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Application of Femtosecond Laser Frequency Combs toward Frequency

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					Application of Femtosecond Laser Frequency Combs
toward Frequency Stability Measurement for DPF Light Source




              Shigeo Nagano, Hiroyuki Ito and Mizuhiko Hosokawa

                       Atomic Frequency Standards Project,
                           Space-Time Standards Group,
    National Institute of Information and Communications Technology (NICT)
                    4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795 Japan




         1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
Motivation
 ・For DECIGO and DECIGO-pathfinder, the laser frequency stability of 1Hz/Hz1/2 level is required from 0.1Hz
 and 1Hz.
 Such high stability was recently achieved in laboratory for clock lasers of the optical frequency standards.


 ・Reliable measurement of the laser frequency stability can provide technical feedbacks to the laser
 development, and thus the measurement can be one of the key point in order to succeed the DECIGO and
 DPF missions.


 ・To exclude the possibility of common mode noise in the frequency stability measurement, beat
 measurements were often taken between the lasers stabilized to the atomic resonance and Fabry-Perot
 cavity.
 ・By means of optical frequency comb based on femtoseond-pulse mode-locked lasers (OFCs), the laser
 frequency stability can be compared with cryogenic sapphire microwave oscillator with the measurement
 accuracy of 1Hz/Hz1/2 level. (Note that the frequency difference between light and microwave is ~105Hz.)
 This is also good application of OFCs developed for the optical frequency standards in National Institute of
 Information and Communications Technology.




             1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
 Optical frequency standards in NICT
・National Institute of Information and Communications Technology (NICT) is
Japanese official institution for the national frequency and time standards.
・Atomic frequency standards in NICT
- NICT-CsF1 : Cesium atomic fountain clock (microwave)
     operation from 2007 with uncertainty of 1.9 x 10-15 and stability of 2 x 10-13 at 1s
                                                                                               Japan standard time(JST)
- Optical clocks based on laser-cooled single 40Ca+ ion
 and cold 87Sr atoms in optical lattice
     potential for higher accuracy (10-18) than microwave standards
     lead a new definition of SI second
     new knowledge regarding fundamental physics
・Optical frequency comb based on femtosecond-pulse mode-locked laser (OFC)                  Cs atomic fountain NICT-CsF1

serves as optical frequency counter and optical synthesizer.
OFC can be also applied to frequency stability measurement for DECIGO and DPF laser
with cryogenic sapphire microwave oscillator

                                                                                            Cryogenic
                                                                                            Sapphire
                                                                                            oscillator



                                                               87
        Single 40Ca+ ion trap        Sr atoms in MOT             Sr optical lattice

                   1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
Frequency stability measurement -principle-

  ・Time domain
  Carrier-envelope phase slip from pulse to pulse   Mode-locked
                                                     fs laser
  Because grope velocity is not equal phase
  velocity (vg≠vp)




  ・Frequency domain
  m-th mode frequency: fm = m frep + fCEO
  frep : pulse repetition frequency
  fCEO =∆φCEO/2πτ : carrier-envelope offset frequency
  m : integer mode number


  For frequency stability measurement, frep and fCEO must be measured and controlled.
  - frep: measured by direct detection of pulse train, controlled with stable microwave reference
  - fCEO : measured by the self-referencing technique, also controlled with stable microwave reference




               1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
Frequency stability measurement -principle-

  ・Time domain
  Carrier-envelope phase slip from pulse to pulse
  because of grope velocity is not equal phase
  velocity




  ・Frequency domain
  m-th mode frequency: fm = m frep + fCEO
  frep : pulse repetition frequency
  fCEO =∆φCEO/2πτ : carrier-envelope offset frequency
  m : integer mode number



  ・Self-referencing technique:
   fceo is a frequency of heterodyne beat
   between the second harmonics of m-th mode and 2m-th mode.

   Need a very broadband spectrum (i.e. an octave)
   Spectrum broadening with photonic crystal fiber




               1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
Frequency stability measurement -principle-

  ・Time domain
  Carrier-envelope phase slip from pulse to pulse
  because of grope velocity is not equal phase
  velocity




  ・Frequency domain
  m-th mode frequency: fm = m frep + fCEO
  frep : pulse repetition frequency
  fCEO =∆φCEO/2πτ : carrier-envelope offset frequency
  m : integer mode number


  For frequency stability measurement, a heterodyne beat measurement
  between CW laser and m-th mode of optical comb.




