KEYWORDS: Optical amplifiers, Transmitters, WDM-PON, Receivers, Signal to noise ratio, Temperature metrology, Signal attenuation, Networks, Connectors, Optical engineering
Dense wavelength division multiplexing has been proposed as a means of implementing communications on aircraft. In such applications, power consumption is a critical consideration. The impact of reducing the transmitter power and recovering losses using a shared amplifier has been investigated. By recovering the power loss using a shared amplifier transmitter, power savings can be made. This network has been modeled and savings of 20% are predicted in a realistic aircraft environment.
Visible light communications (VLC) is a valuable addition to future generations of networks, utilizing light for
illumination for the purposes of advanced service provisioning at high speed. Low energy consumption, license free and
RF interference free operation are compelling advantages. VLC systems are affected by sunlight limiting connection
availability and reliability. The paper presents an analysis of the performance of VLC systems at different locations
around the world over the cycle of a year; the evaluation considers the impact of sunlight as a function of location, time
and for different surfaces over the four seasons of the year.
The demand for fast, secure, energy efficient high capacity networks is growing. It is fuelled by transmission bandwidth needs which will support among other things the rapid penetration of multimedia applications empowering smart consumer electronics and E-businesses. All the above trigger unparallel needs for networking solutions which must offer not only high-speed low-cost "on demand" mobile connectivity but should be ecologically friendly and have low carbon footprint.
The first answer to address the bandwidth needs was deployment of fibre optic technologies into transport networks. After this it became quickly obvious that the inferior electronic bandwidth (if compared to optical fiber) will further keep its upper hand on maximum implementable serial data rates. A new solution was found by introducing parallelism into data transport in the form of Wavelength Division Multiplexing (WDM) which has helped dramatically to improve aggregate throughput of optical networks. However with these advancements a new bottleneck has emerged at fibre endpoints where data routers must process the incoming and outgoing traffic. Here, even with the massive and power hungry electronic parallelism routers today (still relying upon bandwidth limiting electronics) do not offer needed processing speeds networks demands.
In this paper we will discuss some novel unconventional approaches to address network scalability leading to energy savings via advance optical signal processing. We will also investigate energy savings based on advanced network management through nodes hibernation proposed for Optical IP networks. The hibernation reduces the network overall power consumption by forming virtual network reconfigurations through selective nodes groupings and by links segmentations and partitionings.
KEYWORDS: Holograms, Signal to noise ratio, Detection and tracking algorithms, Diffraction, Beam shaping, Diffusion, Algorithm development, Near field optics, Fourier transforms, Electronic filtering
A method for performing optical beam shaping in the near-field region using diffractive optical elements generated by
Fresnel based Phased Optimised General Error Diffusion algorithm (POGED) was developed and investigated by means
of numerical simulations. POGED was found to deliver significantly higher signal to noise ratio than iterative
Gerchberg-Saxton type algorithm.
In this paper we introduce a multi gas sensor system based on refractive index changes in a 2D slab photonic crystal. The
sensor is formed by a L3 resonant cavity sandwiched between two W1.06 waveguides in the photonic crystal. The sensor
configuration is similar to an Add-Drop filter structure. The transmission spectrums of the sensor with different ambient
refractive indices ranging from n = 1.0 to n = 1.1 are calculated. The simulation results show that a change in ambient RI
of Δn = 0.0008 is apparent with a corresponding change in output wavelength of the sensor of 2.4 nm. The properties of
the sensor are simulated using the 3D finite-difference time-domain (FDTD) method. The Q-factor of the sensor is also
optimized, with highest values reaching over 30,000. The sensor system is hybrid integrated with a wireless RF chip
which processes the sensor data and transmits them in effect turning the entire system into a wireless sensor mote.
