Showing papers by "Dino A. Jaroszynski published in 1993"

Coherent Startup of an Infrared Free-Electron Laser

Abstract: Coherent enhancement of the spontaneous undulator radiation by several orders of magnitude has been observed in a free-electron laser at wavelengths from 40 to 100 \ensuremath{\mu}m. The coherent emission can be explained by details of the electron-beam micropulse structure. Furthermore, it has been found that the phase of the optical micropulses is fixed by the electron pulse structure and that the coherence extends over successive optical micropulses, which gives rise to interference effects as a function of the optical cavity length in a laser oscillator.

Experimental observation of limit-cycle oscillations in a short-pulse free-electron laser.

Abstract: The first experimental observation of limit-cycle power oscillations in a short-pulse free-electron laser is presented. These are due to a nonlinear modulation of the optical micropulse shape and phase by the electrons, which leads to the formation of a train of subpulses. Experimentally, the oscillations have been found to depend on the slippage distance and on the desynchronism between optical pulses and electron bunches, comparing well with theoretical predictions.

Optical performance of the CLIO infrared FEL

Abstract: First laser oscillation on the CLIO infrared FEL was obtained in January 1992. This paper describes the layouts of the optical devices used for CLIO, and discusses the optical performances. This machine consists of an rf linear accelerator, described in a companion paper, providing a 30/70 MeV electron beam through a 48 period planar undulator ( K = 0 to 2). The optical cavity is 4.8 m long and uses broadband metal mirrors. The optical beam is extracted with an intracavity CaF 2 or ZnSe plate. Laser oscillation has been obtained thus far in the range of λ =2.5 to 15 μ m at accelerator energies of 32, 40 and 50 MeV. The average power of the laser is about 65 mW for the low duty cycle (6.25 Hz/32 ns) and up to 0.5 W for a duty cycle of 50 Hz/32 ns and should be 5–10 W at maximum repetition rate. The peak power extracted for 8 ps micropulses is 2.5 MW corresponding to 0.4% efficiency. Laser oscillation on the third harmonic of 10 μm has also been achieved (at λ = 3.3 μ m). Application experiments have already been done with CLIO infrared laser (companion paper on nonlinear absorption in InSb), showing the good reliability and overall quality of the laser. The programme now is to operate the accelerator at other energies so as to cover the rest of the designed wavelength range (2–20 μm).

Broadband tunability of a far-infrared free-electron laser

Abstract: A unique property of the free‐electron laser (FEL) is its capability to be tuned continuously over a wide spectral range. This is a major difference with all other high‐power lasers. However, the tunability of first‐generation FELs used to be quite poor (typically 10% or less), due to constraints imposed by the accelerator and the undulator. The free electron laser for infrared experiments (FELIX) uses an undulator with an adjustable gap, which permits wavelength scans over an octave in typically 2 min without the need for any readjustment of the electron beam. Results obtained in operation of the long‐wavelength FEL of the FELIX facility are presented. These involve measurements of the spectral range covered (16–110 μm), the output power, and the influence of the cavity desynchronism. The results are compared with numerical simulations.

Dynamic Desynchronization of a Free-Electron Laser Resonator

Abstract: In a free-electron laser oscillator operating with short electron bunches, the desynchronism between electron bunches and optical pulses resulting in the shortest buildup time to saturation is different from that giving the largest saturated power. In this Rapid Communication we present an experimental demonstration of the dynamic variation of the cavity desynchronism. This is achieved by ramping the electron-bunch repetition frequency during part of each 12-mus machine pulse. A high small-signal gain and a high saturated power are obtained simultaneously.

Dynamic desynchronization of a free-electron laser resonator

Abstract: In a free-electron laser oscillator operating with short electron bunches, the desynchronism between electron bunches and optical pulses resulting in the shortest buildup time to saturation is different from that giving the largest saturated power. In this Rapid Communication we present an experimental demonstration of the dynamic variation of the cavity desynchronism. This is achieved by ramping the electron-bunch repetition frequency during part of each 12-μs machine pulse. A high small-signal gain and a high saturated power are obtained simultaneously

Infrared free-electron laser measurement of power limiting by two-photon absorption in InSb

Abstract: We have performed the first experiment on the free-electron laser (CLIO) at the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique (LURE). In a transmission experiment we observed strong power limiting at wavelengths longer than the absorption edge associated with induced free carrier absorption produced by direct interband two-photon transitions in InSb. We have estimated the two-photon absorption (TPA) coefficient (β) of InSb at 8.9 μm to be 2 cm MW-1, by fitting the power-limiting effect with a simple theoretical model. An important feature of this result is that we are utilizing the broad tunability of the FEL to explore the TPA theory to shorter wavelengths than hitherto available with CO2 lasers. The result is in broad agreement with the longer-wavelength measurements of Sheik-Bahae et al.

Infrared FEL measurements of power limiting by two-photon absorption in InSb and optical pulse length measurements

Abstract: We have performed the first optical experiment using the laser output of the CLIO free electron laser. In a transmission experiment we have observed strong power limiting at wavelengths longer than the absorption edge at 7.5 μm associated with induced free carrier absorption produced by direct interband two-photon transitions in InSb. We have estimated the two-photon adsorption (TPA) coefficient (b) of InSb at 8.9 μm to be 2 cm/MW, vy fitting the power-limiting effect with a simple theoretical model. An important feature of this result is that we are utilising the broad tunability of the FEL to explore the TPA theory to shorter wavelengths than hitherto available with the CO2 laser. The result is in broad agreement with the longer wavelength measurements of Sheik-Bahae et al. [J. Opt. Soc. Am. B4 (1987) 1964], and distinguishes between different theoretical models for the process. Applications that make use of the intrinsic nonlinearity of the interaction include the measurement of picosecond optical pulse lengths using an autocorrelation method, direct measurement of the optical pulse intensity and intensity stabilisation of the FEL source using the power limiting capabilities of the semiconductor.