Division Multiplexing Mechanism Using A Multimode Fibre

Published Date: 02 Nov 2017

The rapid development of telecommunications systems has always been driven by ever-increasing user stresses for different applications as well as nonstop advances in enabling technologies. In the preceding ten years, we have witnessed the tremendous success and dramatic development of the Internet, which has attracted a large number of users surging into the Internet. Individual users are using the Internet constantly for communication, information, and entertainment while business users are progressively relying on the Internet for their daily commercial operations.

As a consequence, Internet traffic has experienced an exponential progress in the preceding ten years, which is consuming additional network bandwidth. On the other hand, the emergence of time-critical multimedia applications, such as Internet telephony, video conferencing, video on demand, and interactive gaming, is also swallowing up a large amount of network bandwidth. All these facts are imposing a tremendous demand for bandwidth capacity on the underlying telecommunications infrastructure.

With massive demand in the usage of internet, there is a large surge in bandwidth requirements in today’s network [1, page 14]. Fibre-optic technology can be considered as a rescuer for meeting these demands because of its potentially limitless capabilities, huge bandwidth with a potentially of nearly 50 terabytes per second, low signal attenuation, low material usage, low power requirement, low signal distortion, small space requirement and low cost. The strategy in designing optical communication systems in order to exploit the fibres vast bandwidth is to present concurrency among various user transmissions into the network architectures and protocols.

In an optical communication network, this may be provided according to either wavelength (WDM), time slots (TDM), or wave shape or spread spectrum (CDM). The optical TDM and CDM are quite futuristic technologies today. In optical TDM, respective end-users should be able to harmonize to within one time rate. The optical TDM bit rate is the combined frequency over all TDM channels in the system, whereas the optical CDM chip rate may be much greater than each user’s data speed. As a result of this, both the CDM chip rate and the TDM bit rate can be much greater than electronic processing speed, which requires that certain part of an end user’s network boundary must operate at a speed greater than electronic speed. Consequently, TDM and CDM are relatively less attractive than WDM since WDM has no such requirement [2]

Nevertheless, because of the short fall in the electronic processing speed, it is doubtful that all the bandwidth of an optical fibre can be exploited by using a singlehanded high capability optical channel or wavelength. For this reason, it is necessary to find an effective technology that can competently exploit the immense potential bandwidth capacity of optical fibres. The development of wavelength division multiplexing (WDM) technology has delivered a practical solution to meeting this challenge. Through WDM technology, several optical signals can be transmitted individually and simultaneously in different optical channels over a single fibre, each one at a rate little gigabits per second, which considerably increases the operating bandwidth of an optical fibre. Recently, commercial WDM systems with up to 160 OC-192 (10 Gbps) channels are available on the market [3].

In addition to the increased usable bandwidth of an optical fibre, WDM also has a number of other advantages, such as reduced electronic processing cost, data transparency, and systematic failure handling [4]. As a result, WDM has become a technology of choice for meeting the enormous bandwidth demand in the telecommunications infrastructure. Optical networks employing WDM technologies are the promising network infrastructure for next-generation telecommunications networks and optical Internet [3-10].

Today, WDM technology stays seen deployed in various types of telecommunications networks. However, the deployment is mainly for point-to-point transmission [4]. All the routing and switching functions remain performed electronically at each network node. Optical signals must pass through optical-electronic (O/E) and electro-optical (E/O) change at each intermediate node as they propagate along an end-to-end path from one node to another node. Consequently, a network node may not be capable of processing all the traffic carried by all its input signals, including the traffic intended for the node as well as the traffic that is just passing through the intersection to other network nodes, causing an electronic bottleneck.

To be able to meet customer needs, telecommunication companies have to develop solutions for increasing their channel capacity at the lowest price possible, and in most cases, wavelength division multiplexing (WDM) seems to be an encouraging solution. It remains expected that the WDM technology will continue seen deployed in all types of telecommunications networks, not only in wide area networks or backbone networks, but also in metropolitan and local area networks [11-15].

Aims and Objectives

The aim of this project work is to create a framework to multiplexing two optical signals generated at different frequencies and transmitted through a plastic multimode fibre. Extending the system to extract the appropriate individual signals remains viewed as a possible addition. The project plan remains divided into four objectives:

To develop a model to deliver the design and define the project requirements and view the available LED, fibre and photo-detector technology

Design and create an optical link, test and discuss the link.

Using a two to one splitter for the combination of two different optical signals and test the system.

