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
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 2
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 3
Seeded X-ray Free Electron Laser Long-term sub-10 fs synchronization over entire facility is required. 4
Seeded X-ray Free Electron Laser 5 J. Kim et al, FEL 2004.
Why Optical Pulses (Mode-locked Lasers)? TR = 1/fR... time fr 2.fR N.fR frequency 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-tooptical 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. 6
200 MHz Soliton Er-fiber Laser 10 cm SMF λ/4 ISO λ/4 10 cm SMF collimator PBS λ/2 collimator 980 nm Pump 50 cm Er doped fiber 10 cm SMF WDM 10 cm SMF 200 MHz fundamentally mode locked soliton fiber laser 167 fs pulses 40mW output power K. Tamura et al. Opt. Lett. 18, 1080 (1993). J. Chen et al, Opt. Lett. 32, 1566 (2007). Similar lasers are now commercially available! 7
Semiconductor Saturable Absorber Modelocked 100MHz - 1GHz Er-fiber Lasers pump (977nm) output WDM EDF Output coupler (coated ferrule) SBR SMF laser cavity 5 x4 x1.5 pump out packaged in a box intensity (a.u.) 1 0.8 0.6 0.4 0.2 0 Optical spectrum sech 2 fit measured 17.5nm 1540 1560 1580 1600 wavelength (nm) 8 RF spectrum 0-20 -40-60 -80 Autocorrelation -100 967.1 967.7 frequency (MHz) 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] IAC (a.u.) 6 4 2 1.54x187fs Long term output power with 270mW pump 0-500 0 500 time delay (fs) 8
Sensitive Time Delay Measurements by Balanced Optical Cross Correlation 9
Single-Crystal Balanced Cross-Correlator Type-II phase-matched PPKTP crystal Transmit fundamental Reflect SHG Reflect fundamental Transmit SHG 10 J. Kim et al., Opt. Lett. 32, 1044 (2007)
Single-Crystal Balanced Cross-Correlator 80 pj, 200 fs 1550nm input pulses at 200 MHz rep. rate In comparison: Typical microwave mixer Slope ~1 µv/fs @ 10GHz 11
ML Fiber Laser Timing Jitter Measurement Modelocked Laser 1 PBS Single crystal balanced crosscorrelator RF-pectrum analyzer Modelocked Laser 2 HWP Detector output (V) 1 0-1 -800 0 800 Oscilloscope Loop filter Time delay (fs) J. Kim, et al., Opt. Lett. 32, 3519 (2007). 12
Timing Jitter in 200 MHz Fiber Lasers Phase Noise (dbc/hz) -110-130 -150-170 -190 Integrated Jitter Erbium Fiber Laser Phase Noise Theory Integrated Measured Theory Noise Floor Measured Jitter Density 6 5 4 3 2 Jitter (fs rms) -210 1 Noise Floor -230 10 3 10 4 10 5 10 6 10 70 7 Frequency (Hz) Ultralow timing jitter (<1 fs) in the high frequency range [100 khz, 10 MHz] J. Kim, et al., Opt. Lett. 32, 3519 (2007). 13
Timing - Stabilized Fiber Links 14
Timing-Stabilized Fiber Links Mode-locked laser isolator PZT-based fiber stretcher Fiber link ~ several hundreds meters to a few kilometers 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 15
2 Link Test System J. Cox et al. CLEO 2008.
Experimental Apparatus ~300 m optical fiber Piezo stretcher 200 MHz Laser In-Loop PPKTP EDFA Out-of-Loop PPKTP Motor Invar Board
Balanced Cross-Correlation Signals Link 1 In Loop S-Curve, slope = 21 mv/fs Signal (mv) 2000 0-2000 Link 1-2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2 time (ms) Link 2 In Loop S-Curve, slope = 18 mv/fs Signal (mv) 2000 0-2000 Link 2-2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2 time (ms) Out of Loop S-Curve, slope = 20 mv/fs Signal (mv) 2000 0-2000 Out of Loop Phase Shift (fs) 8000 6000 4000 2000-2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2 time (ms) 100 Hz PZT Modulation -2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2 time (ms)
Results Timing Jitter Jitter Spectral Density (fs 2 /Hz) 10 4 10 2 10 0 10-2 10-4 10-6 3 2.5 2 1.5 1 0.5 Integrated Jitter (fs rms) 10-4 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 50 Frequency (Hz) 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
Optical-to-Optical Synchronization 20
Optical-to-Optical Synchronization 21
Ti:sapphire Laser + Cr:Forsterite Laser Spanning over 1.5 octaves Ti:sapphire Cr:forsterite 5fs 30 fs 22
Sub-femtosecond Residual Timing Jitter Balanced optical cross-correlator based on GDD (T. Schibli et al, OL 28, 947 (2003)) Long-term drift-free sub-fs timing synchronization over 12 hours 23 J. Kim et al, EPAC 2006.
Optical-to-RF Conversion or Optical-to-RF Locking necessary for Locking OMO to RMO 24
Direct Extraction of RF from Pulse Train E. Ivanov et al, IEEE UFFC 52, 1068 (2005). IEEE UFFC 54, 736 (2007). AM-to-PM conversion, Temperature drift of photodetectors and mixers TR/n TR = 1/fR t... time Photodetector fr 2fR nfr RF frequency 25
Direct Extraction of RF from Pulse Train 55 fs drift in 100 sec TR/n TR = 1/fR A. Bartels et al, OL 30, 667 (2005). t... time Photodetector fr 2fR nfr RF frequency 26 More in: B. Lorbeer et al, PAC 2007.
Balanced Optical-Microwave Phase Detector (BOM-PD) Microwave Signal Electro-optic sampling of microwave signal with optical pulse train J. Kim et al., Opt. Lett. 31, 3659 (2006). 27
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) Modelocked Laser Stable Pulse Train Output 28
Testing Stability of BOM-PDs BOM-PD 1: timing synchronization BOM-PD 2: out-of-loop timing characterization 29
Long-Term Stability: 6.8 fs drift over 10 hours RMS timing jitter integrated in 27 μhz 1 MHz: 6.8 fs J. Kim et al, Nature Photonics 2, 733 (2008). 30
Delay Locked Looop: 2.9 fs drift over 8 hours RMS timing jitter integrated in 0.1 Hz 1MHz, 2.4 fs J. Kim et al, submitted to CLEO 2010. 31
1 GHz diode pumped CrLiSAF Laser: Modelocked Jointly with Jim Fujimoto 32
Prospects for Attosecond Timing Distribution (100 MHz Cr:LiSAF Laser, SSB scaled to 1GHz) Phase Noise (dbc/hz) -130-160 -190-220 -250 Noise Floor Cr:LiSAF Laser Phase Noise Measured Jitter Density Theory Integrated Jitter 250 200 150 100 50 Jitter (as rms) -280 0 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Frequency (Hz) U. Demirbas, submitted to CLEO 2010 33
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. 34