ee 390 guest lecture introduction to fibre optic communication …grover/ee780/haugen_2004...

Fibre Optic Communications Systems Intro-1

EE 390 Guest LectureIntroduction to Fibre Optic Communication Systems

TOPICSBrief history of FOC

Structure of a FOC systemDigital systems considerations

Examples of FOC system calculationsPulse dispersion and attenuation limits

Chris Haugen, Ph. D., P. Eng.TRLabs Research Scientist – Photonics

Adjunct Professor – ECE, U of [email protected]


History and Evolution of Optical Communications...Ancient: 800 BC - Greek signal fires

400 BC - Relay stations150 BC - Alphabetic codesMany forms of optical signals

1960 - Laser invented; concentrated, directional light1966 - Fibre loss = 1000 dB/km! (due to impurities)1970 - Corning: 20 dB/km, competitive with copper cable.1990s - 0.16 dB/km -> 100s of km without repeaters.Late 90’s - present – WDM systems at 10 Gb/s/λ

Communication applications of EM spectrum. [Keiser, Optical Fiber Communications, 3rd ed., 2000]


Elements of a F-O Transmission Link

Major elements of an optical fibre transmission link. The basic components are the light signal transmitter, the optical fibre, and the photodetecting receiver. Additional elements include fibre and cable splices and connectors, regenerators, beam splitters, and optical amplifiers. [Keiser]


Optical fibre attenuation as a function of wavelength. The historical deployment of fibre systems was determined by the low loss “windows.” [Keiser]


Other system evolutions- multimode singlemode- direct detection coherent direct detection coherent?- regenerators vs optical amplifiers

[Agrawal, Fiber-Optic Communication Systems, 2nd ed., 1997]

Future systems features- Wavelength Division Multiplexing (WDM)- All-optical (switches, amplifiers,...)- Solitons (high data rate, long distance)- Fluoride glass (low loss)


Advantages of Optical Fibre CommunicationEnormous potential bandwidth (high freq carrier).

Low Loss (α as low as 0.2 dB/Km for glass).

Repeaters can be eliminated ⇒ low cost and reliability.

Immunity to EMI/Electrical isolation.

Secure; Cannot be tapped without affecting signal.

Small size and weight.

Abundant raw material.

Disadvantages of Optical Fibre CommunicationHigh cost of coupling fibres.

High cost of components.


Point-to-Point Digital Fibre System Considerations

The simplest transmission system is the point-to-point link.

Key system requirements1. Transmission distance2. Date rate/channel bandwidth3. Bit-error-rate (BER)

Fig. 8-1 Simplex point-to-point link.

To meet these requirements, the designer has to choose between a number of components and their associated characteristics I.e.fibre types? Optical sources? Photodetector type? Receiver type?

Two analyses are usually carried out to ensure system performance: link power budget and rise-time budget.


Sample problems:

(a) System capability: The risetimes of the transmitter and the receiver photodiode are 5 ns and 3 ns respectively. The bandwidth of the receiver amplifier is 25 MHz. The total dispersion in the optical fibre is 1.3 ns. What is the maximum bit rate for this system?

- Need to understand ...

- relation between risetime and device bandwidth

- relation between “dispersion” and system bandwidth

- connection between max rate and system bandwidth

(b) System design: Specify initial choices for the transmitter, fibre and receiver for a FOC system with the following specifications: Data rate = 100 Mb/s, Bit Error Ratio (BER) = 10-9, Distance = 500 km.

-Need to understand ...

- - general properties of different components (both attenuation and dispersion)

- amplifier signal/noise ratio (SNR)

- relation of BER and SNR


Attenuation and Link Power Budget

Consider light, of intensity I [W/m2], incident on an elemental volume of material. The change in intensity δI that occurs in distance δx is given by:

pI I xδ α δ= −αp [m-1] is a proportionality constant (attenuation constant) and is dependent on wavelength, λ, I.e. αp = αp(λ). Solving the differential equation shows that the transmitted intensity decreases exponentially with distance (Beer-Lambert Law):

0( ) exp( )pI x I xα= −

The calculations for attenuation are greatly simplified by expressing the various losses encountered with units of decibels(dB). The attenuation coefficient is then expressed in units ofdB/m and this parameter is commonly known as the fibre loss or fibre attenuation, αf.

