Supercontinuum Generation in Photonic Crystal Fiber and Hollow Core Fiber for

Sensing Applications

Dr. Shyamal K. BhadraFiber Optics & Photonics Division

CSIR-Central Glass & Ceramic Research InstituteKolkata, India

IndoIndoIndoIndo----French International Workshop on Glasses and GlassFrench International Workshop on Glasses and GlassFrench International Workshop on Glasses and GlassFrench International Workshop on Glasses and Glass----ceramicsceramicsceramicsceramics

June 6June 6June 6June 6----8, 20128, 20128, 20128, 2012

Lille, France Lille, France Lille, France Lille, France

SuportedSuportedSuportedSuported by CEFIPRAby CEFIPRAby CEFIPRAby CEFIPRA----IFCPARIFCPARIFCPARIFCPAR

Microstructured optical fibers (MOFs): a special class ofoptical fibers made up of only silica glass as the core and an arrayof air holes around the core that runs along the entire fiber length.

Guidance mechanism: Modified total internal reflection

Advantages & Applications:

•Large index contrast between silica andair gives rise to novel waveguidingproperties.

•Single material fiber no dopantsrequired.

•Enhanced design space compared tocylindrical silica fibers: high flexibilityin tailoring the design parameters.

•Extensively used for Supercontinuumgeneration, High power lasers, Bio-sensors etc.( ) 2/1222

FSMcoeff

PCF nna

V −=λ

π

( ) 2/1222clco nn

aV −=

λπ

Salient features & properties of MOFs

�Endlessly single mode behavior: Index difference between coreand cladding is smaller at shorter wavelengths whichcounteracts the trend for multimode behavior and only thefundamental mode is guided even at very short wavelengths.

�Unique dispersion properties: Zero dispersion at visiblewavelengths, multiple zero dispersion wavelengths, ultra-flattened dispersion over a very wide wavelength range.

�Enhanced nonlinearity: high-intensity light guidance in a verysmall core and strong modal confinement due to large indexcontrast between core and cladding.

MostMostMostMost importantimportantimportantimportant designdesigndesigndesign parametersparametersparametersparameters –––– AirAirAirAir holeholeholehole diameterdiameterdiameterdiameter andandandand PitchPitchPitchPitch....HighHighHighHigh flexibilityflexibilityflexibilityflexibility inininin tailoringtailoringtailoringtailoring thesethesethesethese twotwotwotwo parametersparametersparametersparameters leadleadleadlead totototo novelnovelnovelnovelpropertiespropertiespropertiesproperties ofofofof MOFsMOFsMOFsMOFs whichwhichwhichwhich areareareare unachievableunachievableunachievableunachievable inininin conventionalconventionalconventionalconventional fibersfibersfibersfibers....

Microstructured fiber fabrication: facilities at CGCRI

Capillaries of suitable dimensions are stacked in a hexagonal arrayaround a central silica rod using a customized V-groove assembly andput inside a silica jacketing tube. The whole arrangement forms themacroscopic preform.

V-groove assembly helps in compactstacking of capillaries to preventinterstitial spaces and obtain perfectgeometry

Hexagonal array of capillaries

1. Stacking of capillaries

UfAt z=0

At z=z

Ud

z

hole

L

C = 0 Hole collapseC = 1 preform geometry preservedC < 1 Hole contractionC > 1 Hole expansion

Predictive studies of capillary drawing

Collapse ratio ( )

+=2010

2010

/ln hh

hh

UUU

LC

fdfµγ

( )

−−= −

1020

2010

2/1 hhU

Lhheh

fµβγβ

( )

−−= −

1020

1020

2/2 hhU

Lhheh

fµβγβ

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.30.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.30.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.30.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.30.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.30.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.30.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

19500C

20000C........ Uf=8 mm/min

------- Uf=4 mm/min

Uf=2 mm/min 19000C

Inne

r D

iam

eter

(mm

)

Ud (m/min)Ref. – Fitt et al, JLT, vol.-19 (12), 2001

Stacked capillaries in a hexagonal array

Stacked MOF preforms

PCF drawing from the existingconventional fiber drawing tower

3. MOF Drawing

Cane is inserted into a thick silica jacketing tube and thiscomposite arrangement is finally drawn down to fiber.

Final cross-section of a drawn MOF

Theoretical Modeling of MOFs by a commercial software COMSOLMultiphysics implementing the Finite Element Method (FEM).

Steps involved:•Micrograph of the fabricated MOF is traced by CAD software• Imported into the commercial module implementing FEM to solveMaxwell’s equation for that MOF structure

•Refractive index of silica taken into account through Sellmeier’s equation•Finally modal effective indices (neff) of guided modes are obtained

Dispersion in MOFs

•Novel dispersion properties achievable in MOFs: zero dispersionwavelength (ZDW) can be shifted to very short wavelengths, multipleZDWs, ultra-flattened dispersion over a very wide range of wavelength.

