Showing papers in "Journal of Lightwave Technology in 2008"

Compensation of Dispersion and Nonlinear Impairments Using Digital Backpropagation

Abstract: Optical fiber transmission is impacted by linear and nonlinear impairments. We study the use of digital backpropagation (BP) in conjunction with coherent detection to jointly mitigate dispersion and fiber nonlinearity. We propose a noniterative asymmetric split-step Fourier method (SSFM) for solving the inverse nonlinear Schrodinger equation (NLSE). Using simulation results for RZ-QPSK transmitted over terrestrial systems with inline amplification and dispersion compensation, we obtain heuristics for the step size and sampling rate requirements, as well as the optimal dispersion map.

Broadband Information Broadcasting Using LED-Based Interior Lighting

Abstract: Emergence of white-light LEDs allows the combination of lighting and information broadcast functionality in one optical source. We investigate analytically and by Monte Carlo simulations feasible data transmission rates in a moderate-size office room, where we assume illumination conforming to standards and the use of commercially available LEDs and photodiodes. The performances of systems relying on baseband [i.e., pulse-amplitude modulation (PAM)] and discrete multitone (DMT) transmission show that data rates of more than 100 Mbit/s can be expected despite the rather low bandwidth of the system.

Fiber-Optic Sensing: A Historical Perspective

Abstract: Sensing via fiber optics has occupied R&D groups for over 40 years, and some important transitions into the commercial sector have been achieved. We look at the fundamental concepts involved in the various sensing approaches, and the differentiators which have led to commercial impact. We also look to the future of fiber-optic sensors.

Vector Finite Difference Modesolver for Anisotropic Dielectric Waveguides

Abstract: We describe a new full-vector finite difference discretization, based upon the transverse magnetic field components, for calculating the electromagnetic modes of optical waveguides with transverse, nondiagonal anisotropy. Unlike earlier finite difference approaches, our method allows for the material axes to be arbitrarily oriented, as long as one of the principal axes coincides with the direction of propagation. We demonstrate the capabilities of the method by computing the circularly-polarized modes of a magnetooptical waveguide and the modes of an off-axis poled anisotropic polymer waveguide.

Investigation on Single-Mode–Multimode– Single-Mode Fiber Structure

Abstract: This paper presents an investigation on a single-mode-multimode-single-mode fiber structure. A one-way guided-mode propagation analysis for the circular symmetry waveguide is employed to model the light propagation and the approximated formulations are derived and evaluated concerning the accuracy. Phase conjunction of the multimode interference within the fiber structure is revealed. A simple way to predict and analyze the spectral response of the structure is presented through the space to wavelength mapping with the derived approximated formulations. The prediction of spectral response is verified numerically and experimentally.

History, Development, and Applications of High-Brightness Visible Light-Emitting Diodes

Abstract: In a practical sense, the development of high-performance visible-spectrum light-emitting diodes (LEDs) has occurred over a period of over 60 years, beginning with the discovery of the first semiconductor p-n junction in 1940, the development of solid-state electronic band theory in the 1940s, the invention of the first bipolar transistor in 1947, and the demonstration of efficient light generation from III-V alloys in the 1950s and 1960s. This paper reviews some of the major scientific and technological developments and observations that have created the materials and device technologies currently used in the commercial mass production of high-brightness visible-spectrum LEDs and that have culminated in white-light sources exhibiting luminous efficacies of over 150 lm/W, far beyond what has been achieved with conventional lighting technologies.

Optical Networking: Past, Present, and Future

Abstract: Over the past 25 years, networks have evolved from being relatively static with fairly homogeneous traffic to being more configurable and carrying a heterogeneous array of services. As the applications are ultimately the driver of network evolution, the paper begins with a brief history of circuit, packet, and wave services, along with the development of the corresponding transport layers. The discussion then moves to the evolution of network-node architecture, with an emphasis on the optical-electrical-optical and optical-bypass paradigms. Scalability and cost-effectiveness in meeting network demands are two key factors in the discussion. The evolution of networking equipment, along with the development of the optical control plane, has facilitated a configurable optical layer. The enabling technologies, along with their ramifications, are discussed. Finally, the paper speculates on how capacity might evolve in the future, to handle the undoubtedly new services that are on the horizon.