Designing and implementing a system capable of separating the two frequencies from the photo-detector or tone decoder.

Structure of the Report NEEDS UPDATED TO CONFORM WITH TABLE OF CONTENT

The structure of this report takes the following pattern:

The second chapter will entail the deliverables which for this project set out to achieve.

The third chapter covers the fundamental background of the wavelength division multiplexing, its advantages, disadvantages and applications. Then the principle of the WDM transmission and reception will be covered, in addition to the general requirement, for the designing of a WDM system.

Chapter 4 covers the technical approach used to undertake the project to meet the set objectives and the deliverables of the project.

Finally, Chapter 5 covers the outline plan of the procedure for the entire project, allotting the work to be done and the time for it. Also, a graphical illustration of the schedules that help the plan, coordinate and track the tasks in the project will be presented. It again shows the allocation of individual responsibilities, identifies resources required, the time availability at each stage for monitoring and managing the project, have all been duly described.

Chapter 2

2.1 Deliverables

The project will include the following:

Four (4) working circuits (two transmitters and two decoder) using a Multisim and breadboard as a prototype and then finally building the circuit on a copper strip board and indicating the layouts with the parts listed.

Designing two transmitter circuits with the ability to produce the signal power for the two transmitters and multiplex the signals into the multimode fibre using a two to one splitter.

A photo-detector receiver circuitry that is sensitive to light transmitted through the multimode fibre and able to recover the original signals transmitted.

Coupling of the plastic multimode fibre to the transmitters and the photo-detector is a critical requirement for the project and will be access if splicing becomes required at the end of each link

Chapter 3

3.1 Technical Background of WDM

Wavelength division multiplexing (WDM) is an optical multiplexing technique for taking advantage of the giant bandwidth capacity inherent in optical fibres. Conceptually, it is similar to frequency division multiplexing (FDM) that has already been used in radio communication systems for over a century. The basic principle is to divide the large bandwidth of an optical fibre into a number of non-overlapping sub-bands or optical channels and transmit multiple optical signals simultaneously and independently in different optical channels over a single fibre, each signal being carried by a single wavelength [6 page3]

Figure 3.0 Transmission Spectrum of optical fibres [6]

The methodology that can exploit the vast optic-electronic bandwidth mismatch by demanding that each end-user’s equipment functions only at electronic rate, but numerous WDM channels from diverse end-users may be multiplexed on the same fibre can be termed wavelength division multiplexing (WDM). Under WDM the optical transmission spectrum is carved up into a number of non-intersecting wavelength (or frequency) bands, with every wavelength supportive of a single communication channel operating at whatever rates one wants. Therefore, by permitting multiple WDM channels to exist on a single fibre, on can tap into the towering fibre bandwidth, with the corresponding architectures, protocols and algorithms [1 page 20].

In the optical transmission spectrum, there are two common attenuation areas, one at 1300 nanometres (nm) and 1500 nm. The two areas have a variety of about 200nm with an attenuation loss less than 0.5 decibels per kilometre. Theoretically, these two areas can provide a perfect number of 50 terabytes low-attenuation transmission bandwidth. However, the maximum rate at which a target device can read an optical channel remains limited by its electronic processing speed, it is technically impossible to take advantage of all the bandwidth of an optical fibre. Accordingly, a single fibre is theoretically capable of supporting over 1000 optical channels or wavelengths at a few gigabits per second [6 page 4].

Figure 3.1 Simple Block diagram of a WDM system [7]

There is a strong need for the standardisation of WDM systems so that WDM components and equipment from different vendors can inter-operate with one another. The industrial standards for wavelengths been developed under the leadership of the international telecommunication union (ITU), a standard set of wavelength termed the ITU grid, has been defined to correspond with the 1550-nm low-loss area of the fibre.

Table 3.1 WDM transmission system components and their properties [21]

Component

Parameters

Light source

Emission spectrum, peak wavelength, line-width, wavelength dependence on temperature, noise characteristics

Photo-detector

Sensitivity dependence on wavelength, noise characteristics

Multiplexer/de-multiplexer

Loss, number of channels, channel spacing, crosstalk, signal distortion, reflection or scattering

Fibre

Loss and bandwidth dependence on wavelength, polarisation effect, back-scattering

SORTED TO THIS POINT

Figure past and projected future growth of data and voice traffic [2]

3.2 Advantages and Disadvantages of WDM System

The WDM system comes with a good number of advantages which are

Since ordinary WDM multiplexers are quite passive, their reliability is extremely high with exceptionally high mean time between failures (MTBF).

