RF-based Synchronization of the Seed and Pump-Probe Lasers to the Optical Synchronization System at FLASH

RF-based Synchronization of the Seed and Pump-Probe Lasers to the Optical Synchronization System at FLASH Introduction to the otical synchronization system and concept of RF generation for locking of Ti:Sapphire oscillators M. Felber, M. K. Bock, P. Gessler, K. E. Hacker, T. Lamb, F. Ludwig, H. Schlarb, B. Schmidt Deutsches Elektronen Synchrotron - DESY, Hamburg, Germany J. Breunlin, S. Schulz, L. Wissmann Hamburg University, Hamburg, Germany FEL 2010 Conference Malmö, Sweden August 26th, 2010

Motivation Seed Laser The temporal overlap is mandatory for the seeding process Right now there is no high resolution monitor for the synchronization (streak camera has ~1ps resolution) Electron bunch duration with 3rd harmonic module is ~120 fs, HHG pulse ~40 fs Requiring synchronization better than 40 fs rms Pump-Probe Laser The arrival time of the FEL pulse is given by the electron bunch Pump-probe experiments can make use of an electro-optic arrival time monitor to sort the data in time, after the experiment Resolution is in the order of 80 fs but users request more precise synchronization for some experiments If the timing can be set on a10 fs scale, entire movies of a process can be recorded within one burst, without the need of sorting the data

Roadmap for Precise Synchronization Establish a machine reference that is cabable of providing a point-to-point synchronization better than 10 fs Pulsed optical synchronization system Make sure, the beam arrival time is synchronized to the timig reference Beam-based feedback Synchronize external lasers to the same reference First step: RF-based Finally purely optical

General Layout of an Optical Synchronization System The reference timing information is encoded in the precise repetition rate of an optical pulse train RF Master Oscillator RF to Optical Master Laser Oscillator Laser Source locked to machine reference Splitting ( 16 Outputs) Distribution and active length stabilization Fiber Links ( 300 m) Optical Lock Injector Laser Optical to RF Low Level RF Each RF Station.. Optical to RF Low Level RF Beam Diagnostic BAM / EBPM Beam Diagnostic BAM / EBPM Optical Lock / RF Lock Seed Laser Optical Lock / RF Lock Probe Laser Aimed point-to point synchronization: 10 fs GUN LINAC Undulator Pump-Probe Exp.

Optical Synchronization System at FLASH Currenty installed and planned infrastructure Courtesy M.K. Bock

Optical Synchronization System at FLASH Currenty installed and planned infrastructure Courtesy M.K. Bock o Synchronization Hutch Two redundant Master Laser Oscillators (MLOs) locked to the machine reference Free-space laser beam splitting to up to 16 ports Erbium-doped fiber amplifiers at each port Up to 16 Link stabilization units, each supplying one fiber link o Link end-stations Bunch Arrival-time Monitors (BAMs)

Master Laser Oscillator For many years self-built fiber lasers based on self phase modulation have been used 1550 nm telecommunication wavelength repetition rate of 216.66 MHz (1.3 GHz /6) Important Issues Original design: J. Chen et. al., Opt. Lett. 32, 1566-1568 (2007) Recently a commercial SESAM-based laser was installed and tested o Phase Noise o Frequency stability tuning range o Piezo stroke & bandwidth (resonance) o Amplitude Noise o Modulation input range & bandwidth o Output Power o Pulse width o Spectrum (peak and bandwidth) o Robustness & reproducability o Lifetime o Formfactor S. Schulz et al, this conferencethpa05

Link Stabilization Unit Industrialized design in operation for about one year Improved version being manufactured right now Courtesy F. Loehl Courtesy M.K. Bock

