Ajay SinghEngineering Physics Department

Indian Institute of Technology Delhi

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/

Optical Fiber

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Optical Fiber

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Optical Fiber

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The Glass Ceiling: Limits of Silica

Long Distances

High Bit-Rates

Dense Wavelength Multiplexing (DWDM)

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

Compact Devices

…tunability is limited by low index contrastModifications to dispersion, polarization effects?

Breaking the Glass Ceiling:Hollow-core Bandgap Fibers

1000x better

loss/nonlinear limits

(from density)

Photonic Crystal

1dcrystal

Bragg fiber

[ 1978 ]

+ omnidirectional

= OmniGuides

2dcrystal

PCF

[ 1998 ](You can alsoput your stuffs in here …)

Photonic crystal fibers

Photonic crystal fibers combine properties of 2D photonic crystals and classicalfibers.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 (similarto 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 formsEffective low-index material

[1]

Fabrication of PCF

Properties of Single mode PCF

To determine the number of guided modes in SIF usually a normalized frequency V is used. Vis defined as:

In the case of standard fibers,the cladding index is almost wavelength independent and Vincreases when wavelength decreases. It results in multimode operation regime for cut-offnormalized frequency higher than 2.405.

Modal properties of endlessly single mode PCF made ofmulticomponent glass. For any wavelength a mode index value of themodes, higher than the fundamental one, is lower than the effectivecladding index. [1]

Endlessly single mode fibers

Properties of Single mode PCF

To determine the number of guided modes in SIF usually a normalized frequency V is used. Vis defined as:

In the case of standard fibers,the cladding index is almost wavelength independent and Vincreases when wavelength decreases. It results in multimode operation regime for cut-offnormalized frequency higher than 2.405.

Modal properties of endlessly single mode PCF made ofmulticomponent glass. For any wavelength a mode index value of themodes, higher than the fundamental one, is lower than the effectivecladding index. [1]

Endlessly single mode fibers

For PCF a value of the effectiverefractive index of photonic claddingdepends strongly on wavelength. Arefractive index of photonic claddingand therefore stationary value ofnormalized frequency, is defined by thecladding structure, namely by the fillfactor (the ratio of the hole diameter dto the period of the lattice Λ).

A key parameter that describes properties of fibers is a group velocity dispersion (GVD). It isdefined 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

Properties of Single mode PCF

waveguide dispersion can be verystrong

The material dispersion" is modifiedby artificial photonic cladding withthe presence of air-holes.

Varying Λ and air-hole sizes in PCFsa zero-dispersion wavelength can beshifted into the visible region. itautomatically gives a positive(anomalous) dispersion in the visiblerange=> Can be used forCompensation in telecommunicationlines.

PCF with a positive dispersion can beused for dispersion compensation inthe telecommunication lines.

Double-core fibers

[1]

multicapillary fabrication technique it is easier to form multi-core PCFstructures 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 aspolarization splitters

PCF With Special properties

Highly birefringent fibers

Examples of highly birefringent PCF:(a) HB PCF with hexagonal latticeand circular holes (b) test samplesof rectangular-shape HB PCF withrectangular lattice and ellipticalholes of IEMT. [1]

PCF With Special properties

Due to non-axisymmetric distribution of the effective refraction index that depends onthe 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 infiber lines.

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 doublecladding made of different materials mosttypically with a polymer outer cladding.

Effciency of these devices is limited by core size,numerical aperture, and Raman scattering indoped silica.

Double clad of PCF : The inner cladding ensures ahigh NA and is surrounded with a web of silicabridges which are substantially narrower than thewavelength of the guided radiation.

Air-clad fiber with high NA : the diameter of theinner cladding (pump core) can be significantlyreduced while brightness acceptance of the pumpradiation is kept.

Due to high ratio of active core area to innercladding (pump core), the pump light absorptionis improved. It allows us to use inexpensive, highpower broad area emitting pumps.

Large mode area for the single mode signalallows one to obtain a high power output withrelatively low power density.

Highly efficient lasers ….

Fresnel fiber

A concept of the Fresnel fiber after [1]

PCF With Special properties

Free-space diffraction limit inpropagation of high intensity light.

Very diffcult to generate Bessel wave(diffraction less free-space waveform)beyond the Raighley range ofconventional optics.

Micro-structured fiber technique :Fresnelzones determined by the ring of holesspaced at radii such that interstitial holespacing can be significantly larger thanthe propagation wavelength.

The concentric rings of holes (Fresnelzones) have various effectiverefractive indexes and interferesconstructively, forming a peak fieldintensity in the center of the fiber axis.This enables focusing light at theoutput of the fiber at the far fieldwithout any additional lens, while inconventional fibers, light emergingfrom a fiber diffracts and expands.

Advantages:

Endlessly single mode operationLarge mode areaMany-core fiberFiber amplifierDispersionSpecial 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

References

[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=989118

[1] Photonic Crystal Fibers by R. Buczynski , Vol. 106 (2004) ACTA PHYSICAPOLONICA A No. 2

[2]Reconfigurable Optothermal Microparticle Trap in Air-Filled Hollow-CorePhotonic 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

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