Optical Communication - Ray Optics

1. R.M.K. COLLEGE OF ENGINEERING AND TECHNOLOGY
DEPARTMENT OF ECE
OPTICAL COMMUNICATION
DR.K.KANNAN
ASSISTANT PROFESSOR
DEPARTMENT OF ECE

2. ● Optical communication wavelength window is between 800nm to 1300nm.Today
the wave length range is 1550nm
● Characteristics of Light
- Intensity of Light
- Frequency and Wavelength of Light
- Spectral width of Light
● Light act as an Electromagnetic Waves - Polarization
● Light Structure
- Simplest model of ray - RAY Model
- Advanced version of Ray model – WAVE Model
- More advanced Versuin of Wave Model – QUANTUM Model
OPTICAL FIBER COMMUNICATION

3. Ray Optics
Ray is an idealized model of light, obtained by choosing a line that is
perpendicular to the wave fronts of the actual light, and that points in the direction of
energy flow
Types of Ray Optics
- Meridional ray
- Skew ray
- Guided ray
- Leaky ray
RAY THEORY TRANSMISSION
RAY MODEL

4. ● A Meridional ray is a ray that passes through the axis of an optical fiber
● A Skew ray is a ray that travels in a non-planar zig zag path and never crosses the
axis of an optical fiber
● A Guided ray, Bound ray, or Trapped ray is a ray in a multi-mode optical fiber,
which is confined by the core. For step index fiber, light entering the fiber will be
guided if it makes an angle with the fiber axis that is less than the fiber's acceptance
angle
● A Leaky ray or Tunneling ray is a ray in an optical fiber that geometric optics
predicts would totally reflect at the boundary between the core and the cladding, but
which suffers loss due to the curved core boundary
RAY OPTICS

5. ● A meridional ray is a ray that
passes through the axis of an optical
fiber.
● Types
- Bounded Ray - Rays that are
propagated through the fiber by TIR
- Unbounded Ray – Rays that are
refracted from the core
Cladding imperfections
prevent total internal reflection
Unbounded rays eventually
escape from the cable
MERIDIONAL RAY

6. ● Light entry into the tip of the fiber
core
● TIR – Sustained Propagation
- obeys Snell’s law
- n1 > n2 & θ1 > θ2
● Same Plane – Incident ray and fiber
core
MERIDIONAL RAY

7. ● Two ray incidence – Maximum intensity at a meeting point of two ray
during propagation
● Meridional rays take comparatively lesser light ray path because of lesser
acceptance angle
● These ray are confined to the meridional planes of the fiber which are the
planes that contain the axis of symmetry of fibers
MERIDIONAL RAY

8. ● Rays never meet the axis of the fiber
● It does not lie in the plane containing the axis of the fiber and it will go in
different direction
● Ray will go sprile ariund the fiber axis – Multiple reflection in the interface
● Light intensity of this ray is very low – Never meet the axis
● Two possible intensity distribution
● Maximum intensity distribution at the center – axis of the fiber
● Minumum intensity distribution at the center
SKEW RAY

9. ● Depends on light collecting
efficiency of the fiber
● Depends on Optical Source and types
of fiber used
● θ0 and θmax
θmax = Sin-1(n1
2 - n2
2)1/2 - NA
● NA should be large as possible i.e
(n1
2 - n2
2)1/2 should be large
Here n1 is fixed – Glass (1.5) and in
general n2 ≥ 1
SKEW RAY - ACCEPTANCE ANGLE

10. Ray model analysis
✓ Total Internal Reflection explains that the ray completely reflected at the
boundary between core and cladding.
✓ Does not discuss about what happened in the cladding
✓ Zero intensity in the cladding
✓ Doesnot give the correct picture of TIR becose at the interface the
elecromagnetic field interms of light will go to zero
SKEW RAY – MODELANALYSIS

