Lightwave Communication Systems Research Articles

All-optical latches using carrier reservoir semiconductor optical amplifiers

Lightwave communication systems, optical random memories, and photonic encryption/decryption are important applications that rely on all-optical latches. For the first time, the carrier reservoir semiconductor optical amplifiers (CR-SOAs) are employed to simulate two basic optical latches, Set-Reset (SR) latch, and D Flip-Flop, at a data rate of 120 Gb/s. All-optical NAND and NOT logic operations are used to build these latches, which are implemented using Mach-Zehnder interferometers (MZIs) with CR-SOAs. In the presence of the amplified spontaneous emission noise, the variation of the output quality factor (Q-factor) against the CR-SOA key operating parameters is studied. This is achieved by exploiting and numerically solving a set of coupled partial differential equations that describe the CR-SOAs gain and phase dynamics when operated as nonlinear elements and embedded in MZIs. The results demonstrate that CR-SOAs-based MZIs can achieve a high Q-factor while implementing the logical SR latch and D Flip-Flop at a high speed of 120 Gb/s.

A multiple-input-multiple-output visible light communication system based on VCSELs and spatial light modulators

A multiple-input-multiple-output (MIMO) visible light communication (VLC) system employing vertical cavity surface emitting laser (VCSEL) and spatial light modulators (SLMs) with 16-quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing (OFDM) modulating signal is proposed and experimentally demonstrated. The transmission capacity of system is significantly increased by space-division demultiplexing scheme. With the assistance of low noise amplifier (LNA) and data comparator, good bit error rate (BER) performance, clear constellation map, and clear eye diagram are achieved for each optical channel. Such a MIMO VLC system would be attractive for providing services including data and telecommunication services. Our proposed system is suitably applicable to the lightwave communication system in wireless transmission.

Full-duplex lightwave transport systems based on long-haul SMF and optical free-space transmissions

A full-duplex lightwave transport system employing wavelength-division-multiplexing (WDM) and optical add-drop multiplexing techniques, as well as optical free-space transmission scheme is proposed and experimentally demonstrated. Over an 80-km single-mode fiber (SMF) and 2.4 m optical free-space transmissions, impressive bit error rate (BER) performance is obtained for long-haul fiber link and finite free-space transmission distance. Such a full-duplex lightwave transport system based on long-haul SMF and optical free-space transmissions has been successfully demonstrated, which cannot only present its advancement in lightwave application, but also reveal its simplicity and convenience for the real implementation. Our proposed systems are suitable for the lightwave communication systems in wired and wireless transmissions.

Capteur Brillouin réparti à fibre optique à haute résolution et longue portée

Brillouin-based distributed optical fiber sensors have been the subject of intense research in recent years because they offer a unique solution for continuous, real-time monitoring in civil engineering and petroleum industry. These sensors provide strain or temperature measurements with meter spatial resolution over several tens of kilometers. In this work we demonstrate two new Brillouin fiber sensors with enhanced performances based on advanced modulation formats from high-speed lightwave communications systems. We first report a Brillouin distributed sensor with enhanced centimeter resolution using a digital phase-shift keying technique. The second one uses a quadrature phase-shift keying modulator as a single-sideband modulator to balance the pump depletion and the fiber loss by the Brillouin gain. Combined with a specially-designed in-line bidirectional Erbium-doped fiber amplifier, we demonstrate that this technique allows for the achievement of long-range distributed sensing over 100 km.

High Operation Efficiency of Semiconductor Electrooptic Modulators in Advanced Lightwave Communication Systems

Device modeling, Integrated optics, Optical modulator, EO modulator, and Silicon optoelectronics Photonic links have been proposed to transport radio frequency (RF) signals over optical fiber communication systems. External optical modulation is commonly used in high performance RF photonic links. The practical use of optical fiber communication systems to transport RF signals is still limited due to high RF signal loss. In order to reduce the RF signal loss, highly efficient modulators are needed. For many applications, modulators with broad bandwidths are required. However, there are applications that require only a narrow bandwidth. For these narrow band applications, the modulation efficiency can be improved through the resonant enhancement technique at the expense of reduced transmission bandwidth. Therefore we have been investigated to get the best performance of the transmission bit rate capacity and product of different semiconductor materials based electrooptic (EO) modulators over wide range of the affecting parameters.

