Slide Chapter 5

7/31/2019 Slide Chapter 5

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Optical Communication

Chapter 5: Point-to-Point link


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Content

Analog linkDistortion

Carrier to Noise Ratio

Digital link

Attenuation limitDispersion limit

Rise-time budget

Power budget

Basic networks

Power budget in bus topology

Power budget in start topology


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Analog link

Analog link

The modulated signal is analog signal

The most important factor is the Carrier toNoise ratio (CNR)

Applications of analog link: microwavephotonics related field (optical sensor, opticalbeamforming, optical pulse generator, RF over

optical )


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Analog link

Analog link


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Analog link

Signal modulation at the optical source


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Analog link

Signal degradation at the optical source

Harmonic distortioncreate frequency components in the output signal.

Inter-modulation distortion

create frequency components in the output signal.RINContribute to the total CNR

Laser clippingSignal distortion cause by high modulated signal

power


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Analog link

Signal degradation at the optical source

Harmonic distortion: For an input signal ofx(t)=cos(t). If the output y(t) of the opticalsource consists of harmonic frequencies of ,

cause by nonlinear input-output response of thedevice


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Analog link

Inter-modulation distortion

For an input signal ofx(t)=A1cos(1t)+A2cos(2t). The output of theoptical source is

where m and n are integers. The sum of the

absolute values of m and n determines theorder of the intermodulation distortion


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Analog link

Carrier-to-Noise Ratio (CNR)

Carrier-to-noise ratio (CNR) is the ratio of rmscarrier power to rms noise power at the input ofthe RF receiver following the photodetection

process.For N signal-impairment factors the total CNR is

given by


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Analog link

Carrier power

The output optical power of a laser can be expressedas:

s(t) is the time-varying analog drive signalPt is the optical output power at the bias current levelm is the modulation index typically between 0.25 and 0.5

For sinusoidal signal, the carrier power (in A2) at theoutput of the receiver is:

R0 is the unity gain responsivity of the photodetectorM is the photodetector gain

Pis the averaged optical power


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Analog link

Source noise

Relative Intensity Noise (RIN): RIN is due to therandom fluctuation of intensity. Themeansquare noise current is


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Analog link

Photodetector and Preamplifier noise

Photodiode noise:

q is the electron chargeIp is the primary photocurrent

ID is the detector bulk dark current

M is the photodiode gain (M=1 for pin photodiodes)

F(M) is the excess photodiode noise factor Mx(0


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Analog link

Photodetector and Preamplifier noise

Preamplifier noise:

Req is the equivalent resistance of the photodetectorload and the preamplifier

Ft is the noise factor of the preamplifier

Shot noise: intensity fluctuation due to thequantum noise effect. Its also the lower limit ofa laser source noise


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Analog link

Total CNR


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Analog link

Limiting Conditions Preamplifier noise is the dominant noise when the received

optical power is low. Therefore, the carrier-to-noise ratiobecomes:

The photodetector (quantum) noise is the dominant noise whenthe received optical power is at intermediate level. For well-designed photodiodes, the bulk and surface dark currents are

small as compared with the quantum noise. In this case


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Analog link

Limiting Conditions RIN / Reflection noise is the dominant noise when the RIN value

is large (Back-reflected signals can increase the RIN by 10-20dB). The CNR can be expressed as:


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Analog link

Limiting Conditions


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Digital link

Digital link

The modulated signal is digital signal

Have to consider several factor: attenuationlimit, dispersion limit, power budget, rise-time

budgetApplications of digital link: data transmission in

optical networks


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Digital link

Point-to-Point digital links

For different application, the link length can varyfrom one km to thousands of km

For data links between terminals with relative short

distance (


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Digital link


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Digital link


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Digital link Regenerators can be used to compensate for fibre loss.

A regenerator is a receiver-transmitter pair.

The receiver detects the incoming optical signal and recoversthe electrical bit stream.

The transmitter converts the electrical bit stream back into anoptical bit stream.

Optical amplifiers can be used to compensate for fibreloss by amplifying the optical bit stream directly. Due to the dispersion effect, optical amplifiers can not be

cascaded indefinitely. Regenerators do not suffer from this

problem, as they regenerate the original bit stream and thuscompensate for both fibre loss and dispersion.


