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Acknowledgments

All-Fiber, Phase-Locked Supercontinuum Source for Frequency Metrology and Mo lecular Spectro scopy Brian R. Washburn National Institute of Standards and Technology Optoelectronics Division 815.03 325 Broadway Boulder, CO 80305. Acknowledgments. S. A. Diddams, N. R. Newbury,

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Acknowledgments

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  1. All-Fiber, Phase-Locked Supercontinuum Source for Frequency Metrology and Molecular Spectroscopy Brian R. Washburn National Institute of Standards and Technology Optoelectronics Division 815.03 325 Broadway Boulder, CO 80305

  2. Acknowledgments S. A. Diddams, N. R. Newbury, S. L. Gilbert, W. Swann National Institute of Standards and Technology J. W. Nicholson and M. F. Yan OFS Laboratories, USA C. G. Jørgensen OFS Fitel Denmark I/S, Denmark

  3. Introduction: Output of a Mode-Locked Laser Output of a Mode-Locked Laser 10 ns Power Time Domain 10 fs time 0.1 THz 10 MHz Frequency Domain Power Frequency Small Dt => Broad Spectral Coverage

  4. Supercontinuum Ti:sapphire Laser Microstructure Fiber

  5. Optical Frequency Metrology Nonlinear Optical Fibers Radio Frequency Standards Mode-Locked Lasers Molecular Spectroscopy

  6. Outline Part One: Mode-locked Fiber Lasers • Compare/contrast fiber lasers to free-space lasers • Fiber Dispersion and Nonlinearities • Mode-locking in fiber lasers Part Two: Optical Frequency Metrology • Components of the all-fiber supercontinuum source • Phase-locking a fiber laser • System performance Part Three: Molecular Spectroscopy

  7. Passively Mode-locked Lasers Elements of mode-locked lasers • Pump source • Gain element • Saturable absorber for mode-locking • Dispersion compensation for shortest pulses

  8. Fiber Lasers: Advantages and Disadvantages • Advantages • Easy to align fiber laser cavity • Less sensitive to misalignment • Passive optical elements are inexpensive • Uses less power than Ti:sapphire laser • More compact • Disadvantages • More sensitive to environment (polarization) • Optical fiber limits total laser power • All fiber cavity limits ability to easily experiment with laser design • Careful dispersion and nonlinearity management is needed for proper laser design

  9. Gain Medium: Erbium-Doped Fiber (EDF) EDF Gain Bandwidth • Use a fiber that is highly doped with Er as the gain element of the laser • This fiber exhibits normal dispersion : D=-70 ps/nm-km

  10. Saturable Absorber for Mode-Locking • A saturable allows the laser cavity to “favor” high peak power, ultrashort pulses • An absorber created by Kerr lensing is typically used in solid state lasers • Fiber nonlinearities are used in fiber lasers Need a complete understanding of fiber dispersion and nonlinearities

  11. Fiber Dispersion and Nonlinearities Fiber Group Velocity Dispersion (GVD)

  12. Fiber Dispersion and Nonlinearities Fiber Self Phase Modulation (SPM)

  13. Characterizing Dispersion and Nonlinearity in an Optical Fiber • Assume single mode and no birefringence • Concerned with phase matched nonlinearities • Assumptions leads to SPM and GVD only

  14. Characterizing Dispersion and Nonlinearity in an Optical Fiber • The dispersion length (LD) is the length of fiber where a Gaussian pulse to temporally broadens by Sqrt(2) • The nonlinear length (LNL) is the length of fiber for which a pulse gains a phase of 1 radian

  15. The Nonlinear Schrödinger Equation A beautiful equation which accurately describes a highly nonlinear optical system An understanding of this equation provides the ability to design and predict the behavior of active fiber devices • Fiber lasers • Erbium doped fiber amplifiers • Nonlinear loop mirrors/switches • TOADs

  16. Mode-locking in Fiber Lasers • Active Mode-locking • Typically use AOM or Mach Zehnder to achieve mode locking • Sigma laser (Duling et al, Opt Lett Vol 21, 21 1996) • Advantage: Can achieve high repetition rates (10 GHz) • Passive Mode-locking • Interferometric designs based on gain and saturable absorber sections • Figure eight lasers (Sacnac switch) • Stretched Pulse Lasers • Advantage: sub-picosecond, high energy pulses

  17. Figure Eight Laser Output

  18. Nonlinear Loop Mirror: Linear Operation Linear Operation: No phase shift between interferometer arms

  19. Nonlinear Loop Mirror: Nonlinear Operation Nonlinear Operation:

  20. Figure Eight Laser Performance AMP Temporal FWHM <100 fs Average Power= 100 mW Center Wavelength= 1560 nm

  21. Stretched Pulse Laser Normal Dispersion Anomalous Dispersion

  22. Nonlinear Polarization Rotation Ey Ex

  23. Nonlinear Polarization Rotation Ey Ex

  24. Nonlinear Polarization Rotation Ey Ex

  25. Nonlinear Polarization Rotation Ey Ex

  26. Nonlinear Polarization Rotation Ey Ex

  27. Stretched-Pulse Operation intensity time

  28. Stretched-Pulse Fiber Laser WDM 980 nm Pump 90/10 Splitter EDF Isolator Output Polarizer Polarization Controllers Polarization Controllers

