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| FS... |
 Posted: Thu Oct 15, 2009 5:47 am |
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This is a simple problem-I send a collimated beam of white light down
one path. It strikes a absorptive coated mirror and returns back down
the same path. Can I detect the new color/spectrum of the light on the
returned beam? I would obviously use a splitter to retrieve the beam.
Would there be any interference/SNR problems with the original beam
performing this experiment
I can repeat the experiment sending the broadband beam down an optical
fibre and back.
I cannot think of any reason why this would not work-the beams of
light are directed with different Poynting vectors and are two
independent waves with no phase correlation, so I expect they would
each carry their own frequency and modulation information independent
of one another. I will try to get around to this in the lab this month
but I am interested in any real world experience here.
Thanks,
Fritz |
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| Uncle Al... |
| Posted: Thu Oct 15, 2009 6:47 am |
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FS wrote:
[quote:246689d4e3]
This is a simple problem-I send a collimated beam of white light down
one path. It strikes a absorptive coated mirror and returns back down
the same path. Can I detect the new color/spectrum of the light on the
returned beam? I would obviously use a splitter to retrieve the beam.
Would there be any interference/SNR problems with the original beam
performing this experiment
I can repeat the experiment sending the broadband beam down an optical
fibre and back.
I cannot think of any reason why this would not work-the beams of
light are directed with different Poynting vectors and are two
independent waves with no phase correlation, so I expect they would
each carry their own frequency and modulation information independent
of one another. I will try to get around to this in the lab this month
but I am interested in any real world experience here.
[/quote:246689d4e3]
KISS: Short single pulse it instead of CW - the pulse still passes
through itself. Train of pulses and use phase-lock detection.
"Optical correlation with totally incoherent light," Opt. Lett. 24
1469 (1999)
http://www.optik.uni-erlangen.de/jabe/pdf/page_22.pdf
Photons are clever and naughty. Assume nothing.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2 |
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| FS... |
| Posted: Thu Oct 15, 2009 7:50 am |
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Hmm-thanks Al- however:
you sent me the article "Super Resolution through Turbulence"-not
sure it applies here-perhaps the wrong article?
We bouncing off a target at less than on 1cm. The turn around time
difference would be sub nanosecond which would stretch our hardware
development limits here. I run a LIBS laser spectrometer system and
can usually crank within 20ns-not better- and that is some pretty
heavy, power sucking pulse gens and a qswitched laser. We are looking
at something compact that can run on batteries here.
Hence my quest to use continuous light (not necessarily a laser).
Thanks,
Fritz |
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| Uncle Al... |
| Posted: Thu Oct 15, 2009 9:41 pm |
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FS wrote:
[quote:feac2ae320]
Hmm-thanks Al- however:
you sent me the article "Super Resolution through Turbulence"-not
sure it applies here-perhaps the wrong article?
We bouncing off a target at less than on 1cm. The turn around time
difference would be sub nanosecond which would stretch our hardware
development limits here. I run a LIBS laser spectrometer system and
can usually crank within 20ns-not better- and that is some pretty
heavy, power sucking pulse gens and a qswitched laser. We are looking
at something compact that can run on batteries here.
Hence my quest to use continuous light (not necessarily a laser).
[/quote:feac2ae320]
Point taken. A miniature xenon strobe from a reusable camera; an LED,
a laser diode. High intensity CW operation means heat. Pulsed will
get you there with less dissipation.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2 |
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| Rich L.... |
| Posted: Fri Oct 16, 2009 6:36 am |
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There should be no problem doing this as you propose. Photons do not
interact with each other, so there will be no interference between the
two beams. The only possible effect would be incident light
scattering off the beam splitter and into the detector. You didn't
say how much light would be absorbed and reflected. If very very
little light is reflected you may need to be a little bit clever about
separating the incident light scattered at the beam splitter from the
signal you are trying to observe. If necessary you could use
polarized light, a polarized beam splitter and a polarization rotator
between the splitter and the sample, assuming the sample does not
rotate the polarazation itself. Even better would be to conduct the
experiment at a small angle so that the reflected light is physically
separated from the incident beam, if the physical setup allows that.
