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In an optical network a signal can only travel a maximum distance [d.sub.max] before its quality deteriorates to the point that it must be regenerated by installing regenerators at nodes [...]

The motivation behind this study is the essential need for survivability in the telecommunications networks. An optical signal should find its destination even if the network experiences an occasional fiber [...]

A metal film perforated by a regular array of subwavelength holes shows unexpectedly large transmission at particular wavelengths, a phenomenon known as the extraordinary optical transmission (EOT) of metal hole arrays (1). EOT was first attributed to surface plasmon polaritons, stimulating a renewed interest in plasmonics (2-4) and metallic surfaces with subwavelength features (5-7). Experiments soon revealed that the field diffracted at a hole or slit is not a surface plasmon polariton mode alone (8). Further theoretical analysis (9) predicted that the extra contribution, from quasi-cylindrical waves (10-13), also affects EOT. Here we report the experimental demonstration of the relative importance of surface plasmon polaritons and quasi-cylindrical waves in EOT by considering hole arrays of different hole densities. From the measured transmission spectra, we determine microscopic scattering parameters which allow us to show that quasi-cylindrical waves affect EOT only for high densities, when the hole spacing is roughly one wavelength. Apart from providing a deeper understanding of EOT, the determination of microscopic scattering parameters from the measurement of macroscopic optical properties paves the way to novel design strategies., Understanding EOT quantitatively has been one of the main challenges in plasmonics in the past decade (see ref. 7 for a review). An important contribution to EOT may be the [...]

The bandwidth and versatility of optical devices have revolutionized information technology systems and communication networks. Precise and arbitrary control of an optical field that preserves optical coherence is an important [...]

Traditional lens-based photonic Fourier beam-forming systems can be used to steer multiple beams for narrowband radio-frequency (RF) phased-array antennas. For wideband RF phased-array antennas, such Fourier beam-forming systems suffer from frequency-dependent beam steering, known as beam squint. We present a novel squint-free Fourier-based photonic multibeam-forming system for wideband two-dimensional RF phased-array antennas using a lens and frequency-mapped modulation. In this new beam-forming system, we modulate the receiving wideband RF signals onto a broadband light source in a frequency-mapped manner by a traveling-wave tunable filter at each antenna element. These modulated signals are launched in a miniaturized topology of the RF antenna array, and the wavelength-scaling factor in the lens Fourier transform exactly compensates the frequency dependence of beam steering. Heterodyne detection at the Fourier plane between the focused modulated multicolor spots and the broadband laser reference beams from the same light source recovers the received RF signals. An analysis with numerical simulations and then demonstrated with preliminary experimental results of this beam-forming system is presented. OCIS codes: 320.7120, 280.4750, 280.5110, 070.1060.

The implementation and performance of network scheduling schemes for time-domain wavelength interleaved networks. The efficiency of p roposed distributed schemes is comparable to centralized schedulers at over 90%.

An external-cavity laser (ECL) operating in a chaotic state is usually used in a chaotic optical secure communication system and its feedback length (FL) is often regarded as an additional key. Our analyses show that an eavesdropper's (Eve) laser can synchronize with a transmitter (Alice) without any knowledge of the FL by simply increasing the injection strength. A sequence of a 1 Gbit/s nonreturn-to-zero message encoded by the FL as the key is successfully eavesdropped. The reason for the synchronization deviation between Alice's and Eve's lasers is given. Our results indicate that the FL as a key cannot enhance the security of chaotic optical communication using long-ECLs. OCIS codes: 140.5960, 140.1540, 060.4510.

An inherent polarization sensitivity of a volume grating, in general, can be used to determine all the Stokes parameters of input beams. We demonstrate the polarization-dependent bandgap in a photonic crystal in principle at any wavelength using the finite-difference time-domain method. We show how this bandgap can be used to realize a volume-grating Stokesmeter. We also present an explicit design for a high-speed version of such a Stokesmeter and identify design constraints that arise in this context. Finally, we describe a spectrally resolved volume-grating Stokesmeter based on the tunability of a photonic bandgap material. OCIS codes: 050.7330, 110.5405, 230.0230.

High-fidelity 10-Gd/s optical-proximity-communication (OPxC) is described by using reflecting mirrors micro-machined into silicon and co-integrated with low-loss silicon-on-insulator waveguides for packaged chip-to-chip communication. RMS jitter and amplitude metrics for the eye quality are nearly identical when OPxC hops associated with the package is inserted into 10-Gb/s links, thus enabling chip packages for several classes of proximity communication.

