Generation of frequency-chirped optical pulses in a large-slippagefree-electron laser

We have investigated the optical output of the free-electron laser for infrared experiments (FELIX) when it is driven by an electron beam with a ramped energy. We show that the applied slow ramp on the electron beam energy leads to a frequency chirp on each picosecond optical pulse. Typical values for the chirp are 0.2% frequency sweep across a 1.5-ps-long optical pulse. The optical pulses were analyzed with a double-grating pair and with a second-order autocorrelator. The pulse duration was reduced in the double-grating pair by 20%. A linear dependence of the chirp on the cavity desynchronization was measured     

Short-pulse effects in a free-electron laser

The Free-Electron Laser for Infrared eXperiments (FELIX) offers a unique combination of short electron bunches and long wavelengths, i.e. a slippage parameter μ_c ranging up to 10. As a consequence, pronounced short-pulse effects can be observed. In this paper the experimental observation of two of these effects is discussed, namely the occurrence of limit-cycle oscillations and the feasibility of tuning of the micropulse duration. The stable limit-cycle oscillation of the macropulse power is due to a modulation of the optical micropulse shape. This is a consequence of a combination of high optical power and short pulses. The former causes synchrotron oscillations of the electrons and the effect is, therefore, closely related to spiking phenomena. The short-pulse nature of FELIX ensures that the oscillations do not evolve into the chaotic behavior normally associated with spiking and the sideband instability. Experimental results are compared with numerical simulations     

Differences between intra- and extra-cavity pulse time structure ina hole-coupled free-electron laser

In the strong-slippage regime of a free-electron laser, the optical pulse inside the resonator is composed of a series of subsequently growing and decaying subpulses due to a limit-cycle oscillation. The picosecond time structure of the outcoupled pulses can be quite different from that of the intracavity pulse, in case of outcoupling through a hole and for specific resonator parameters. This is demonstrated by autocorrelation measurements and corroborated by simulations     

Continuously tunable, high-power, single-mode radiation from a short-pulse free-electron laser

This paper gives the first demonstration of high-power, continuously tunable, narrowband radiation that is produced by means of a free-electron laser (FEL) in the far-infrared region of the electromagnetic spectrum. A Fox-Smith intracavity étalon was used to induce phase coherence between the 40 optical micropulses that were circulating in the laser cavity. The corresponding phase-locked spectrum consisted of a comb of discrete frequencies separated by 1 GHz. A pair of external Fabry-Pérot étalons was used to filter out a single line from this spectrum. The power in the selected narrow line at 69 microm wavelength was equal to 250 mW during the macropulse of the laser. The spectral width of the selected line is as small as that of a single cavity mode, i.e., a fraction of 25 MHz, in single macropulses of the laser. The average bandwidth of 25 MHz is determined by mode hopping of the phase-locked FEL. The selected frequency hops over 25 MHz between the extrema of this band. The influence of partially coherent spontaneous emission and mode hopping on the final linewidth was studied. The narrow-linewidth radiation was scanned in frequency over 1 GHz. We show that the possibilities to scan over smaller or larger frequency intervals are unlimited.     

Hole coupling in free electron lasers

Numerical simulations of the performance of a far-infrared free electron laser with an aperture in the upstream cavity mirror are presented. Two different applications of mirror apertures are studied: broadband extraction of radiation and injection of the electron beam. The emphasis is on the effect of the aperture on the amplitude of the higher-order transverse modes. It is shown that the mode distribution at saturation can be greatly influenced by variation of the mirror radius of curvature. A simple formula which is quite useful for predicting the dominant higher-order modes is derived. This permits optimization of the cavity with respect to the application in question     

Phase locking in an infrared short-pulse free-electron laser

The feasibility of the phase-locking procedure for the external selection of a single mode with reasonable power has been tested by simulation of the optical pulse evolution in a short-pulse free-electron laser using a model based on the self-consistent solution of the equations of motion for the electrons and the wave equation driven by single-particle currents. The simulations show that a high degree of coherence between successive pulses can be induced by a low-finesse etalon. Saturated operation in a greatly reduced number of modes, but with the same total power, is attained with a slight delay in the growth of the power as compared to the case without phase locking     

Influence of a step-tapered undulator field on the optical pulseshape of a far-infrared free-electron laser

The optical output of the free-electron laser for infrared experiments (FELIX), which operates in the regime of strong slippage, consists of picosecond pulses. Depending on the amount of cavity desynchronization, the optical pulse can develop substantial structure in the form of multiple subpulses. We present second-order autocorrelation measurements of the subpulses at several far-infrared wavelengths while applying a step-taper in the undulator field. The operation with a step-tapered undulator prevents the electrons from reabsorbing the optical field energy, leading to a smooth optical pulse. For different settings of the undulator the measured pulse shape and corresponding power spectrum are discussed. It is possible without decreasing the small-signal gain to produce a smooth high-power optical pulse during the whole saturated part of the machine pulse in an FEL oscillator with a reverse-step tapered undulator