Birefringence and Bragg grating control in femtosecond laser written optical circuits
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This document summarizes research on controlling birefringence and Bragg gratings using femtosecond laser writing in optical circuits. It describes using this technique to fabricate integrated wave plates with polarization contrast up to 35 dB and polarization beam splitters with extinction ratios from 19-24 dB. The research demonstrates controlling birefringence from 10-6 to 10-4 by manipulating laser stress fields and polarization during writing. Future work aims to reduce device sizes and losses for applications in quantum photonics and sensing.
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Birefringence and Bragg grating control in femtosecond laser written optical circuits
- 1. www.inescporto.pt www.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. 2 Outline Nonlinear absorption process Bragg grating spectral control Integrated wave plates Polarization beam splitters Birefringence control
- 3. 3 The advantages of nonlinear absorption
- 4. 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 Nonlinear photopolymerization Laser ablation Waveguide writing in insulators Nonlinear absorption process
- 5. 5 Femtosecond laser writing Laser ablation 10 μm under the glass surface Waveguide end facet Localized refractive index change 10 μm Green light filtered
- 6. 6 Fiber Laser 1044 nm, 300 fs SHG Z Position Stage X Y Position Stages 500 kHz Pulse Energy = 90 nJ to 160 nJ Wavelength = 522 nm Scan Speed = 0.27 mm/s Femtosecond waveguide writing Shah et al. [Optics Express 13, 6, 2005]
- 7. 7 Fiber Laser 1044 nm, 300 fs SHG PSO Control AOM X Y Position Stages 500 kHz 500 Hz Pulse Energy = 90 nJ to 160 nJ Wavelength = 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
- 8. 8 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
- 9. 9 Fabrication of Phase-Shifted BGWs Normal Bragg Grating Defect in the grating structure
- 10. 10 Spectral shaping applications Chirped gratings for pulse shapingMultiple phase shifts for spectral shaping
- 11. 11 The intensity controls the light matter interaction Suspended core fibers
- 12. 12 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, 2009 Fernandes et al. IEEE Photonics Technology Letters, 24, 7, 554-556, 2012 Dolgaleva et al. Optics Letters, 36, 22, pp. 4416-4418, 2011 Grenier et al. Optics Letters, 37, 12, pp. 2289-2291, 2012 Fernandes et al. Oral presentation OSA Frontiers in Optics, 2009
- 13. 13 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
- 14. 14 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)]
- 15. 15 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)]
- 16. 16 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
- 17. 17 B Polarizers Detector L The birefringence splits the Bragg reflection into two polarization modes Δ n= ΔλB 2 Λ
- 18. 18 BGW measurement of birefringence removes the ambiguity of the cross polarizer technique. Polarizers Detector ±45º 45º L I p= Ii 2 1cos I c= Ii 2 1−cos n= 2 L δ=±cos−1 (I p −Ic Ii )+ m2π m=0,1,2,... Waveguide retardance: Parallel polarizers Crossed polarizers Measuring birefringence with crossed polarizers
- 19. 19 160 nJ =±cos −1 I p −Ic I 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
- 20. 20 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 writing Waveguide length = 25.4 mm Pulse Energy = 160 nJ Integrated wave plates demonstration 45º
- 21. 21 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, 2011 Fernandes et al. Oral presentation in Frontiers in Ultrafast Optics, SPIE Photonics West, 2012
- 22. 22 r= P2 P1+P2 P1 P2 r (λ)=Asin 2 [ κ(λ) L+ϕ(λ)] Using directional couplers to design polarization beam splitters
- 23. 23 Coupling is polarization dependent Δ r=sin [(K V −KH ) 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)]
- 24. 24 Definition of a more convenient parameter Δ r=sin[( KV −K H ) L+ ϕV −ϕH ]×sin[( KV + K H ) L+ ϕV + ϕH ] Δ r=∣rV−rH∣Polarization splitting contrast ratio:
- 25. 25 19 dB and 24 dB extinction ratio polarization splitting Polarization beam splitters Parallel polarization writing Interaction length = 19.2 mm Separation = 8 μm S-Bends radius = 100 mm
- 26. 26 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
- 27. 27 Controlling the birefringence with stress
- 28. 28 The stress fields are dependent on the geometry
- 29. 29 Independent birefringence control is possible with the strength of the stress fields and with the writing polarization Strength of the stress fields Writing polarization
