A.N. Matveenko

Visualization of radiation from a high-power terahertz free electron laser with a thermosensitive interferometer

A thermosensitive interferometer based on a plane-parallel glass plate is used for visualization of a high-power terahertz radiation. The plane wavefront of visible radiation emitted by a semiconductor laser is reflected from the two surfaces of the plate and forms on a screen an interference pattern recorded by a digital video camera. Terahertz radiation being measured is incident on the outer surface of the plate and heats a thin surface layer, which causes a shift of interference fringes. For K8 glass, a shift by one fringe corresponds to an absorbed energy of 5.1 J/cm2. The problem of determining the sign of the phase shift was solved by comparing the interference patterns with the images obtained with an infrared imager sensitive to near IR radiation. The processing of interference patterns makes it possible to determine the power density distribution over the beam cross section of the Novosibirsk free electron laser. In these measurements, the absolute value of the beam power determined by integrating over the cross section was 65 ± 7 W for a 130-μm wavelength. Visualization of the complex image with a spatial resolution no worse than 1 mm and a frame repetition rate of 25 Hz is demonstrated.

Status of the Novosibirsk high power free electron laser

The first stage of Novosibirsk high power free electron laser (FEL) was commissioned in 2003. It is based on normal conducting CW energy recovery linac. Now the FEL provides electromagnetic radiation in the wavelength range 120-180 micron. The average power is 100 W. The measured linewidth is 0.3%, which is close to the Fourier-transform limit. The assembly of user beamline is in progress. Plans of future developments are discussed.

Harmonic Generation in the Novosibirsk Terahertz Free Electron Laser

At the present time, the radiation of the main first harmonic of the Novosibirsk terahertz free electron laser (FEL) lies in the range of 120-230 mum with power level of up to 400 W. The radiation of second and third harmonics was experimentally found in the FEL. This high harmonic FEL radiation extends its spectral range by a factor of 3. Rather high second harmonic radiation unusual for typical FEL allows uninterrupted radiation in the range of 40-230 mum.

Quasi-continuous sub-millimeter optical discharge on Novosibirsk free electron laser: experiments and elementary theory

The first quasi-continuous optical discharge in near sub-millimeter spectral range was demonstrated on high-power Novosibirsk free electron laser. Experimental results and elementary theory of this phenomenon are presented.

Novosibirsk terahertz free electron laser: status and survey of experimental results

During the past year, generation characteristics of the Novisibirsk terahertz free electron laser were growing up. It generates now coherent radiation tunable in the range 120-170 /spl mu/m at the repetition rate of 2.8-11.2 MHz. Maximum average output power reaches 400 W at 11.2 MHz. Laser radiations are transmitted through a 14-m optical beamline to the user stations. Experiments on chemistry, biology, spectroscopy, imaging and holography with employment of terahertz radiation are in progress.

High power THz applications on the NovoFEL

High power THz applications on the Novosibirsk terahertz free electron laser are described.

High Speed Terahertz Imaging Using Thermosensitive Elements

We have studied sensitivity, linearity, dynamic range, space and time resolution of three imaging systems for the visualization of high-power radiation from 0.12 to 0.25 mum: an IR-thermograph, a phosphor screen sensitive to thermal quenching of luminescence, and a glass-plate visible-light interferometer.

Observation of Sideband Instability in the Novosibirsk Terahertz Free Electron Laser

Sideband instability was observed very clearly in the long-pulse regime of the Novosibirsk terahertz free electron laser (FEL) by a Bruker vacuum Fourier spectrometer IFS-66v. For positive frequency detuning, simultaneous generation of six sideband modes was observed. Mode competition and mode switching have been registered. Negative frequency detuning decreased the number of sideband modes and an single-mode generation with the narrowest classical spectrum has been obtained for sufficiently large detuning .

Highly sensitive fast Schottky-diode detectors in experiments on Novosibirsk free electron laser

Fast Schottky-diode detectors is used for measurement and optimization of main parameters of Novosibirsk free electron laser (FEL) such as losses in optical resonator, small signal gain, optimal output coupling. Possibility of applying of the device for measurement of coherence time and FEL pulse duration is considered. We plan also to calibrate free electron laser wavelengths by this detector and etalon universal gas laser. High sensitivity of the detector allows apply of it in various users experiments too.

Modulation instability at the Novosibirsk terahertz free electron laser: Study and suppression

Different mode regimes versus the extent modulation instability stabilization were investigated with a complex diagnostics system. Slippage due to frequency detuning of electron and light pulses turned out to be the main stabilization factor. Spectral and time parameters of the laser, especially the radiation of high harmonics, have been shown to strongly depend on the stabilization.