EEET1133 Assignment optical fibre RMIT Lanka Bogoda

8/6/2019 EEET1133 Assignment optical fibre RMIT Lanka Bogoda

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SCHOOL OF ELECTRICAL AND COMPUTER ENGINEERING

Optical Fibre Technology

TECHNOLOGY DISCUSSION ASSIGNMENT

Subject : Optical interconnects

Name : Lanka Bogoda

Student Number : S3224908

Lanka N Bogoda - S3224908 Page 1 of 8


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Optical interconnects

Optical communications system made up of optical components and they play very importantrole in delivering the optimum service to meet the today's high speed and high performance

requirements. By 2020 fast CPU need to deliver 256 core or more which enable 20Tera-flops (The

FLOPS is a measure of a computer's performance )and require 40 Tera-Flops bandwidth to support flat

programming model. To support such huge amount of bandwidth, more than 32,000 electricalconnections are required for core to core communication. This make the routing become complicated

on-chip and off-chip. Though some interconnect innovation such as having more metal routing layer

can improve the metal congestion issue, however this increase the routing length and eventually hit theelectrical channel capacity ratio.

When more bits are sent through the electrical lines in a given period, timing factor is limiting

the overall performance. Electrical line loss in high frequency due to skin effect is critical at any high

speed input and output designs. Signal dispersion that cause Inter symbol Interference (ISI) is anotherhigh speed design issue that needs to be handled carefully. When more wires are packed, cross-talk

becomes dominant and further degrade quality of the signal. Signal termination mismatch is anothercommon issue in any electrical line signal integrity engineering. Termination mismatch can cause the

signal reflection from receiver end to driver end and keep reflecting back and forth until the energy isdissipated in the communication channel. All the channel limitation discussed above require additional

circuitry such as equalization, termination compensation, pre-emphasis etc which increase diameter

size and dissipate more power. To resolve the electrical transmission line limitation, a new interconnectmaterial need to be considered such as optical interconnect.[1]

Optical interconnect is considered a solution for channel bandwidth limitation. This is as aresults of optical interconnect has different characteristic compare to electrical interconnect. Optical

interconnect has negligible signal latency for distance communication, very high communication

bandwidth with light as channel carrier and absence of electromagnetic effects such as impedancematching, crosstalk, and inductance effects. With these characteristic optical interconnect can be use toeliminate the disadvantages of electrical interconnect. The optical transceivers are expected to be much

smaller and simpler compare to electrical link transceivers as most of the complicated circuits can be

removed. With Wavelength Division Multiplexing (WDM), multiple wavelengths can be transmitted byusing one single optical link hence improve bandwidth and latency efficiency. Therefore optical

interconnect can be used as one to one connection on global clock distribution network.

The optical devices of many fiber networks requires a number of functional devices, some of

which can be fabricated using small optical components. These optical components are made up of

parts which have linear dimensions in the order of a few millimeters to nanometers. The completed

functional device may occupy a space a few centimeters on one side. Components to be discussed inthis report have the common feature that the fiber transmission link is opened and small devices are

inserted into the gap between the fiber ends to produce a functional component. Network components

constructed entirely of fibers or constructed in integrated-optic forms.[2]



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Optical interconnect components

Attenuators

Attenuators reduce the amount of power to the required level flowing through the fiber system.Both fixed and variable attenuators are available depending on the application. The applications

include testing of receiver sensitivities (varying the attenuation changes the amount of power

incident on the receiver) and protecting a receiver from saturating due to excess incident power.Attenuation from a few tenths of a decibe to more than 50 dB.

Figure 1 Gap attenuator showing relative displacement of the fibers to vary the insertion loss

Power Splitters and Directional Couplers

These devices distribute input power from a single fiber to two or more fibers and these

components are designed to control the fraction of power delivered to the output ports.Applications include power distribution in local area networks and in subscriber networks.

Figure 2.Power splitter and (with Port 4 added) four port directional coupler



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Isolators

An isolator is a one way transmission line and permits the flow of optical power in just single

forward direction. Applications include protection of a transmitting laser diode from back

reflections. these reflections increase the noise in the system by disrupting the diodes operation.Isolators also enhance the stability of fiber amplifiers by reducing the possibility of

feedback,which causes unwanted oscillations in such devices.