Simple WDM systems do not require cooling and their activity do not require electrical power that make the process environmental friendly.

The system will typically not require configuration, adjustments, service and are easy to be instated.

Its operation are full duplex over one fibre or two fibre spans and are available for a WDM multiplexer.

When the WDM link structure is fully passive, and the transceiver discharge energy level is below +50dBm (typically of a standard single mode fibre), then any addition or removal of a different wavelength will not have any effect on the other existing signal wavelength in the fibre.

The WDM system also comes with some bottlenecks, which are

The impossibility of remote supervision and composition due to simple nature of WDM

Difficulty in the growth of multiplexers as most has fixed channels, and it requires that the proper capacity of the WDM unit is considered from scratch.

There is a limitation on their operation due to transceiver optical budget and complete connection loss. Typically WDM systems are spread out in a distance of less than 100km [8 pages 178].

3.3 Applications of the WDM System (NOT COMPLETED)

Optical systems and networks have evolved enormously in the last three decades with the creation of next-generation optical components, subsystems, systems and networks that are now utilized in all aspects of the network structure starting from the in-house/building and access networks, all the way up to the backbone and ultra-long haul infrastructures.

WDM/SONET network application provides a natural migration route to support traffic development, where every wavelength is a separate SONET ring and every single ring can be set to one of the numerous line speeds. In this application, the line speed should be high if there is enough traffic realise the economy of scale [9]. There are two types of SONET rings, the Unidirectional Path Switched Rings (UPSR) and Bidirectional Line Switched Rings (BLSR) [3]. In UPSR, traffic transmitted is always in one direction and the ring consists of 2 fibres where one is dedicated for protection

purpose. In BLSR traffic can be either transmitted in the clockwise direction or routed in the counter-clockwise direction. BLSR consists of either two or four fibres, commonly referred to as BLSR/2 and BLSW/4 respectively. UPSR is commonly used in Metro access networks, whereas BLSR is commonly used in Metro backbone networks [10].

Long-haul terrestrial network

Very long-haul undersea network

Short-haul to medium-haul local exchange network of the regional carriers in the United States

Metropolitan-area networks deployed by large enterprise in the United States as well as in Europe.

3.4 WDM System facilitating technologies

The concepts for WDM started being explored in the laboratory more than three decades ago, the enabling technology for the cost-effective implementation of WDM was the creation and perfection of various passive and active optical components used to distribute, isolate, combine and amplify optical powers at diverse wavelengths.

WDM systems are separated into diverse wavelength forms, dense (DWDM) and conventional/coarse (CWDM). CWDM systems offer up to 8 channels in the third (3rd) transmission window (C-Band) of silica fiber around 1550 nm. Dense wavelength division multiplexing (DWDM) uses a similar transmission window but by means of a denser channel spacing. Channel plans differ, but a typical arrangement would be 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacingPassive devices need no external control for their process, so they are slightly limited in their application in WDM networks.

These components are mainly used to split and combine or tap off optical signals. The actions of active devices can be controlled electronically, in this manner providing a high proportion of system flexibility. Active WDM network components include tunable optical filters, tunable sources, and optical amplifiers [11]. The figure 3.4 will describe a typical WDM transmission system.

Figure 3.4 Implementation of a typical WDM transmission system [11]

The wavelength multiplexer/de-multiplexer is classified into unconventional and conventional and operates as wavelength-selective or wavelength-independent. A lens which is a wavelength independent device and as such can only be a multiplexer in the sense that it can combine (focus) many wavelength to a single point whiles for a wavelength separation or de-multiplexing requires dispersive elements such as a grating.

Due to the principle of reversibility in optics, a multiplexer can be used as a de-multiplexer as in the case of a grating element. The grating devices in various configurations are the most popular WDM elements in practice [12]. Also other factors or parameters to be taken into consideration for a WDM transmission system includes

Light source with respect to emission spectrum, peak wavelength, line-width, wavelength dependence on the temperature and noise characteristics.

Photo-detector with respect to sensitivity dependence on wavelength and noise characteristics.

Multiplexer/De-multiplexer with respect the loss, number of channels, channel spacing, crosstalk, signal distortion and reflection/scattering.

Fibre with respect to loss and bandwidth dependence on the wavelength, polarisation effect and back-scattering.

