When the electric field component of a laser is fixed in one direction, we call it linearly polarized. When it keeps circling, we call it circly polarized. Changing between the two is done with a quarter wave plate. A quarter wave plate is a glass plate with different refractive index in two directions called fast and slow axis. Quarter wave means that light along one axis is a quarter of a wavelength delayed with respect to the other one. Laser light is most commonly linearly polarized, but it's always possible to disassemble it in two components, one along the fast axis and one along the slow axis. By rotating the quarter wave plate with its axis at 45 degrees to the incoming beams polarization, both components have then the same amplitude and leave the quarter wave plate with a pi over two phase lag. The light is now circularly polarized. When circularly polarized light hits a quarter wave plate, it's linearly polarized again. This is my setup using a laser and a polarizer to make linearly polarized light and a quarter wave plate to make it circular. I insert a polarizing beam splitter and a power meter in the reflected arm. To align the polarizer with the beam splitter, I rotate the polarizer until the measured power is minimal. Now I position the quarter wave plate such that the back reflection overlaps with the incoming beam. I add a back reflecting mirror, so the input circular polarization is reversed on its way back. Currently, it's elliptical, but by putting the power meter in the again reflected arm and now maximizing the power, we make our light circularly polarized. The quarter wave plates are useful for all kinds of applications. Combine it with a polarizer and you have an optical isolator. Combine it with a focal cell and you build yourself a fast polarization tater. Stay healthy and see you next time.