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watson.fawkes
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Quote: Originally posted by un0me2  | [...] while also trying to fuck aroudn with the control panel for the camera, playing with exposure, gamma, etc.
When I remove the camera (or more appropriately, when the camera removed itself) and look through the viewport I'm seeing extremely narrow, very well
resolved lines (such as I've come to expect), but capturing them on the CCD is rather more difficult than I had expected. |
What's your focus setting? You'll almost certainly need to use a manual setting.
As for calibration, the cheapest molecular source I know of is a sodium vapor lamp. It has a doublet line that's particularly amenable to software
recognition. For automatic re-calibration (say, in the field), some LED's would suffice, but you'd need to characterize them first.
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unome
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Here's one using the other camera with a long-life, smart-bulb
For the Raman, I am seriously considering taking one of those super-bright Luxeon diodes (Royal Blue, 700mW) and then buying one of the diode to fiber
couplers, then using the bought one as the basis for making a cobalt-blue one in acrylic.
That will (1) colimate the light and (2) remove everything but a small bandwidth from getting through. One filter on the other side and boom, we'll
know whether or not it has any functionality at all.
[Edited on 27-2-2010 by unome]
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unome
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Same bulb, but using a transmission grating - ~70' front to back (ie. front is closer & @ the bottom), slit is about 40mm long, about 1mm thick.
Another one, starting to get the resolution I'm after FINALLY
That last one actually splits the two reds and then splits the blue into about 3-4 parts, so the resolution would have to be up there wouldn't it?
[Edited on 1-3-2010 by unome]
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unome
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Right, I took this narrow slice out of a transmission grating shot - notice the resolution? There are some very narrow, VERY finely resolved lines,
even the green is resolved into two in the 8-bit version (greyscale).
And here is the 3D image of it from ImageJ
Check out that doublet in the green
Woo-fucking-hoo
Now, if I lay that slice horizontally and increase it's width to match the graph (just stretch the image), plus swap it around so that the 400nm
region is on the left (instead of the right), then we have something recognisable as a graph of wavelength v intensity . Now all I need to do is get some decent, fairly low power LED's (high powered
white LED's are too fucking bright for this little baby, they wash out the spectra) and play around a little so I can start getting absorption spectra
from colored solutions, then we can play...
After that, try and get a laser to excite the fuck out of organics and get the Stokes/Anti-Stokes (fairly high-powered NIR diode lasers are easier to
get than blue/green and a whole lot cheaper - plus the IR-filter on the CCD should really block the fucking laser line if one chose wisely).
[Edited on 5-3-2010 by unome]
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not_important
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| Quote: | | As for calibration, the cheapest molecular source I know of is a sodium vapor lamp. |
As I've said before, neon lamps will give you a reference for less than a Euro - including the control chip to let you switch it on and off using a
printer port pin. Several of the papers on making a laser diode based Raman spectroscope used neon lamps for their references. For about twice the
cost you can get a similar argon lamp. giving several strong lines in the violet end of the spectrum.
| Quote: | | For the Raman, I am seriously considering taking one of those super-bright Luxeon diodes (Royal Blue, 700mW) and then buying one of the diode to fiber
couplers, then using the bought one as the basis for making a cobalt-blue one in acrylic |
Way too broad emission, it'll be quite weak by the time you've filtered it down to a narrow enough band.
| Quote: | | After that, try and get a laser to excite the fuck out of organics and get the Stokes/Anti-Stokes (fairly high-powered NIR diode lasers are easier to
get than blue/green and a whole lot cheaper - plus the IR-filter on the CCD should really block the fucking laser line if one chose wisely). |
Similar problem, the VNIR laser diodes are fairly broad band emitters for a laser, multimode as anything, and drift badly. For almost purpose they are
used to drive some solid to lase to get a useful light source, not directly as lasers.
The IR filter on cameras generally doesn't have that sharp of an edge, nor are they dense enough to significantly attenuate the laser frequency. On
top of that they'd absorb in the region of the Stokes emissions. Also the camera's CCD will not be sensitive to part to much of the Stokes lines -
they'll be too far to the IR side. And the anti-Stokes emission is much weaker than the Stokes.
