email: bppalphysics'iitd'ernet'in

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Photonic Bandgap Bragg Fibers A new platform for
realizing application specific Specialty
Optical Fibers and All-Fiber Components
IEEE-LEOS Distinguished Lecture (2005-2007) Part-I
email bppal_at_physics.iitd.ernet.in
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Indian Institutes of Technology
IIT Roorkee(2001)
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Vision of IIT Delhi

• Excel in scientific and technical education and
  research
• Serve as a valuable resource to industry
• Remain a source of pride for all Indians

4
Structure of programmes
Ph. D. (3 to 5 yrs, typically)
2 yr M.Tech. or MS (Research) (Admission thru
GATE)
2 yr M.B.A.
5 yr dual degree (B. Tech. M. Tech.) Integrated
M.Tech
2 yr M.Sc.
4 yr B. Tech. (Admission thru JEE)
UG 2200 Students PG 2800 Students

• After 12 years of school education (focus on
  Maths, Science)
• Over 300K students write JEE and 3500 get
  admitted in all 7 IITs together

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Part I Photonic bandgap Bragg fibers
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Acknowledgement

• Sonali Dasgupta
• Dr. M. R. Shenoy

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Agenda

• Introduction
• PCF Index Guided PBG Structures
• Bragg fibers
• Modeling and fabrication
• Specific dispersion tailored designs various
  applications
• Conclusions

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Theoretical BW of an Optical Fiber

• 1280 nm (235 THz) to 1650 nm (182 THz)
• ? 53 THz
• gt 12 billion telephone channels (_at_ 64 Kbit/s for
  one ch)!

25 30 THz is usable today through DWDM!
Till the IT bubble burst

• O.F. installed _at_ 3000 m/hr ? 3 times around the
  globe/day !

• Wave Star 400 Gb/s systems (commercially
  available)
• ? 12000 Encyclopedia volumes/sec!

• Indian Telecom Co.s are experiencing growing
  market!

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International and National Connectivity India
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Undersea Fiber cable drop off and National
Network Connectivity India
(VSNL)
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Two most important Tx characteristics

• Dispersion
• Loss

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Pulse dispersion with propagation in a fiber
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Chirping resultant signal distortion
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Dispersion spectrum

• Standard G.652 fiber

D 17 ps/km.nm _at_ ? 1550 nm
D ????c? d2ne?d?2
Anomalous dispersion region
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Attenuation spectrum

• AllWave Fiber Lucent Tech.
• SMF-28e Corning Inc.

Low water peak fiber (LWPF)
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Optical nonlinear effects

• For intense electromagnetic fields (comparable to
  inter-atomic fields), the response of the medium
  to incident light becomes nonlinear
• P ?o( ?(1)E ? (2)EE ? (3)EEE....)
• ?o vacuum permittivity
• ?(n) nonlinear susceptibility

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Nonlinear characteristics of silica fiber

• ?(2) ? 0 ? Does not contribute to
  nonlinear effects
• ?(3) ?0 ? Major contributor to the
  nonlinear processes. Results in the
  intensity dependent variation of the
  refractive index

In silica, n2 2.2 x 10-20 m2/W
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Nonlinear effects in an optical fiber

• Stimulated Scattering effects
• Stimulated Brillouin Scattering
• Stimulated Raman Scattering
• Soliton Self Frequency Shift

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Question

• Is this the end of fiber development?

NO

• Application-specific specialty fibers emerged

• Fibers in which material loss is not a limiting
  factor

• Nonlinearity and/ dispersion properties could
  be tailored to achieve characteristics otherwise
  impossible in conventional fibers

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Emergence of Microstructured Optical Fibers

• Wavelength scale periodic refractive index
  features

• Opened up lots of avenues not necessarily for
• telecom alone!

