power budget - problem set 2

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1

2

3

4 (a) Describe the operation of an Erbium Doped Fibre Amplifier (EDFA). Indicate the advantages of fibre amplifier technology in comparison with Semiconductor Laser Amplifiers (SLAs). List the main techniques for flattening of the amplifier gain curve of an optical amplifier.

[7 marks]

(b) Briefly discuss non-linear properties of optical fibre. Explain concisely what is meant by the terms “four-wave mixing”, “stimulated Brillouin scattering” and “stimulated Raman scattering”. Describe the impact of the phenomena on DWDM system operation and the techniques used to reduce the effects.

[6 marks]

5 An optical fibre system is used to distribute multiplexed digital video signals from a single transmitter to N geographically dispersed receivers. An N-port optical splitter is located just after the transmitter and the maximum distance from the splitter to any of the N receivers is 39 km. The system specifications and design information are as follows:

The bit rate is 622 Mbits/sec. The worst case transmitter output power is +1.5 dBm and the transmitter spectral width is less than 1.5 nm. The splitter has N output ports, with an excess loss less than 0.3 dB per port.

Singlemode dispersion shifted fibre is used which meets ITU-T recommendation G.653 with a dispersion coefficient of 3.5 ps/nm.km and a worst case fibre attenuation at the operating wavelength of 0.25 dB/km.

Throughout the system cable spans with an average length of 740 m are fusion spliced together with a maximum splice attenuation of 0.07 dB. There are four connectors used between the transmitter and each receiver. The maximum connector attenuation is 0.25 dB.

All of the receivers have an identical specification and have a worst case

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sensitivity of –27 dBm at 622 Mbits/sec. The power margin for repair, upgrade etc. must not be less than 2.4 dB, between the transmitter and each receiver.

Based on the information above:

(i) Sketch a block diagram for the system [4 marks]

(ii) What is the value of the worst-case dispersion penalty in the system?[4 marks]

(iii) What is the maximum value of the splitter ratio N with the system configured as above?

[12 marks]

(iv) Furthermore to increase the value of N the use of an EDFA, to be placed between the transmitter and the N-port splitter, is being considered by the designers. Explain why an amplifier is used. If the value of N is to be raised to 100 what minimum amplifier gain in dB is needed? You may neglect noise and connections losses for the EDFA.

[5 marks]

6 (i) Discuss briefly the main issues to be considered while designing optical communication systems in the WDM environment.

[6 marks]

(ii) List and briefly discuss the most significant non-linear effects in fibre transmission.

[7 marks]

7 Briefly discuss non-linear properties of optical fibre. Explain what is meant by the term four-wave mixing, describe the impact of the phenomenon on DWDM system operation and techniques for reducing the effect.

8 (a) Outline five common reasons why power margins are employed in the design of optical transmission systems.

[6 marks]

(b) A 2.4 Gbit/s optical transmission system is to operate at a wavelength of 1550 nm over a distance of 72 km. Two different transmitter types are available, type 1 and 2,

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with specifications shown in Table 1:

Transmitter type

Minimum output power

Worst case spectral width

A +2 dBm 0.6 nmB +1.2 dBm 0.5 nm

Table 1

A common receiver is used, which has a worst-case sensitivity of -24 dBm. Singlemode fibre is to be used with a dispersion coefficient of 3.5 ps/km.nm and an attenuation 0.23 dB/km. The average distance between fusion splices is 800 m. The worst case connector loss is 0.36 dB, while the worst case fusion splice loss is 0.056 dB. Two connectors are used per system.

1. Determine the dispersion penalty for each transmitter type2. Hence determine which transmitter type will offer the highest power margin for the installed system.

[12 marks]

(c) For the purpose of an upgrade to operation at 5 Gbits/s a transmitter with a worst case output power of +8 dBm is available, while the receiver sensitivity is reduced to -23.2 dBm. Assuming a repair margin of 1.5 dB minimum is needed, what worst case source spectral width could be tolerated for the upgrade transmitter?

[7 marks]

Solutions

1. Using a block diagram describe a typical unidirectional dense wavelength division multiplexed (DWDM) transmission system.

[5 marks]

Hence answer the following questions related to DWDM:

(i) What are the typical channel spacings used in a DWDM system and what wavelength ranges are used?

