vcsel clock.smith 8.chou 1
DESCRIPTION
CLEO 2009: High Frequency Polarization Switching VCSEL Clock Using Subwavelength Quarter-Wave Plateby:Clinton J. Smith, Wen-Di Li, Shufeng Bai, and Stephen Y. ChouTRANSCRIPT
NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY
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 LabPrinceton University
Supported in part by DARPA
NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY
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Outline
Motivations
VCSEL polarization self-switching
Form birefringence of subwavelength quarter-wave plate (QWP)
Optical clock built with VCSEL, subwavelength QWP, & partial reflector (PR)
Demonstration of optical clock oscillations
Summary
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Motivations: All Optical Clock Source for Atomic Clocks
GPS Handheld & satellite
Telecommunications High-speed all optical clock signal
Current atomic clocks are bulky and power hungry
www.garmin.com
50 W operating power13 x 42 x 52 cm50 kgwww.symmetricom.com
Goal: Create a power-efficient, compact atomic clock
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Comparison to Atomic Clocks Developed by Knappe & Jau
Y. Y. Jau, E. Miron, A. B. Post, N. N. Kuzma, and W. Happer, "Push-Pull Optical Pumping of Pure Superposition States," 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, "A microfabricated atomic clock," APPLIED PHYSICS LETTERS, vol. 85, pp. 1460-1462, 2004.
Knappe Jau Smith
Size < 1 cm3 * optical bench top ~1.7 cm3
Number of Optical Elements
6 8 3
Power Consumption
5 mW N/A 5-10 mW
Operating PrincipleCurrent
ModulationLaser Intensity
ModulationPolarization Self-
Switching
Frequency 4.6 GHz 3.4 GHz ** 4.6 GHz
Cs/Rb Resonance Lock
Yes Yes N/A
*Does not include current modulation electronics** Designed for Rb resonance lock
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Goals: Self-Switching, 4.6 GHz, All-Optical VCSEL Clock
Atomic clocks rely on Cs excitation
4.6 GHz goal frequency modulation via polarization self-switching
Compact design with Nanoimprint
1 cm3 goal volume
Use nanoimprinted subwavelength QWP with PR integration
Power efficient
30 mW goal total power consumption via polarization self-switching
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VCSELs’ Cavity Symmetry Leads to Polarization Self-Switching
VCSELs have isometric cavity & circular aperture Lase with modes in both horizontal and vertical polarizations Corresponds to [011] & [01/1] crystal directions
Isometry can lead to semi-random polarization self-switching Like polarization “mode-quenching” Usually occurs at ~100% above threshold current
6 μm
SEM image of Avalon Photonics single-mode 850nm VCSEL
Typical Optical Power vs. Drive Current curve of a VCSEL that polarization self-switches.
NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY
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P P||
VCSEL
P||
PP||
0
50
100
150
200
Po
lari
za
tio
n R
atio
250
4 6 8 10 12 14Current (mA)
16
c
b
a
• Demonstrated, for the first time, polarization control (e.g. fixing, enhancing and switching) using subwavelength grating
• Suitable for large scale integration
• Allow individually control of each VCSEL
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).
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Form Birefringence of a Subwavelength Quarter-Wave Plate:Birefringence from material properties AND structure
Parallel Polarization||2
||1 EE
)1()1(
)1(21||
2||1
||22
||11
|| ffEffE
fEfE
)1(22
21|| fnfnn
111 ED 222 ED
Perpendicular Polarization 21 DD
)1(
)1(
)(
)(
21
21
fEfE
fDfD
Eaverage
Daverage
212
2
1
1
21
11
)1(
)1(
ff
fD
fD
fDfD
22
21
1
1
nf
nf
n
nnn ||
S. Bai, "Nanophotonic devices, applications and fabrication by nanoimprint lithography," Thesis Submitted to Princeton University, November 2007.
200nm 200nm
Form birefringence at 850 nm Use as QWP for PS-VCSEL Clock
Common materials α-Si grating on FS substrate
Compact 200 nm period, 178nm high grating 50% duty cycle: 100nm linewidth
Inexpensive & easy to fabricate Compared to conventional QWP (e.g.
quartz)
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Polarization Switching of VCSEL Using Subwavelength Quaterwave Plate
10 ns/div
1.55 GHz
1 GHz
10 GHz
100 GHz
1 THz
10 THz
1 10
Frequency
100 1000 10000 100000
Cavity Length (m )
RR LL
PR
QWP
VCSEL
|| ||||
RL
• First demonstration of polarization switching VCSELs using a thin (only 240 nm thick) subwavelength grating quarter waveplate
• Terahertz frequency and tunable
• Suitable for large scale integration
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
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Create an Atomic Clock Using VCSEL Polarization Self-Switching Behavior
4.6 GHz modulations create sidebands separated by Cs hyperfine frequency
Use frequency (f=c/4L) and Cs absorption in feedback loop to maximize resonance
Can fine tune oscillations to match resonance by changing cavity length
Cs Vapor CellPRQWPVCSEL
RR
LL
||||
||
RL
|| Clock f=c/4L
R ||
L
||
Feedback Loop
L
QWP POL
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
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VCSEL Clock Oscillation Frequency Governed by Cavity Length
Cavity Length
VCSEL Drive
Current
Theoretical Oscillation Frequency
Measured Oscillation Frequency
FWHM SNR
1.64 cm 4.28 mA 4.58 GHz 4.6 GHz 8.5 MHz 25 dB
2.04 cm 3.45 mA 3.67 GHz 3.88 GHz 20 MHz 25 dB
Independent Component Mount Integrated Component Mount
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VCSEL Clock Oscillation Frequency Changed With Drive Current
Cavity Length
Theoretical Oscillation Frequency
VCSEL Drive
Current
Measured Oscillation Frequency
FWHM SNR
2.04 cm 3.67 GHz 3.45 mA 3.88 GHz 20 MHz 25 dB
2.04 cm 3.67 GHz 2.97 mA 5.63 GHz 6 MHz 25 dB
2.04 cm 3.67 GHz 5.58 mA 7.22 GHz 6 MHz 30 dB
3.45 mA Drive Current 5.58 mA Drive Current2.97 mA Drive Current
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Summary
Form birefringence of α-Si 200 nm grating on FS substrate was used to create subwavelength QWP for 850 nm light
VCSEL polarization self-switching property was combined with external cavity QWP & PR to create optical clock VCSEL clock oscillation governed by f=c/4L 3.88 & 4.6 GHz oscillations demonstrated 8.5 MHz FWHM 1.7 cm3 volume achieved 5-10 mW power consumption demonstrated
VCSEL drive current can change VCSEL clock oscillation frequency 5.63 & 7.22 GHz oscillations demonstrated 6 MHz FWHM
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Acknowledgements
Wen-di Li & Dr. Shufeng Bai for their contributions to this project
Prof.’s Prucnal, Gmachl, Arnold, & Wysocki for helpful advice and discussions
All fellow group members for being helpful both with equipment maintenance and in discussions