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The birth of PHOTONIC CRYSTALS
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Photonic Crystal (PhC) concept

• 1987 Eli Yablonovitch, PRL, vol. 58, pp.
  2059-2062 (1987)
• 1987 S. John, PRL, vol. 58, pp. 2486-2489 (1987)

(Same year in which EDFA was discovered at U of
Southampton!)

• Lattice of dielectrics with right spacing and
  different optical properties can generate an
  optical bandgap (similar to electronic bandgap
  in semiconductors)

• Light in this optical bandgap will not propagate
  in the crystal structure except where a defect
  disrupts regularity of the lattice (akin to
  change in semiconductor properties by dopants)

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BPPal
PhC
Square lattice of air holes in a high-index
dielectric
Spatial periodicity is a natural analog to solid
state crystals
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Historical quotes!
Yablonovitchs quotes
that people were going to read my article and
say, Oh, thats so obvious why didnt I
think of it!
There were no citations to speak of in the
first couple of years I guess it wasnt
obvious to anybody
Since 1993, no. of papers using the phrase
photonic bandgap was growing _at_ 70 each year
.
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Experimental demonstrations

• Impressive breakthrough demonstrations e.g.
• - 3D photonic crystals (PhC) operating at
  optical ?s
• - PhC waveguide that steers light around sharp
  corners

Soon other groups notably MIT (USA) and Univ
of Bath (UK) initiated work on applying PhC
principles / concepts to fibers and realize
photonic crystal fibers (PCF)/ Photonic
bandgap fibers (PBGF) Microstructured fibers
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PC Fibers Fabrication
Fibers with an internal periodic structure formed
through capillaries filled with air, typically
laid to form a hexagonal lattice
a) A stack of glass tubes is formed as a
macroscopic preform b) Fused at 1800 2000
?C c) Drawn into a fiber in a fiber draw
tower
P. Russell, Science, 299,Jan 2003
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Guidance in Conventional Fiber

• Total Internal Reflection at the core-cladding
  interface

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Microstructured Fibers

• Broad classification depending on light
  confinement
• mechanisms

- Index Guided e.g. holey fibers
- Photonic bandgap guided
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Guidance in PC Fibers
- Index Guided

• Guidance is due to a modified TIR arising from
  the air-
• filled holes forming cladding of smaller
  effective r.i.

?
Holey fibers
Periodicity is not essential!
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Guidance in a 2-D PBGF
PBG guided
Core defect realized through a central air
capillary (usually a bigger diameter)
PhC cladding

• Bragg scattering from the dielectric interfaces
  blocks certain ?s from propagating into the
  structure thereby creating a PBG effect

Periodicity is essential!
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Assortment of PCF's
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Photonic crystal Fibers

• offer huge design freedom

• Several parameters to manipulate
• - lattice pitch
• - air hole diameter shape
• - r.i. of the glass
• - pattern of lattice

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Tailored characteristics

• Anomalous, zero or low normal dispersion even in
  the visible region

• Flattened dispersion over a large wavelength
  range

• Exceptional nonlinear optical properties
  (through anomalous dispersion and small MFD)

• Large solid or air core single-mode fiber
  feasible

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Chronology of PCF development
After R. Buczynski, Acta Physica Polonica A,
vol. 106 (2004)
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Bragg Fibers
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Guidance in 1-D PBG fiber
Bragg fiber
Low index core ( few microns)
Periodic cladding (sub micron layer thickness)
Refractive index profile of Bragg fiber
Cross-sectional view of Bragg fiber
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Air-core Bragg fiber

• Originally proposed by Yarivs group in 1978 at
  Caltech
• MIT group eventually fabricated it in 2003 and
  named it as
• OmniGuide
• Omniguide fiber has been cleared by American
  Drug Admin.
• for trials in humans as a medium to pump in
  high-power CO2
• laser as a diagnostic tool (due to the air
  core, material damage
• threshold is very high!

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Conventional, metallic and Bragg fibers
Source OmniGuide
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Functional Principle

• Analogous to a planar stack of alternate high
  and low index media
• Characteristic parameters n1, l1 and n2, l2
  l1 l2 ?

