Dear All
I agree with Johannes that a beam will not result from the output of a
small fibre. What will emerge is a cone of light with a X-section that
depends on the modes present. If the fibre is small enough (say < 5x
the wavelength) then a Gaussian X-section will result. This will
certainly produce a local illumination of a cell or bunch of cells. I
cannot thee the prblem with coupling the laser in unless 1) the
desired laser power is high or 2) you are terribly worried about
efficiency. A UV microscope lens should work just fine for this
purpose since there is no real nead for diffraction limted
performance. The tapering of a gradient index fibre should help
maintain overall throughput. I believe that an alternative approach
might be to use a small GRIN lens which should not asbsorb all the UV
if short enough. This should also produce a local spot, but of course
the illumination will still be a cone.
I hope this helps.
Regards Mark Cannell
On Tue, 25 Feb 1997 17:24:47 +0100 Johannes Helm wrote:
> From: Johannes Helm <[log in to unmask]>
> Date: Tue, 25 Feb 1997 17:24:47 +0100
> Subject: Re: Light microspot and some more...
> To: [log in to unmask]
>
> In answer to the following letter by Dr M Cannell:
> >
> > Sorry, but I kind of disagree with Johannes.
> >
> > You can use a fused silica fibre. These are available in various
> > dimaters but I am not sure if single mode fibres are available. It
> > might be possible to draw the fibre in a flame to the 2 um or so
you
> > may need.. Drawing the fibre willl reduce throughput.
> >
> > For coupling the laser to the fibre, use a microscope objective
with
> > low numerical aperture -similar to the fibre NA.
> >
> > I hope this helps.
> >
> > Regards
> >
> > Mark Cannell
> >
> >
>
> Good afternoon.
>
> If I understand Mark's letter correctly, Mark identifies the
diameter
> of the core on the output side (you write 2um, Mark) with what
Susana
> calls "spotsize". So the idea is to use the micromanipulator in
order
> to push the output end of the fiber very close to the region of
interest
> in the cell and then assume that fluorescence is more or less
exclusively
> generated in a small region around this output end. (Do I get this
in the
> way you meant it Mark, or did I understand it in a wrong way?
Please,
> correct me in this case.) I do, however, have some doubts about the
> validity of this assumption. Might be I am, again, too pessimistic
> because thinking too much in terms of optics and not facing the
> experimental problem realistically, but as far as I know, the laser
light
> looses its beam-properties if a standard fiber from fused silica
> is applied.
>
> Having looked for UV fibers in the catalogues of several producers
of
> fibres, both multi mode and single mode, polarization conserving or
not,
> I found that no single mode and polarization conserving fibers are
available
> for wavelengths in the UV. Of course, as Mark has written, one can
> purchase fibers from fused silica, but they won't transport the
laser
> beam as a laser beam; what will remain of the "beam" upon
outcoupling
> are simply speckles. Therefore, I fear, it will be difficult to
> calculate the size of a "spot" of the light which will be coupled
> out of the fiber. Since Susana was asking for a possibly small
> size of a light spot and was considering to use a fiber, I expressed
> my doubts about the feasibility. What I think is that the spot size
> which can be generated using a high NA water immersion lens for UV
> will be smaller than the spot(s) or the array of spots which will
> be generated using a fiber. Especially, while it will be difficult
in this
> special case to calculate or estimate the size of the area or volume
> illuminated by the fiber-technique, the PSF of the objective will
allow to
> properly do this estimation.
>
>
> As what concerns the coupling of a UV laser beam into a single mode
fiber:
> As far as I know it has not yet been done. My state of information
is,
> however, three or four years old. At that time, I had talked to Dr.
> Wodrich from the Schott Glass Factories, Mainz and Wiesbaden, FRG,
> Dpt. of Fiber Optics. They had at that time decided to not continue
> developmental work on single mode UV fibers due to the difficulties
which
> had come up upon trying to couple the laser beam into the fiber.
>
> It might well be that this information is outdated and that somebody
> who knows more about fiber optics will be so kind and tell us. Thank
> you in advance.
>
> Another alternative to minimize a spot-size would, of course, be to
use
> a CSLM and illuminate the specimen via a high-NA lens with IR light
from
> a TiSap laser generating 2-photon excitation. Several papers have
been
> published about successful experiments applying this technique. It
is,
> however, a very expensive method.
>
> Best regards
>
> Johannes
> --
>
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*******
> Paul Johannes Helm
>
> Mailadress: Department Physics 4
> The Royal Institute of Technology
> S-100 44 Stockholm
> Sweden
>
> Visitingadress: Department Physics 4
> The Royal Institute of Technology
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>
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