In a Raman set up we need very collinear pulsed nanosecond beams. A colleague with experience recommending using two cylindrical lenses (one for each beam before a combining optic), which we can put into the beam path to focus the light to a line. If these lines are collinear they should appear dim due to the destructive interference (they will not be overlapped in time nor do they need to be). Does anyone have experience with this or recommendations for creating very collinear beams? Is this as simple as my colleague made it sound? What sort of focal lengths do you use? Logistical concerns here?
Our use: We plan to use a combining optic to transmit one pulsed ns beam and reflect another ns beam off this combining optic. We will subsequently align our two beams through two sets of irises (this is pretty typical I believe). The irises may not get sufficiently collinear light or may take many iterations and I would like to know other ways people address this problem.
When you say "colinear," what do you mean, exactly? To me this means two parallel but not coincident optical axes. But perhaps you mean that you want to create two parallel strips of illumination? A diagram would help.
A logistical concern might be how closely you can place the two cylindrical lenses vs. how closely you need the images to be.
If your intention is to make the two beams parallel to each other, and they are close enough to put thru a single spherical lens, the best method is to overlap the beams at the focal plane of the lens. The focal plane of a lens is where the angle of a beam entering the lens is totally transformed into position on that plane (Fourier transform of angle to position). Use a lens with a long enough focal length so that spherical aberration is not a factor. Depending on the intensity of the beams you could use first surface reflection off a quality uncoated optical flat (preferably wedged) to reduce the intensity to the point where you can examine the focused spots. Make sure that you observation is at the focal of the lens for the laser wavelength. This will be the position of the smallest spot size only if the beams are well collimated. There is also a slight difference between the focal plane and the position of the smallest spot from a collimated beam, but this should be negligible if you are using a high f# configuration. Focal length / diameter of 20 will do this, as well as keep you away from spherical aberrations.
Thank you for your responses. I was using collinear, it would have been less ambiguous to say coincident. Dan, I think that your idea of using a long focal length lens and combining the beams as the focal length of the lens will work for these purposes.
cylindrical lens is not the only option to get collinear beams, there are others options also.
Yes it is the best controller to insert wave optics in fiber
but there are other ways also can be applied
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