11. Wave model analysis
➢ In the cladding medium the light intensity is not zero, the fields are present in
the second medium and it is exponentially decay in the cladding
➢ The field in the cladding must be consider otherwise it will disturb the field inside
the core – Protect the field in the cladding
➢ Proper cladding should be done to die out the field and that should not go away to
the outside the world
Therefore at TIR
- Standing wave type of fields in the core
- Decaying fields in cladding
- The ray undergo phase change at the reflecting boundary

12. ● Sustained constructive
interference by the phase front
is achieved by separation
between them
● The separation between these
should be multiples of 2𝜋
● Then only the phase front
moving in this direction and
gets reflected(constructive
interference)
SKEW RAY - TIR

13. ● For a sustained constructive
interference, the distance
between these two phase -
fronts must be multiples of 2𝜋
● S1 = d/Sinθ & S2 = AD Cosθ
S2 = (Cos2θ – Sin2θ)d/Sinθ
2𝝅n1/λ(S1-S2) + 2𝜹= 2𝝅m
2𝝅n1dSin𝜽/λ + 𝜹 = 𝝅m
● The above condition is must
satisfied for the light ray
propagated inside the fiber
SKEW RAY CONT…

14. ● For sustained propagation incident angle of the ray < 𝜃0𝑚𝑎𝑥 (TIR)
● With the above the incident ray must satisfy the following condition is
2𝝅n1dSin𝜽/λ + 𝜹 = 𝝅m
● Otherwise the ray can't propagate inside the fiber
● Here m = integer and 2𝝅n1dSin𝜽/λ gives discrete angles
● Departure from a solid cone of light in continuous
𝜽 𝐝𝐨𝐦𝐚𝐢𝐧 𝐢𝐧𝐭𝐨 𝐝𝐢𝐜𝐫𝐞𝐭𝐞 𝜽 domain
● This change will lead to MODES inside the optical fiber – Discrete patterns of
intensity of light inside the optical fiber
SKEW RAY CONT…

15. ● Skew ray follows helical path through the fiber.this path traced through the
fiber gives a change in direction of 2 𝛾 (γ + 𝛾) at each reflection
● The number of Reflections inside the core will give the output distribution not
depends on the input conditions of the fiber
● Non uniform light input – Uniform light output by multiple reflection
● The incident and reflected rays at the point B are in the same plane represented
by Cos 𝜃
SKEW RAY – ACCEPTANCE ANGLE

16. ● Thus to resolve the ray path AB relative to the radius BR in these two perpendicular
planes requires multiplication by Cos 𝛾 and Sin 𝜃
SKEW RAY – ACCEPTANCE ANGLE

19. ● Depend on Cos 𝛾 value, the axial angles are larger than meridional rays
● In meridional rays Cos 𝛾 = 1 and 𝜃 𝑎𝑠 = 𝜃𝑎
● Hence, Skew rays tends to propagate only in annular region near the outer
surface of the core and do not fully utilize the core for transmission

20. COMPARISION BETWEEN
MERDIONALAND SKEW RAYS
MERIDIONAL RAY SKEW RAY
Meridional rays enter into the optical
fiber through it's axis
Skew rays enter into the optical fiber not
it's axis
These rays cross the fiber axis at each
reflection
Skew rays do not cross the fiber axis and
propagate around the optical fiber axis
Acceptance angle is minimum Acceptance angle is maximum
Light collecting efficiency is
minimum
Light collecting efficiency is maximum
Propagation in Zig Zag Path Propagation in Helical Path

21. ● A leaky mode or leaky rays or tunneling
mode is a mode having an electric field
that decays monotonically for a finite
distance in the transverse direction
● The propagation of light through optical
fiber by skew rays suffer only partial
reflection while meridional rays are
completely guided
● Thus the modes allowing propagation of
skew rays are called leaky modes. Some
optical power is lost into clad due to
these modes.
LEAKY RAYS

