on the so-called silicon-organic hybrid (SOH) platform have only recently shown frequency responses up to 100 GHz, high-speed operation beyond 112 Gbit/s with fJ/bit power consumption. In this paper, we review the SOH platform and discuss important devices such as Mach-Zehnder and IQ-modulators based on the linear electro-optic effect. We further show liquid-crystal phase-shifters with a voltage-length product as low as VπL = 0.06 V·mm and sub-μW power consumption as required for slow optical switching or tuning optical filters and devices.

High-Power Operation of Terahertz Oscillators With Resonant Tunneling Diodes Using Impedance-Matched Antennas and Array Configuration

Abstract: We report the theoretical and experimental results of an examination of the structure needed to achieve high output power in resonant tunneling diode (RTD) oscillators in the terahertz range. An offset-fed slot antenna and antenna width adjustments were employed in a single oscillator to increase the output power by increasing the radiation conductance and impedance matching. A high output power oscillation (~400 μW) at 530-590 GHz was obtained by RTDs with a large negative deferential conductance (NDC) region and offset-fed slot antennas. The maximization of the output power that was obtained by adjusting the antenna width was attributed to the impedance matching between the RTD and antenna. An output power of >;1 mW is theoretically expected in an oscillator that combines an RTD with a large NDC region, offset-fed slot antenna, and antenna width adjustment. In an array configuration, oscillators with an offset structure were employed for array elements and connected together with the metal-insulator-metal stub structure. A single peak was observed in the oscillation spectrum, and combined output powers of 610, 270, and 180 μW at 620, 770, and 810 GHz were obtained in a two-element array.

Plasmonic Resonances in Nanostructured Transparent Conducting Oxide Films

Abstract: Transparent conducting oxides (TCOs) are emerging as possible alternative constituent materials to replace noble metals such as silver and gold for low-loss plasmonic and metamaterial (MM) applications in the near infrared regime (NIR). The optical characteristics of TCOs have been studied to evaluate the functionalities and potential of these materials as metal substitutes in plasmonic and MM devices, even apart from their usual use as electrode materials. However, patterning TCOs at the nanoscale, which is necessary for plasmonic and MM devices, is not well studied. This paper investigates nanopatterning processes for TCOs, especially the liftoff technique with electron-beam lithography, and the realization of plasmonic nanostructures with TCOs. By employing the developed nanopatterning process, we fabricate 2-D-periodic arrays of TCO nanodisks and characterize the material's plasmonic properties to evaluate the performance of TCOs as metal substitutes. Light-induced collective oscillations of the free electrons in the TCOs (bulk plasmons) and localized surface plasmon resonances are observed in the wavelength range from 1.6 to 2.1 μm. Well-defined resonance peaks are observed, which can be dramatically tuned by varying the amount of dopant and by thermally annealing the TCO nanodisks in nitrogen gas ambient while maintaining the low-loss properties.

Fabrication of a Graded-Index Circular-Core Polymer Parallel Optical Waveguide Using a Microdispenser for a High-Density Optical Printed Circuit Board

Abstract: A simple fabrication method for multimode polymer optical waveguides with graded-index (GI) circular cores is introduced for use in optical printed circuit boards (O-PCBs). The new method, named “Mosquito method,” utilizes a microdispenser to dispense a viscous monomer directly onto the substrates. By optimizing the dispensing conditions, 12-channel parallel waveguides with circular GI-cores (core diameter of 40 μm) are successfully fabricated using the Mosquito method. The advantages of GI-core waveguides for O-PCB applications are discussed by comparing the optical characteristics of the fabricated waveguides with those of conventional step-index (SI) square-core polymer waveguides, and even with those of silica-based GI multimode fibers (MMFs), as an ideal case. To the best of our knowledge, this is the first comparison of SI- and GI-core multimode polymer waveguides that are composed of the same polymer materials and that have similar core and pitch sizes. We experimentally demonstrate that the GI circular-core polymer waveguides fabricated by the Mosquito method have sufficiently low propagation loss (0.033 dB/cm at 850 nm), low connection loss with GI-MMFs, and low interchannel crosstalk. We observe approximately -50 dB of interchannel crosstalk in the 250-μm pitch GI-core waveguide fabricated, which is almost 10 dB lower than in the SI counterpart. Furthermore, sufficiently low crosstalk is maintained in a half-pitch GI-core waveguide fabricated by the Mosquito method.

