MTCA.4 Phase Detector for Femtosecond-Precision Laser Synchronization

For time-resolved experiments at FELs such as the European XFEL an accurate synchronization of the machine is essential. The required femtosecondlevel synchronization we plan to achieve with an optical synchronization system, in which an inherent part is the master laser oscillator (MLO) locked to the electrical reference. At DESY we develop a custom rear transition module in MTCA.4 standard, which will allow for different techniques of phase detection between the optical and the electrical signal, as well as locking to an optical reference using a cross-correlator. In this paper we present the current status of the development, including two basic solutions for the detection to an RF. One of the methods incorporates an external drift free detector based on the so-called MZI setup. The other one employs the currently used down-converter scheme with subsequent improvements. The module can serve for locking a variety of lasers with different repetition rates. INTRODUCTION TO OPTICAL SYNCHRONIZATION SYSTEM AT THE EUROPEAN XFEL The optical synchronization system planned for the European XFEL has a range of uses. It is employed in these locations of the facility, where the most demanding synchronization precision is necessary. First of all, it serves as a reference for a number of lasers in the machine, including pump-probe laser for time resolved experiments. The lasers can be directly locked to the optical synchronization system using laser-to-laser locking stations (L2L) described in [1]. Another applications are precise bunch arrival-time monitors or a support for the 1.3 GHz coaxial cable based timing distribution, suffering from the drifts arising with a distance from the signal source in the cables because of their thermal expansion. The electrical synchronization system, mainly supplying the LLRF stations along the FEL, comprises of so-called interferometer links which task is to extend the possible synchronization distances [2]. Nevertheless, to achieve the required stabilities of the timing signal at different locations along over 3km long accelerator, the system has to be resynchronized to the optical reference. At DESY, we have developed a module called REFM-OPT, which promises to achieve sub-10fs synchronization precision between electrical and optical signal over longer time periods [3]. The REFM-OPT utilizes the laser-to-RF phase detector (L2RF) ∗ Ewa.Janas@desy.de Figure 1: Optical synchronization scheme of the European XFEL. Description of system components: Master Oscillator (MO), Master Laser Oscillator (MLO), Slave Laser Oscillator (SLO), Free-Space Distribution (FSD), Link Stabilization Unit (LSU), Laser-to-Laser synchronization (L2L), Laser-to-RF synchronization (RF), Bunch Arrival-time Monitor (BAM) beam diagnostics directly using the laser pulse train, Photon Arrival-time Monitor (PAM). Figure source: courtesy of Cezary Sydlo of an extraordinary performance presented in [4], where we obtained 3.6 fs peak-to-peak phase drift over 24 h. An overview of the optical synchronization system is shown in Figure 1. It incorporates the current solution used at DESY’s mother facility FLASH (about ten times smaller than the European XFEL), with subsequent improvements and extensions. The signal source is a master laser oscillator (MLO), which is a commercial mode-locked laser with pulse duration of 200 fs at a repetition rate of 216.67 MHz. The MLO in turn is locked to the master oscillator (MO), the TUP045 Proceedings of FEL2015, Daejeon, Korea ISBN 978-3-95450-134-2 474 C op yr ig ht © 20 15 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s FEL Technology and HW: Electron Diagnostics, Timing, Sync. , and and Controls Figure 2: MTCA.4 components for laser locking application. Description of the terms: RTM-Rear Transition Module, a board connected to a specific AMC from the rear side of a MTCA.4 crate, AMC-Advanced Mezzanine Card, LASY-LAser SYnchronization board, SIS8300-L/SIS8325digitizer board, where the controller is implemented, PZT44 channel piezo driver to control the piezo of the laser, FMC20-dual FPGA mezzanine card, here used for backplane and RTM interconnections. ultra-stable signal source of a six times higher frequency (1.3 GHz) for the electrical reference distribution system. The signal distribution is done through the free-space distribution (FSD) and link stabilization units (LSUs) and it has been presented in more detail in [1], whereas the MLO locking scheme together with supplementary locking schemes implemented on the same hardware for another lasers is a subject of this paper. LASER LOCKING SCHEME BASED ON MTCA.4 STANDARD Laser synchronization in the European XFEL will utilize the MTCA.4 standard [5]. In the current development state, the system is built out of MTCA.4 cards, from which the phase detector item DRTM-DWC10 was designed to fulfill LLRF system needs and adapted for laser locking purpose with external RF components [6]. In the near future, the setup will be enriched by a dedicated rear transition module DRTM-LASY, first mentioned in [7]. It will exchange the currently used external RF components and DRTM-DWC10 board, and provide a set of new functions. The laser locking MTCA.4 setup supplemented with this module is presented in Figure 2. In order to lock the laser to the reference (which can be either optical link end or RF signal), the phase difference between these two signals is first detected on LASY board (in one of the methods described below) or by a connected to it drift-free detector. The resulting error signal is processed to its Advanced Mezzanine Card (AMC). There it is digitized and the phase difference is stabilized by a controller, which acts on the laser’s frequency/phase via piezo driver board called DRTM-PZT4 [8]. Alternatively, an external actuator can be used, but it has naturally a disadvantage of more external components. Taking into account the flexibility of the setup, the photo detectors to convert the laser signal to RF are not placed on LASY. However, there is foreseen a regulated power supply to drive an external photo diode. Additional feature enabling extension of board usage are two DAC outputs, which are planned to be added at the front panel. Having discussed what the complete synchronization setup looks like, the following sections move on to review various phase detection methods supported by LASY RTM. DOWN-CONVERTER LOCKING SCHEME The laser lock using a down-converter scheme is the most straightforward and bases on two-steps locking procedure described in [6]. All the RF components placed before out of a MTCA.4 system are now integrated on the LASY PCB (Figure 3). In order to achieve a flexibility over possible LO frequencies, independent of available reference, the board provides an LO generation functionality. Similarly, there is a clock generation section foreseen, thus an optimal clock frequency at ADC for non-IQ detection may be derived. For the sake of better performance in case of noise and phase drift, two compensation methods are implemented.