Optical Synchronization at ELBE
The linear electron accelerator ELBE at Helmholtz-Zentrum Dresden-Rossendorf can deliver short electron pulses with 150 fs duration and bunch charge up to 300 pC. This drives two THz-sources, one broadband CTR/CDR and a narrowband undulator source.
To enable highly resolved pump-probe experiments with table top laser sources pump and probe beams have to be synchronized on a 100 fs time scale.
In collaboration with DESY, Hamburg a synchronization system based on a mode locked laser as an optical master oscillator is used to ensure a timing stability on the few 10 fs scale [1,2]. The laser will be locked to the accelerators radio frequency (RF) master oscillator and the pulses will be distributed via optical fibers to the remote stations. To detect delay changes in the optical fibers caused by temperature drifts and mechanical stress, part of the laser light is reflected at the far end and sent back to the near end of the transmission line. Phase changes are measured using a balanced optical cross correlator which generates an error signal of a few mV per femtosecond. The error signal is fed into a fast digital controller to compensate timing variations in two steps. Fast changes are minimized using a fiber piezo stretcher while slow changes are compensated by an optical delay stage offering a broader tuning range.
For the Optical master oscillator a commercial solution was chosen. The NKT Origami 15 produces laser pulses at very low phase noise . It is locked to the accelerators RF using the custom made phase lock electronics. The measured phase noise of the HZDRs Origami was below 6 fs [1 kHz; 10 MHz].
The link stabilizer is using polarization maintaining fibers and contains the optical cross correlator, the actuators for the phase correction and dispersion compensation.
DESY kindly provided these mechanics including the necessary knowledge and support to HZDR to build up a similar system .
To operate the link stabilizer a fast digital controller is necessary. Together with fast ADCs and DACs it reads the balanced detectors output and controls the two actuators to compensate for phase changes. At ELBE a National Instruments compact Rio system is used. A typical chassis contains a National Instruments (NI) Realtime Controller and a FPGA-Board extended by fast analog inputs and outputs.
Using National Instruments Hardware makes it beneficial to use NI LabView for the programming of the control loops and the user interface. As mentioned above the programming is split into two parts. The fast controller for the link stabilization is done with the FPGA module for LabView and a slow part for polarization control, data logging and the user interface. This allows starting to program in both tasks without interfering each other. Only the way of exchanging data has to be defined beforehand.
Bunch arrival time measurement
The laser pulses, provided by the synchronization system, are used as a timing reference for the arrival time measurement. For this, a pickup signal generated by the electric field of the passing electron bunches is overlapped in time with one of the laser pulses inside an electro-optic intensity modulator (EOM). Depending on their temporal relation, the laser pulses are modulated in amplitude. That means the arrival time information is coded into the amplitude variation of one laser pulse. Laser pulses that do not overlap with one of the pickup signals are used as an amplitude reference. The intensity relation between modulated and reference pulses can be measured and calibrated for different time delays. This calibration plot is used for calculating the relative delay during the measurement. The electro- optic technique allows single bunch arrival-time measurements for pulsed and CW machines [2,6].
 J. Kim et al., “Drift-free femtosecond timing synchronization of remote optical and microwave sources”, Nat. Photon 2, 733 (2008).
 F. Loehl et. al., “Electron Bunch Timing with Femtosecond Precision in a Superconducting Free-Electron Laser”, Physical Review Letters PRL 104, 144801 (2010)
 M. Kuntzsch et. al., “Status of the femtosecond synchronization system at ELBE”, BIW2012, Newport News, USA, MOBP03
 M.K. Bock et. al., “Report on the Redesign of the Fibre Link Stabilisation Units at FLASH”, FEL 2011, Shanghai, China, WEPA19
 M.K. Bock et. al., “Recent Developments of the Bunch Arrival Time Monitor with Femtosecond Resolution at FLASH”, IPAC 2010, Kyoto, Japan, pages 2405-2407, WEOCMH02