It is shown that monopolar optical spin orientation of free carriers in zinc-blende structure based quantum wells (QWs) causes an electric current which reverses its direction upon changing the helicity of the radiation from left to right circular polarization resulting in a circular photogalvanic effect. The monopolar non-equilibrium population of spin-up and spin-down states has been achieved by far-infrared optical excitation ofp- andn-type GaAs/AlGaAs QWs structures. Two methods are introduced allowing to determine spin relaxation times. One is based on the Hanle effect in magnetic field induced circular photogalvanic effect, the other is spin sensitive bleaching of absorption. In contrast to usually applied methods of optical spin orientation, in the present case of terahertz excitation only one kind of charge carriers is involved in spin orientation and relaxation processes.