frequency combs - the budker group

Frequency Combs250 Special TopicsJennie Guzman

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Frequency Comb

http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm

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Ultrafast Laser

Kerr-lens mode-locked Ti:sapphire laser

Output Coupler

Pump

Ti: sapphire crystal

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Kerr Effect•Index of refraction is proportional to intensity

•Kerr medium acts like a lens for high intensity

n(I) = n0 + n2I

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Kerr Effect

Cundiff, Steven T., Ye, Jun, Reviews of Modern Physics, Vol. 75, 2003

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Ultrafast Laser

Kerr-lens mode-locked Ti:sapphire laser

Output Coupler

Pump

Ti: sapphire crystal

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Dispersion•Spectral dispersion due to variation in refractive index

•Results in temporal spreading•Longer wavelengths travel faster than shorter ones (normal dispersion)

•2-Prism sequence

•The first prism spatially disperses the pulse•Long wavelength components travel through more glass of prism 2

•Net effect is to generate “anomalous” dispersion to counteract the normal dispersion of the crystal

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fs Laser Spectrum

f = nfrep + f0


Laser Cavity•f =c/2d, where d is the path length in the cavityrep

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Definition of the Second

9191631770 Hz6 S1/2

F = 4

F = 3

•Defined with respect to oscillation frequency between hyperfine levels in Cs133

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Absolute Frequencies

0 10 20 30 40 50

f =n f + fn 0rep

•Reference to Cs133

•Two measurements: f and f n 0

•f can be measured using a fast photodiode

rep

•f requires that the comb span one octave0

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Spectral Broadening

•air-silica microstructure fiber

•small diameter ~•nonlinear medium leading to nonlinear optical effects, ie: four-wave mixing, self-phase modulation

1.7µm

Ranka, J. K. et al., Optics Letters 25, 1 (2000)

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Spectral Broadening


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Stabilizationrep•f can be stabilized via microwave reference

Cavity Length PZT

fs Laser

RF Source Cs Clock

To Fiber

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Stabilization0•f can be stabilized through self-referencing

•“self-referencing” refers to second harmonic generation to compare extremes of the comb


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Frequency Comb

fs laser

Pump Laser Photonic Crystal Fiber Interferometer

Detector

Detector

Hansch, T. W. et al., Nature 416, 2002

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Stability•Cs clock short-term stability: at 1second

•fs optical comb short-term stability: at 1

second

•Commerical fs combs:

•Stability: at 1 second

•Accuracy:

5 × 10−12

4 × 10−14

5 × 10−13

≈ 10−14

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Frequency Comb Numbers

•Pulse duration <6 fs

•

•

•700 - 1000 nm (before spectral broadening)

•400-1200 nm

•KLM Ti:sapphire laser

10MHz < frep < 10GHz

•Erbium - doped fiber lasers: 1000-2400 nm

•Ytterbium-doped fiber lasers: 600-1600 nm

−frep

2< f0 <

frep

2

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Spectral Coverage

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Astro-Comb

•accuracy and long-term stability are limited by wavelength calibration sources

•Current Limitations

•Sources limited in red - near IR•Iodine absorption cells

•few spectral lines•thorium-argon lamps

•limited spectral lines•unstable bright features - saturate detectors

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Astronomical Spectrograph

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Astro-Comb•Possible solution - Frequency comb

•absolute frequency is determined by rep. rate and offset frequency

•can be referenced to rf sources•current fractional stability exceeds needs• 3 × 10

−11

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Astro-Comb

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Astro-Comb

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Astro-Comb Proposal

LRIS: Home page http://www2.keck.hawaii.edu/inst/lris/

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http://www2.keck.hawaii.edu/inst/lris/


References


Jun, Ye et al. IEEE Journal of Selected Topics in Quantum Electronics, Vol. 9, No. 4, July/August 2003

Ippen, E. P., Appl. Phys. B. 58, 159-170 (1994)

Diddams, S. A. Phys. Rev. Lett. 84, 22, 2000

Ranka, J. K. et al., Optics Letters 25, 1 (2000)

Hansch, T. W. et al., Nature 416, 2002

Thorpe, M. J., arxiv: 0803 - 4509v

LRIS: Home page http://www2.keck.hawaii.edu/inst/lris/


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frequency combs - the budker group

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