Posted by
Craig Brideau on
URL: http://confocal-microscopy-list.275.s1.nabble.com/405-laser-intensity-at-the-objective-is-0-15-of-actually-intensity-is-this-normal-tp7587264p7587292.html
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Pete's estimates feel optimistic to me when you take into consideration
that multiple wavelengths are typically coupled down one single mode fiber
for microscopy applications. For one-laser-one-fiber applications Pete is
quite correct as the vendor can optimize the fiber and coupling optics for
one wavelength. For multiple lasers down a single fiber, however,
compromises must be made, and coupling efficiency <50% are not uncommon for
diode lasers. As Zdenek says, some fibers are not single mode for 405nm. To
expand on this, single-mode fibers are only single mode to a rated
wavelength known as the cutoff wavelength. Below the cutoff wavelength, the
core of the fiber is actually too wide to act as a single mode waveguide
for a short wavelength. Typically 405nm suffers from this, as again most
systems were initially designed for ~450nm. Zdenek's test of checking the
pattern for each line out of the fiber should give you some idea of how
well it performs for all the laser lines. In my own experience, at least on
slightly outdated coupling systems, 405nm often goes multi-modal in
extended band single mode fibers. You will see two lobes in the output
instead of a single Gaussian spot if this should happen. You may also
notice strange behavior if you bend the fiber in different ways as the
stresses will alter the properties of the fiber, especially impacting
shorter wavelength boundary cases where the wavelength of light is near the
cutoff for single mode behavior in the fiber.
Craig
On Mon, Sep 11, 2017 at 10:16 AM, <
[hidden email]> wrote:
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>
> Also, some single mode fibers are actually not single mode at 405 nm. The
> higher modes won't be useful for fluorescence excitation in a confocal
> microscope... So just check the far filed pattern from your fiber when
> aligning the 405 line to make sure you're only coupling to the TEM(0,0)
> mode
> - it should be nice gaussian when you point the bare fiber at a sheet of
> common white (fluorescent ) paper.
> --
> Zdenek Svindrych, Ph.D.
> W.M. Keck Center for Cellular Imaging (PLSB 003)
> Department of Biology,University of Virginia
> 409 McCormick Rd, Charlottesville, VA-22904
>
http://www.kcci.virginia.edu/> tel: 434-982-4869
>
> ---------- Původní e-mail ----------
> Od: Peter Brunt <
[hidden email]>
> Komu:
[hidden email]
> Datum: 11. 9. 2017 10:45:05
> Předmět: Re: 405 laser intensity at the objective is 0.15% of actually
> intensity ... is this normal?
> "*****
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>
> Hi Emmanuel,
> For singlemode fibre coupling across multiple wavelengths ranging from 405-
> 642nm I'd typically expect >60% coupling efficiency for all lines in the
> worst case. For 405nm, 488nm and 561nm I'd actually expect closer to ~70%
> on
> the basis that most fibre coupling systems (industry wide) aren't entirely
> achromatic and the closer the wavelengths, the easier it is to adjust the
> coupling optics to maximise the efficiency.
>
> Still, if we assume there is still some loss on the dichroics, you should
> still expect ~54% average power out of the fibre. I think there is a good
> chance your lasers / fibre coupler need realigning. This is somewhat
> evidence by the fact that your coupling seems to be biased towards 561nm.
>
> I hope this helps.
>
>
> Pete Brunt
>
> AVR Optics
> "
>