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Progress in femtosecond timing distribution and synchronization for ultrafast light sources

Progress in femtosecond timing distribution and synchronization for ultrafast light sources. John Byrd Lawrence Berkeley National Laboratory. John Staples, LBNL Russell Wilcox, LBNL Larry Doolittle, LBNL Alex Ratti, LBNL Franz Kaertner, MIT Omar Illday, MIT Axel Winter, DESY

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Progress in femtosecond timing distribution and synchronization for ultrafast light sources

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  1. Progress in femtosecond timing distribution and synchronization for ultrafast light sources John Byrd Lawrence Berkeley National Laboratory

  2. John Staples, LBNL Russell Wilcox, LBNL Larry Doolittle, LBNL Alex Ratti, LBNL Franz Kaertner, MIT Omar Illday, MIT Axel Winter, DESY Paul Emma, SLAC John Corlett, LBNL Mario Ferianis, ST Jun Ye, JILA David Jones, U of B.C. Joe Frisch, SLAC Bill White, SLAC Ron Akre, SLAC Patrick Krejcik, SLAC Acknowledgements John Byrd, BIW2006

  3. A great intro to fsec lasers Femtosecond Optical Frequency Comb: Principle, Operation and Applications Jun Ye (Editor), Steven T. Cundiff (Editor) John Byrd, BIW2006

  4. Synchronicity Stabilized link Stabilized link Stabilized link FEL seed laser Stabilized link PC drive laser user laser Stabilized link EO laser LLRF • Next generation light sources require an unprecedented level of remote synchronization between x-rays, lasers, and RF accelerators to allow pump-probe experiments of fsec dynamics. • Photocathode laser to gun RF • FEL seed laser to user laser • Relative klystron phase • Electro-optic diagnostic laser to user laser Master John Byrd, BIW2006

  5. Overview • Motivation: LCLS example • Ultrastable clocks • Stabilized distribution links • Synchronizing techniques • Measuring synchronization John Byrd, BIW2006

  6. Lots of FEL activity John Byrd, BIW2006

  7. Small things A gnat’s ass 100 femtoseconds • 100x10-15 sec • 30 microns • 0.8 mrad@1.3 GHz • 0.045 deg@1.3 GHz • 1.8 mrad@2856 MHz • 0.1 deg@2856 MHz • (10 TeraHertz)-1 • 20*(1.5 micron) John Byrd, BIW2006

  8. Motivation: LCLS • Final energy 13.6 GeV (stable to 0.1%) • Final peak current 3.4 kA (stable to 12%) • Transverse emittance 1.2 mm (stable to 5%) • Final energy spread 10-4 (stable to 10%) • Bunch arrival time (stable to 150 fs) Critical LCLS Accelerator Parameters P. Emma (stability specifications quoted as rms) John Byrd, BIW2006

  9. Electron Bunch Compression d DE/E d d under-compression szi ‘chirp’ z z z sz sdi Dz = R56d V = V0sin(kz) P. Emma RF Accelerating Voltage Path-Length Energy- Dependent Beamline John Byrd, BIW2006

  10. Compression Stability d RF phase jitter becomes bunch length jitter… Compression factor: Df d z P. Emma John Byrd, BIW2006

  11. LCLS Machine Schematic 250 MeV z  0.19 mm   1.6 % 4.30 GeV z  0.022 mm   0.71 % 13.6 GeV z  0.022 mm   0.01 % 6 MeV z  0.83 mm   0.05 % 135 MeV z  0.83 mm   0.10 % Linac-X L =0.6 m rf= -160 rf gun Linac-1 L 9 m rf  -25° Linac-2 L 330 m rf  -41° Linac-3 L 550 m rf  0° 23-m Linac-0 L =6 m undulator L =130 m 21-1b 21-1d 21-3b 24-6d 25-1a 30-8c X ...existing linac BC1 L 6 m R56 -39 mm BC2 L 22 m R56 -25 mm DL1 L 12 m R56 0 LTU L =275 m R56  0 1 X-klys. 3 klystrons 1 klystron 26 klystrons 45 klystrons research yard SLAC linac tunnel P. Emma John Byrd, BIW2006