Optical CDMA is a flexible technology for efficient and scalable multiple access networks. It also offers increased
physical layer privacy and on-demand bandwidth sharing management. Highly scalable approach to incoherent OCDMA
system was developed and demonstrated. Error-free operation with a BER less than 10-12 including complete elimination
of the MAI noise was achieved using 2ps time gating. The approach introduces a negative power "penalty" which helps
to improve an overall system power budget and performance. It is shown that two picosecond time gate can significantly
improve number of simultaneous users in the system by more than tree times without any degradation of BER.
Semiconductor-based optical amplifiers (SOAs) offer solutions to a variety of amplification needs covering wavelengths ranging from of 0.6 to 1.6 µm. Gain adjustment, through the bias current, enables automatic power control to be implemented. However, this requires knowledge of the signal strength. The amplified spontaneous emission power, particularly in high gain SOAs, can be significant with respect to the signal strength, and therefore additional components may be required to derive an accurate measure of the signal strength. This increases both the complexity and cost of implementing automatic power control (APC). We report on a method for estimating the signal strength based on measurement of the total output power and the SOA drive current. The method is extendable to other methods of optical amplification, e.g., erbium-doped fiber amplifiers.
The semiconductor optical amplifier (SOA) is a versatile component that can be deployed to meet the expanding applications associated with the introduction of additional functionalities at the optical level in wavelength division multiplexed systems. The future network requires low cost, small footprint, directly controllable amplification throughout the different application layers from long haul through to metro; the intrinsic size and integration capability advantages will ensure that the SOA plays a key role in this evolution. In multi-wavelength gating/amplification applications the gain dynamics, oscillating at timescales comparable to that of the data which is being amplified, introduce issues of pattern dependent waveform distortion (patterning) in single channel, and inter-channel cross-talk in multi-wavelength cases which require management through careful SOA design and understanding of the network application scenarios. In this paper, an optical linear amplifier (OLA) architecture with the unique capability to provide variable gain whilst maintaining linear operation at high output saturation powers will be described. Initial characterisation results for the OLA will be presented.
A novel buffering mechanism for IP packets over WDM links is proposed by exploiting the packet nature. ACK packets which do not carry any payload can be singled out to be processed and stored purely in the electronic control plane without getting through the optical path which consists of the switching matrix and optical delay line buffers. An optical path is then reserved at the output for these ACK packets by extending the optical delay lines to be integral of ACK packets. Simulation results have shown an improvement both in the packet loss probability and the mean delay for using this scheme.
In this paper, we introduce deflection routing for irregular meshed networks in an optical packet based environment. By using the MPLS (Multiprotocol Label Switching) control plane to manage the deflected paths, we offer a relatively simple way of controlling the otherwise complicated deflected paths. This is beneficial due to the way it simplifies the optical packet switch so it only needs to deal with simple forwarding and thereby relaxing the need for buffering. The latter is becoming increasingly more important in optical networks since there is no practical optical memory insight for the near future. By introducing deflection routing along with statistical wavelength multiplexing, the need for buffers is greatly reduced while the network resources are better utilized. MPLS is emerging as a main protocol for future networks, whether being based on packet switched or circuit switched technology, hence rendering this approach to be compatible with most modern and future systems.
The increasing volume of Internet packet traffic in the telecommunications network has instigated a revolution in traffic modelling techniques, where self-similar instead of Markovian traffic models are now used to produce a realistic representation of data traffic. This paper presents a brief introduction to self-similar traffic and its impact on the performance of an optical node, in terms of packet loss probability and delay. It is demonstrated from simulation results that by employing multiple wavelength channels within the switch and incorporating void-filling, a packet loss probability of 10-6 can be attained. The generated traffic was subject to statistical tests, and its Hurst parameter was determined using parameter estimation to verify the theoretical assumptions inherent in using infinite variance ON/OFF sources to capture self-similarity.