In this paper we study and present an in-depth analysis on the operability and the viability of a typical wireless optical CDMA (OCDMA) local area network. Three receiver structures for OCDMA systems, using optical orthogonal codes (OOC) with minimum auto and cross-correlations as signature sequence, namely, correlation, correlation with hard-limiter, and chip-level detection are studied and proposed for such a network. For the synchronization circuit design the performance of two algorithms for OOC based OCDMA networks, namely, simple serial-search and multiple-shift in the context of wireless OCDMA LAN are studied. Furthermore, we study a synchronization method based on matched filtering and show that it presents a much better performance in our wireless OCDMA system. The effect of sampling rate and its performance on tracking circuit is analyzed. Bit-error-rate (BER) analysis is performed by photon counting methodology. Multi-user interference (MUI), ambient light, and photodetector dark current are considered in our analysis. Our analysis strongly indicates the viability and practicality of such systems in certain important wireless optical communications systems. Index Terms--OCDMA, wireless optical LAN, optical orthogonal codes, synchronization, acquisition, tracking, sampling effect.

Given K simultaneously active users, an Optical CDMA (OCDMA) receiver needs to discriminate among K codes. Since the number of available codes C is always C >> K at any given time for any truly asynchronous OCDMA system that employs quasi-orthogonal codes, there is always a set of unused codes. We here propose to exploit these unused codes to increase the spectral efficiency of the system. This can be accomplished by exclusively assigning to each user a set of M codes which represent a [log.sub.2](M)-tuple of bits so that each user effectively uses a multi-dimensional modulation (multiple information bits per code ate conveyed). In this paper, we analyze the performance of such a system and find in closed form an upper bound on the average probability of error. Moreover, we report for the first time that, under lightly loaded, non interference-limited conditions, the multi-dimensional OCDMA seheme here proposed allows us to obtain both higher spectral efficiency and lower bit-error probability (BEP) than a conventional OCDMA system, which employs one code only per user. The analysis here presented allow us to formulate a useful system design criterion that yields truly asynchronous OCDMA systems with very high spectral efficiency. Index Terms--Multi-dimensional modulations, optical communications, optical code division multiple access (OCDMA), performance analysis.

In order to analyze the effect of beam type on free space optical communication systems, bit error rate (BER) values versus signal-to-noise ratio (SNR) are calculated for zero order and higher order general beam types, namely for Gaussian, cos-Gaussian, cosh-Gaussian, and annular beams. BER analysis is based on optical scintillation using log-normal distribution for the intensity, which is valid in weak atmospheric turbulence. BERs for these beams are plotted under variations of propagation length, source size, wavelength of operation, and order of the beam. According to our graphical outputs, at small source sizes and long propagation distances, the smallest BER value is obtained for the annular beam. On the other hand, at large source size and small propagation distance, the smallest BER value is obtained for the cos-Gaussian beam. Moreover, our study of the order of the beam shows that higher order beams have lower BER values than the zero order beams at longer propagation distances. But this drop compared with the order seems to be incremental. OCIS codes: 200.2605, 010.1300, 010.1330, 290.5930.

A new scheme for chaotic cryptography of an optical signal transmitted in a free-space optics technology for line-of-sight communication links is presented. The results from simulations for both attenuation of the optical signal in a line-of-sight configuration and noise for security demonstrated the effect of the parameter mismatch between transmitter and receiver on synchronization and message recovery.

Though it promises high bandwidths, the optical medium is not popular in local area networks. This is because current optical networks do not offer the ease of use and setup that an uncoordinated multiple access network such as Ethernet offers. In this paper, we propose a novel nonlinear trellis code designed for multiple access among uncoordinated nodes in an optical communications system. This code has been shown to have an efficiency of 30%. We have implemented the codes on Xilinx FPGA's for a 6 user optical system, transmitting data on a single wavelength. The above system was set up using commercial off-the-shelf components and we demonstrated BER of < [10.sup.-9] for three users, each running at a channel rate of 2 Gbps. Demonstration of this system required the design of new channel codes, architectural optimizations for the implementation of the channel cedes for high speed with limited resources and electrical/optical optimizations to realize the optical channel. Index Terms--High-speed architectures, multiple access network, nonlinear trellis code, optical network, uncoordinated access network.

