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Transcript of Tunable Laser Spectroscopy Referenced with Dual Frequency Combs International Symposium on Molecular...
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)
(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