Interband Cascade Lasers With Low Threshold Powers and High Output Powers

Abstract: The midwave infrared interband cascade laser (ICL) can operate at threshold power densities 30 times lower than those of the quantum cascade laser. This is ultimately attributable to the much longer interband carrier lifetime, rather than to specifics of the cavity dimensions and mirror reflectivities. The ICL is therefore an attractive candidate for insertion into the portable, battery-powered chemical sensors now being developed for this spectral region. We review the characteristics of ICLs operating at wavelengths from 2.9 to 5.5 μm, and show that their Auger coefficients vary by less than a factor of 3 throughout this range. Consequently, the ICL performance degrades only modestly with increasing wavelength. We report that an epitaxial-side-down-mounted ICL ridge of width 30 μm and λ = 3.7 μm emits more than 300 mW of continuous wave (CW) output power at room temperature with M2 ≤ 3.1. A distributed-feedback ICL with a fourth-order grating etched into its corrugated sidewalls produces 55 mW of CW power in a single spectral mode at T = 25 °C.

Long-Wavelength VCSEL Using High-Contrast Grating

Abstract: Recent advances in high-contrast grating (HCG) vertical-cavity surface-emitting lasers (VCSEL) emitting at 1550 nm is reported in this paper. The novel near-wavelength HCG has an ultrathin structure and broadband reflectivity. It enables a monolithic, simple fabrication process for realizing InP-based VCSELs emitting at ~1550 nm. We report 2.4-mW single-mode output under continuous-wave operation at 15°C. We show that, despite broadened by the Brownian motion, the HCG-VCSEL has a total linewidth of 60 MHz or a coherent length of 5 m in air, and an intrinsic linewidth <;20 MHz. Transmission of directly modulated 10 Gbps over 100-km dispersion-compensated single-mode fiber is demonstrated. Tunable HCG-VCSEL is demonstrated with the HCG integrated with a micro-electro-mechanical structure. Continuous wavelength tuning as wide as 26.3 nm is achieved. The tunable VCSEL was used as a source for external modulation for 40-Gbps differential-phase-shift-keyed signal and transmitted over 100-km dispersion-compensated link with negligible power penalty.

50-Gb/s Direct Modulation of a 1.3-μm InGaAlAs-Based DFB Laser With a Ridge Waveguide Structure

Abstract: We demonstrate 50-Gb/s direct modulation by using 1.3-μm distributed-feedback lasers with a ridge waveguide structure. We employed InGaAlAs material for a multiple-quantum well to obtain a low damping factor K, and fabricated a ridge waveguide structure buried in benzocyclobutene to realize a structure with a low parasitic capacitance. In addition, to obtain high maximum frequency relaxation oscillations fr, we designed the cavity length L), and achieved a 3-dB-down frequency bandwidth of 34 GHz. We realized 50-Gb/s clear eye openings with a back-to-back configuration, and achieved a mean output power of over 5.0 dBm, and a dynamic extinction ratio of 4.5 dB. We measured the 50-Gb/s transmission characteristics, and obtained clear eye openings for transmissions over 20-, 40-, and 60-km single-mode fibers (SMF). We also measured the bit-error-rate performance, and obtained an error-free operation and a power penalty of less than 0.5 dB after a 10-km SMF transmission.

Polymer-Based Hybrid-Integrated Photonic Devices for Silicon On-Chip Modulation and Board-Level Optical Interconnects

Abstract: The accelerating increase in information traffic demands the expansion of optical access network systems that require the cost reduction of optical and photonic components. Low cost, ease of fabrication, and integration capabilities of low optical-loss polymers make them attractive for photonic applications. In addition to passive wave-guiding components, electro-optic (EO) polymers consisting of a polymeric matrix doped with organic nonlinear chromophores have enabled wide-RF-bandwidth and low-power optical modulators. Beside board level passive and active optical components, compact on-chip modulators (a few 100 μm to a few millimeters) have been made possible by hybrid integration of EO polymers onto the silicon platform. This paper summarizes some of the recent progress in polymer-based optical modulators and interconnects. A highly linear, broadband directional coupler modulator for use in analog optical links and compact, and low-power silicon/polymer hybrid slot photonic crystal waveguide modulators for on chip applications are presented. Recently, cost-effective roll-to-roll fabrication of electronic and photonic systems on flexible substrates has been gaining interest. A low-cost imprinted/ink-jet-printed Mach-Zehnder modulator and board-to-board optical interconnects using microlens integrated 45° mirror couplers compatible with the roll-to-roll fabrication platforms are also presented.

