Re: Is there simple device producing circular polarized light?

Hello, Chao.

  Well, I'm not sure I can give you exactly what you want, but I'll do my 
best.

  The only way I can think of to get broad bandwidth, circularly-polarized 
light is to do exactly what you mentioned in your post: a polarizer 
followed by a quarter waveplate.  You can get very broad bandwidth 
polarizers using a variety of methods, but obtaining a broad bandwidth 
quarter waveplate is harder.

  However, they're out there.  You can do a web search on "achromatic 
waveplates" and find several vendors who provide such an optical element.

  For example, one vendor (I feel I should strive for objectivity, and 
refrain from endorsing a particular company) quotes a quarter waveplate 
that will work from 465-610 nm, with less than a 1/100 wave deviation from 
1/4 wave over that whole bandwidth.  Another company quotes a similar 
element, without the tolerance specification, that they say works over the 
400-700 nm band (though I suspect that it's performance is identical to 
the first company's product).  Since this covers almost the whole visible 
spectrum, I imagine it would be suitable for what you have in mind.

  These things aren't cheap, generally costing hundreds of dollars US.  
Unfortunately, I cannot think of an affordable way to come up with 
anything similar.  The commercial products are based on the combination of 
two specially-tailored, specially-packaged optical flats.  There's simply 
no way to duplicate that without going through a similarly-complicated 
effort.

  Sorry I couldn't come up with better, but if I come across anything in 
the near future I'll try to let you know.

  - Guy