[4 marks]

(ii) Explain why are Erbium Doped Fibre Amplifiers (EDFAs) a key component of DWDM systems? Mention two disadvantages associated with the use of EDFAs in DWDM systems.

[4 marks]

(iii) A long range DWDM system that spans several hundred km will require a chain of optical amplifiers to operate successfully. Explain with the aid of suitable diagrams why such systems are not limited in operating span by a lack of power budget but rather by the cumulative deterioration of the optical signal-to-noise ratio. State your assumptions as appropriate.

[7 marks]

1 (a)

Simple unidirectional DWDM system

The outputs of a number of transmitters, each tuned to a well defined and precise wavelength are combined together using a DWDM multiplexer. A power amplifier boosts the signal level, while further line amplifiers maintain the signal level to the receiver. An Add/drop mux/demux is also shown to allow for the possibility of adding or dropping individual wavelengths. At the receiver, after a EDFA used as a preamp, the various wavelengths are correctly split apart and routed to receiver. Each wavelength behaves as if it has sole access to the fibre, a so called virtual fibre.

(i) Channel separation is set by the International Telecommunications Union (ITU) at: 50, 100 and 200 GHz, equivalent to approximate wavelength spacings of 0.4, 0.8 and 1.6 nm. Channels presently lie in the range 1530.3 nm to 1567.1 nm (so-called C-Band). Operation in the so called L-band from 1570 to 1620 is currently becoming available.

Solutions

[4 marks]

(ii) EDFAs are required to allow for longer fibre spans and to compensate for losses in DWDM multiplexers and demultiplexers. However the use of EDFAs does introduce some problems. EDFAs do not have a flat response across the C-band, leading to gain tilt (level variations between channels) and requiring the use of gain flattening filters. EDFAs also introduce excess noise into signals, which can degrade signal quality and can be problematic for example were a restoration path following a failure introduces a larger number of EDFAs into a system span.

[4 marks]

(iii) Optical amplifiers allow one to extend link distance for a DWDM system to several hundred km. To do so optical amplifiers are used to compensate for attenuation after a given span of fibre, typically 80 km. For example in Graph #1 below we have assumed a sample system that uses 0.25 dB atten fibre, 80 km fibre sections and amplifiers with a gain of 19 dB. (student is not expected to calculate values – just give indicative values). The loss in DWDM multplexers and demultiplexers is assumed to be compensated for by power and preamp optical amps for simplicity.

Graph #1

From the graph we can see that as each amplifier restores the signal level to a value almost equivalent to the level at the start of a fibre section, that in principle system reach is extended to 700 km +. Thus by setting the gain of the amplifiers appropriately one could envisage that power budget limitations (eg . where the optical power level at the destination DWDM receiver falls below the receiver sensitivity level) will not apply.

However an amplifier cannot compensate for dispersion (and crosstalk in DWDM systems).Amplifiers also introduce noise, as each amplifier reduces the Optical SNR by a small

amount (noise figure). For the same system above if we assume amplifiers with a reasonable noise figure of 5 dB the OSNR level versus distance plot becomes (Graph

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#2)

Graph #2

In this system the starting SNR, the transmitter SNR is assumed to be 50 dB while the amplifier noise figure is 5 dB. It is clear that the optical SNR drops with distance, so that if we take 30 dB as a reasonable limit, the max distance between a DWDM T/X and R/X is only 300 km., much smaller than the limit (in as much as it exists – see above) set by the power budget alone.

[7 marks]

[10 Marks]

2. A 2.488 Gb/s optical transmission system is to operate at a wavelength of 1550 nm over an unrepeatered distance of 74 km. The transmitter available has a minimum coupled output power of 0 dBm, while the receiver has a worst-case sensitivity of -26 dBm.

The fibre dispersion is known to be less than or equal to 2 ps/km. The fibre and connector attenuation values are shown in Table 1.

Table 1

Type Average attenuation

Standard deviation

Worst case attenuation

Fibre 0.19 dB 0.02 dB 0.25 dBConnector 0.2 dB 0.04 dB 0.5 dB

Two connectors are to be used in the system and all remaining joints are fusion splices. The typical intersplice distance is 1.5 km. The minimum power margin required is 2 dB. Hence:

(i) Show that with a worst-case power budget it is not possible to achieve the power margin required.

