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Slide Chapter 5

Apr 05, 2018

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Nguyễn Đức
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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

    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

    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