|Abstract:Free space transmission is an important topic in terahertz technology research. Beam collimation is the key to free space transmission. Compared with visible and infrared lasers, the wave front of terahertz beams can be simplified to neither a plane nor spherical wave. It is a Gaussian beam with a big divergence angle. Theoretical derivation from the classical electromagnetic theory and ABCD laws makes a good agreement: firstly, the positive lens makes the terahertz beam converge in front of the focal plane which is quite different from that it makes the visible or infrared lasers converge on the focal plate; secondly, the positive lens used in front of the detector can improve the energy utilization ratio but does not substantially enhance the beam collimation; thirdly, the negative lens which is match with the radius of the Gaussian beam’s wave front is more appropriate for terahertz beam collimation. The parameter for negative lenses is determined by , where is the distance from terahertz crystal to the lens, is the beam waist and is the center wavelength of the terahertz beam. Experiments show that a negative lens with at the match place, , can improve the terahertz beam collimation from 6° to 0.1°, while a positive lens with at the best position, , which is determined by classical optic, only achieves beam convergence at with an angle of 2.5°. We obtain 20-m terahertz space transmission with a negative lens that has a very simple optical scheme. The signal obtained is seven times greater than that obtained with the positive lens scheme. This discovery will play an important role in free space transmission and terahertz imaging.