Phase Noise Effects on High Spectral Efficiency Coherent Optical OFDM Transmission

Abstract: There are three major advantages for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission using digital signal processing. First, coherent detection is realized by digital phase estimation without the need for optical phase-locked loop. Second, OFDM modulation and demodulation are realized by the well-established computation-efficient fast Fourier transform (FFT) and inverse FFT. Third, adaptive data rates can be supported as different quadrature amplitude modulation (QAM) constellations are software-defined, without any hardware change in transmitter and receiver. However, it is well-known that coherent detection, OFDM, and QAM are all susceptible to phase noise. In this paper, theoretical, numerical, and experimental investigations are carried out for phase noise effects on high spectral efficiency CO-OFDM transmission. A transmission model in the presence of phase noise is presented. By using simulation, the bit error rate floors from finite laser linewidth are presented for CO-OFDM systems with high-order QAM constellations. In the experiments, the phase noise effects from both laser linewidth and nonlinear fiber transmission are investigated. The fiber nonlinearity mitigation based on receiver digital signal processing is also discussed.

Radio-Over-MMF Techniques—Part II: Microwave to Millimeter-Wave Systems

Abstract: Microwave to mm-wave radio carriers are commonly employed for creating high-capacity picocell wireless networks. Advanced radio-over-fiber (RoF) techniques can efficiently generate and transport such carriers, and deliver them to simplified antenna stations. As in in-building networks multimode fiber is predominantly used, adequate radio-over-multimode fiber (RoMMF) techniques are required to overcome the modal dispersion in multimode fiber links. The optical frequency multiplying technique is introduced; it is relatively simple to implement, yet it is shown to be robust against the modal dispersion, and it is able to generate very pure microwave carriers while requiring only moderate speed electronics. Thus, it can convey high data rates in comprehensive modulation formats on multiple-GHz carriers in MMF networks. It offers simultaneous operation at multiple radio standards, and capabilities for dynamic adaptation of the radio link parameters such as carrier frequency, transmit power, and other antenna site functions by means of an embedded control channel. Moreover, in combination with optical routing it enables dynamically adjustable network configurations for flexible wireless service delivery.

Dual-Frequency Laser at 1.5 $\mu$ m for Optical Distribution and Generation of High-Purity Microwave Signals

Abstract: We describe the stabilization of the beatnote of an Er,Yb:glass dual-frequency laser at 1.5 mum with and without an external microwave reference. In the first case, a classical optical phase-locked loop (OPLL) is used, and absolute phase noise levels as low as -117 dBrad2/Hz at 10 kHz from the carrier are reported. In the second case one or two fiber-optic delay lines are used to lock the frequency of the beatnote. Absolute phase noise levels as low as -107 dBrad2/Hz at 10 kHz from the carrier are measured, fairly independant of the beatnote frequency varying from 2 to 6 GHz. An analysis of the phase noise level limitation is presented in the linear servo-loop theory framework. The expected phase noise level calculated from the measurement of the different noise sources fits well with the predictions.

Studies of Phosphor Concentration and Thickness for Phosphor-Based White Light-Emitting-Diodes

Abstract: The dependence of luminous efficacy on phosphor concentration and thickness for high-power white light-emitting-diode (WLED) lamps is investigated by employing three-dimensional ray-tracing simulations. The simulations show that the brightness or luminous efficacy of WLED lamps highly depends on the combination of phosphor concentration and phosphor thickness (or phosphor-matrix composite volume). The package with lower concentration and higher phosphor thickness has higher luminous efficacy because the light trapping efficiency is lower with the low phosphor concentration. At the correlated color temperature (CCT) value of around 4000 K, ray-tracing simulation and experimental results show 20% and 23% improvement in lumen, respectively, with a 1.8-mm-phosphor package over a 0.8-mm-phosphor package. A package with convex lens can improve the lumen output over flat lens, but this improvement is small, and it requires higher amount of phosphor, up to 25%, to achieve same CCT value.

