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.