Download - Frequency Combs - The Budker Group
Frequency Combs250 Special TopicsJennie Guzman
1
Frequency Comb
http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm
2
Ultrafast Laser
Kerr-lens mode-locked Ti:sapphire laser
Output Coupler
Pump
Ti: sapphire crystal
3
Kerr Effect•Index of refraction is proportional to intensity
•Kerr medium acts like a lens for high intensity
n(I) = n0 + n2I
4
Kerr Effect•Index of refraction is proportional to intensity
•Kerr medium acts like a lens for high intensity
n(I) = n0 + n2I
4
Kerr Effect
Cundiff, Steven T., Ye, Jun, Reviews of Modern Physics, Vol. 75, 2003
5
Ultrafast Laser
Kerr-lens mode-locked Ti:sapphire laser
Output Coupler
Pump
Ti: sapphire crystal
6
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
7
fs Laser Spectrum
f = nfrep + f0
Cundiff, Steven T., Ye, Jun, Reviews of Modern Physics, Vol. 75, 20038
Laser Cavity•f =c/2d, where d is the path length in the cavityrep
9
Laser Cavity•f =c/2d, where d is the path length in the cavityrep
9
Definition of the Second
9191631770 Hz6 S1/2
F = 4
F = 3
•Defined with respect to oscillation frequency between hyperfine levels in Cs133
10
Definition of the Second
9191631770 Hz6 S1/2
F = 4
F = 3
•Defined with respect to oscillation frequency between hyperfine levels in Cs133
10
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
11
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)
12
Spectral Broadening
Cundiff, Steven T., Ye, Jun, Reviews of Modern Physics, Vol. 75, 2003
13
Stabilizationrep•f can be stabilized via microwave reference
Cavity Length PZT
fs Laser
RF Source Cs Clock
To Fiber
14
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
15
Frequency Comb
fs laser
Pump Laser Photonic Crystal Fiber Interferometer
Detector
Detector
Hansch, T. W. et al., Nature 416, 2002
16
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
17
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
18
Spectral Coverage
19
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
20
Astronomical Spectrograph
21
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
22
Astro-Comb
23
Astro-Comb
24
Astro-Comb Proposal
LRIS: Home page http://www2.keck.hawaii.edu/inst/lris/
25
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
26