Performance Characterization of Ultrahigh SpeedOptically Amplified Spectral-Phase EncodedOCDMA Systems with Second-Harmonic-Generation Effect in Thin and Thick Crystal Receiver Structures

In this paper we study and evaluate the performance of a spectral-phase encoded optical CDMA (SPE-OCDMA) system using advanced receiver structures based on second harmonic generation (SHG) effect imposed in a Thick or Thin crystal. This receiver structure is employed as the nonlinear pre-processor prior to the conventional low-speed photodetector. In our performance evaluation amplified spontaneous emission (ASE) of the optical amplifiers, receiver front end thermal noise and photodetector shot noise which are effective in low power conditions, and multiple access interference (MAI) noise which is the dominant source of noise in high power conditions in any coherent OCDMA communications system have been considered. We begin by studying the statistical behavior of Thick crystals in an optically amplified digital lightwave communication system in the context of a SPE-OCDMA communication system. The error probability for Thick crystal receiver structure is evaluated using Saddle-Point approximation. Furthermore, we obtain a closed form approximation for the probability density function (pdf) of the decision variable in SPE-OCDMA systems with Thin crystal SHG receiver. In this analysis, to obtain approximate probability density function, we employ Gram-Charlier expansion based on the corresponding first order moments of the decision variable. The first three moments are obtained in a closed form approximation and consequently a closed form expression for the error probability is obtained by integrating the approximated pdf of the decision variables for transmitting bit "1" and "0" in Thin crystal receiver structure. The precision of the approximation is verified by the Monte-Carlo simulation. Finally the performance of SPE-OCDMA network for both Thin and Thick SHG crystals are analytically evaluated and discussed for different number of interfering users and different code-lengths and different speed of the conventional photodetectors. We deduce that the performance of Thin SHG crystal receiver is more sensitive to the transmitted power level especially in low power conditions. It is concluded that in low power conditions where ASE, thermal and shot noises are the dominant noise sources, Thick SHG crystal receiver outperforms the Thin crystal receiver. However, in high power conditions where MAI noise is the most effective noise term, Thin SHG crystal receiver structure outperforms the Thick crystal receiver.

Digital Lightwave Receivers: An Experimentally Validated System Model

In this letter, an analog-to-digital converter (ADC) that is typically employed in current state-of-the-art high bit rate lightwave communication systems experiments is characterized for a broad range of input powers in order to develop a reliable model for simulation purposes. We show that the receiver performance is dominated by the digitizer noise, especially at low powers. This is particularly important for burst-mode receivers that have to cope with a large variation in input power. Simulation results show a good agreement with experimentally received intensity modulated data at 10.7 Gb/s with powers ranging from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-$</tex> </formula> 6 to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-$</tex> </formula> 18 dBm.

Editorial: Optical computing circuits, devices and systems

Information forms the basis of modern technology. To meet the ever-increasing demand for information, means have to be devised for a more efficient and better-equipped technology to intelligibly process data. Advances in photonics have made their impact on each of the four key applications in information processing, i.e., acquisition, transmission, storage and processing of information. The inherent advantages of ultrahigh bandwidth, high speed and low-loss transmission has already established fiber-optics as the backbone of communication technology. However, the optics to electronics inter-conversion at the transmitter and receiver ends severely limits both the speed and bit rate of lightwave communication systems. As the trend towards still faster and higher capacity systems continues, it has become increasingly necessary to perform more and more signal-processing operations in the optical domain itself, i.e., with all-optical components and devices that possess a high bandwidth and can perform parallel processing functions to eliminate the electronic bottleneck.

Optimum and suboptimum memoryless nonlinearities for the detection of ultrashort light pulses in Gaussian noise

Detection of ultrashort light pulses with conventional bandlimited photodetectors can lead to substantial performance degradation in digital lightwave communication systems. All-optical nonlinear processing of the received field prior to the optical to electrical conversion can greatly overcome this problem. In this letter, through a decision-theoretic framework and using a basic invariance property of the photodetector, we obtain the optimal zero-memory nonlinear element and evaluate its performance. Possible suboptimum nonlinearities are also investigated and their performance is examined. The results in this letter strongly justify the intuitive use of power-law devices, such as those implemented via two photon absorption (TPA) or second harmonic generation (SHG), in the detection of ultrashort light pulses and show how by using them almost optimal performance can be achieved.

COUPLING EFFICIENCY OF METAMATERIAL MAGNETOOPTICAL INTEGRATED ISOLATOR

Nonreciprocal devices such as isolators are well-known in light wave communication systems. The integrated optical isolator is presented where the magnetic field modes propagate perpendicular to the in-plane magnetization of a planar magnetooptical waveguide. The cladding and film of optical isolator are made of magnetic materials with magnetization adjusted to be parallel to their plan. The substrate is filled with new artificial metamaterials (MTMs). The dispersion equation of the transverse magnetic (TM) fields is derived. The difference Δβ between the phase constant for forward and backward propagation is calculated numerically for different values of MTMs permittivity (εs) and permeability (μs). Results show that the cutoff thickness is different for forward and backward propagation and varies with the MTMs parameters. The cutoff thickness of the isolator is selected such that the coupling coefficients of the isolator with optical fiber are obviously different for forward and backward propagation.