Remember that white LEDs have a strong output in the blue, plus a broad yellowish peak; it's no where near black body (like a quartz-halogen) much
less flat (like a good xenon arc). You'll need to calibrate the base spectrum intensity and use that to tweak the absorption spectrum, or build a rig
that illuminates the CCD with 2 bands - raw white LED output and that transmitted through the sample, then let the software 'difference' the two. Add
a third band for light from a neon bulb and you've got calibration at the same time.
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unome
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Nice to hear from you not_important...
Yeah, I've been looking at the little neon bulbs that can be slotted into circuits (some even come on boards, just connect 'em), for calibration
purposes, they draw fuck all current and aren't going to wash out the spectrum in the minispectrometer...
You say that the Luxeon LED's are too wide a wavelength, but the datasheets seem to suggest that at most there is a half-width of about 10nm and that at full-power, the dominant emission is fairly narrow. Given
the massive (compared to 5mW laser pointers) amount of light emitted from the 0.7W Blue LED's (dominant wavelength ~440-460nm @ 700mA), I still
suspect it is worth a try... Put that through the relevant filter and if the specs are telling the truth then I'd be expecting at least half of the
total emission to fall within the necessary waveband, if it doesn't work - nothing ventured = nothing gained
If I can narrow the output band from the LED's down with filters, I will, but I'm working backwards from what Sir C.V. Raman did, a blue filter to get
only blue light (obviously He concentrated a shitload more light than I'm going to, but if 5mW lasers can do it, I cannot see why this cannot). The
main purpose behind trying to utilize LED's rather than laser diodes, is the reported problems with them for this application and the costs involved
in trying to source 'proper' lasers... For proof of concept, ANY Stokes excitation that can be measured will suffice, if it is recognisable that is a
bonus. If that point can be reached using low-budget, low-tech gadgetry and some workarounds, then that gives me hope (in much the same way that being able to produce a half-decent graph of the
wavelength v intensity, using freeware and way out of date, cheap as shit digital cameras did above).
I am sorry to hear that about the NIR lasers, I was hoping that I could utilize the insensibility of the CCD sensor to the NIR emissions to remove the
laser line and the Stokes-excitation (which is increased wavelength from the laser line if I have it right, ie. NIR too), leaving only the Anti-Stokes
(lower wavelength than the laser line - ie. visible), which given sufficient power in the excitation laser, I presumed would be able to be utilized
(pretty much using the weaknesses/nature of the accessible OTC equipment to improve the capability of an OTC sensor). Is there any way to improve the
utility of the 200mW NIR (<800) Laser's?
I was thinking if the Stokes radiation from a 5mW laser is measurable, there has to be some hope that the Anti-Stokes radiation, from a much more
powerful excitation laser would be at least measurable (and thus useable). The IR filters on the cameras don't have that sharp an edge, but if the
laser is above it, then it wouldn't register would it? Neither would the Rayleigh Scattered or Stokes Excitation? Leaving only the Anti-Stokes to
contend with? Thus capitalizing on the inherent weakness of the chosen sensor?
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unome
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Righto, I've worked out what I want to do to try and prototype this and see if it works - I can (by the looks of a local site), get a small kit to run
one/two of those high-powered blue-LED's off a standard 9V battery, the same place sells neon bulbs and "white" LEDs (apparently, by the data sheet
not yellow phosphor on blue), which I intend to wire up to run off battery power too.
That saves me fucking around trying to set up breadboards/PCB's while prototyping.
I have some ideas for the filter(s), a dark blue filter will restrict the wavelength going into the solution and a yellow filter will allow everything
but blue out of the solution, perfect for my needs.
I'm experimenting here, it is far from a foregone conclusion it will work I'm
basing the concept upon what I've read (standing high and dry on the shoulders of some MAJOR giants). When I finally get all this shit together I'll post the details and probably some pictures of the setup and the
results (good or bad).
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not_important
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| Quote: Originally posted by unome | ...