3.4.1 Single and multimode Fibre

Optical fibres are dielectric waveguides designed to guide lightwaves along their length. For easy fabrication and implementation, silica preform is usually drawn into fibres of circular cross section. Optical fibres consist of a cylindrical core with reflective index , surrounded by a cladding with an index , with > . The border between the core and the cladding may be sharp or graduated. Optical fibres are of two types the single mode fibre (SMF) and the multimode fibre (MMF) [13]

Optical fibre possesses many characteristics that make it an excellent physical medium for high-speed networking Centred at approximately 1300 nm is a range of 200 nm in which attenuation is less than 0.5 dB per kilometre. The total bandwidth in this region is about 25 THz. Centred at 1550 nm is a region of similar size with attenuation as low as 0.2 dB per kilometre. Combined, these two regions provide a theoretical upper bound of 50 THz of bandwidth. The dominant loss mechanism in good fibres is Rayleigh scattering, while the peak in loss in the 1400-nm region is due to hydroxyl-ion (OH) impurities in the fibre. Other sources of loss include material absorption and radiative loss [14].

A single mode fibre has the following parameters, refractive index of the core , refractive index of the cladding , core radius ρ, source wavelength λ and a fibre parameter V. The fibre parameter is defined as

V = × …………… (1)

The single mode fibre also known as the fundamental mode usually has a core size of about 10µm. A step-index fibre will support a single mode if V in (1) is less than 2.4048. The single mode fibre eliminates intermodal dispersion and can hence support transmission over much longer distances.

Figure 3.4.1.1 Single mode optical fibre [14] Figure 3.4.1.2 Single mode step index fibre

The angles for which waves do propagate correspond to modes in a fibre. If more than one mode may propagate through a fibre, the fibre is called multimode. In general, a larger core diameter or high operating frequency allows a greater number of modes to propagate. The number of modes supported by a multimode optical fibre is related to the normalized frequency V. In multimode fibre, the number of the modes m is given approximately by

m ≈ ………………. (2)

The multimode fibre typically has a core size of 50-100µm

Figure 3.4.1.3 Graded index fibre[14] Figure 3.4.1.4 Multimode optical fibre [14]

The advantage of multimode fibre is that its core diameter is relatively large; as a result, injection of light into the fibre with low coupling loss can be accomplished by using inexpensive, large-area light sources, such as light-emitting diodes (LED’s). The disadvantage of multimode fibre is that it introduces the phenomenon of intermodal dispersion which does not occur in single mode fibres.

In multimode fibre, each mode propagates at a different velocity due to different angles of incidence at the core-cladding boundary. This effect causes different rays of light from the same source to arrive at the other end of the fibre at different times, resulting in a pulse that is spread out in the time domain. Intermodal dispersion increases with the distance of propagation.

The effect of intermodal dispersion may be reduced through the use of graded-index fibre, in which the region between the cladding and the core of the fibre consists of a series of gradual changes in the index of refraction. Even with graded-index multimode fibre, however, intermodal dispersion may still limit the bit rate of the transmitted signal and may limit the distance that the signal can travel. One way to limit intermodal dispersion is to reduce the number of modes. The reduction in the number of modes can be accomplished by reducing the core diameter, reducing the numerical aperture, or increasing the wavelength of the light [14].

Figure 3.4.5 Numerical aperture of a fibre [14]

3.4.2 Light emitting diode (LED) CHECKED FOR PLAGIARISM & GRAMMER

Light emitting diodes assumed the role of an old technology that in recent times has undergone various improvements resulting in its use as ubiquitous as a mobile phone. Taking a ride through the city, from traffic lights to flashlights, examining the floor lighting on a modern aircraft, automotive lighting, aviation lighting and advertising, there is a greater probability of encountering LED based products. Nick Holonyak in 1962 remained associated with the invention of the first LED.

Gallium arsenide phosphide (GaAsP) compound semiconductors were the first light emitting diode (LED) to have emitted in the visible wavelength region with external efficiencies of 0.2% but evolved with time. As LEDs based on aluminium gallium indium phosphide (A1GaInP) have external efficiencies exceeding 50% and emitting at amber, orange and yellow wavelengths, other semiconductors based on A1GaInN technologies emits efficiently in the violet, blue, UV, cyan and green wavelength range. With reasonably high efficiencies, all colours of the visible spectrum become enclosed.

Figure 3.4.2.1 the evolved performance of commercial LED [15]

The LED is an optical diode that emits light when in the forward biased condition. Figure 3.4.2.2 shows the symbol of an LED which is equivalent to p-n junction diode without the two arrows signifying that the device emits light energy. In the n-type semiconductor electrons move across the p-n junction, when the p-n junction is in forward biased.

