Chapter 5 Transmission System Engineering. Design the physical layer Allocate power margin for each impairment Make trade-off . 5.1 System Model. Only digital systems are considered Using NRZ codes BER is the measurement factor. 5.2 Power penalty. Power penalty

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Chapter 5 Transmission System Engineering

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5.1 System Model • Only digital systems are considered • Using NRZ codes • BER is the measurement factor

5.2 Power penalty Power penalty • The increase in Signal power required (dB) to maintain the same BER in the presence of impairment. • The reduction in SNR due to a specific impairment (used in this course) Recall (p.260) For PIN receiver with Gaussian noise where the decision threshold is optimal

5.4 Receiver Key system parameters: sensitivity and overload Dynamic range: Pmax-Psen Sensitivity is usually measured at BER = 10-12, and using a pseudo-random 223-1 bit sequence.

5.5 Optical Amplifiers C-band and L-band EDFAs, Raman Amplifiers are available. EDFAs have BW=35nm at 1550nm and they can amplify multiple wavelength in a WDM system. Impairments of EDFA • Inducing noise • Nonlinear gain (depending on power) • Nonflat gain profile

The saturation power (~10mw to 100mw) is • proportional to pump and other parameters) • Operating an EDFA in saturation has no • fundamental problem • Practically it is operated in saturation 5.5.2 Gain Equalization in EDFAs The gain flatness becomes an important issue in WDM systems with cascaded amplifiers • Preequalization (preemphasis) • Equalization at each stage

5.5.3 Amplifier Cascades Let the loss between two stages = where α: attenuation coefficient : amplifier spacing In general G ≧ gain > loss Recall

Consider the case There are amplifiers (Fig 5.5) Using the equation (4.5), we have the total noise power at the output as

5.5.4 Amplifier Spacing Penalty In a cascaded Amplifiers WDM system, If is small we may use a small gain amplifier. In this section, we will study the relation between penalty and spacing. The ASE noise power at the output of a cascade of amplifiers is

Ideally when G=1 the minimum noise power is achieved. (perfectly distributed gain) (N=∞ NInG=αL) The power penalty for using lumped amplifier is given

For α = 0.25dB/km We reduce the spacing from 80km→40km However we have double the number of amplifiers Recall (4.11) page. 257 Noise figure Fn=2nsp If an amplifier with Fn=3.3dB is used It can be viewed as having an effective NF = 3.3dB - 13.3dB = -10dB

5.5.6 Lasing Loops In ring networks, if the amplifier gain is larger than the loss, the ring may lase. Lasing may occur even for a single wavelength Solutions： • Gain is less than the loss being compensated for => degrade SNR • No loop

5.6 Crosstalk Filters, Mux/Demuxs, switches, optical amplifiers and fibers can induce crosstalk. Two kinds of crosstalk：(a) interchannel crosstalk, (b) intrachannel crosstalk (coherent crosstalk) Crosstalk results in a power penalty. 5.6.1 Intrachannel Crosstalk Causes：(a) reflection (b) leakage The penalty is high when the polarization is matched or out of phase.

5.7 Dispersion • Intermode dispersion (multimode fibers) • Polarization mode dispersion (imperfect core) • Chromatic dispersion (different wavelengths) 5.7.1 Chromatic Dispersion Limits: NRZ Modulation Let the pulse spreading due to chromatic dispersion be a fraction of the bit period. is specified by ITU(G.957) and Telcordia(GR-253) for 1dB and 2dB penalty

Narrow Source Spectral Width For SLM DFB lasers, the unmodulated lasers Δλ≦50MHz Ideally a directly modulated laser, Δλ≈ bit rate e.g 2.5GHz for 2.5Gb/s ook (B=1/2 Be) When chirping occurs. Δλ≈ 10GHz Reducing reflection, Isolator or reducing extinction ratio can reduces Δλ For external modulated lasers Δλ≈ 2.5 × bit rate

5.7.3 Dispersion Compensation Methods to reducing the impact of dispersion • External modulation (reduce chirping) • Small dispersion fiber • Dispersion compensation fiber

If 80km fiber with 17 ps/nm-km dispersion is used, we have 1360 ps/nm dispersion. Then 13.5km DCF fiber with 100 ps/nm-km can compensate the dispersion to zero as shown in Fig 5.20. However DCF fiber has high loss about 0.5dB/km 0.5dB/km × 13.6km=7dB Figure of merit (MOF) for DCF fiber is If the DCF fiber has -100 ps/nm-km dispersion and loss = 0.5dB/km then FOM = = 200 ps/nm-dB Larger FOM is desirable.

Chirped Fiber Bragg Gratings In a regular fiber, chromatic dispersion introduces larger delays for the lower frequency components in a pulse, we can design a chirped grating fiber with larger delays for the higher frequency components to compress the pulse.

5.7.4 Polarization-Mode Dispersion (PMD) Because of the ellipticity of the fiber core, different polarizations travel with different group velocities. Polarization changes with time. So PMD varies with time. The time-averaged differential time delay is given by