The Gravitational Waves group at University of Birmingham has a library of SVG graphics:
http://www.gwoptics.org/ComponentLibrary/
These can be used, for example, in Inkscape (which is free). I think you need only cite the creative commons license if you use them in a publication (or thesis).
I know its not free but most people have access to MS Office and Power Point can be used to draw decent diagrams, in fact I prefer it to professional paid drawing packages like Corel Draw.
I drew the attached schematic drawn in 3D of a laser tweezers system using Power Point
If you want something free you should try Inkscape for regular drawings like the one in the answer before. If you want fancy 3D you could use POV-Ray, but you have to invest quite a lot of time...
@sebastian: I will try Inkscape, thanks for the reply !
@alexander: That is a quite decent diagram for powerpoint. Infact I was going to draw a lazer tweezer system itself to set up before I start with. Did you use any ray tracing software for setting this up ?
This diagram was drawn as part of a guide to new users of the tweezer system as although specific ray optics are not accurately shown, ALL optical elements are shown with their relative positions and orientations.
Attached are ray diagrams for both the trapping optics and illumination optics. again both drawn in Power Point without any other software.
The Gravitational Waves group at University of Birmingham has a library of SVG graphics:
http://www.gwoptics.org/ComponentLibrary/
These can be used, for example, in Inkscape (which is free). I think you need only cite the creative commons license if you use them in a publication (or thesis).
I too would recommend using inkscape along with freely available optical components icons. Or else, you can create your own icons in inkscape. Just try to learn how to use pen tool.