The free electrons which are at a higher energy level with respect to holes in the valence band, when the free electrons recombines with hole, it falls from the conduction band to a valence band, the energy level associated with its transforms from greater value to a lesser value. The corresponding change between the greater level and the lesser level gets freed by an electron whiles travelling from the conduction band to the valence band. With its unique material, this energy gets released as photons which radiate the light energy.

Thus, the energy freed in the form of light rely on the energy matching that of the forbidden gap, which defines the wavelength of the radiated light be it an invisible or visible (infrared).

Figure 3.4.2.2 Symbol of an LED [15] Figure 3.4.4.3 Basic LED circuit [16]

The outward arrows related with a diode specify that it is an LED. The resistor Rs is the current limiting resistor, due to this resistor the current through the circuit gets restricted and disallowed from surpassing the maximum current rating of the diode.

= Supply voltage

= Drop across LED

Then applying KVL to the LED circuit as

………………………. (3)

………….……………(4)

Voltage drop across a conducting LED is about 2-3V when in forward biased mode which is significantly larger than that across a normal silicon and germanium diode. The contemporary range of commercially available LED’s is 10 to 80 mA. Unless and otherwise specified, while analysing the LED circuits, the drop voltage across LED is . The brightness of the LED depends on the current passing through it; then it is high when gets much larger than . The breakdown voltage of LED is about 3-10V; hence the LED can be easily destroyed if the reverse biased voltage in the circuit is high.

Figure 3.4.4.4 LED output characteristic and I-V characteristics [17]

The amount of power output translated in light is directly proportional to the forward current, and the actual colour of the LED gets ascertained by the wavelength of the light radiated, which in turn gets determined through the actual semiconductor compound used in creating the PN junction throughout production. Tests have proven the LED is reliable than semiconductor lasers under identical operating conditions. LED’s can be used advantageously because they are cheap and have longer life span [18].

3.4.3 Optical connectors and couplers

Optical fibre connectors provide a means for jointing the cut ends of two fibre optic cables. Such a joint is not an everlasting one, but it can be closed and opened several times. There is the need of optical fibre connectors at the points of a system in which, it is essential to have the flexibility in terms of structure arrangement and test access [19]. An optical connector may have different sets of necessity as a splice joint, low insertion loss, stable in insertion losses after a great number of connect and disconnect actions, low in cross-talk among different connectors and must be interchangeable with other connectors. Optical connectors come in two categories, the butt-coupled (without lenses) and the lens coupled [13 pages 89].

Fibre optic connectors have application in all types of networks, at the output and input ports of the communication systems and for connecting testing equipment. The effects of introducing of an optical connector in an optical cable are that the transmitted signal gets attenuated, and part of the signal gets reflected [19].

Optical fibre connectors made of lens coupled type have a centrepiece of moulded lens shape. They are comprised of a moulded, plastic bi-conical middle element with an optical fluid in each of the two concave cavities, and it requires accurate transverse alignment of the fibres. Optical connectors of the butt-coupled type consist of a ferrule for each fibre, a precision sleeve into which the ferrules fit and a lid to keep the connection. The types of butt-coupled optical connectors available are:

Tube alignment connectors

Tapered sleeve connectors

Jewel bushing connectors

Resilient ferrule connectors

Optical fibre connectors can also be classified on the basics of the fibre alignment system, fibre end face finish, coupling mechanism, number of jointed fibres and the outer diameter of their ferrule (2.5 mm or 1.25 mm).

Some fibre optical links requires more than a straightforward point-to-point link and may need multi-port types of connections. In most cases, these types of systems will require a fibre optic module that can redistribute (combine or split) the optical signals throughout the system. Fibre optic couplers could be either passive or active devices. The distinguishing characteristic between passive and active couplers is that a passive coupler apportions the optical signal without optical-to-electrical conversion.

Electronic devices in the form of active couplers partition or merge the signal electrically and use fibre optic detectors as sources for input and output. A perfect coupling element should have the following characteristics:

It should assign the light power among the divisions of the system without scattering losses. Finite scattering loss at the coupler could limit the number of branches usable in a network.

It should be insensitive to the wavelength within the window for which it stays designed

Their accomplishment should not be influenced by either the light power dispersion among the fibre modes.

It must be flexible in its activity to allow the physical arrangement of different network configurations for the optimal utilization of cable or optical power.

