EIE 696/ENE 623 Optical Communication

1. EIE 696/ENE 623 Optical Communication Lecture 2

2. Optical loss or attenuation • Pin = 1 mW, Pout = 0.1 mW L(dB) = 10 dB • Pin = 1 mW, Pout = 10-5mW  L(dB) = ___ dB

3. Optical loss or attenuation • L(dB) = 10  Pout = 0.1 Pin • L(dB) = 50  Pout =

4. Optical loss or attenuation • Fiber loss at 1550 nm is about 0.2 dB/km If the length is 100 km, the loss will be …..

5. dBm • Decibels with respect to 1 mW. • For example, P = 1mW  -30 dB = ……. dBm

6. Excess Loss (multiport devices) • Multiport device such as directional coupler.

7. Excess Loss (multiport devices) • For example, • Therefore, Pout/Pin = 0.8  loss  1 dB Pout/Pin = 0.5  loss  3 dB

8. Coherence length • The length in space corresponding to the bandwidth of the source’s spectrum. where vg = group velocity B = bandwidth

9. Birefringence and beat length • In a long fiber, a slight birefringence causes a shift of relative phase shift. This leads to a change in relative strengths of Ex and Ey. • Beat length is the length which wave travels before the phase shift completes a change of 2.

10. Birefringence and beat length where Lbeat= beat length = 2n ‘0’ = ordinary wave ‘e’ = extraordinary wave Source: ARC Electronics http://www.arcelect.com/fibercable.htm

11. Fiber Network Topologies • Star Network • Linear Bus Network • Tree Network

12. Star Network Source: Fiber Optic Network Paul E. Green, Prentice Hall.

13. Fiber Network Topologies • For an ideal star coupler (with no excess loss), power splits equally among terminals.

14. Linear Bus Network • Directional couplers are used to tap data from the bus.

15. Directional Coupler • Consider case of input at port 1 • Not all input at port 1, necessarily emerges at remaining ports, this leads to definition of losses encountered in DC. P1 P2 P3 P4

16. Directional Coupler • Throughput loss(LTHP): This is the loss encountered in going straight through expressed in dB. • TAP loss (LTAP): This is the loss encountered in crossing over expressed in dB.

17. Directional Coupler • Excess loss (LE): If P1 P2 + P3 then • Directionality loss (LD): If P4 0 then

18. Directional Coupler • Sometimes, DC is specified by their splitting ratio. (i.e. P2/P3) - For example, splitting ratio =1:1 P2/P3 = 1;therefore, P2= P3 splitting ratio =8:1 P2/P3 = 8;therefore, P2= 8P3

19. Directional Coupler • An ideal DC is one with P1 = P2 + P3  LE = 0 dB P4 = 0 LD =  dB • Suppliers provide DCs by describing their LTAP. • For example, LTAP = 3 dB implies P3 is 3-dB down, or LTAP = 10 dB implies P3 is 10-dB down.

20. Example 1 • Light source gives 107 photons/bit interval while a receiver requires at least 103 photons/bit interval. If a star coupler and directional coupler used in this network are having excess loss of 10 dB and 1 dB, respectively. How many terminals could this network have?

21. Power Budget Source: Optical Fiber Communications, G.Keiser, McGraw Hill.

22. Combiners • N x 1 coupler • Ideal case: Pout = Pin with multimode fiber at output  no excess loss. • With a single mode fiber, the best possible result is Pout = Pin /N. Excess loss will be 10log10N.

23. Splitters • 1 x N coupler • (Pout)j = Pin/N. • Loss = 10log10N for both single and multi-mode fibers. • Excess loss will be …. dB.

24. Tree Network Topology

25. Data Transmission Formats • WDMA = Wavelength Division Multiple Access • TDMA = Time Division Multiple Access • CDMA = Code Division Multiple Access

26. Optical power and numbers of photons. • It is important to understand the relationship between an optical power and number of photons/time or number of photons/bit.

27. Optical power and numbers of photons. • For λ = 1.24 μm, hν = 1 eV or 1.6 x 10-19 J. • This replies that 1 W of optical power give the same number of photons per sec as 1 A of electrons per second. • 1 A = 1/e = 6.3 x 1018 electrons/s

28. Reflections at plane boundary • Normal incidence The reflection coefficient, , can be written as where  = the ratio of the reflected electric field to the incident electric field

29. Reflections at plane boundary Reflectance or Reflectivity *From the conservation of energy, R + T = 1 where T = transmittance.

30. Reflections at plane boundary Ex. Calculate transmittance, T, into fiber from air Soln T + R = 1 air Fiber; n = 1.5

31. Reflections at plane boundary • Oblique incidence • If the electric field is polarized perpendicular to the incident plane, it is called “s-polarization”. • If the electric field is polarized parallel to the plane of incidence, this is called “p-polarization”.

32. Reflections at plane boundary Fresnel’s laws of reflection

33. Reflections at plane boundary • Zero reflection (R=0) occurs only for the p-polarization at the angle called “Brewster angle”. • There is no incident angle that will make s = 0.

34. Reflections at plane boundary • In case of =1, which occurs at • Therefore, critical angle can be found as

35. Numerical Aperture • NA identifies the largest angle which light can be coupled to the waveguide, so that rays will be guided as modes in the waveguide.

36. Numerical Aperture • Snell’s law:

37. Numerical Aperture • As we know no TIR for  < c (cutoff at  = c): cis a critical angle.

38. Optical Fibers Single-mode fiber Multi-mode fiber Source: Fiber Optic Network Paul E. Green, Prentice Hall.

39. Optical Fibers • Three properties of fibers give them an edge over other media as a communication technology • Large bandwidth • Low attenuation • Small size • Immune to EM

40. Optical Fibers • Fibers are made from one of the most plentiful materials on earth which is ……… • This is a win-win situation in both cost and environment. • “The same ton of coal required to produce 90 miles of copper wire can turn out 80,000 miles of fiber.” A. Toffler, The 3rd Wave, 1980.

41. Optical Fibers • Refractive index (n): This relates to a phase velocity in a medium. • where c = speed of light in free space (air) = 3x108 m/s v = light velocity in any medium • Note: The frequency will not change when the medium is changed, but the wavelength will do.

42. Optical Fibers Snell’s law:

43. Optical Fibers