               1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
Femtosecond laser frequency combs in NICT
 ・ FC8003 (Menlo Systems GmbH)
      Photonic crystal fiber for spectrum broadening over one octave
      Straightforward now
      Not reliable for long-term operation (> a few hours)


 ・Venteon OS (NanoLayers GmbH)
      Pulse repetition frequency : 200MHz                                   FC8003 (Menlo system GmbH)

      Output spectrum coverage : 1 octave
      Reliable for long-term operation


 ・ Gigajet 20W (Gigaoptics GmbH)
      Pulse repetition frequency : 1GHz
      Output spectrum coverage : 2/3 of one octave
                                                                           Venteon OS (NanoLayers GmbH)
      Reliable for long-term operation




                                                                            Gigajet 20W (Gigaoptics GmbH)

            1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
Gigajet20W based optical frequency comb
                                                          ・Kerr-lens mode-locked
                                                          femtosecond-pulse Ti:sapphire laser
                                                          ・Pulse repetition frequency : 1GHz
                                                          ・Average output power : 800mW@8.5W pump
                                                          power
                                                          ・Pulse width : 26fs @951nm
                                                          ・Output spectrum : 2/3 of one octave
                                                          ・2f-to-3f self-referencing technique :
                                                          beat measurement of 960nm (THG)
                                                          and 640nm (SHG)
                                                          ∵3 fm -2 f3m/2 = 3(m frep+fceo) – 2(3m/2 frep+fceo)
                                                                         = fceo
                                                          ・Stable long-term operation over 19 hours
                                                          without any alignment




       1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
Cryogenic sapphire oscillator
  ・High quality HEMEX single-crystal sapphire
  ・Temperature : 7.3K
  ・High unloaded quality factor : ~109
  ・Whispering gallery resonant mode : WGH16,0,0

  ・Oscillation frequency : 11.2005GHz
                                                                                       F. Riehle, Frequency Standards,
       Synthesis chain generates 1GHz signal                                           Wiley-VCH Verlag GmbH (2004)


  ・Excellent short-term stability : <10-15 between 2s~700s, minimum 5x10-16 at 30s~60s
  ・Good frequency reference for optical frequency comb in order to evaluate the DPF light source
   Originally introduced as flywheel oscillator for Cs atomic fountain
  ・Developed in University of Western Australia (UWA)




             Sapphire and niobium cavity          Baffles                Liquid-helium dewar

              1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
              Frequency stability measurement by optical comb
                                                                                         ・Femtosecond laser frequency comb referenced
                                                                                         to cryogenic sapphire oscillator
                                                                                         ・Frequency stabilized laser for observation of
                                                                                         atomic resonance (clock laser)
                                                                                         Wavelength 729nm, output power few mW
Instability




                                                                                         High-finesse Fabry-Perot cavity with ultralow
                                                                                         expansion glass (ULE) spacer (F~150000)
                                                                                         Narrow line width (<25Hz)
                                                                                         ・Measured frequency stability
                                                                                              5 x 10-15@1s~10s




              Allan variance is a measure of frequency stability of oscillator
                                    〈 y − y i 2 〉
                                                                    1 t   t 
                          2 =       i1
                                                                      ∫
                                                                         i

                           y                         ,   here y i≡
                                                                                   dt
                                           2                          t i   0
              Corresponding Allan variance of
              power spectral density of frequency noise δν(f)

                                     1   f  2
                          2 =
                           y                  
                                    2   0

                                                                                                    Vacuum chamber for ULE cavity

                                1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)
 Summary
・National Institute of Information and Communications Technology plans to measure the frequency stability of the
DPF light source by femtosecond laser optical frequency comb (OFC) referenced to cryogenic sapphire microwave
oscillator. 


・The frequency stability of clock laser for optical frequency standard was measured by the OFC, and it was found
to be 5 x 10-15 in root Allan variance between 1s~10s.
This is the first measurement with 10-15 level by the OFC referenced the CSO, to the best of our knowledge.


・Further improvement of the measurement accuracy is required to confirm the frequency stability of 1Hz/Hz1/2.




                 1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

				
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posted:12/21/2011
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