1. Optical Link Design for Digital Communication Systems

2. Fig: Simple point-to-point link
POINT-TO-POINT LINKS
The simplest transmission link is a point-to point line having a transmitter at one end and a receiver on the other, as shown below:
Fig: Simple point-to-point link
The design of an optical link involves many interrelated variables such as the fiber, source, and photodetector operating characteristics, so that the link design and analysis may require several iterations before they are working satisfactorily.

3. POINT-TO-POINT LINKS
The key system requirements needed in analyzing a link are:
1. The desired (or possible) transmission distance
2. The data rate or channel bandwidth
3. The bit error rate (BER)
To fulfill these requirements the designer has a choice of the following components and their associated characteristics:
1. Multimode or single-mode optical fiber
(a) Core size
(b) Core refractive-index profile
(c) Bandwidth or dispersion
(d) Attenuation
(e) Numerical aperture or mode-field diameter
2. LED or laser diode optical source
(a) Emission wavelength
(b) Spectral line width
(c) Output power
(d) Effective radiating area
(e) Emission pattern
(f) Number of emitting modes
3. pin or avalanche photodiode
(a) Responsivity (~quantum efficiency)
(b) Operating wavelength
(c) Speed
(d) Sensitivity

4. System Considerations:
Wavelength of operation: In carrying out a link power budget, we first decide at which wavelength to transmit and then choose components operating in this region. If the distance over which the data are to be transmitted is not too far, we may decide to operate in the 800- to 900-nm region. On the other hand, if the transmission distance is relatively long, we may want to take advantage of the lower attenuation and dispersion that occurs at wavelengths around 1300 or 1550 nm.
Source: The system parameters involved in deciding between the use of an LED and a laser diode are signal dispersion, data rate, transmission distance, and cost. The spectral width of laser output is much narrower than that of an LED. Since laser diodes typically couple from 10 to 15 dB more optical into a fiber than an LED, greater repeaterless transmission distances are possible with a laser. This advantage and the lower dispersion capability of laser diodes may be offset by cost constraints. Not only is a laser diode itself is expensive than an LED, but also the laser transmitter circuitry is much complex.

5. System Considerations:
(c) Photo Detector: In choosing a particular photodetector, we mainly need to determine the minimum optical power that must fall on the photodetector to satisfy the bit error rate (BER) requirement at the specified data rate. A pin photodiode receiver is simpler, more stable with changes in temperature, and less expensive than an avalanche photodiode receiver. In addition, pin photodiode bias voltages are normally less than 50 V, whereas those of avalanche photodiodes are several hundred volts. However, the advantages of pin photodiodes may be overruled by the increased sensitivity of the avalanche photodiode if very low optical power levels are to be detected.

6. System Considerations:
(d) Optical fiber: For the optical fiber we have a choice between single-mode and multi-mode fiber, either of which could have a step- or a graded-index core. This choice depends on the type of light source used and on the amount of dispersion that can be tolerated. Light-emitting diodes (LEDs) tend to be used with multimode fibers (although special LEDs called edge-emitting LEDs can launch sufficient optical power into a single-mode fiber for transmission at data rates up to 560 Mb/s over several kilometers. The optical power that can be coupled into a fiber from an LED depends on the core-cladding index difference , which, in turn, is related to the numerical aperture of the fiber (for  = 0.01 the numerical aperture NA = 0.21). As  increases, the fiber-coupled power increases correspondingly. However, since dispersion also greater with increasing , a tradeoff must be made between the optical power that can be launched into the fiber and the maximum tolerable dispersion.

7. System Considerations:
(d) Optical fiber (continued): Either a single-mode or a multimode fiber can be used with a laser diode A single-mode fiber can provide the ultimate bit-rate-distance product, with values of 30 (Gb/s).km being achievable. A disadvantage of single-mode fibers is that the small core size (5 to 16 um in diameter) makes fiber splicing more difficult and critical than for multimode fibers having 50 um core diameters.

8. Two types of design and analysis procedures are normally carried out for digital optical systems:
(I) Link power budget analysis
(power margin calculations between the transmitter and the receiver considering attenuation, connectors, splices and other losses)
(II) rise-time budget analysis
(rise time calculations and speed of response of system considering various dispersive effects)

9. (i) Link power budget analysis
The optical power received by the receiver depends on the power transmitted and on the various losses occurring over the fiber. The power received at the output must be sufficiently higher than the receiver sensitivity (after accounting for safety margin).