Rich L.
========== Moderator's note ==========================================
There are some misunderstandings in this posting, concerning em. waves
(light):
First of all interference has nothing to do with interaction. It is an
effect of the superposition of wave. If two beams of light show interference
effects of not, is a question whether they are coherent or not.
Second, to be very picky, in fact photons are interacting. However that is a
very weak quantum correction with a cross section of fourth order in \alpha_em. |
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| Rich L.... |
| Posted: Sun Oct 18, 2009 10:57 pm |
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On Oct 16, 11:36 am, "Rich L." <ralivings... at (no spam) sbcglobal.net> wrote:
[quote]There should be no problem doing this as you propose. Photons do not
interact with each other, so there will be no interference between the
two beams. The only possible effect would be incident light
scattering off the beam splitter and into the detector. You didn't
say how much light would be absorbed and reflected. If very very
little light is reflected you may need to be a little bit clever about
separating the incident light scattered at the beam splitter from the
signal you are trying to observe. If necessary you could use
polarized light, a polarized beam splitter and a polarization rotator
between the splitter and the sample, assuming the sample does not
rotate the polarazation itself. Even better would be to conduct the
experiment at a small angle so that the reflected light is physically
separated from the incident beam, if the physical setup allows that.
Rich L.
========== Moderator's note ==========================================
There are some misunderstandings in this posting, concerning em. waves
(light):
First of all interference has nothing to do with interaction. It is an
effect of the superposition of wave. If two beams of light show interference
effects of not, is a question whether they are coherent or not.
Second, to be very picky, in fact photons are interacting. However that is a
very weak quantum correction with a cross section of fourth order in \alpha_em.
[/quote]
Dear Moderator,
I understand all that. As a practical matter (the original poster was
asking about an experimental set up) there would be no significant
interaction between the outgoing and returning beam (as was implied by
Uncle Al's post), nor would there be any interference effects at the
detector as the poster was suggesting a beam splitter to separate the
outgoing and returning beams. The suggestion that super short pulses
be used to avoid such effects is unnecessary and would be a huge
complication to the experimental set up. That is the point of my
reply.
I realize that there is a theoretical photon-photon interaction in
vacuum, but to the best of my knowledge this has not been observed in
the absence of some non-linear material in addition to the vacuum. As
an experimental issue it is not relevant, especially at ordinary
optical power densities.
Rich L. |
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| ... |
| Posted: Tue Oct 20, 2009 5:45 am |
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Rich L. <ralivingston at (no spam) sbcglobal.net> wrote:
[quote][...]
========== Moderator's note ==========================================
[/quote]
Mod> Second, to be very picky, in fact photons are interacting. However
Mod> that is a very weak quantum correction with a cross section of fourth
Mod> order in \alpha_em.
But earlier, FS said:
FS> I can repeat the experiment sending the broadband beam down an
FS> optical fibre and back.
... and in an optical fibre, its third order nonlinearity would
couple the forward and backward beams. And there'd even be some
weaker nonlinear effect in air, which I'd guess would dominate
that of \alpha_em, but I haven't looked at the numbers.
Or did I miss the point of the fibre remark?
--
---------------------------------+---------------------------------
Dr. Paul Kinsler
Blackett Laboratory (Photonics) (ph) +44-20-759-47734 (fax) 47714
Imperial College London, Dr.Paul.Kinsler at (no spam) physics.org
SW7 2AZ, United Kingdom. http://www.qols.ph.ic.ac.uk/~kinsle/ |
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| Rich L.... |
| Posted: Wed Oct 21, 2009 7:46 pm |
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On Oct 20, 10:45 am, p.kins... at (no spam) ic.ac.uk wrote:
[quote]Rich L. <ralivings... at (no spam) sbcglobal.net> wrote:
[...]
========== Moderator's note ============================================
Mod> Second, to be very picky, in fact photons are interacting. However
Mod> that is a very weak quantum correction with a cross section of fourt=h
Mod> order in \alpha_em.
But earlier, FS said:
FS> I can repeat the experiment sending the broadband beam down an
FS> optical fibre and back.
