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Design of POINT -TO - POINT OPTICAL FIBER LINK

An optical link between two points with a total of 16 channels each with a capacity of 5 Gbps. One end consists of 16 transmitters and a multiplexer to send and on the other, a demultiplexer and 16 receivers to receive. The multiplexer and demultiplexer is connected by optical fibre. Along this link, there are amplifiers to amplify the signal and dispersion compensation modules to compensate the dispersion in the fibre link.

Channel It is a communication path along which the signal is sent over. Through multiplexing a number of channels voice and data channels can be sent over an optical channel. The number of channels to be used is 16 in this project.

Bit Rate It is number of bits that are transferred between several devices in a specified amount of time. It is same as Data rate. The Data rate per channel is 5 Gbps

Distance between nodes It is important to determine the distance between the two cities, Coventry and Paris, before commencing the design of the optical fiber link. Base on the result from the RAC route planner the distance between Coventry and Paris is 479 km (279 miles). The link can be established as shown in MAP. This link will be a transoceanic link as it passes through water. While designing the parameters like dispersion compensation and losses compensation will have to be given top priority. Table of given specifications Parameters Specifications 1 Number of channels (M) 16 2 Bit Rate per channel (B) 5 Gb/s 3 Distance between nodes (L) 479 km

Mathematical Analysis System capacity The total number of channels and the system bandwidth that a system can handle is the system capacity or we can say the maximum number of channels that a cable system can carry simultaneously. It determines the minimum bandwidth requirement for the whole system and an important factor taken into account while selecting an optical fiber. This link has 16 channels at 5Gb/s. System capacity = M*B = 16 * 5 Gb/s = 80Gb/s The total system capacity is 80Gb/s. 3.2 Bit-Rate distance Product The bit rate distance product of an optical fiber is a figure of merit equal to the product of fiber’s length and it predicts the effective fiber bandwidth for other lengths and for concatenated fibers. System capacity *distance = B*L = 5 Gb/s* 479km = 2,395Gb/s*km Since, the bit rate distance product is 2.395 Gb/s*Mm is low than ≤ 10(Gb/s)*Mm it is a low specification link. 3.3 Channel spacing It is the minimum frequency separation between two adjacent WDM signals. An inverse proportion of frequency versus wavelength of operation calls for different wavelengths to be introduced at each signal. The optical amplifiers bandwidth and receivers ability to identify two close wavelength sets the channel spacing.

The unique wavelengths passing through the amplifier are restricted by inter channel cross talk. The significance of having adequate channel spacing is to avoid any kind of cross-talk due to the interference between adjacent channels. This is usually done by giving a guard band between adjacent channels which acts as a buffer and prevents any kind of interaction between adjacent channels. Smaller channel spacing leads to better system capacity. δλ={1-2} nm for 8≤ m ≤ 32 Considering channel spacing δλ = 1nm as the link is of 16 channels Or Frequency bandwidth δf = 100GHz Channel line width λ = 1550nm Speed of light C = 3*10 8 m/s Therefore the channel spacing equals = [(1550nm) 2 /3*10 8 ] * 100GHz = 0.8nm ≈ 1nm 3.4 Spectral window It is a band of wavelengths at which a fibre is sufficiently transparent for practical use. It can be estimated from the calculation of spectral window the requirement of source and the handling capacity of the optical fiber. Spectral window ∆λ ∆λ = m* δλ = 16 * 1*10 -9 ∆λ = 16nm typically, which is greater than 2B i.e. 10Gbps 7 4. Design Specifications and Selection criteria Based on the calculations above for system capacity, channel spacing and with the knowledge of attenuation we can select the components required for our link. The standard specifications that should be in all the components are: 1. Operational at 1550nm wavelength 2. Functional in 5 Gbps data rate range 1. Optical Fiber Corning SMF-28e® Photonic Fiber The key optical performance parameters for single-mode fibers are attenuation, dispersion, and mode-field diameter. Optical fiber performance parameters can vary significantly among fibers from different manufacturers in ways that can affect your system’s performance. It is important to understand how to specify the fiber that best meets system requirements.

Impairments in performance: Attenuation Attenuation is decrease in the signal strength in a fiber optic cable because of absorption and scattering. It is the loss of optical power as light travels down a fiber and measured in decibels (dB/km). Over a set distance, a fiber with a lower attenuation should be opted will allow more power to reach its receiver than a fiber with higher attenuation. While low-loss optical systems are always desirable, it is possible to lose a large portion of the initial signal power without significant problems. A loss of 50% of initial power is equal to a 3.0 dB loss.