- 30. 30 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)
- 31. 31 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
- 32. 32 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
- 33. 33 Fundação para a Ciência e Tecnologia (FCT) Canadian Institute for Photonic Innovations Natural Sciences and Engineering Research Council of Canada Acknowledgments
- 34. www.inescporto.pt www.fc.up.pt http://photonics.light.utoronto.ca/ Thank you lfernandes@fc.up.pt
- 35. 35 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]
- 36. 36 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
Editor's Notes
- The title of my PhD thesis is: Birefringence and Bragg grating control in femtosecond laser written optical circuits
- I am going to first: Introduce the fundamentals of ultrafast/femtosecond light matter interaction And then: Present experimental results namely: And the demonstration of devices based on waveguide birefringence
- Fig. 1.2 Illustration of the interaction physics of focused femtosecond laser pulses in bulk fused silica. (a) The laser is focused below the sample surface resulting in a high intensity in the focal volume. (b) The energy is nonlinearly absorbed and a free electron plasma is created by multiphoton/tunneling and avalanche photoionization. (c) The plasma transfers its energy to the lattice on a 10 ps time scale resulting in one of three types of permanent modification (d): isotropic refractive index change at low pulse energy, sub-wavelength birefringent nanostructures at moderated energy and empty voids at high pulse energy. K. Itoh, W. Watanabe, S. Nolte, C.B. Schaffer, MRS Bull. 31(8), 620 (2006)
- Pulse energy between 90 nJ and 160 nJ From the threshold for significant change for waveguide To the threshold for damage Low loss waveguiding inside this window
- Anisotropies are responsible for the birefringence. Influenced by the pulse energy. Influenced by the laser polarization. Define the polarization of the writing laser in respect to the scanning direction. Relate the nanograting with the polarization of the writing laser and birefringence value. Effects unique to fused silica.
- Waveguide birefringence in fused silica has been studied before. Found asymmetries in the fs writing Devices have been demonstrated Some author have found that fs written waveguides in fused silica have birefringence due the the asymmetric properties of the nonlinear absorption process . In some cases birefringence may be a problem but it can also be attractive to exploit its potential
- Nanogratings are found to be responsible for some of the birefringence. One of such properties is the formation of nanogratings which have been a topic of many research papers and also of many presentation at this conference. We are going to study theses birefringent properties and used them to fabricate polarization dependent devices.
- Another recent development was the demonstration of Quantum entanglement measurements on a chip. Offering more stability and scalability. The picture shows an example of a integrated directional coupler preforming quantum interference. Author have stressed the importance of polarization dependent integrated devices for these applications. Like polarization beam splitter (submitted paper ) and integrated wave plates (the topic of this presentation) to replace the bulk components used in previous demonstrations. Highlight the components that can me integrated with polarization dependent devices. Develop control and exploit the birefringence for device fabrication (Ex. Wave plates and polarization beam splitters.)
- Remind about the proper axis for the polarization modes. Define the Slow and the Fast axis. Use input polarizer to determine Birefringence with BGW shift between V and H modes. BGWs are sharp and deep spectral features
- Distinguish from the BGW method (from previous slide) Define Ip and Ic Explain equations for retardance Ambiguity from order m And from +- degeneracy Half wave plate will have a retardance of Pi Stress the ambiguity on the cross polarization results, and the possibility of solving the ambiguity with the BGW measurement.
- Show the comparison between the writing laser polarization. Show pulse energy dependency. Birefringence peak shift to higher wavelength for higher pulse energies.
- Explain the axis of the plot The x axis is the Analyzer angle.
- Maximum coupling (A =1) for symmetric coupling Coupling ratio r Coupling coefficient k Phase accumulated in the bending region ( phi )