Fog is a highly dispersive medium at optical wavelengths, and the received pulse waveform may suffer significant distortion. Thus it is desirable to have the impulse response of the propagation channel to recover data transmitted through fog. The fog particle density and the particle size distribution both strongly influence the channel impulse response, yet it is difficult to estimate these parameters. We present a method using a dual-wavelength free-space optical system for estimating the average particle diameter and the particle number density and for approximating the particle distribution function. These parameters serve as inputs to estimate the atmospheric channel impulse response using simulation based on the modified vector radiative transfer theory. The estimated channel response is used to design a minimum mean-square-error equalization filter to improve the bit error rate by correcting distortion in the received signal waveform due to intersymbol interference and additive white Gaussian noise. OCIS codes: 010.1300, 010.3310, 010.5620, 060.2605, 200.2605, 290.1090.

We propose three types of narrowband optical filters based on a Fox-Smith resonator. We demonstrate that by choosing the appropriate combination of coating materials on each prism facet, one can design either a high reflectance or a high transmittance optical filter, suitable for low bit rate optical communication applications with International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) standards. We also show the possibility of designing an optical filter having a desirable finite reflectance/transmittance ratio with simultaneous peaks at ITU-T standard wavelengths. Such filters could be suitable for wavelength tuning applications. OCIS codes: 060.4510, 130.7408, 230.1480, 230.5480, 230.7408, 310.1210.

As PMD compensation has become a very intense subject of research in recent years, the achievement of simple PMD compensators based on a feedback signal is a key issue. As PMD compensation uses a feedback indicator for system performance evaluation, such feedback signals must reproduce the DGD variations with time. In this paper, the correlation between DGD and different feedback signals is studied with a new method. This method is applied for various modulation schemes, leading to well-known results. By applying the method to the electrical and optical duobinary formats, it is shown that for duobinary signals, the DOP feedback signal shows the best correlation with instantaneous DGD value. Such a behavior of duobinary format is very interesting since duobinary modulation schemes are now considered for systems upgrades or mixed architectures (10 Gbit/s-40 Gbit/s on the same link). In such systems, PMD compensation has to be bit-rate independent which is available with the DOP as feedback indicator. Index Terms--Optical communications, modulation coding, polarization mode dispersion (PMD).

Free space optical communications (FSOC) is a method by which one transmits a modulated beam of light through the atmosphere for broadband applications. Fundamental limitations of FSOC arise from the environment through which light propagates. This work addresses transmitted light beam dispersion (spatial, angular, and temporal dispersion) in FSOC operating as a ground-to-air link when clouds exist along the communications channel. Light signals (photons) transmitted through clouds will interact with the cloud particles. Photon-particle interaction causes dispersion of light signals, which has significant effects on signal attenuation and pulse spread. The correlation between spatial and angular dispersion is investigated as well, which plays an important role on the receiver design. Moreover, the paper indicates that temporal dispersion (pulse spread) and energy loss strongly depend on the aperture size of the receiver, the field-of-view (FOV), and the position of the receiver relative to the optical axis of the transmitter. OCIS codes: 060.4510, 290.1090, 290.4210.

Optical beamforming networks (OBFNs) are an interesting alternative for the design of wideband antenna arrays, mainly due to their low losses and weight, their high parallel processing capabilities and their electromagnetic immunity. Nevertheless, for a practical implementation, different technological and architectural tradeoffs need to be assessed. In this paper, the performance of an OBFN system is analytically studied and experimentally demonstrated. The study forms part of the optical beamforming network project, a multigroup research project supported by the European Space Agency (ESA). Differently from bulky free-space geometries, the architecture is based on fiber-optic delay lines for the time delays generation and on a spatial light modulator for the phase shifts generation. Experimental results for an X-band prototype are provided, showing beam steering accuracy, multibeam capability, amplitude distribution weighting and wideband operation. Index Terms--Microwave photonics, optical beamforming, phased array antennas, spatial light modulator (SLM).