ASIDE: Decibels

A convenient method of relating the signal levels at various in a fibre optic link is to reference it to some value or noise level. This is often done in terms of a power ratio measured in decibels (dB):

where P1 and P2 are optical (or electrical) power. Due to the logarithmic nature of dB, concatenated losses expressed in dB can be simply added together.

2

1

Power ratio in dB 10log PP

= P1 P2


Attenuation

ASIDE: Decibels, cont.

While decibels provide a relative power ratio measurement, the concept can be extended to provide absolute power measurements by referring the power ratio to a specific value, most commonly 1 mW:

where a measurement of 0 dBm = 1 mW.

Finally, we relate the attenuation constant, αp, to the fibre loss, αf.

Power in dBm 10log1 mW

P=

4.343f pα α=

As with all communications channels, there is a loss associated with optical fibres. The size of the loss depends on both the optical wavelength and the type of fibre being used.Typical values

• 1.55 µm SMF – αf = 0.2 dB/km• 800 nm MMF – αf = 2-3 dB/km


Graded-Index Multimode

Attenuation

Step-Index Singlemode

Fibre loss as a function of wavelength for multimode and singlemode fibre. [Keiser]


Attenuation

Must also consider the losses of the fibre couplers (associated with the system components) and fibre splices (in long runs) occurring in the system.

Optical power loss model for point-to-point link [Keiser]

Attenuation limit: Distance the optical signal can propagate before it becomes to weak for the system to work reliably.

The attenuation limit is determined by a number of factors including system margin. System bit-rate, photodetector/receiver circuit, desired BER also help to determine the attenuation limit and these a frequently quantified by a parameter know as the receiver sensitivity, Prec. Receiver sensitivity is defined as minimum average received power, by the photodetector, required to achieve a given BER at a given bit rate.


Link Power Budget

Receiver sensitivities for BER = 10-9 [Keiser]

The link power budget considers the total optical power loss, PT, that is allowed between the source, PS, and the photodetector, Prec, in the receiver.

rPT s ecP P= −

T l sP C M= +

Where Cl is the channel loss (due to connectors, attenuation, splices, etc.) and Ms is the system margin. The system margin is added to account for system aging, temperature fluctuations, andthe addition of components at a later time. A margin of 6 –8 dB is typical for systems that are not expected to have additional components added to them.


Rise-time BudgetBasic Pulse Code Modulation (PCM) Concepts

Pulse code modulation (PCM) is the technique used almost universally to convert analog data to a digital format for transport on optical communication systems. A binary code is used to convert each sampled analog value in a string of “1” and “0” bits.

Transmitted “Bit Stream”

The number of bits, m, needed to represent each sample depends on the number of quantized signal levels, M.

22 logmM or m M= =

The bit rate, B, associated with a PCM signal is then given by (making use of the Nyquist criterion):

2(2 ) logs sigB mf f M= ≥ ∆Where fs is the sampling frequency and ∆fsig is the bandwidth of the analog signal.


Rise-time Budget

The PCM encoded data must then be channel or line encoded into its final optical format for transmission down the optical link. In most optical systems, simple Amplitude Shift Keying (ASK) (On-Off Keying - OOK) is used to transmit the digital bits. Two of the most common line encoding schemes for ASK are the non-return-to-zero (NRZ) and return-to-zero (RZ) formats.

