Optical Fibres

22
COMMUNICATION OPTIC FIBRE TRANSMISSION

description

Slideshow on optical fibre

Transcript of Optical Fibres

Page 1: Optical Fibres

COMMUNICATION

OPTIC FIBRE TRANSMISSION

Page 2: Optical Fibres

TOTAL INTERNAL REFLECTION

Page 3: Optical Fibres

Total Internal Reflection

• When a ray of light travels from a denser to a rarer medium such that the angle of incidence is greater than the critical angle, the ray reflects back into the medium. This is called total internal reflection.

Page 4: Optical Fibres

Optical Fibres

• Consists of a very thin glass core surrounded by a material of slightly lower refractive index called cladding

• The thin fibre can be bent without breaking and a ray of light can be sent down the fibre’s core

• Total internal reflection takes place at the boundary of the core and the cladding

Page 5: Optical Fibres

Acceptance angle

• The maximum angle of incidence that a ray can make that will result in total internal reflection is called acceptance angle

Page 6: Optical Fibres

PROBLEM

1. The refractive index of the core of an optical fibre is 1.50 and that of the cladding is 1.40.Calculate the acceptance angle of the fibre. Ans:330

2. The refractive index of the core of an optical fibre is 1.50 and the critical angle of the core- cladding boundary is 750.Calculate the refractive index of the cladding. Ans: 1.45

Page 7: Optical Fibres

Material Dispersion

• Light of different wavelengths have different refractive index and hence come out of the fibre at different times. This is called material dispersion

Page 8: Optical Fibres

Modal Dispersion

Page 9: Optical Fibres

Modal dispersion

• Rays that undergo many internal reflections are said to follow a high order mode paths

• Rays undergoing fewer reflections follow low order mode paths

• Set of rays having same wavelength reach the end at different times due to different paths taken. This is called modal dispersion

Page 10: Optical Fibres

Monomode & Multimode fibre

• In multimode fibres, the core has a diameter of about 100μm and the cladding is about 20μm thick

• The rays passing through multimode fibres undergo material as well as modal dispersion

• In monomode fibres, the core has a diameter of about 8 -10μm and the cladding is about 125μm thick.

• Rays follow just one path eliminating modal dispersion

Page 11: Optical Fibres

Step Index fibre

• The refractive index of core is constant• The refractive index of cladding is constant• The refractive index of cladding is slightly

lower than that of core

Page 12: Optical Fibres

Graded index fibre

• Refractive index of core decreases smoothly from the centre to the outer edge

• Refractive index of cladding is constant

Page 13: Optical Fibres
Page 14: Optical Fibres

ATTENUATION

• Attenuation in an optic fibre is caused by the impurities of the glass core. The amount of attenuation depends on the wavelength of the light being transmitted.

• Power loss in decibels is defined as• Power loss = 10log (Pfinal / Pinitial) in dB• Thus a power loss of 16 decibels means

that the initial power of, say,8.0mW has been reduced to 0.2mW

Page 15: Optical Fibres

PROBLEM

3. An amplifier amplifies an incoming signal of power 0.34mW to a signal of power 2.2mW.Calculate the power gain of the amplifier in decibels. Ans: 8.1dB

4. A signal of power 12mW is input to a cable of specific attenuation 4.0 dB/km. Calculate the power of the signal after it has travelled 6.0km in the cable. Ans: 0.048mW

Page 16: Optical Fibres

VARIATION OF SPECIFIC ATTENUATION WITH

WAVELENGTH

Page 17: Optical Fibres

Attenuation & Wavelength

• The specific attenuation ( power loss in dB per unit length ) actually depends on the wavelength of the radiation travelling along the optic fibre

• The graph shows minima at 1310nm and 1550nm, which implies that these are desirable wavelengths for optimal transmission

• These are infra red wavelengths

Page 18: Optical Fibres

DETECTION

• The light that enters an optic fibre travels down the length of the fibre and the arrival of light is registered by a photodiode

• In the absence of any light, falling on the photodiode, the current is zero

• When light of a specific wavelength falls on the photodiode, a current flows. The magnitude of the current is proportional to the intensity of light

Page 19: Optical Fibres

A light detector circuit with a photodiode

Page 20: Optical Fibres

NOISE

• Source of noise in a cable:• Random motion of electrons which creates

additional electric fields contaminating the signal. This increases with temperature.

• Lightning• Charged particles emitted by the sun

during intense solar activity

Page 21: Optical Fibres

NOISE IN OPTICAL FIBRES

• Main source is the dark current of the photodiode. This is the small current that flows even when the photodiode is dark

• Signal to noise ratio (SNR) is defined as • SNR = 10log Psignal / Pnoise

Page 22: Optical Fibres

PROBLEM

• The minimum SNR considered acceptable for a certain signal is 30dB.If the power of the noise is 2.0mW, calculate the least acceptable signal power.

• The SNR in a certain signal is 10dB.the signal passes through an amplifier of gain 6.0dB.What will be the signal to noise ratio after amplification?