Fiber Optics Basics

Principal Fiber Optic Transmission

the electrical signal processing is according to international standards

the conversion into the "optical frequency band" enables to use the advantages coming up with F.O. transmission

electricalsignalprocessing

ElectricalTransmission

E / O -Conversion

ElectricalTransmission

O / E -Conversion

electricalsignalprocessing

OpticalTransmission

Fiber as transmission medium

Fiber Principles

A ray of light enters into the fiber at a small angle .

The capability (maximum acceptable value) of the fiber cable to receive light on its core is determined by its numerical aperture NA:

Fiber Principles

•where: 0: maximum angle of acceptance(i.e limit between reflection and refraction)

•n1: core refractive index•n2: cladding refractive index

Light propagation.

• If > 0: the ray is fully refracted and not captured by the core.

• If < 0: the ray is reflected and remains in the core.

Advantage of F.O. Transmission

Enormous bandwith --> Broadband services Very low attenuation --> Long repeater distance No crosstalk --> Immunity,

good quality Resistance against --> Widely applicable

environment NO RFI, EMI --> High

reliability Leight weight of fiber --> Airplane, Sea... SiO2 --> No

availability restrictions

FormulaPhase velocity c c = x f wavelength

f frequency

Phase velocity c0 c0 = 300 000 Km/sin vacuum

Refractiv index ni ni = c0 : ci

ni refractiv index in medium ic0 velocity in vacuumci velocity in medium i

Snell´s law (refraction law)

sin 1 / sin 2 = n2 / n1 = c1 / c2 1 angle of incident in medium 12 angle of transition medium 2n1 refractive index in medium 1n2 refractive index in medium 2 c1 velocity in medium 1c2 velocity in medium 2

Factors Causing Attenuation Light Absorption: Intrinsic absorptions (due to

fiber material and molecular resonance) and extrinsic absorptions (due to impurities such as OH- ions at around 1240 nm and 1390 nm). In modern fibers, extrinsic factors are almost negligible.

Rayleigh scattering: Scattering causes the light energy to be dispersed in all directions, with some of the light escaping the fiber core.

Bending losses Micro bending: Caused by light escaping the core due to

imperfections at the core/clad boundary Internal angle of acceptance: The angle of

incidence of the light energy at the core/cladding boundary exceeding the Numerical Aperture

Macro bending: Bending of the fiber

Types of Dispersion

Modal dispersion: when a very short pulse is injected into the fiber within the numerical aperture, all of the energy does not reach the end of the fiber at the same time. Different modes of oscillation carry energy down the fiber down different paths and thus travel further.

Chromatic dispersion: the pulse sent down the fiber is usually composed of a small spectrum of wavelengths. This means they go through the fiber at different speeds.

Fiber Types: Multimode Fiber

Fiber Types: Single mode Fiber

Transmission tests End-to-end optical link loss Rate of attenuation per unit length Attenuation contribution to splices,

connectors, couplers (events) Length of fiber or distance to an event Linearity of fiber loss per unit length

(attenuation discontinuities) Reflectance or optical return loss

Attenuation of Different Fiber Components 0.2 dB/km for single mode fiber loss at

1550 nm; 0.35 dB/km for single mode fiber loss at

1310 nm; 0.05 dB for a fusion splice 0.1 dB for a mechanical splice; 0.2 - 0.5 dB for a connector pair; 3.5 dB for a 1 to 2 splitter (3 dB splitting

loss plus 0.5 dB excess loss).

The OTDR depends on two types of optical phenomena: Rayleigh Backscattering and Fresnel Reflections:

Rayleigh scattering is intrinsic to the fiber material itself and is present along the entire length of the fiber.

Fresnel reflections are "point" events and occur only where the fiber comes in contact with air or another media such as at a mechanical connection/splice or joint.

Principles of an OTDR

Rayleigh scattering

Fresnel reflection

OTDR block diagram

OTDR Components Laser diodes: Laser diodes are selected according to the

wavelength of the test. Pulse generator with laser diode: A pulse generator controls

a laser diode which sends powerful light pulses (from 10 mW to 1 Watt) into the fiber.

Photodiode: OTDR photodiodes are especially designed to measure the extremely low levels of backscattered light, at 0.0001% of what is sent by the laser diode.

Time base and control unit: The control unit is the brain of the OTDR. It takes all the acquisition points, performs the averaging, plots them as a log. function of time and then displays the resulting trace on the OTDR screen. The time base controls the pulse width, the spacing between subsequent pulses and the signal sampling.

OTDR specifications Dynamic range: The dynamic range is one of the most

important characteristics of an OTDR, since it determines the maximum observable length of a fiber and therefore the OTDR suitability for analyzing any particular network.

Dead Zone: The length of fiber which is not fully characterized during the recovery period after an event is termed the dead zone.

Resolution Accuracy Wavelength