10. (i) Link power budget analysis
The optical power received by the receiver depends on the power transmitted and on the various losses occurring over the fiber. The power received at the output must be sufficiently higher than the receiver sensitivity (after accounting for safety margin).
Ptx = Prx + CL + Ms
where Ptx is the transmitter power, Prx is the sensitivity of the receiver, CL is the total link loss or channel loss (including fiber splice and connector loss), and Ms is the system’s safety margin.
Channel Loss may be expressed as :
CL = f L + con + splice
where f is the fiber loss (dB/km), L is the link length, con is the sum of the losses at all the connectors in the link, and splice is the sum of losses at all splices in the link.

11. Design Example:
An engineer plans to design a 2.5-Gbps SONET OC-48 (or equivalently, an SDH STM-16 link) link over a 30-km path length. For the 30-km cable span, there is a splice with a loss of 0.1 dB every 5 Km (a total of 5 splices). The engineer selects a laser diode that can launch -2 dBm of optical power into the fiber and an InGaAs avalanche photodiode (APD) with a -32 dBm sensitivity at 2.5 Gbps. A short jumper cable is needed at each end, assume that each cable introduces a loss of 1.5 dB. In addition, there is a 0.6 dB connector loss at each fiber joint (total of 4 connectors in all including two for jumper cables and two for fiber ends). The problem of the engineer is to determine whether he can operate the link at 1310 nm, or to use more costly 1550nm equipment. Find out.
(Take fiber attenuation as 0.6 dB/km and 0.3 dB/km at 1310 nm and 1550 nm respectively.)

12. Spreadsheet for calculating the 1310-nm SONET link power budget
Solution:
Spreadsheet for calculating the 1310-nm SONET link power budget
Component/Loss parameter Output/sensitivity/loss Power margin, dB
Coupled laser diode output dBm
APD sensitivity at 2.5 Gbps -32 dBm
Allowed loss (-2 -(-32))
Jumper cable loss (2 x 1.5 dB) dB
Splice loss (5 x 0.1 dB) dB
Connector loss (4 x 0.6 dB) dB
Cable Attenuation (30 km) -18 dB (final margin)
Since the final margin is adequate (keeping in view the safety margin which is usually 3 dB), it is feasible to operate this link at 1310 nm from the point of view of power calculations (rise-time budget needs to be also calculated to ascertain final suitability).

13. (ii) Rise-time budget analysis
Rise time budget analysis deals with calculating the temporal response of the various system components.
The system design considerations must take into account the temporal response of the system components, because the temporal response is directly related to the pulse dispersion and hence the bit rate of the optical fiber channel.
The finite bandwidth of the optical system may result in overlapping of the received pulses or ISI, giving a reduction in sensitivity at the optical receiver. Therefore, either a worse BER must be tolerated, or the ISI must be compensated by equalization within the receiver.

14. What is rise time?
It is used to calculate the dispersion limitation of an optical fiber link. This is particularly useful in digital systems at higher bit rates. The rise time of a linear system tr is the time in which output changes from 10% to 90% of the maximum output value when the input is a step function.

15. Transfer function of RC circuit
Therefore the 3 dB bandwidth for the circuit is:
Combining this equation with tr, the 3 dB bandwidth for the circuit is:

16. In a fiber optic communication system, the three building blocks (i. e
In a fiber optic communication system, the three building blocks (i.e. the transmitter, the fiber (channel) and the receiver) each has its own response time (or rise time) associated with it.
The total rise time of the system tsys is given as:
tsys = [ ttx2 + tf2 + trx2 ] 1/2
The rise time of the optical fiber includes contributions from intermodal dispersion (tintermodal) and intramodal dispersion (tintramodal) through the relation: tf = [ tintermodal2 + tintramodal2 ] 1/2
For a low-pass system like an fiber optic link (analogous to an RC circuit), the total rise time and the bandwidth f are related by the standard expression: tsys = 0.35 / f
For an RZ format, f = B and for NRZ format, f = B/2, where B=bit rate.
Therefore, for digital systems, tsys should be below its maximum value, i.e.

17. Thus the upper limit on tsys should be less than 35% of the bit interval for an RZ pulse format and less than 70% of the bit interval for an NRZ pulse format.

