Vcsel Clock.Smith 8.Chou 1

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

2

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

3

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

4

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

5

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

6

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

7

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).

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

8

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)

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

9

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

10

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

11

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

12

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

13

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

NanoStructures LaboratoryNanoStructures Laboratory PRINCETON UNIVERSITY

14

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