It should provide the flexibility for connecting to a number of branches even with varying fibre types.

3.4.4 Multiplexer and De-multiplexer

Optical multiplexers and de-multiplexers are optical devices used to either combine distinct wavelength channels into a composite signal or split a composite signal into their channel constituents. There are several types of optical multiplexers in use today, based on either the diffraction or the interference effect [20].

Optical multiplexers and de-multiplexers can either be active or passive devices and can be unconventional and conventional. Passive devices use prisms, gratings or fixed filters, whereas active designs work with tunable filters. The main challenge in designing a (de)multiplexer is to get a high channel separation and low cross talk. The isolation between the channels should be at least 20dB. This means that each neighbouring channels gets damped by at least a factor of 100 when detecting the wanted channels

3.4.4.1 Multiplexer and Demultiplexer Based On a Prism

A prism can be used to split up light into its different component’s, and can be used to de-multiplex different wavelengths. Systems based on prisms seem to be highly complicated for standard telecommunication applications. Using prisms to (de)multiplex WDM channels of polymer optical fibres (POF) based systems, seems to be promising as those fibres have a large core (around 1mm). They get used in the aerospace sector, in the medical sector and the automotive industry.

3.4.4.2 Multiplexer and Demultiplexer Based On Interference Filters

Interference filters (also called dielectric filters, multilayer interference filters or thin film filters) usually built out of two different materials with a different refractive index to each other. They take advantage of the effect that each change in the refractive index results in some part of the light reflected, and part transmitted.

Some wavelengths interference with their reflected parts are constructive, whereas others interference are destructive. Those wavelengths that interfere constructively can pass the filter, whereas the others get reflected. Besides the material parameters, the incident angle plays a vital role as each layer gets relatively thicker when the filter gets tilted. De-multiplexing using interference filters: Each of those filters let’s pass one wavelength and reflects all the others.

Designing a (de)multiplexer out of interference filters causes several problems and to be able to use the filters, the light has first to be extracted from the fibre, focused using a lens, passed through the filter, collected by another lens and finally fed back into the fibre. It is obvious that a process including many different elements is expensive to produce and difficult to align.

Another problem is that the filters do not reflect 100% of the incident light at the "wrong" wavelengths. This is no problem for 8 channels, but leads to significant losses for 16 channels and above. To reduce such losses, other setups can decrease the amount of reflections. A possibility is to use a filter with a broader band first and sort half of the wavelengths out.

3.4.4.3 Multiplexer and Demultiplexer Based On Bragg Gratings

Bragg gratings are the exact opposite of filters: Instead of allowing a single wavelength to pass, they reflect one single wavelength. Built of fibre, they are easy to couple to existing systems (splicing). The main problem is the reflected light. This problem can be solved using Optical Circulators. In most cases, it is not done because of the extraordinary cost and complexity of the used optical circulators bragg grating based multiplexers and de-multiplexing can be used for WDM.

3.4.4.4 Multiplexer and Demultiplexer Based On Fused-Fibre Couplers

The operation of fused-fibre couplers is dependent on the used wavelength. Shorter input wavelengths couple in a shorter distance than longer ones. Once coupled to the other fibre, the wavelengths couple back again to the initial fibre. If two wavelengths be present, a coupler can be designed to separate the wavelengths into the two outputs of the coupler. If run in the opposite direction, the device can be used to multiplex two known wavelengths together. Coupler based de-multiplexers works well if the spacing between the channels is sufficiently large.

3.4.5 Photodetector

The photodetector is an essential component of an optical communication system where an optical signal gets converted to an electrical signal to be subsequently amplified by the receiver electronics. A good photodetector must have high sensitivity at the operating wavelength, high fidelity, large optical to electrical conversion efficiency, high response speed, large signal to noise ratio at the output, high reliability and low sensitivity of performance to ambient conditions. Light detectors are broad band devices hence the channels need to be de-multiplexed first, before they can be detected.

Optical detectors can use preamplifiers and filters. There are two main technologies for detecting light coming out of a fibre: PIN diodes and avalanche photodiodes (APD). PIN photodiodes gets built like normal diodes, but with an intrinsic layer in the middle. Detectors based on PIN diodes usually have waveguides built in to get a maximum amount of light from the fibre to the detector. PIN diodes can operate up to around 50 GHz.

