Optical- Simple Optical Fibres Flashcards
What does reflectivity measure?
Percentage of incident light that is reflected
Path of light ray through boundary between two materials of different refractive index
Comes in at angle θ to the normal. If n2>n1, pass some reflected at same angle and some through at angle φ to normal which is less than θ. If n2
Snell’s law
n1Sinθ=n2Sinφ
Graphs of reflectivity vs angle θ
For n2>n1: 4% at 0 θ curving up exponentially to 100% at θ=90 (line almost vertical at end)
For n2
How do refractive indexes of core and cladding compare?
Cladding ri lower than that of core
When does TIR occur?
When angle of incidence of light at core-cladding interface is greater than the critical angle
Numerical aperture
Equal to sin(θmax) so angle of incidence is equal to critical angle and light is guided along core-clad interface.
Measure if the light gathering power of the fibre.
Independent of fibre diameter
Relating refractive indexes to angle of incidence formula
n2^2-n1^2=sin^2(θ)
What does a fibre with numerical aperture of 1 mean?
It can gather light from any incoming angle
What does efficiency of optical fibres depend on?
Area of core relative to fibre cross-section as cladding is a poor light-guide (cladding thickness must be more than wavelength).
Packing efficiency of circular tubes (surface tension prevents manufacture of square or hexagonal fibres).
Finding packing efficiency of fibres
If arranged in rows all touching and rows slot in with each other.
Unit cell rhombus with vertices at centres of circular cross-section. 1 full fibre in each unit cell with area πa^2.
Area of unit cell is 2a x rt(3)a
Percentage area occupied is 91%
Cladding can reduce this as not all of CSA is core
Incoherent fibre bundle
Entry point of light at one end of fibre bundle doesn’t match exit point. This is suitable for light transmission
Coherent fibre bundle
1:1 relationship between entry and exit point for fibres within a bundle and allow image transmission. Example is the fibre in the centre of the bundle at the entry must be in the middle at the exit.
Rod in tube method for fibre manufacture
3cm diameter rods suspended above a furnace inside thin tubes made of a cladding glass. This cladding glass has lower r.i, expansion coefficient matched to core, appropriate viscosity-temperature curve. Core may be lead glass (high r.i) and cladding a soda-lime-silica glass (low r.i). Tubes are collapsed onto the core at viscosities around 10^6 poise (water is 10^-3) and stretched to fibres of diameter around 60μm
Just after fibre manufacture
Fibres coated with organic lubricant (nm thick) to reduce subsequent damage and loss of strength. Cleanliness vital to maintain efficacy of core-clad interface as a reflector and preserve strength of finished fibre
