Clinton J. Smith, Wen-Di Li, Shufeng Bai and Stephen Y. Chou NanoStructures Laboratory, Princeton University CLEO/IQEC 200...
Outline <ul><li>Motivations  </li></ul><ul><li>VCSEL polarization self-switching </li></ul><ul><li>Form birefringence of s...
Motivations: All Optical Clock Source for Atomic Clocks <ul><li>GPS </li></ul><ul><ul><li>Handheld & satellite </li></ul><...
Comparison to Atomic Clocks Developed by Knappe & Jau Y. Y. Jau, E. Miron, A. B. Post, N. N. Kuzma, and W. Happer, &quot;P...
Goals: Self-Switching, 4.6 GHz, All-Optical VCSEL Clock <ul><li>Atomic clocks rely on Cs excitation </li></ul><ul><ul><li>...
VCSELs’ Cavity Symmetry Leads to Polarization Self-Switching <ul><li>VCSELs have isometric cavity & circular aperture </li...
<ul><li>Demonstrated, for the first time, polarization control (e.g. fixing, enhancing and switching) using subwavelength ...
Form Birefringence of a Subwavelength Quarter-Wave Plate: Birefringence from material properties AND structure Parallel Po...
Polarization Switching of VCSEL Using Subwavelength Quaterwave Plate   10 ns/div 1.55 GHz <ul><li>First demonstration of  ...
Create an Atomic Clock Using VCSEL Polarization Self-Switching Behavior <ul><li>4.6 GHz modulations create sidebands separ...
VCSEL Clock Oscillation Frequency Governed by Cavity Length Independent Component Mount Integrated Component Mount 25 dB 2...
VCSEL Clock Oscillation Frequency Changed With Drive Current 3.45 mA Drive Current 5.58 mA Drive Current 2.97 mA Drive Cur...
Summary <ul><li>Form birefringence of  α -Si 200 nm grating on FS substrate was used to create subwavelength QWP for 850 n...
Acknowledgements <ul><li>Wen-di Li & Dr. Shufeng Bai for their contributions to this project </li></ul><ul><li>Prof.’s Pru...
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Vcsel Clock.Smith 8.Chou 1

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CLEO 2009: High Frequency Polarization Switching VCSEL Clock Using Subwavelength Quarter-Wave Plate
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Clinton J. Smith, Wen-Di Li, Shufeng Bai, and Stephen Y. Chou

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Vcsel Clock.Smith 8.Chou 1

  1. 1. Clinton J. Smith, Wen-Di Li, Shufeng Bai and Stephen Y. Chou NanoStructures Laboratory, Princeton University CLEO/IQEC 2009 High Frequency Polarization Switching VCSEL Clock Using Subwavelength Quarter-Wave Plate NanoStructures Lab Princeton University Supported in part by DARPA
  2. 2. Outline <ul><li>Motivations </li></ul><ul><li>VCSEL polarization self-switching </li></ul><ul><li>Form birefringence of subwavelength quarter-wave plate (QWP) </li></ul><ul><li>Optical clock built with VCSEL, subwavelength QWP, & partial reflector (PR) </li></ul><ul><li>Demonstration of optical clock oscillations </li></ul><ul><li>Summary </li></ul>
  3. 3. Motivations: All Optical Clock Source for Atomic Clocks <ul><li>GPS </li></ul><ul><ul><li>Handheld & satellite </li></ul></ul><ul><li>Telecommunications </li></ul><ul><ul><li>High-speed all optical clock signal </li></ul></ul><ul><li>Current atomic clocks are bulky and power hungry </li></ul>www.garmin.com 50 W operating power 13 x 42 x 52 cm 50 kg www.symmetricom.com Goal: Create a power-efficient, compact atomic clock
