Tunable Laser Spectroscopy Referenced with Dual Frequency Combs International Symposium on Molecular Spectroscopy 2010 Fabrizio Giorgetta, Ian Coddington, Esther Baumann, William Swann, Nathan Newbury NIST, Boulder, CO

Introduction Motivation: calibrated, fast, broadband, and sensitive spectroscopy Tunable laser spectroscopy –Highly sensitive –Broadband measurement –‘Jump’ between spectral regions of interest Dual comb spectroscopy –High frequency accuracy –Broadband measurement Combine both for –High sensitivity –Broadband measurement –High frequency accuracy Gas cell

Measuring rapidly tuned cw lasers Goal: measure instantaneous laser frequency spectrum –Accuracy & resolution = instantaneous laser linewidth Conventional OSA, FTIRs: inadequate resolution, accuracy, and speed Comb assisted diode laser spectroscopy [Del’Haye et al., Nat. Phot. 3, 529 (2009)] –Highly linear frequency sweep with MHz resolution –Significant improvement over etalon techniques, but –Only works with continuously tuned lasers Dual comb spectroscopy of dynamic laser –Absolute frequency accuracy of a referenced frequency comb –Frequency resolution can be time-bandwidth limited –Fast update times (down to 30 μs demonstrated) –Can support arbitrary cw waveforms –Works with frequency-switchable lasers (MG-Y, SG-DBR)

(f r ) -1 J.-L. Peng et al. Appl. Phys. B 92, 513 (2008). Coddington et al. Phys. Rev. Lett. 100, (2008) Wideband Intensity Measurement - Resolution: f r (100 MHz) - Accuracy: ~ 1 kHz - Update: T U = 1/  fr (300 μs) - Span: 1/ (2  T)=T U  f r 2 /2 (1.5 THz) Dynamic cw laser

f r +  f ~ 100 MHz + 3 kHz E LO Tight phase locking of both combs Require residual linewidth << Δf (3 kHz): Phase lock combs to two cw lasers –For high frequency accuracy, lock cw lasers to cavity Achieves sub-radians optical coherence f r ~ 100 MHz E signal 1535 nm 1560 nm Cavity stabilized Lasers frequency field

High Resolution Measurement cw T U : 300 μs Dynamic cw laser

Instantaneous optical frequency Instantaneous frequency on fast time scale Inst freq. FFT Time Voltage ~ field Frequency Intensity Frequency domain comb picture  Measured data

Frequency Measurement of Tuned Laser Measurements mutually independent Arbitrary / discontinuous waveforms

Gas Spectroscopy with a Step-tuned Laser Step-tune laser frequency –Fine steps across line –One coarse step between lines Frequency comb accuracy Span: 5.2 THz (42 nm) ADC Dynamic cw laser

Gas Spectroscopy Total measurement time: ~ 5 s (limited by cw laser) Frequency axis accuracy ~ 1 kHz Line center frequency accuracy ~ 1 MHz (fit residual) HiTran data CO (115 Torr) CO 2 (1000 Torr) C 2 H 2 (50 Torr)

Conclusion Demonstrated coherent dual-comb spectroscopy of tunable cw laser –High frequency resolution (3 kHz) and accuracy (~1 kHz) –High time resolution (down to T U = 30 μs) –Minimal a priori frequency knowledge required: 3 THz for T U = 300 μs Future improvements –Higher comb repetition rate to track faster lasers –Other spectral regions –Real-time processing Applications –Metrology of lasers with fast frequency tuning / hopping –Dynamic, wide band optical sensing Thank you

Frequency combs T. W. Hänsch, Rev. Mod. Phys. 78, 1297 (2006). J. L. Hall, Rev. Mod. Phys. 78, 1279 (2006). Stabilization of any two degrees of freedom → Entire comb stabilized → Entire pulse train stabilized t 1/f repetition  offset Passively Mode-locked Laser Ti:sapphire Cr:forsterite Er Fiber Yb fiber… f cw =f beat + f ceo + N  f r cw

Data II

Instantaneous optical frequency Instantaneous frequency on fast time scale Inst. freq FFT Time Field