J.M Berset

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).

Activities of the CLIO infrared facility

Abstract The infrared CLIO FEL has operated as a user facility since mid-1992. About 2400 h of laser beam time are now produced annually, of which 800 h are dedicated to FEL physics and optimisation and 1600 h for laser users. The user beam time is allocated by a programme committee, the demands exceeding the available beam time by a large factor. CLIO spectral range spans 3 to 50 μm and its peak power is several MW in 0.2 to 5 ps long pulses. It is based on a dedicated radiofrequency linear accelerator. The repetition rate of the micropulses can be varied from 32 to 4 ns during 10 μs long macropulses (at up to 50 Hz) and average power up to a few Watt is achievable. Besides the laser facility, CLIO is used internally to study FEL physics. Several new studies have been successfully conducted at CLIO: ultrashort FEL pulses, 2-colors simultaneously generated, undulator step tapering, harmonic generation and self amplified spontaneous emission (SASE) at 5 μm. Several of these studies have resulted in the improvements of the user facility. The application of CLIO are in the following fields: - Semiconductors and quantum wells physics - Near field infrared microscopy - Vibrational energy transfers in molecules in rare gas matrices - Pump-probe study of vibrational relaxation of molecules - Surfaces and interfaces studies by sum frequency generation (“SFG”) - Medicine. Ancillary equipment is provided to the users. In particular optical parametric oscillators (“OPOs”) can provide powerful tunable infrared source between 2 and 8 μm to supplement CLIO.

Bunch length measurements on CLIO

Abstract The lengths of both electron and laser micropulses have been measured under various conditions. The width of the electron bunches varies between 8 and 16 ps FWHM for standard operation of the linac in our usual 32–50 MeV range. The laser micropulse length varies between 1.5 and 6 ps at 8 μm, depending on the detuning of the optical cavity length. Shorter pulses of subpicosecond duration have been produced by running our linear accelerator in a configuration where the electron bunches acquire a quasi-linear time-energy relationship while travelling on the side, rather than on the crest, of the accelerating high-frequency wave.

CLIO, an infrared free electron laser facility

Abstract The “CLIO” Free Electron Laser is a tunable infrared laser that is principally dedicated to users. This laser uses a 30–60 MeV relativistic electron beam, provided by a linear accelerator, and an undulator to provide the optical gain needed for the laser process. This facility has been operating since January 1992, producing laser pulses of about 10 MW in 2 ps, with a tunable wavelength range of 2 to 17.5 μm. The average power depends on the repetition rate of laser pulses, and presently reaches about 2 W (at 1 4 of the maximum repetition rate). Four user rooms are available for general experiments using the CLIO laser beam. Three experiments have already succesfully taken place. Study of two photons absorbtion in InSb and InAs as a function of wavelength; infrared microscopy by tunnelling effect, by peaking the evanescent wave of the laser probe on a sample with an optical fiber, for different wavelengths; and surface spectroscopy using the “Sum Frequency Generation” technique, i.e. by mixing the tunable CLIO infrared laser with a synchronized Nd-Yag laser in order to observe the resonance vibration on a surface. This paper presents the general performances of the accelerator, of the laser and describes the principal application experiments which are now in progress.

Short pulses and broadband measurement by autocorrelation with a sum-frequency generation setup

Abstract Generation of subpicosecond light pulses on the CLIO FEL has been reported previously, at a wavelength λ ≈ 9 μ m. These results are especially striking when compared to the electron bunch, measured to be about 10 ps long. In this paper, we present experimental data giving us some clues to the understanding of this effect. Autocorrelation measurements are performed using two different systems, amongst which one is based on sum-frequency generation on surfaces. This allows broadband operation and study of the laser temporal characteristics with wavelength.

Sub-picosecond laser pulse generation on the CLIO FEL

Abstract Laser pulse length measurements have been carried out on the CLIO infrared free-electron laser using a Michelson interferometer with a Te crystal as a second order autocorrelation device. Simultaneously spectral measurements have been carried out showing that the laser pulse is close to the Fourier transform limit. A minimum laser pulse length of δ t = 400 fs, and a line width of δ;λ = 500 nm have been found for λ = 9.2 μm, giving a peak power of 70 MW in the full spectral bandpass. We also examine, in this paper, the laser spectral brightness as a function of the cavity length detuning, and the laser pulse length with the linac operating off the crest of the accelerating wave.

TEMPORAL STRUCTURE OF THE CLIO LASER MICROPULSES IN THE TWO-COLOR REGIME

Abstract By using sum-frequency generation in a Michelson arrangement, we have directly measured the temporal overlap between single-frequency micropulses in a two-color free-electron laser. As expected, the average separation appears to be related to the slippage length between the electrons and light when passing through the undulator. The two colors are then partly simultaneous, although there is not necessarily two-frequency bunching in the electron beam.

New results of the CLIO infrared FEL using a new undulator and hole coupling extraction

Abstract The “CLIO” laser spectral range has been extended towards longer wavelengths. With this aim, the previous undulator of CLIO has been replaced [O. Marcouille et al., A new undulator for the extension of the spectral range of the CLIO FEL, these proceedings, Nucl. Instr. and Meth. A 375 (1996) 465], and consists now of N = 38 periods of λ 0 = 5.04 cm, instead of N = 48 and λ 0 = 4 cm. A larger undulator vacuum chamber has also been installed to avoid diffraction. The extraction plate output coupling system has been replaced by a hole coupler to allow lasing beyond 17 μm. This paper summarises the general performance of CLIO with this new configuration, and shows that two colour operation has also been observed with hole coupling [D.A. Jaroszynski et al., Step tapered operation of the FEL: efficiency enhancement and two colour operation, these proceedings, Nucl. Instr. and Meth. A 375 (1996) 647].

Observation of self-amplified spontaneous emission in the infrared free electron laser “CLIO”

Abstract A solution has been proposed about ten years ago to reach the X-ray range: the principle is to operate the FEL in the Self-Amplified Spontaneous Emission (SASE) configuration. In the high gain regime, the spontaneous emission is amplified along the undulator in a single pass configuration and without optical cavity. We report here the observation of SASE at the shortest wavelength, the mid-infrared range. A spectral analysis of the SASE has been carried-out in the start-up regime of SASE, far from the saturation level.