  4. 4. Comparison to Atomic Clocks Developed by Knappe & Jau Y. Y. Jau, E. Miron, A. B. Post, N. N. Kuzma, and W. Happer, &quot;Push-Pull Optical Pumping of Pure Superposition States,&quot; Physical Review Letters, vol. 93, p. 160802, 2004. S. Knappe, V. Shah, P. D. D. Schwindt, L. Hollberg, J. Kitching, L.-A. Liew, and J. Moreland, &quot;A microfabricated atomic clock,&quot; APPLIED PHYSICS LETTERS, vol. 85, pp. 1460-1462, 2004. *Does not include current modulation electronics ** Designed for Rb resonance lock 4.6 GHz 3.4 GHz ** 4.6 GHz Frequency N/A Yes Yes Cs/Rb Resonance Lock Polarization Self-Switching Laser Intensity Modulation Current Modulation Operating Principle 5-10 mW N/A 5 mW Power Consumption 3 8 6 Number of Optical Elements ~1.7 cm 3 optical bench top < 1 cm 3 * Size Smith Jau Knappe  
  5. 5. Goals: Self-Switching, 4.6 GHz, All-Optical VCSEL Clock <ul><li>Atomic clocks rely on Cs excitation </li></ul><ul><ul><li>4.6 GHz goal frequency modulation via polarization self-switching </li></ul></ul><ul><li>Compact design with Nanoimprint </li></ul><ul><ul><li>1 cm 3 goal volume </li></ul></ul><ul><ul><li>Use nanoimprinted subwavelength QWP with PR integration </li></ul></ul><ul><li>Power efficient </li></ul><ul><ul><li>30 mW goal total power consumption via polarization self-switching </li></ul></ul>
  6. 6. VCSELs’ Cavity Symmetry Leads to Polarization Self-Switching <ul><li>VCSELs have isometric cavity & circular aperture </li></ul><ul><ul><li>Lase with modes in both horizontal and vertical polarizations </li></ul></ul><ul><ul><li>Corresponds to [011] & [01/1] crystal directions </li></ul></ul><ul><li>Isometry can lead to semi-random polarization self-switching </li></ul><ul><ul><li>Like polarization “mode-quenching” </li></ul></ul><ul><ul><li>Usually occurs at ~100% above threshold current </li></ul></ul>SEM image of Avalon Photonics single-mode 850nm VCSEL Typical Optical Power vs. Drive Current curve of a VCSEL that polarization self-switches. 6 μ m
  7. 7. <ul><li>Demonstrated, for the first time, polarization control (e.g. fixing, enhancing and switching) using subwavelength grating </li></ul><ul><li>Suitable for large scale integration </li></ul><ul><li>Allow individually control of each VCSEL </li></ul>No grating P // grating P grating Control of Polarization of VCSELs using Subwavelength Grating SW grating S.Y. Chou, S. Schablitsky, and L. Zhuang , “Application of Amorphous Silicon Sub-wavelength Gratings in Polarization Switching Vertical-cavity Surface-emitting lasers,” J. Vac. Sci & Technol. B, 15 ( 6), 2864 (1997). P P || VCSEL P || P P ||
  8. 8. Form Birefringence of a Subwavelength Quarter-Wave Plate: Birefringence from material properties AND structure Parallel Polarization Perpendicular Polarization S. Bai, &quot;Nanophotonic devices, applications and fabrication by nanoimprint lithography,&quot; Thesis Submitted to Princeton University, November 2007. 200nm 200nm <ul><li>Form birefringence at 850 nm </li></ul><ul><ul><li>Use as QWP for PS-VCSEL Clock </li></ul></ul><ul><li>Common materials </li></ul><ul><ul><li>α -Si grating on FS substrate </li></ul></ul><ul><li>Compact </li></ul><ul><ul><li>200 nm period, 178nm high grating </li></ul></ul><ul><ul><li>50% duty cycle: 100nm linewidth </li></ul></ul><ul><li>Inexpensive & easy to fabricate </li></ul><ul><ul><li>Compared to conventional QWP (e.g. quartz) </li></ul></ul>