You say that the Luxeon LED's are too wide a wavelength, but the datasheets seem to suggest that at most there is a half-width of about 10nm and that at full-power, the dominant emission is fairly narrow. Given
the massive (compared to 5mW laser pointers) amount of light emitted from the 0.7W Blue LED's (dominant wavelength ~440-460nm @ 700mA), I still
suspect it is worth a try... Put that through the relevant filter and if the specs are telling the truth then I'd be expecting at least half of the
total emission to fall within the necessary waveband, if it doesn't work - nothing ventured = nothing gained |
By the datasheet you referenced, the typical half-width for the royal-blue is 24 nm, a 2nm wide filter might get 1/5 the power output. The
distribution for the peal over a range of 20 nm, meaning there's a good chance that the peak is several nm off the typical; if the filter weere
centered on the value of the typical peak it would not be unlikely to find the actual LED's peak does not fall within the filter passband.
The light is emitted over a wide cone of 160 degrees, a bit of trickyness is needed to collect most of it. BTW, do be careful not to push on the
plastic dome covering the LED chip, some of these are fairly sensitive to mechanical stress applied there.
Raman's original work used near mono-chromatic line derived from sunlight, but most of what he did used a line in the mercury spectrum, thus a quite
narrow source.
Are you just trying to demonstrate the Raman effect, in which as simple and cheap gear can work, or actually use it as an analytical device?
You're tossing about filter choices, but do you know the expected shifts? I think that for a 500 nm excitation you're looking at shifts in the range
of 20 to 100 nm, so 520 to 600 nm would be the range to isolate detect.
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a_bab
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http://ledmuseum.candlepower.us/led/ - this site has lots of spectra measured for all kinds of LED's, lasers and such. You may pick up something
interesting there.
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unome
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So sorry, been busy (got work being done on the house & sundry other productions)...
First and foremost, I want to show that it can be done... Following that, I intend to work something out as an analytical tool, but first I would like
to demonstrate (mainly to myself more than anything) that it can in fact be done and from there, we'll take it as it comes.
What about the cheaper bandpass filters (half-width of 5nm, ie. full width of 10nm), that would take in the majority of the light from the diodes
cited wouldn't it? At a price of losing resolution, but yeah... I want to get something working, it can be be remade/remodeled later, I need a
prototype
There are LED optics (fucking cheap too) that are designed to work specifically with the Luxeon and/or Cree LEDs, they take the gaussian emission and
narrow it down to a 10' arc or even collimate it for fiber optics. I intend trying both, a 10' arc, through a microscope objective and then fiber
optics.
I was just thinking, given that slit width maketh the width of the spectrum, I could mount spectrometer so it is divided into three compartments with
only one grating, but three slits. All three spectra would show on the CCD image (sideways - so the long axis would pick up the spectra of all 3).
Build a neon bulb into the middle and each and every picture would be automatically able to be alligned (alignment with every use - that removes all
the mirrors, fancy tricks and everything - plus no moving parts), based upon the known peaks of the neon bulb.
Anyway, with the filters available now (or even just including a simple monochromator) I could utilize a 12V automotive spotlight bulb (tungsten
halide IIRC) and just take the wavelengths I want from that - given that they are 100-250W, that would give several watts of light in a given
wavelength, but I'm trying to be gentle The monochromator would be nothing more
nor less than a blue filter, which would then be narrowed further by filtration.
As it has been demonstrated that miniscule amounts of light are capable of causing measurable Stokes-shifted excitation, then that too would work
(probably cheaper than what I have outlined into the bargain). As the light source would not be operating continually for any real length of time,
heat would be manageable.
But not_important, I am merely trying to do what I think is possible, I appreciate your assistance and am not trying to argue with you, your knowledge
on this point surpasses mine by so great a margin as to be disturbing. All I am doing is showing that (a) lasers aren't NEEDED; and (b) that this will
be possible at home, sooner rather than later.
My reason for aiming at the bottom end of the visible scale is simple, that gives me the greatest chance of capturing the Stokes-shifted radiation
within the known limitations of the CCD/CMOS sensors. My reason for trying to avoid laser-diodes, is that they eat too much power and according to you
(and others) are of doubtful utility for this application.