Nonequilibrium Green’s Function Model for Simulation of Quantum Cascade Laser Devices Under Operating Conditions

Abstract: A simulation scheme based on nonequilibrium Green's functions for biased periodic semiconductor heterostructure devices is presented in detail. The implementation can determine current and optical gain both for small and large optical fields. Specific results for superlattices, quantum cascade lasers, and quantum cascade detectors are shown which demonstrate the capabilities of the approach.

Information Processing Using Transient Dynamics of Semiconductor Lasers Subject to Delayed Feedback

Abstract: The increasing amount of data being generated in different areas of science and technology require novel and efficient techniques of processing, going beyond traditional concepts. In this paper, we numerically study the information processing capabilities of semiconductor lasers subject to delayed optical feedback. Based on the recent concept of reservoir computing, we show that certain tasks, which are inherently hard for traditional computers, can be efficiently tackled by such systems. Major advantages of this approach comprise the possibility of simple and low-cost hardware implementation of the reservoir and ultrafast processing speed. Experimental results corroborate the numerical predictions.

InAs/GaAs Quantum-Dot Lasers Monolithically Grown on Si, Ge, and Ge-on-Si Substrates

Abstract: The realization of semiconductor lasers on Si substrates will enable the fabrication of complex optoelectronic circuits. This will permit the creation of the long-dreamed chip-to-chip and system-to-system optical interconnects. This paper reports recent developments in our work on InAs/GaAs quantum-dot (QD) lasers monolithically grown on Si, Ge, and Ge-on-Si (Ge/Si) substrates. A thin AlAs nucleation layer (NL) was first investigated for the growth of InAs/GaAs QDs on Si substrates. The AlAs NL enables more defects to be confined in the interface between the GaAs epitaxial layer and Si substrate, and hence leads to higher photoluminescence intensity for InAs/GaAs QDs. Room-temperature lasing at 1.29 μm with a threshold current density of 650 A/cm2 was demonstrated with the use of an AlAs NL. The growth of InAs/GaAs QDs on Ge and Ge/Si substrates was further studied. A low threshold current density of ~200 A/cm2 for 1-mm long QD lasers has been demonstrated for QD lasers grown on Ge substrates by using Ga prelayer technique. This growth technique has also been explored for Ge/Si substrates. Room-temperature lasing at 1.28 μm with threshold current density of ~164 A/cm2 and lasing operation up to 84°C has been demonstrated for a 3-mm long device.

Two-Dimensional Optical Beam Steering With InP-Based Photonic Integrated Circuits

Abstract: Two-dimensional optical beam steering using an InP photonic integrated circuit has been demonstrated. Lateral beam steering controlled by a 1-D phased array has been made easier through on-chip interferometer monitors. Longitudinal beam steering controlled by the input wavelength has demonstrated an efficiency of 0.14 °/nm. Very fast beam steering (>107 °/s) in both dimensions has been demonstrated as well. As the latest development, a widely tunable sampled-grating distributed Bragg reflector laser has been monolithically integrated and 2-D beam steering has been demonstrated with this on-chip tunable laser source.

Theoretical Analysis of GeSn Alloys as a Gain Medium for a Si-Compatible Laser

Abstract: In this paper, a theoretical analysis of unstrained GeSn alloys as a laser gain medium was performed Using the empirical pseudopotential method, the band structure of GeSn alloys was simulated and verified against experimental data This model shows that GeSn becomes direct bandgap with 655% Sn concentration The optical gain of GeSn alloys with 0-10% Sn concentration was calculated with different n-type doping concentrations and injection levels It is shown theoretically that adding Sn greatly increases the differential gain owing to the reduction of energy between the direct and indirect conduction bands For a double-heterostructure laser, the model shows that at a cavity loss of 50 cm-1, the minimum threshold current density drops 60 times from Ge to Ge09Sn01, and the corresponding optimum n-doping concentration of the active layer drops by almost two orders of magnitude These results indicate that GeSn alloys are good candidates for a Si-compatible laser

What is Laser Threshold

Abstract: The question of “what is a laser” is closely related to that of what is laser threshold. We analyze possible definitions of laser threshold through the L-I curve and conclude that there is no consistent definition of a threshold when the spontaneous emission factor is large, as in the case of micro- and nano-lasers. All these definitions based on L-I curve are not consistent with the evolution of statistical properties of light output with pumping. Thus, we argue that the threshold definition based on L-I curve has to be abandoned for nanolasers, where the fraction of spontaneous emission into a lasing mode becomes quite large.