18. Design Example:

19. The
Let us calculate the total rise time tsys.
we get

20. Power Penalties (Advanced issue in Power Budget Analysis)
More sophisticated power budget calculations will include power penalties.
A power penalty is defined as the increase in receiver power needed to eliminate the effect of some undesirable system noise or distortion
Typically, power penalties may result from:
Dispersion.
Dependent on bit rate and fibre dispersion,
Typical dispersion penalty is 1.5 dB
Reflection from passive components, such as connectors.
Crosstalk in couplers.
Modal noise in the fibre.
Polarization sensitivity.
Signal distortion at the transmitter (analog systems only).

21. Dispersion Penalty Defined as:
The increase in the receiver input power needed to eliminate the degradation in the BER caused by fibre dispersion
Typical value is about 1.5 dB.
Several analytic rules exist:
Low pass filter approximation rule
Allowable pulse broadening (Bellcore) rule

22. Dispersion Penalty
Defined as the increase in the receiver input power needed to eliminate the degradation caused by dispersion
Defined at agreed Bit Error Probability, typically 1 x 10-9
In the sample shown the receiver power levels required at 1 x 10-9 with & without dispersion are dBm &
-33.1 dBm respectively
The dispersion penalty is thus 2.1 dB

23. Low pass filter approximation rule for the Dispersion Penalty
Simple analytic rule of thumb for calculating the dispersion penalty Pd
Based on two assumptions:
that dispersion can be approximated by a low pass filter response.
the data is the dotting pattern.
B is the bit rate in bits/sec and Dt is the total r.m.s impulse spread caused by dispersion over the fibre.
To keep Pd < 1.5 dB, the B.Dt product must be less than 0.25 approximately.

24. Calculating the Dispersion Penalty (Low pass filter approx rule)
Total Chromatic Dispersion, Dt = Dc x S x L
where:
Dc is the dispersion coefficent for the fibre (ps/nm.km)
S is transmitter source spectral width (nm)
L is the total fibre span (km)
􀁹 Assuming single-mode fibre so there is no modal dispersion
􀁹 Does not include polarization mode dispersion
􀁹 Typically the dispersion coefficent will be known
􀁹 For Eg., ITU-T Rec.G.652 for single-mode fibres at 1550 nm (typical):
􀂾 Attenuation < 0.25 dB/km
􀂾 Dispersion coefficent is 18 ps/(nm.km)
􀁹 Let’s take 50 km of single-mode fibre meeting ITU G.652
􀁹 Let’s take 1550 nm DFB laser with a spectral width of 0.1 nm

25. Calculating the Dispersion Penalty (Low pass filter approx rule)
Total Dispersion, Dt = Dc x S x L
= 18 ps/nm.km x 0.1 nm x 50 km
= 90 ps total dispersion
System operating at 2.5 Gbits/sec
Total Dispersion, Dt = 90 ps
Dispersion Penalty:
The Penalty is thus = 1.2 dB

26. Calculating the Dispersion Penalty graphically
(Low pass filter approx rule)

27. Dispersion Penalty for STM-1 (155.52 Mbps)

28. Dispersion Penalty for STM-4 (622.08 Mbps)

29. Dispersion Penalty for STM-16 (2488.32 Mbps)

30. EXAMPLE: Calculating the Dispersion Penalty
(Low pass filter approx rule):
EXAMPLE:
􀀹 An optical fibre system operates at 1550 nm at a bit rate of 622 Mbits/sec over a distance of 71 km
􀀹 Fibre with a worst case loss of 0.25 dB/km is available.
􀀹 The average distance between splices is approximately 1 km.
􀀹 There are two connectors and the worst case loss per connector is 0.4 dB.
􀀹 The worst case fusion splice loss is 0.07 dB
􀀹 The receiver sensitivity is -28 dBm and the transmitter output power is +1 dBm
􀀹 The source spectral width is 0.12 nm and the fibre dispersion meets ITU recommendations at 1550 nm
􀀹Determine worst case power margin, taking account of a power penalty
(Use the Low Pass Filter Approximation rule)

31. EXAMPLE: Calculating the Dispersion Penalty
(Low pass filter approx rule):
EXAMPLE:
Step 1: Find the Dispersion Penalty
􀁹 71 km of singlemode fibre meeting ITU G.652
􀁹 1550 nm DFB laser with a spectral width of 0.12 nm
􀁹 System operating at 622 Mbits/sec
Total Dispersion = ps
Dispersion Penalty = 0.2 dB

32. EXAMPLE: Calculating the Dispersion Penalty
(Low pass filter approx rule):
EXAMPLE:
Step 2: Develop the Power Budget and find the power margin

33. System Performance with varying bit rate

34. Receiver sensitivities vs bit rate
The figure shows receiver sensitivities as a function of bit rate. The Si pin, Si APD, and InGaAs pin curves are for a 10-9 BER. The InGaAs APD curve is for a BER.

