Optical Communication Unit 1 - Part 2
- 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 ● 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 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