Optical- Dispersion Flashcards
What are telecommunications digitally coded as?
Light pulses
How does height of pulses change over distance travelled?
Decreases exponentially due to loss of intensity by absorption and scattering
What is dispersion?
Where light signals spread out as they travel because of many different light paths
How do short pulses change from being launched to emerging?
Height much less and broader. Over longer distance broader and signals begin to merge. Even longer, light still emerges but information from original signal is lost as pulses merge to make flat line
Formula for extra path length with distance of 1 along core for maximum end-to-end light path
(1/sinC)-1=(1-sinC)/sinC
Extra fractional length is (n2-n1/n1)
C is critical angle
How is dispersion measured?
Measure spread of arrival time of signal after 1km
What is bandwidth?
The information carrying capacity (MHz) times the fibre length (km) and is the reciprocal of the dispersion (s/km).
How does bandwidth vary with length?
Optical loss and dispersion increase with length so bandwidth decreases with length
What unit is MHz equal to?
Mbits/s
How to improve bandwidth using refractive indexes
Reducing refractive index differences between core and cladding. Resulting data transmission rates still too slow to be useful just using this to improve it.
Graded fibre index
Aims to get light travelling in different paths to have same average velocity along fibre. Have low refractive index at the outside, medium closer to centre and high at centre. This means high velocity near the edge of the core and lower one near the centre. Requires a parabolic refractive index profile. Can improve transmission rates over 1km to GHz. Hard to make exact profile needed.
What happens if the core diameter is reduced to the magnitude of the wavelength of light?
Need to use wave equations for electrons in atoms. Only limited number of solutions (propagation modes) for the light exist. The light electric field not tightly confined to the core and penetrates a significant distance into cladding so need much thicker cladding compared with cheap, low-grade fibre
What is basic propagation mode?
1st order mode. Corresponds to light travelling straight along fibre core. The condition that this is the only mode (fibre is monomode) is:
V=2πrxn.a./λ < 2.405
This eliminates signal dispersion
Can achieve transmission rates of 1000GHz for 1km
What increases number of modes?
Increasing fibre core diameter. Increasing Δn
Other factors leading to dispersion and how to counteract them
Material dispersion (variation of refractive index with wavelength) Waveguide dispersion (dépendent on exact refractive index profile of fibre) Material dispersion close to 0 at 1.3μm transmission window Waveguide dispersion can be designed to partially cancel material dispersion.
