Laser Frequency Combs for Precision Radial Velocity ......laser frequency comb Diode laser...
Transcript of Laser Frequency Combs for Precision Radial Velocity ......laser frequency comb Diode laser...
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