[Edited on 8-3-2010 by unome]
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not_important
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There' a big difference between demonstrating the Raman effect, and making a useful piece of equipment. A demonstration can get away with an
excitation source of 10 or 20 nm bandwidth, taking an actual spectrum needs to be an order of magnitude narrower.
You need to understand the wavelength and intensity differences involved. Filter pass/block differences of a thousand are marginal, you need blocking
of optical density 5 or 6 for passable performance with better gear getting OD7 and above. At the wavelength of that blue LED, 4-acetamindo-phenol has
7 peaks within a 10 nm range; excitation light of more than a nm or so width will smear or merge those bands.
Laser diodes can work, but the data analysis needs to deal with their instabilities. There are Raman systems that use NIR laser diodes, 785 nm being
common, but those are special diodes rather than the low cost ones. The cheap ones are multimode, especially when operated much below maximum current;
even when operated in the single mode range they mode hop, meaning the output shifts several nm in wavelength. Special diodes are used, they include
methods that stabilise a single mode.
The roll-off from CCD limitations and its IR filter are not quick enough to block out the laser frequencies and lower Raman shift ranges. The roll-off
is enough to lose most of the C-H bond information, though.
Shorter wavelengths give higher Raman scattering values because of the wavenumber^4 proportionality. But they also give increased problems with
fluorescence, which can completely drowned out the Raman bands. Shorter wavelength also means narrower spectra, as the Raman shifts are fixed
wavenumber values regardless of the exciting light's wavelength. The Raman spectrum for 400 nm light is half the wavelength spread as that at 800 nm,
this means increased resolution is needed at the shorter wavelengths.
The attached spreadsheet lets you pick a wavelength and presents the resulting wavelengths of the Raman shift for a number of useful molecular
fragments.
And if you really want to learn more, someone seems to have uploaded Raman Spectroscopy for Chemical Analysis as a zip at http://www.filedropper.com/zooter
Attachment: Raman bands.ods (19kB)
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Polverone
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not_important, thanks for your contributions to this thread! Although the topic is obviously unome's baby I have a strong interest in his experiment
too, since I think bringing the cost of analytical instruments down could be a great boon for home chemistry.
Based on the holography pages a_bab linked to earlier, I get the impression that unstabilized single mode lasers may hop modes relatively slowly (i.e.
100 ms+ between hops). This is no good for holography but might be tolerable for spectroscopy: with fine enough temporal resolution during image
capture, you get fine spectral structure, frequency shifted when the mode hops, and could perform alignment and stacking on a spectral image
collection in software (cheap) to avoid the hardware expense and complexity of keeping the laser in a single mode during the full data acquisition
period.
So far I haven't been able to find clearer information than a_bab's holography links about the typical time to hop modes in diode lasers. I will do
some more searching but I wondered if you knew the information I seek or likely search terms and locations to try.
PGP Key and corresponding e-mail address
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un0me2
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Ok, couple of quick questions
All green (532nm) laser pointers are pumped (excited) with infra-red type laser diodes?
They all contain the Vanadate Crystal & the KTP crystal?
So what is stopping an individual purchasing one of these, taking it apart and multi-pumping the bejesus out of it with several 1064nm laser diodes
through standard fiber?
I've been reading up (this page is good, here is a decent picture of what the insides can/do look like and Sam's provides a shitload of information...).
What I find interesting, is it appears that solid, near-single mode lasers can be made using multiple laser diodes, given the Vanadate crystal and the
KTP (plus an IR filter), this should be around about the line we want?