A multichannel free-space optical (FSO) communication system based on orbital angular momentum (OAM)-carrying beams is studied. We numerically analyze the effects of atmospheric turbulence on the system and find that turbulence induces attenuation and crosstalk among channels. Based on a model in which the constituent channels are binary symmetric and crosstalk is a Gaussian noise source, we find optimal sets of OAM states at each turbulence condition studied and determine the aggregate capacity of the multichannel system at those conditions. OAM-multiplexed FSO systems that operate in the weak turbulence regime are found to offer good performance. We verify that the aggregate capacity decreases as the turbulence increases. A per-channel bit-error rate evaluation is presented to show the uneven effects of crosstalk on the constituent channels. OCIS codes: 060.2605, 050.4865, 010.1300, 010.3310, 060.4230, 050.7330, 010.7060.

In this paper we examine the optical sources of noise that degrade high-performance microwave photonic links. In particular, we study the residual phase noise due to laser frequency fluctuations and the detector nonlinearity on microwave signals transmitted on an optical fiber, or generated in the opto-electronic oscillator (OEO). Based on experimental findings, we identify a significant reduction of the relative intensity noise of the laser if the received optical power saturates the photodiode. Furthermore, we suggest the use of a semiconductor optical amplifier in saturation as yet another means to reduce the phase noise induced by laser intensity fluctuations. We also identify the use of multiple photodetectors to reduce the influence of associated 1/f noise, which adds to the phase noise of a transmitted microwave signal, and is the ultimate limitation to the phase noise of the high-performance OEO. Reduction of noise that is due to optical interferences is also addressed. Index Terms--AM noise, optical communication, oscillators, phase noise.

We report error-free underwater optical transmission measurements at 1 Gbit/s ([10.sup.9] bits/s) over a 2 m path in a laboratory water pipe with up to 36 dB of extinction. The source at 532 nm was derived from a 1064 nm continuous-wave laser diode that was intensity modulated, amplified, and frequency doubled in periodically poled lithium niobate. Measurements were made over a range of extinction by the addition of a Mg[(OH).sub.2] and Al[(OH).sub.3] suspension to the water path, and we were not able to observe any evidence of temporal pulse broadening. Results of Monte Carlo simulations over ocean water paths of several tens of meters indicate that optical communication data rates > 1 Gbit/s can be supported and are compatible with high-capacity data transfer applications that require no physical contact. OCIS codes: 010.04455, 010.4458, 060.2605, 060.4510, 190.0190, 290.7050.

Nowadays, techniques for education are based on the combination of theoretical formulas, experimental measurement and simulation approaches. Theoretical analysis, experimental and simulation approaches based educations allow developing simple and non expensive tools for the aim of studying complex problems in optical network communication. Theoretical formulas, used to describe the non linear effects on digital lightwave links, may by simplified with the use of simulation models so that real and industrial applications can be approached. In this study, we use theoretical model taking into account system parameters, physical penalty and transmission speed, for the purpose of developing simulation tools. Using these simulation techniques, it is possible to investigate various compensation techniques and demonstrate their effects on system performances. Key words: Theoretical analysis, simulation techniques, compensation techniques, transmission optical system, DWDM technology, INTRODUCTION Rapid growth of the global communication network is being made possible by the availability of new optical fiber technologies. Dense Wavelength Division Multiplexing system DWDM saves the network operators [...]

A second-order small-signal model is used for describing the nonlinear redistribution of noise in a saturated semiconductor optical amplifier (SOA). Findings reveal the importance of proper inclusion of second-order noise terms for the description of the change in the skewness of the noise distributions.

An all-optical polarization-insensitive clock recovery system consisting of a mode-locked laser diode (MLLD) and a newly-developed polarization-diversity circuit is examined. Results suggest the successful achievement of small polarization sensitivity of less than 0.5 dB remaining at low timing jitter of less than 0.17 ps and at clear clock output with no changes in the pulse characteristics.

To assess the coherent detection of an optical signal perturbed by atmospheric turbulence, the loss in the mean signal-to-noise ratio (SNR) is usually invoked although it constitutes a limited description of the signal fluctuations. To produce statistical distributions of the SNR, we generate random optical fields. A 5/3-power law for the phase structure function is considered. The benefit of a wavefront tilt correction is assessed. Based on the 1%-probability fade, an optimum receiver size is found. For phase fluctuations only, a similarity between the signal distribution and the beta distribution is observed. Phase and amplitude are assumed independent, and the influence of amplitude perturbations is assessed with a scintillation index of 2. Turbulence impairments are compared for a coherent receiver and a direct-detection receiver. OCIS codes: 010.1300, 010.1330, 060.1660, 060.4510.