Fiber Connectors and Splices

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




Splice loss: 5x0.2dB =-1dB -33.8dBm


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




Splice loss: 5x0.2dB =-1dB -18.8dBm


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.

Fiber Connectors and Splices

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