If not, I'd really like some input into how to make a single-mode, 532nm laser using components that (a) won't break the budget; (b) will do the job
without mode-skipping and (c) are sufficiently powerful to provide us with Raman spectra
Any and all advice and every suggestion will be looked into
Attachment: Ch.3.Laser.Source.pdf (26kB)
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Attachment: Williamson.Holehouse.Demystifying.Fiber.Laser.Construction.pdf (347kB)
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not_important
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Yes, the mode hopping tends to be fairly slow. Some NIR Raman spectroscopes, particularly those from before the modifications that gave more stable
performance, did just as you suggest - watch the captures and adjust them if a mode hop occurred. If the laser line is several times greater than any
Raman line it's fairly easy to track it, and a bit more complicated to tweak the rest of the spectrum (dispersive systems tend to be linear based on
wavelength while Raman shifts are fixed wavenumber offsets)
The idea of effectively having several spectroscopes in one and recording several spectra at the same time can be useful for such an application, as
well as simplifying the calibration of the instrument. You duplicate the sample area and permanently place a reference sample in the 2nd. The software
locates the laser line, then the known peaks in the reference, and assigns wavenumber values to the subject based on those; as they are taken with the
same excitation source at the same time the correlation is meaningful. Using a neon lamp as a 3rd reference allows assigning absolute wavelengths as
well.
This assumes that the diode laser is running single mode, even if frequency hopping. For many diodes this means running them at higher currents to
kick them out of multimode operation; multimode gives multiple overlapping Raman spectra that have little use outside of demonstrating the principle.
Temperature control for the diode may be needed to maintain stability.
Note that this is more difficult when using a CCD camera to record the spectra than when using a standalone CCD under your control. The camera is
designed to take pictures under a certain range of conditions that do not closely resemble those inside a Raman spect machine. You can get results
that let you see strong Raman bands, but weaker ones may be inaccessible.
| Quote: | | So what is stopping an individual purchasing one of these, taking it apart and multi-pumping the bejesus out of it with several 1064nm laser diodes
through standard fiber? |
First off the Nd:YVO4 generates 1064 nm when pumped with 808 nm diode lasers. Among other things it allows the combining of the output of a number of
diodes to give a single beam; you can't combine the diodes into such an intense beam using passive (filber, lenses, &ct) means. The loses aren't
too bad in doing so. You want to do this because laser diodes are temperature dependent, broad linewidth, difficult to impossible to get really
tightly focused beams, - all of which makes for a poor match to the doubler NLO KTP. You would get some green output, but conversion efficiencies will
run well under 1%.
The cheap DPSSFD lasers such as pointers end pump the Nd:YVO4, for higher power you need to do side pumping and I doubt the vanadate crystal has been
ground for side pumping. After that putting too much power into it will damage or destroy the crystal; the same is true for the KTP.
You'll need to focus the diodes output into spots a few microns in diameter; fiber just makes it easier to stack beams from multiple diodes closely
side by side.
As you increase the power going into the laser and NLO crystals the power dissipation in them increases, and they will need more cooling. At higher
power levels better control of the crystal's temperature is needed, not just simple cooling but maintaining it within a narrow range. This means TEC
temperature controllers, and understanding the paramters of the optical crystals.
On top of that you really don't want too high of power, especially if you're not willing to invest a lot of effort in enclosures and reflection
control. The IR lasers, both diode and vanadate, in a 5 mw green laser pointer are strong enough to damage your eye in a moment. Power levels above a
few hundred milliwatts start fires; catching such a beam in your eye results in your hearng a 'pop' or 'crack' sound and then noticing you seem to be
having trouble seeing out of one eye.
Keeping down stray reflects gets real difficult. Black paint isn't really that black, it reflects several percent of the light hitting it. With high
power lasers you're faced with both the problem of the absorbed energy and dealing with the few percent now bouncing around. Filters may not handle
the power levels, instead you use reflective grating and beam dumps to dispose of unwanted light.
Laser pointer levels of intensity will generate Raman spectra, the ratio between exciting and shifted intensities is a major difficultly. Boosting the
intensity of the exciting light may make detection easier, but introduces problems with sample heating and human safety.
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un0me2
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Ok - so what if I use a basic laser diode (like the one out of a DVD-RW, as extracted here and pump that straight into the back of the Nd:YVO4/KTP crystals?
I am awaiting a price on these crystals, they appear to be fairly cheap and can even be purchased in sets
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not_important
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Very little, to maybe something.
DVD diodes are ~650 nm, CD drives use ~780 nm. The absorption spectrum for the 0.5% doping level is attached. The weak bands become stronger at
higher doping levels, so in part the answer depends on the doping level, and in part on the size of the Nd:YVO4. You can pump big slabs off the peaks,
that way the pump energy can penetrate deep enough into the slab. But with a bit a few mm in cross section much of the pump energy will just go
through it.