.. and in an optical fibre, its third order nonlinearity would
couple the forward and backward beams. And there'd even be some
weaker nonlinear effect in air, which I'd guess would dominate
that of \alpha_em, but I haven't looked at the numbers.
Or did I miss the point of the fibre remark?
--
---------------------------------+---------------------------------
Dr. Paul Kinsler
Blackett Laboratory (Photonics) (ph) +44-20-759-47734 (fax) 47714
Imperial College London, Dr.Paul.Kins... at (no spam) physics.org
SW7 2AZ, United Kingdom. http://www.qols.ph.ic.ac.uk/~=
kinsle/[/quote]
I have to admit I was thinking free space rather than fiber when I
wrote my comments. I don't have direct experience with this sort of
thing in optical fibers. Would the third order non-linearities be
strong enough to be a practical experimental concern? I suppose the
answer to that really depends on the ratio of the incident light
relative to the reflected signal he is trying to measure, and to the
overall power levels involved. My own intuition (which may be
ignorant) is that at ordinary light levels (milliwatts or less) that
the nonlinearities are still insignificant. If you (or any one else)
knows otherwise I'd appreciate hearing about it.
Rich L. |
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| Keith Blow... |
| Posted: Thu Oct 22, 2009 7:05 am |
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On Thu, 22 Oct 2009 06:46:50 +0100, Rich L. wrote:
<snip>
[quote]
I have to admit I was thinking free space rather than fiber when I wrote
my comments. I don't have direct experience with this sort of thing in
optical fibers. Would the third order non-linearities be strong enough
to be a practical experimental concern? I suppose the answer to that
really depends on the ratio of the incident light relative to the
reflected signal he is trying to measure, and to the overall power
levels involved. My own intuition (which may be ignorant) is that at
ordinary light levels (milliwatts or less) that the nonlinearities are
still insignificant. If you (or any one else) knows otherwise I'd
appreciate hearing about it.
Rich L.
[/quote]
The rule of thumb for standard single mode fibre is a nonlinear pi-phase
shift requires 1Wkm. So for delivery over a few meters your need kW peak
powers before you notice anything.
The separation of forward and backward signals is a standard task in
fibres. A simple 50:50 coupler will do this extremely well. This sort of
thing is done in optical time domain reflectometry which is used to
measure the length dependent fibre loss via the Rayleigh backscattered
light.
--
Keith Blow |
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| Rich L.... |
| Posted: Mon Oct 26, 2009 12:32 pm |
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On Oct 22, 12:05 pm, Keith Blow <keithH... at (no spam) HIDEbackblow-
loft.demon.co.uk> wrote:
[quote]On Thu, 22 Oct 2009 06:46:50 +0100, Rich L. wrote:
snip
I have to admit I was thinking free space rather than fiber when I wrote
my comments. I don't have direct experience with this sort of thing in
optical fibers. Would the third order non-linearities be strong enough
to be a practical experimental concern? I suppose the answer to that
really depends on the ratio of the incident light relative to the
reflected signal he is trying to measure, and to the overall power
levels involved. My own intuition (which may be ignorant) is that at
ordinary light levels (milliwatts or less) that the nonlinearities are
still insignificant. If you (or any one else) knows otherwise I'd
appreciate hearing about it.
Rich L.
The rule of thumb for standard single mode fibre is a nonlinear pi-phase
shift requires 1Wkm. So for delivery over a few meters your need kW peak
powers before you notice anything.
The separation of forward and backward signals is a standard task in
fibres. A simple 50:50 coupler will do this extremely well. This sort of
thing is done in optical time domain reflectometry which is used to
measure the length dependent fibre loss via the Rayleigh backscattered
light.
--
Keith Blow
[/quote]
If I understand this correctly, using ultra fast pulses in the fiber
might actually make the non-linear effects worse since the peak powers
would be so high (a 1mW average power consisting of 100fs pulses at
80MHz would give a peak power of about 125KW peak power in the pulses,
so that even over a very short length of fiber there might be a
significant non-linear effect. It would clearly be much better to do
this CW!
Rich L. |
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