This paper describes a new approach for implementing optical switching in a MPLS (Multiprotocol Label Switching) domain where optical packet switching is deployed by using deflection routing. By implementing wavelength multiplexing along with deflection routing, the need for optical buffers is reduced considerably, thereby alleviating of the problems associated with optical packet switching. Using MPLS traffic engineering extensions that naturally facilitate the use of alternative routes furthermore eases the control of deflection routing. The following assumes knowledge of MPLS terminology and functionality.
A novel architecture is proposed for future multi-terabit IP (internet protocol) routers, employing multiple cascaded stages of optical switching and buffering. WDM is used within the node to facilitate its operation. External synchronization is not required, and a void-filling algorithm is used to simplify hardware requirements. Packet priorities are not implemented in the current version of the switch, and the issue of header table lookup is not considered. Performance with respect to packet loss is studied by simulation, demonstrating that this multi-stage concept results in substantial hardware reduction.
This review details the approaches adopted to implement classical code division multiple access (CDMA) principles directly in the optical domain, resulting in all optical derivatives of electronic systems. There are a number of ways of realizing all-optical CDMA systems, classified as incoherent and coherent based on spreading in the time and frequency dimensions. The review covers the basic principles of optical CDMA (OCDMA), the nature of the codes used in these approaches and the resultant limitations on system performance with respect to the number of stations (code cardinality), the number of simultaneous users (correlation characteristics of the families of codes), concluding with consideration of network implementation issues. The latest developments will be presented with respect to the integration of conventional time spread codes, used in the bulk of the demonstrations of these networks to date, with wavelength division concepts, commonplace in optical networking. Similarly, implementations based on coherent correlation with the aid of a local oscillator will be detailed and comparisons between approaches will be drawn. Conclusions regarding the viability of these approaches allowing the goal of a large, asynchronous high capacity optical network to be realized will be made.
Optical packet switches offer high speed, fine granularity, flexibility and transparency to data rate and format. There has been much work on the design of optical packet switches each having distinct advantages and disadvantages. Nevertheless, their common limitation is optical splitting loss, which is compensated by optical amplifiers, further degrading performance because of the induced amplifier noise. Hence, it is desirable to design an optical packet switch with a low optical splitting loss. This study has focused on the ALCATEL broadcast-and-select switch, which has significant optical splitting and combining losses for large switches. Arrayed-waveguide gratings (AWG) have been chosen to reduce the switch splitting loss replacing the demultiplexers and Semiconductor Optical Amplifier gates (SOA gates) in the ALCATEL switch. The switch still has the same functionality with an AWG which can be used as an interconnect, and has been demonstrated with insignificant crosstalk of approximately 30 dB. In this paper, three optical packet switches using AWGs are studied; the broadcast-and-select switch, the feed-forward delay switch and the feed-back delay switch. An additional novel feature is their use of wavelength division multiplexed inputs and outputs. Here, their optical performance is investigated with respect to bit error rate and power penalty, and compared with the ALCATEL broadcast-and-select switch using SOA gates.
Coherent optical pulse CDMA systems based on noncoherent demodulation of M-ary orthogonal signals are proposed. Each of the J information bits is encoded and transmitted as a M (equals2J) symbol word of a family of Hadamard-Walsh orthogonal sequences. A pulsed laser is employed at the transmitter, modulated by the symbol sequence and encoded by an optical tapped delay-line encoder to generate a unique optical pseudo-random sequence (bipolar code). At the receiver, a pulsed local oscillator followed by a tapped delay-line encoder is employed to produce the optical code sequence of the intended user. Correlation between the received signal and the local code is executed through a coherent optical correlator comprising a 3 dB coupler and balanced detector. Noncoherent demodulation of the M-ary orthogonal signals based on the maximum-likelihood criterion is used to recover the information bits. After a description of the network implementation, the performance of the system is theoretically analyzed and its numerical evaluation given. It is shown that the ratio of the number of available users to the practical code gain at a BER equals 10-9 is approximately 3.7% - 5%. Although a fewer number of users can be supported compared to full coherent reception (10%), the system no longer requires phase locking whilst keeping the advantages of the coherent approach. Hence the linewidth requirement of the laser sources is relaxed. The performance and the implementation of the system are still significantly better than conventional incoherent optical pulse CDMA systems utilizing unipolar codes.
