www.inescporto.ptwww.fc.up.pt

http://photonics.light.utoronto.ca/

Birefringence and Bragg grating control in femtosecond laser written optical circuits

lfernandes@fc.up.pt

Luís A. Fernandes

2

Outline

Nonlinear absorption process

Bragg grating spectral control

Integrated wave plates

Polarization beam splitters

Birefringence control

3

The advantages of nonlinear absorption

4

Itoh et al. [MRS Bull. 31(8), 620, 2006]

Ultrafast oscillators: ~ 100 fs ~ nJ ~ 100 MHz

Amplified systems: ~ 100 fs ~ μJ to mJ ~ kHz

Nonlinearphotopolymerization

Laser ablationWaveguide writing in insulators

Nonlinear absorption process

5

Femtosecond laser writing

Laser ablation 10 μm under the glass surface

Waveguide end facet

Localized refractive index change

10 μm

Green light filtered

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Fiber Laser1044 nm, 300 fs

SHG

Z Position Stage

X Y Position Stages

500 kHz

Pulse Energy = 90 nJ to 160 nJWavelength = 522 nm

Scan Speed = 0.27 mm/s

Femtosecond waveguide writing

Shah et al. [Optics Express 13, 6, 2005]

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Fiber Laser1044 nm, 300 fs

SHG

PSO Control

AOM

X Y Position Stages

500 kHz

500 Hz

Pulse Energy = 90 nJ to 160 nJWavelength = 522 nm

Scan Speed = 0.27 mm/s

Zhang et al. [Opt. Lett. 32, 2559-2561 (2007)]

Bragg Grating Waveguide (BGW) Fabrication

Z Position Stage

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Positive index change provides confinement

SMF28 Fiber Waveguide BGW

Bragg Grating Waveguides (BGWs)Waveguide propagation loss

Waveguide end facet

Typical coupling loss of 0.05 dB/facet

10 μm

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Fabrication of Phase-Shifted BGWsNormal Bragg Grating

Defect in the grating structure

10

Spectral shaping applications

Chirped gratings for pulse shapingMultiple phase shifts for spectral shaping

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The intensity controls the light matter interaction

Suspended core fibers

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Bragg grating waveguides summary

Tunable interferometry inscription of gratings in suspended core fibers

With applications in temperature and strain independent sensing

Flexible grating fabrication for prototyping and spectral shaping

Phase control for arbitrary shifts along a grating

With sharp features (22 pm linewidth)

Design of chirped gratings with pulse shaping applications

Becker et al. IEEE Photonics Technology Letters, 21, 1453-1455, 2009Fernandes et al. IEEE Photonics Technology Letters, 24, 7, 554-556, 2012Dolgaleva et al. Optics Letters, 36, 22, pp. 4416-4418, 2011Grenier et al. Optics Letters, 37, 12, pp. 2289-2291, 2012Fernandes et al. Oral presentation OSA Frontiers in Optics, 2009

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Hnatovsky et al. [Appl. Phys. A 84, 47–61, (2006)]

Laser polarization: (formation of nanogratings / form birefringence)

Laser pulse energy / Stress induced anisotropy

Scanning speed, etc

Measured birefringent values ~ [10-5,10-4]

Birefringence in Femtosecond Written Waveguides

Δ n=nV−nH

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Opportunities to use birefringent waveguides

Waveguide birefringence in fused silica:

Bhardwaj et al. [Optics Letters 29, 1312–1314 (2004)]

Yang et al. [J. Appl. Phys. 95, 5280 (2004)]

Bellouard et al. [Optics Express 14, 8360-8366 (2006)]

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Opportunities to use birefringent waveguides

Waveguide birefringence in fused silica:

Bhardwaj et al. [Optics Letters 29, 1312–1314 (2004)]

Yang et al. [J. Appl. Phys. 95, 5280 (2004)]

Bellouard et al. [Optics Express 14, 8360-8366 (2006)]

Nanogratings formation and form birefringence:

Bricchi et al. [Optics Letters 29, 119-121 (2004)]

Hnatovsky et al. [Appl. Phys. A 84, 47–61, (2006)]

Taylor et al. [Optics Letters 32, 2888-2890 (2007)]

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Opportunities to use birefringent waveguides

Waveguide birefringence in fused silica:

Bhardwaj et al. [Optics Letters 29, 1312–1314 (2004)]

Yang et al. [J. Appl. Phys. 95, 5280 (2004)]

Bellouard et al. [Optics Express 14, 8360-8366 (2006)]

Nanogratings formation and form birefringence:

