EIE 696/ENE 623 Optical Communication Lecture 2

Optical loss or attenuation  P in = 1 mW, P out = 0.1 mW  L(dB) = 10 dB  P in = 1 mW, P out = mW  L(dB) = ___ dB

Optical loss or attenuation  L(dB) = 10  P out = 0.1 P in  L(dB) = 50  P out =

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 …..

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

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

Excess Loss (multiport devices)  For example,  Therefore, P out /P in = 0.8  loss  1 dB P out /P in = 0.5  loss  3 dB

Coherence length  The length in space corresponding to the bandwidth of the source’s spectrum. where v g = group velocity B = bandwidth

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 E x and E y.  Beat length is the length which wave travels before the phase shift completes a change of 2 .

Birefringence and beat length Source: ARC Electronics where L beat = beat length  = 2  n ‘0’ = ordinary wave ‘e’ = extraordinary wave

Fiber Network Topologies  Star Network  Linear Bus Network  Tree Network

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

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

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

 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. P1P1 P2P2 P4P4 P3P3 Directional Coupler

 Throughput loss (L THP ): This is the loss encountered in going straight through expressed in dB.  TAP loss (L TAP ): This is the loss encountered in crossing over expressed in dB. Directional Coupler

 Excess loss (L E ): If P 1  P 2 + P 3 then  Directionality loss (L D ): If P 4  0 then Directional Coupler

 Sometimes, DC is specified by their splitting ratio. (i.e. P 2 /P 3 ) - For example, splitting ratio = 1:1  P 2 /P 3 = 1;therefore, P 2 = P 3 splitting ratio = 8:1  P 2 /P 3 = 8;therefore, P 2 = 8P 3 Directional Coupler

 An ideal DC is one with P 1 = P 2 + P 3  L E = 0 dB P 4 = 0  L D =  dB  Suppliers provide DCs by describing their L TAP.  For example, L TAP = 3 dB implies P 3 is 3-dB down, or L TAP = 10 dB implies P 3 is 10-dB down. Directional Coupler

Example 1  Light source gives 10 7 photons/bit interval while a receiver requires at least 10 3 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?

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

Combiners  N x 1 coupler  Ideal case: P out = P in with multimode fiber at output  no excess loss.  With a single mode fiber, the best possible result is P out = P in /N. Excess loss will be 10log 10 N.

Splitters  1 x N coupler  (Pout) j = P in /N.  Loss = 10log 10 N for both single and multi-mode fibers.  Excess loss will be …. dB.

Tree Network Topology

Data Transmission Formats  WDMA = Wavelength Division Multiple Access  TDMA = Time Division Multiple Access  CDMA = Code Division Multiple Access

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.

Optical power and numbers of photons.  For λ = 1.24 μm, h ν = 1 eV or 1.6 x 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 electrons/s

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

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

Ex. Calculate transmittance, T, into fiber from air Sol n T + R = 1 air Fiber; n = 1.5 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”.

Fresnel’s laws of reflection 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. Reflections at plane boundary

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

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.

 Snell’s law: Numerical Aperture

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

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

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

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 3 rd Wave, 1980.

Optical Fibers  Refractive index (n): This relates to a phase velocity in a medium.  where c = speed of light in free space (air) = 3x10 8 m/s v = light velocity in any medium  Note: The frequency will not change when the medium is changed, but the wavelength will do.

Optical Fibers Snell’s law:

Optical Fibers