35. Transmission distance vs data rate First Window Transmission Distance
Transmission-distance limits as a function of data rate for an 800-MHz.Km fiber, a combination of an 800-nm LED source with a Si pin photodiode, and an 800-nm laser diode with a Si APD.

36. Transmission distance vs data rate First Window Transmission Distance
Figure shows the attenuation and dispersion limitation on the repeater-less transmission distance as a function of data rate for the short-wavelength (770 to 900-nm) LED/pin combination. The BER was taken as 10-9 for all data rates. The fiber-coupled LED output power was assumed to be a constant -13dBm for all data rates up to 200 Mb/s. The attenuation limit curve was then derived by using a fiber loss of 3.5 dB/km and the receiver sensitivities shown in earlier Fig. Since the minimum optical power required at the receiver for a given BER becomes higher for increasing data rates, the attenuation limit slopes downward to the right.

37. Transmission distance vs data rate First Window Transmission Distance
The dispersion limit depends on material and modal dispersion. Material dispersion at 800 nm is taken as 0.07 ns/(nm.km) or 3.5 ns/km for an LED with a 50-nm spectral width. The curve shown is the material dispersion limit in the absence of modal dispersion. This limit was taken to be the distance at which tmat is 70 percent of a bit period. The modal dispersion limit was taken to be the distance at which tmod is 70 percent of a bit period. The achievable repeaterless transmission distances are those that fall below the attenuation limit curve and to the left of the dispersion line, as indicated by the hatched area. The transmission distance is attenuation-limited up to about 40 Mb/s, after which it becomes material-dispersion-limited.

38. Transmission distance vs data rate First Window Transmission Distance
Greater transmission distances are possible when a laser diode is used in conjunction with an avalanche photodiode. Let us consider an AlGaAs laser emitting at 850 nm with a spectral width of 1 nm which couples 0 dBm (1 mW) into a fiber flylead. The receiver uses an APD with a sensitivity depicted in Fig. depicted earlier. In this case the material-dispersion-limited curve lies off the graph to the right of the modal-dispersion-limit curve, and the attenuation limit (with an 8-dB system margin) is as shown in Fig. above. The achievable transmission distances now include those indicated by the shaded area.

39. Transmission distance vs data rate Third Window Transmission Distance
NRZ
Transmission-distance limits as a function of data rate for 1550-nm laser diode, an InGaAs APD, and a single-mode fiber with D = 2.5 ps/(nm . km) and a 0.3-dB/km attenuation.

40. Transmission distance vs data rate Third Window Transmission Distance
NRZ
For the third window (1550 nm), let us examine a single-mode link operating at 1550 nm. In this case the dispersion in the fiber is due only to GVD effects, since there is no modal dispersion. We take the dispersion to be D = 2.5 ps/(nm . km) and the attenuation to be 0.30 dB/km at 1550 nm. For the source we first choose a laser which couples 0 dBm of optical power into the fiber and which has a large spectral width ( = 3.5 nm). Then we select a laser with  = 1 nm as a second example. The receiver can use either an InGaAs avalanche photodiode (APD) or a InGaAs pin diode.

41. Transmission distance vs data rate Third Window Transmission Distance
The attenuation-limited transmission distances for these two photodiodes are shown in Fig. with the inclusion of an 8-dB system margin.
For the dispersion limit we examine two cases. First, for the ( = 3.5 nm) case, Fig. shows limit for NRZ data where the product DL is equal to 70 percent of the bit period. Second, for RZ data the product DL is equal to 35 percent of the bit period. These curves are for the ideal case. In reality various noise effects leading to laser instabilities coupled with chromatic dispersion in the fiber can decrease the dispersion-limited distance.

42. Assignments
Reference Books (pl see annexure)
Quiz (expected next week)