Optical communications

Chapter 2.2Chapter 2.2

Pham Quang Thai

Pqthai.hmcut@gmail.com

Content

• Geometrical optics

• Optical propagation in fiber

• Fiber fabrication

• Signal degradation in fiber• Signal degradation in fiber

• Types of fiber

2

Basic parameters

3

Light: an electro-magnetic (EM) wave and

discrete energy packets (photon)

• Different power distribution. Why?

– Video: multi mode fiber

4

5

Johann Carl Friedrich Gauss

Michael Faraday André-Marie Ampère

James Clerk Maxwell

6

• Very low loss and axial circular symmetry of fiber give

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• Ez inside the core:

Bessel function

• Ez outside the core:

modified Bessel

function

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Optical propagation in fiber

9

Fiber mode

• There are several modes when light propagates in fiber

– Transverse electric (TE) mode: EZ=0

– Transverse magnetic (TM) mode: HZ=0– Transverse magnetic (TM) mode: HZ=0

– Hybrid mode: EZ≠0 and HZ≠0

• Hybrid mode is denoted HE or EH depending on whether HZ or EZ is larger

– For each l, Bessel’s functions have m roots, so each mode is denote as MODElm (TE01, TM01, HE11

etc.)

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The number of modes

depends on

• The value of V for a mode to appear is its “cut-off frequency”11

Conclusion

• Electromagnetic field distributes in several modes.

• The field are not confined to the core, high order

modes penetrate further into the cladding

• TE and TM modes only appear when l=0, all other • TE and TM modes only appear when l=0, all other

modes are hybrid modes.

• There is always at least one mode (HE11)

• More modes appear as V increases

• The value V=2.405 is the condition for one mode or

multi-mode

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Examples for chapter 2.2

• Problems 2-19:

– Determine the normalized frequency (V) at 820

nm for a step index fiber having a 25-μm core

radius, n1=1.48 and n2=1.46 radius, n1=1.48 and n2=1.46

Content

• Geometrical optics

• Optical propagation in fiber

• Fiber fabrication

• Signal degradation in fiber• Signal degradation in fiber

• Types of fiber

14

Fiber fabrication

• Refer to section 2.8 in text book [1]

15

Fiber fabrication

• Video.

16