RF generation from optical pulse train Direct Conversion (gating lower frequencies) + Drift: 10.7 fs over >15 h @ 1.3 GHz (M. Felber, PAC09, TH6REP088) + Jitter: 3.3 fs [1kHz,10MHz] @ 3 GHz (S. Hunziker, DIPAC09, TUPB43) + small and robust + 5-10 mw P opt sufficient + relatively cheap (<2k ) Small output power vs. amplifier drift Am-to-PM conversion: 1-4 ps/mw Temperature dependency ~350 fs/ C Power [dbm] 0-10 -20-30 -40-50 -60-70 ER80-8/125 70cm here: small bandwidth PD ER110-4/125 79cm SMF -80 0 1 2 3 4 5 6 7 Freq [GHz]

Concept for RF-based synchronzation of lasers WDM gain fiber WDM FRM 90/10 coupler 99/01 coupler 50/50 future use for optical cross-correlator Pump-Probe oscillator frequency: 108 MHz Optical Link End Photo Diode >10 GHz Splitter 1 BPF 108 MHz BPF 1.3 GHz Pump Laser Diode Amplifier Amplifier Directional Coupler In Cpl Out Iso Ti:Sa 108 MHz Monitor Directional Coupler LDD Phase Detector Directional Coupler Out Iso LPF 1.9 MHz Mixer In Cpl Reference 108 MHz Monitor Phase Shifter Directional Coupler LPF 1.9 MHz DC LNA Optical Power Monitor Amplifier BPF 108 MHz Photo Diode (slow) Frequency Divider :2 Amplifier BPF 216 MHz BPF 1.3 GHz 1 Splitter Photo Diode >10 GHz HHG laser oscillator frequency: 81 MHz, likely to be upgraded to 108 MHz Three frequencies are generated from referenceand Ti:Sa pulse trains S 2 3 BPF 9.1 GHz Amplifier In Cpl In Cpl Out Iso Ti:Sa 1.3 GHz Monitor Directional Coupler Out Iso Out Iso LPF 1.9 MHz Mixer In Cpl Directional Phase Coupler Shifter Amplifier Out Iso Reference 1.3 GHz Monitor In Cpl BPF 9.1 GHz 2 3 S First adjustment with 1.3 GHz IQ modulator, then set other phases Ti:Sa Oscillator Piezo Driver RF locking components DAC Digital Controller ADC ADC ADC Ti:Sa 9.1 GHz Monitor LPF 1.9 MHz Reference 9.1 GHz Monitor LO I Q RF Vector Modulator 1.3 GHz RF Reference Lock from coarse to fine in steps at 108 MHz, 1.3 GHz, and 9.1 GHz

Phase Noise and Timing Jitter of the Seed Laser at 1.3 GHz

First measurements of refernce signal in HHG laboratory Amplitude noise of optical pulse train Monitor Output Power Spectral Density [dbv/hz] Voltage Noise of Optical Power Monitor at Link End -60 link in 'open loop' mode -80-100 -120-140 -160-180 1k 10k 100k 1M 10M Relative Intensity Noise Frequency of Optical [Hz] Pulse Train at Link End 2 0.73 1.75 0.02 0.08 0.07 RIN Integrated [%] 1.5 1.25 1 0.75 0.5 link in 'open loop' mode 0.25 0 1k 10k 100k 1M 10M Frequency [Hz]

First measurements of refernce signal in HHG laboratory Amplitude noise of optical pulse train Monitor Output Power Spectral Density [dbv/hz] Voltage Noise of Optical Power Monitor at Link End -60 link in 'open loop' mode -80 OXC signal found, feedback off -100-120 -140-160 -180 1k 10k 100k 1M 10M Relative Intensity Noise Frequency of Optical [Hz] Pulse Train at Link End 2 1.75 0.73 0.02 0.08 0.07 1.12 0.49 0.08 0.06 RIN Integrated [%] 1.5 1.25 1 0.75 0.5 link in 'open loop' mode OXC signal found, feedback off 0.25 0 1k 10k 100k 1M 10M Frequency [Hz]