Any time fibers are joined together there will be some loss. Losses for fusion splicing and for mechanical splicing are typically 0.2 dB or less. Dispersion Dispersion is the time distortion of an optical signal that results from the time o flight differences of different components of that signal, typically resulting in pulse broadening.

Impact of Dispersion In digital transmission, dispersion limits the maximum data rate, the maximum distance, or the information-carrying capacity of a single-mode fiber link. In analog transmission, dispersion can cause a waveform to become significantly distorted and can result in unacceptable levels of composite second-order distortion (CSO). single-mode fiber that eliminated severe multimode fiber related dispersion and left only chromatic dispersion and polarization mode dispersion to be dealt with.

Chromatic dispersion It represents the fact that different colors or wavelengths travel at different speeds, even within the same mode. Chromatic dispersion is the result of material dispersion, waveguide dispersion, or profile dispersion. Figure below shows chromatic dispersion along with key component waveguide dispersion and material dispersion.

The example shows chromatic dispersion going to zero at the wavelength near 1550 nm. This is characteristic of bandwidth dispersion-shifted fiber. Standard fiber, single-mode, and multimode have zero dispersion at a wavelength of 1310 nm. Every laser has a range of optical wavelengths, and the speed of light in fused silica (fiber) varies with the wavelength of the light. Since a pulse of light from the laser usually contains several wavelengths, these wavelengths tend to get spread out in time after travelling some distance in the fiber. The refractive index of fiber decreases as wavelength increases, so longer wavelengths travel faster. The net result is that the received pulse is wider than the transmitted one, or more precisely, is a superposition of the variously delayed pulses at the different wavelengths. Further complication is that lasers, when they are being turned on, have a tendency to shift slightly in wavelength, effectively adding some Frequency Modulation (FM) to the signal. This effect, called “chirp,” causes the laser to have an even wider optical line width. The effect on transmission is most significant at 1550 nm using non-dispersion-shifted fiber because that fiber has the highest dispersion usually encountered in any real-world installation. Polarization mode dispersion It is another complex optical effect that can occur in single-mode optical fibers. Single-mode fibers support two perpendicular polarizations of the original transmitted signal. If it is perfectly round and free from all stresses, both polarization modes would propagate at exactly the same speed, resulting in zero PMD.

However, practical fibers are not perfect; thus, the two perpendicular polarizations may travel at different speeds and, consequently, arrive at the end of the fiber at different times. Figure below illustrates this condition. The fiber is said to have a fast axis, and a slow axis. The difference in arrival times, normalized with length, is known as PMD (ps/km 0.5 ). Excessive levels of PMD, combined with laser chirp and chromatic dispersion, can produce timevarying composite second order distortion. Like chromatic dispersion, PMD causes digital transmitted pulses to spread out as the polarization modes arrive at their destination at different times. For digital high bit rate transmission, this can lead to bit errors at the receiver or limit receiver sensitivity. Single mode fiber is the most suitable choice for this link. A laser is used to launch light into this fiber, which have a small core and diameter. Corning SMF- 28® Photonic fiber is a single mode fiber designed for optical customisation and component applications, has low manufacturing cost, standardised processes and improved performance. The key technical features and optical performances of this fiber are listed below: 1. Good optical and geometric specifications 2. Exceptional performance and splice- ability 3. Low loss and high effective area 4. Attenuation <= 0.2 dB/km 5. Dispersion <=18[ps/(nm*km)] 6. Functional at low temperature like –60 0C to upto +85 0C.

Polarisation mode Dispersion <=0.2(ps/\/km)

Also it can be used as a cost-effective fiber for the periodic in-line dispersion compensation that is usually required. Specification sheet of Manufacturer for Corning SMF-28e® Fiber

Specification sheet of Manufacturer for Corning SMF-28e® Fiber

Transmitter MAP 1550 nm Optical Transmitter An optical transmitter is used to convert the electrical signal into optical form and to launch the resulting optical signal into the optical fibre. Semiconductor lasers do the encoding to allow an optical output of 850nm, 1330nm or 1550nm. There are 16 channels in the link from Coventry to Paris and thus 16 transmitters; one for each link is required. The Multiple Application platform (MAP) 1550nm Optical Transmitter is used in this link. It is an externally modulated 1550nm transmitter. Manufacturer’s Specification Sheet of MAP 1550 nm Optical Transmitter

The keys features associated with this Transmitter that makes it the most suitable choice is: 1. High output power 2. Wide frequency range 3. Operational from 155 Mb/s to 12.5 Gb/s data rates 4. Functional Optical wavelength 1550nm 5. Extinction ratio 11dB 6. Various options for Optical connector FC/PC, SC/PC 3. Receiver

