slide chapter 5
TRANSCRIPT
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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
Basic networks
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Basic networks
Basic networks
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Basic networks
Basic networks
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Basic networks
Basic networks
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Basic networks
Basic networks
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Basic networks
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Basic networks
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