  9. 9. Polarization Switching of VCSEL Using Subwavelength Quaterwave Plate 10 ns/div 1.55 GHz <ul><li>First demonstration of polarization switching VCSELs using a thin (only 240 nm thick) subwavelength grating quarter waveplate </li></ul><ul><li>Terahertz frequency and tunable </li></ul><ul><li>Suitable for large scale integration </li></ul>Subwavelength grating quarter waveplate Laser pulse Spectrum for a 4.8 cm cavity S.Y. Chou, S. Schablitsky and L. Zhuang, “Subwavelength Transmission Gratings and Their Applications in VCSELs,” SPIE, Vol. 3290, pp73-81, 1997 R R L L PR QWP VCSEL || || || R L
  10. 10. Create an Atomic Clock Using VCSEL Polarization Self-Switching Behavior <ul><li>4.6 GHz modulations create sidebands separated by Cs hyperfine frequency </li></ul><ul><li>Use frequency (f=c/4L) and Cs absorption in feedback loop to maximize resonance </li></ul><ul><li>Can fine tune oscillations to match resonance by changing cavity length </li></ul>D.K. Serkland, G.M. Peake, K.M. Geib, R. Lutwak, R.M. Garvey, M. Varghese, & M. Mescher, “VCSELs for atomic clocks,” Proceedings of the SPIE , vol. 6132, pp. 66-76, 2006 Cs Vapor Cell PR QWP VCSEL R R L L || || || R L || Clock f=c/4L R || L || Feedback Loop L QWP POL
  11. 11. VCSEL Clock Oscillation Frequency Governed by Cavity Length Independent Component Mount Integrated Component Mount 25 dB 20 MHz 3.88 GHz 3.67 GHz 3.45 mA 2.04 cm 25 dB 8.5 MHz 4.6 GHz 4.58 GHz 4.28 mA 1.64 cm SNR FWHM Measured Oscillation Frequency Theoretical Oscillation Frequency VCSEL Drive Current Cavity Length
  12. 12. VCSEL Clock Oscillation Frequency Changed With Drive Current 3.45 mA Drive Current 5.58 mA Drive Current 2.97 mA Drive Current 30 dB 6 MHz 7.22 GHz 5.58 mA 3.67 GHz 2.04 cm 25 dB 6 MHz 5.63 GHz 2.97 mA 3.67 GHz 2.04 cm 25 dB 20 MHz 3.88 GHz 3.45 mA 3.67 GHz 2.04 cm SNR FWHM Measured Oscillation Frequency VCSEL Drive Current Theoretical Oscillation Frequency Cavity Length
  13. 13. Summary <ul><li>Form birefringence of α -Si 200 nm grating on FS substrate was used to create subwavelength QWP for 850 nm light </li></ul><ul><li>VCSEL polarization self-switching property was combined with external cavity QWP & PR to create optical clock </li></ul><ul><ul><li>VCSEL clock oscillation governed by f=c/4L </li></ul></ul><ul><ul><li>3.88 & 4.6 GHz oscillations demonstrated </li></ul></ul><ul><ul><li>8.5 MHz FWHM </li></ul></ul><ul><ul><li>1.7 cm 3 volume achieved </li></ul></ul><ul><ul><li>5-10 mW power consumption demonstrated </li></ul></ul><ul><li>VCSEL drive current can change VCSEL clock oscillation frequency </li></ul><ul><ul><li>5.63 & 7.22 GHz oscillations demonstrated </li></ul></ul><ul><ul><li>6 MHz FWHM </li></ul></ul>
  14. 14. Acknowledgements <ul><li>Wen-di Li & Dr. Shufeng Bai for their contributions to this project </li></ul><ul><li>Prof.’s Prucnal, Gmachl, Arnold, & Wysocki for helpful advice and discussions </li></ul><ul><li>All fellow group members for being helpful both with equipment maintenance and in discussions </li></ul>

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