Fiber Optic Communication Fundamentals

Fiber Optic Communication Fundamentals  Section

1 Fiber optics -- Snell’s law -- Total Internal Refraction -- Wave Theory -- Model Dispersion -- Material (Chromatic) Dispersion -- Advantage of Fiber Optic Communication

Section 2 Laser Physics -- Quantum Theory -- Spontaneous Emission -- Stimulated Emission -- Population Inversion -- Cavity Resonance -- Laser Beam

Section 3 Photo-detectors -- Photo-electron effect -- Photomultipliers -- Semiconductor Photodiodes

Section 1 Fiber Optics -- Significance of Optics Fiber * It keeps the light inside the fiber and sends it to where we want it to go Optics Fiber

Sun

Earth

-- Reflection and Refraction, Snell’ Law  Snell’s

Law * n1 Sinθ I = n2 Sinθ

r

n1 n2

-- Total Internal Refraction  Critical

Angle * The incidence angle which has 900 refraction angle n2 n1 * Sin θ r = (n1/ n2) Sinθ

I

-- Wave Theory  Maxwell’s

Equations * ∇ × E = - B/t * ∇ × H = D/t *∇ •D=0 *∇ •B=0 * Optical fiber is a wave guide. Different fiber just imposes different boundary conditions to the equations.

-- Model Dispersion  Modes

of optical fiber * One typical thing for modes is that they have different velocities of propagation along the fiber. mode 1 mode 2

-- Model Dispersion  Model

Dispersion, Multi-mode fiber * For multi-mode fiber, signal riding on difference modes for propagation. * The velocity difference of modes creates model dispersion

-- Model Dispersion  Single

mode fiber * Only one mode is allowed to exist for propagation. * The diameter of the core is small enough to only allow the fundamental mode to survive. * No model dispersion but the energy flow along the fiber is limited because of the small core diameter.

-- Material (Chromatic) Dispersion

white

Prism

red blue

•

Refraction Index is frequency dependent. Sin θ r = (n1(f)/ n2(f)) Sinθ I

•

V = C/n(f)

•

-- Material (Chromatic) Dispersion  Difference

of refraction index causes difference of propagation velocity of light along the fiber.  Even for a single mode fiber, if the light source is not monochromatic, there still exists dispersion due to frequency dependence of the refraction index of material.  Low line width laser beam is important as the light source of fiber optic communication.

-- Advantage of Fiber Optic Communication 

Huge Capacity * Capacity for one channel (correspondence to one wavelength) 1542nm wavelength, f=c/(1542nm) = 1.95 x 1014 Hz = 195 THz. Possible bit rate ≤ 2f. * Capacity due to DWDM

 

No Electrical Connection, No Electromagnetic Interference Distance between repeaters, regenerators *Direct connection distance cable typical bandwidth Thick-net coax 10-100Mbps Multimode fiber 100 Single-mode-fiber 100-2400Mbps

distance 500 2km 40km

Section 2 Laser Physics  LASER

*Light Amplified Stimulated Emission Radiation

-- Quantum Theory 

 

Light is particle which is composed of photons. * The energy of a single photon = hf. Internal energy of atoms and molecules are quantized called energy levels. A photon emission or absorption corresponding to a transition of energy levels. E2 * hf = E2 – E1 E1

-- Spontaneous Emission  Particles

(atoms, molecules) in their higher energy levels tend to jump back to the ground state (lower energy level states) by themselves.

-- Stimulated Emission  Jump

Stimulated by another photon whose energy equals to the gap of the energy levels. E2 hf = E2 – E1 E1

 Same

kind of photon, same direction.

-- Population Inversion  More

populated in higher energy level instead of ground state (lower energy levels).  Optical pump

-- Cavity Resonance  Selected

wavelength meets the condition of resonance of the cavity.

-- Laser beam  Same

frequency (narrow line width), same direction.

Section 3 Photo-detectors  Change

the photon signal to electrical.  Have region of wavelength sensitivity.

-- Photon electron effect  If

the energy of a photon is larger than the work function of the metal. Than one photon electron is created.



hf ≥ Eg

-- Photomultipliers  Avalanche

effect. Dynodes Anode

Cathode -v

Output

-- Semiconductor Photodiode  Depletion

Light

region and photo-current v p -

intrinsic

n +

d End-illuminated p-I-n photodiode

End