The Diagram of Feedback Regimes Revisited

Abstract: We revisit the well-known Tkach and Chraplyvy (T-C) diagram of feedback regimes in semiconductor lasers. Our aim is twofold: first, extending the classification of feedback effects in the T-C diagram to short and long external cavities, and to coherent and incoherent interactions; second and more important, identifying in the diagram feedback phenomena that have been meanwhile studied and developed to noteworthy applications, namely, self-mixing, period-1 and multiperiodicity, intermittency and chaos. We complement the feedback diagram with application regions, so as to describe not only feedback effects detrimental to a laser used as the transmitter of an optical link, but also feedback effects in the weak and strong regime of interaction, developed into applications for instrumentation and communications in recent years.

Terahertz-Wave Generation Using Graphene: Toward New Types of Terahertz Lasers

Abstract: This paper reviews recent advances in terahertz-wave generation in graphene toward the creation of new types of terahertz lasers. First, fundamental basis of the optoelectronic properties of graphene is introduced. Second, nonequilibrium carrier relaxation or recombination dynamics in optically or electrically pumped graphene is described to introduce a possibility of negative dynamic conductivity in a wide terahertz range. Third, recent theoretical advances toward the creation of current-injection graphene terahertz lasers are described. Fourth, unique terahertz dynamics of the 2-D plasmons in graphene are described. Finally, the advantages of graphene materials and devices for terahertz-wave generation are summarized.

Back-End Deposited Silicon Photonics for Monolithic Integration on CMOS

Abstract: We present the vision of back-end deposited silicon photonics (BDSP) and review works that have been done in this field. Individual aspects of BDSP platform including excimer-laser-annealed polycrystalline silicon, low-loss plasma-enhanced chemical vapor deposition silicon nitride waveguide, modulator, detector, electrical interface, back-end CMOS compatibility, and benefits of the platform are discussed in detail.

Dependence of Ion-Implant-Induced LBIC Novel Characteristic on Excitation Intensity for Long-Wavelength HgCdTe-Based Photovoltaic Infrared Detector Pixel Arrays

Abstract: In this paper, experimental results of laser-irradiance-dependent polarity inversion of laser beam induced current (LBIC) for As-doped long-wavelength HgCdTe pixel arrays grown on CdZnTe are reported. Models for the ion-implant-induced junction transformation are proposed, and demonstrated using numerical simulations. The novel trap-related p-n junction transformation induced by ion implantation is observed under typical laser irradiances for low temperature. The implantation-induced traps and Hg interstitial diffusion are key factors for inducing the LBIC coupling, polarity reversion, and junction broadening at different laser irradiances. The trap type, trap density, and junction configuration are extracted from the measured experiment data. The results provide the near room-temperature HgCdTe photovoltaic detector with a reliable reference on the junction reversion and broadening around implanted regions, as well as controlling the n-on-p junction formation for very long wavelength HgCdTe infrared detector pixels.

Ultralow Operating Energy Electrically Driven Photonic Crystal Lasers

Abstract: The introduction of the photonic crystal (PhC) wavelength-scale cavity as a laser cavity enables us to obtain both ultralow threshold current and operating energy. These parameters are essential when using the transmitters in chip-to-chip and on-chip interconnections. To improve the device performance, we employ an ultracompact embedded active region that we call a lambda-scale embedded active-region PhC laser or LEAP laser. We have developed an electrically driven LEAP laser, which operates under room-temperature continuous-wave conditions. To fabricate the electrically driven LEAP laser, we used Zn thermal diffusion and Si ion implantation, respectively, for p-type and n-type doping in an undoped InP layer. However, with previous fabricated devices there was a large leakage current through the substrate and the threshold current was 0.39 mA, which is larger than the expected threshold obtained by optical pumping. To reduce the leakage current, we propose using an InAlAs sacrificial laser instead of an InGaAs layer. The leakage current path through the substrate is effectively suppressed, and as a result, the threshold current is reduced to 7.8 μA, which is the lowest threshold current reported for any laser. Furthermore, the LEAP laser operates at up to 95 °C by using an InGaAlAs-based multiple quantum well structure. We also describe the dynamic characteristics of the laser. The LEAP laser exhibits a maximum 3-dB bandwidth of 16.2 GHz and the modulation current efficiency factor is 53.8 GHz/mA0.5 or 1.7 GHz/μA0.5, which is four times that of a vertical cavity surface-emitting laser. The device is directly modulated by a 12.5-Gb/s nonreturn-to-zero signal with a bias voltage of 1.6 V and a bias current of 109 μA, resulting in an energy cost of 14.0 fJ/b. This is the smallest operating energy for any laser. These results indicate that the LEAP laser is highly suitable for use as a transmitter in computercom applications.