[8 marks](ii) If the average splice attenuation is 0.02 dB what is the maximum permissible value of the standard deviation for the splice attenuation, assuming a two standard deviation statistical

Solutions

power budget? [12 marks]

2 (a) (i) Before considering the power budget we need to determine the dispersion penalty for the system. For a bit rate B of 2480 Mbits/sec and t of 2 ps/km RMS impulse spread the power penalty is calculated using the standard rule-of-thumb:

P Bd t 10 110

2 212 Log

Using the formula:System Span (km) 74.00

Minimum power margin (dB) 2.00

Dispersion penalty calculator Total penalties

Bit rate: 2.4880E+09 Polarisation 0.00

RMS impulse spread 2.0E-12 Reflection 0.00Total RMS impuse spread 148.0E-12

Dispersion 4.81Dispersion Penalty (dB): 4.805

Total Penalties 4.81

The dispersion penalty at 2.48 Gbits/sec is 4.81 dB

(ii) For a worst case power budget calculation we do not know the worst case splice attenuation, but even with the absence of this information it is possible to show that the power margin is 1.69 dB. Splice loss can only make this power margin lower still, thus we can assert that it is not possible to reach a margin of 2 dB.

Worst CaseBasic Infomation Derived Information

System Span in km 74.00

Transmitter Output Power (dBm) 0.00

Number of Connectors 2.00

Connector Loss (dB) 0.50 Total Connector Loss (dB) 1.00

Fibre Attenuation 0.25 Total fibre attenuation (dB) 18.50

Maximum fibre length available (km) 1.50 No. of fibre lengths needed 49.33

Worst case loss per splice (dB) Unknown Total splice loss (dB) Unknown

Penalities 4.81

Receiver sensitivity (dBm) -26.00 Available power margin excluding splice loss (dB) 1.69

(ii) We now use a statistical approach (eg. the losses for the fibre and the connectors are replaced by a value equal to the average plus two standard deviations) to calculate the total permissible splice attenuation, assuming the 2 dB power margin. Then since we know the average splice loss we can find the maximum std dev, using the total number of fibre lengths figure, calculated as part of the power budget.

Solutions

Basic Infomation Derived Information


Transmitter Output Power (dBm) 0.00



Fibre Attenuation 0.23 Total fibre attenuation (dB) 17.02

Maximum fibre length available (km) 1.50 No. of fibre lengths needed 49.33

Average Loss per splice (dB) 0.02 Total average splice loss (dB) 0.97

Penalities 4.81 Available margin WITHOUT splices 3.61

Receiver sensitivity (dBm) -26.00 Maximum splice loss 1.61

Maximum Std Dev for splice loss 0.0066

Two Std Deviation Statistical Case

As calculated above the maximum standard deviation for the splice loss is 0.0066 dB

[10 Marks]

3. (a) Sketch the major elements of a fibre amplifier and describe the operation of the device. Indicate the advantages of fibre amplifier technology in comparison with that associated with SLAs.

[7 marks]

(b) (i) Explain the term “all-optical switch” and indicate the main benefits of the all-optical switching technology.

(ii) List the main optical switching technologies available, discuss their strengths, weaknesses and potential applications in fibre communication systems.

[7 marks]

(c) Explain what is meant by the term “four-wave mixing”; describe the impact of the phenomenon on DWDM system operation and techniques for reducing the effect.

[6 marks]

3 (a) (a) A fibre amplifier consists of a short (typically ten metres or so) section of fibre which has a small controlled amount of the rare earth element (usually erbium) added to the glass in the form of an ion (Er3+).Operation is as follows:1. A (relatively) high-powered beam of light is mixed with the input signal using a wavelength selective coupler. The input signal and the excitation light must of course be at significantly different wavelengths. This beam of light constantly keeps the erbium ions in an excited state. (The power level of the pump is often controlled through a feedback loop.)2. The mixed light is guided into a section of fibre with erbium ions included in the core.3. This high-powered light beam excites the erbium ions to their higher-energy state.4. When the photons belonging to the signal (at a different wavelength from the pump light) meet the excited erbium atoms, the erbium atoms give up some of their energy to the signal and return to their