Fiber-to-the-Home: 1977–2007

Abstract: Fiber has been envisioned for delivering broadband services to the residential customer for over 30 years, yet it has only recently entered the mainstream. Currently, fiber-to-the-home (FTTH) is being installed in many countries at remarkable rates (even though it still constitutes only a fraction of all broadband lines in most countries). Other lightwave transmission technologies have progressed far faster. What has held FTTH up for so long? What improvements along the way have occurred? What recent changes have made it successful? This article follows the progress in moving fiber toward the home and major architectural changes that have reduced costs while increasing capabilities to meet today's needs.

Evaluation of Sensitivity of the Digital Coherent Receiver

Abstract: In this paper, we investigate the sensitivity of the digital coherent receiver both theoretically and experimentally. The receiver sensitivity close to the shot-noise limit is demonstrated in the 10-Gbit/s binary phase-shift keying system with the help of a low-noise optical preamplifier. We also introduce polarization diversity into our receiver. Maximal-ratio polarization combining in the digital domain makes the receiver sensitivity independent of the state of polarization of the incoming signal without power penalty.

100-Gb/s DQPSK Transmission: From Laboratory Experiments to Field Trials

Abstract: We discuss the generation, detection, and long-haul transmission of single-polarization differential quadrature phase shift keying (DQPSK) signals at a line rate of 53.5 Gbaud to support a net information bit rate of 100 Gb/s. In the laboratory, we demonstrate 10-channel wavelength-division multiplexed (WDM) point-to-point transmission over 2000 km on a 150-GHz WDM grid, and 1200-km optically routed networking including 6 reconfigurable optical add/drop multiplexers (ROADMs) on a 100-GHz grid. We then report transmission over the commercial, 50-GHz spaced long-haul optical transport platform LambdaXtremereg. In a straight-line laboratory testbed, we demonstrate single-channel 700-km transmission, including an intermediate ROADM. On a field-deployed, live traffic bearing Verizon installation between Tampa and Miami, Florida, we achieve 500-km transmission, with no changes to the commercial system hardware or software and with 6 dB system margin. On the same operational system, we finally demonstrate 100-Gb/s DQPSK encoding on a field-programmable gate array (FPGA) and the transmission of real-time video traffic.

The Data Vortex Optical Packet Switched Interconnection Network

Abstract: A complete review of the data vortex optical packet switched (OPS) interconnection network architecture is presented. The distributed multistage network topology is based on a banyan structure and incorporates a deflection routing scheme ideally suited for implementation with optical components. An implemented 12-port system prototype employs broadband semiconductor optical amplifier switching nodes and is capable of successfully routing multichannel wavelength-division multiplexing packets while maintaining practically error-free signal integrity (BER < 10-12) with median latencies of 110 ns. Packet contentions are resolved without the use of optical buffers via a distributed deflection routing control scheme. The entire payload path in the optical domain exhibits a capacity of nearly 1 Tb/s. Further experimental measurements investigate the OPS interconnection network's flexibility and robustness in terms of optical power dynamic range and network timing. Subsequent experimental investigations support the physical layer scalability of the implemented architecture and serve to substantiate the merits of the data vortex OPS network architectural paradigm. Finally, modified design considerations that aim to increase the network throughput and device-level performance are presented.

Design of Photonic Crystal Nanocavity With $Q$ -Factor of ${{\sim}10^{9}}$

Abstract: Photonic crystal nanocavities are expected to make great contributions in areas of physics and engineering such as the slowing and stopping of light and optical quantum information processing. We first review approaches to the goal of increasing the quality factor of two-dimensional photonic crystal nanocavities. Losses from a cavity can be suppressed, with a consequent increase of the quality factor, by containing the electromagnetic field in the form of a Gaussian envelope function. We then propose a new method of analytical cavity design, based on the detailed investigation of waveguide modes, in order to realize Gaussian cavity mode fields. This has enabled us to achieve a quality factor of while maintaining a cavity volume of one cubic wavelength.

