1. FLS 2010 Workshop March 4 th, 2010 Recent Progress in Pulsed Optical Synchronization Systems Franz X. Kärtner Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology Cambridge, MA, USA

2. 2 Acknowledgement Students Hyunil Byun Jonathan Cox Anatoly Khilo Michelle Sander Postdocs: J. Kim (KAIST, Korea) Amir Nejadmalayeri Noah Chang Colleagues and Visitors: E. Ippen, J. G. Fujimoto, L. Kolodziejski, F. Wong and M. Perrott DESY: F. Loehl, F. Ludwig, A. Winter

3. 3 Outline  Synchronization System Layout for Seeded FEL  Advantages of a Pulsed Optical Distribution System  Low Noise Optical Master Oscillators  Timing Distribution Over Stabilized Fiber Links  Optical-to-Optical Synchronization  RF-Extraction and Locking to Microwave References  Prospects for sub-fs timing distribution

4. 4 Seeded X-ray Free Electron Laser Long-term sub-10 fs synchronization over entire facility is required.

5. 5 Seeded X-ray Free Electron Laser J. Kim et al, FEL 2004.

6. 6 Why Optical Pulses (Mode-locked Lasers)?  Real marker in time and RF domain, every harmonic can be extracted at the end station.  Suppress Brillouin scattering and undesired reflections.  Optical cross correlation can be used for link stabilization or for optical-to- optical synchronization of other lasers.  Pulses can be directly used to seed amplifiers, EO-sampling, ….  Group delay is directly stabilized, not optical phase delay.  After power failure system can auto-calibrate. frequency …... fRfR 2.f R N.f R T R = 1/f R time

7. 7 200 MHz Soliton Er-fiber Laser 200 MHz fundamentally mode locked soliton fiber laser 167 fs pulses 40mW output power J. Chen et al, Opt. Lett. 32, 1566 (2007). Similar lasers are now commercially available! K. Tamura et al. Opt. Lett. 18, 1080 (1993).

8. 8 Semiconductor Saturable Absorber Modelocked 100MHz - 1GHz Er-fiber Lasers Compact, long-term stable femtosecond laser source at GHz reprate SBR burning problem solved by SMF buffer and pump-reflective coating on SBR 380mW pump  27.4mW output Optical spectrum FWHM: 17.5nm Pulse width: 187fs Repetition rate: 967.4MHz Intensity noise: 0.014% [10Hz,10MHz] Optical spectrum RF spectrum Autocorrelation 5”x4”x1.5” pump out Long term output power with 270mW pump

9. 9 Sensitive Time Delay Measurements by Balanced Optical Cross Correlation

10. 10 Reflect fundamental Transmit SHG Transmit fundamental Reflect SHG Type-II phase-matched PPKTP crystal Single-Crystal Balanced Cross-Correlator J. Kim et al., Opt. Lett. 32, 1044 (2007)

11. 11 In comparison: Typical microwave mixer Slope ~1  V/fs @ 10GHz Single-Crystal Balanced Cross-Correlator 80 pJ, 200 fs 1550nm input pulses at 200 MHz rep. rate

12. ML Fiber Laser Timing Jitter Measurement Modelocked Laser 1 Modelocked Laser 2 HWP PBS Single crystal balanced cross- correlator Oscilloscope RF-pectrum analyzer Loop filter J. Kim, et al., Opt. Lett. 32, 3519 (2007). 12

13. 13 Ultralow timing jitter (<1 fs) in the high frequency range [100 kHz, 10 MHz] J. Kim, et al., Opt. Lett. 32, 3519 (2007). Timing Jitter in 200 MHz Fiber Lasers Noise Floor Integrated Jitter Measured Jitter Density Theory

14. 14 Timing - Stabilized Fiber Links

15. 15 PZT-based fiber stretcher Mode-locked laser Fiber link ~ several hundreds meters to a few kilometers isolator Timing Comparison Faraday rotating mirror Cancel fiber length fluctuations slower than the pulse travel time (2nL/c). 1 km fiber: travel time = 10 μs  ~100 kHz BW Timing-Stabilized Fiber Links

16. 2 Link Test System J. Cox et al. CLEO 2008.