  12. Phase, Amplitude, and Charge Sensitivities John Byrd, BIW2006 P. Emma

  13. Optical metrology • A revolution is going on in optical metrology due to several coincident factors: • development of femtosecond comb lasers • breakthroughs in nonlinear optics • wide availability of optical components 2005 Nobel Prize in Physics awarded to John L. Hall and Theodor W. Hänsch "for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique" This technology is nearly ready for applications in precision synchronization in accelerators John Byrd, BIW2006

  14. A brief history of timekeeping • 1949 Ramsey's separated oscillatory field technique • 1955 First caesium atomic clock • 1960 Hydrogen maser • 1967 Redefinition of the second in terms of caesium • 1975 Proposals for laser cooling of atoms and ions • 1978 Laser cooling of trapped ions • 1980s GPS satellite navigation introduced • 1985 Laser cooling of atoms • 1993 First caesium-fountain clock • 1999 First optical-frequency measurement with femtosecond combs • 2001 Concept of an optical clock demonstrated John Byrd, BIW2006

  15. Mode-locked Lasers Locking the phases of the laser frequencies yields an ultrashort pulse. John Byrd, BIW2006

  16. Locking modes Intensities John Byrd, BIW2006

  17. Femtosecond combs diode detection John Byrd, BIW2006

  18. Example:Ti:Sapph MLL Repetition rate given by round trip travel time in cavity. Modulated by piezo adjustment of cavity mirror. Passive mode locking achieved by properties of nonlinear crystal Modern commercial designs include dispersion compensation in optics Comb spectrum allows direct link of microwave frequencies to optical frequencies John Byrd, BIW2006

  19. Self-referencing stabilizer CEO frequency can be directly measured with an octave spanning spectrum and stabilized in a feedback loop. This allows direct comparision (and or locking) with optical frequency standards. John Byrd, BIW2006

  20. Master Oscillator: Passively Mode-Locked Er-fiber lasers Ippen et al. Design: Opt. Lett. 18, 1080-1082 (1993) • diode pumped • sub-100 fs to ps pulse duration • 1550 nm (telecom) wavelength for fiber-optic component availability • repetition rate 30-100 MHz John Byrd, BIW2006

  21. Master Oscillator Timing Jitter Agilent Signal Analyzer 5052a f0=1 GHz • Scaled to 1 GHz • Limited by photo • detection • Theoretical limit ~1 fs Very stable operation over weeks ! John Byrd, BIW2006

  22. Why fiber transmission? • Fiber offers THz bandwidth, immunity from electromagnetic interference, immunity from ground loops and very low attenuation • However, the phase and group delay of single-mode glass fiber depend on its environment • temperature dependence • acoustical dependence • dependence on mechanical motion • dependence on polarization effects • These are corrected by reflecting a signal from the far end of the fiber, compare to a reference, and correct fiber phase length. • Two approaches: CW and pulsed John Byrd, BIW2006

  23. Stabilized fiber link Frequency-offset Optical Interferometry Principle: Heterodyning preserves phase relationships 1 degree at optical = 1 degree RF 1 degree at 110 MHz = 0.014 fsec at optical Gain 105 leverage over RF-based systems in phase sensitivity Technique used at ALMA 64 dishes over 25 km footprint, 37 fsec requirement John Byrd, BIW2006

  24. Detailed configuration Control channel Monitor channel • Phase errors,drifts in 110 MHz RF circuits insignificant • Reflections along fiber don't contribute: only frequency-shifted reflection beats with outgoing laser line to produce error signal • Low power cw signals, linear system, commodity hardware John Byrd, BIW2006