• Physics of waveguidance is understood in terms
  of formation of
• bandgap decaying of Bloch waves in a
  multilayer planar stack

Guided Wave Optical Components and Devices
Basics, Applications and Technology, B.P. Pal
(ed.), Elsevier (USA), October 2005.
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Photonic Bandgap
Photonic bandgap forbids propagation of light (in
the medium) whose frequency falls within the
bandgap region
OmniGuide
Typical bandgap diagram of a 1D-periodic planar
stack
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Modeling of Bragg fibers

• Minimization Procedure

P. Yeh, A. Yariv, E. Marom, J. Opt. Soc. Amer.
68, 1196 (1978)

• Finite Difference Time Domain Method

• Less computational time and valid for kjr gtgt
  1 where

nj is the refractive index of the cladding layers

• Fairly accurate results (lt 2 error) for air-core
  Bragg fibers with large index contrast

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Semi-asymptotic Matrix Approach
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Condition for optimum confinement
? Round trip phase through one period (?)is 2?
l1 , l2 thickness of cladding layers n1 ,
n2 refractive index of cladding layers
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Air core Bragg fiber

• Typical dispersion modal field amplitude

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Effect of cladding thickness variation on field
confinement
Choice of physical parameters is critical for
achieving desired propagation characteristics
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Dispersion compensating Bragg fiber
S. Dasgupta, B.P. Pal, and M.R. Shenoy, Optics
Letters, Vol. 30, 1917-1919 (2005) cited in
Virtual J. Nano-scale Sc Tech. , AIP, July 25
issue (2005)
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Dispersion compensating fiber
DTLT DDLD 0
Dispersion coeff. (D) L1(d??d?) ?
??0?c)d2neff?d?2
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Bragg fiber designs for dispersion compensation
Reported designs

• Exploit HE11 mode
• Introduce defect layer in the cladding
• (TE01 mode)

G. Ouyang, Y. Xu, and A. Yariv, Opt. Exp., vol.
10, 899, (2002) T. D. Engeness, M. Ibanescu, S.
G. Johnson, O. Weisberg, M. Skorobogatiy, S.
Jacobs and Y. Fink, Opt. Exp., vol. 11, 1175,
(2003)
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Proposed design for dispersion compensating Bragg
fiber (DCBF)

• Proposed design

?0 central wavelength of the bandgap
?o 20 greater than operating wavelength
(?op) ?op Operating wavelength at the band
edge at which negative dispersion is
desired
Trade off between Low loss High negative
dispersion
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Dispersion and loss spectrum of proposed DCBF
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Advantages of proposed DCBF

• Large negative dispersion of TE01 mode (in
  perfectly periodic Bragg fiber)
• High FOM Two orders of magnitude higher than
  that of conventional DCFs
• Non-degenerate TE01 mode ? Polarization mode
  dispersion is absent
• Adaptable design for any desired ?op

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Bragg fiber for metro neworks
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Fiber glut

• In the metro sector the glut is much smaller

• Focus shifted to Metro Optical Network (MON)

• Challenge is to develop bandwidth efficient MON
  at low cost

• Economics is dictated by number of components,
• architectural simplicity, and repair costs

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Metro-fiber design requirements

• Accommodate unpredicted traffic growth
• Transparent network (120 200 km) with
  flexibility to add/drop individual
  signals at any node(s) before regeneration
• Dispersion loss are key design issues

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Reported Metro specific fiber designs
Span length of 100 km _at_10 Gb/s, without the
need for a dispersion compensating device (avg D
7 8 ps/km.nm)
1 I. Tomkos, B. Hallock, I. Roudas, R. Hesse, A.
Boskovic, J. Nakano, R. Vodhanel, IEEE Photon.
Technol. Lett., vol. 13, 735 (2001) 2
http//www.alcatelcable.com/Products/Fiber/data-sh
eets/6911_ds_rev0.pdf
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MetroCor (Corning)
-10 ? D (ps/nm.km) ? -1 (1530 1605 nm)
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Metro Fiber (Alcatel)
6 ? D (ps/nm.km) ? 13 (1530 1650 nm)
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Proposed metro specific Bragg fiber design
Span length of 100 km without any dispersion
compensator and amplifier
We exploit