Mathematical Modeling and Statistical Analysis of Second Harmonic Generation Effects With Thin and Thick Crystals in Ultrahigh-Speed Optically Amplified Digital Lightwave Communication Systems

In this paper, we present an in depth analysis and discussions on the behavior and performance of using either thin or thick second harmonic generation (SHG) crystals prior to photodetector in ultrahigh-speed optically amplified digital lightwave communication systems. Our study begins by considering a conventional low-speed optically amplified lightwave receiver and study its performance in ultrahigh-speed regime and show its substantially degraded behavior, indicating the need to use advance optical nonlinear elements such as SHG prior to photodetection. In studying SHG effects, we begin by discussing the mathematical models of thin and thick SHG crystals in the context of statistical digital lightwave communication systems. In the case of thin SHG crystals and in the regime of high-speed digital systems, where Gaussian assumption is not necessarily valid, we use advanced numerical simulation based on Monte Carlo simulation to obtain its corresponding performance. However, for thick SHG crystals by invoking an advanced theorem due to Papoulis on Gaussianity approximation and a lemma due to Turin, we obtain the moment generating function of the sampled output with which, we obtain its corresponding performance using saddle point approximation method. Finally in our performance analysis, we consider electronically induced thermal noise and sketch the performance for various system parameters. The results in this paper indicate that the use of SHG crystals can enhance the system performance, when compared with the conventional receiver, by a substantial amount, i.e., five orders of magnitude. Furthermore, we highlight the existence of an optimum length for thick SHG crystals in the presence of thermal and optical amplifier noise.

Effect of phase noise on optical minimum-shift keying transmission systems

Phase-modulated (PM) signals have recently been used in long-haul lightwave communication systems to reach record distances. Added directly to the signal phase, linear and nonlinear phase noise cause performance degradation in PM systems. Minimum-shift keying (MSK), as a special format of PM signals, presents some different features of phase noise tolerance. This paper investigates the effect of phase noise on the performance degradation in the 10 Gb/s optical MSK systems and analyzes the contribution of phase noise to the bit error rate (BER), compared with the conventional PM format, 50% duty cycle optical return-to-zero differential phase-shift keying (RZ-DPSK) signals, in several typical local fiber dispersion transmission systems. The effect of intensity noise on the performance degradation in the corresponding transmission systems is also investigated.

News Briefs

Purdue University researchers have developed a way to finely control ultra-fast laser pulses' spectral properties to create more powerful optical-communications technologies, The laser pulses last a trillionth to a quadrillionth of a second-picosecond to a femtosecond. In essence, each pulse could serve as a data hit in future high-speed optical-communications systems. Picosecond pulses could yield data rates ot one terabit per second, much higher than those of today's lightwave communications systems.

Fabrication of Nanocomposites

A series of block copolymers containing polyesteramides as hard block and polyethylene glycol (PEG) or hydroxy terminated polybutadiene (HTPB) as soft block have been synthesized by different methods. These diblock copolymers are made into composite with metal salt and then assembled into ordered periodic structure at molecular level (∼5 to 50 nm). The polymers are characterized by solution viscosity, FTIR, 1H-NMR spectroscopy, TGA, and Differential Scanning Calorimetry (DSC). The nanopatterns of the polymers as well as of the composites are studied under Scanning Electron Micrographs (SEM) and Transmission Electron Microscope (TEM). They can be used for fabricating biological and chemical sensor, engineering microelectronic and optoelectronic devices, light-wave communication systems, etc.

Comparison of microwave and light wave communication systems in space applications

The performances of optical and radio frequency communication systems are compared for long distance applications, such as deep space communications, where the signal-to-noise ratio (SNR) is crucial. We compare an optical communication system operating at 0.8 µm using intensity modulation and direct detection with an avalanche photodiode, an optical communication system operating at 1.5 µm using on-off keying and an optical preamplifier, and a radio frequency communication system operating in the X band. Assuming typical system parameters for the link budget analysis, we find that for distances between the transmitting and receiving antennas (R) of 107 km, the SNR for the optical systems is proportional to R?4, while for the radio frequency system it is always proportional to R?2. The maximum data rate achievable with the radio frequency system is higher than that with the optical systems for distances beyond 108 km. For near-Earth communication links, an optical system with optical preamplification is preferable when the data rate is higher than several gigabits per second. Clearly our results are based on specific system parameters. However, the equations involved and the method of comparison will be applicable for a wide range of system parameters.

