Use of Fringe-Resolved Autocorrelation for the Diagnosis of the Wavelength Stability of a Free Electron Laser

Abstract

Although monitoring the wavelength stability of a free-electron laser provides users valuable useful information, a single-shot measurement of the laser spectrum is not always an easy task in the mid/far-infrared wavelength region due to the lack or very high cost of an array-type photodetector, etc. We propose to use fringe-resolved autocorrelation to monitor the wavelength stability of an oscillator-type free-electron laser. The idea is verified by the numerical experiments for intensity and fringe-resolved autocorrelations which include shot-to-shot fluctuations of intensity, duration, and central wavelength of micropulses as well as the chirps of both micro/macro pulses.

Pulse duration and wavelength stability measurements of a midinfrared free-electron laser

“…1 of [8]. This implies that we can correlate the narrowing of the FRAC signal width with the wavelength stability [8]. Actually the above argument is very similar to that explaining why the FRAC signal width of chirped pulses appears narrower than that of transform-limited pulses with the same pulse duration.…”

“…See Fig. 1 of [8]. This implies that we can correlate the narrowing of the FRAC signal width with the wavelength stability [8].…”

“…In the following, we show that we can obtain the pulse duration, regardless of the presence of the unknown amounts of chirp and wavelength stability, only from the FRAC signals. We would like to emphasize that this is not as obvious as it sounds, because the width of the FRAC signals would be significantly narrowed by the presence of chirp and wavelength stability [8], both of which are unknown.…”

“…In this Letter, we report the measurements of the pulse duration and wavelength stability of KU-FEL by the fringe-resolved autocorrelation (FRAC) method [4][5][6][7]. Note that the FRAC signals are influenced by not only the pulse duration and the chirp, which is well-known, but also the wavelength stability, which was first discussed, to our knowledge, in our previous paper [8]. Under the presence of unknown amounts of chirp and wavelength stability, extracting the pulse duration from FRAC signals is not a very trivial issue for a low repetition laser such as the KU-FEL.…”

“…Then, using the pulse duration we obtain, we estimate the wavelength stability. This is an improved version of the method we have proposed in a recent paper [8], where we have assumed that the pulse duration is known in advance by the separate intensity autocorrelation (IAC) measurement.…”