Chapter 8 Basic System Design The basic system

Chapter 8 Basic System Design

+ The basic system design verification can be done through: 1. Power budget 2. Risetime budget. + The power budget involves the power level calculations from the transmitter to the receiver. 1. 2. 3. 4. Attenuation Coupled power Other losses Equalization penalty (DL) 5. 6. 7. 8. SNR requirements Minimum power at detector BER Safety margin (Ma)

The optical power budget is then assembled taking into account ALL these parameters. Pi = (Po + CL + Ma + DL) d. B where Pi = mean input power launched in the fiber Po = mean optical power required at the receiver CL = total channel loss DL =dispersion-equalization or ISI penalty, *The sensitivity of the detector is the minimum detectable power.

+ Risetime budget following: includes the 1. Risetime of the source, TS 2. Risetime of the fiber (dispersion), TF 3. Risetime of the amplifier, TA 4. Risetime of the detector, TD

The risetime budget is assembled as: Tsyst = 1. 1(TS 2 + TF 2 + TD 2 + TA 2)1/2 For non-return-to-zero (NRZ) data For return-to zero (RZ) data

Example 8. 1 We need to design a digital link to connect two points 10 -km apart. The bit rate needed is 30 Mb/s with BER = 10 -12. Determine whether the components listed are suitable for the link. Source: LED 820 nm Ga. As. Al; couples 12µW into 50µm fiber; risetime 11 ns Fiber: Step Index fiber; 50µm core; NA = 0. 24; 5. 0 d. B/km loss; dispersion 1 ns/km; 4 connectors with 1. 0 d. B loss per connector Detector: PIN photodiode; R = 0. 38 A/W; Cj = 1. 5 p. F, Id = 10 p. A; risetime = 3. 5 ns; minimum mean optical power = - 86 d. Bm Calculate also the SNR of the link if RL given is 5. 3 kΩ

Solution : For this example, 3 factors need to be considered: a) Bandwidth b) Power levels c) Error rate (SNR) Risetime Budget We start with the risetime budget. Assume using NRZ coding, the system risetime is given by: Also: Tsyst = 1. 1(TS 2 + TF 2 + TD 2)1/2

Now we can assemble the total system risetime: Total system risetime = 23. 3 ns Risetime of the source, TS = 11. 0 ns Risetime of the fiber (dispersion), TF 10 x 1. 0 ns = 10. 0 ns Allowance for the detector risetime, TD

Power Budget Total power launched into fiber = -19 d. Bm Losses: Fiber attenuation 5 d. B/km x 10 = 50 d. B 4 connectors 1 d. B x 4 = 4 d. B Power available at detector =[( -19 d. Bm – 50 d. B- 4 d. B)] = -73 d. Bm Since power available at the detector is – 73 d. Bm, the sensitivity of the detector must be less than this. The safety margin, Ma = -73 -(-86) d. B = 13 d. B

The choice of components are suitable because; a) b) TD calculated is greater than TD given Total power available at the detector is greater than the minimum power required by the detector i. e Ma is positive.

Example 8. 2 An optical link is to be designed to operate over an 8 -km length without repeater. The risetime of the chosen components are: Source: 8 ns Fiber: Intermodal 5 ns/km Intramodal 1 ns/km Detector 6 ns From the system risetime considerations estimate the maximum bit rate that may be achieved on the link using NRZ code.

Solution: Tsyst = 1. 1(TS 2 + TF 2 + TD 2) = 1. 1 [82 + (8 x 5)2 + (8 x 1)2 + 62)1/2] = 46. 2 ns Max bit rate = Maximum bit rate = 15. 2 Mbps Or 3 d. B optical BW = 7. 6 MHz

Example 8. 3 The following parameters were choosen for a long haul single mode optical fiber system operating at 1. 3µm. Mean power launched from laser = - 3 d. Bm Cabled fiber loss = 0. 4 d. B/km Splice loss = 0. 1 d. B/km Connector loss at transmitter and receiver = 1 d. B each Mean power required at the APD When operating at 35 Mbps(BER = 10 -9) -55 d. Bm When operating at 400 Mbps(BER = 10 -9) -44 d. Bm Required safety margin = +7 d. B

Estimate: a) b) c) maximum possible link length without repeaters when operating at 35 Mbps. It may be assumed that there is no dispersion-equalization penalty at this rate. maximum possible link length without repeaters when operating at 400 Mbps. the reduction in the maximum possible link length without repeaters of (b) when there is dispersionequalization penalty of 1. 5 d. B.