• Interplay of higher order dispersion coefficients and various nonlinearphenomena determines the spectral bandwidth and brightness of thegenerated Supercontinuum.

2

2

λλλλλλλλ

d

nd

cD

eff−=Dispersion is calculated using the expression:

Comparison of theoretical and experimental data of dispersion for two fabricated MOFs

Tailoring dispersion by tuning the design parameters

constant pitchand varying d/Λ

constant d/Λ and varying pitch

ΛΛΛΛd

Sunlight (/white light):

• Broad spectrum

• Low brightness

• Low directionality

• Low temporal coherence

Laser:

• Narrow spectrum

• High brightness

• High directionality

• High temporal coherence

• Broad spectrum

• High brightness

• High directionality

• High temporal coherence

Sunlight Laserknown as

Supercontinuum

Supercontinuum (SC) Generation: Phenomenon in which an ultra-short laser pulse undergoes extreme nonlinear spectral broadening togive a broadband spectrally continuous output due to the interplaybetween higher order dispersion and various nonlinear processes.

Applications: OCT, ultra-short pulse generation, designing multiwavelengthsources, optical frequency metrology, spectroscopy, etc.

Nonlinear processes in SCG:•Self phase modulation•Raman scattering•Soliton self-frequency shift•Four wave mixing•Cross phase modulation

Fundamental NLSE

Kerr Nonlinearity

12

2002 ===

βγ TP

L

LN

NL

D

=

0sec),( T

ThTzA

Solution is of the form:

=

0sec),( T

ThNTzA

Higher Order Solitons

N is the soliton order

N = 1 Fundamental solitonN = 2 Second order solitonN = 3 Third order soliton And so on……………….

Fundamental soliton

Theoretical aspects of femtosecond SC generation

Evolution of higher order solitons

N=2

N=3

N=4

N=5

The form of the generalized nonlinear Schrödinger equation is ……….

Nonlinear response function Raman response function

τ1 = 12.2 fs; τ2 = 32 fs

Dispersion term

Self-Steepening termSPM & Raman term

Design criteria for MOFs for efficient supercontinuum generationusing ultrashort pump pulses

• High nonlinearity & tight modal confinement� High air-filling fraction/ High index contrast� Small core size

• Zero dispersion wavelength close to the pulsed pump wavelength• Pump wavelength in the anomalous dispersion regime

Experimental conditions:

Pulse width: 110 fs (FWHM)Pump wavelength: 1060 nm

Comparison of the experimental and simulated femtosecond SC spectrum

Related publications:Applied Optics, vol. 48, G12-G20 (2009), Ghosh et al.Optics Commun, vol. 283, 3081-3088 (2010), Roy et al.Physical Review A, vol. 79, 023824 (2009), Roy et al.Optics Letters, vol. 34, 2072-2074 (2009), Roy et al.

Visible part of the SC spectrum obtained from the MOF fabricated atCGCRI dispersed by a prism. The experiment was done at Heriot-WattUniversity, Edinburgh, UK.

Femtosecond SC generation: Experimental Results

Parameters ValuesAir-filling fractionAir hole diameterPitchCore diameterZero dispersion wavelength

0.955.80µm6.07µm4.28µm1019nm

PCF-19D (3) PCF-19D (4)

Parameters ValuesAir-filling fractionAir hole diameterPitchCore diameterZero dispersion wavelength

0.935.24µm5.66µm3.31µm954nm

High average power SC generation

�39 W average output power → maximum reported till date for pulsed

sources.

�MOF design with comparatively larger core and high core-claddingindex contrast led to greater coupling efficiency (80%), highdamage threshold, improved power transmission and rapid onsetof visible SC with blue spectral components extending to about 400 nm.

SC output power vs input poweralong with the output mode profileand prism-separated white light

Evolution of continuum in 2m length ofthe MOF for 21 ps pulses at differentpump powers and rep. rate of 28 MHz

Published in Optics Express, 18 (6), 5426-5432 (2010), Chen et al.

Blue-enhanced SC generation by group index matching technique

IDFiber dia.

(µm)Core dia.

(µm)Air-filling fraction

λZDW

(nm)

ABC

11510090

4.373.683.22

0.920.910.92

984970945

0.4 0.6 0.8 1 1.2 1.4 1.61.47

1.48

1.49

1.5

1.51

1.52

1.53

1.54

Wavelength (µµµµm)G

roup

inde

x (n

g)

CBA

ng = n – λ(dn/dλ)To generate wavelengths further into theblue the group index curve is to bemodified to rise more steeply withincreasing wavelength in the infraredregime in order to match the limitinginfrared wavelength to a deeper blue.

Ref.: Opt. Exp., vol. 16 (4), 2008, Stone and Knight.

