Review questions for HW 1


1. What is figure of merit for communication systems?

Date rate `times` distance or bandwidth `times` distance

2. What are the advantages of optical communication?

Wide bandwidth, compact fiber, immunity from EM interference.

3. Why a transparent optical network is better than an opaque optical network?

Transparent (all-optical) network avoids bottlenecks from E/O and O/E conversions as well as the limitation of 40GHz from electronic switching.

4. Name 2 multiplexing schemes used in optical network.

TDM and WDM

5. Name 3 approaches to increase network capacity.

1. increase number of cables or fibers.
2. increase bandwidth of a channel.
3. carry more channels per cable or fiber.

6. A multiplexing system has 2 channels with data rate of 10Gb/s and 5 channels with data rate of 5Gb/s. What is the minimal data rate for the system?

`2 times10G + 5 times 5G = 45`Gb/s

7. A system has a total bandwidth of 100GHz. Each channel requires 500MHz. How many channels can be supported by the system ideally?

# of Channels = 100G/0.5G = 200

8. A plane wave with $\omega=10^{14}$ rad/s propagates in air along x axis. Its H field in in y directions. Find i) its wave number and ii) polarization direction.

i) k = $\omega$ / c = $\frac {10^{14}} {3 \times 10^8} = 10^6/3$ (rad/m)
ii) z

9. A plane propagates in air and has electric field e = 5 $\cos ( 10^{11} t - k y ) \hat z$ (V/m). Find i) k, ii) amplitude of its magnetic field, iii) its average Poynting vector.

i) k = $\frac {10^{11}} {3 \times 10^8} = 10^3/3$ (rad/m)
ii) $H_o= E_o/ \eta$ = 5/377 = 0.01326 (A/m)
iii) $\vec S_{ave} = \hat y 0.5 |E_o|^2/ \eta = \hat y 0.5 5^2 / 377 = \hat y 0.03316(W/m^2)$

10. A laser emitting photons at wavelength of 1.5 micron. Find its photon energy in eV.

E(ev) = 1.24 / 1.5 = 0.8267 (eV)

11. If the laser in question 10 has linewidth of 1nm, find its bandwidth in Hz.

bandwidth = $\nu \frac {\Delta \lambda} \lambda = c \frac {\Delta \lambda} {\lambda^2} = \frac {3 \times 10^8 \times 10^{-9}} {(1.5 \times 10^{-6})^2} = 1.333 \times 10^{14}$ (Hz)

12. If the laser in question 10 has power of 1mW, how many photons are generated per sec?

# photons/sec = $P_{op}/E(J) = 10^{-3}/(q 0.8267)=7.56 \times 10^{15}$ (1/s)

13. If it takes 8 electrons to generate 1 photons, how much current is required to sustain the power in question 12?

I = q (# electrons/sec) = $q 8 \times 10^{-3}/(q 0.8267)$ = 9.677(mA)


Last Modified: January 29, 2018
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