Adjustment of optics resources

[silin]

Hi everyone,

who knows where to find a good introduction into adjustment of optical components, like making sure that light correctly centered on a lens, so that there are no aberrations, adjustment of mirrors under certain angles and such. I've been looking for some advice on adjustment, but couldn't find it on this general level. Did anybody see such stuff in books or on the web somewhether. I'll be very very thankful.

 

[Claude Bile]

How you adjust an optical component depends on the mount. I suggest looking up various manufactureres of optical compoenents and mounts, for example Thor Labs, Melles Griot and Spectra Physics have product catalogues that cover this kind of thing, although I'm not sure how much of the catalogue is available online.

Also, are you using a laser as your light source? Laser optics follows different rules (especially safety rules) to other types of optics.

Claude.

 

[silin]

Thank you for the reply. The mounts I´m using are microbench (Linos) and optical rail (Owis). I´ve checked the manufacturers you mentioned, only Melles Griot has some about laser beam positioning. I do use laser light. I´m actually specifically interested in adjustment of a self-made confocal microscope. Does anybody know any sources about that? Thanks.

 

[Claude Bile]

I googled confocal microscopy and found a few interesting sites, including this one;

http://www.physics.emory.edu/~weeks/confocal/

The links in particular I think are worth having a look at. If you have any specific questions about aligning optics, post them here and I'll try to answer them as best I can.

Claude.

 

[silin]

I have a couple of concrete questions by now:

- how to ensure that laser beam is exactly parallel to the edge of the optical table?
- what are the options of centering the beam in Linos microbench system (I know about using a quadrant photodiode, iris diaphragms or pinholes in the light path, are there other options?)?
- how to measure laser beam diameter except for marking on paper how broad it is (seems like not really exact)?
- when you bend a beam with a mirror, how to make sure that the angle is exactly 90 degrees?

Thanks in advance.

 

[Claude Bile]

To ensure the beam is parallel to the table, simply measure the height of the beam and two different points along the path. Adjust the laser beam until the heights at two different points match. Obviously, the further apart these two points are (in terms of beam path length) the more accurate your alignment will be.

I am not familiar with the Linos microbench system unfortunately, so I can't really answer your second question.

Accurate beam spot measurements are done using a CCD array similar to what you would find in a digital camera. With a CCD array, each CCD acts as a detector, representing a single pixel on an image. From there it is just a matter of extracting the numerical data and performing whatever analysis you require.

The easiest way to ensure a beam is bending at a right angle, is to measure a 90 degree angle and align the beam to where you think it ought to be. If you have some way of ensuring you have a surface that is exactly parallel to the incident beam, you can get fancy and use interference patterns to align the mirror (provided the mirror is only partially reflecting).

Claude.

 

[silin]

Thank you, Claude. I have one more question: does anybody know how to make sure that a parallel laser beam goes exactly through the center of a convex lens without having a CCD camera or any fancy laser profile monitor? What I´ve tried so far is to project the widened beam (after convex lens) on a distant wall and see how round it is. But I don`t know how exact this is.

 

[Claude Bile]

Easy, if the beam is not traveling through the centre of the lens, it will be deflected. Mark where the spot is without the lens, once you insert the lens in the beam path, it is a matter of lining up the lens so the spot is centred on where it originally was without the lens.

This is all assuming that your lens surface is perpendicular to the beam path. If there is a slight tilt, it shouldn't matter if you have a good lens. If the beam noticably distorts (usually due to the presence of coma), simply realign it to minimise the abberations in your spot (i.e. trying to get it as circular as possible). This is pretty much what you described in your previous post.

It probably goes without saying that a good lens mount will help enormously.

Claude.