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

fabrizio.giorgetta@nist.gov

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)

(fr)-1

J.-L. Peng et al. Appl. Phys. B 92, 513 (2008).Coddington et al. Phys. Rev. Lett. 100, 013902 (2008)Wideband

Intensity Measurement

- Resolution: fr (100 MHz)

- Accuracy: ~ 1 kHz

- Update: TU = 1/ fr (300 μs)

- Span: 1/ (2T)=TU fr2/2

(1.5 THz)

Dynamic cw laser

fr+f ~ 100 MHz + 3 kHz

ELO

Tight phase locking of both combsRequire 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

fr ~ 100 MHz

Esignal

1535 nm1560 nmCavity stabilized Lasers

frequency

field

High Resolution Measurement

cw

TU : 300 μs

Dynamic cw laser

Instantaneous optical frequency

Instantaneous frequencyon fast time scale

Inst freq. FFT

TimeVol

tage

~ f

ield

Frequency

Inte

nsity

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)

AD

CDynamic 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)

CO2

(1000 Torr)C2H2

(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 TU = 30 μs)

– Minimal a priori frequency knowledge required: 3 THz for TU = 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 combsT. 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/frepetition

offset

PassivelyMode-locked LaserTi:sapphireCr:forsteriteEr FiberYb fiber…

fcw=fbeat+ fceo + N fr

cw

Data II

Instantaneous optical frequency

Instantaneous frequencyon fast time scale

Inst.freq

FFTTime

Fie

ld