Guided Modes: Step-index and Graded Index Fibers

• Number of Modes:

– Step index fiber: V2/2

– Graded index fiber (parabolic ref.index profile) : V

2/4

Photonic Crystal Fibers • New optical fiber structure (1990s) • Initially called Holey fibers, later termed Photonic

crystal fibers, or microstructured fibers. • Basic difference between PCF and a conventional fiber

– cladding region (and sometimes the core regions) of a PCF contain air holes, which run along the entire length of the fiber.

• Differences in the light guiding properties: – Conventional fiber: material properties of the core and cladding – PCF fiber: the structural arrangement creates an internal

microstructure, which enables another dimension for light control

PCF fibers…..

• Light guiding characteristics: determined by the size

and spacing (pitch) of the holes in the microstructure and the refractive index

• PCF categories: – Index guiding fibers – Photonic bandgap fibers

• Index guiding PCF: has a high index core surrounded

by a lower-index cladding; light transmission

mechanism similar to that in a conventional fiber. Effective refractive index depends on the wavelength,

and the size and pitch of the holes

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Index Guiding PCF

Index Guiding PCF….

Source: Gerd Keiser, Optical

Fiber Communications, 4th

ed.,

Tata McGraw-Hill, 2008:

Chapter 2.

•Fiber has a solid core

surrounded by a cladding

region containing air holes

running along the length of

the fiber •Core and cladding made of

the same material (say, pure

silica) •The air holes lower the

effective refractive index of

the cladding region (n=1 for

air, 1.45 for silica) • The above microstructural

arrangement creates a step-

index optical fiber

• Advantages

– Very low losses – High resistance to darkening effects from

nuclear radiation - Can support single mode operation over

wavelengths in the range 300 nm to 2000 nm. – Mode field area of the PCF is much higher than

that of a conventional fiber – Can transmit high optical power levels

without encountering nonlinear effects

Photonic Bandgap (PBG) Fiber

PBG Fiber ….

Source: Gerd Keiser, Optical

Fiber Communications, 4th

ed.,

Tata McGraw-Hill, 2008:

Chapter 2.

•Fiber has a hollow core

surrounded by a cladding

region which contains air

holes running along the

length of the fiber

•Functional principle of PBG

fiber analogous to the role of

the periodic crystalline

lattice in a semiconductor

(blocks electrons from

occupying a bandgap

region) •Hollow core acts as a

defect in the photonic

bandgap structure – creates

a region for the light to

propagate

• Features (Source: John Senior, Optical Fiber Communications, 3

rd

edition, Pearson Education, 2010, chapter 2.) – Periodic arrangement of cladding air holes provides for the

formation of a photonic bandgap in the transverse plane of the fiber

– Wavelengths within the bandgap cannot propagate perpendicular to the fiber axis (i.e. in the cladding)

– Light can be confined to propagate within a region where the ref index is lower than the surrounding material

– PBG fibers guide only in certain wavelength bands – Possible to have wavelengths at which higher order modes are

guided while the fundamental mode is not – Propagation losses are not as low as other fibers – Tend to be highly dispersive with narrow transmission windows

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