Principles of light guidance

1
Principles of light guidance
Early lightguides
Luminous water fountains with coloured films over
electric light sources built into the base were
used for early public displays. These fountains
use the same basic principle of light guidance as
modern optical fibres. The same idea is still
used today in fountains, advertising displays,
car dashboards...
Paris Exposition of 1889
2
Principles of light guidance
How water can guide light

• The water in the jet has a higher optical
  density, or refractive index, than the
  surrounding air.
• The water-air surface then acts as a mirror for
  light propagating in the water jet.
• Rays of light travel in straight lines and are
  reflected back into the jet when they reach its
  outer surface.
• Hence the light rays follow the jet of water as
  it curves towards the ground under gravity.

3
Tyndalls demonstration for the classroom
or more accurately, Colladons demonstration
4
Optical fibres threads of glass
Human hair for comparison
50 80 mm
Typical refractive indices Cladding ncl
1.4440 Core nco 1.4512 Light is guided
along the core by Total Internal
Reflection Cladding helps isolate light from edge
of fibre where losses and scattering are high
5
Total Internal Reflection
Rays striking an interface between two
dielectrics from the higher index side are
totally internally reflected if the refracted ray
angle calculated from Snells Law would otherwise
exceed 90.
If n1 1.470 and n2 1.475, say, then qcrit
85.28 within a fibre core
6
Bound rays vs Refracting Rays
Bound rays zig-zag indefinitely along a fibre or
waveguide
Refracting rays decay rapidly as they propagate
7
Visible laser focused into an optical fibre
This Argon laser excites both leaky modes and
bound modes
Spatial transient glow due to leaky modes
scattering from acrylate jacket material
Several Watts _at_ l 514nm
Light scatters off the air itself !
Bound modes continue for many km along the fibre,
and are seen here due to Rayleigh scattering, the
main cause of attenuation in modern fibres
Rayleigh scattering is much reduced at longer
infrared wavelengths
8
Applying Snells Law Numerical Aperture
Numerical Aperture is defined as NA n0 sina
where a is the cone half-angle for the emerging
light rays, and n0 is the external index. In a
simple way, NA characterises the light gathering
ability of an optical fibre.
Ray diagram
Knowing the critical angle inside the fibre helps
us calculate a, and hence the NA by successive
applications of Snells Law
9
An optical communications link
So, where does this optical fibre fit into the
overall picture?
10
Digitisation of analogue data
Voice is sampled digitally about 8000 times every
second, and each sample needs 8 bits of data to
encode it, so a telephone conversation requires
64,000 bits/sec. Quality does not suffer by
discrete sampling if it is fast enough. This is
analogous to projecting 25 discrete movie frames
per second, which fools the eye into seeing a
continuous picture sequence.
11
Dispersion - a problem in step profile fibres
Different rays propagate along step profile
fibres at different rates - this is known as
multimode dispersion. Pulse distortion is greater
for fibres with many modes, and gets worse as the
fibre length increases.
12
Dispersion in step profile fibres How bad?
A simple calculation can tell us how much
dispersion to expect in a step profile multimode
fibre. Consider a representative segment
The speed of light in the core is c / ncore.
Hence the transit time through the segment for
the axial ray is
Of course, thats the fastest possible time. The
slowest is for the critical ray...
The critical ray travels a distance S, where
Hence, the transit time for the critical ray is
We are interested in the time delay
Or more particularly, the time delay per unit
length along the fibre
For typical multimode fibres, ncore 1.48, nclad
1.46 , so DT/L 67 ns / km by this
calculation. In fact, practical fibres exhibit
DT/L 10 - 50 ns / km, due to mode mixing.
13
Variation on a theme Graded index fibre
Shortest Path (physically) travels through the
highest index region and is therefore
slow. Longest Path (physically) travels through
lower index some of the time and is faster With
the correct graded index profile, all rays can
have identical transit times, eliminating
multimode dispersion !! Caution There are still
other types of dispersion present !
14
Chromatic Dispersion
Even if we eliminate all types of multimode
dispersion, pulses of light having different
wavelengths still travel at different velocities
in silica, so pulse spreading is still possible
if we use a spread of wavelengths. This is called
Material Dispersion and is responsible for
rainbows etc.
Together, Material Dispersion and Waveguide
Dispersion are termed Chromatic Dispersion. The
pulse spread is proportional to fibre length L
and wavelength spread Dl.
In the fundamental mode, the light spreads out
differently into the cladding depending on
wavelength. Hence, different wavelengths have
different effective refractive indices. This is
Waveguide Dispersion.
15
Transmission through an optical fibre
Telecommunications engineers quantify the
transmission of light through a system using
logarithmic units called decibels
(dB) Transmission in dB 10 log10 (Pout / Pin)
GAIN
LOSS
Lasers, amplifiers etc...
Fibres, passive components etc...
16
The telecommunications windows
17
Attenuation mechanisms
Several effects lead to loss of light in fibres
Absorption by impurity ions and atoms of the pure
glass (eg, OH- ion)
Absorption by vibrating molecular bonds (eg Si -
O)
Rayleigh Scattering by inhomogeneities frozen
into the glass structure itself
Each of these effects has a strong spectral
dependence!