This document discusses photonic crystal fibers (PCFs). PCFs are composed of nanostructures that affect photon propagation through periodic refractive indices, similar to how semiconductor crystals affect electron motion. PCFs can guide light through two mechanisms: index guiding and photonic bandgap guiding. They have properties like endless single mode operation, large mode areas, and tunable dispersion. Special PCFs include double core fibers, highly birefringent fibers, and hollow core bandgap fibers. PCFs offer advantages over standard fibers like flexibility in core size and wavelengths used. Challenges include difficult fabrication and limited operating frequencies.

1. Ajay Singh
Engineering Physics Department
Indian Institute of Technology Delhi

2. Photonic crystals ??
Crystals composed of nanostructures, of periodic dielectric/refractive
index that affect the propagation of photon in the same way as
the periodic potential in a semiconductor crystal affects the electron
motion by defining allowed and forbidden electronic energy bands.
(Periodicity leads to allowed and forbidden states)
http://www.physics.buffalo.edu/phy514/w08/

3. Optical Fiber
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4. Optical Fiber
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5. Optical Fiber
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6. The Glass Ceiling: Limits of Silica
Loss: amplifiers every 50–100km
…limited by Rayleigh scattering (molecular entropy)
…cannot use “exotic” wavelengths like 10.6μm
Nonlinearities: after ~100km, cause dispersion, crosstalk, power limits
(limited by mode area ~ single-mode, bending loss)
also cannot be made (very) large for compact nonlinear devices
…tunability is limited by low index contrast
Modifications to dispersion, polarization effects?
Long Distances
High Bit-Rates
Compact Devices
Dense Wavelength Multiplexing (DWDM)

7. Breaking the Glass Ceiling:
Hollow-core Bandgap Fibers
1000x better
loss/nonlinear limits
(from density)
Photonic Crystal
1d
crystal
Bragg fiber
[ 1978 ]
+ omnidirectional
= OmniGuides
2d
crystal
PCF
(You can also [ 1998 ]
put your stuffs in here …)

8. Photonic crystal fibers
Photonic crystal fibers combine properties of 2D photonic crystals and classical
fibers.lattice pitch, air hole shape and diameter, refractive index of the glass,
and type of lattice determines the properties of the fiber. (Generally Silica)
There are two guiding mechanisms in PCF: index guiding mechanism (similar
to the one in classical optical fibers) and the photonic bandgap mechanism.
Solid core and Hollow core PCFs (http://spie.org/x31636.xml)
solid core
holey cladding forms
Effective low-index
material

9. [1]
Fabrication of PCF

10. Properties of Single mode PCF
Endlessly single mode fibers
To determine the number of guided modes in SIF usually a normalized frequency V is used. V
is defined as:
In the case of standard fibers,the cladding index is almost wavelength independent and V
increases when wavelength decreases. It results in multimode operation regime for cut-off
normalized frequency higher than 2.405.
Modal properties of endlessly single mode PCF made of
multicomponent glass. For any wavelength a mode index value of the
modes, higher than the fundamental one, is lower than the effective
cladding index. [1]

11. Properties of Single mode PCF
Endlessly single mode fibers
To determine the number of guided modes in SIF usually a normalized frequency V is used. V
is defined as:
In the case of standard fibers,the cladding index is almost wavelength independent and V
increases when wavelength decreases. It results in multimode operation regime for cut-off
normalized frequency higher than 2.405.
Modal properties of endlessly single mode PCF made of
multicomponent glass. For any wavelength a mode index value of the
modes, higher than the fundamental one, is lower than the effective
cladding index. [1]
For PCF a value of the effective
refractive index of photonic cladding
depends strongly on wavelength. A
refractive index of photonic cladding
and therefore stationary value of
normalized frequency, is defined by the
cladding structure, namely by the fill
factor (the ratio of the hole diameter d
to the period of the lattice Λ).

12. Properties of Single mode PCF
A key parameter that describes properties of fibers is a group velocity dispersion (GVD). It is
defined as:
Dispersion characteristics in PCFs can be easily shaped due to the flexibility of varying air-hole
size and the position in the photonic cladding.
A comparison of dispersion in SIF and in an index-guiding PCF [1]
Dispersion properties
 waveguide dispersion can be very
strong
 The material dispersion" is modified
by artificial photonic cladding with
the presence of air-holes.
 Varying Λ and air-hole sizes in PCFs
a zero-dispersion wavelength can be
shifted into the visible region. it
automatically gives a positive
(anomalous) dispersion in the visible
range=> Can be used for
Compensation in telecommunication
lines.
 PCF with a positive dispersion can be
used for dispersion compensation in
the telecommunication lines.

