laser frequency combs for precision radial velocity ......laser frequency comb diode laser...
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Laser Frequency Combs for Precision Radial Velocity Measurements in Astrophysics
David F. Phillips
A collaboration between astronomers (CfA), physicists (CfA) and electrical engineers (MIT)
~15 scientists
“Astro-Comb”
•!Precision spectroscopy essential for advances in astrophysics
•!Currently limited by spectrograph wavelength calibration
•!Laser frequency combs can solve calibration problem
•!Rapid progress in last 3 years:! concept => lab demo => operation at observatories
•!Coming soon: discovery & characterization of Earth-like planets
•!10-20 years: direct measurement of cosmological dynamics
Diddams et al., Nature 445, 627 (2007).Murphy et al., MNRAS 380, 839 (2007).
Existing calibratorTh:Ar lamp
~1 m/s stability
Unique Opportunity
Over the next decade, we will:
•!Discover the first Earth-like planets around other stars
•!Search these planets forchemical signatures of life
•!Image planets directly Is the Earth special, or just another planet?
=> Complete Copernican revolution?
Origins of Life in the Universe
http://origins.harvard.edu/
Extrasolar Planets
Habitable zone
Hot Jupiters
Super-Earths
Jupiter mass planets close to their stars
(D. Sasselov, Nature 451, 29 2008)
> 400 exoplanets found to date• Primarily hot Jupiters• “Super Earths” now being found• Soon, earth-like planets Super Earths
Transit Method
HD 189733BHot jupiter63 light-years awayMass = 1.13 MJ
Radius = 1.14 RJ
T=1000 KP = 2.2 days
primary eclipse
secondary eclipse
Spitzer space telescope at 8 µm
Kepler satellite has hundreds of exoplanet candidate systems.
Need confirmation!
Heather Knutson & Dave Charbonneau (2007)
http://www.kepler.arc.nasa.gov/
arXiv:1001.0268v2 [astro-ph.EP]
Radial Velocity Method
•!Heavier planet causes larger Doppler shift in star light
•!Gives planet mass and orbital period & radius, not size
•!Doppler-shift from Earth-mass planet is VERY SMALL
Unseenexoplanet
Pre-comb sensitivity
Δλ ΔRV
10–7 Å 1 cm/s
10–6 Å 10 cm/s
10–5 Å 1 m/s
0.01 Å 1 km/s
Earth-mass planetaround Sun-like star
“Hot Jupiter”
Doppler shift:Δλ/λ = ΔRV/c
v = 1 km/sv = 1 GHzλ = 2 10-3 nmx = 1 pixel = 20 µm
KRV = 28.4�
P
1 year
�−1/3 �MP sin i
MJ
� �M�
M⊙
�−2/3
m/s
Stellar velocity due to planetary pull
100,000 absorption linesfew GHz linewidths
Stellar Spectrum
SpectrographSmall frequency shifts => Use echelle spectrograph! => broad spectral coverage & high-resolution
corrector
collimator
slit
2DCCD array
From telescope
cross disperser(low-dispersion)
echelle grating(high dispersion)
Slit is often a multimode fiber, allowing the spectrograph to reside in an environmentally controlled room, away from the telescope
v = 1 km/sv = 1 GHzλ = 2 10-3 nmx = 1 pixel = 20 µm
http://www.eso.org/sci/facilities/lasilla/instruments/harps/index.html
SpectrographSmall frequency shifts => Use echelle spectrograph! => broad spectral coverage & high-resolution
corrector
collimator
slit
2DCCD array
From telescope
cross disperser(low-dispersion)
echelle grating(high dispersion)
State-of-the-art astrophysical spectroscopy is broadband, photon-starved => spectrograph resolution R = λ/Δλ ~ 100,000 => minimum resolution element Δν > 5 GHz
Slit is often a multimode fiber, allowing the spectrograph to reside in an environmentally controlled room, away from the telescope
v = 1 km/sv = 1 GHzλ = 2 10-3 nmx = 1 pixel = 20 µm
http://www.eso.org/sci/facilities/lasilla/instruments/harps/index.html
Octave-Spanning CombOctave-spanning comb => atomic clock accuracy & stability for all comb lines
~300 nm
RF beat frequencies =>lock ωr & ωCE
to atomic clock
• 105 comb lines• narrow lines (<kHz) • coherent• equally-spaced• intense (10 µW/line)
ωr ~ 1 GHz
nH = 2nL
τpulse < 4 fs
We use Ti:Sapphire laser
Cr:Forsterite lasers and fiber lasers also used.
Broadband gain, Kerr effect => octave spanningKerr-lens mode-locking => stabilize pulsing Cavity sets Trep ~1 GHz
Double-chirped mirror pairs => compensate broadband dispersionhttp://nobelprize.org/nobel_prizes/physics/laureates/2005/index.html
Astro-Comb
PID lock box
λ/2
Photo-detector
Fabry-Perot cavity
PZT
Octave-spanning 1-GHzlaser frequency comb
Diode laser
Astro-comb beam
EOM
Synthesizer
Photo-detector
SynthesizerPhase lock box
Filter out most comb lines with a stabilized Fabry-Perot Cavity to yield line-spacing up to ~40 GHz
Li, et al., Nature 452, 610-612 (2008).