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lower-energy state.This doesn't happen for all wavelengths of signal light. There is a range of wavelengths approximately 24 nm wide that is amplified.5. A significant point is that the erbium gives up its energy in the form of additional photons, which are exactly in the same phase and direction as the signal being amplified. So the signal is amplified along its direction of travel only. Thus all of the additional signal power is guided in the same fibre mode as the incoming signal.6. There is usually an isolator placed at the output to prevent reflections returning from the attached fibre. Such reflections disrupt amplifier operation and in the extreme case can cause the amplifier to become a laser.The erbium doped fibre amplifier has the following advantages over semiconductor optical amplifier technology:

The erbium doped fibre amplifier is a default traveling wave amplifier (no gain ripple)No polarization dependence (SOAs are highly polarization sensitive)Small coupling lossLow noise figure (3-7 dB depending on pump wavelength)High gain (up to 50 dB possible)High output saturation power (>100 mW)No chirpLong excited state population lifetime ( 10 ms) – no crosstalk problem.

[7 marks]

(b) (i) An all-optical switch is a device that enables signals in optical fibres or integrated optical circuits (IOCs) to be selectively switched from one circuit to another without conversion of the signal to electrical form.Up to now, the switching in optical networks has been laid almost entirely in electronics. In every switching node, optical signals are converted to electrical form, buffered electronically, and subsequently forwarded to their next hop after being converted to optical form again. However, as the network capacity increases, electronic switching nodes are unable to keep up. Apart from that, electronic equipment is strongly dependent on the data rate and protocol, and thus, any system upgrade results in the addition and/or replacement of electronic switching equipment. All-optical switching promises to eliminate both these drawbacks.(ii) Table below suggests appropriate network applications for various switching technologies.

Platform Scheme Strengths Weaknesses Potential applications

Opto-mechanical

Employ electromechanical actuators to redirect a light beam

Optical performance, “old” technology

Speed, bulky, scalability

Protection switching, Optical Add/Drop Multiplexing, Optical spectral monitoring

MEMS Use tiny reflective surfaces (subcategory of optomechanical switches)

Size, scalability Packaging, reliability

Cross-Connect, Optical Add/Drop Multiplexing, Optical spectral monitoring

Thermo-optical Temperature control to change index of refraction

Integration, wafer-level manufacturability

Optical performance, power consumption, speed, scalability

Cross-Connect, Optical Add/Drop Multiplexing

Liquid Crystal Processing of polarisation states of light

Reliability, optical performance

Scalability, temperature dependency

Protection switching, Optical

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Add/Drop Multiplexing, Optical spectral monitoring

Gel/oil based A subset of thermo-optical technology

Modular scalability

Unclear reliability, high insertion loss


Magneto-optics Faraday Speed Optical performance

Protection, Optical Add/Drop Multiplexing, Optical spectral monitoring, packet switching

Acousto-optic Acousto-optic effect, RF signal tuning

Size, speed Optical performance


Electro-optic Dielectric Speed High insertion loss, polarisation, scalability, expensive

Cross-Connect, Optical Add/Drop Multiplexing, Optical spectral monitoring

SOA-based Speed, loss compensation

Noise, scalability

Cross-Connect

[7 marks](c) The four-wave mixing (FWM) is a non-linear effect, a result of interaction of light with the material in optical fibre. The interaction of light with the material in optical fibre is typically very small and thus interactions between different signals on the same fibre are also very small. However, since the signal travels long distances on fibre, very small effects have the opportunity to build up into large at power levels of few mW. The four-wave mixing is ”elastic” effect where although the optical wave interacts with and affected by the presence of matter there is no energy exchange between the two.FWM occurs when two or more waves propagate in the same direction in the same (single-mode) fibre. The signals mix to produce new signals at wavelengths, which are spaced at the same intervals as the mixing signals.

A signal at frequency 1 mixes with a signal at frequency 2 to produce two new signals one at frequency 21-2 and the other at 22-1. The effect can also happen between three or more signals.There are a number of significant points.

The effect becomes greater as the channel spacing is reduced. The closer the channels are together the greater the FWM effect.

FWM is non-linear with signal power. As signal power increases the effect increases exponentially.

