Recap_OFC_19_08_14.pptx

Optical Fiber Communication

Review Unit1

10EC72

ECE Department ,CMRIT

Overview of OFC

Generic Optical Comm. System

• _{Wavelength: 0.85, 1.3, 1.55,}

DWDM

• _{Transverse mode: SM vs. MM} • _{Longitudinal mode: DFB,}

VCSEL vs. FP, DBR

Considerations:

•_{Wavelength: 0.85, 1.3, 1.55,}

DWDM

•_{Transverse mode: SM vs. MM} •_Dispersion

Optical fiber cable structure

 _{Fiber consists of mainly three parts:}

1) Core

2) Cladding

Numerical Aperture

 _{It defines the light collecting capability of the}

fiber

 _{More the difference between n1 and n2,}

greater is the numerical aperture  More light collecting

 _{Relative refractive index,}

Modes in optical fiber

 _{Mode basically defines the no of rays going inside the fiber.}

 _{Single mode fiber has no dispersion or pulse broadening whereas}

NA Vs Dispersion

 _{Less pulse broaden  n1 should be close to n2}

 _{More collecting of light  n1 should be greater}

than n2

 _{High BW application (n1-n2  Less), Sensor}

application (n1-n2  More)

Step Index Fiber

Multi Mode step index fiber

Single mode step index fiber a= 40-60 um

Graded index fiber

 _{Graded index fiber has low dispersion due to}

Number of Modes

 _{For graded index fiber}

 _{For Step index fiber}

Meridional Ray Vs Skew ray

Meridional Ray

Numerical aperture for skew rays

 _{The reflection at point B at an angle φ may be}

given by

cos γ sin θ = cos φ

Single Mode Fiber

 _{Allow propagation of only one mode, Other modes}

are attenuated by leakage or absorption

 _{Fundamental mode exists with 0≤V≤2.405}  _{Single mode operation can be possible using}

a) Smaller core diameter  Need highly directional beam of light

b) Reducing relative refractive index  Difficult for fabrication

_{Graded index fiber can also be designed for}

single mode operation.

𝐿 𝑃₀₁𝑜𝑟𝐻 𝐸₁₁

𝑉 _𝐶=2.405

(

𝛼

)

Cut off wavelength

 _{Normalized cut off frequency}

or,

is the wavelength where a particular fiber becomes single mode So,

Or,

𝑉 _𝐶= 2 𝜋 𝑎

𝜆_𝑐 ( 𝑁𝐴 )=

2 𝜋 𝑎

𝜆_𝑐 . 𝑛1 ( 2 𝛥)

1/2

𝜆_𝑐=2𝜋 𝑎𝑛1

𝑉𝐶

(2𝛥)1/2

𝜆_𝑐

𝜆_𝑐 𝜆 =

𝑉 𝑉 _𝐶

Mode field diameter (MFD)

 _{Mode field radius or spot size (W₀) is defined}

as the radial distance from the core center to the 1/e² (power) or 1/e (intensity) point of the Gaussian profile.

 _{MFD=2W₀ characterizes the functional}

properties of single mode fiber𝐸(𝑟)=𝐸₀exp

(

2

Practical example of MFD

 _{Corning SMF-28 has MFD}

 _{9.2 µm at 1310 nm wavelength}  _{10.4 µm at 1550 nm wavelength}

 _{Core diameter = 8.2 µm, MFD>Core diameter}

MFD for graded index fiber

 _{MFD for graded index fiber can be calculated}

by

Why MFD is standardized?

 _{Because mismatch in mode field diameter}

increases the fiber loss while splicing two fibers

Effective refractive index

 _{Propagation constant for fundamental LP₀₁}

mode= β

 _{Β gives the increase in phase angle per unit}

length which is directly related to wavelength of LP₀₁ mode λ₀₁ by the factor 2π.

or,

 _{In convenient we define an effective}

refractive index for single mode fiber

sometimes referred to as phase index or normalized phase change coefficient.

where,

𝛽.𝜆₀₁=2 𝜋

𝜆01=

2 𝜋 𝛽

𝜂𝑒𝑓𝑓 =_𝑘𝛽 _𝑘

=2 𝜋 𝜆 𝛽=2 𝜋

Fiber Materials

 _{Fundamental requirements for choosing fiber}

material

 _{Must be possible to make long, thin, flexible fiber}

 _{Must be transparent for the desired optical wavelength}  _{Slightly different core and cladding RI should be}

available

Materials satisfying these requirements are “Glass” and “Plastic”

 _{Majority of fibers are made of glass}

containing either silica (SiO₂) or silicate

 _{Plastic fibers are less widely used as it has}

Types of fibers

Types of Fiber

Glass Fiber

Halide Glass Fiber

Chalcogenide

Glass Plastic-Clad Glass fiber

Glass Fiber

 _{Most common is silica (SiO₂) which has a}

refractive index of 1.458 at 850 nm

 _{To produce slightly varying RI various oxides}

Halide Glass and Charcogenide Glass

 _{Fluoride glasses have extremely low}

transmission loss at mid-infrared wavelength. This material belongs to family of halide

glasses. (F, Cl, Br, I)

 _{The charcogens Sulpher (S), Selenium (Se),}

Plastic-clad Glass and Plastic fibers

 _{Core with silica and cladding with polymer}

(Plastic) material. Called as PCS. Selected cladding material is natural quartz with RI=1.405 at 850 nm.

 _{Example of plastic fibers are:}

 Polysterene core (n₁=1.60) and a methyl

metherylate cladding (n₂=1.49) to give NA=0.6

 A polymethyl metherylate core (n₁=1.49) and a

Photonic Crystal Fiber

 _{Uses photonic crystals to form the cladding around the}

core of the cable

 _{Photonic crystal is a low-loss periodic dielectric medium}

constructed using a periodic array of microscopic air holes that run along the entire fiber length

 _{Mainly two types: high index guiding fibers and low index}

guiding fibers.

 _{High index guiding fibers are guiding light in a solid core}

by the Modified Total Internal Reflection (M-TIR) principle.

 _{Low index guiding fibers guide light by the photonic band}

High index guiding fibers

 _{It relies on a high index core region, typically pure silica,}

surrounded by a lower effective index provided by the microstructured region.

 _{The refractive index of the microstructured cladding in}

PCFs exhibits a wavelength dependency very different from pure silica - an effect which allows PCFs to be designed

THE BANDGAP EFFECT – LOW INDEX GUIDING FIBERS

 _{The periodic microstructure in the PBG fiber cladding}

results in a so called photonic bandgap, where light in certain wavelength regions cannot propagate

 _{The core is created by introducing a defect in the PBG}