Johannes is correct that there are no single mode UV fibers out there - or
at least as of a year ago no one claimed any soon to be available.

I know some guys at Bell Labs that tried making NSOM (Near Field Scanning
Optical Microscope) tips from UV grade fused silica about 1.5 years ago any
could not get it to work.

Certainly, you can buy single mode fibers for the visible.  But using them
in an NSOM fashion is painful for the uninitiated.  Since a cell is "large",
a far field focused optical probe sounds like a much better approach.

----------------------
At 05:24 PM 2/25/97 +0100, you wrote:
>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
>--
>*****************************************************************************
>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
>                Teknikringen 14/4tr.
>                S-100 44 Stockholm
>                Sweden
>
>Voice:          +46 8 790 7219
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>

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