Bricchi et al. [Optics Letters 29, 119-121 (2004)]

Hnatovsky et al. [Appl. Phys. A 84, 47–61, (2006)]

Taylor et al. [Optics Letters 32, 2888-2890 (2007)]

Quantum entanglement measurements:

Marshall et al. [Optics Express 17, 12546–12554 (2009)]

Sansoni et al. [Phys. Rev. Lett. 105, 200503 (2010)]

Lobino and O’Brien [Nature 469, 43-44 (2011)]

Sensor applications:

Polarization sensitive devices

Integrated Lasers

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B

Polarizers

Detector

L

The birefringence splits the Bragg reflectioninto two polarization modes

Δ n=Δλ B

2 Λ

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BGW measurement of birefringence removes the ambiguity of the cross polarizer technique.

Polarizers

Detector

±45º

45º

L

I p=I i2

1cos I c=I i2

1−cos n=

2

L

δ=±cos−1( I p−I cI i )+ m2πm=0,1,2, ...

Waveguide retardance:

Parallel polarizers Crossed polarizers

Measuring birefringence with crossed polarizers

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160 nJ

=±cos−1I p− I cI i

m2 with m=0,1,2, ...

25.4 mm long waveguide

Dependence on writing energy and wavelength

n=

2

L

Energy dependence for parallel writing

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λ/2 - 35 dB polarization contrast(linear polarized output)

λ/4 - 5% variation(circular polarized output)

Zero-order half-wave plate (m = 0)

Parallel polarization writingWaveguide length = 25.4 mmPulse Energy = 160 nJ

Integrated wave plates demonstration

45º

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Integrated wave plates summary

Birefringence measured with BGWs and with crossed polarizers provided accurate and unambiguous values

Successfully demonstrated integrated wave plates:

Less than 5% output variation in a quarter-wave plate

35 dB linearity contrast for a half-wave plate

Broadband (66 nm) wave plates

As small as 4 mm with as low as 0.8 dB loss

Fernandes et al. Optics Express, 19, 19, 18294-18301, (2011)Fernandes et al. Oral presentation in CLEO, 2011Fernandes et al. Oral presentation in Frontiers in Ultrafast Optics, SPIE Photonics West, 2012

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r=P2

P1+P2

P1

P2

r (λ )=Asin2[ κ(λ)L+ϕ(λ )]

Using directional couplers to design polarization beam splitters

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Coupling is polarization dependent

Δ r=sin [(K V−K H )L+ϕV−ϕH ]×sin [(K V+K H )L+ϕV+ϕH ]

Polarization splitting contrast ratio: Δ r=∣rV−rH∣

Separation = 8 μm

r (λ )=Asin2[ κ(λ)L+ϕ(λ )]

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Definition of a more convenient parameter

Δ r=sin [(KV−K H )L+ ϕV−ϕH ]×sin [(KV+ K H )L+ ϕV+ ϕH ]

Δ r=∣r V−rH∣Polarization splitting contrast ratio:

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19 dB and 24 dB extinction ratio polarization splitting

Polarization beam splitters

Parallel polarization writingInteraction length = 19.2 mmSeparation = 8 μmS-Bends radius = 100 mm

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Polarization beam splitters summary

Birefringence provided polarization dependent coupling in directional couplers

Successfully demonstrated integrated polarization beam splitters:

19 dB to 24 dB of polarization contrast splitting

Relatively large devices (19 mm), but higher birefringence can reduce this value

Fernandes et al. Optics Express, 19, 13, 11992-11999, (2011)Fernandes et al. Oral presentation SPIE LASE, Photonics West, 2010

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Controlling the birefringence with stress

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The stress fields are dependent on the geometry

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Independent birefringence control is possible with the strength of the stress fields and with the writing polarization

Strength of thestress fields

Writing polarization

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Birefringence control summary

Birefringence is controllable over a range of < 4 x 10-6 to 4.35 x 10-4 with laser induced stress and writing polarization

Distinction between form birefringence (nanogratings) and stress birefringence

This birefringence control can reduce the size and total loss of wave plates

Minimum of 2 mm with 0.2 dB loss

Ability to design low and high birefringence elements with one step fabrications

Fernandes et al. Optics Express, 20, 22, 24103-24114, (2012)

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Conclusions

Improved Bragg grating fabrication control:

Demonstrated Bragg gratings in pure silica suspended core fibers

Improvements towards flexible spectral shaping and fast prototyping

Demonstrated pulse shaping applications

Demonstrated control and definitive birefringence determination:

