Terahertz Spectrum Modulation With Deformable Mirror

Terahertz (THz) wave with modulated spectrum has been required in many fields, so the technology of THz spectrum modulation is of crucial importance in research. In this thesis, the femtosecond laser with wavefront controlled by a Micromachined Membrane Deformable Mirror (MMDM) has been applied to produce modulated THz waves in the mode of air plasma. The pumping femtoseond laser with different wavefronts can generate THz waves with different spectrum. The work in this thesis is summaried as follows:1. The method of the reconstruction of disturbed phase of the femtosecond laser reflected from the Micromachined Membrane Deformable Mirror is proposed. Firstly, a simple Mach-Zehnder interferometer is used to form interference fringes and a deformable mirror is used for spatial shaping of the femtosecond laser pulse on the object beam path. With the algorithms of interference fringe analysis, Fresnel diffraction and phase unwrapping, the disturbed phase information is obtained. Secondly, with two basic properties of the deformation, this method has been verified, which shows an effective way for wavefront phase reconstruction and record of femtosecond laser pulse.2. The air plasma filament is modulated by controlled wavefront of laser pulse. At first, the spatial shaping of femtosecond laser wavefront is achieved by micromachined membrane deformable mirror, the changes of the plasma filament which is induced by ionization are observed by CCD after focusing through a convex len. In the experiment, we modulate the filament in three aspects:the position of the filament, the intensity of the filament and the length of the filament. Based on the aboved method, the reconstruction of disturbed phase of the femtosecond laser is obtained. With the algorithm of Fresnel diffraction, the field distribution of focus is obtained after the disturbed laser wavefront propagating through the convex len. The changes of the focal distance, the intensity of the focus and the length of the focus are obtained by the simulation. Combined with dynamic compensation model, the modulation results of filaments are theoretically analyzed. 3. Terahertz spectrum is modulated by the controlled wavefront of the femtosecond laser pulse via the modulated filaments. At first, a terahertz time-domain spectroscopy system is set up to get Thz time domain signals, a micromachined membrane deformable mirror on the pumb path is used for the spatial shaping of the femtosecond laser pulse. By changing the distribution of the filament, the purpose to modulate the THz spectrum is achieved. In the experiment, THz spectrum is modulated in two aspects:the change of the intensity of the spectrum and the shift of the spectral peak position. The modulation results of THz spectrum are theoretically analyzed from three aspects:the length of the filament, the intensity of the filament and the length of the filament.