22. An optical fiber in air has an NA of 0.4. Compare the acceptance angle for meridional rays
with that for skew rays which change direction by 100° at each reflection.
Solution: The acceptance angle for meridional rays is given by
NA = n0 sin θa = (n12 − n2 2 )1/2
For Air medium n0 = 1
NA = θa = sin−1 NA = sin−1 0.4
= 23.6°
The skew rays change direction by 100° at each reflection, therefore γ = 50°.The acceptance
angle for skew rays is:
θas = sin−1 (NA/Cos γ) = sin−1 (0.4/Cos 50°) = 38.5°
In this example, the acceptance angle for the skew rays is about 15° greater than the
corresponding angle for meridional rays
PROBLEM

23. TYPES OF OPTICAL FIBER –
REFRACTIVE INDEX

24. ● The refractive index of the core is uniform throughout and undergoes on abrupt
change at the core cladding boundary
● The RI Profile n(r) = n1 ; r < a - Core
= n2 ; r ≥ a - Cladding
STEP INDEX FIBER

25. ● The diameter of the core is about 50-200μm in the case of multimode fiber and
10μm in the case of single mode fiber
● Lower bandwidth and lower attenuation
● The light ray propagation is in the form of meridional rays(Zig Zag) and it
passes through the fiber axis

26. Single mode or Monomode step index fiber
- Only one EM Mode and the core diameter in the order of 2 to 10𝜇m
- Low intermodal dispersion(Broadening of transmitted light pulse)
and lower tolerance requirements on fiber connectors
- Maximum bandwidth
- Larger NA,Larger core diameter and easier coupling to optical source
SINGLE MODE STEP INDEX FIBER

27. A multimode step-index fiber has a core of radius (a) and a constant refractive
index n1. A cladding of slightly lower refractive index n2 surrounds the core.
- Diameter of the core is 50 - 200𝜇𝑚
- Lower bandwidth
- Light propagation in Zig Zag path along the fiber
- Short distance communication
- Large dispersion
- Low performance
MULTI MODE STEP INDEX FIBER

28. ● The refractive index of the core is made
to vary gradually such that it is
maximum at the center of the core.
● The diameter of the core is about 50μm
in the case of multimode fiber
● The path of light is helical in manner
● Attenuation is less
● This fiber has higher bandwidth
● The light propagation is in the form of
skew rays and it will not cross fiber
axis.
GRADED INDEX FIBER

29. Refractive Index profile of Graded index fiber is
Here ∆ = Relative RI deference
𝛼 = Profile parameter
𝛼 = 1 - Triangular profile
= 2 - Parabolic profile
= ∞ - Step Index profile
GRADED INDEX FIBER

30. ● Here you can describe the wrong
uses of your logo
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uses of your logo
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uses of your logo
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uses of your logo
GRADED INDEX FIBER

31. ● Here you can describe the wrong
uses of your logo
● Here you can describe the wrong
uses of your logo
● Here you can describe the wrong
uses of your logo
● Here you can describe the wrong
uses of your logo
GRADED INDEX – MULTI MODE
FIBER

32. STEP INDEX AND GRADED INDEX FIBER

33. COMPARISION
Feature Step-Index Fiber Graded-Index Fiber
Bandwidth Size Lower bandwidth Higher bandwidth
Diameter of the Core 50-200 µm About 50 µm
Application Scenarios
Normally used in short-distance (within
a few kilometers) and low-speed (8
Mb/s or less) communication systems
Usually used in medium-distance
(10~20 km) and relatively higher-
speed (34~140 Mb/s)
communication systems
Data Transmission Form
Light propagates in the shape of a
zigzag along the fiber/core axis
Light travels forward in the form
of sinusoidal oscillation/curves
Modal Dispersion
Affects the transmission capacity of the
fiber and limits the relay distance
Greatly decreased dispersion than
step-index multimode fiber,
making a higher bandwidth
Numerical Aperture High Low
Reflection Loss Yes No
Attenuation Maximum Minimum