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Frequency Comb
http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm
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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•Index of refraction is proportional to intensity
•Kerr medium acts like a lens for high intensity
n(I) = n0 + n2I
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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
Cundiff, Steven T., Ye, Jun, Reviews of Modern Physics, Vol. 75, 20038
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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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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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
Cundiff, Steven T., Ye, Jun, Reviews of Modern Physics, Vol. 75, 2003
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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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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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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 Proposal
LRIS: Home page http://www2.keck.hawaii.edu/inst/lris/
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References
Cundiff, Steven T., Ye, Jun, Reviews of Modern Physics, Vol. 75, 2003
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/
http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm
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