Solutions

The effect is strongly influenced by chromatic dispersion. FWM is caused when signals stay in phase with one another over a significant distance. The greater the dispersion, the smaller the effect of FWM - because chromatic dispersion ensures that different signals do not stay in phase with one another for very long. Thus to reduce the FWM effect the system architects sometimes employ a balanced structure where sections of dispersive fibre with different dispersion characteristics are joined to form the span. The idea here is that no section of fibre has zero dispersion but that different sections have dispersion of opposite sign so that the total at the end of the link (span) is zero (dispersion compensation).

If the WDM channels are evenly spaced then the new spurious signals will appear in signal channels and cause noise. Another method of reducing the effect of FWM is to space the channels unevenly.

[6marks]

[10 Marks]4. (a) Describe the operation of an Erbium Doped Fibre Amplifier (EDFA). Indicate the advantages of fibre amplifier technology in comparison with Semiconductor Laser Amplifiers (SLAs). List the main techniques for flattening of the amplifier gain curve of an optical amplifier.[7 marks]

(b) Briefly discuss non-linear properties of optical fibre. Explain concisely what is meant by the terms “four-wave mixing”, “stimulated Brillouin scattering” and “stimulated Raman scattering”. Describe the impact of the phenomena on DWDM system operation and the techniques used to reduce the effects.[6 marks]

4 (a) A an Erbium Doped Fibre Amplifier fibre amplifier consists of a short (typically ten metres or so) section of fibre which has a small controlled amount of the rare earth

element (erbium) added to the glass in the form of an ion (Er3+).Operation is as follows:1. A (relatively) high-powered beam of light is mixed with the input signal using a wavelength selective coupler. The input signal and the excitation light must of course be at significantly different wavelengths. This beam of light constantly keeps the erbium ions in an excited state. (The power level of the pump is often controlled through a feedback loop.)2. The mixed light is guided into a section of fibre with erbium ions included in the core.3. This high-powered light beam excites the erbium ions to their higher-energy state.4. When the photons belonging to the signal (at a different wavelength from the pump light) meet the excited erbium atoms, the erbium atoms give up some of their energy to the signal and return to their lower-energy state.

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This doesn’t happen for all wavelengths of signal light. There is a range of wavelengths approximately 24 nm wide that is amplified.5. A significant point is that the erbium gives up its energy in the form of additional photons, which are exactly in the same phase and direction as the signal being amplified. So the signal is amplified along its direction of travel only. Thus all of the additional signal power is guided in the same fibre mode as the incoming signal.6. There is usually an isolator placed at the output to prevent reflections returning from the attached fibre. Such reflections disrupt amplifier operation and in the extreme case can cause the amplifier to become a laser.The erbium doped fibre amplifier has the following advantages over semiconductor optical amplifier technology:

The erbium doped fibre amplifier is a default 12ravelling wave amplifier (no gain ripple)

No polarization dependence (SOAs are highly polarization sensitive)Small coupling lossLow noise figure (3-7 dB depending on pump wavelength)High gain (up to 50 dB possible)High output saturation power (>100 mW)No chirpLong excited state population lifetime ( 10 ms) – no crosstalk problem.

Flattening of the Gain Curve Techniques:1. Operating the device at 77° K. This produces a much better (flatter) gain curve

but it’s not practical.2. Introducing other dopant materials (such as aluminium or ytterbium) along with

the erbium into the fibre core. 3. Amplifier length is another factor influencing the flatness of the gain curve. 4. Controlling the pump power (through a feedback loop) is routine to reduce

amplified spontaneous emission. 5. Adding an extra WDM channel locally at the amplifier (“gain clamping”). 6. Manipulating the shape of the fibre waveguide within the amplifier.

At the systems level there are other things that can be done to compensate:1. Using “blazed” fibre Bragg gratings as filters to reduce the peaks in the

response curve. 2. To transmit different WDM channels at different power levels to compensate

for later amplifier gain characteristics.