First measurements of refernce signal in HHG laboratory Amplitude noise of optical pulse train Monitor Output Power Spectral Density [dbv/hz] RIN Integrated [%] -60-80 -100-120 -140-160 Voltage Noise of Optical Power Monitor at Link End link in 'open loop' mode OXC signal found, feedback off OXC signal found, feedback on -180 1k 10k 100k 1M 10M Relative Intensity Noise Frequency of Optical [Hz] Pulse Train at Link End 2 1.75 0.73 0.02 0.08 0.07 1.12 0.49 0.08 0.06 1.5 0.69 0.45 0.57 0.07 link in 'open loop' mode 1.25 OXC signal found, feedback off 1 OXC signal found, feedback on 0.75 0.5 0.25 0 1k 10k 100k 1M 10M Frequency [Hz]

First measurements of refernce signal in HHG laboratory Phase noise of electrical signal after photo diode SSB Phase Noise [dbc/hz] -60-80 -100-120 -140-160 1.3 GHz Phase Noise at Link End Link in 'open loop' mode Locking bandwidth of MLO can further be reduced Integrated Timing Jitter [fs] -180 10 100 1k 10k 100k 1M 10M 1.3 GHz Phase Frequency Noise [Hz] at Link End 300 50.87 49.63 27.91 4.91 3.33 9.35 14.59 250 200 Link in 'open loop' mode 150 100 50 0 10 100 1k 10k 100k 1M 10M Frequency [Hz]

First measurements of refernce signal in HHG laboratory Conversion of the amplitude noise to phase noise in photo diodes SSB Phase Noise [dbc/hz] Integrated Timing Jitter [fs] -60-80 -100-120 -140-160 1.3 GHz Phase Noise at Link End Link in 'open loop' mode Link stabilization fb on DRO free running -180 10 100 1k 10k 100k 1M 10M 1.3 GHz Phase Frequency Noise [Hz] at Link End 300 50.87 49.63 27.91 4.91 3.33 9.35 14.59 51.64 51.46 32.53 52.08 59.82 9.76 13.92 250 534 52.88 5.2 2.29 2.04 1.66 3.58 200 150 100 50 Link in 'open loop' mode Link stabilization fb on DRO free running Locking bandwidth of MLO can further be reduced The amplitude noise of the electrical pulse train degrades the phase noise of the stabilized link by about 75 fs [1 khz 10 MHz] A low noise DRO provides the possibility to filter out this noise 0 10 100 1k 10k 100k 1M 10M Frequency [Hz]

First measurements of refernce signal in HHG laboratory Conversion of the amplitude noise to phase noise in photo diodes SSB Phase Noise [dbc/hz] Integrated Timing Jitter [fs] -60-80 -100-120 -140-160 1.3 GHz Phase Noise at Link End Link in 'open loop' mode Link stabilization fb on DRO free running DRO locked to stbilized link -180 10 100 1k 10k 100k 1M 10M 1.3 GHz Phase Frequency Noise [Hz] at Link End 300 50.87 49.63 27.91 4.91 3.33 9.35 14.59 51.64 51.46 32.53 52.08 59.82 9.76 13.92 250 534 52.88 5.2 2.29 2.04 1.66 3.58 56.28 62.78 42.83 2.68 1.94 0.95 2.8 200 Link in 'open loop' mode Link stabilization fb on 150 DRO free running 100 50 DRO locked to stbilized link 0 10 100 1k 10k 100k 1M 10M Frequency [Hz] Locking bandwidth of MLO can further be reduced The amplitude noise of the electrical pulse train degrades the phase noise of the stabilized link by about 75 fs [1 khz 10 MHz] A low noise DRO provides the possibility to filter out this noise The locked DRO follows the reference and provides low phase noise at higher offset frequencies

Conclusion and Outlook The optical synchronization system at FLASH is continuously improved Optical reference link to HHG laser is almost finished RF components are investigated and ready for assembly Link to pump-probe laser will be installed before end of this year Two-color balanced optical cross-correlator is already installed and tested at the injector laser It will extend the RF-based synchronization of the seed and pump-probe lasers until summer 2011 Thank you!