Quantum Optics meets Astrophysics Frequency Combs for High Precision Spectroscopy in Astronomy T. Wilken, T. Steinmetz, R. Probst T.W. Hänsch, R. Holzwarth, Th. Udem Max-Planck-Institute of Quantum Optics, Garching G. Locurto, A. Manescau, L. Pasquini European Southern Observatory, Garching

T. Wilken et al.Quantum Optics meets Astrophysics Overview Frequency Combs High Precision Astronomy A Multi-GHz Comb System in the Visible Spectrograph Calibration

T. Wilken et al.Quantum Optics meets Astrophysics Why measuring frequencies?

T. Wilken et al.Quantum Optics meets Astrophysics What’s wrong with wavelengths? - Wavefront distortions - Non-perfect mode matching

T. Wilken et al.Quantum Optics meets Astrophysics Optical Frequencies 500 nm  500 THz oscillations per second Fastest electronics: few 10 GHz Gearing needed with gear ratio of 10 5 – 10 6 !

T. Wilken et al.Quantum Optics meets Astrophysics Frequency combs Fourier transformation of a pulse train results in a comb like spectrum. Characteristic parameters f r, n, f CE Comb = Phase stabilization f r ~ few MHz to few GHz, f CE < f r n ~ 10 5 to 10 6 (gear ratio!) locked to an atomic clock Every mode as accurate and stable as the reference f0f0 f rep

Nobel Poster Measuring the Frequency of Hydrogen with a Laser Comb Nobel Prize in Physics 2005, Nobel Poster

Applications Precision Spectroscopy CEO Phase Control: Attosecond Physics Direct Comb Spectroscopy Infrared / THz Spectroscopy Fourier Transform Spectroscopy Optical Atomic Clocks Calibration of Spectrometers Distance Measurement Dissemination of Time and Frequency Frequency T. Wilken et al.Quantum Optics meets Astrophysics

T. Wilken et al.Quantum Optics meets Astrophysics Frequency Combs High Precision Astronomy A Multi-GHz Comb System in the Visible Spectrograph Calibration

T. Wilken et al.Quantum Optics meets Astrophysics Understanding Astronomers… …or: which frequency is measured in m/s ? Most astronomical spectral data are Doppler shifted lines due to relative motion between the celestial object and the Earth! Quantity of interest: radial velocity measured in m/s E.g. corresponds to 0.5 m/s

T. Wilken et al.Quantum Optics meets Astrophysics High Precision Astronomy Spectroscopy of the solar surface Detection of extrasolar Earths Acceleration of cosmic expansion Calibration must be reproducible over years / decades! Required precision 4x10 -9 ~1 m/s~2 MHz 2x ~5 cm/s ~100 kHz 4x ~1 cm/s ~20 kHz

T. Wilken et al.Quantum Optics meets Astrophysics Calibration Sources Traditional calibration lamps (e.g. Th-Ar spectral lamps) achieve presently ~2x10 -9, but have major drawbacks: Th-Ar lamp Frequency comb Lines differ in intensity and spacing Uniform mode spacing and intensity Fixed line positions Reference to atomic clock: does not change with time Lamp ages Precision limited to ~10 -9 Precision limited to ~ 

T. Wilken et al.Quantum Optics meets Astrophysics Calibration Sources A specifically designed frequency comb overcomes all these problems and has the potential for sub-kHz precision! Th-Ar lamp Frequency comb Lines differ in intensity and spacing Uniform mode spacing and intensity Tunable line positions Ar ions are sensible to lamp pressure changes. Reference to atomic clock: does not change with time Lamp ages Precision limited to ~10 -9 Precision limited to ~ 

T. Wilken et al.Quantum Optics meets Astrophysics Calibration Sources A specifically designed frequency comb overcomes all these problems and has the potential for sub-kHz precision!

T. Wilken et al.Quantum Optics meets Astrophysics Frequency Combs High Precision Astronomy A Multi-GHz Comb System in the Visible Spectrograph Calibration

T. Wilken et al.Quantum Optics meets Astrophysics Astrocomb Requirements Two major criteria must be fulfilled to calibrate a spectrometer: –Covering the spectrometers bandwidth ( nm) –Resolvable with dedicated high precision instruments (  f modes > 10 GHz)

T. Wilken et al.Quantum Optics meets Astrophysics Yb-fiber Oscillator Standard fs-fiber laser – c = 1040 nm (FWHM > 60 nm) – f rep = 250MHz –  = 60 fs (after external compression) – P > 200 mW (uncompressed)

T. Wilken et al.Quantum Optics meets Astrophysics Mode Filter Cavities Fabry-Perot resonator acts as filter:

T. Wilken et al.Quantum Optics meets Astrophysics Broadening the Comb Amplification and compression of the pulses close to the Fourier limit (~100 fs, ~5W optical Power) Generation of green light in a SHG stage (~530nm) Coupling to a strongly nonlinear fiber (courtesy of P. Russell, MPL) Spectral coverage (at present): nm (-20dB)

T. Wilken et al.Quantum Optics meets Astrophysics Present Status (of the comb system for the VTT)

T. Wilken et al.Quantum Optics meets Astrophysics Frequency Combs High Precision Astronomy A Multi-GHz Comb System in the Visible Spectrograph Calibration

T. Wilken et al.Quantum Optics meets Astrophysics Our Mission to HARPS (2010/11) Wavelength range: 530 ± 50 nm Mode spacing: GHz (High Accuracy Radial velocity Planet Searcher)

T. Wilken et al.Quantum Optics meets Astrophysics Real Spectra on the CCD

T. Wilken et al.Quantum Optics meets Astrophysics Real Spectra on the CCD

T. Wilken et al.Quantum Optics meets Astrophysics Fitting the Comb Lines Fit functions: 350 – 450 individual Gaussians

T. Wilken et al.Quantum Optics meets Astrophysics Calibration Characteristics The calibration’s repeatability is determined by the standard deviation of successive measurements of the same line’s position on the CCD. RepeatabilityAbsolute calibration The deviation of a model of the pixel-to-frequency map from the measured line positions determines the calibration’s absolute accuracy.

T. Wilken et al.Quantum Optics meets Astrophysics Calibration Repeatability Photon noise limited detection! SD: 6.7 cm/s PNL: 6.4 cm/s

T. Wilken et al.Quantum Optics meets Astrophysics Limits of the Repeatability In a stable series the repeatability averages down to ~2.5 cm/s. comb - ThAr

T. Wilken et al.Quantum Optics meets Astrophysics Conclusion Frequency combs provide the optimal calibration tool for high precision astronomy Our Yb-fiber comb system now delivers >100 nm bandwidth and was tested at the HARPS spectrograph Photon noise limited repeatability was demonstrated at the 2cm/s level but unidentified systematic effects remain. CCD imperfections could be calibrated for the first time

T. Wilken et al.Quantum Optics meets Astrophysics Conclusion & Outlook Frequency combs provide the optimal calibration tool for high precision astronomy. Our Yb-fiber comb system now delivers >100 nm bandwidth and was tested at the HARPS spectrograph. Photon noise limited repeatability was demonstrated at the 2cm/s level but unidentified systematic effects remain. CCD imperfections could be calibrated for the first time. Optimize broadening the comb to span the entire visible range. Implement spectral flattening to achieve minimum photon noise. Identify the systematic effects to enable long-term calibration.