[10 Marks]

4 (b) Nonlinear effects are the result of interaction between light and matter and they increase exponentially as the level of optical power in the fibre is increased (above 3 mW). Typically they are very small, but since the signal travels long distances on fibre, very small effects have the opportunity to build up into large ones. Nonlinear effects can be grouped into two classes. “Elastic” effects where although the optical wave interacts with and is affected by the presence of matter there is no energy exchange between the two (four-wave mixing). “Inelastic Scattering” is where there is an energy transfer between the matter involved and the optical wave (Stimulated Brillouin Scattering and Stimulated Raman Scattering). Nonlinear effects are nearly always undesirable. After attenuation and dispersion they provide the next major limitation on optical transmission. The four-wave mixing (FWM) is an “elastic” effect where although the optical

Solutions

wave interacts with and affected by the presence of matter there is no energy exchange between the two. FWM occurs when two or more waves propagate in the same direction in the same (single-mode) fibre. The signals mix to produce new signals at wavelengths, which are spaced at the same intervals as the mixing signals. A signal at frequency w1 mixes with a signal at frequency 2 to produce two new signals one at frequency 21-2 and the other at 22-1. The effect can also happen between three or more signals.The effect becomes greater as the channel spacing is reduced. The closer the channels are together the greater the FWM effect.FWM is non-linear with signal power. As signal power increases the effect increases exponentially.The effect is strongly influenced by chromatic dispersion. FWM is caused when signals stay in phase with one another over a significant distance. The greater the dispersion, the smaller the effect of FWM – because chromatic dispersion ensures that different signals do not stay in phase with one another for very long. Thus to reduce the FWM effect the system architects sometimes employ a balanced structure where sections of dispersive fibre with different dispersion characteristics are joined to form the span. The idea here is that no section of fibre has zero dispersion but that different sections have dispersion of opposite sign so that the total at the end of the link (span) is zero (dispersion compensation).If the WDM channels are evenly spaced then the new spurious signals will appear in signal channels and cause noise. Another method of reducing the effect of FWM is to space the channels unevenly. Stimulated Brillouin Scattering (SBS) is a scattering of light backwards towards the transmitter caused by mechanical (acoustic) vibrations in the transmission medium. SBS can be a major problem in three situations: 1) in long distance systems where the span between amplifiers is great and the bit rate low (below about 2.5 Gbps); 2) in WDM systems (up to about 10 Gbps) where the spectral width of the signal is very narrow; 3) in remote pumping of an erbium doped fibre amplifier (EDFA) through a separate fibre. EDFA pumps typically put out about four lines of around only 80 MHz wide. Stimulated Raman Scattering (SRS) is caused by a similar mechanism to the one, which produces SBS. However, the interactions involved are due to molecular vibrations rather than acoustic ones. Scattered light can appear in both the forward and backward directions. SRS is a not an issue in single-channel systems but can be a significant problem in WDM systems. When multiple channels are present, power is transferred from shorter wavelengths to longer ones. The effect of SRS becomes greater as the signals are moved further and further apart (within some limits). This is a problem as we would like to separate the signals as much as we can to avoid four-wave mixing effects and when we do we get SRS.SRS increases exponentially with increased power. At very high power it is possible for all of the signal power to be transferred to the Stokes Wave.

[10 Marks]

5.

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5 An optical fibre system is used to distribute multiplexed digital video signals from a single transmitter to N geographically dispersed receivers. An N-port optical splitter is located just after the transmitter and the maximum distance from the splitter to any of the N receivers is 39 km. The system specifications and design information are as follows:

The bit rate is 622 Mbits/sec. The worst case transmitter output power is +1.5 dBm and the transmitter spectral width is less than 1.5 nm. The splitter has N output ports, with an excess loss less than 0.3 dB per port.

Singlemode dispersion shifted fibre is used which meets ITU-T recommendation G.653 with a dispersion coefficient of 3.5 ps/nm.km and a worst case fibre attenuation at the operating wavelength of 0.25 dB/km.

Throughout the system cable spans with an average length of 740 m are fusion spliced together with a maximum splice attenuation of 0.07 dB. There are four connectors used between the transmitter and each receiver. The maximum connector attenuation is 0.25 dB.

All of the receivers have an identical specification and have a worst case sensitivity of –27 dBm at 622 Mbits/sec. The power margin for repair, upgrade etc. must not be less than 2.4 dB, between the transmitter and each receiver.

Based on the information above:

(i) Sketch a block diagram for the system [4 marks]

(ii) What is the value of the worst-case dispersion penalty in the system?[4 marks]

(iii) What is the maximum value of the splitter ratio N with the system configured as above?

[12 marks]

(iv) Furthermore to increase the value of N the use of an EDFA, to be placed between the transmitter and the N-port splitter, is being considered by the designers. Explain why an amplifier is used. If the value of N is to be raised to 100 what minimum amplifier gain in dB is needed? You may neglect noise and connections losses for the EDFA.

