User:Andy Maloney/Notebook/Lab Notebook of Andy Maloney/2009/03/30/Optical tweezers
So the element that produces the photons in our laser diode is shaped like a rectangle and it produces elliptical beams. Originally, I thought that an asphere would be able to collimate the LD with no problems. However, I learned today that the best way to collimate a source that is shaped like a line, is to use cylindrical lenses and not spherical ones.
Of course this seems obvious and something that I should have taken into consideration when I spent my GRD money because I would have gotten more of them. I originally purchased the cylindrical lenses as a way to turn the elliptical beam into a circular one. I also purchased a set of anamorphic prisms to do this, but I thought it best to get both just in case I was not able to get the anamorphic prisms to work. Thankfully the cylindrical lenses have come in handy to collimate the beam and not at all what I intended to use them for initially.
Since the LD produces an elliptical beam, there are two axis that need to be collimated, the parallel axis and perpendicular axis. See the figure below.
We have setup collimation for the parallel axis but we need another cylindrical lens to complete the collimation in the perpendicular axis.
To collimate the perpendicular axis, we need a cylindrical lens with a focal length that is larger than the focal length of the first lens. Right now we have a shutter on the LD (important and I don't want to remove it) that takes up space. Why do I care about space? Well, we only have 2 10mm and 1 30mm lenses. Right now we are using the 30mm lens to collimate the parallel axis. I am going to try and collimate the parallel axis with the 10mm lens and the perpendicular axis with the 30mm lens. This way we don't have to purchase anything. But, we loose the usage of the shutter. I don't like this. I'll have to think about a way to do it without loosing the shutter.
What's more irritating is that if we use the anamorphic prisms, we need to make the input beam 4x larger in one direction to get a circular beam out. To do this, we make need to make a lens telescope or use the proper cylindrical lenses to get this parameter correct. I'm beginning to think the prisms aren't worth their time. We shall see though.