60-GHz Photonic Millimeter-Wave Link for Short- to Medium-Range Wireless Transmission Up to 12.5 Gb/s

Abstract: In this paper, a 60-GHz photonic millimeter-wave link system for short- to medium-range broadband wireless data transmission is investigated. The system employs advanced mm-wave photonic components and radio-over-fiber (RoF) techniques for the generation of a DSB-SC optical mm-wave carrier and its subsequent on-off-keying modulation and transmission. For short-range applications, we have constructed a compact wireless RoF transmitter consisting of a high-frequency photodiode and a mm-wave antenna only. This system achieved error-free (BER=10-9, 231-1 PRBS, NRZ) in-door transmission of 12.5-Gb/s signals over wireless distances up to 3.1 m with a receiver sensitivity as low as - 45.4 dBm . For fixed wireless access (FWA) requiring a bit error rate of 10-4, the maximum transmission distance for 12.5 Gb/s is increased up to 5.8 m. For medium-range broadband wireless transmission an electrical radio-frequency (RF) amplifier was employed in the RoF transmitter. Here we achieved 7.5-Gb/s error-free transmission in out-door line-of-sight experiments over wireless distances of up to 36 m. Based upon the experimental results, we expect that the maximum wireless distance the system could accommodate for 12.5 Gb/s is in the kilometer range when using high-gain antennas and an RF transmitter amplifier with a sufficient bandwidth.

High Dynamic Range Microwave Photonic Links for RF Signal Transport and RF-IF Conversion

Abstract: Two different techniques were used to extend the dynamic range of analog optical links operating at microwave frequencies. The link noise figure was reduced by adjusting the bias point of an external Mach-Zehnder intensity modulator. The link distortion was reduced by linearizing the transfer function of the intensity modulator. Frequency conversion was implemented using an additional optical modulator and electrical local oscillator rather than a conventional electronic mixer. A microwave frequency signal transport link was demonstrated with a spur-free dynamic range (SFDR) of 71 dB in a 500-MHz noise bandwidth. A link with down-conversion from a microwave frequency to an intermediate frequency was demonstrated with an SFDR of 64 dB in a 500-MHz noise bandwidth.

Microstructure Fiber Based Optical Parametric Oscillators

Abstract: The current status of fiber based optical parametric oscillators is presented with a focus on pulsed systems employing microstructure fibers. It is shown based on standard expressions for parametric processes in optical fibers that systems employing short (less than a few cm) optical fibers lead to superior performance in terms of wavelength tunability and output power. Practical guidelines for realizing a working system are given. These devices are now practical as ultrafast pulsed-light sources and for extending the wavelengths of operation of existing mode-locked fiber lasers.

Frequency Quadrupling and Upconversion in a Radio Over Fiber Link

Abstract: In this paper, a novel technique to realize frequency quadrupling and upconversion in a radio over fiber (RoF) link is proposed and experimentally demonstrated. The frequency quadrupling is achieved by using two cascaded Mach-Zehnder modulators (MZMs) that are biased at the minimum transmission point, with a tunable optical delay line placed between the MZMs. By properly adjusting the time delay between the two MZMs, a pair of optical wavelengths with a wavelength spacing corresponding to four times the frequency of the microwave drive signal is generated. The two wavelengths are then sent to a third MZM to which an intermediate-frequency (IF) signal is applied. At the output of the third MZM, a frequency-upconverted signal at the millimeter-wave (mm-wave) band is obtained. The advantages of the technique are that a relatively low-frequency local oscillator (LO) signal is used to generate a high-frequency LO signal and the upconverted signal is more tolerant to the dispersion-induced power fading compared with a conventional RoF link based on double-sideband (DSB) modulation. Experiments are performed to verify the technique.

High-Capacity Optical Transmission Systems

Abstract: The 25 years since the founding of the Journal of Lightwave Technology have seen more than three orders of magnitude increase in the capacity of optical transmission systems. This dramatic increase was brought about by the deployment of WDM and advances in high-speed transmission technologies.

Silicon-on-Insulator Microring Add-Drop Filters With Free Spectral Ranges Over 30 nm

Abstract: We demonstrate highly compact optical add-drop filters based on silicon-on-insulator microring resonators. The microring resonators have a small radius of 2.5 mum and a very large free spectral range ~ 32 nm at the 1.55 mum communication band. The propagation loss in such small micoring resonators was experimentally determined and shown to be extremely important in designing microring add-drop filters with low add-drop crosstalk, low drop loss, and maximally flat drop passband. For box-like channel dropping responses, second-order optical add-drop filters with two coupled microring resonators are designed and demonstrated, and the simulation matches well with the experiment. Devices were patterned with electron-beam lithography. Two fabrication procedures utilizing different polarity of resists were introduced and compared, and the process with negative resist resulted in much smaller sidewall roughness of waveguides, thus reducing the propagation loss in microring resonators.

