OPTO−ELECTRONICS REVIEW 22(4), 218–223 DOI: 10.2478/s11772−014−0200−4
Practical application of cross correlation technique to measure jitter of master-oscillator-power-amplifier laser system J. MŁYŃCZAK*1, K. SAWICZ−KRYNIGER2, A.R. FRY3, J.M. GLOWNIA3, and S. LEEMANS4 1Institute
of Optoelectronics, Military University of Technology, 2 Kaliskiego Str., 00–908 Warsaw, Poland 2Department of Chemical Engineering and Technology, University of Technology, 24 Warszawska Str., 31–155 Cracow, Poland 3Stanford Linear Accelerator Center, National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA 4University of California Santa Cruz, 1156 High Street, Santa Cruz, Ca 95064, USA
The Linac coherent light source (LCLS) at the SLAC National Accelerator Laboratory (SLAC) is the world’s first hard X−ray free electron laser (XFEL) and is capable of producing high−energy, femtosecond duration X−ray pulses. A common tech− nique to study fast timescale physical phenomena, various “pump/probe” techniques are used. In these techniques there are two lasers, one optical and one X−ray, that work as a pump and as a probe to study dynamic processes in atoms and mole− cules. In order to resolve phenomena that occur on femtosecond timescales, it is imperative to have very precise timing be− tween the optical lasers and X−rays (on the order of ~20 fs or better). The lasers are synchronized to the same RF source that drives the accelerator and produces the X−ray laser. However, elements in the lasers cause some drift and time jitter, thereby de−synchronizing the system. This paper considers cross−correlation technique as a way to quantify the drift and jitter caused by the regenerative amplifier of the ultrafast optical laser.
Keywords: jitter, cross−correlation, laser amplifier, pump−and−probe technique.
1. Introduction Chemical and biological reactions, on the level of atoms and molecules, recently became one of the most thoroughly studied physical phenomena [1–7]. One of the well−known ways to investigate them is the pump−and−probe technique [1–9]. This technique had not been used so widely in this field before different lasers generating short pulses of elec− tromagnetic radiations were developed [10–14]. Not until then were special facilities built at SLAC National Acceler− ator Laboratory (SLAC) where scientists carry out such experiments. In the pump−and−probe technique applied at SLAC there are two types of lasers. The first one, which is solid−state mode−locked master−oscillator−power−amplifier (MOPA) laser system generating NIR radiation at the wavelength of around 800 nm, works as a pump. It initiates a reaction in a sample. The second one, which is free−electron laser [Linac Coherent Light Source (LCLS)] generating X−rays at the wavelength of around 1 nm, works as a probe, e.g., thro− ugh X−ray diffraction, spectroscopy, or imaging. SLAC’s revolutionary X−ray laser is revealing intimate details of atoms and chemical reactions and making stop−motion mo− vies of this tiny realm with the goal of doing the same for *e−mail:
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living cells [15]. The idea of the pump−and−probe technique is presented in Fig. 1. The resolution of the temporal evolution of the system under investigation is mainly determined by the duration of pulses generated by the lasers as well as by the accuracy of synchronization of these pulses. The synchronization is
Fig. 1. Idea of the pump−and−probe technique. Opto−Electron. Rev., 22, no. 4, 2014
realized by a RF oscillator and by a delay line that delays the pumping pulse so it hits the sample just before the probing pulse. The shorter the pulses and more accurate synchroni− zation the higher the resolution. Even though the optical pulses are extremely short (~40 fs) and synchronization is very accurate (
