Fibre optics
• Modes of Propagation
• Types of optical fibre
• Losses in optical fibre
Modes of propagation:
Modes can be seen as the possible number of paths of light in an optical fibre. The paths are all zigzag
paths excepting the axial direction. Light ray travelling through a fibre are axial rays and zigzag rays.
As a ray gets repeatedly reflected at the walls of the fibre, phase shift occurs, consequently the waves
travelling along certain zigzag paths will be in phase and intensified while the waves travelling along
certain other paths will be out of phase and diminish due to destructive interference. The light ray paths
along which the waves are in phase inside the path are known as modes the number of modes that a
fibre will support depends on the ratio d/λ.d is diameter of the core.
Modes are designated by an ‘’order’ number ‘m’. In a fibre of fixed thickness, the higher order modes
propagates at smaller angles than the lower order modes. The zero order ray travels along the axis and
known as axial ray.
n2
Low order
High order
n1
Zero
Order
n1
Low & High order ray paths in multimode
fibre
Zero order ray path ((axial
ray)
in a single mode fibre
Types of Optical
Fibre:
Optical fibres are of two types: SMF and MMF
SMF has a smaller core diameter and can support only one mode of propagation .
MMF has larger core diameter, it is further divided on the basis of index profile. An index profile
is a plot of refractive index on X axis versus the distance from the core drawn on Y axis. The
index profile of a MMF can be either a step index (SI) type or graded index (GRIN) type.
The index profile of a SMF is usually a step index type.
SMSIF:
It has a very thin core(dia.of 4µ m) of uniform refractive index of a higher value which is
surrounded by a cladding of lower refractive index. The R.I. Changes abruptly at the core cladding
boundary because of which it is known as a step index fibre. light travels in SMF along a single
path,i.e. Along the axis, it is the zero order mode that is supported by a SMF. A SMF has a Δ=
.002.
MMSIF:
It is similar with SMSIF except that its core is of larger diameter(100µm) which is very large
compared to the λ being tranmitted.Light follows zigzag path . Δ of a MMF is 0.3 as core dia. is
large.
Types of Optical
Fibres
Optical fibres are of two types: SMF and MMF
SMF has a smaller core diameter and can support only one mode of propagation .
MMF has larger core diameter, it is further divided on the basis of index profile. An index profile is a
plot of refractive index on X axis versus the distance from the core drawn on Y axis. The index
profile of a MMF can be either a step index (SI) type or graded index (GRIN) type.
The index profile of a SMF is usually a step index type.
SMSIF:
It has a very thin core(dia.of 4µ m) of uniform refractive index of a higher value which is
surrounded by a cladding of lower refractive index. The R.I. Changes abruptly at the core cladding
boundary because of which it is known as a step index fibre. light travels in SMF along a single
path,i.e. Along the axis, it is the zero order mode that is supported by a SMF. A SMF has a Δ= .002.
MMSIF:
It is similar with SMSIF except that its core is of larger diameter(100µm) which is very large
compared to the λ being tranmitted.Light follows zigzag path . Δ of a MMF is 0.3 as core dia. is
large.
GRINF:
It is a MMF with a core consisting of concentric layer of different refractive indices (R.I). Therefore R.I.
Of core varies with distance from the fibre axis. it has a high value at centre and falls off with
increasing radial distance from the axis. Such a profile causes a periodic focussing of the light
propagating through the fibre. In GRINF , accpt. angle and NA decreases with radial distances from the
axis.
The NA for parabolic index profile= n1( 2 Δ)1/2 √1- (r/a)2
Conventionally the size of optical fibre is denoted by SIF 50/125,100/140 and for GRIN fibres
62.5/125 and 85/125 .
V number is known as normalized frequency of the fibre.
V = 2πa *√n12- n22/ λ where a is radius of core
V = 2πa/ λ*(NA)
V = 2πa/ λ * n1 ( 2 Δ)1/2
The maximum number of modes Nm supported by SI fibre is
Nm = ½ V2
Thus for V = 10, Nm = 50
continued
Types of Optical Fibres ( Reference slide for images)
n
2
n
n
1
2
n
2
n
1
n
1
Single mode Step Index
Fibre
n
2
n
n
1
2
n
θ
2
n
1
n
1
Multi mode Step Index
Fibre
n
n
2
n
2
n
1
n(r
)
2
a
Graded Index
Fibre
For V< 2.405, the fibre can support only one mode and is classified as a SMF. MMFs
have value of V > 2.405 and can support many modes simultaneously.
The λ corresponding of V= 2.405 is known as cut-off wavelength.
λc = V λ/2.405
In case of GRINF for large value of V
Nm =V2/4
ATTENUATION:
It is the loss of signal intensity when signal transmitted in long distances i.e. The
amount of light lost b/w input and output. Generally it is due to medium components
are cables, connectors etc. it is expressed in dB/km
α = 10/L * log Pi /Po
Where Pi is input power , Po is output power and L is length of fibre used.
For ideal fibre Po = Pi
Fibre losses:
In optical fibre mainly 3 mechanism are responsible for losses in output signal
– Absorption: Impurities are a major sources of losses in fibres
– Rayleigh scattering: Glass is a disordered structure having local microscopic variations in density
which cause variation in refractive index ,causes scattering losses in output.
– Geometric effects: these are due to manufacturing defects.
Dispersion: it causes widening the pulse as the light travels through the fibre. Amplitude of the light
signal pulse decreases .Dispersion is measured in ns/km.
There are 3 mechanism causes distortion of the light pulse in a fibre:
1. material dispersion
2. waveguide dispersion
3. modal dispersion:
a) Intermodal
b) Intramural
Broadening of the signal due to
dispersion
continued
Fibre losses: (continued)
Material dispersion: It is a pulse spreading due the variation of refractive index of the core material as
a function of wavelength.
Material dispersion (Dm) = λ(Δ λ) /c * L d2n/d λ2 ,
where λ is peak wavelength ,Δ λ = spectral wavelength, L is length of the core and n is refractive index
of core.
Material dispersion can be reduced using more n more monochromatic source.
Wave guide dispersion: wave guide dispersion arises from the guiding properties of the fibre.
A single mode fibre confines only about 80% of the optical power to the core ,dispersion thus
arises because 20% light propagating in the cladding travels faster than light confined to the
core.
• Intermodal dispersion: When light pulse enters fibre it is breakdown into small pulses carried
by individual modes. Parts of the waves arrive output before other parts leading to a spread
(broadening)of input pulse. This is known as intermodal dispersion. It does not depend on the
spectral width of the source.
• Intramodal dispersion: Pulse broadening within a single mode fibre is called intramodal or
chromatic dispersion. since this phenomenon is wavelength dependent and group velocity is
function of wavelength it is also known as group velocity dispersion.
Dispersion limits the bandwidth of a fibre. The information flow i.e. Pulse rates must be slow
enough that dispersion does not cause adjacent pulses to overlap.