Narrow Linewidth CW Laser Phase Noise Characterization Methods for Coherent Transmission System Applications

Abstract: Several techniques for phase noise PSD measurement of continuous wave (CW) lasers to be used in coherent transmission systems are analyzed. Between them, we evaluate two novel techniques. The first employs a homodyne optical phase-locked loop, while the second uses a signal source analyzer. Experimental results obtained by these two methods are compared with classical linewidth measurement methods like self-heterodyne and Michelson interferometer. Limits and accuracy of each method are discussed. Furthermore, the comparison shows that, for coherent transmission system applications, only a subset of the analyzed methods is useful for laser phase noise characterization.

Impact of Nonlinear Transfer Function and Imperfect Splitting Ratio of MZM on Optical Up-Conversion Employing Double Sideband With Carrier Suppression Modulation

Abstract: Generation of optical millimeter-wave (mm-wave) signal using a Mach-Zehnder modulator (MZM) based on double-sideband (DSB), single-sideband (SSB), and double-sideband with carrier suppression (DSBCS) modulation schemes have been demonstrated for various applications, such as broadband wireless signals or optical up-conversion for wavelength-division-multiplexing (WDM) radio-over-fiber (RoF) network, wideband surveillance, spread spectrum, and software-defined radio. Among these schemes, DSBCS modulation offers the best receiver sensitivity, lowest spectral occupancy, the least stringent requirement of electrical bandwidth, and the smallest receiving power penalty after long transmission distance. Nonetheless, the inherent nonlinear E/O (electrical/optical) conversion response of a MZM is such that the signal quality of the optical mm-wave suffers. Fabrication tolerances make a balanced 50/50 splitting ratio of the MZM's y-splitter particularly difficult to achieve. As a result, imbalanced MZMs have a finite extinction ratio (ER) and degrade the optical carrier suppression ratio (OCSR) using DSBCS modulation. In this paper, the effect of the MZM nonlinearity and imbalanced y-splitter on optical mm-wave generation by DSBCS modulation is theoretically and experimentally investigated. A novel approach with better performance and greater cost-effectiveness than dual-electrode MZM (DD-MZM) is presented to realize a DSBCS modulation scheme based on a single-electrode MZM (SD-MZM).

Just-in-Time Scheduling for Multichannel EPONs

Abstract: We investigate optical network unit (ONU) grant scheduling techniques for multichannel Ethernet passive optical networks (EPONs), such as wavelength division multiplexed (WDM) EPONs. We take a scheduling theoretic approach to solving the grant scheduling problem. We introduce a two-layer structure of the scheduling problem and investigate techniques to be used at both layers. We present an extensive ONU grant scheduling simulation study that provides: 1) insight into the nature of the ONU grant scheduling problem and 2) indication of which scheduling techniques are best for certain conditions. We find that the choice of scheduling framework has typically the largest impact on average queueing delay and achievable channel utilization. An offline scheduling framework is not work conserving and consequently wastes channel resources while waiting for all ONU REPORT messages before making access decisions. An online scheduling framework, although work conserving, does not provide the best performance since scheduling decisions are made with the information contained in a single ONU REPORT. We propose a novel online just-in-time (JIT) scheduling framework that is work conserving while increasing scheduling control by allowing the channel availability to drive the scheduling process. In online JIT, multiple ONU REPORTs can be considered together when making scheduling decisions, resulting in lower average queueing delay under certain conditions and a more effective service differentiation of ONUs.

Terahertz Photonics: Optoelectronic Techniques for Generation and Detection of Terahertz Waves

Abstract: Optoelectronic techniques for generation and detection of terahertz (THz) signals have been reviewed. The operation principles of THz photomixer sources and THz parametric sources have been studied and recent developments in these areas have been presented. The performances of developed THz optoelectronic sources have been discussed and compared.