  29. Stretched-Pulse Laser Performance AMP Temporal FWHM <100 fs Average Power= 100 mW Center Wavelength= 1560 nm

  30. Part Two: Optical Frequency Metrology Optical Frequency Metrology Nonlinear Optical Fibers Radio Frequency Standards Mode-Locked Lasers New way to connect microwave and optical frequencies

  31. Electric Field from a Mode-locked Laser Df Df repetition frequency fr I(f) fo = frDf/2p f fn = nfr + fo 0 Time domain (Pulses in time) Time domain (Pulses in time) 2Df 2Df E(t) E(t) Carrier-envelope phase slip from pulse to pulse because group and phase velocities differ Carrier-envelope phase slip from pulse to pulse because group and phase velocities differ t t repetition rate repetition rate Frequency domain (Comb of lines) • Stable frequency comb if • Repetition rate (fr)locked • Offset frequency (f0) (phase slip) locked

  32. Acoustic Frequency Metrology: Guitar Tuning Known Frequency, fk Unknown Frequency, fun Df=0.5 Hz Df=0.01 Hz

  33. Optical Frequency Metrology RF Beat fun repetition frequency fr I(f) fo = frDf/2p f 0 fn = nfr + fo 1550 nm 193,548,387,096,774.2 Hz 10,000,000.0 Hz Locking Electronics Stability and accuracy of RF standard passed to optical frequencies Cesium Time Standard ~9 GHz Hydrogen Maser 10 MHz

  34. Measure offset frequency fo as shown and lock to zero Phase-lock fr directly to an rf synthesizer Stabilize frequency comb bySelf-reference frequency locking fr I(f) fo f 0 fn = n fr+ fo x2 f2n = 2nfr+ fo fo To Lock comb to an RF oscillator

  35. Supercontinuum Frequency Comb upercontinuum

  36. A Fiber Laser-Based Frequency CombTranslate Ti:Sapphire results to Fiber-based system • Most existing frequency combs limited to Ti:Sapphire laser-based systems • No self-referenced frequency combs from a mode-locked fiber laser in use • Locking of a fiber laser to other stabilized sources have been achieved* • Until recently a full octave from fiber laser not available* • A fiber-based frequency comb can provide • Compact, inexpensive design • Potential for stable “hands-free” operation • Optical frequency metrology in the IR * References F. Tauser et al, Opt. Express 11, 594 (2003) F.-L. Hong et al, Opt. Lett. 28, 1 (2003) J. Rauschenberger et al., Opt. Express 10, 1404 (2002)

  37. All-Fiber Supercontinuum Source HNLF 1480 nm pump 1480 nm pump Er fiber SMF pigtail 980 nm pump Er fiber isolator Continuum after 20 cm DF - HNLF

  38. Highly Nonlinear Fiber (HNLF) nonlinearity : 8 to 15 1/W-km Effective Area : 13 mm2 loss : 0.7 to 1 dB/km dispersion (1550 nm) : -10 to +10 ps/nm-km dispersion slope (1550 nm) : 0.024 ps/nm2-km splice loss (to SMF) :0.18 dB splice loss (to HNLF) :0.02 dB

  39. f-to-2f Interferometer An octave of supercontinuum allow the generation of CEO beat frequencies with a SNR of 30 dB

  40. Phase-locked Frequency Comb • Oscillator: 20 nm FWHM pulses, 50 MHz rep rate • Amplifier: 100 mW output, FWHM < 100 fs • Supercontinuum generation in highly nonlinear fiber (HNLF), 23 cm of length • f-to-2f Interferometer

  41. Fiber Laser-Based Frequency Comb f-to-2f interferometer Supercontinuum Source (all fiber)

  42. Frequency Stability - Optical comb phase locked to RF source- Any optical comb line known absolutely by fn = nfr + fo

  43. Phase Noise Measurements • CEO frequency lock : integrated phase error for 2.07 MHz signal (DC to 25 MHz) was ~10 mrad • Repetition rate lock : integrated phase error (DC to 25 MHz) was <1 mrad

  44. Optical Frequency Metrology Molecular Spectroscopy Nonlinear Optical Fibers Radio Frequency Standards Mode-Locked Lasers

  45. Standard Reference Materials Standards for Wavelength Division Multiplexing

  46. Spectroscopy of Acetylene Wavemeter Frequency Uncertainty: ~1.8 MHz Typical wavemeters: ~20 MHz (0.15 pm at 1550 nm) Reference: Swann and Gilbert, JOSA B Vol. 17,7, (2000)

  47. Metrology with Supercontinuum Comb fun 2fr fr repetition frequency fr I(f) fo = frDf/2p f 0 fn = nfr + fo Fiber-based Frequency Comb ESA Computer Tunable CW Laser 12C2H2 RF Beat

  48. Conclusions • Stabilized frequency combs have revolutionized optical clocks • Previous systems limited to 400 nm to 1300 nm • Fiber laser-based frequency comb demonstrated • Potentially more robust than Ti:sapphire laser based frequency comb • Extend phase-lock frequency combs into the IR • Permit unprecedented accuracy in IR frequency metrology • Can lock frequency comb to Cesium time standard or other atomic standard

  49. Thank you for your time! Brian R. Washburn National Institute of Standards and Technology Optoelectronics Division 815.03 brianw@boulder.nist.gov

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