17. Experimental Apparatus ~300 m optical fiber Piezo stretcher 200 MHz Laser EDFA Invar Board Motor In-Loop PPKTP Out-of-Loop PPKTP

18. Balanced Cross-Correlation Signals Link 1 Link 2 Out of Loop

19. Results – Timing Jitter 360 as (rms) timing jitter from 1 Hz to 100 kHz 3.3 fs (rms) timing jitter from 35 μHz to 100 kHz  Out-off loop jitter limited by quantum noise

20. 20 Optical-to-Optical Synchronization

21. 21 Optical-to-Optical Synchronization

22. 22 Ti:sapphire Laser + Cr:Forsterite Laser Ti:sapphire Cr:forsterite Spanning over 1.5 octaves 5fs 30 fs

23. 23 Sub-femtosecond Residual Timing Jitter J. Kim et al, EPAC 2006. Long-term drift-free sub-fs timing synchronization over 12 hours Balanced optical cross-correlator based on GDD (T. Schibli et al, OL 28, 947 (2003))

24. 24 Optical-to-RF Conversion or Optical-to-RF Locking necessary for Locking OMO to RMO

25. 25 Direct Extraction of RF from Pulse Train AM-to-PM conversion, Temperature drift of photodetectors and mixers … RF frequency …... fRfR 2f R nf R T R = 1/f R time Photodetector t T R /n E. Ivanov et al, IEEE UFFC 52, 1068 (2005). IEEE UFFC 54, 736 (2007).

26. 26 Direct Extraction of RF from Pulse Train RF frequency …... fRfR 2f R nf R T R = 1/f R time Photodetector t T R /n 55 fs drift in 100 sec A. Bartels et al, OL 30, 667 (2005). More in: B. Lorbeer et al, PAC 2007.

27. Microwave Signal 27 Balanced Optical-Microwave Phase Detector (BOM-PD) J. Kim et al., Opt. Lett. 31, 3659 (2006). Electro-optic sampling of microwave signal with optical pulse train

28. 28 Optoelectronic Phase-Locked Loop (PLL) Regeneration of a high-power, low-jitter and drift-free microwave signal whose phase is locked to the optical pulse train. Balanced Optical- Microwave Phase Detector (BOM-PD) Regenerated Microwave Signal Output Tight locking of modelocked laser to microwave reference Balanced Optical- Microwave Phase Detector (BOM-PD) Stable Pulse Train Output Modelocked Laser

29. 29 Testing Stability of BOM-PDs BOM-PD 1: timing synchronization BOM-PD 2: out-of-loop timing characterization

30. RMS timing jitter integrated in 27 μHz – 1 MHz: 6.8 fs 30 Long-Term Stability: 6.8 fs drift over 10 hours J. Kim et al, Nature Photonics 2, 733 (2008).

31. 31 Delay Locked Looop: 2.9 fs drift over 8 hours J. Kim et al, submitted to CLEO 2010. RMS timing jitter integrated in 0.1 Hz – 1MHz, 2.4 fs

32. 32 1 GHz diode pumped CrLiSAF Laser: Modelocked Jointly with Jim Fujimoto

33. Prospects for Attosecond Timing Distribution (100 MHz Cr:LiSAF Laser, SSB scaled to 1GHz) U. Demirbas, submitted to CLEO 2010 33

34. 34 Conclusions  Long term stable (10h) sub-10 fs timing distribution system is completed.  True long term stability (forever): Implement Polarization Control  Master Oscillators commercially available + amplifier >400mW of output power > 10 links)  300 m Fiber Links, over 10h < 5 fs ( < 1fs possible)  Optical-to-Optical Synchronization, over 12h < 1fs  Optical-to-Microwave Synch., over 10h < 7fs ( < 1fs possible)  Solid-State Lasers show timing jitter [1kHz – 10 MHz] < 200as (<50as)  Continued development of this technology seems to enable < 100as long term stable timing distribution.