Broadband-Frequency-Tunable Sub-Terahertz Wave Generation Using an Optical Comb, AWGs, Optical Switches, and a Uni-Traveling Carrier Photodiode for Spectroscopic Applications

Abstract: We present a monochromatic sub-terahertz signal generation technique using an optical comb signal, arrayed waveguide gratings (AWGs), and a uni-traveling carrier photodiode (UTC-PD) for spectroscopic applications. This scheme offers random or continuous frequency tuning in the range between 100 GHz and up to 1 THz. In addition, since a RF synthesizer is employed as a reference signal source of the photonic frequency multiplier, frequency locking with external instruments and reliable operation are offered. Highly coherent optical comb signal for the photonic frequency multiplication provides a narrow linewidth and very low phase noise in the generated sub-terahertz signal. For 125 GHz, the phase noise is approximately -92 dBc/Hz at the offset frequency of 10 kHz. This is larger than that of the 25-GHz RF source by about 13 dB and agrees well with the theory regarding phase noise multiplications due to frequency multiplication. For generating monochromatic signals, unwanted spurious signals are suppressed in the optical domain over a wide range with two AWGs, and the suppression ratio is expected to be better than 46 dBc. Utilizing the implemented sub-terahertz signal generator with a J-band UTC-PD module, absorption lines of N2O were measured in the frequency range between 240 and 360 GHz and compared with theoretical calculations.

All-Optical 500-Mb/s UWB Transceiver: An Experimental Demonstration

Abstract: We propose and demonstrate experimentally, for the first time, a prototype for all-optical ultra-wideband (UWB) transceiver at 500 Mb/s. We report 1) UWB pulse optimization that takes into account the transmitter RF front end and the U.S. federal communications commission (FCC) spectral mask, 2) a new approximate match filter receiver using optical signal processing, and 3) modulation at 500 Mb/s. Our previous optimization of UWB pulse shapes was based only on the FCC spectral mask, without taking into account the frequency response of the RF components (amplifier and antenna) in the UWB transmitter. Here, we modify our pulse optimization technique to ensure that the equivalent isotropic radiated power (EIRP) from the transmitter meets FCC specifications. For the RF hardware used, we achieve 63.6% efficiency over the FCC mask, which yields an 11.6- and a 5.9-dB improvement over Gaussian monocycle and doublet pulses, respectively. We also introduce simple optical signal processing at the receiver that allows the incoming RF signal to be matched against a square pulse whose duration is adapted to the channel. The exact matched filter would require a new optimized pulse that would include not only hardware frequency response but channel effects that vary with antenna placement as well. The proposed approximation allows a simple variation of the pulse duration: an increase to account for pulse expansion in the channel but an upper limit to combat multipath effects. Finally, we demonstrate the optimized pulse and approximate match filter receiver at 500 M/s. We attain a 10-6bit error rate at a 65-cm separation line of sight (LOS) link with simple on-off keying and no forward error correction.

RF Photonics

Abstract: In parallel with the development of fiber optics for transmission of digital information, the 1970s and 1980s saw the development of techniques for the transmission of wide-bandwidth analog radio-frequency (RF) signals over optical fibers. The simultaneous need for high linearity and high signal-to-noise in RF photonic links spurred the development of lasers, modulators and detectors matched to the special needs of such links. RF photonics was also explored for the processing of RF signals by means of optically implemented filters and variable-delay lines. Commercial applications of RF photonics included cable television signal distribution and subcarrier-multiplexed digital signals.

Transmission Characteristics of 120-GHz-Band Wireless Link Using Radio-on-Fiber Technologies

Abstract: The transmission characteristics of a 120-GHz-band millimeter-wave wireless link are described. The wireless link uses photonic technologies for generation, modulation, and transmission of millimeter-wave signals. This configuration enables set up of the photonic millimeter-wave generator and transmitter core separately; therefore, the wireless link can be used as a kind of radio-over-fiber system. The effects of transmitting 120-GHz-band optical subcarrier signals through single-mode fibers were theoretically and experimentally investigated. It was confirmed that the time shift of the code edges, because of chromatic dispersion, limits the transmission distance. A data stream at 10-Gbit/s was successfully transmitted over the 120-GHz-band millimeter-wave wireless link, with a bit error rate (BER) below 10-12 over a distance of 250 m. The results also demonstrated the stability of the wireless link, which satisfied the 10-Gb Ethernet standard under clear weather conditions.