Re: Astigmatism aberration as a function of distance

Posted by Michael Giacomelli on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Astigmatism-aberration-as-a-function-of-distance-tp7582827p7582838.html

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Hi Lu,

Regarding your collimator, using short focal length achromats is usually a
bad idea unless you are using a very broadband laser (E.g. short pulse
ti:saph or supercontinuum).  Instead, aspheric couplers are usually used as
an achromat will have significant spherical aberration.  If you do use an
achromat, make sure that do not use it backwards (always minimize
glass-wavefront curvature - flatter face into diverging wavefront, curved
face into collimated wavefront) and if possible simulate in zemax to be
sure before trying it.  Just simulating the AC254-30-A, putting the lens in
backwards results in a massively abberated beam.  I would double check that
first.  Alternatively, buying an aspheric may be a better idea. Technically
the AC254-30-A is ok, but only just, only if perfectly aligned.  You have
little margin for error.

If you haven't already, I recommend aligning the coupler to the grid of
your table, and running the beam quite far out and ensuring that it is
truly parallel to that grid (and thus perpendicular to lens face).  Because
of the short focal length, you must be very precise here, with an error of
about a quarter of a millimeter in centration introducing noticeable
astigmatism.  I recommend a good 3 axis kinematic.

Regarding tilt of the fiber face, usually fibers are angle cleaved, and
then mounted in a coupler with a matching tilt.  Make sure that if you used
an APC fiber, you have an APC mount, and if you used a flat cleaved fiber,
you have an un-angled mount.

Regarding mirrors, a standard thorlabs mirror used in one of their mounts
will have negligible astigmatism when used with a beam of your diameter.
It is true that mirrors can and do introduce aberration into beams, but
with such a narrow beam diameter, even a relatively poor mirror will not
introduce noticeable phase error.  These problems are much more common at
2" and above.

By the way, do you have access to a beam profiler?  Taking an image of this
focal spot and posting it might give some clues.

Mike




On Wed, Nov 5, 2014 at 1:11 AM, Yan, Lu <[hidden email]> wrote:

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>
> Hi Mike,
>
> Thanks for promote reply. Like you said for an on axis system (especially
> simple as mine), astigmatism should not be there, and that why it confused
> me a lot. I am currently using a Thorlabs air-spaced achromatic lens
> (f=30mm) (http://www.thorlabs.com/thorproduct.cfm?partnumber=ACA254-030-A)
> for the collimation. The fiber I have tried are SMF600 and RBG 400, both of
> which are single mode at 632 nm. I usually looked at the back reflection
> (from surfaces of the lens) formed interference pattern (concentric rings)
> to align my fiber w.r.t. my fiber facet.
>
> For your comments on my questions:
>
> 1) I am using the multimode fiber as the pinhole to achieve confocal.
> 2) Looking at the beam spot after the collimation lens, it was not changing
> much even at several meters away from the lens. The spot changed rapidly
> around the focal plane if the beam was reimaged through another lens (e.g.
> L2 or L3 in the linked page). I think the NA is large enough for the fiber
> I am using.
> 3) I was thinking maybe the fiber tip is tilted with respect to the
> collimation lens plane? Would that cause problem? OR would that still give
> me concentric rings pattern centered at my fiber tip (if the fiber is
> tilted w.r.t. the lens plane)?
>
> Thanks,
> Lu
>
> -----------------------------------------------------
> Lu Yan
> Nanostructured Fibers and Nonlinear Optics Laboratory
> Electrical and Computer Engineering
> Boston University
> 8 St. Mary St., Boston, MA, 02215
> [phone]
> [hidden email]
> -----------------------------------------------------
>
> On Wed, Nov 5, 2014 at 12:37 AM, Michael Giacomelli <[hidden email]> wrote:
>
> > *****
> > To join, leave or search the confocal microscopy listserv, go to:
> > http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> > Post images on http://www.imgur.com and include the link in your
> posting.
> > *****
> >
> > Hi Lu,
> >
> > In an on axis system like you have drawn there should be no astigmatism
> if
> > the fiber is well centered on the optic.  Assuming you've correctly
> aligned
> > the collimator, I would think it is some other aberration you are seeing.
> >
> > Regarding your questions in that linked page:
> >
> > 1)  It depends on what you want to collect.  4f (or some other imaging
> > condition) will give you maximum light collection, which is likely what
> you
> > want if you have selected a multimode fiber.  Alternatively, if this is a
> > confocal system, it is probably not necessary.
> >
> > 2)  The diagram shows a collimated single mode fiber.  That should be
> > independent of distance.  If you find that your spot is changing rapidly
> > with distance, likely something is wrong with the collimation.  What are
> > you using a collimator?  Is it suitable for the NA and
> wavelength/bandwidth
> > of your source?  Is it well aligned?  What is the exact model of fiber
> you
> > are using.
> >
> > 3)  Most likely it is a problem with your coupler.
> >
> > Mike
> >
> > On Tue, Nov 4, 2014 at 11:36 PM, Yan, Lu <[hidden email]> wrote:
> >
> > > *****
> > > To join, leave or search the confocal microscopy listserv, go to:
> > > http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> > > Post images on http://www.imgur.com and include the link in your
> > posting.
> > > *****
> > >
> > > Hi folks,
> > >
> > > I am building a fiber based confocal microscopy setup (with sample
> stage
> > > scanning). But I always got some astigmatism aberration in PSF
> > measuremnts.
> > > The similar aberration was there even I replaced the objective lens
> with
> > a
> > > regular lens and imaged my illumination beam through that lens with a
> > > camera. I got elongated beam 'spot' on both sides of the focal plane,
> and
> > > the orientation of the two 'spot' were orthogonal. I think that is
> > > astigmatism aberration if I am not mistaken. I draw a schematic in
> > Evernote
> > > so I can include it here. Here is the link:
> > >
> > >
> >
> https://www.evernote.com/shard/s275/sh/55130807-98d4-4748-a4a9-64d19650b695/be0756284a13da18fe6d1f7f419cbcfe
> > > (copy and paste if the link does not work in email)
> > >
> > > I tried to adjust both lens in xy to avoid off-axis incident, but the
> > > aberration would go away. So I got confused where they came from. I
> hope
> > > someone here could lead me a direction to further look into it.
> > >
> > > Thanks very much,
> > > Lu
> > > -----------------------------------------------------
> > > ​​
> > >
> > > Lu Yan
> > > Nanostructured Fibers and Nonlinear Optics Laboratory
> > > Electrical and Computer Engineering
> > > Boston University
> > > 8 St. Mary St., Boston, MA, 02215
> > > [phone]
> > > -----------------------------------------------------
> > >
> >
>