The optical drive lasers are a few mw for the read laser, and around 100 mw for the write laser. I beleave the pump laser in a 5 mw green pointers is
around a quarter watt, certainly at least 100 mw ( ~80% up conversion efficiency, maybe 10% doubling)
[Edited on 10-3-2010 by not_important]
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un0me2
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So I'm going to need to find a decent source of the 1W Sony 808nm diodes?* OK, trouble is our nanny state has banned anything serious laser wise here
(goons were pointing them at pilots on approach at the main airports), so I'd (1) have to find a supplier and (2) get one who would write on the
package - electronic parts assorted.
It is illegal to purchase anything over 1mW without licensing and a whole lot of paperwork in terms of finished pointers, diodes are something else
again, they are electro-optics and appear to be exempt (at present). I'm still awaiting replies from various suppliers of the Vanadate:KTP
coupled-crystals, although given that the epoxy(??) that is used to join them doesn't like high pump power, I may have to go with separate crystals
in-line.
Now, what, in your opinion, would be the 'BEST' strength of 532nm light for amateur Raman Spectroscopy?
... I don't mean the lowest possible and I don't need beast-design, I mean something that is going to give reasonable results, time and time again,
with the least issues (from crystal heating, diode breakdown, etc.). Remember this is amateur, I'd like to resolve as many of the spectral
characteristics as possible, doesn't mean I'll be able to, but that is the goal.
Now, the other question would be, how will building these solid-state mitigate the mode-hopping behavior?
* If anyone has a decent source of these things please share, I'm getting overwhelmed with the sheer volume of so-called honest salesmen that I'm
dealing with at present, many of whom seem to speak a bizarre blend of pigin english and pseudo-tech geek
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not_important
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Pulling a functional system together out of components isn't going to be real easy. You're going to need _good_ 3 wavelength safety goggles, the 808
and 1064 will be on the order of 100 to 1000 milliwatts and quite capable of damaging your eye in no time at all. The upper end of that range and
above is getting a bit tricky to work with, not to be lightly undertaken.
Putting one together yourself might allow you to pick a diode that you know can be brought into single mode operation. Any diode that doesn't have the
potential for mode hopping is quite likely to be rather expensive, it's better to design a system that lets you detect a mode hop and deal with it in
software. Using a CCD camera is going to limit performance anyway, so there's little incentive for really good lasers anyway. Good optics is as
important as the laser itself, filtering to clean up the beam, suppressing it in the collected Raman emissions, tight control of stray light inside
the dispersive optics; all that helps the performance.
http://www.dealextreme.com/details.dx/sku.26887
20mW 532nm Green Laser Module (3V : 250mA 11.9mm)
Pick up a 20-pack of UV LEDs, a couple of 10-packs of the 14000 mcd whites, something else out of the DIY category, something computer related, maybe
a digital scale (I'll accept the 3 Kg/0.1g one), whatever else is useful and fills out your order.
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unome
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Ok, I have ordered the suggested diode-laser modules - any suggestions on where to get the safety goggles?
I mean, given the numbers, you are saying that we would be dealing with Class IV (>500mW) levels of the 808/1064 and IIIb (5-500mW) levels of the
532nm? Surely the better idea would be to prevent the escape of the 808/1064nm light altogether? Presumptively there are filters that WILL remove them
from the scene (the 1064 is a real hazard, given it is non-visible).
On that subject, if something like this were built (from the attached paper) - using straight out CD/DVD type reflective gratings, we could (provided
we could work out a way to absorb the stray wavelengths) narrow the beam considerably, perhaps even more than with the laser-line filters?
From what I can make of the design, the reflective grating acts as a monochromator - effectively only transmitting (provided the stray wavelengths are
absorbed) the line that it is calculated to reflect on that angle (dependent upon grating and the size of the mirror/hole...).. While this looks
somewhat advanced for what I am trying to do, a modified version, with a moveable reflective grating, totally enclosed with the ONLY vision being via
CCD camera (I have no real wish for cataracts or permanent blindness), should allow for fairly accurate analysis (especially when put through a
transmission grating).
