optical fiber concepts
TRANSCRIPT
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