Fig. 8-9 NRZ and RZ formats

The relationship between the system bandwidth and bit-rate will depend on the line encoding used. In RZ, each data bit is encoded as 2 optical code bits I.e. it will require twice the bandwidth as the NRZ signal. Making use of the Nyquist criterion:

NRZ B ~ 2∆fRZ B ~ ∆f


System with finite

bandwidth

Input Output

Impulse response

Impulse

Square Square response

Rise-time Budget

For any linear system, there is an inverse relationship between the bandwidth, ∆f, and rise-time, tr, of the system. If one considers a simple RC circuit, the time and frequency response of the circuit can be described as:

10( ) 1 exp ( ) (1 2 )tV out V H f j fRC

RCπ − = − − → = +

1

LinearSystem

t

Vin

t

Vout

1

0.1

0.9

tr

Risetime, tr, associated with a bandwidth- limited linear system.

If the rise-time is defined as the time for the output response to rise from 10% to 90% of its final value, it can be shown that:

0.35rt f=

∆


Rise-time Budget

ASIDE:This result is generally valid. For example, if the bandwidth of a receiver is 25 MHz, the rise-time of the receiver will be 14 ns.

The result of this is that there are different rise-time constraints for NRZ and RZ encoded optical signals.

0.35 for RZ format

0.7 for NRZ formatr

Bt

B

≤

A rise-time budget analysis is used to determine the dispersion (pulse spreading) limitations of an optical fibre link. The total system rise-time, tsys, is the root sum square of all the contributors to pulse rise-time degradation.

1/ 22 2 2 2 1/ 2

1( )

N

sys i tx fibre rxi

t t t t t=

= = + + ∑

The major contributors are the rise-times of the transmitter and receiver, ttx and trx, and the fibre itself, tfibre. There are a further two components which determine the fibre rise-time. These are group velocity dispersion, tGVD, and in multimode fibre, modaldispersion, tmod.

2 2 2modfibre GVDt t t= +


Rise-time Budget

Pulse spreading, dispersion, can lead to inter-symbol interference. [Keiser]

tGVD arises due to the fact that the group velocity of an optical pulse is wavelength dependent and it is given by

GVDt D L λσ≈

Where D is the dispersion of the fibre [ns/nm km or ps/nm km], L is the length of the fibre [km], and σλ is the spectral width of the source [nm]. D is dependent on the fibre being used. Since in practice a fibre link rarely consists of a single continuous fibre, an average value of D should be used.

Modal rise-time is very difficult to predict given the order of fibres placed in a long link can have an effect. For single mode systems operating at 1.55 µm, the modal rise-time is 0 by definition.


Loss and Dispersion Limited Links LengthsAs an optical pulse propagates along a fibre, it loses power due to attenuation and it becomes distorted due to dispersion. Both ofthese mechanisms independently limit the ultimate link length.

maxr

10 logP

atten s

f ec

PLα

=

max 2

1 1 or dispLB B

∝

Long wavelength window transmission. 1550 nm ILD, σλ = 3.5 and 1.0 nm, with InGaAs APD, SMF with D = 2.5 ps/nm-km, α= 0.3 dB/km. [Keiser]


Loss and Dispersion Limited Links Lengths

Data Rate

Tran

smis

sion

Dis

tanc

e

L1

L2

Loss Limited Link

DispersionLimited Link

B

The system described by the graph shown above is operated with a data rate B. The loss limited transmission distance is L1. The transmission distance can be extended by making up for the loss in the link I.e. amplify the signal using an optical amplifier. Eventually, the system will reach distance L2, the dispersion limit length. At this point, the pulses are so distorted that they must be regenerated in a repeater. Repeaters convert the optical signal to an electronic signal and process it to reshape and retime the pulses. The electronic pulses are then converted back into an optical signal and sent onto the fibre.

ROpticalAmplifier

OpticalAmplifier

OpticalAmplifierRepeater

OpticalAmplifier

L1

T

L1

L2

Note: Optical amplifiers do not correct dispersion.

ee 390 guest lecture introduction to fibre optic communication …grover/ee780/haugen_2004...

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