From 10-6 to 10-4 birefringence control measured as a function of wavelength, pulse energy and

writing laser polarization

Integrated wave plate demonstration:

λ/2 with 35 dB polarization contrast and λ/4 with 5% variation; Possible broadband operation

Polarization beam splitters with 19.2 mm:

From 19 dB to 24 dB extinction ratios

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Polarization beam splitter

Wave plates

Millimeter length devices with[10-6 to 10-4] birefringence low loss

Future Work

Spectral control:

BGWs for pulse shaping and rapid prototyping

Polarization Control:

Smaller polarization splitters and wave plates

Applications in quantum photonics

Single step fabrication and prototyping

Integration of devices for quantum measurements

Optical circuits in fiber cladding

Implementation of polarization devices

Distributed sensing

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Fundação para a Ciência e Tecnologia (FCT)

Canadian Institute for Photonic Innovations

Natural Sciences and Engineering Research Council of Canada

Acknowledgments

www.inescporto.ptwww.fc.up.pt

http://photonics.light.utoronto.ca/

Thank you

lfernandes@fc.up.pt

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List of publications

Bragg gratings in suspended core fibers

● ''Inscription of Fiber Bragg Grating Arrays in Pure Silica Suspended Core Fibers''

Becker, Fernandes, et al. [IEEE Photonics Technology Letters, 21, 1453-1455, 2009]

● ''Temperature and Strain Sensing With Femtosecond Laser Written Bragg Gratings in Defect and Non-Defect Suspended-Silica-Core Fibers,''

Fernandes et al. [IEEE Photonics Technology Letters, Vol. 24, Issue 7, pp. 554-556, 2012]

BGW spectral control (phase shifts, chirps) and application in pulse shaping

● ''Femtosecond Laser Fabrication of Phase-Shifted Bragg Grating Waveguides in Fused Silica,''

Grenier, Fernandes et al. [Optics Letters, Vol. 37, Issue 12, pp. 2289-2291, 2012]

● ''Integrated Optical Temporal Fourier Transformer Based on Chirped Bragg Grating Waveguide,''

Dolgaleva, Fernandes et al. [Optics Letters, Vol. 36, Issue 22, pp. 4416-4418, 2011]

Birefringence measurements, control and applications

● ''Stress induced birefringence tuning in femtosecond laser fabricated waveguides in fused silica,''

Fernandes et al. [Optics Express, Vol. 20, Issue 22, pp. 24103-24114, 2012]

● ''Femtosecond laser writing of waveguide retarders in fused silica for polarization control in optical circuits,''

Fernandes et al. [Optics Express, Vol. 19, Issue 19, pp. 18294-18301, 2011]

● ''Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica,''

Fernandes et al. [Optics Express, Vol. 19, Issue 13, pp. 11992-11999, 2011]

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List of communications● ''Femtosecond laser writing of polarization devices for optical circuits in glass,''

Fernandes et al. Proc. SPIE, 82470M, SPIE LASE: Frontiers in Ultrafast Optics, Photonics West, 2012

● ''Femtosecond laser direct fabrication of integrated optical wave plates in fused silica,''

Fernandes et al. Oral presentation in CLEO:2011 - Laser Applications to Photonic Applications, paper CWO6, 2011

● ''Integrated Temporal Fourier Transformer Based on Chirped Bragg Grating Waveguides,''

Dolgaleva et al. Oral presentation in CLEO:2011 - Laser Applications to Photonic Applications, paper CThHH6, 2011

● ''Femtosecond laser fabrication of birefringent directional couplers in fused silica,''

Fernandes et al. 7584-22, SPIE LASE: Laser Applications in Microelectronic and Optoelectronic Manufacturing XV, Photonics West, 2010

[Received Student Award for best oral presentations]

● ''Flexible tailoring of femtosecond laser-written Bragg grating waveguides,''

Grenier et al. SPIE MOEMS-MEMS: Advanced Fabrication Technologies for Micro/Nano Optics and Photonics III, Photonics West, 2010

● ''Femtosecond Laser Writing of Phase-Shifted Bragg Grating Waveguides in Fused Silica,''

Fernandes et al. OSA Frontiers in Optics, paper LMTuC5, 2009

● ''Temperature and strain characterization of Bragg gratings impressed with femtosecond laser radiation in suspended-silica-core fibers,''

Fernandes et al. Proc. SPIE, 73861N, Photonics North, 2009

● ''Fiber Bragg grating inscription with DUV femtosecond exposure and two beam interference,''

Becker et al. Proc. SPIE, 73862Y, Photonics North, 2009