Avalanche photodiodes gets used in the long haul networks because they have a superior sensitivity compared to PIN diodes (up to a factor of 10 better). APD’s are stronger reverse biased PN-diodes but are slower than PIN diodes. Impact ionization by photons creates an avalanche of free carriers (junction breakdown), which can then be measured. They get used up to 2.5 GHz. APD’s up to 10 GHz are possible but usually too expensive [22 pages 308].

3.4.6 Optical amplifier

Optical fibre systems have limitations caused by fibre losses due to their transmission distance. Re-generators get used to overcome the loss limitations in a long-haul system by converting the optical signal into an electric current. This then gets regenerated by using a transmitter. In wavelength division multiplex system, these re-generators turn out to be quite expensive and complex. Optical amplifiers offer a different approach to compensate for these losses, by amplifying the optical signal without having to convert it into the electrical domain.

The conventional laser principle technique is what optical amplifiers devices operations based on. They accept one or many optical signals, each inside a window of optical frequencies and concurrently amplify all the wavelengths. That is they coherently discharge more photons at every wavelength.

There are several types of optical amplifiers, and the most common are erbium-doped fibre amplifier (EDFA), semiconductor optical amplifiers (SOA), and Raman amplifiers. Optical amplifiers need optical or electrical energy to stimulate the state of electron-hole pairs. Energy gets characteristically provided by injecting optical light in EDFA or electrical current (in SOA) and optical light in Raman amplifiers [23].

The EDFA consists of a silica fibre core doped with and the fibre gets pumped with an optical signal from another laser operating at a wavelength of 980 or 1480nm [22 pages 469]. The physical mechanism which provides gain in semiconductor optical amplifiers (SOA’s) varies in many aspects from that of the above EDFA amplifiers.

Fundamentally, SOA’s are semiconductor lasers short of the optical cavity feedback and so the population inversion gets produced by an electrical current in the active region. The stimulated radiation of photons occurs along electron-hole recombination procedures induced through the signal photons (at wavelengths comprised of the amplification band of the semiconductor material).

High-power pumps are exploited by stimulated Raman scattering (SRS) amplifiers to benefit from non-linear distinctive attribute of a fibre that are non-doped fibre amplifiers. The signal and pump get injected into the fibre via a fibre coupler. The energy gets shifted from the pump ray of light to the signal beam through SRS as the two beams co-transmit inside the fibre. The signal and the pump beam counter-transmit in the regressive-pumping configuration usually used in practice. Raman amplifiers can also be called discrete or distribute based on their design [23 pages 121].

Chapter 4

4.1 Technical Approach Introduction

In the study of communication networks, there are three basic suitable approaches for the investigating the performance of a system. These three techniques are identified as analytical modelling, simulation and measurement.

Mathematical analysis is used in analytical modelling to evaluate a given system and has the advantage that it is usually the quickest method. It is also quite inexpensive as it does not need the use of expensive instruments and equipment’s. However, analytical modelling generally requires many assumptions in order to simplify the difficult to a level that makes analysis practical. The level of accuracy is therefore often open to question.

Simulations are used to model a system and its behaviour, usually by means of a computer program, in a form suitable for deriving information about the system’s characteristics and attributes that are of interest to the study. Simulation techniques are usually more accurate as they can incorporate more details and require lesser assumptions than analytical modelling and, thus, are more often closer to reality. Moreover, the ever-increasing power of computer systems and simulation software has made the approach an attractive and viable option.

The third technique, measurements, involves testing actual working systems and networks, and it is usually the most expensive method as it often requires the use of expensive measurement instruments alongside with the cost of actually building a system prototype. It is generally very accurate as every detail can be included in the study.

The design of this system started with a list of specific desired requirements and constraints. The inputs for the design of a wavelength division multiplexing (WDM) system, the link length and the number of terminals, the type of signal or data to be transmitted and the speed of transmission or bandwidth demand of the system. The design had some other constraints which may include the fidelity of the system needed to satisfy the signal to noise ratio, coupling efficiency and the fact that the system must be cost effective. Assessment was carried out on the ready availability of products on the market and the compatibility with the design specification to achieve the desired output. The approach will seek to address and describe the proposed design specification, methodology and technical errors.

CHECKED FOR PLAGIARISM & GRAMMER

4.1 Design specification

In other to achieve the project objective, the following design specifications were outlined as the parameters

Two optical transmitter circuitries were required to transmit signals at different frequencies and with a wavelength in the range of 650-700nm.

A two to one passive combiner to act as a multiplexer which will combine the transmitted signals.