• Quarter wave stack condition for low loss

• Large air-core for small positive dispersion and
  small dispersion slope

B. P. Pal, S. Dasgupta and M. R. Shenoy, Opt.
Express, vol. 13, 621-626 (2005)
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Dispersion spectrum of proposed Bragg metro-fiber
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A negative dispersion Bragg metro-fiber

• Through a defect layer in the cladding

-9.4 ? D (ps/nm.km) ? -0.2 (1520 1600 nm)
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SOLID-CORE BRAGG FIBERS
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Why solid core Bragg fibers ?
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Nonlinear effects in the optical fiber

• Stimulated Scattering effects
• Stimulated Brillouin Scattering
• Stimulated Raman Scattering
• Soliton Self Frequency Shift

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Supercontinuum (SC) generation
Broad coherent spectrum, extending over tens of
nanometers, which results from the broadening of
the spectrum of optical pulses in a nonlinear
medium.

• The spectral broadening occurs due to an
  interplay
• of the various nonlinear processes
  occurring in a
• medium

2. SC spectrum critically depends on the
dispersion profile of the fiber
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Supercontinuum Light
P. Russell, Science, 299,Jan 2003
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Supercontinuum Light
Courtesy Wayne Knox and Parama Pal, Institute
of Optics, University of Rochester, NY
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Supercontinuum Light

• Utility in
• Optical Coherence Tomography
• Optical Metrology
• High speed spectroscopy

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Requirements for generating supercontinuum

• High intensity electromagnetic (EM) field
• Appropriate dispersion characteristics of the
  medium

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MODELING OF LOW-INDEX CONTRAST BRAGG FIBERS
LP approximation is valid for low-index contrast
Bragg fibers

• Obtaining the eigen value equation
• Incoming field component in the last cladding
  layer is zero
• Eigen function Lorentzian
• Location of peak Effective index
• FWHM Loss

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Nonlinear pulse propagation in optical fibers
Dispersion effects
Nonlinear effects
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Numerical simulations Split Step Fourier Method
Dispersive terms
Nonlinear terms
GP Agrawal, Nonlinear F.O., Academic Press
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Dispersion Decreasing Bragg fiber (DDBF) for
supercontinuum light
B.P. Pal, S. Dasgupta, M.R. Shenoy, and A.
Sysoliatin, Optoelectronics Letters (15th
Sept.,2006)
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DDBF

• Lop of the DDBF is order of magnitude smaller
  than DDF

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Supercontinuum pulse

• 100 fs pulse, peak power 5kW

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Temporal spectrum
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Advantages

• NdYAG laser pump at 1060 nm ? Easy to achieve
  high powers
• SC pulse at 1060 nm is not possible through HNLFs
  and DSFs
• 150 nm 25-dB bandwidth (useful in OCT) with
  DDBF of length lt 1m
• Aeff 55 ?m2 ? easier coupling of light
• Silica core DDBF should be easy to fabricate
  using well-known MCVD process (net change in dia
  3)
• more economic than PCFs

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Issues involved

• Tradeoff between small effective area and small
  dispersion slope
• Flattened dispersion characteristics

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Conclusions

• Bragg fiber-based Dispersion Compensator using
  multiple quarter wave stack condition to achieve
  very high FOM
• Metro-centric Bragg fiber design to achieve
  uncompensated span length of 100 km
• Tapered Bragg fiber for enhanced nonlinear
  interactions as an alternative technology
  platform for supercontinuum generation

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All-Fiber Components
IEEE-LEOS Distinguished Lecture (2005-2007) Part
- II
email bppal_at_physics.iitd.ernet.in
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Branching components
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All-fiber component Technologies Advantages
Much reduced insertion loss vis-à-vis micro-optic
I.O. components