Given the reflective indices for CD/DVD's are known (or can be worked out), then it should be possible to narrow the wavelength down dramatically.
Worth a shot anyway, unless you have objections to the scheme?
PS EXACTLY the same schematic is used to describe the narrowing of a diode beam in the second PDF.
Attachment: Turner.etal.Frequency.Noise.Characterisation.of.Narrow.Wavelength.Diode.Lasers.pdf (219kB)
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[Edited on 13-3-2010 by unome]
Attachment: Hawthorn.Weber.Scholten.Littrow.Configuration.Tuneable.External.Cavity.Diode.Laser.with.Fixed.Direction.Output.Beam.pdf (111kB)
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watson.fawkes
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| Quote: Originally posted by unome | | Ok, I have ordered the suggested diode-laser modules - any suggestions on where to get the safety goggles? |
Phillips Safety carries them.
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unome
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Well fuck me... If I haven't stumbled onto something, this is apparently PRECISELY what the big boys do... Diode onto a holographic grating, then the
required wavelength is diffracted onto a mirror, they call it line-locking / laser-locking / wavelength-stabilization
Presumably the other, stray and/or unwanted wavelengths are absorbed into some material, but that is the basic system - ie. no need for precision
narrow-bandpass filters, ONLY the required wavelength can escape, thus even if it mode-skips, only the intensity will change, not the wavelength.
Attachment: Shine.Day.Focus.Tunable.Diode.Lasers.pdf (62kB)
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Attachment: Volodin.Wavelength.Stabilization.and.Spectrum.Narrowing.of.High.Powered.Multimode.Laser.Diodes.and.Arrays.by.Use.of.Volu (882kB)
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The latest paper compares Volume Holographic Gratings and Volume Bragg Gratings (way too expensive) with heatsinks and achieves wavelength widths of
.3nm which is fucking spectacular and way beyond our requirements...
Attachment: Kohler.etal.Wavelength.Stabilized.High.Power.Diode.Laser.Modules.pdf (497kB)
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Let's go back to Mr Koppen's site, and look at the mathematics (which I kinda suck at, but
anyhow)... The numbers of interest here are already semi-established, 550nm comes off a CD @ ~20' & off a DVD @ ~48', whereas the equivalent
numbers for 500nm are 18.5' & 42.5', so right away we see that for this to work with 532nm the angle of the DVD grating is going to have to be
around about 45', which works wonderfully (I'll work on my maths & get an absolute angle).
The actual angle is going to be worked out according to His equation "sin(a) = l/D", with D=0.74um, l=532nm, which allows us to solve for sin(a) =
0.74/532 or some such.... If the number it spits out is between 44 & 46.5 it should be about right...
Trouble is, when I solve for sin(45)=0.850903525 and 0.74/532=0.00139097744, thus there is not going to be a sleep-filled night ahead of me
Alternatively, we could use an indexed screw to perform fine adjustment (when we get the thinnest green line possible, that is probably it) starting
from the point given by the 'virtually' linear progression of the wavelengths we can see that by dividing the difference between 500 and 550 (5.5')
/50 and then multiplying that by 32 = 3.52 (then adding that to the angle for 500nm) - ie. 42.5 + 3.52 = 46.02' we should be able to get close)
What would we use to absorb the stray light-radiation? Any ideas?
The other good thing is, the fact that this is Raman Spectroscopy and we want the laser to go in from the side of the sample, while we collect the
filtered light from right-angles thereto (IIRC), so we can forgo the mirror and just put the light straight into the sample through a slit/lens, which
simplifies fabrication somewhat.
[Edited on 14-3-2010 by unome]
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not_important
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Not much time right now, so just quick notes.
Yeah, you can do it this way.
Remember to see if equations are using degrees or radians.
It's tough to get good light absorption. Black paint runs 95-97%, NiP 98-99%; all slightly less than 2 orders of magnitude. Raman Stokes is 5 to 7
orders of magnitude down, anti-Stoke 1 to 2 below that. You need careful design to get multiply reflections for dumped unwanted light, sometimes
filters are still needed, or multiple dispersion designs, to really clean up the light.
