Optical Synchronization at ELBE
The linear electron accelerator ELBE has been upgraded in order to produce short electron pulses with 150 fs duration and capable for highly charged bunches of up to 1 nC. Several short pulse secondary radiation sources are driven by these ultrashort electron bunches. In order 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 is locked to the accelerators radio frequency (RF) master oscillator and the pulses are 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 Onefive Origami 15 is a robust system showing very low phase noise . It is locked to the accelerators RF using the available phase lock electronics. The measured phase noise of the HZDRs Origami was below 6 fs [1 kHz; 10 MHz].
The link stabilizer contains the optical cross correlator, the coarse phase measurement, the actuators for the phase correction, dispersion compensation and the polarization control, designed by DESY, Hamburg .
Since the link mechanics was engineered and improved in several iterations it shows very good performance and long term stability at the Free-Electron Laser in Hamburg (FLASH). Still there is a lot of research and development going on to reach even higher phase stability. In future numerous links of this kind will be installed at the European XFEL currently under construction in Hamburg.
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. Different to DESY at ELBE a NI PXI- based System is used. A typical chassis contains a National Instruments (NI) Realtime Controller, a Board with a FPGA extended with fast analog inputs and outputs. A second board with slow ADCs is used to monitor power values and to do the slow polarization control feedback. The motor controller is connected to the board with an extended bus structure.
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].
The stabilized laser pulses are transmitted to the BAM electronics via singlemode optical fibers. Since the slope (i.e. the signal bandwidth) of the bunch signal is directly contributing to the measurement resolution, one has to avoid long cabling between EOM and beamline pickup. Therefore the BAM-frontend has been installed underneath the beamline what leads to a cable length of 2 meters. To protect the electronics from radiation it was covered with 0.1 m of lead in any dimension.
The BAM-frontend houses two electro optic modulators, one for the fine and one for the coarse channel. To adjust the timing overlap between laser and pickup signal an optical delay line (ODL) is installed. A second one is used to compensate for differences in cable and fiber length in both channels. To recover the phase information of the laser signal a coupler is used to tap of a small fraction of the intensity signal. This signal is sent to the readout electronics and used to clock the ADC.
 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