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Week-9. Distribution networks and Fiber Components. Introduction. We have discussed so far unidirectional and point to point links of the optical fiber communications.

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Week 9


Distribution networks and Fiber Components

Bahria University


  • We have discussed so far unidirectional and point to point links of the optical fiber communications.

  • However versatility of the fiber optics makes possible the design of the bidirectional systems (but with Multimode optics).

  • Distribution of the information over multiple terminals is also important phenomena and the mostly LAN ( local area network) is deployed by using fiber optics for this distribution.

  • Fiber optics LANs have advantages over traditional wires include improved security, Smaller size, Lower weight and broader bandwidth.

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  • In this session we will study the basic system configurations and components for distributing and controlling information over the fiber cables in various ways that are not restricted to the single optical channel, unidirectional link connecting a single transmitter to a single receiver.

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Distribution networks
Distribution networks

  • A directional coupler forms the basics of many distribution networks.

  • The direction of the allowed power flow are indicated by the figure shown below:

    On white board

  • Let, we consider the working of the directional coupler, the power P1 is incident on the port 1 of the coupler.

  • This power will divide between the ports 2 and 3 according to the desired splitting ratio.

  • Ideally no power will reach at the port 4, called isolated port.

  • Without loss of generality we can assume that the power emerging from the port 2 is equal to or greater than the power emerging from the port 3 ( depending upon the splitting ratio).

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Characteristic coupler losses
Characteristic Coupler losses

  • Throughput Loss:

    Specifies the amount of transmission loss between the input

    port and the favored port (Port 2).

  • Tap Loss:

    Specifies the amount of transmission loss between the input and the tap (Port 3).

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  • Directionality:

    Specifies the loss between the input port and the port we wish to isolate.

  • Excess Loss:

    Specifies the power lost within the coupler. It radiates radiation, scattering, absorption and coupling to the isolated port.

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  • In ideal Coupler no power reaches port 4 ( Directionality loss= infinite)

  • Additionally, no power is lost so that the total power emerging from port 2 and 3 equals the input power (P2+P3=P1), making the excess loss zero.

  • Good directional couplers have excess losses less than 1 dB and directionality greater than 40 dB.

  • Splitting Ratio: is the P2/P3 , “the ratio of the powers at the two output ports”.

  • Couplers are described by their Tap loss, e.g 10 dB coupler means 10 dB tap loss it means that it has the 10 dB transmission onto the desired port.

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On White Board

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  • If the Loss (Throughput) and Loss (Tap) are the losses of an ideal directional coupler having a given splitting ratio, then the losses of an actual coupler having the same splitting ratio but with excess loss are:

  • As we have shown the bidirectional coupler because any one of the port can serve as the input. ( Possible couplings have been shown through the arrow signs)

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Duplexing networks
Duplexing Networks

  • In most straight forward way for transmitting and receiving at the both ends of a point to point link two fibers are used.

  • One carries information in one direction and other carries information other direction ( opposite).

  • A full duplex system (one permitting simultaneous transmission in both directions along the same fiber) conserves the fiber, a particular advantage for the longer distance.

    On White board

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Tee network
Tee Network

On White board

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  • The tee network shown in the previous figure connects many terminals, where each terminal contains a transmitter and the receiver.

  • The trunk fiber also known as the bus or data bus carries the information between the taps (Connections).

  • Taps are provided by the Tee couplers, where Tee couplers allows the bidirectional flow in the bus fiber (Tee coupler constitute of the two directional coupler).

  • A network with many terminals requires a large splitting ratio means the throughput power >>> tapped power for Tee couplers.

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Distribution loss
Distribution Loss

  • Consider the total loss between the terminals 1 and N, assuming that the directional couplers that attach to the bus fiber have the throughput and tap losses.

  • The signal must pass through the N-1 Terminalsbefore reaching the coupler at the receiver so loss will be:

  • So, “total loss increases with the increase in the terminals”.

  • Above mentioned loss is the ideal case but in actual practice each coupler input and output port requires a connector, so that there are 2N connectors in path between 1 to N. If Lc is the loss per connector then the total loss will become:

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Star network
Star Network

On white board

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  • An alternative to the tee network for multi terminal network is the “star configuration” that has been shown in the previous figure.

  • The star coupler interconnects N terminals, and the coupler has 2N ports.

  • The star coupler distributes the power equally to each of the receiver ports from any one of the transmitter ports.

  • So, that the transmission efficiency of each port equals to the 1/N , so the loss will be:

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  • Generally star coupler provides significantly higher efficiency than the Tee coupler.

  • This is because the logarithmic loss variations of the star configuration increases much slower with N than with the linear change of the tee.

  • Example:

    On white board

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Ring network
Ring Network

  • Fibers can also connect numerous terminals in the ring networks.

  • The ring is actually a serial connection of independent point to point links.

  • Each ring node contains an optical transmitter and receiver.

