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					     Development of
Femtosecond Laser Comb
  for Optical Frequency
        Metrology
       Yu-Ping Lan
       2005.09.05
       Outline

•Introduction
•Femtosecond Laser Comb
•Fiber Laser Comb
•Summary
             Introduction

• 1999: Prof. Hänsch first used mode-locked
  laser on optical frequency measurement.
• 2000: Bell Lab. extended the spectrum of
  the femtosecond laser from violet to
  infrared by using the microstructure fiber.
Spectra of A Mode-locked Laser
• Time domain




• Frequency domain
 Femtosecond Laser Spectrum
 Laser spectrum
                                              Microstructure fiber




• Center at 800 nm, repetition rate = 1 GHz         diameter ~1.7 μm
• Spectrum range 500-1180 nm
• Average power > 750 mW at 5 W pump power.
Femtosecond Laser Comb
    Frequency Stabilization
                  AOM               Loop         Offset Frequency
    AOM                                          Stabilization
                  driver            filter

     f-2f self-
                    Prescaler                Prescaler    Synthesizer
    referencing


Femtosecond           Repetition                1 GHz     Low noise
   laser              frequency                  filter   synthesizer

              Repitition Frequency
   PZT                                                        Rb
              Stabilization
   HV                      Loop
                           filter              Cs clock       GPS
   AMP
Optical Frequency Measurement
• 633 nm Iodine stabilized He-Ne laser
   (127I2 R(127) 11-5 a15, g line)

Offset frequency (MHz)     73.1385
Beat frequency (MHz)       267.2573
Comb number                473612
Measured frequency (kHz)   473 612 340 396 ± 5
                           (473 612 340 399 ± 11.6)
                                                                  Fiber Laser Comb
                         980 nm
                         pump

                                    x P-APM Ring                980 nm                                                          980 nm
                                                                                                                                              SMF-28
                                            fiber laser         pump                                                            pump
                                Er




                                                                                                                                                                         HNLF
                                   :fib




                                                             PZT                      Er:fiber
                                                  x
                                       e




                                                                           x                                                       x                   x
                                     r




                           Fiber laser oscillator                                 Er:fiber amplifier


                                                                                                                       1
                                                                                                                            9
                                                                                                                            8

                          3.0                                                                                               7                                                        Laser oscillator
                                                                                                                            6
                                                                                                                                                                                     after amplifier
                                                                                                                                                                                     after HNLF
                                                                                                                            5
                          2.5
                                                                                                                            4
Intensity (arb. units)




                                                                                                 Power (Arb. units)



                          2.0                                        FWHM=77 fs
                                                                                                                            3




                          1.5
                                                                                                                            2



                          1.0


                          0.5                                                                                         0.1
                                                                                                                            9
                                                                                                                            8

                          0.0                                                                                               7


                                           -400       -200       0          200        400                                  1000       1200   1400   1600         1800        2000      2200            2400
                                                             Delay (fs)                                                                                     Wavelength (nm)
         Specifications
Laser oscillator
•    turn-key self mode-locking
    (P-APM : polarization-additive pulse mode-locking)
•    center wavelength: 1560 nm
•    pulsewidth < 200 fs
•    average power ~5 mW
•    repetition rate: 100 MHz
•    PZT tunning range ~2 kHz for max. voltage of 150 V

After amplification and compression
•   pulsewidth <100 fs
•   average power ~150 mW

After HNLF
•   Spectrum: 1050-2100 nm
•   Power ~130 mW
         Fiber Laser Comb



                           14x26.5x4.5 cm3(2nd version)
25x25x5 cm3(1st version)
                Frequency Stabilization
                                             SMF        HNLF
Er:fiber laser              x            x          x          x
oscillator                  Amplifier              PC              SMF                               PD3
                                                                                 MgO:PPLN filter

                                                                    PD1
Pump LD




                                                                                  Single Beam f-2f
          PZT




                     SYN1                      BPF

                                                                    PD2
                    HV          Loop           SYN2
                    AMP         filter                                                SYN4

                                  Frequency                LPF
                                  Counter                                  LPF                    BPF


                Current           Loop              Phase                 Prescaler
                driver            filter            detector              ÷16              SYN3
                                                                     Performance
                                                                                                                                     -20
                   0.6
                                                                     1σ=0.12 mHz
                                                                                                                                     -30
                   0.4
Frequency (mHz)




                                                                                                                                     -40




                                                                                                                   Power (dBm)
                   0.2
                                                                                                                                     -50

                   0.0                                                                                                               -60


                   -0.2                                                                                                              -70

                                                                                                                                     -80
                   -0.4

                                                                                                                                           70     80    90            100          110         120         130
                              0    20       40     60     80     100      120       140                                                                         Frequency (Mhz)
                                                   Time (minutes)

                          2
                                                                                                                                 2
                                                                                                                                                                                         1σ=0.52 mHz
                        -13
                   10
                          8
                                                                                                                                 1
 Allan Deviation




                          6


                                                                                                 Frequency (mHz)
                          4

                                                                                                                                 0
                          2


                        -14
                   10                                                                                                       -1
                          8
                          6

                          4
                                                                                                                            -2
                                   2    3 4 56         2   3 4 5 6        2     3 4 5 6
                               1                 10                100                    1000                                        0    20     40   60          80        100         120         140
                                                  Averaging Time (s)                                                                                        Time (minutes)


                              Out-of-loop repetition frequency                                                                                  In-loop offset frequency
Nd:YAG Laser /I2 Frequency Measurement

                  91.749
                               1064 nm Nd:YAG= 281630 GHz +   f=91747.042±0.23 kHz Ti:S laser comb
                                                              f=91747.021 ±0.22 kHz fiber laser comb
                  91.748
Frequency (kHz)




                  91.747


                  91.746


                  91.745

                           0                  5                10                     15
                                                          Number


                   The difference is only 21 Hz.
                Summary
• The field of optical frequency
  metrology has been revolutionized.
• Mode-locked femtosecond laser can
  be used as a tool linking optical and
  microwave frequency.
• CMS has developed a very compact
  fiber laser comb.
 (Contact: Yu-PingLan@itri.org.tw, JLPENG@itri.org.tw)