A novel synchronization architecture is presented for packet based optical networks, exhibiting low insertion loss and reduced crosstalk, taking advantage of the characteristics of AWG (arrayed waveguide grating) filters. Thus far, only logarithmic delay lines have been investigated rigorously, the generic structures either based on 2 X 2 switches or on a 1-to-m splitter combined with semiconductor optical amplifier gates. The first scheme introduces not only high loss but also crosstalk, producing a high amount of interferometric noise. The latter has a lot of splitting loss to accommodate but overall, fewer stages are necessary to achieve the same delay as more than two paths per stage can be set up. In this paper, AWGs in combination with wavelength converters replace the splitter/SOA-gate geometry minimizing the optical loss, ensuring that higher levels of optical power remain to traverse the adjacent switching matrix. This novel set-up ensures that only low levels of amplification are needed minimizing concomitant noise accumulation. An incoming cell or data stream will be converted to a distinct wavelength determined by an evaluation circuit in the electronic domain. The chosen wavelength maps the input to a length of fiber which in turn represents the necessary delay, effectively executing path length (and hence time alignment) equalization of different incoming packets. Once the wavelength conversion is executed the cell is fed into an AWG, governed by a `hardwired' translation-table (input/output), ensuring cells propagate to the correct output. Finer delays are realized by cascading the principle stage.
This paper describes the development of an optical packet transport network, known as WAvelength Switched Photonic NETwork (WASPNET)--a collaboration between Strathclyde, Essex and Bristol Universities as well as BT, Fujitsu and GPT. One of its main objectives is to reduce packet contention at each node. Normally, this is resolved using node deflection routing or optical delay loops (i.e. the solution is focused at the node design strategy). However in WASPNET, this problem is considered not only as a node design problem but also as a network control and management issue. Although suitable node design can reduce packet loss performance, an appropriate network control can reduce the probability of contentions, hence, improve the network throughput and node cost. This suggests that the network management strategy also influences the node design. A possible network control methodology, the SCattered- Wavelength-Path (SCWP), has been identified to support WASPNET implementation. The paper presents some of the comparison studies that were carried out. These include comparing its limitations, control complexity, packet loss performance and buffer requirements against another technique--the Shared-Wavelength-Path. It highlights solutions to problems encountered by the SCWP. Although the studies performed were intended for WASPNET transport system, the findings are invaluable for those involved in WDM network design.
Coherence multiplexing (CM) exploits the coherence properties of optical sources so that many users may be multiplexed onto a single physical channel giving a secure, asynchronous mode of transmission, free of central network control. In this paper, for the first time, the CM system performance limitations owing to the impact of interference from unwanted users is presented and the suitability of LED, single mode Fabry-Perot laser and DFB sources are considered. It is concluded that the single mode Fabry-Perot is preferred, offering readily achievable interferometer length tolerances and near complete RF rejection of interferometric noise at the receiver. However, interferometric noise still limits the network capacity: a linewidth of lnm permits 17 users to be simultaneously offered a single video channel with a BER<10-9.
This paper is a study of routing in optical WDM grid networks, these being chosen since they represent good idealizations of less regular networks, suggest hypotheses about them, can be used for local area and computer networks, and are analytically tractable. Due to performance, cost, and physical effects in the fibers, it will be necessary to have several fibers between each pari of connected nodes supporting some small number of wavelengths on each. This is modeled analytically and compared with simulation for a variety of cases. It is shown that the simplified analytical routing compares well with the heuristic simulation algorithm, and that when multiple fiber links are used, wavelength conversion offers little reduction in network requirements.