  • The node function is that of an active regenerator, after receiver detects the delivered message and station reads it, the data are regenerated and then retransmitted to the next station.

  • As here Ring network gives the concepts of active generator so they can interact with more terminals than with the Tee and star.

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Dual ring architecture
Dual Ring Architecture

  • If any one of the node in the ring fails, then the entire system shutdown.

  • Similarly, the entire system fails if there is a break in any of the transmission fiber in the loop.

  • Several modifications to the basic ring solve this issue.

  • An optical bypass switch can be inserted that will be discussed in next section ( Optical switches).

  • Installing a second loop is another practical modification of the basic ring, that adds some redundancy to the system.

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Multifiber system
Multifiber system

  • N terminal system can be implemented by the directly connected each terminal to all others in the same manner.

  • At each transmitter, a single source illuminates a fiber bundle containing N-1 fibers.

  • To obtain greater efficiency, the source area equals that of the fiber bundle.

  • Each fiber leads to the one of the remote receivers, at receiver one fiber arrives from one of the distant transmitter.

  • Although this architecture uses a lot of fibers but it does have some advantages as the power launched doesn’t suffer losses from the couplers and power distribution losses.

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Multifiber architecture
Multifiber architecture

On White board

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Directional couplers
Directional Couplers

  • In this section we describe the design of the several four-port directional couplers,each uses a different concept to achieve the desired coupling.

  • These are as follows:

  • Fused Biconical tapered directional coupler

  • Offset Butt-joint directional coupler

  • Beam splitting Directional Coupler

  • Wave front- dividing directional coupler

  • Cascaded directional coupler

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Fused biconical tapered d irectional c oupler
Fused Biconical Tapered Directional Coupler

  • The fused Biconical tapered directional coupler has been sketched as shown below:

    On white board

  • The construction is fairly simple, here two single mode or multimode fibers are twisted around one another and put under tension.

  • The junction is heated, softening the fibers and causing their cladding to fuse.

  • Pulling on the softened fibers forms a Biconical taper at each of the four ports.

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Internal operation
Internal operation

  • In multimode fibers, coupling occurs because higher ordered mode, no longer strike the core-cladding interface beyond the critical angle in the tapered region.

  • Now, this has been shown in the previous figure that these modes are trapped by the total reflection at the outer surface of the cladding.

  • They have been converted into cladding modes.

  • Then due to the gradual bending and variation throughout the bend convert the cladding modes back into core-guided waves.

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Operation for single mode fiber
Operation for Single mode fiber

  • Single-mode operation of the tapered and fused coupler is explained by the “exchange of the energy in the overlapping evanescent fields associated with the two fibers.”

  • The taper brings the two core closer together to each other.

  • The taper also decreases the fiber core diameter thus lowering the normalized frequency (V parameter).

  • As, we know that decreasing the V parameter increases the spot size and enhance the overlapping over evanescent fields, improving the coupling.

  • As, the reference the simple directional coupler where input is at Port-1 and output is at port 2 and 3 so the equation becomes:

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Offset butt jointed directional coupler
Offset Butt jointed Directional Coupler

  • An offset butt joint can be used to form a four-port directional Coupler in the manner illustrated in the figure:

    On white board

  • With an input at the port-1, the favored port collects the amount of the power determined by the lateral offset.

  • And the portion of the incident light travels from the joint to the tap (Port 3) along a planner curved plastic waveguide.

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Beam splitting directional coupler
Beam splitting Directional Coupler

  • For conventional optic systems, a beam splitter ( Partial Reflector) serves as a simple directional coupler as illustrated in the figure:

    On white board

  • A beam splitting plate consists of a thin partially reflective coating on a transparent substrate.

  • The thickness and composition of the coating determine the splitting ratio.

  • The beam splitting plate displaces the transmitted beam laterally with respect to the incident beam. This displacement may be removed by using Cube that comprises of the two prism.

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Beam splitter using grin directional couplers
Beam splitter using GRIN directional couplers

  • Basically the space occupied by the splitter is equivalent to a gap, and we have already discussed that the gaps produced the large losses.

  • The refocusing of the rays using GRIN may solve this problem, like as shown in the following figure:

    On white board

  • A little variation in this in terms as , two quarter-pitch GRIN rod lenses, separated by the partially reflective coating, make up the coupler.

  • Consider an input at the port 1 the combined lenses image light onto the fiber at the port 2 and reflective surface reflect this onto the Port 3.

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Wave front dividing directional coupler
Wave-front Dividing directional coupler

  • All beam splitting couplers are amplitude division devices.

  • They distribute light by dividing the amplitude of the incident wave into the desired properties.

  • Couplers can be developed that can be produced by the wave front division, dividing the wave front into the several parts and directing the separated waves to the desired ports.

  • This coupler has been shown in the following figure, the input light from the port 1 diverges, the upper half of the wave is imaged onto the fiber at the port 2 by the concave reflector and the lower half of the wave is imaged onto the fiber at the port 3.

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