OPTICAL RECEIVER MO10GB1550 A receiver is a fibre-optic device that is responsible for converting the weakened signal back to an electrical signal. It accepts optical signals from the optical fiber and converts it into electrical signal. A typical one consists of optical detector, a low noise amplifier and other circuitry used to produce the output electrical signal. Optical receiver MO10GB1550 is the receiver used in this link. The key features of this receiver are: 1. Receiver sensitivity >-19dBm 2. Maximum Optical Input Power 2dBm 3. Low power consumption 4. Low cut off frequency 50 KHz 5. Supports upto 10Gbps Data rate 6. Maximum output power >+6.5dBm

Manufacturer’s Receiver Specification sheet Property Unit Worst Case Typ Comments Receiver Sensitivity dBm 17.5 >-19 10Gb/s BER at 1X10 -10 λ=1.5um Receiver Transimpedence Gain Ω 2K Max Optical Input Power dBm +1 +2 PIN Responsivity A/W 0.75 >0.85 Receiver 3dB Bandwidth GHz 8 >8.5 Small signal Low Frequency Cutoff KHz 50 Phase Linearity Deviation Degree 20 <10 Amplitude Peaking dB 2.5 <1.5 Input Optical Reflection dB -25 -30 Output Return Loss dB -10 -15 Total Power Consumption mW 550 <400 PIN Diode Bias V +5 Amplifier Bias V 3.5/5.5 Total DC Current mA 100 Out Power dBm +5 >+6.5

Multiplexer AOC 100/200 GHz Configurable MUX Module The multiplexing technique used for this system is DWDM (Dense Wavelength division multiplex). Since the link has 16 channels and DWDM increases the capacity signal of embedded fiber i.e. the incoming optical signals are assigned to specific wavelengths within a designated frequency band then multiplexed on to a single fiber.

This process allows multiple video, audio and data channels to be transmitted over one fiber while maintaining system performance and enhancing transport systems.

TM DC Fixed Dispersion Compensator Dispersion is the dominating factor limiting transmission performance in the optical systems and in trans oceanic links it is the most important factor to come over. Dispersion is the time distortion of an optical signal, i.e. each spectral component of the mode takes a different time to travel through the fibre, typically resulting in pulse broadening. Dispersion can limit the maximum data rate, the maximum distance, or the information carrying capacity of a SM fibre. The compensating devices are designed to have dispersion of the opposite sign to that of the fibre in the link so as to eliminate the delay difference between spectral components.

The optical amplifiers are used to boost transmitter power, eliminate the need for electronic regenerators and improve receiver sensitivity, to increase the capacity of fibre-optic networks, opening up new wavelength windows for WDM such as 1300nm, 1550nm etc. Some of the technical advantages are improved noise figure and reduced non-linear penalty of fibre system, allowing longer amplifier spans, higher bit rates, closer channel spacing and operation near the zerodispersion wavelength. Optical amplifiers can be placed at intervals along a fiber link to provide linear amplification of the transmitted optical signal. It provides much simpler solution, which can be used for any kind of modulation at any transmission rate. Moreover, if it is sufficiently linear it may allow multiplex operation at different wavelength. Since the link is nearly 500km long it definitely needs intermediate amplifiers, which will boost up the travelling signal.

Manufacturer’s Data Sheet Amplifiers The main feature of the amplifier are listed below: 1. High Power up to 23 dBm 2. Operates at various wavelengths 3. Low Noise 4. Automatic Gain Control 5. Wide Signal Bandwidth 6. Transient Control 7. Excellent Gain Flatness 8. Dynamic Gain & Power Control

Demultiplexer

MRV SFP Media Connect At the receiver end, the demultiplexer will then separate the signals according to its wavelength. The demultiplexer used is MRV SFP Media Connect and has the following specifications:

Connector All the design components are standing individually. Hence, to set-up the optical fiber link, connectors are needed to connect them together. FIS SC/APC Connector (part number: F1-3069APC) from Fiber Instrument Sales Incorporated is used in this design. This connector is designed for top optical performance and greatly reduces termination time. The connector features are pre-radiused zirconia ferrue, pre-assembled body and precision moulded plastic body. This SC connector achieves low optical loss with high performance physical contact and maximum repeatability.