13. PCF With Special properties
Double-core fibers
 multicapillary fabrication technique it is easier to form multi-core PCF
structures than the traditional step-index ones.
 Two solid cores are separated by a single air hole.
 Used as directional couplers, wavelength multiplexers/demultiplexers, and
band sensors Highly birefringent double-core PCFs are also used as
polarization splitters
[1]

14. Highly birefringent fibers
Examples of highly birefringent PCF:
(a) HB PCF with hexagonal lattice
and circular holes (b) test samples
of rectangular-shape HB PCF with
rectangular lattice and elliptical
holes of IEMT. [1]
PCF With Special properties
 Due to non-axisymmetric distribution of the effective refraction index that depends on
the size and spatial distribution of holes
 Extremely high birefringence in comparison to standard optical fibers
 A highly birefringent dispersion compensating microstructure optical fiber (MOF)
 Highly insensitive to temperature => Sensing applications
 Due to this immunity highly birefringent PCFs are very attractive for sensing and for
telecommunication applications as a compensator of polarization mode dispersion in
fiber lines.

15. Fiber lasers and amplifiers
Double clad PCF. A solid core is surrounded
with low filling factor cladding (inner one),
which plays a role of a pump core since the
pump field is confined by a second high filling
factor cladding (outer one). [1]
PCF With Special properties
 Conventional SIFs lasers :core and double
cladding made of different materials most
typically with a polymer outer cladding.
 Effciency of these devices is limited by core size,
numerical aperture, and Raman scattering in
doped silica.
 Double clad of PCF : The inner cladding ensures a
high NA and is surrounded with a web of silica
bridges which are substantially narrower than the
wavelength of the guided radiation.
 Air-clad fiber with high NA : the diameter of the
inner cladding (pump core) can be significantly
reduced while brightness acceptance of the pump
radiation is kept.
 Due to high ratio of active core area to inner
cladding (pump core), the pump light absorption
is improved. It allows us to use inexpensive, high
power broad area emitting pumps.
 Large mode area for the single mode signal
allows one to obtain a high power output with
relatively low power density.
 Highly efficient lasers ….

16. Fresnel fiber
A concept of the Fresnel fiber after [1]
PCF With Special properties
 Free-space diffraction limit in
propagation of high intensity light.
 Very diffcult to generate Bessel wave
(diffraction less free-space waveform)
beyond the Raighley range of
conventional optics.
 Micro-structured fiber technique :Fresnel
zones determined by the ring of holes
spaced at radii such that interstitial hole
spacing can be significantly larger than
the propagation wavelength.
 The concentric rings of holes (Fresnel
zones) have various effective
refractive indexes and interferes
constructively, forming a peak field
intensity in the center of the fiber axis.
This enables focusing light at the
output of the fiber at the far field
without any additional lens, while in
conventional fibers, light emerging
from a fiber diffracts and expands.

17. Advantages:
Endlessly single mode operation
Large mode area
Many-core fiber
Fiber amplifier
Dispersion
Special fibers
•PCF with high-index core is more flexible than conventional fiber:
- Possible to make very large core area to send high power
- Possible to make core very small compared to conventional
fibers. Designer wavelengths possible.
•Air-guiding PCF (hollow core of fiber):
- Possible to send high power
- No entrance or exit reflectance (loss goes down)
Challenges:
•PCF is difficult to fabricate
•PCF is limited to specific frequencies

18. References
[1] Photonic Crystal Fibers by R. Buczynski , Vol. 106 (2004) ACTA PHYSICA
POLONICA A No. 2
[2]Reconfigurable Optothermal Microparticle Trap in Air-Filled Hollow-Core
Photonic Crystal Fiber by O. A. Schmidt, M. K. Garbos, T. G. Euser, and P. St.
J. Russell, PRL 109, 024502 (2012) DOI: 10.1103/PhysRevLett.109.024502
[3]http://en.wikipedia.org/wiki/Photonic_crystal
[4]http://mpl.mpg.de/research-groups/jrg/research/ENO.html
[5] http://www.reference.com/browse/photonic+crystal+fibers
[6]http://www.menardjm.com/photonic-crystal-fibers.html
[7]http://en.wikipedia.org/wiki/Photonic-crystal_fiber
[8]http://www.fiberoptics4sale.com/Merchant2/optical-fiber.php
[9]http://www.google.com/patents/US6243522
[10]http://en.wikipedia.org/wiki/Photonic-crystal_fiber
[11]http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=98
9118

20. !