This narrated power point presentation attempts to explain the fundamental principles of Photonic Crystal Fibers. The material will be useful for KTU final year students who prepare for the subject EC 405, Optical Communications.

1. Photonic Crystal Fibers
MEC

2. Contents
• Fiber Bragg Grating.
• Photonic Crystal Fibers.
- Double Clad Photonic Crystal Fiber.
- Index-Guided Microstructures.
- Photonic Bandgap Fibers.
• Nanostructure Core Fiber.

3. Fiber Bragg Grating
• Distributed Bragg Reflector constructed in a
short segment of optical fiber, reflects particular
wavelengths of light, transmits all others.
• Periodic variation in refractive index of the fiber
core generates a wavelength-specific dielectric
mirror.
• Used as inline optical filter to block certain
wavelengths, as a wavelength-specific reflector.

4. Fiber Bragg Grating

5. Photonic Crystal Fibers
• Idea presented by Yeh et al. in 1978, proposed
to clad a fiber core with Bragg grating.
• Photonic crystal fiber made of 2D photonic
crystal with an air core invented by P. Russell in
1992.
• First PCF reported at the Optical Fiber
Conference (OFC) in 1996.
• Combine properties of 2D photonic crystals and
classical fibers.

6. Photonic Crystal Fibers

7. Photonic Crystal Fibers
• Also called holey fibers, cladding contains
air holes.
• Microstructured optical fiber containing a
fine array of air holes longitudinally down
the fiber cladding.
• Microstructure within the fiber is highly
periodic due to the fabrication process.
• Two distinct guidance mechanisms.

8. Photonic Crystal Fibers
• Index guided fibers - guided modes trapped in a
fiber core with higher average index than
cladding containing the air holes - modified total
internal reflection.
• Photonic bandgap fibers - guided modes
trapped in a core of either higher, or indeed
lower, average index - photonic bandgap effect.
• Existence of two different guidance mechanisms
makes PCFs versatile.

9. Photonic Crystal Fibers
• Used to realize optical components and devices
- long period gratings, multimode interference
power splitters, tunable coupled cavity fiber
lasers, fiber amplifiers, multichannel add/drop
filters, wavelength converters, wavelength
demultiplexers.
• Low Transmission losses essential - Increased
homogeneity control, use of highly purified
silicon as the base material - loss of 0.3 dB/km
at 1.55 μm for a 100 km span.

10. Double Clad Photonic Crystal Fiber
• Made of silica with two photonic
claddings with different
properties.
• Solid core surrounded with low
filling factor cladding.
• Inner cladding ensures high
numerical aperture(>0.8),
surrounded with a web of silica
bridges substantially narrower
than wavelength of the guided
radiation.
• Rare-earth ion doping medium of
a fiber laser introduced into the
core of the PCF.

11. Index-Guided Microstructures
• Greater index contrast, cladding contains air
holes with a refractive index of 1, normal silica
cladding index = 1.457, germanium-doped core
index = 1.462.
• Interaction of guided mode with cladding region -
first order and wavelength independent in
conventional fibers.
• Large index contrast and small structure
dimensions - effective cladding index a strong
function of wavelength.

12. Index-Guided Microstructures
• Short wavelengths - effective cladding index
only slightly lower than core index.
• Longer wavelengths - mode samples more of
the cladding, effective index contrast is larger.
• Wavelength dependence results in unusual
optical properties.
• High index contrast enables the PCF core to be
reduced to less than 1 μm, increases light
intensity in the core, enhances nonlinear effects.

13. Index-Guided Microstructures
solid-core region

14. Index-Guided Microstructures
• Air fill fraction is low (< 0.4) - single-moded at all
wavelengths - WDM - broadband applications.
• Holey region more than 20% of the fiber cross-
section - index-guided PCFs display interesting
range of dispersive properties - dispersion-
compensating / dispersion-controlling fiber
components.
• High optical nonlinearity per unit length - modest
light intensities can induce substantial nonlinear
effects - 2R data regeneration with just 3.3 m.

15. Index-Guided Microstructures
• Filling the cladding holes with polymers or liquid
crystals allows external fields to be used to
dynamically vary fiber properties.
• Temperature sensitivity of polymer within the
cladding holes employed to tune a Bragg grating
written into the core.
• Index-guided PCFs with small holes and large
hole spacings provide very large mode area
(low optical nonlinearities) - applications in high-
power delivery (e.g. laser welding & machining),
as high-power fiber lasers and amplifiers

16. Index-Guided Microstructures
• Useful for collection and transmission
of high optical powers where signal
distortion is not an issue.
• PCFs can be spliced to conventional
fibers - integration with existing
components and subsystems.

17. Photonic Bandgap Fibers
• Photonic band-gap materials / photonic crystals -
materials with periodic dielectric profile, prevent
light of certain frequencies or wavelengths from
propagating in single or multiple number of
polarisation directions within the materials.
• Microstructured fiber - periodic arrangement of
air holes required to ensure guidance.
• Periodic arrangement of cladding air holes -
formation of photonic bandgap in the transverse
plane of the fiber.
• Band gap forbids propagation of certain
frequency range of light.

18. Photonic Bandgap Fibers
• Exhibits a two-dimensional bandgap -
wavelengths within this bandgap cannot
propagate perpendicular to the fiber axis.
• Confined to propagate where refractive
index is lower than the surrounding
material.
• Light can be guided within a low-index, air-
filled core region, can also guide light in
regions with higher refractive index.

19. Photonic Bandgap Fibers
• Index-guiding fibers usually have a guided
mode at all wavelengths, PBG fibers only
guide in certain wavelength bands.
• Possible to have wavelengths at which
higher order modes are guided, while the
fundamental mode is not.

20. Photonic Bandgap Fiber Structures

21. Photonic Bandgap Fiber Structures

22. Nanostructure Core Fiber
• Solid silica-based photonic crystal fiber - guides
light using PBG mechanism.
• Two-dimensional periodic array of germanium
doped rods in the core region.
• Minimum attenuation of 2.6 dB/km at wavelength
of 1.59 μm.
• Greater bending sensitivity, smaller index
difference b/w core and leaky modes.
• Applications in optical sensing of curvature
and stress.

23. Solid Silica Structures
• All-solid silica structure facilitate fiber
fabrication using existing technology.
• Birefringence of the order of 10−4
achievable with large mode field
diameter up to 10 μm.
• Use within fiber lasers & gyroscope.

24. References
• John M Senior, “ Optical Fiber
Communications Principles and Practice ”,
Chapter 2, Optical Fiber Waveguides,
Pearson Education/Prentice Hall, Third
Edition, 2009, pp : 75 – 78.
• https://www.youtube.com/watch?v=3gd5u
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25. Thank You