Design and Evaluation of a Flexible-Bandwidth OFDM-Based Intra-Data Center Interconnect

Abstract: Data center networks are facing growing challenges to deliver higher bandwidth efficiency, lower latency, better flexibility, and lower cost. Various optical interconnect schemes have been proposed to take advantage of the high bandwidth capacity and low power consumption offered by optical switching. However, these schemes cannot offer flexible bandwidth sharing due to the large granularity in optical circuit switching, and they require costly optical components. In this paper, we introduce a novel data center network architecture based on cyclic arrayed waveguide grating device and multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing technology with parallel signal detection (PSD). This architecture offers flexible bandwidth resource sharing at fine granularity. Other features include high-speed switching, low and uniform latency, and the ability to change the data rates dynamically. By eliminating costly optical components and keeping the core optical router passive and static, the power consumption, hardware cost, and operation cost are reduced. The fine granularity bandwidth sharing and MIMO switching through PSD are verified experimentally. We also propose and evaluate efficient subcarrier allocation schemes to achieve high bandwidth utilization. Finally, we present the implementation of an efficient scheduler for the bandwidth allocation of the proposed scheme.

Linewidth Sharpening via Polarization-Rotated Feedback in Optically Injected Semiconductor Laser Oscillators

Abstract: Combining optical injection and polarization-rotated optical feedback in a semiconductor laser can induce self-referenced periodic output that is widely tunable by simply varying the dc-bias points of the system's master and slave lasers. We observed a feedback-induced reduction of the fundamental period-one oscillation linewidth by more than two orders of magnitude relative to the injection-only case. Performance was found to be negatively affected by the interference between the external injection signal and the residual feedback in the same polarization. The nonlinear dynamics of the optically injected semiconductor laser can be used to minimize sensitivity to fluctuations in the operating points. However, the use of the nonlinear dynamics at high oscillation frequencies is limited by the decreasing strength of the interaction between the circulating intracavity optical field and the carrier density.

Theoretical Analysis of Hybrid Plasmonic Waveguide

Abstract: We investigate the properties of the modes supported by the hybrid plasmonic waveguide consisting of a metal surface separated from a high-index slab by a low-index spacer. We examine the variations of the effective mode indices and field profiles of the hybrid modes for various choices of waveguide dimensions. We show that the observed variations of the modal properties can be explained from the fact that these modes result from the coupling of the surface plasmons, supported by the metal-dielectric interface, and the dielectric waveguide mode, supported by the high-index slab. The method of analysis is very general and can be used to explain the modal properties of the hybrid plasmonic waveguides for a wide range of material properties and waveguide dimensions.

Energy Efficiency of Directly Modulated Oxide-Confined High Bit Rate 850-nm VCSELs for Optical Interconnects

Abstract: The design, fabrication, and performance of the presently most energy-efficient oxide-confined 850 nm vertical-cavity surface-emitting lasers (VCSELs) for optical interconnects are presented. We employ a novel current spreading layer to reduce differential resistance. Compared to our previous designs, a higher indium content is used in the InGaAs quantum wells to increase the differential gain at low injected current densities. The influence of the oxide aperture diameter on the energy efficiency is determined by comparing the key performance parameters for a batch of VCSELs produced on the same epitaxial wafer, but with varying aperture diameters from 2.5 to 9.0 μm. The static light output power-current-voltage characteristics, small-signal modulation response, and large signal performance of our VCSELs are investigated in detail. The parameters important for energy-efficient operation are analyzed including threshold current, differential quantum efficiency, and differential resistance. We observe that our single-mode VCSELs are more energy efficient than our multimode VCSELs, although our multimode VCSELs typically exhibit a larger maximum static wallplug efficiency. Error-free (defined as a bit error ratio <;1 × 10 -12) data transmission at 25 Gb/s with a record-low dissipated heat energy of only 56 fJ/bit is achieved using a single-mode VCSEL with an oxide aperture diameter of 3.5 μm.