Docstoc

Direct Frequency Comb Spectroscopy Optical Frequency Measurements

Document Sample
Direct Frequency Comb Spectroscopy Optical  Frequency Measurements Powered By Docstoc
					                                                Femtosecond
                                              Frequency Combs
Direct Frequency Comb             Time domain

 Spectroscopy: Optical                E(t)                     Δφ                  2Δφ

                                                 Δτ ~10-14 s
       Frequency                                                                             t

  Measurements with
                                                                    Δt ~10-9 s
   Ultra-short Pulses                 Frequency domain

                                  Ι
          Jason Stalnaker                    νn = n frep + f0
                                      f0
 Optical Frequency Measurements
              Group
  Time and Frequency Division
                                       0                                                 ν
               NIST
                                                                            frep

                                                                       Δφ
                                       Offset frequency:   f0 = frep
                                                                       2π
                                                                                 What about direct
Octave-Spanning Laser
                          Tara Fortier, Scott Diddams and Albrecht Bartels      comb spectroscopy?
        Frequency Comb Spectrum                                              Simpler experimental setup
                                                                                  Reduce number of extra lasers
 Relative Power (dB)




                        0
                       - 10                                                        Only need one stable reference
                       - 20
                       - 30                                                  Simultaneous access to 100’s of THz
                       - 40                                                       Can simultaneously excite multiple
                       - 50                                                        transitions
                              600       800       1000       1200     1400
                                       Wavelength (nm)
                                                                                   Access to wavelengths otherwise difficult
                                                                                   to produce
     Comb parameters:
     • Total Output Power ≅ 850 mW
                                                                             Main Drawback
     • Number of Modes ~105                                                       Low power per mode < 1 μW
     • frep~ 1 GHz
           Spectroscopy with                                       Direct Comb
              short pulses                                         Spectroscopy
                                                                Experiments at NIST
                     Light On Resonance   Light Off Resonance

                                                                • Spectroscopy of calcium clock transition
Electric Field
   Light




                                                                using cold atoms
                                                                      Tara Fortier, Yann LeCoq, Jason Stalnaker, Davi Ortega,
                                                                      Scott Diddams, Chris Oates and Leo Hollberg
Evolution
 Atomic




                                                                • Two-photon spectroscopy of cesium atoms
                                                                in a vapor cell
Excited-State
 Population




                                                                      Vela Mbele, Jason Stalnaker, Vladi Gerginov, Tara Fortier,
                                                                      Scott Diddams, Carol Tanner and Leo Hollberg


                            Time                  Time          • Spectroscopy of excited calcium transitions
                                                                using cw-comb spectroscopy
                 e                                                    Jason Stalnaker, Yann LeCoq, Tara Fortier, Scott Diddams,
                                                                      Chris Oates and Leo Hollberg
      Energy




                 g
   Calcium Clock
                                                       Power limitations
   Spectroscopy
                                                        Wavelength good for comb:
                              4s4p 1P1                                         Frequency Comb Spectrum




                                                         Relative Power (dB)
                                                                                            657 nm
                                                                                0
                                          4s4p   3P
                                                   2
    423-nm                                                                     - 10
    Cooling/Trapping                      4s4p   3P
                                                   1
    Transition                            4s4p   3P
                                                   0                           - 20
                                                                               - 30
                       657-nm
                       Clock Transition                                        - 40
                       γ0 = 400 Hz
   4s2 1S0                                                                     - 50
                                                                                      600     800    1000   1200   1400
                                                                                             Wavelength (nm)
 Calcium Clock Setup
  Ca atoms cooled and trapped on                       Laser output ≈ 2.4 μW per mode
  4s2 1S0 → 4s4p 1P1 transition
                                                       Lose ≈ 10 with fiber coupling and switching optics
  Trap parameters:                                     Can deliver ≈ 240 nW to atoms
     ~6 107 atoms
     Load Time ≅ 2 ms                                     ⇒ ΩR ≈ 4 kHz
     T ≅ 2 mK (≅ 3 MHz Doppler width)
                                                       Use cold atoms
  Population transfer detected through
                                                        ⇒ Long interrogation times
  ground-state depletion
Can we see the clock transition in Ca with             Amplification of comb
               the comb?
Amplification & Linewidth of
           Comb                                                                Experimental Setup
What happens if we injection lock a diode laser with
the comb output?
                                                                                                    trapping beams
Near lasing threshold current:
         Many teeth amplified                                                                 λ/4              λ/4   probe
                                                                                                                     beams                                 high-finesse
         Gain per tooth ~10                                                                                                                            534-nm optical cavity

Well above lasing threshold current:                                                          λ/4              λ/4
                                                                                                                                                ×2
        One tooth preferentially amplified
                                                                                                     Ca MOT
        Gain for one tooth ~1000                                                                                                  fiber laser
                                                                                                                                   1068 nm


                                                                                                                                                              to Ti:S laser
                                                                                         DL     AOM                                        fb                 cavity length
 Transition linewidth ~ 400 Hz
          ⇒ Comb modes need to be narrow
 Stabilize mode of comb to narrow cw laser (1068 nm)
          ⇒ Stability transferred throughout comb
                                                                                                                             _
                                  Comparison of stable cw light at 657 nm to                    frep                                            ×2
                                  comb locked to cw light at 1068 nm
                                                                               H-maser                                                               to Ti:S laser
         Normalized Beat Signal




