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Lecture 16 – Joining Fibres and Analysing Links Optical Fibres and Telecommunications – Real World Networks Introduction • Where are we? • Joining optical fibres – Mechanical splices – Fusion splicing – Connectors • OTDR • Power budgets Optical Fibres and Telecommunications – Real World Networks 1 Last time • Semiconductor optic amplifiers • FP and TW designs – Bandwidth considerations – Gain ripple • • • • • Cross talk and cross saturation Polarisation dependent gain Gain clamping Real amplifiers Introduction to real world networks Optical Fibres and Telecommunications – Real World Networks Joining Optical Fibres • Optical fibres are usually made in spans of a few 10’s km. • This is obviously not a great enough distance in many cases. • Fibres must be joined together. • Matching similar fibres is done by ‘splicing.’ • Matching dissimilar fibres, or fibres with other components (eg. amplifiers) can be done using connectors. • Aim is to keep losses as low as possible through this process. • First step is normally fibre stripping and cleaving. Optical Fibres and Telecommunications – Real World Networks 2 Cable structure • Stripping involves removing the coating from the optical fibre. • Can be done chemically, mechanically or thermally. • Next step is cleaving the fibre. Optical Fibres and Telecommunications – Real World Networks Fibre cleaving • Edge of fibre is scored or nicked with a metal / diamond blade. • Fibre is then broken under tension. • Very good quality, flat faces can be achieved. • Best possible results are achieved with polishing. Picture credit: http://www.tpub.com/neets/tm/108-3.htm Optical Fibres and Telecommunications – Real World Networks 3 Fibre Splicing • In a fibre splice two bare fibre ends are joined together. • Losses can occur in a variety of ways (intrinsic and extrinsic losses). • Can either use mechanical (direct butting) or fusion splicing. • Loss can be ~ 0.1dB/Splice. • Careful preparation of the fibre is required. Optical Fibres and Telecommunications – Real World Networks Intrinsic Splice Losses Core Diameter Mismatch 2r2 2r1 Numerical Aperture Mismatch Loss = 10 log10((r2/r1))2 2θ2 Mode Field Diameter Mismatch ω1 2θ1 Loss = 10 log10((NA2/NA1))2 ω2 Loss = 10 log10(4/(ω2/ω1+ω1/ω2)2) Optical Fibres and Telecommunications – Real World Networks 4 Intrinsic Splice Losses II • Intrinsic losses occur due to differences in the two fibres being spliced. • Can have a combination of these effects. • Worst case scenario – splicing single mode to multimode fibre. Loss > 20dB ! • Have to ensure the fibres are well matched. Optical Fibres and Telecommunications – Real World Networks Extrinsic Splice Losses Lateral Misalignment All formulas are for single mode fibre ! x End Separation Loss = 10log10(exp(-x/w0)2) z Angular Misalignment Loss = 10log10[1/(S2+1)] where: S=λz/(2πnw0) θ Loss = 10log10(exp(-nπw0sin(θ)/λ)2) Optical Fibres and Telecommunications – Real World Networks 5 Extrinsic Splice Losses & Reflection Losses • Extrinsic losses are caused by imperfections that can be eliminated. • Misalignment losses are illustrated in the previous slide. • Also problems with bad cleaves, dirt, errors in fibre fabrication. • Reflection losses are caused by the air space that may remain between the fibres. Total loss is called the Fresnel loss. • Loss=-10log10(1-R) where is given by the Fabry-Perot reflection of the airspace, length z: – R=R1+R2-2(R1R2)0.5cos(4πz/λ) – R1, R2 are the reflections of the fibre/air interfaces. Optical Fibres and Telecommunications – Real World Networks Making Splices - Mechanical v-groove • Fibres are aligned in specially machined v-groove. • Index matching fluid applied to fibre tips. • Splice is covered. • Fibres are placed in a capillary tube. • Index matching fluid can be applied. • Fibre rotated until maximum signal power observed • Use mechanical splices where a relatively low number of splices required. • Relatively low skill level required. Optical Fibres and Telecommunications – Real World Networks 6 Making Splices – Fusion Splicing Graphic Credit: www.corningcablesystems.com Optical Fibres and Telecommunications – Real World Networks Aligning fusion splices Can use video camera to align splice. Or use output power – bend loss !! Graphic GraphicCredit: Credit: www.corningcablesystems.com www.corningcablesystems.com Optical Fibres and Telecommunications – Real World Networks 7 Fusion Movies Fusion Splice Alignment Process Movies from: http://floti.bell.ac.uk/MATHSPHYSICS/splicing.htm Optical Fibres and Telecommunications – Real World Networks Factors affecting a good splice. Graphic Credit: www.corningcablesystems.com Splice loss can be <0.1dB !!! Optical Fibres and Telecommunications – Real World Networks 8 Connectors • In some situations cleaving is not suitable. • eg. Temporary joins, joining to standardised equipment. • In this case use connectors – ‘Pulg and Play.’ • Tend to suffer from higher losses than good quality cleaves. Picture credit: http://www.tpub.com/neets/tm/108-9.htm Optical Fibres and Telecommunications – Real World Networks Making a connector • Stripped optical fibre is place in ceramic capillary. • Fibre is then glued into place. • Fibre tip is polished back to the top of the ferrule. Picture credit: http://www.tpub.com/neets/tm/108-9.htm Optical Fibres and Telecommunications – Real World Networks 9 Connector Losses • • • • • Loss occurs in much the same way as for splices. Need to ensure connectors are kept clean. Connector loss normally ~0.5-1dB. Staff need little training in operation. Can add to network costs. Optical Fibres and Telecommunications – Real World Networks Analysing the link - OTDR • In order to characterise our link and find connector / splice losses we can use Optical Time Division Reflectometry (OTDR). • This technique sends pulses of light down the fibre and looks at the reflected and backscattered light from the fibre. • This information allows a plot of intensity versus distance to be performed for the link. • Can use this information to perform fibre link analysis over distances of 100’s of km. • Broken fibres are easily spotted. Optical Fibres and Telecommunications – Real World Networks 10 OTDR Plot Optical Fibres and Telecommunications – Real World Networks Understanding OTDR I Single fibre with no splices / connectors Dead Zone Beginning of a fibre Dead zone gives short distance limit. Picture Credit www.agilent.com Optical Fibres and Telecommunications – Real World Networks 11 Understanding OTDR II End of the fibre Fibre Break Picture Credit www.agilent.com Optical Fibres and Telecommunications – Real World Networks Understanding OTDR III Fusion Splice Connector Picture Credit www.agilent.com Optical Fibres and Telecommunications – Real World Networks 12 Building a Network – The Power Budget • In order for any fibre-based system to work, it’s essential that enough power is received at the receiver to allow signal detection. • The amount of power received ABOVE the receiver sensitivity is called the System Margin. • If the power received is less than the sensitivity then the amount of power BELOW the sensitivity is called the System Deficit. • Note that there is sometimes sign confusion over the system deficit. • In order to calculate an optical power budget, we must take into account all of the sources of power and loss in the system. Optical Fibres and Telecommunications – Real World Networks Point to Point Link Tx Rx LED Transmitter P1550nm=-20dBm 30km Fibre in 10km spans: Loss = 0.2dB/km. Splice Loss = 0.2dB Connector loss = 0.8dB Receiver sensitivity = -30dBm Calculate the system margin. Optical Fibres and Telecommunications – Real World Networks 13 Point to Point Link II Rx Tx Power in: -20dBm Connector loss: -0.8dB Fibre loss: 30x0.2dB=-6dB Splice loss: 2x0.2dB =-0.4dB Connector Loss: -0.8dB System Margin: -2828-(-30)dBm -20dBm -20.8dBm -26.8dBm -27.2dBm -28.0dBm 2dB System will function, but system margin is low – need to aim for 5-10dB Optical Fibres and Telecommunications – Real World Networks Point to Point Link III Rx Tx It is now decided to increase the link to 60km – what is the system margin ? Power in: -20dBm -20dBm Connector loss: -0.8dB -20.8dBm Fibre loss: 60x0.2dB=-12dB -32.8dBm Splice loss: 5x0.2dB =-1dB -33.8dBm Connector Loss: -0.8dB -34.6dBm System Deficit: -34.634.6-(-30)dBm -4.6dB System Cannot Operate !!! Optical Fibres and Telecommunications – Real World Networks 14 Point to Point Link IV Rx Tx One answer could be to replace the LED with a laser. Output Power = -5dBm. Power in: -5dBm -5dBm Connector loss: -0.8dB -5.8dBm Fibre loss: 60x0.2dB=-12dB -17.8dBm Splice loss: 5x0.2dB =-1dB -18.8dBm Connector Loss: -0.8dB -19.6dBm System Margin: -19.619.6-(-30)dBm 10.4dB This is much healthier! Optical Fibres and Telecommunications – Real World Networks Homework1: a. Calculate the system margin (deficit) for a 300km fibre link, made up of 10km fibres (Loss=0.25dB/km). Signals go through two connector pairs in a patch panel at each end. Assume splice loss=0.1dB/splice. Laser power = 0.0dBm. Receiver sensitivity =-32.0dBm. b. In order to improve performance, it is decided to place amplifiers with a gain of 30dB at the 100km and 200km points. The amplifiers are placed in the link using one connector at each end (loss=0.8dB/connector.) Calculate the system margin (deficit) in this case. 1. Example from Understanding Fiber Optics, Hecht Optical Fibres and Telecommunications – Real World Networks 15 Conclusions • • • • • • Joining and cleaving optical fibres Factors affecting splices Fusion and mechanical splices Connectors OTDR plots Power budgets – System margin – System deficits • Power budget calculations Optical Fibres and Telecommunications – Real World Networks 16