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Femtosecond optical synchronization systems for XFELs

Femtosecond optical synchronization systems for XFELs. A. Winter, F. Ö. Ilday, J. Chen, F. Kärtner, H. Schlarb, F. Ludwig, P. Schmüser DESY Hamburg, MIT 19.9.2005 LLRF Workshop, CERN Oct. 2005. Overview. Requirements Optical synchronization systems Conclusion and Outlook.

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Femtosecond optical synchronization systems for XFELs

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  1. Femtosecond optical synchronization systems for XFELs A. Winter, F. Ö. Ilday, J. Chen, F. Kärtner, H. Schlarb, F. Ludwig, P. Schmüser DESY Hamburg, MIT 19.9.2005 LLRF Workshop, CERN Oct. 2005

  2. Overview • Requirements • Optical synchronization systems • Conclusion and Outlook • Optical master oscillator • Optical timing distribution • RF reconversion • Synchronization of other laser systems • Test in accelerator environment

  3. Requirements • Provide stability ~x-ray pulse width (down to ~10 fs) • Amplitude and phase stability in cavities (10-4, 0.01deg) in most critical sections • Provide ultra-stable reference to select locations with femtosecond stability • Synchronize probe systems to reference with ~10 fs stability • Precise reference frequency generation and distribution system required • “femtosecond” synchronization means systems follow reference with low added timing jitter and have intrinsic high-frequency timing jitter both, ~fs

  4. Synchronization System Layout remote locations fiber couplers RF-optical sync module Master Laser Oscillator stabilized fibers RF-optical sync module low-bandwidth lock low-noise microwave oscillator low-level RF Optical to optical sync module Laser • A master mode-locked laser producing a very stable pulse train • The master laser is locked to a microwave oscillator for long-term stability • length stabilized fiber links transport the pulses to remote locations • other lasers can be linked or RF can be generated locally

  5. Output coupler Timing stabilized fiber links PZT-based fiber stretcher SMF link 1 - 5 km 50:50 coupler Master Oscillator isolator “coarse” RF-lock Faraday Mirror <50 fs fine cross- correlator ultimately < 1 fs • transmit ~500 fs pulses via dispersion compensated fiber links • assuming no fluctuations faster than T=2nL/c. • L = 1 km, n = 1.5 => T=10 µs, fmax = 100 kHz

  6. TR/n t … .. f f (n+1)fR nfR fR 2fR nfR Direct Detection to Extract RF from the Pulse Train TR = 1/fR BPF LNA Photodiode Optical Pulse Train (time domain)

  7. Timing jitter measurements • signal converted to electronic domain by photodector • harmonic (~1GHz) or repetition rate filtered • phase noise measured with Signal Source Analyzer

  8. Amplitude noise of various fiber laser 0.03% rms for Er-doped fiber laser (EDFL) 0.04% rms for Yb-doped fiber laser (YDFL) 0.1% rms for Ti:Sapphire Some of the quietest lasers around (3x-10x better than typical TiSa)

  9. Timing Jitter of fiber lasers • All measurements scaled to 1 GHz • Noise floor limited by photodetection • Theoretical noise limit ~1 fs

  10. Direct seeding laser systems Direct seeding a Ti:sapphire amplifier after pulse shaping with self-compression to ~100 fs (~1 nJ @ 1550 nm & ~0.3 nJ @ 775 nm) pump coupler Er-doped fiber single-mode fiber SHG input pulse (~500 fs) PPLN Nonlinear pulse shaping (amplifier & compression) ~100 fs @ 775 nm to Ti:Sapphire amplifier 975 nm pump diode

  11. Direct seeding laser systems Direct seeding a Ti:sapphire amplifier after pulse shaping with self-compression to ~100 fs (~1 nJ @ 1550 nm & ~0.3 nJ @ 775 nm) pump coupler Er-doped fiber single-mode fiber SHG input pulse (~500 fs) PPLN Nonlinear pulse shaping (amplifier & compression) ~100 fs @ 775 nm to Ti:Sapphire amplifier 975 nm pump diode 975 nm pump diode Amplification to high energy with nonlinear pulse shaping (~1 J @ 1550 nm, low repetition frequency) pump coupler input pulse Er-doped fiber bulk grating compressor (high energy) 10 uJ, ~100 fs pulses at 1550 nm stretcher fiber 975 nm pump diode OR air-core photonic crystal fiber (< 1 uJ)

  12. Very little amplitude noise added 0.033% 0.018%

  13. Very little phase noise added 29 fs 27 fs

  14. System Test in Accelerator environment • Can lab results be transferred to real environment ? • Test done at MIT Bates laboratory: • Locked EDFL to Bates master oscillator • Transmitted pulses through 1km total of laid out fiber • Close loop on fiber length feedback ~ 500 meters

  15. Setup • 1km return of fiber • Passive temperature stabilization of 500 m • RF feedback for fiber link • EDFL locked to 2.856 GHz Bates master oscillator

  16. Results • Fiber link extremely stable without closing loop (60 fs for 0.1 Hz…5 kHz) • Closing feedback loop reduces noise (12 fs for 0.1 Hz .. 5kHz) • No significant noise added at higher frequencies • Can be stabilized to eventually sub-fs level using optical cross-correlation.

  17. Transmitted frequency • Added jitter due to phase lock: ~30 fs (10 Hz..2 kHz) • Total jitter added (link, phase lock, increase at high frequency) < 50 fs • Overall improvement 272 fs vs. 178 fs (10Hz .. 20 MHz) • Spurs are technical noise!

  18. Conclusion and Outlook • Successful demonstration of complete system in accelerator environment over 1 km fiber link • sub-20 fs jitter added during transmission (0.1Hz .. 20MHz) • 178 fs absolute phase noise limited by MO (10Hz .. 20 MHz) • Most timing jitter is technical noise which can be eliminated. • Stable, uninterrupted operation over weeks. • Following a few years of development: < 10 fs

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