                                  1.0                                                                                        fo                      pump power

                                  0.8                                                          Counter


                                  0.6
                                                           3 Hz
                                  0.4

                                  0.2
                                  0.0
                                        -200   -100    0     100    200
                                        Frequency relative to carrier (Hz)
Optical Ramsey-Bordé                                                                            Ramsey-Bordé
       Fringes                                                                                  Spectroscopy
 With large amplification Ramsey-Bordé fringes possible                                          20
                                                                                                         T= 14 μs
                                                                                                 10




                                                                                  Signal (mV)
                                                                                                  0                         11.6 kHz
                                        π/2                π/2                                  -10
                                                                                                -20
                                                                     Absorption
                  T/2                             T/2                                           -30
                                                                                                -40
Position




                                                                                                                -50           0              50

                                Stimulated
                                                  Stimulated
                                                   Emission
                                                                                                         T= 104 μs                2.3 kHz




                                                                                     Signal (mV)
                                 Emission                                                          10
             Absorption
                                                                                                    0
                                                                 Stimulated
                                Absorption    Absorption          Emission
                                                                                                   -10

                                                                                                         -10          -5           0              5
           π/2            π/2
                                      Time                                                                                             1.2 kHz
                                                                                                         T= 203 μs
   Population in Ground State ∝ Cos(2 π (νCa - ν)T + ΔΦ)
                                                                                                     4




                                                                                     Signal (mV)
                                                                                                     2
                                                                                                     0
                                                                                                    -2
                                                                                                    -4
                                                                                                               -5             0           5
                                                                                                                      Relative Frequency (kHz)
                                                            Cs structure
 Calcium Results                                 Rich atomic structure: 4 electronic states accessible

                                                 Resonant enhancement through 6 P states

                                                 Hyperfine structure: 14 states to study
Observed narrow features (~1 kHz) with comb
spectroscopy
                                                    9S 1/2 F=3-4
Spectroscopic demonstration of stability
transfer                                            7D 5/2 F=1-6
                                                    7D 3/2 F=2-5
       ⇒ Only need one optical oscillator
                                                    8S 1/2 F=3-4
Amplification of comb in two different regimes                                                    7P 3/2
                                                                                                  7P 1/2

Possible extensions to colder atoms
(Yb optical lattice clock) and/or XUV regime
                                                    6P 3/2 F=2-5
                                                                                           Detection
                                                    6P 1/2 F=3-4
                                                                                           Fluorescence




                                                    6S 1/2 F=3-4
          Experimental                              Use Resonant
             Setup                                  Enhancement
                                                                                      1 1
                                                Two-Photon Transition Probability ∝
    Filter                                                                            Δ2 Δ2
                                                                                          2
                  PMT




   455 nm                                                                              1


                Cs Cell                                                   Δ2
                                                                               ν2



                                                                         Δ1

                                                          ν1


                          _
               frep            ×2

H-maser                   fo    to Ti:S laser
                                pump power
                                                   How to make Δ1 and Δ2 zero?
          Computer
                                                       → Use velocity selection
Velocity selective                                                                                   Experimental
double resonance                                                                                       spectrum
                                     c
                                 ν2=(1-v/c)(f0+ n2 fRep)




                                                           450-nm Fluorescence Signal
                                     b
       ν1=(1+v/c)(f0+ n1 fRep)


                                     a



                                                                                        1 0 0 0 .7 7 2 0   1 0 0 0 .7 7 3 0          1 0 0 0 .7 7 4 0
                                                                                                                   f re p [M H z ]
                     c
 atomic population




                                                                      ≈100 resonance peaks

                                                                      All allowed combinations of ground-intermediate-
                     b
                                                                      excited states observed

                                                                      Filtering comb selects specific transitions

                     a

                            velocity
                        Theoretical Modeling
                            and Results                                                           Acknowledgments
Model spectrum using standard two-photon formula

                         6S1/2→ 6P3/2→ 9S1/2
                                                                                                  Calcium            Cesium
                                                                                                  Tara Fortier       Vela Mbele
                        Experimental Spectrum
                                                                                                  Yann Le Coq        Vladi Gerginov
Fluorescence Spectrum




                                                          6S1/2 F = 3→ 6P3/2 F'→ 9S1/2 F '' = 3   Davi Ortega        Tara Fortier
                                                                             F' = 3               Scott Diddams      Scott Diddams
                                                              F' = 2                              Chris Oates        Carol Tanner
                                                                                                  Leo Hollberg       Leo Hollberg
                                                                                      F' = 4
                        Modeled Spectrum                                                          (With help from
                                                                                                    Till Rosenband
                                                                                                    David Hume
                                                                                                    Jim Bergquist)
                        1000.7720    1000.7730         1000.7740
                                          frep [MHz]




Measurement of absolute energies and hyperfine constants
for four states: 8S1/2, 9S1/2, 7D3/2, 7D3/2


Uncertainties are 50 - 100 kHz for optical frequencies
       ~ 100 x improvement for 9S1/2, 7D3/2, 7D3/2 states

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:21
posted:11/19/2011
language:English
pages:10