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Natural Science Forum / Physics / Optics / April 2005optics of choice
When constructing a Galilaen beam expander which is the lens of choice, a Plano-concave or Bi-concave? Thanks Steve Neither. You need a converging group and a diverging group. > Neither. You need a converging group and a diverging group. Well, you need to take them of course but the question, as I understood its meaning was - should he take a symmetrical "lens" or not - and I vote for symmetrical - for both, the converging as well as for the diverging group rgds George Nyman > > Neither. You need a converging group and a diverging group. > > [quoted text clipped - 3 lines] > diverging group > rgds George Nyman Not quite. Aberrations are less if one surface is plano on each lens -- AND the flat side of each lens is facing the other lens: Narrow beam in --> ----- )| < |) ===== Expanded beam out (This is what Bill almost said, but refrained from saying in the interest of making people think about it.) Mark >When constructing a Galilaen beam expander which is the lens of choice, >a Plano-concave or Bi-concave? > >Thanks >Steve Laser beam enters the smaller lens from the flat sides and exits from the concave side (which needs to be a conic to remove spherical) Then diverging beam stikes a convex surface (also a conic) and leaves from the final flat side. Conic constants are determined using an optical design program or through a lot of algebra to determine the conics which work. Jim Klein > Laser beam enters the smaller lens from the flat sides and exits from > the concave side (which needs to be a conic to remove spherical) [quoted text clipped - 6 lines] > > Jim Klein Okay, if the O.P. is willing to spend the time and money on aspheric lenses matched to his/her beam set-up, you're right. But for inexpensive off-the-shelf spherical lenses, the lens surface orientation is the reverse of what you say. Mark >> Laser beam enters the smaller lens from the flat sides and exits from >> the concave side (which needs to be a conic to remove spherical) [quoted text clipped - 14 lines] > >Mark Optimal shape factor is a function of N so using a simple design program to check out the performance of each simpler design (using plano-c or bi-type lenses is critical. You can do it for free with the demo version of ODP www.westcoastengineering.com or OSLO EDU If the power is high, checking the positions of first order ghost images is also a really good idea. Using a negative group and a positive group almost always demands a design code to evaluate combinations of off-the-shelf designs. Jim Klein |
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