  25. Drift Results Compare phase at the end of fiber with reference to establish stability. Measure slow drift (<1 Hz) of fiber under laboratory conditions Compensation for several environmental effects results in a linear drift of 0.13 fsec/hour and a residual temperature drift of 1 fsec/deg C. Lab AC cycle Environmental factors • Temperature: 0.5-1 fsec/deg C • Atmospheric pressure: none found • Humidity: significant correlation • Laser Wavelength Stabilizer: none • Human activity: femtosecond noise in the data John Byrd, BIW2006

  26. Laser Standard Clock • Laser provides absolute standard for length of transmission line • Narrow-line (2 kHz) Koheras Laser (coherence length > 25 km) • For single fringe stabilization over 150 m, laser frequency must be stabilized to better than 1:108 • Use frequency lock with acetylene cell Frequency lock loop on acetylene (C2H2) 1530.3714 nm absorption line John Byrd, BIW2006

  27. Thermal control of critical components Peltier Coolers Baseplate Aluminum Chamber Some components Complete Insulating Jacket John Byrd, BIW2006

  28. RF signal transmission RF (S-band) may be modulated directly onto the optical carrier with a zero-chirp Mach-Zehnder modulator and recovered directly at the far end of the fiber. Any modulation pattern is acceptable. Critical to minimize added phase noise at demodulation. Modulation of CW carrier has signal S/N advantages over pulsed modulation. John Byrd, BIW2006

  29. An advantage of AM pulse train spectrum RF out optical in f 150ps t 100MHz T two methods 3GHz 1/f • Diode has an average current limit before saturation • At saturation, high frequencies drop in power • Diode bandwidth is chosen to be equal to RF frequency, and pulse width is 1/bandwidth • For t=150ps, T=10ns and f=3GHz, AM has 15db more power in the transmitted frequency John Byrd, BIW2006

  30. Group and Phase Velocity Correction Interferometric technique stabilizes phase delay at a single frequency . At a fixed T, simple a 1.6% correction for 1 km cable. Possible fixes: measure group velocity from the differential phase velocity at two frequencies. Correction can be applied dynamically or via a feedforward scheme. John Byrd, BIW2006

  31. Pulsed distribution system Low-noise microwave oscillator low-bandwidth lock 1 4 3 fiber couplers Optical to RF sync module Master laser oscillator 2 stabilized fibers Low jitter modelocked laser Optical to RF sync module 5 low-level RF Optical to optical sync module Laser Demonstration of complete link with ~ 50fs jitter (1-4) and ~ 20fs jitter from (2-4) John Byrd, BIW2006

  32. Stabilized Fiber Links: pulsed PZT-based fiber stretcher SMF link 500 km 50:50 coupler Master Oscillator isolator OC coarse RF-lock <50 fs fine cross- correlator ultimately < 1 fs Optical cross correlator enables sub-femtosecond length stabilization, if necessary John Byrd, BIW2006

  33. RF-Transmission over Stabilized Fiber Link • Passive temperature stabilization of 500 m • RF feedback for fiber link • EDFL locked to 2.856 GHz Bates master oscillator John Byrd, BIW2006

  34. RF-Synchronization Module John Byrd, BIW2006

  35. Summary so far RF: Jitter: Dtrms[10Hz,1MHz] Drift: Dtp-p[>8hours] Optical: Jitter: Dtrms[10Hz,1MHz] Comparison of RF phase over independent transmission lines now in progress for CW and pulsed approaches John Byrd, BIW2006

  36. RF transmission design • RF transmission has looser requirements on jitter • LLRF system can integrate between shots to reduce high frequency jitter John Byrd, BIW2006

  37. Synching mode-locked lasers Trep Trep slave master n*frep n*frep BP BP ML Laser m*frep+fceo n*frep Df H Detection and bandpass filter carrier/envelope offset repetition rate 0 frequency Shelton (14GHz) Bartels (456THz) Shelton et al, O.L. 27, 312 (2002) Bartels et al, O.L. 28, 663 (2003) present work (5THz) ML Laser John Byrd, BIW2006

