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 • 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 • 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
When impairments appear, let denote the received powers and noise standard deviations, at the same SNR, we have
5.3 Transmitter Design parameters • Output optical power (laser output, eye safety) • Rise/fall time • Extinction ratio (ER) • Modulation type • Side-mode suppression ratio • Relative intensity noise (RIN) • Wavelength stability and accuracy
In general the output power of a laser is about 1mw~10mw Output power of optical amplifiers~50mw ER is defined as
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
At the first a few stages, the input power (signal + noise) to a stage increases as the number of stages increases, consequently, the amplifiers begin to saturate and gain drops. (Fig 5.3)
It reached steady-state condition where the amplifier output power, , and gain remain the same from stage to stage The total input power +ASE = the total output power
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
5.5.5 Power Transients and Automatic Gain Control If some of channels fail, input power and the amplifier gain In Fig 5.7 λ8 will be amplified unusually => receiver overloaded => We need an AGC.
(b) (c) monitoring wavelength
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.
In the worst case (polarization matched, out of phase) Let be the average received signal power and be the average crosstalk power from other signal channel. The electrical field at the receiver is
5.6.2 Interchannel Crosstalk Source：leakage of filter, switches, Mux/Demux
5.6.3 Crosstalk in Networks Crosstalk may accumulate.
5.6.4 Bidirectional Systems The near end crosstalk is more severe than the far end crosstalk.
5.6.5 Crosstalk reduction • For switches 1. better switch device 2. spatial dilation 3. wavelength dilation
For Mux/Demux add a filter between the demux and the mux
5.6.6 Cascaded Filters Required 1. wavelength stability 2. wavelength accuracy
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.
For WDM systems, we need to use a different grating for each wavelength as shown in Fig 5.22.
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