Two optical transmitters and a receiver/detector with an efficient coupling efficiency to reduce the amount of losses. The optical receiver must have the following characteristic: fast response, high sensitivity, low cost, low noise, high reliability and its size must be compatible with the fibre core size.

A plastic multimode fibre to serve as a transmission link between the coupler, transmitter and a receiver. The core diameter of the fibre cable should be 0.75-1.5mm and about 1.0-1.5m long from transmitter to coupler and from the coupler to the photo-detector.

Two tone decoding circuit will be designed with the ability to retrieve the original signals at their respective frequencies.

4.3 Methodology

To get the project going two techniques were adopted, the simulation and measurement techniques. The first phase of was to design a transmitter circuit and ensure it did conform to the parameters set. Two transmitter circuits were first designed with 555 timers in the Astable mode on a multisim software and simulated to see the output frequencies on an oscilloscope before the hardware was built on a on a copper strip board. The initial plan was to use two LED’s flashing at the set frequencies of the LED driver circuit, then the plastic multimode fibre will be mounted in close proximity to the LED’s so enough light from the LED could be transmitted through the fibre cable.

However, as there was a greater tendency of loss and coupling inefficiency, a second approach was taken. Two sharp transmitters with the product description name GP1FAV50TK0F and referred to OPIC devices, which have an inbuilt LED and with a peak light emission wavelength of 660nm did suit the design specification. Due to the extinction ratio and spectral linewidth of optical transmitters a direct modulation approach was ideal with the transmitters on hand. The LED in the transmitters was of the A1GaInP type semiconductor.

Three one meter (1m) lengths of toslink multimode fibre cable where used in linking the two transmitters to the inputs of the coupler and from the output of the coupler to the photodetector/photodiode. The photodector/photodiode was also a sharp product with description GP1FAV50RK0F.

4.3.1 Transmitter Circuitry

The transmitter circuits were designed on the bases of the Astable mode of operation of a 555 timer. In this mode of operation, the 555 timer produced a string of pulses at the output (pin 3). To produce the string of pulses the 555 timer was constantly retriggered automatically by connecting the trigger (pin 2) to the threshold (pin 6).

A resistor was connected between the control voltage (pin 5) and the supply voltage of the 555 timer, the on time can be made longer and shorter but the resistor will not affect the off time or time between the pulses. The resistor-capacitor network is used to control the frequency of the pulses [24]

Figure 4.3.1 Astable Mode Configuration & 555 timer internal architecture [ ]

In the Astable mode of operation, the capacitor charges and discharges between 1/3 and 2/3 . When in the triggered mode, the discharge and charge times, and consequently the frequencies are not influenced by the supply voltage.

The charge time (output high) is given by:

t1 = 0.693 (RA + RB) C ……………………………….. (5)

And the discharge time (output low) by:

t2 = 0.693 (RB) C……………………………………… (6)

Thus the total period is:

T = t1 + t2 = 0.693 (RA +2RB) C ………………………… (7)

The frequency of oscillation is:

f = = …………………………………. . (8)

The duty cycle is:

D = ………………………………………………. (9)

4.3.2 Led driver

Type of LED used is A1GaInP

Receivers

The receiver in the system essentially consists of a photo-detector at the front end plus an amplifier to amplify the detected photocurrent and some additional signal processing circuit. For optical fibre links it is recommended that low noise, high gain, high sensitivity and wide dynamic range photo-receivers are used as the front ends of the receiver to convert photocurrent to voltage.

4.3.5 Coupler and fibre cable

The coupler and the plastic optical multimode fibre used for the transmission of the signals are of the toslink type. The SPDIF optical cable is designed to transmit audio data up to sampling rates of 192(kHz). With the ability of the spdif toslink to transmit two channels of digital data signal once it suited the project specification. The design transmission speed of the sharp GP1FAV50TK0F of 13.2Mb/s

Toslink fibre optical cables are made of fibre-optic cabling (usually plastic fibre). They transmit the digital audio as pulses of laser-generated light, not as electrical signal. The optical in the connector accepts standard TOSlink cables that transmit audio digital form through light pulses. Toshiba link (TOSLink) is a digital interconnection standard that allows transmission of S/PDIF data by sending light over optical TOS cables, and standardizes the physical jacks and connectors used. The plug itself is squarish and has grooves around it to allow it to fit into an optical jack with a nice click.

100Ω resistor was connected in series to the LED to limit the current to a safer value.

Multi-strand

Chapter 5

5.1 Results and Discussion

5.2 Conclusions and Recommendations

Chapter 6