• Coupling losses between the Tx fiber and the
  component
• Absence of Fresnel reflection losses due to
  mismatch in r.i.
• Easy integration through fiber splicing

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Platforms
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Fuse-pull-taper Technique
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Schematic of the microprocessor-controlled FBT
coupler fabrication set up at IIT DELHI
OXY BUTYLENE FLAME
PULLING MECHANISM
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Metric for BW utilization in a DWDM system

• Useful metric

• Problem TDM beyond 40 Gb/s is difficult!

• Best near-term option smaller channel spacing
  (??)!

? Tighter tolerance on components like wavelength
filter!
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Wavelength Interleaver
A wavelength interleaver is a device that
combines two, or more streams of wavelength
channels ( , ) with constant spacing
in the frequency domain, into a single
dense stream of channels with separation
at the output.
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Applicability of the Device
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Wavelength Division Multiplexing (WDM)

• Dense WDM (DWDM)

• DWDM requires ITU compliant channel spacings
• ?
• 193.1 THz ? 0.1 ITHz I an integer
• e.g. 200 GHz (? 1.6 nm), 100 GHz (? 0.8 nm), 50
  GHz (? 0.4 nm)

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All-fiber Unbalanced MZI Based Wavelength
Demutiplexer
?? channel spacing ?1, ?2 propagation
constants at signal wavelengths ?1 and ?2
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Two-stage MZI Configuration for Flattop Response
?L1
Port 1
Port 5
K1
K2
K3
Port 6
?L2
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Optimization of the Splitting Ratios of the
Couplers
Criteria for flatness
Flatness parameter ? 0.05 dB
and
Criteria for minimum loss
Minimum loss ? 0.05 dB
and
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Experimental Set-up for Realizing Flattop
Interleaver Based on Two-stage Unbalanced MZI
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Experimental Results
Experimentally measured flattop wavelength
response.
Simulated flattop wavelength response.
FSR 0.5 nm, and flattened over 0.1 nm
spacing Validates our algorithm !
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Gain Flattening Filters
N. Kumar, M.R. Shenoy, and B.P. Pal, Standard
single-mode fiber-based loop mirror as a gain
flattening filter, IEEE Photon. Tech. Letts. vol.
17, pp. 2056-2058 (October, 2005)
98
Principle of gain flattening
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Fiber Loop Reflector
Port-1
CW
Coupler
CCW
Port-2
T Transmittance, CW Clockwise R
Reflectance, CCW Counter-Clockwise
The key ??? phase shift suffered by the coupled
light
S. Li, K.S. Chiang, and W.A. Gambling, Gain
flattening of an EDFA using a high-birefringence
fiber loop mirror, IEEE PTL, vol. 13, p. 942
(2001)
100
Role of Polarization

• Some birefringence is present in the loop due to
  bends and twists etc.

are Jones matrix elements
where,
? Orientation of the wave plate ?
Birefringence
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Simulation Results
Simulated spectral response at the transmitted
port of the loop mirror realized with
over-coupled coupler having FSR 60 nm.
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Experimental Set-up for Gain Flattening of EDFA
PC Polarization Controller
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Gain-Flattening of EDFA
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Side-polished fiber half-coupler-based GFF
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Side-polished fiber half-coupler technology
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Tunable fiber coupler
Phase resonance leads to power coupling through
evanescent tails
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Side-polished fiber half-coupler-based GFF
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Physical Principle

• Side-polished SMF with a MM Overlay Waveguide

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Modeling
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Measured transmission spectrum
Measured Spectrum

Present method
Uniform cladding
approximation

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Experimentally measured flattened gain spectrum
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Conclusions

• All-fiber MZI-based Wavelength Interleaver
• All-fiber loop mirror-based Gain-flattening
  filter
• Side-polished fiber with a MM waveguide-based
  GFF