Download - Step Index and Graded Index Fiber
-
8/13/2019 Step Index and Graded Index Fiber
1/23
STEP INDEX AND GRADED
INDEX FIBER
Made by-
Aparna Garg
Ashish Arya
Abhinav Agrawal
Aftab Ali Rai
-
8/13/2019 Step Index and Graded Index Fiber
2/23
Single mode and Multimode Fiber
Core has uniform refractive index
Refractive index of cladding is also uniform butsmaller than Core
R.I.core>R.I.claddingStep change in Refractive Index at core cladding
interface
STEP INDEX FIBER
-
8/13/2019 Step Index and Graded Index Fiber
3/23
STEP INDEX FIBER
Refractive Index variation in Step Index Fiber
-
8/13/2019 Step Index and Graded Index Fiber
4/23
Fabricated from doped silica
Normalized Frequency (V)
-
8/13/2019 Step Index and Graded Index Fiber
5/23
2 Types:
Matched Cladding
Depressed Cladding
Matched Cladding:
Core: Constant refractive index
Cladding: Single homogeneous layer of dielectric
SINGLE MODE STEP INDEX FIBER
-
8/13/2019 Step Index and Graded Index Fiber
6/23
Depressed Cladding:
Core: Constant Refractive Index
Cladding: Made of two regions
SINGLE MODE STEP INDEX FIBER
-
8/13/2019 Step Index and Graded Index Fiber
7/23
Fabricated from multicomponent glass or doped Silica
Large core diameter and numerical aperture
MULTIMODE STEP INDEX FIBER
-
8/13/2019 Step Index and Graded Index Fiber
8/23
Normalized Frequency (V)>2.405
Attainable bandwidth: 300MHz- 3GHz km
High attenuation
Numerical aperture,
MULTIMODE STEP INDEX FIBER
-
8/13/2019 Step Index and Graded Index Fiber
9/23
GRADED INDEX MULTIMODE FIBER
Graded index fiber is an optical fiber whose core has arefractive index that decreases with increasing radialdistance from the fiber axis.
-
8/13/2019 Step Index and Graded Index Fiber
10/23
Higher Order ModesLarger propagation length
Travel farther into the claddingSpeed increases with distance away from the core(decreasing index of refraction)
Relative difference in propagation speed is less
-
8/13/2019 Step Index and Graded Index Fiber
11/23
Refractive index profile
where, = Relative refractive index difference
arnn
ara
r
n
rn
21
2
1
21
21
-
8/13/2019 Step Index and Graded Index Fiber
12/23
If , = Step index profile
= 2 Parabolic profile= 1 Triangular profile
Variations in R.I. with
-
8/13/2019 Step Index and Graded Index Fiber
13/23
Light rays follow Sinusoidal paths down the fiber.
The intermodal dispersion is smaller than for step index multimodefiber
Shorter wavelengths of rays travelling near fiber axis
4
2
1
inter
c
nD
g
-
8/13/2019 Step Index and Graded Index Fiber
14/23
Propagation of Ray
Propagation through Reflection and Total Internal
Reflection Due to gradual decrease in cores R.I. from the center of
the fiber ray reflects many times
-
8/13/2019 Step Index and Graded Index Fiber
15/23
The ray that travels directly along the axis reaches the end ofthe fiber at the same time as the higher order ray .
Modal dispersion
speed of guided light changes with fiber cores refractive index.
Farther the light goes from the centre of the fiber, the faster it is.
Speed difference compensate for the longer paths followed bythe light rays that go farthest from the centre of the fiber. This
equalizing of transit times of different modes greatly reducesmodal dispersion.
-
8/13/2019 Step Index and Graded Index Fiber
16/23
Step model of graded index fiber
Model shows the refraction at different layers of core
Ray refracts at higher angle at every layer and total internal
reflection occurs before it reaches at boundary of core and
cladding
-
8/13/2019 Step Index and Graded Index Fiber
17/23
NA(Numerical aperture) of gradedindex fiber
NA is at its maximum value at fiber axis
Axial Numerical Aperture [NA(0)]= n1(2 )1/2
NA decreases with increase in r
Becomes zero for r>a
-
8/13/2019 Step Index and Graded Index Fiber
18/23
No. of modes in graded index fiber
Depends upon NA, Relative R.I. difference, Profileparameter(alpha) and normalized frequency (V).
Have typically over one hundred modes
Accept less light than multimode single index fiber for samecore and delta
-
8/13/2019 Step Index and Graded Index Fiber
19/23
Basic design parameter
Standard multimode graded index fiber core and cladding
sizes are 50/125 m, 62.5/125 m, 85/125 m and 100/400 m.
Typical values of delta are around 0.01 to 0.02
62.5/125 m design with 0.02 delta offers the best overallperformance
-
8/13/2019 Step Index and Graded Index Fiber
20/23
Difference between Step index and Graded index
1. Step index fiber is of two types viz; mono mode fiber and multi mode fiber.Graded index fiber is of only of one type that is multi mode fiber.
2. The refractive index of the core of the step index fiber is constant throughout the core.
The refractive index of the core of the graded index fiber is maximum at thecentre of the core and then it decreases towards core-cladding interface.
-
8/13/2019 Step Index and Graded Index Fiber
21/23
3. Number of modes for step index fiber N = V2/2, where V is cut offfrequency or normalized frequency or V- numberNumber of modes for graded index fiber is N = V2/ 4.
4. V number can be less that 2.405 or more that 2.405 for step indexfiberV number is always more than 2.405 for graded index fiber.
-
8/13/2019 Step Index and Graded Index Fiber
22/23
5. Step-index multimode fibers are mostly used for imaging andillumination.
Graded-index multimode fibers are used for data communications andnetworks carrying signals moderate distances - typically no more
than a couple of kilometers.
-
8/13/2019 Step Index and Graded Index Fiber
23/23