Fast and Accurate Comb-based Spectral Analysis of a Frequency

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Fast and Accurate Comb-based Spectral Analysis of a Frequency Powered By Docstoc
					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                        Gas

    – Highly sensitive                               cell
    – 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
           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)
                 J.-L. Peng et al. Appl. Phys. B 92, 513 (2008).
 Wideband     Coddington et al. Phys. Rev. Lett. 100, 013902 (2008)
  Intensity
Measurement
 Dynamic
 cw laser




                                      - Resolution: fr (100 MHz)

                                      - Accuracy: ~ 1 kHz
                  (fr)   -1


                                      - Update: TU = 1/ fr (300 μs)

                                      - Span: 1/ (2T)=TU  fr2/2
                                              (1.5 THz)
                     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
 field




                 •   Achieves sub-radians optical coherence
     frequency
                                           fr ~ 100 MHz
 Esignal
 ELO




                                         fr+f ~ 100 MHz + 3 kHz
Cavity stabilized
Lasers                     1560 nm                           1535 nm
           High Resolution Measurement


Dynamic
cw laser




                                         TU :
                                         300 μs




               cw
                   Instantaneous optical frequency
        Frequency domain comb picture                      Measured data




                                          Voltage ~ field
Intensity




                                                                 Time
            Frequency

                                                                   FFT


                                                                          Instantaneous
                                                                           frequency
                                                                           on fast time
                                                                           scale
Frequency Measurement of Tuned Laser




                           Measurements
                            mutually independent
                           Arbitrary /
                            discontinuous
                            waveforms
   Gas Spectroscopy
with a Step-tuned Laser




                                                 ADC
Dynamic
cw laser



            • Step-tune laser frequency
               – Fine steps across line
               – One coarse step between lines
            • Frequency comb accuracy
            • Span: 5.2 THz (42 nm)
                  Gas Spectroscopy
         HiTran data




                   CO                  CO2                  C2H2
                (115 Torr)          (1000 Torr)           (50 Torr)




• Total measurement time: ~ 5 s (limited by cw laser)
• Frequency axis accuracy ~ 1 kHz
• Line center frequency accuracy ~ 1 MHz (fit residual)
                               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
                                      T. W. Hänsch, Rev. Mod. Phys. 78, 1297 (2006).
                                         J. L. Hall, Rev. Mod. Phys. 78, 1279 (2006).
   Frequency combs
                                                foffset
                   t



                                                            1/frepetition


      Passively
   Mode-locked Laser
   Ti:sapphire
   Cr:forsterite                                               fcw=fbeat+ fceo + N  fr
   Er Fiber
   Yb fiber…                 n


Stabilization of any two degrees of
freedom → Entire comb stabilized
→ Entire pulse train stabilized

                                                                        cw
Data II
    Field    Instantaneous optical frequency




                                     FFT
            Time
Inst.
freq                                        Instantaneous
                                             frequency
                                             on fast time
                                             scale

				
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