Photonic-crystal fiber (PCF) is a new class of optical fiber based on the properties of photonic crystals. Because of its ability to confine light in hollow cores or with confinement characteristics not possible in conventional optical fiber, PCF is now finding applications in fiber-optic communications, fiber lasers, nonlinear devices, high-power transmission, highly sensitive gas sensors, and other areas. In this project, photonic crystal fibers and far field measurement technique was described. The project also focused on the development of analytical formulae and a method to characterize PCF from its far field radiation pattern using effective index approach considering PCF to be similar to single mode step index fiber. This project was an explanation of an already published research paper

1. CHARACTERIZATION OF PHOTONIC
CRYSTAL FIBER USING FAR FIELD
METHOD
PRESENTED BY:
SURBHI VERMA
(2K11/EP/072)

2. PHOTONIC CRYSTAL FIBER - Guiding
Mechanism
Modified TIR and Photonic Bandgap
Fabrication of PCF
Characterization using Far Field
Measurement

3. 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

4. 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 materia
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

5. CLASSIFICATION OF PCF
High Index Guiding Fiber Low Index Guiding Fiber
(use modified total internal reflection) (use photonic band gap effect)
Endlessly Single Mode

6. DIFFERENT DESIGNS OF PCF WAVEGUIDE
(J. C. Knight, “Photonic Crystal Fibres”, Nature, Vol. 424, pp. 847-851, Aug. 2003)

7. FABRICATION
STACK AND PULL METHOD

8. 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

9. 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”.

10. 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)=
𝑢(Ɵ)
𝑢(0)
=
[ 0
1
𝐽0 𝑈 Ζ
𝐽0
𝛼 Ζ Ζ DΖ+
𝐽0 𝑈
𝐾0 𝑊 1
∞
𝐾0 𝑊 Ζ
𝐽0
𝛼 Ζ Ζ DΖ]
𝐽1 𝑈
𝑈𝑊2 (𝑈2+𝑊2)
• Using standard integrals to solve, expression of intensity as a function of 𝜃 is
obtained and is then plotted.
   0u C , exp{ik m }d d
 
 
         l

11. 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

12. 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

13. 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.