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Fiber Connectors and Splices
Need for fibre joints
• Optical fibres are usually made in spans of a few 10’s km, which is obviously not a great enough distance in many cases.
• Connect fibers as a result of cuts
• Matching similar fibres is done through splicing
• Matching dissimilar fibres, or fibres with other components (eg. amplifiers) can be done using connectors.
• The aim is to minimize losses while joining two fibre cables
Types of fibre joints
• Use of optical fibre connector;- demountable connection - Involves the use of mechanical or optical device that provides a demountable connection between two fibers or a fiber and a source or detector
• Fusion splice;- Permanent fibre joint/connection – Involves the actual melting (fusing) together the ends of two pieces of
fiber. The result is a continuous fiber without a break.
• Mechanical splice;- Semi-permanent – Ends of two pieces of fiber are cleaned and stripped, then carefully
butted together and aligned using a mechanical assembly. A gel is used at the point of contact to reduce light reflection and keep the splice loss at a minimum. The ends of the fiber are held together by friction or compression, and the splice assembly features a locking mechanism so that the fibers remained aligned.
NB: Both Fusion and mechanical splicing are capable of introducing splice losses in the range of 0.15 dB (3%) to 0.1 dB (2%).
Mechanical splicing
Procedure • 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
NB: Mechanical splices are used in cases where a relatively low number of splices required Relatively low skill level required for this type of slicing
Fusion splicing
• Before splicing, plastic buffer coatings on both fibres are removed on both fiber ends.
• The ends are then cleaved (a deliberate, controlled break, intended to create a perfectly flat end-face, perpendicular to the longitudinal axis of the fiber) and cleaned using isopropyl alcohol
• Cleaved fibre ends are fused permanently together using an electric arc • During splicing, fibres area held in V-grooves for alignment • A variety of splicers have developed to cater for multimode and single-mode fibre
Steps in fusion splicing Aligning the fibre ends on the V-groove
Optical connectors
• Stripped optical fibre is place in ceramic capillary
• The fibre is then glued into place and the fibre tip polished back to the top of the ferrule
• To minimize fibre losses, there is need to ensure connectors are kept clean
• Connector loss are normally ~0.5-1dB
Insertion loss (Attenuation)
• Represents the typical attenuation for a mated pair of optical connectors (approx 0.35 dB)
• It is the most common type of fibre measurement
Return Loss
• Fiber losses (magnitude of reflections) due to optical power being directed back toward the source
• Represents the most common source of reflection is a fibre joint
• Return loss is specified in negative dB, Example: A return Loss of -60 dB is better than a return Loss of -50 dB.
Optical measurements: Optical Time Division Reflectometry (OTDR)
• OTDR is a technique used to analyse an optical link to find connector / splice losses
• 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
• This information can be used to perform fibre link analysis over distances of 100’s of km
• Broken fibres are easily spotted.
OTDR plot
Multiplexing in fibre networks
(Dense)Wave Division Multiplexing ((D)WDM)
• Achieved through refraction and diffraction technique for combining and separating optical signals of different wave lengths.
• Closely spaced wavelengths are used
• Results in higher system capacity
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
• In order to calculate an optical power budget, we must take into account of all the sources of power and loss in the system.
Power Budget
• Consider a Point to Point (P2P) link shown below;
Assuming the link has the following parameters,
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
Power Budget
Solution:
Effective power
Power in: -20dBm: -20dBm
Connector loss: -0.8dB -20.8dBm
Fibre loss: 30x0.2dB=-6dB -26.8dBm
Splice loss: 2x0.2dB =-0.4dB -27.2dBm
Connector Loss: -0.8dB -28.0dBm
System Margin: -28-(-30)dBm 2dB
Note: System will function, but system margin is low – need to aim for 5-10dB
Power Budget
Assuming the link distance is increased to 60km, calculate the system margin
Effective power
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 Margin: -34.6-(-30)dBm -4.6dB
Note: System will not operate with such a margin
Power Budget
One possible solution would be to replace the LED optical source with a laser
Effective power
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: -19.6-(-30)dBm -10.4dB
Assignment 2
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. 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.