Solution a)35 Mbps Pi – Po = [(Fiber cable loss + Splice losses ) x L + Connector loss + Ma ]d. B [-3 d. Bm – (-55 d. Bm)] = (0. 4 + 0. 1)L + 2 + 7 0. 5 L = 52 – 2 -7 L = 86 km b) 400 Mbps Pi – Po = [(Fiber cable loss + Splice losses ) x L + Connector loss + Ma ]d. B [-3 d. Bm – (-44 d. Bm)] = (0. 4 + 0. 1)L + 2 + 7 0. 5 L = 41 – 2 -7 L = 64 km

c) Including dispersion-equalization penalty of 1. 5 d. B Pi – Po = [(Fiber cable loss + Splice losses ) x L + Connector loss + DL + Ma]d. B [-3 d. Bm – (-44 d. Bm)] = (0. 4 + 0. 1)L + 2 + 1. 5 + 7 0. 5 L = 41 – 2 -1. 5 - 7 L = 61 km Note: a reduction of 3 km in the maximum length without repeaters when DL is taken to account.

Example 8. 4 An optical link was designed to transmit data at a rate of 20 Mbps using RZ coding. The length of the link is 7 km and uses an LED at 0. 85µm. The channel used is a GRIN fiber with 50µm core and attenuation of 2. 6 d. B/km. The cable requires splicing every kilometer with a loss of 0. 5 d. B per splice. The connector used at the receiver has a loss of 1. 5 d. B. The power launched into the fiber is 100µW. The minimum power required at the receiver is – 41 d. Bm to give a BER of 10 -10. It is also predicted that a safety margin of 6 d. B will be required. Show by suitable method that the choice of components is suitable for the link.

Solution The power launched into the fiber Minimum power required at the receiver Total system margin Fiber loss 7 x 2. 6 Splice loss 6 x 0. 5 Connector loss Safety margin 100µW = -10 d. Bm - 41 d. Bm - 31 d. Bm 18. 2 d. B 3. 0 d. B 6. 0 d. B 28. 7 d. B Excess power margin = -31 d. Bm - 28. 7 d. B = 2. 3 d. Bm Based on the figure given, the system is stable and provides an excess of 2. 3 d. B power margin. The system is suitable for the link and has safety margin to support future splices if needed. .

Example 8. 5 An optical communication system is given with the following specifications: Laser: = 1. 55µm, = 0. 15 nm, power = 5 d. Bm, tr = 1. 0 ns Detector: t. D = 0. 5 ns, sensitivity = -40 d. Bm Pre-amp: t A = 1. 3 ns Fiber: total dispersion (M+Mg) = 15. 5 psnm-1 km-1, length = 100 km, = 0. 25 d. B/km Source coupling loss = 3 d. B Connector (2) loss = 2 d. B Splice (50) loss = 5 d. B System: 400 Mbps, NRZ, 100 km

Solution For risetime budget system budget, Tsyst = source fiber detector pre-amp for receiver, ts t. F t. D t. A = 1. 0 ns = = 0. 25 ns = 0. 5 ns = 1. 3 ns = = 1. 39 ns System risetime from (1), (2) and (3) = = 1. 73 ns = 1. 75 ns …(1) …(2) total …(3)

Since the calculated Tsyst is less than the available Tsyst the components is suitable to support the 400 Mbps signal. For the power budget: Laser power output Source coupling loss Connector loss Splice loss Attenuation in the fiber Total loss 5 d. Bm 3 d. B 2 d. B 5 d. B 25 d. B 35 d. B Power available at the receiver = (5 d. Bm -35 d. B) = -30 d. Bm The detector’s sensitivity is -40 d. Bm which is 10 d. B less. Therefore the chosen components will allow sufficient power to arrive at the detector. Safety margin is +10 d. B,