Investigation and Comparison of Postweld-Shift Compensation Technique in TO-Can- and Butterfly-Type Laser-Welded Laser Module Packages

The postweld-shift (PWS) compensation techniques employing a laser displacement meter (LDM) in the transistor outline (TO)-Can-type laser-welded laser module package and a high-magnification camera with image capturing system (HMCICS) in the butterfly-type laser-welded laser module packages are quantitatively investigated and compared. The results show that the fiber alignment shifts due to the PWS can be aligned back closer to their original optimum position after applying the proposed compensation techniques, and hence the coupling powers loss due to the PWS could be regained significantly. Comparing with the measured coupling power and the PWS, the results show that the PWS are in the ranges of 0.2-1.6 mum and 2.1-4.2 mum for the TO-Can- and the butterfly-type laser packages, respectively. The reason for having a larger PWS in the butterfly laser package is that the fiber ferrule is not solidly constrained in the vertical direction. In this paper, a coupled thermal-elasto-plasticity model of a finite-element method (FEM) analysis is employed to evaluate the PWS in the TO-Can- and the butterfly-type laser module packages. The simulated results are in good agreement with the measured results and these show that the PWS in the butterfly-type laser packages are larger than those in the TO-Can packages. A combination of the experimental and numerical results has provided a practical design guideline for fabricating reliable and high-yield laser-welded TO-Can- and butterfly-type laser module packages for use in lightwave communication systems

Cost-Effective Optoelectronic Packages Using Powder Metallurgy

In this paper, we propose powder metallurgy for optoelectronic packages in order to reduce the cost of laser packages. Conventional die compaction (DC) and metal injection molding (MIM) are used for the shaping of powder metallurgy. The distinguishing types of inherent porosity in DC and MIM steels have their respective effects on the defect mechanisms of laser welding joints. For DC steel with 85% relative density and continuous porosity, the rising gas pressure pushes the molten metal out of the welding regions, resulting in weak and unstable welding joints. For MIM steel with more than 95% relative density, the laser power dominates the defect mechanisms, and the defect-free welding joints with optical spot size of 400 mum can be attained by using laser power of less than 1.0 kW. Although elimination of inherent porosity in MIM steel under optimum welding condition can give rise to additional postwelding-shift (PWS), it is still controlled to less than 2 mum, resulting in optical coupling loss of less than 15%. The advantage of applicability of the MIM method to several materials makes it possible to employ SS316L, Kovar, and Invar, which have the characteristic of better property for laser welding but difficult machining, as the metal housing in low-cost lightwave communication system. Thus, using MIM Invar with low coefficient of thermal expansion can minimize the tracking error, which is an important issue for bidirectional and triple-directional optoelectronic packages. The reliability data demonstrate that the laser modules using MIM steel as construction housing are stable and reliable

Optical phase-shift-keyed transmission

Differential-phase-shift keying (DPSK) has recently been used to reach record distances in long-haul lightwave communication systems. This paper will review theoretical, as well as implementation, aspects of DPSK, and discuss experimental results.

Frequency characteristics of stationary optical pulses in lightwave communication systems with periodical gain created by semiconductor amplifiers

The propagation and evolution are studied of optical pulses in fiber optics transmission lines with both dissipation and periodical gain created by semiconductor optical amplifiers. Using numerical simulations, conditions that permit formation of averaged and dissipative optical solitons are considered. Their basic properties, such as shift of the frequency spectrum and mean frequency change are also examined. The qualitatively different tendencies are shown of the shift of the mean frequency for the two different types of stationary pulses. The influence is determined of the saturation on the change of the mean-frequency-shift velocity.

Design of multi-terabit/s terrestrial transmission systems facilitated by simple analytical tools

For 30 years, the capacity distance product of lightwave communication systems has continuously doubled every 16 months, to reach more than 10 Pbit/s.km in 2002. Along with this growth, the complexity of system engineering has increased as linear and nonlinear transmission impairments have become more stringent. Dispersion management is widely accepted as a powerful technique to mitigate some of them, but requires numerous experiments or time-consuming numerical simulations to be performed optimally. We exhibit two analytical rules that provide meaningful insight on this matter. Then we show how these rules helped us to improve the transmission distance of a recent 5 Tbit/s transmission experiment from 1,200 km to 1,500 km.