  38. Idealized example 80 th harmonic Achieved 4.3 fsec jitter over 160 Hz BW for 10 seconds. John Byrd, BIW2006

  39. Two-frequency synch scheme m-n2s m- s s s m- s) - m- s) = 0 m- m) - s- s) = 0 m m master clock frequency transmitted frequencies synched laser } } 5THz 5THz Lock two frequencies within the frequency comb separated by 5 THz. For a 1degree error in phase detection, temporal error is <0.6 fsec John Byrd, BIW2006

  40. Two-frequency synch layout frep CW 2 frep interferometer CW 1 master clock split interferometer  mux stabilized fiber + interferometer synched laser demux interferometer John Byrd, BIW2006

  41. Direct seeding laser systems Amplification to high energy at low repetition frequency a) All fiber: ~1 J @ 1550 nm b) Grating compressor: ~10 J @ 1550 nm c) OPCPA: ~100 J – 1mJ @ 1550 nm pump coupler air-core photonic crystal fiber (< 1 uJ) input pulse a) 1 uJ, ~100 fs Er-doped fiber stretcher fiber b) 10 uJ, ~100 fs 975 nm pump diode bulk grating compressor (high energy) c) 100mJ-1mJ, ~20 ps OPCPA 1 um, 1mJ, 20ps Regen. Ampl. John Byrd, BIW2006 PPLN

  42. Conceptual system design • Laser synch for any popular modelocked laser • RF transmission via modulated CW, and interferometric line stabilization • RF receiver is integrated with low level RF electronics design John Byrd, BIW2006

  43. Details, details… • Actual performance depends on many technical details: • thermal and acoustic environment of cable layout • design of feedback loops • gain limited by system poles (i.e. resonances in the system) • multiple audio BW feedback loops suggests flexible digital platform • feedback must deal with drift and jitter (separate loops?) • AM/PM conversion in photodiode downconversion John Byrd, BIW2006

  44. Example: Menlo EDFL plate piezo mirror motorized stage old new amplitude • Piezo driven cavity end mirror controls reprate • Was a 10mm long piezo on a light Al plate • Replaced with 2mm piezo on steel plate phase John Byrd, BIW2006

  45. AM-to-PM conversion in a photodiode var. atten. CW laser modulator var. delay EDFA 1.1Vpp power meter network analyser • Measured at 3GHz using a network analyser • Modulation was 100% AM on 1530nm CW carrier • From 1mW to 0.5mW on a 15GHz photodiode, phase shift was 87fs/mW • In this test, phase noise from 10Hz to 3kHz was 92fs p-p. The noise was averaged over 100ms to determine AM/PM shift • CW power stability through 100m fiber <10% p-p variation over 16h (low polarization dependent loss) • This variation results in 8.7fs p-p • Conclusion: for RF transmission, AM-to-PM is not an issue John Byrd, BIW2006

  46. Measurement techniques • How do we characterize the achieved synchronization on the electron or photon beam? • Use “classic” approaches: • time to angle or position • time to frequency • time to amplitude • Deflecting cavity • Electro-optic sampling • Streak camera • Laser tagging • X-ray/laser cross correlator John Byrd, BIW2006

  47. Time to Position RF ‘streak’ 2.44 m V(t) sy e- sz D 90° bc bp Electron bunch measurements using a transverse RF deflector P. Emma S-band V0 20 MV sz 50 mm, E 28 GeV John Byrd, BIW2006

  48. EO Sampling Electro-Optic Sampling encodes electron pulse shape on a laser pulse A. Cavalieri EO Crystal John Byrd, BIW2006

  49. time polarizing beamsplitter integrated intensity time; space time integrated intensity John Byrd, BIW2006

  50. EOS data from SPPS A. Cavalieri Timing Jitter Data (20 Successive Shots) Single-Shot w/ high frequency filtering iCCD counts shot time (ps) color representation time (ps) John Byrd, BIW2006

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