Posted by
Matthew Nicholas-2 on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Liquid-light-guides-for-lasers-tp7580735p7580739.html
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Thanks very much for the information, Zdenek. Based on the behavior of
multimode fibers you mentioned, I wondered whether LLGs might exhibit
similar behavior. I'm actually interested in how lasers propagate through
LLGs in relation to illumination for widefield fluorescence microscopy
(TIRFM, to be specific). We have a setup with multiple lasers, which we
currently plan to guide through a broadband, single-mode fiber in order to
clean up the beam profiles (i.e. using the fiber as a spatial filter),
followed by collimation with a parabolic mirror. However, the power
capacity of the fiber is somewhat limited, and it's been my impression that
LLGs have far better throughput efficiency, at least for noncoherent,
extended sources like lamps (perhaps specifically because LLGs support
multiple transverse modes?). In addition, the Guassian profile is actually
somewhat undesirable, since it does not provide even illumination without
substantial beam expansion (and thus loss of significant power density in
the object field). I was doing some research on beam shaping and came
across a couple references that seemed to suggest the output of a LLG is
more uniform (i.e. a "top-hat" distribution, rather than Gaussian), and
that LLGs tend to eliminate coherence, e.g.
http://www.microscopyu.com/articles/livecellimaging/automaticmicroscope.html:"The most widely used and practical method of coupling a light source to
the microscope, while also reducing coherence, is to focus the light into a
flexible length of single-mode optical fiber or a liquid light guide (see
Figure 9). Thermal motion in the liquid light guide constantly alters the
optical path and scatters light so that both spatial and temporal coherence
are effectively eliminated. In the case of a coiled single-mode optical
fiber, the cladding reflections constantly change because the fiber flexes
slightly, producing an exit beam that is effectively uniform in intensity
over time and space. The technique of vibrating the fiber (at a frequency
of up to 100 kilohertz) is also effective in scrambling the light. The
light phase is scrambled due to the varying path lengths of light waves
passing through the fiber, although the high radiance and monochromaticity
are preserved. The exit beam is described by a *top-hat* intensity profile
rather than the Gaussian profile that is characteristic of laser light...."
So basically I was wondering if a LLG might provide more even illumination
and fewer interference effects (speckle, etc.). My intuition told me "no,"
especially since I've never seen this done by others, but I was curious.
Thanks very much for sharing your experience.
Cheers,
Matt
On 7/20/2013 1:48 AM, Zdenek Svindrych wrote:
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Dear Matt,
I don't have much experience using LLGs with lasers... But as fat as I know
you can get TEM00 only from a single mode fiber (that is some five to ten
microns in diameter). With thicker fibers more modes can propagate and the
output beam profile depends on which modes you excite, e.g. it depends on
the angle of input beam. The far filed output of a 300 micron (pretty thick)
fiber can easily look like a ring with dark center.
With LLGs it's even more severe, as they are far thicker (3 to 5
millimeters). You may try it yourself by shooting your cheap green pointer
(these usually have surprisingly nice mode structure, crapped only by the
plastic lenses inside) into an LLG. The output is far from gaussian...
Still there is some coherence in the output, and probably you'll get a lot
of speckle. It depends on your application, but most of the time it is not
practical to use LLGs with lasers. You loose all the nice features lasers
(diffraction-limited performance) and the bad features remain (speckle).
So, the last bit: Why do you want that?
Cheers,
zdenek svindrych
---------- Původní zpráva ----------
Od: Matthew Nicholas <
[hidden email]>
<
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Datum: 19. 7. 2013
Předmět: Liquid light guides for lasers?
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Dear list,
I'm wondering if anyone has any experience using liquid light guides with
lasers? In particular, I'm interested in what the beam profile exiting the
fiber typically looks like (assuming a Gaussian/TEM_00 input), and whether
the emitted light is still spatially coherent. Any experience anyone could
share (including products with which you've had success) would be quite
helpful and much appreciated! I'd also be very interested if anyone can
suggest a good reference on optical properties of liquid light guides that
includes discussion of laser applications. So far, I have not been able to
find much, and unfortunately I have zero personal experience in this area.
Thanks in advance for any help you can offer,
Matt