[5 marks]

(a) (i) Block diagram

[4 marks]

Solutions

5

(ii) The dispersion penalty is found from the standard worst case formula

This expresses the power penalty Pd in terms of the bit rate B and the mean square impulse spread t

2. The worst case dispersion penalty arises for the longest system span which is 39 km, thus:

Dispersion penalty calculation Bit rate: 622.0E+6

Fibre dispersion in ps/nm.km 3.50Transmitter spectral width (nm) 1.5System Span (km) 39.00Calculated total dispersion (ps) 204.8 ps

Worst case Dispersion Penalty (dB): 0.362

[4 marks]

(iii) Solution is via a power budget for a 39 km link, using the dispersion penalty and then determining the maximum split loss from input to output port (excluding excess loss) which allows us to estimate the maximum value of N. The calculations are:

Power Budget Calculation

Basic Infomation Derived Information


Transmitter Output Power (dBm) worst case 1.500



Fibre Attenuation worst case 0.250 Total fibre attenuation (dB) 9.75

Average inter-splice distance (km) 0.740 No. of fibre lengths needed assuming avg intersplice distance

52.70

Loss per splice (dB) 0.070 Total splice loss (dB) 3.62

Dispersion Penalities 0.362

Solutions

Receiver sensitivity (dBm) worst case -27.000 Available power budget (dB) 28.50

Minimum power margin (dB) 2.40

Maximum splitter insertion loss (dB) 11.3684 Calculated as balance of power budget

The budget available is 28.5 dB. Once power penalties, losses, etc (including the power margin) are accounted for, from the above the balance available is 11.36 dB

Ths is the maximum loss per port that can be tolerated and is dependent on the split ratio and the excess loss.

Based on the figure above, the maximum split ratio only loss per port is 11.07 dB.

For various split ratios the loss is:

For a split ratio of 13:1 (ie 13 fibres out) the split ratio loss is 11.14 dB, which is too high. For a split ratio of 12:1 (ie 12 fibres out) the split ratio loss is 10.79 dB, which is just right

Thus for the present conditions the maximum value of N is 12

(iv) Here an EDFA is used as a power amplifier prior to the splitter to allow for a larger split ratio (eg. more receivers) by overcoming splitter insertion loss by using a higher input power level to the splitter.

For the value of N to rise to 100 then the splitter loss in dB excluding insertion loss will be per port 20 dB. This greatly exceeds the unamplified allowance of 11.36 dB allowed by the power budget. The amplifier must make up the shortfall between 20 dB and 11.36 dB, which means a that an amplifier with a minimum gain of at least 8.64 dB is needed.

[10 Marks]

6 (a) (i) Engineering of a communication system in the WDM environment requires taking many devices and integrating them into a system. This is a non-trivial exercise as interactions between different types of equipment and their operation in a field environment must be well understood.There are many issues to be considered:

1) Determining the width and spacing of wavebands.2) Stabilising the wavelength of wavelength-sensitive components.3) Filter alignment in cascades of filters.4) Control of non-linear effects.5) Control of dispersion.6) Control of cross-talk.7) Dynamics of optical amplifiers.

Solutions

8) Control of system noise (especially for amplified spontaneous emission sources).[6 marks]

(ii) Various non-linear effects prevalent in fibre transmission need consideration and control for the system to operate successfully.Four-Wave Mixing (FWM)FWM occurs when two or more waves propagate in the same direction in the same (single-mode) fibre. The signals mix to produce new signals at wavelengths, which are spaced at the same intervals as the mixing signals. This is a most serious issue for WDM systems. It is influenced very strongly by two factors:

1) Channel spacing;2) Fibre dispersion.

Stimulated Brillouin Scattering (SBS)SBS is really a single channel phenomenon but nevertheless it must be taken into account in WDM situations. SBS depends on a number of things:

1) Signal linewidth. The narrower the linewidth the more of a problem SBS becomes. SBS is generally not a problem with channel bandwidth greater than 100 MHz;

2) Signal power. There is a threshold value below which SBS will not cause a problem;

3) Fibre core size. The narrower the core, the higher the power concentration and thus the greater effect from SBS;

4) Wavelength. SBS has a bigger effect in the 1550 nm band than in the 1310 nm band.Stimulated Raman ScatteringSRS occurs between signal channels and between groups of channels.To keep degradation due to SRS down to acceptable levels the product of total power and total optical bandwidth must be less than 500 GHz/W. The bandwidth quoted is the spread of the WDM channels.