Link Design

From the Data sheets of manufacturers Transmitter power Pt =0.2dBm Receiver sensitivity Pr = -19dBm Attenuation = 0.2 db/km Line Losses Attenuation is the reduction in optical power as light travels through the fibre. The main causes of optical attenuation in fibres are: coupling loss, splice loss, optical fibre loss and connector loss, and also scattering, absorption of the light, irregularities in the glass structure. Apart from actual losses suffered, while designing the system it is also important to incorporate a margin of 6 -8 dB to account for losses from splices or other components that may have to be added at a future date and also to allow for any deterioration of components due to aging. For the given link, which has attenuation loss of 0.2dB, the fibre loss is calculated as follows. A [dB] = α * L = 0.2dB/km * 479km = 95.8dB (for the whole link) This shows the requirement of deploying amplifiers in the link to make up for the lost power. This lost power must be recovered so that the output power should be high. Since, ∆λ< 50 nm therefore, multiple combination of amplifiers are not required. Amplifier spacing This is the space between two adjacent amplifiers in the link. Amplifier spacing = LA[km] LA[km] = Pt [dBm] –Pr [dBm] α [dB/km] = [0.2 + 19] / 0.2 = 96km

Number of Amplifiers The number of amplifier required (N) = L/LA Where L = distance for the link (479 km) N = 479 /96 = 4.99 ~5 amplifiers The link requires 5 amplifiers with 96 km spacing.

Based on the above readings the following graph is plotted The above graph shows as the distance is increased the dispersion also increases and thus dispersion compensators come into the picture. To reduce the overall dispersion the dispersion compensators are deployed in equal numbers and intervals with the amplifiers.

Conclusion As a technology Optical communication has proven to become one of the fastest growing segments of the telecommunications industry worldwide. Designing a fiber optic system needs a whole length of specifications and considerations related to power, dispersion, capacity etc. The components were selected on the basis of data rate 5Gbps and wavelength 1550nm of the laser source. Source was selected as laser as it has the potential to carry the signal in long distance fibers.

While designing a system the major parameters to be taken into account are BER (Bit error rate) and SNR (Signal to Noise ratio). Designs should be flexible so as to ensure system upgrade. The transmitter laser used in this experiment is capable of 10Gb/s data rate where as link only requires 5Gb/s for 16 channels. The extra 5Gb/s bandwidth could be used cost effectively as the system is upgraded. For multi-channel transmission, WDM is used to combine and separate all the wavelengths. Since there will be loss and dispersion in the fiber optic link, amplifier such as EDFA is used to amplify the signal and DCM for dispersion compensation. The design for the optical link built, satisfies the requirement for this project.

Web References http://www.fiber-optics.info http://www.rad.com http://www.corning.com http://www.globalspec.com http://www.jdsu.com http://www.nuphoton.com http://www.teraxion.com http://www.mrv.com/technology/ Technical References 1) G.P.Agarwal,”Optical Fiber Communications” 2) G. Keiser, “Optical Fiber Communications”, McGraw-Hill Inc., 2000.

LED Structures in Optical Fiber Communication
LED Structures

Five major type: 1-Planar LED 2-Dome LED Dome LED 3-Surface emitter LED s 4-Edge Emitter LED 5-Super luminescent LED s

Only two have use in Optical Fiber Communication (SLED and ELED)

Surface-Emitting LEDs

The surface-emitting LED is also known as the Burrus LED in honor of C. A. Burrus, its developer. In SLEDs, the size of the primary active region is limited to a small circular area of 20 &mm to 50 &mm in diameter. The active region is the portion of the LED where photons are emitted. The primary active region is below the surface of the semiconductor substrate perpendicular to the axis of the fiber.

A well is etched into the substrate to allow direct coupling of the emitted light to the optical fiber. The etched well allows the optical fiber to come into close contact with the emitting surface.

In addition, the epoxy resin that binds the optical fiber to the SLED reduces the refractive index mismatch, increasing coupling efficiency.

Edge-Emitting LED s

The demand for optical sources for longer distance, higher bandwidth systems operating at longer wavelengths led to the development of edge-emitting LED s.

It shows the different layers of semiconductor material used in the ELED. The primary active region of the ELED is a narrow stripe, which lies below the surface of the semiconductor substrate. The semiconductor substrate is cut or polished so that the stripe runs between the front and back of the device.

The polished or cut surfaces at each end of the stripe are called facets.

In an ELED the rear facet is highly reflective and the front facet is antireflection-coated. The rear facet reflects the light propagating toward the rear end-face back toward the front facet. By coating the front facet with antireflection material, the front facet reduces optical feedback and allows light emission. ELEDs emit light only through the front facet. ELEDs emit light in a narrow emission angle allowing for better source-to-fiber coupling. They couple more power into small NA fibers than SLEDs. ELEDs can couple enough power into single mode fibers for some applications. ELEDs emit power over a narrower spectral range than SLEDs. However, ELEDs typically are more sensitive to temperature fluctuations than SLEDs.