The capacity of local area networks (LAN) can be substantially increased by amalgamating different access techniques. Wavelength Division Multiplexing (WDM) and Code Division Multiple Access (CDMA) both offer and asynchronous mode of communications. Integration of WDM and CDMA could offer increases in the network capacity whilst still attaining asynchronous operation. Here, a different kind of integration is proposed, similar to the hybrid system used in electrical CDMA. A novel optical CDMA concept using wavelength hopping and time spreading is described employing prime sequences for both hopping and spreading and proof is given that the autocorrelation function has zero side-lobes and that the crosscorrelation function is at most 1. Code generation and correlation is achieved efficiently, the number of possible stations is greatly increased as is the number of simultaneous users. It is shown that the system offers unprecedented security leading to a very secure communications. The system is suitable for truly asynchronous highly secure LAN applications.
Crosstalk induced interferometric noise has been found to constrain the performance of optical switching networks. A computer design and simulation package has been used to analyze the effect of this noise on optical TDM crossconnects. This software tool is able to calculate all possible routes that a signal may traverse within a crossconnect for a given input/output assignment, and also incorporates a theoretical analysis of interferometric noise. It has been found that for a crossconnect capable of switching 512 independent channels, the constituent 2 x 2 crosspoints must possess isolations better than -20 dB, and the switch architecture must be dilated.
KEYWORDS: Waveguides, Optical signal processing, Signal attenuation, Directional couplers, Photonic integrated circuits, Optical networks, Glasses, Signal processing, Silica, All optical signal processing
Silica-on-silicon waveguiding components are of great importance in the construction of an all optical signal processing network. By using arsenosilicate glass (ASG) waveguides a tight optical confinement can be achieved thus allowing a high device packing density. This paper examines differences in waveguide curvature and reflow conditions then illustrates how they affect the device loss.
A fiber-optic intensity modulator was constructed based on coupling from a side polished single-mode fiber to a high index multi-moded electro-optic polymer overlay. Resonant coupling from the fiber to the film is controlled by applying an electric field to the polymer film causing modulation of fiber throughput intensity. The polymers used were cross linkable azo dye based species exhibiting a pockels co-efficient of up to 8 picometer/volt. Extinction ratios greater than 15 dB with insertion losses less than 0.6 dB were achieved. The effects of electrode thickness, overlay index and thickness on device performance are addressed. Wavelength selective filters were also fabricated using polymers giving linewidths of 7 nm (FWHM).
KEYWORDS: Switches, Network architectures, Signal attenuation, Switching, Time division multiplexing, Waveguides, Telecommunications, Optical switching, Networks, Control systems
This paper describes some novel architectures for optical TDM switching, and an experimental system working at 720 Mb/s; this includes a complete BER characterization. The architectures are composed of 2 X 2 optical switches and delay lines. The trade-offs between the various factors affecting their design are explored, and a wide range of architectures are presented suitable for different applications.
The theory of polarization effects in an optical passive ring resonator gyro (OPRG) will be discussed using Jones matrices. The consequences of a mismatch in the states of polarization incident to the optical fiber ring resonator (OFRR) and a misalignment in the principal axes of its directional coupler shall be considered. Particular attention will be paid to ways of improving the gyro's reciprocity by means of an in-loop polarizer.
Evanescent wave sensors based on single mode waveguides are attractive but have rather low sensitivities for gaseous species and suffer degradation from surface contaminants. We propose the use of a thin high-index loading film on a silica waveguide to enhance the field strength at the guide surface and to correct measurements for surface contamination by launching the two polarisation modes. Resonant structures also enhance sensitivities when matched to the absorption characteristics of a gas.
The use of optical waveguides as chemical sensors requires a referencing system to compensate for the effect of contamination on the surface of the sensor. In this paper, we propose the use of two waves with different penetration depths for their evanescent fields in order to correct measurements. Several schemes are considered, including two-wavelength, two-mode and polarisation discrimination techniques. It is shown that good results can be obtained using TE0 and TM0 modes in a high index film.
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