[7 marks]

[10 Marks]

7 (a) The four-wave mixing (FWM) is a nonlinear effect, a result of interaction of light with the material in optical fibre. The interaction of light with the material in optical fibre is typically very small and thus interactions between different signals on the same fibre are also very small. However, since the signal travels long distances on fibre, very small effects have the opportunity to build up into large at power levels of few mW. FWM occurs when two or more waves propagate in the same direction in the same (single-mode) fibre. The signals mix to produce new signals at wavelengths, which are spaced at the same intervals as the mixing signals.

Solutions

A signal at frequency 1 mixes with a signal at frequency 2 to produce two new signals one at frequency 21-2 and the other at 22-1. The effect can also happen between three or more signals.There are a number of significant points. The effect becomes greater as the channel spacing is reduced. The closer the

channels are together the greater the FWM effect. FWM is non-linear with signal power. As signal power increases the effect

increases exponentially. The effect is strongly influenced by chromatic dispersion. FWM is caused when

signals stay in phase with one another over a significant distance. The greater the dispersion, the smaller the effect of FWM - because chromatic dispersion ensures that different signals do not stay in phase with one another for very long. Thus to reduce the FWM effect the system architects sometimes employ a balanced structure where sections of dispersive fibre with different dispersion characteristics are joined to form the span. The idea here is that no section of fibre has zero dispersion but that different sections have dispersion of opposite sign so that the total at the end of the link (span) is zero (dispersion compensation).

If the WDM channels are evenly spaced then the new spurious signals will appear in signal channels and cause noise. Another method of reducing the effect of FWM is to space the channels unevenly.

[10 Marks]

8 (a) Outline five common reasons why power margins are employed in the design of optical transmission systems:

To allow for ageing of sources and other components. To allow for variations in the system performance induced by temperature To cater for extra splices, when cable repair is carried out. To allow for extra fibre, if rerouting is needed in the future. To allow for upgrades in the bit rate or advances in multiplexing. Any 4 out or 5 will do, [1.5 marks each]

[10 Marks]

8 (b) The dispersion penalty for each fiber type is found from the standard worst case formula

This expresses the power penalty Pd in terms of the bit rate B and the mean square impulse spread t2. The basic system info is

System Span in km: 72.00 Bit rate (bits/s): 2.40E+09Receiver sensitivity (dBm) worst case: -24.00

The power budget info for connectors, fiber and splices is:

Solutions

The dispersion penalty and the repair margin for transmitter type 1 and 2 is shown below:

Based on the above it is clear that even though the output power of Transmitter type 2 is lower, the lower spectral width results in a lower dispersion penalty and thus the repair margin at 2.47 dB is better and thus transmitter type 2 is selected.

[10 Marks]

8 (c) For the purpose of the upgrade to operation at 5 Gbits/s a transmitter with a worst case output power of +8 dBm is available, while the receiver sensitivity is reduced to -23.2 dBm. The problem is that even though the power budget has increased (from say 26 dB for the type 1 transmitter above) to 31.2 dB, the increased bit rate will absorb this increase as a higher dispersion penalty unless the source spectral width is improved significantly from the 2.4 Gb/s transmitter case. We know the repair margin is to be 1.5 dB minimum is needed. The next steps are:

1. From the power budget and the repair nargin determine (see power budget below) that we could tolerate a maximum dispersion penalty of 9.72 dB and still just achieve a repair margin of 1.5 dB.

2. Using the maximum tolerable dispersion formula, adapt the formula for the dispersion penalty above to yield from the dispersion the maximum pulse dispersion which is 85.1 ps (see below) over 72 km of fiber. The maxium dispersion per km is thus 1.18 ps. Using the fiber dispersion coefficient of 3.5 ps per km.nm, the resultant maximum spectral width is 0.34 nm.

Solutions

In conclusion then, based on a maximum dispersion penalty of 9,72 dB, at 5 Gbits/s, the maximum tolerable transmitter output spectral width is 0.34 nm.

[10 Marks]

power budget - problem set 2

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