CHARACTERIZATION OF PHOTONIC CRYSTAL FIBER USING FAR FIELD METHOD

PRESENTED BY:SURBHI VERMA(2K11/EP/072)

PHOTONIC CRYSTAL FIBER - Guiding Mechanism

Modified TIR and Photonic Bandgap

Fabrication of PCF

Characterization using Far Field Measurement

WHAT IS PHOTONIC CRYSTAL FIBER?

• PCFs are optical fibers that employ a microstructured arrangement of material in a background material of different refractive index. the background material is often undoped silica and a low index region is typically provided by air voids running along the length of the fiber.

• The first fiber with a photonic crystal structure was first reported by Russell and his colleagues in 1995

Guiding Mechanism in PCF• As with any material, a 2D photonic crystal has a

maximum β value that can propagate.• At a particular frequency, this corresponds to the

‘fundamental’ mode of an infinite slab of the material, and this value of β defines the ‘effective refractive index’ of the material

• Modal index, nm= β/k • The allowed values (blue) of modal index

CLASSIFICATION OF PCF

High Index Guiding Fiber Low Index Guiding Fiber (use modified total internal reflection) (use photonic band gap effect)

Endlessly Single Mode

DIFFERENT DESIGNS OF PCF WAVEGUIDE

(J. C. Knight, “Photonic Crystal Fibres”, Nature, Vol. 424, pp. 847-851, Aug. 2003)

FABRICATIONSTACK AND PULL METHOD

EFFECTIVE INDEX METHOD(S.K. VARSHNEY AND R.K. SINHA, “CHARACTERIZATION OF PHOTONIC CRYSTAL FIBER FROM FAR FIELD MEASUREMENTS”,

JOURNAL OF MICROWAVES AND OPTOELECTRONICS)

• Effective index method is used to calculate the effective index (i.e. neff) and effective normalized frequency (i.e. Veff) of the infinite periodic photonic crystal cladding.

• Geometry of PCF ~ Geometry of SIF

Cross-section of triangular photonic crystal with air hole Circular approximation of hexagonal unit cell diameter ‘d’ and spacing between two air holes as ‘λ’ composed of hexagonal unit cell

FAR FIELD MEASUREMENT• Far field is the region outside near field region where field distribution is

independent of distance from source.• If the source/aperture has a maximum overall dimension d (where d>>),

then far field region exist at distance z greater than 2d2/ from source/aperture where is the wavelength.

• Far field is the fourier transform of near field pattern and is also called as “fraunhofer diffraction”.

FAR FIELD – MATHEMATICAL PART• For a near field pattern (x,y), the far field pattern is given by

• the relative far field pattern is given by:Ũ(Ɵ)==

• Using standard integrals to solve, expression of intensity as a function of is obtained and is then plotted.

0u C , exp{ik m }d d

l

FAR-FIELD INTENSITY PATTERN (S.K. VARSHNEY AND R.K. SINHA, “CHARACTERIZATION OF PHOTONIC CRYSTAL FIBER FROM FAR FIELD

MEASUREMENTS”, JOURNAL OF MICROWAVES AND OPTOELECTRONICS)

Far-field intensity pattern of photonic crystal fibers having d/λ= 0.20, 0.30, 0.40 and core radius 0.64λ at a wavelength of :

1.55µm 1.3µm

Variation of effective normalized frequency (Veff) as a function of normalized air hole size (d/λ)

Universal curves depicting the variation of sinθx/sinθh and k0ρ sinθh with veff for equivalent step index fiber for given pc

CHARACTERIZATION OF PCF

• (a) Measure the value of θx and θh from above graph where, θx is the angle in the far field pattern at which the intensity within the main lobe is minimum and θh is the angle at which intensity is half of the maximum value.(b) obtain the ratio sinθx/sinθh and use fig.11 to obtain Veff.(c) further, with the knowledge of Veff of PCF, the normalized half intensity angle αh can be determined and hence the core radius ρ can be determined.

• Since, ρ=0.64λ, therefore, air hole spacings λ of a given PCF can be obtained.