Circulators

In a circulator, power feed into the first port comes out from the second, while power into the

second port emerges from the third (Figure 3). This behavior repeats at each successive inputport until power into the last port emerges from the first. Practical circulators are typically three-

or four-port devices. Using a circulator, efficient two-way transmission (full-duplex) along a

single fiber at a single wavelength can be make possible. [3]

Figure 3. An optical circulator separates transmitted and received messages at a terminal

Multiplexers/Demultiplexers/Duplexers

The multiplexer and demultiplexer are greatly used in fiber-optic wavelength-divisionmultiplexed (WDM) systems. The multiplexer combines beams of light from the differenttransmitters (each at a slightly different wavelengths) onto the single transmission fiber. The

demultiplexer separates the individual wavelengths transmitted and guides them to

separate channels to the appropriate optical receivers. Requirements for

multiplexers/demultiplexers include combining and separating independent channels lessthan a nanometer apart, accommodating 100s of channels. losses can be as low as a few tenths

of a decibe and isolation are of 40 dB or more. The duplexer allows for simultaneous two-

way transmission along a single fiber. The wavelengths are different for the transmittingand receiving light beam. [4]

Mechanical Optical SwitchesOperationally, an optical switch is similar to an electrical switch. Mechanical movements of

some parts (as shown in Figure 4) cause power entering one port to be dispatched to one

of two or more output ports. Such devices are useful in testing of fiber components and

systems and in other applications, such as bypassing inoperative nodes in a local areanetwork. Insertion losses less than 0.10 dB and isolation is greater than 50 dB are

acceptable requirements.



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Figure 4. Mechanical optical switch

Figure 5Moveable reflecting prism switch

Prisms

Because of the dispersion in glass prisms, they can operate as multiplexers, demultiplexers,and duplexers. Right-angle glass prisms also act as excellent reflectors owing to perfectreflection (total internal reflection) at the glass-to-air interface. The beam-splitting cube,

shown in Figure 6, consists of two right-angle prisms attached together with a thin

reflective coating between them. This beam splitter has the advantage over a flat reflectivelayer in that no angular displacement occurs between the input and output beam directions.

This simplifies the alignment of the splitter with the input and output fibers.[5]

Figure 6. Beam-splitting cube



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Gratings

Ruled reflection gratings are used in multiplexers and demultiplexers as well. As illustrated

in Figure 7 the dispersion characteristics of the grating carry out the wavelength

separation function needed for a demultiplexer. The grating has much greater dispersive powerthan a prism, allowing increased wavelength spatial separation. The relationship between the

incident and reflected beams, for an incident parallel light beam. [6]

Figure 7.Blazed reflection grating operated as a demultiplexer.

Figure 8Two-channel demultiplexer

Filters

Dielectric layered filter, consisting of very thin layers of various dielectrics fabricated onto aglass substrate, are used to construct multiplexers, demultiplexers, and duplexers. Filters

have unique reflectance and transmittance characteristics. They can be designed to reflect

at certain wavelengths and transmit at others, thus spatially separating (or combining)

different wavelengths as required for WDM applications. [7]

Beam Splitters

A beam-splitting plate as shown in Figure 5 is a partially silver plated glass layer. The

thickness of the silvered plate determines the fraction of light transmitted and reflected. In

this way, the input beam can be divided in two parts of any desired ratio.