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Digital link

The bit rate-distance product (BL) is generallyused as a measure of the system performancefor point-to-point links.

BL product depends on operating wavelengthbecause both fibre loss and fibre dispersion are

wavelength dependent.First lightwave system: 0.85m BL product about 1(Gb/s)-km

Second lightwave system: 1.3 m BL product about 25(Gb/s)-km

Third lightwave system: 1.55 m BL product about 1000(Gb/s)-km


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Digital link

Attenuation LimitFibre attenuation plays an important role in the system

design.The signal received by an optical receiver requires a

minimum average power Pr. If the average transmitterpower is Pt , then the maximum transmission distance is

limited by:

Where tot (in dB/km) is the total loss of fibre cablewhich including splice and connector losses.


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Digital link

Attenuation Limit

The maximum transmission distance, L, is also limitedby the bit rate, B, because of the linear dependence ofthe received power on the bit rate as follows:

Where h is the photon energy and Np is the averagenumber of photons per bit required by the receiver.

Therefore, at a given operating wavelength, L decreasesas B increases.


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Digital link


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Digital link

Dispersion Limit

Due to pulse broadening, fibre dispersion limitsthe bit rate-distance product BL.

The system is dispersion limited if the

dispersion-limited transmission distance isshorter than the loss-limited distance.


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Digital link

For the 0.85m lightwave system, intermodal dispersion

is the most limiting factor for multimode fibres. The first generation of terrestrial telecommunication

systems uses multimode graded-index fibres andbecame operational in 1978 at 50-100 Mb/s with

repeater spacing ~10km.


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Digital link

For multimode step-index fibre, BL=c/(2n1).

For n1=1.46 and =0.01, multimode systems are dispersionlimited even at a low bit rate of 1Mb/s and the transmissiondistance is limited to 10km.


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Digital link

For multimode graded-index fibre, BL=2c/(n12).

For n1=1.46 and =0.01, 0.85m lightwave systems are losslimited.


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Digital link

The 1.3m lightwave systems used single-mode

fibres near the 1.3m minimum-dispersionwavelength.

Dispersion-induced pulse broadening is the limiting

factor for such systems. The bit rate-distance product isthen limited by

BL (4|D|)-1

Where D is the dispersion parameter and is the rms

width of the source spectrum. The value of |D| is around1-2ps/(km-nm)

For |D| =2ps/km, BL 125 (Gb/s)-km.


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Digital link The second generation lightwave systems are generally

loss limited for bit rates up to 1Gb/s but becomedispersion-limited at high bit rates.


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Digital link

The 1.55m lightwave systems take the

advantage of the smallest fibre lossAround this wavelength, D is typically 15ps/(km-nm) for

conventional fibres. Therefore, fibre dispersion becomes

a major problem for such systems.

The limit for such a lightwave system can be expressedas:

B2L < (16|2|)-1

Where 2 is the group velocity dispersion. D and 2 canbe related by the following equation.

D=-(2c/2) 2


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Digital link For B2L = 4000 (Gb/s)2-km, ideal 1.55m lightwave

systems become dispersion limited for B> 5Gb/s.


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Digital link Using dispersion-shifted fibres, both dispersion and loss

are minimum around 1.55m.


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Digital link

Rise-time budget


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Digital link

Rise-time budget


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Digital link

Rise-time budget


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Digital link


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Digital link

Example


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Digital link Power budgetThe power arriving at the detector must be sufficient to

allow clean detection, with few errors. This usuallymeans that signal power must be larger than the noisepower present at the receiver.

This power level is called the receiver sensitivity, and it

is related to the bit-error-rate (BER).The power received depends on:

Power from the light sourceSource to fibre coupling lossFibre lossConnector and splicing lossFibre to detector coupling loss


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Digital link


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Digital link


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Digital link


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Digital link

Examples


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Digital link

Examples


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Digital link


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Basic networks

Basic topologies


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Basic networks

Basic topologies


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Basic networks


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Basic networks


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Basic networks


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Basic networks Power budget


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks


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Basic networks

Conclusion


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Conclusion

Design consideration for point-to-point link

CNR (or SNR): noise contributed by all opticaldevices in the link. Depending on theapplication, different noise limits are set.

Power budget is calculate to estimate thesystem attenuation level and received power

Rise-time budget is calculate to estimate the

modulation limit caused by dispersion

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