Results: Observations and Optimization

λp = 1060 nmT = 600 psL = 10 m

400 600 800 1000 1200 1400 1600-40

-35

-30

-25

-20

-15

-10

-5

0

Wavelength (nm)

Sp

ectr

al P

ow

er (

dB

)

ABC

350 400 450 500 550 600 650 700 750 800-40

-38

-36

-34

-32

-30

-28

-26

-24

-22

-20

Wavelength (nm)

Sp

ectr

al P

ow

er (

dB

)

ABC

� Extent of SC spectra of MOFs Band C are almost same. Wetherefore seem to reach a limitto the short wavelength edge ofthe SC spectra.

� Although decrease in core sizeleads to shorter wavelengthgeneration, yet spectral powerof the continuum in the blueside shows steady reduction.

� No such remarkable powerreduction is observed in thelonger wavelength side.

� Optimization of the corediameter and position of thepump w.r.t. ZDW is necessaryfor generating wavelengthsdeeper into the blue region withenhanced spectral powerdensity.

MOF – B generates maximum blue-enhancedSC spectra with optimum spectral powerdensity in the short wavelength side.

Published in J. of Lightwave Technology, vol. 29, 146-152 (2011), Ghosh et al.

Initial laser cavity design

Lasing at 2µm; Inset: ASE

30% output coupling fibermetal coated mirror

Index matching gelLMA-PCF

M1M2

Gefilter

DM2

DM1

M3

O1

O3

O2

Yb:Tm = 2.5:1Fiber diameter ≈ 130µmCore diameter ≈ 6.21µmBridge width ≈ 0.586µm

Yb-Tm co-doped air-clad fibers for 2µm laser application

Hollow core PCF: Light guidance in air via photonic bandgap

Geometrical parameters:Fiber diameter ≈ 140 µmCore diameter ≈ 9.50 µmHole diameter ≈ 5.5 µmPitch ≈ 6.10 µm

Possible applications:�Chemical and biological sensors�Gas-filled Raman amplifier�Laser induced micro particle guidance�Laser frequency metrology�Quantum optics

Recent development

Cross-section of a hollow core PCF fabricated at CGCRI, Kolkata

central hollow coreCane

Fiber cross-sectional view and TEM picture of Si nano-particles doped preform

Activities at Fiber Optics & Photonics Division, CGCRI

�Photonic crystal fibers

� High power optical fiber laser

Yb-doped high yttrium alumino-silicate nano-particlesbased pentagonal-shaped fibers (core dia.≈35µm aredeveloped through the conventional MCVD process withsolution doping technique. The lasing performance(81% efficiency, 17.5W output power), measured atORC, Southampton, UK is shown alongside.

�MCVD system with chelate delivery facility

The system, from Nextrom Inc., Finland has the facilityto deliver rare-earth elements in vapour phase.

�Fiber Bragg grating based sensors and devices

A prototype CCD based FBG interrogationsystem with user interface software andpackaged weldable FBG strain sensors.

0.1

0.2

0.3

0.05

0.1

0.15

0.2

Blue component due to dispersive wave in presence of 3OD

3OD present 3OD absentFrequency Domain

No Radiation

Effect of 3OD on Dispersive wave

δ3 = 0.02 δ3 = 0.03

600 700 800 900 1000 1100-80

-70

-60

-50

-40

-30

-20

-10

0

λ (nm)

Pow

er(

dB

)

600 700 800 900 1000 1100-80

-70

-60

-50

-40

-30

-20

-10

0

λ (nm)

Po

we

r (d

B)

N=3

What happen if δ3 < 0

The radiation falls in the opposite side

Frequency domainδ3 = -0.01

Low amplitude pedestal

N=2Time domain

Positive and Negative Third order dispersion (3OD)

δ3 = 0.06 δ3 = -0.06

Radiation

Radiation

δ1 = Normalised Delay

Reference

-3-2

-10

0

1

2

3

40

0.5

1

1.5

2

(ν-ν 0)T 0

ξ

Inte

nsity

NSR

Conjugate dispersive waves

Dual radiation

Effect of 4OD on temporal and spectral pulse evolution

L=0.25 unit L=0.50 unit

L=0.75 unit L=1 unit

Spectrogram of dual radiation at various distance (N=2)

2nd ZDP

Blue

Red

Results

Evolution of frequency position and amplitude of dispersive waves as a function of fourth order dispersion for δ3 = 0 (blue), δ3 = 0.01 (red) and δ3 = 0.02 (black).

The solid curves in fig (a) represent modified phase matching condition considering higher order effects

(a) (b)

1 2 3 4 5 6 7 8 9

x 10-3

-80

-70

-60

-50

-40

-30

-20

-10

0

δ4

Pea

k po

wer

(dB

)

N=2

1 2 3 4 5 6 7 8 9

x 10-3

-6

-5

-4

-3

-2

-1

0

1

2

3

4

δ4

∆νd

T0

N = 2

Reference

All odd higher order dispersion (HOD) terms generate a single peak on the blue or red side depending on their signs. All positive even HOD terms, on the other hand, generate conjugate radiations on blue and red side. No radiation is observed for negative values of those parameters.