Optical Networks - _Problems4_70

Optical Networks - _Problems4_70 - Problems 355...

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Problems 355 transmission and, at the time of this writing, is increasingly being deployed in long- haul networks. Problems 5.1 In an experiment designed to measure the attenuation coefficient α of optical fiber, the output power from an optical source is coupled onto a length of the fiber and measured at the other end. If a 10-km-long spool of fiber is used, the received optical power is 20 dBm. Under identical conditions but with a 20-km-long spool of fiber (instead of the 10-km-long spool), the received optical power is 23 dBm. What is the value of α (in dB/km)? If the source-fiber coupling loss is 3 dB, the fiber-detector coupling loss is 1 dB, and there are no other losses, what is the output power of the source (expressed in mW)? 5.2 The following problems relate to simple link designs. Assume that the bit rate on the link is 1 Gb/s, the dispersion at 1.55 μ m is 17 ps/nm-km, and the attenuation is 0.25 dB/km, and at 1.3 μ m, the dispersion is 0 and the attenuation is 0.5 dB/km. (Ne- glect all losses except the attenuation loss in the fiber.) Assume that NRZ modulation is used. (a) You have a transmitter that operates at a wavelength of 1.55 μ m, has a spec- tral width of 1 nm, and an output power of 0.5 mW. The receiver requires 30 dBm of input power in order to achieve the desired bit error rate. What is the length of the longest link that you can build? (b) You have another transmitter that operates at a wavelength of 1.3 μ m, has a spectral width of 2 nm, and an output power of 1 mW. Assume the same receiver as before. What is the length of the longest link that you can build? (c) You have the same 1.3 μ m transmitter as before, and you must achieve an SNR of 30 dB using an APD receiver with a responsivity of 8 A/W, a gain of 10, an excess noise factor of 5 dB, negligible dark current, a load resistance of 50 ± , and an amplifier noise figure of 3 dB. Assume that a receiver bandwidth of B/ 2 Hz is sufficient to support a bit rate of B b/s. What is the length of the longest link you can build? (d) Using the same 1.3 μ m transmitter as before, you must achieve an SNR of 20 dB using a pin receiver with a responsivity of 0.8 A/W, a load resistance of 300 ± , and an amplifier noise figure of 5 dB. Assume that a receiver bandwidth of 2 Hz is sufficient to support a bit rate of B b/s. What is the length of the longest link you can build? 5.3 Compute the dispersion-limited transmission distance for links with standard single- mode fiber at 1550 nm as a function of the bit rate (100 Mb/s, 1 Gb/s, and 10 Gb/s)
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356 Transmission System Engineering for the following transmitters: (a) a Fabry-Perot laser with a spectral width of 10 nm, (b) a directly modulated DFB laser with a spectral width of 0.1 nm, and (c) an externally modulated DFB laser with a spectral width of 0.01 nm. Assume that the modulation bandwidth equals the bit rate and the dispersion penalty is 2 dB. Assume that NRZ modulation is used.
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Optical Networks - _Problems4_70 - Problems 355...

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