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Figure 9.Beam-splitting plate

Ongoing research

The world's first mid-infrared light amplifier was fabricated on IBM's Yorktown Heights, New

York-based pilot line in a standard CMOS process, but using the optical photonic structures that

were originally designed for telecommunication optical interconnects. The telecommunicationsindustry uses near-infrared wavelengths of 1,550 nanometers, but IBM bumped this up to the

mid-infrared band at 2,200 nanometers to fabricate its optical amplifier. [8]

Conclusion

A detail physical characteristic of optical channel is explained which shows overwhelm

advantages over electrical interconnect in terms of good signal integrity. Then comparison of opticalinterconnect with copper interconnect is shown across different process node. Optical interconnect

show great improvement in bandwidth/latency efficiency for global signal route than local route.However in term of clock distribution, not much advantage can be offered from optical interconnect

unless photo detector responsiveness can be further improved.[9] Optical interconnect has tremendouspotential in simplify I/O transceiver design. This is important in order to implement thousand of optical

I/O array on die if transceiver is small enough. Intel has published a complete optical transceiver

solution which can be taken further close to the technology requirement on I/O bandwidth in order tosupport multi-core demand.



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Comparison of the technology of optical fiber and my belief

The literature survey that lead to this technology discussion assignment of RMIT made me

understand that rapid growth of the technology has not been discussed enough at general public leveland limited to only academic layers of universities and research institutions. This can be attributed to

the fact that a work related training I got on optical fiber systems where the instructor was not aware of

full duplex optical fiber communication, optical amplifiers , optical multiplexers and full duplex opticalcommunication in the same wavelength using optical circulators.

The existence and advance of optical fiber communications is based on the invention of thesemiconductor junction laser, the invention of low-loss optical fibers, and related technologies, such as

integrated optics. We should never forget that it took more than 25 years from the early, pioneering

ideas to the first large-scale commercial deployment of optical communications systems.

Deployment of optical telecommunication systems is when the revolution got started in the

marketplace, and when optical fiber communications began seriously to impact the way information is

transmitted. The market demand for higher-capacity transmission was helped by the fact that computers

continued to become more powerful and needed to be interconnected. This is one of the key reasonswhy the explosive growth of optical fiber transmission technology parallels that of computer

processing and other key information technologies. These technologies have combined to meet theexplosive global demand for new information services including data, Internet, and broadband services.

Another milestone in the optical communications revolution is the deployment of the firsttransatlantic fiber system in 1988. It was the time that researchers began to explore the next step

forward optical fiber amplifiers and WDM transmission. The use of Wavelength Division

Multiplexing (WDM) has other advantages, such as the tolerance of WDM systems of the high

dispersion present in the low loss window of embedded fibers, WDMs ability to grow the capacityincrementally, and WDMs ability to provide great simplicity and flexibility in the network.

This survey further allowed me to expand my knowledge on technology on all aspects of opticalinterconnects devices and their production of theses devices in the industry. Most impotently when you

compare the optical fiber technology with its old rival copper we can find alternative replacement for

each old device such amplifiers, multiplexers and directional couplers are available in optical form aswell.



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Reference

[1] Bob Chomycz, Planning Fiber Optic Networks,2009, McGraw Hill CHAPTER 2, optical power

[2] GhafouriShiraz, H. Distributed feedback laser diodes and optical tunable filters, 2003, John Wiley

& Sons Ltd, Principles of Distributed Feedback Semiconductor Laser Diodes: Coupled Wave Theory,

[3] Prof. Dr. Fedor Mitschke,Fiber Optics, Physics and Technology, Springer publishing, 2009,

Chapter 8. Components for Fiber Technology

[4] Achyut K. Dutta, WDM TECHNOLOGIES, OPTICAL NETWORKS Volume IIIChapter 2, OTDM

and WDM for Large-Scale Photonic Networks

[5] Prof. Dr. Fedor Mitschke,Fiber Optics, Physics and Technology, Springer publishing, 2009,

Chapter 8. Components for Fiber Technology

[6] Herwig Stange,HANDBOOK OF FIBER OPTIC DATA COMMUNICATION,2002, ACADEMICPRESS Chapter 5, Function of the Optical Subassembly

[7] John Crisp Barry Elliott,3rd Edition, 2009, Newnes,Introduction to Fiber Optics Chapter 3, Mixing

rod couplers

[8][R. Colin Johnson, 08-05-10,"Technology for change" www.smartertechnology.com]

[9] http://kevintham.blogspot.com/2009/04/optical-interconnect.html


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EEET1133 Assignment optical fibre RMIT Lanka Bogoda