Most of the applications requiring small diameter laser spots are generated by focusing of laser light and that has a theoretical diffraction limited minimum diameter equal to the wavelength. Does anyone know what is the smallest fully collimated beam (small divergence say single digit milli or micro rads are ok) optical (particularly laser) beams? I know they can generate 1um diameter synchrotron x-ray beams for diffraction experiments. What about for lasers? What kind of lens assembly do they need to accomplish this? I assume they dont use classical lens and start using fresnel lenses to achieve this?