Astro-Comb
PID lock box
λ/2
Photo-detector
Fabry-Perot cavity
PZT
Octave-spanning 1-GHzlaser frequency comb
Diode laser
Astro-comb beam
EOM
Synthesizer
Photo-detector
SynthesizerPhase lock box
wavelength
inte
nsity
Astro-Comb
PID lock box
λ/2
Photo-detector
Fabry-Perot cavity
PZT
Octave-spanning 1-GHzlaser frequency comb
Diode laser
Astro-comb beam
EOM
Synthesizer
Photo-detector
SynthesizerPhase lock box
€
FSR = m × frep
wavelength
inte
nsity
Astro-combAstro-comb tested using a 1.5 m telescope with high-resolution echelle spectrograph (TRES)
Astro-combAstro-comb tested using a 1.5 m telescope with high-resolution echelle spectrograph (TRES)
Astro-combAstro-comb tested using a 1.5 m telescope with high-resolution echelle spectrograph (TRES)
TRES spectrographR~40,000
Astro-combAstro-comb tested using a 1.5 m telescope with high-resolution echelle spectrograph (TRES)
TRES spectrographR~40,000
Calibration Spectrum
Th:Ar calibrator
Astro-comb spectrum
Red astro-comb and Th:Ar spectrumTRES spectrograph on 1.5 m Tillinghast reflector at Mt. Hopkins2 dimensional spectrum on spectrograph CCD
echelle grating(high dispersion)
cross disperser(low-dispersion)
lens
2D CCD
100 µm fiber
Red Astro-Comb
• 100 nm bandwidth.• 32 GHz spacing.• 10 GHz FWHM on spectrograph.• 100 nW per line.• 25000-40000 counts in 5 minutes through
integrating sphere.
Red Astro-Comb SNRδν = A
FWHM
S/N ×√
n
A ≈ 0.7FWHM ≈ 10 GHz(S/N)300 s ≈ 200(S/N)max ≈ 300n ≈ 6 pixels∆ν ≈ 15 MHz or 15 m/s for one peak250 peaks/order: ∆ν ≈ 1 MHz in one order
Fit Model
CCDModel as flatTails from charge diffusion don't appear to help
Comb∆ν = 1 GHz S • δI/IS = side mode suppression ∆ν = 3 MHz • δI/I Negligible at the 1 m/s level
Fiber100 µm fiberModeled as flat top in 2DHalf circle in 1DAdd skew
OpticsModeled as Hermite-GaussiansUp to 16 terms
One order of comb lines superimposed Residuals of fit
near shot noise floor
Calibration
! !
!"#$%&"'$()*+$',*-.#./0(1.*2'"'$()"&3
455*67/*/,$8'/9*:$;./9116*/,$8'/*$)*',.*/1.0'&(<&"1,
•Calculate wavelength calibration order by order.•Compare calibration to reference frame.•Calculate deviation of all orders from mean drift.
! !
!"#$%&"'$()*+$',*-.#./0(1.*2'"'$()"&3
Blue Astro-Comb
• 1 GHz Ti:Sapphire source laser• BBO doubling crystal• 50 nm bandwidth
• Fabry-Perot Cavity:• Bragg Mirror Stack• Bandwidth limited by air
Dispersion to ~15 nm• Improved flux to spectrograph• Phase scrambling
A. Benedick, Optics Express, 18, 19175-19184 (2010).
Blue Astro-CombOne order of comb lines superimposed
Residuals of fit
Much closer to shot noise!420 nm with less fringing.Fiber shaking active.
50 cm/s resolution
faster data rate
Green Astro-Comb
PID lock box
λ/2
Photo-detector
Fabry-Perot cavity
PZT
Octave-spanning 1-GHzlaser frequency comb
Diode laser
Astro-comb beam
EOM
Photo-detector
SynthesizerPhase lock box
Synthesizer
Photonic Crystal Fiber (PCF)High nonlinear coefficientZero-dispersion wavelength ~700 nmFour-wave mixingNear Gaussian mode profile
FP CavityComplementary-Chirped Mirror Pair
Green Astro-Comb• 45 GHz green astro-comb• Various Ti:Sapphire settings• Astro-comb lines not resolved on OSA• Lab demo. Next, test at telescope
• 240 GHz green astro-comb• demo for low-resolution OSA• Side suppression minimal
What’s Next?
Harvard, Smithsonian, Geneva Observatory
• Observe Earth candidates found by Kepler • Use Doppler-shift method to distinguish
new Earths from exotics
William Herschel telescope,Canary Islands, 4.2 meter mirror
HARPS clone fornorthern hemisphere
Astro-combcalibrator =>ΔRV < 10 cm/s