Part of what determines how well this works is grating quality. Surface defects give simple scattering, meaning off frequency stray light shows up at
the wavelength you want. That's why multiple dispersion gets used, and _may_ be needed with low grade gratings like CD/DVD surfaces. A reflective
pre-recorded DVD may give the best result, reflective Al coating on the lines.
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watson.fawkes
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The traditional cheap beam dump is a stack of single edge razor blades, new, untouched (clean), and bolted together, with sharp edges facing the beam.
It's the combination of all the sharp, diffractive edges and a conductive-but-lossy material and a geometry that leads to lots of reflections that do
the trick.
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unome
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Nice to hear, I intend to be super-careful in choosing the reflective grating - generally, especially with DVD's (because they have effectively 3
layers, not 2) the reflective coating is lifted/warped along the edge of the cut - so I'll cut a much larger bit than is needed and mask the
unnecessary portion with black card, ensuring only clean, non-damaged reflective surfaces are exposed to the beam.
That said, this is an amateur forum and this is aimed (no pun intended) at the amateur scientist and what CAN be done with minimal outlay and some
ingenuity, so if I can avoid major expenses like line-filters I intend to do so (not being a clown - just working to a plan, or trying to). I'm
tempted to make a deep slit - several mm's thereby narrowing the beam further - the further the beam has to travel in between parallel edges, reduces
the chance of stray light, plus the further the slit is from the grating, the more surface area is available to ensure good diffractive outcomes... I
wonder, what about straight carbon/silicon? What, given their color, are they like at absorbing light? (heat would be a minor issue - it would be far
enough away from the module that it shouldn't effect the mode)... What about black cardboard? It is just if the entirety of that end of the
monochromator is non-reflective (or as non-reflective as possible), the distance between the grating and the narrow, deep slit is as far as is
practicable, we give ourselves the best possible chance of making it work...
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not_important
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Beam dumps are generally used when you want to absorb a beam (duh). The fundamental frequency beam emerges from the NLO at a difference angle than the
generated overtone, so you position a beam dump to suck up that unwanted energy (part of the reason experimental setups are large, to give space for
the beams to separate).
In this case for safety reasons, and considering the relatively low power in terms of heating stuff, it's likely better to use simple filters to drop
the IR beam levels to to safer levels, then a grating to fan out everything followed by the slit to isolate, and lots of absorbers to keep stray light
down. It would be a lot of work to make enough beam dump stacks, and the power level just isn't there.
There's the 808 amd 1064 nm beams, the LED-like radiation of the diode laser than can extend down into the green, and Raman shifted light from all
three laser frequencies generated in any material they pass through including filters, optical fibers, and so on. Light is Rayleigh scattered, as
well as by inhomogeneous areas of the optics, surface imperfections in the grating and other optics, and dust. It doesn't appear as a neat spot offset
from the desired light after dispersion, but from the entire optical path at all angles plus reflections.
I don't think a deep slit really helps, else you would see it used a lot. Instead double or triple monochromators are used, each stage giving roughly
a reduction in unwanted light to roughly 1/1000 the incoming level. That's with good replica gratings and other optics and using collimating mirrors
rather than lenses, I'd not expect quite so good results with DVD gratings.
Also remember that the CD/DVD grating is not truly parallel lines, but rather a portion of a spiral and thus an array of curved lines.
Bulk silicon is not a good absorber of visible light. It appears grey, semimetallic if the finish is smooth. Bulk carbon is difficult to work with,
the cheap forms dust off badly, the hard forms have some transparency. Carbon black in a carrier is OK. Micro-rough surfaces are better than smooth
ones, thus dyes and thin paints that don't fill the spaces between fibers in paper products give better results than thick paint.
Optics Communications
Volume 270, Issue 2, 15 February 2007, Pages 262-272
doi:10.1016/j.optcom.2006.08.038
Common black coatings – reflectance and ageing characteristics in the 0.32–14.3 μm wavelength range
black fabric is pretty good, running about 96-98 percent.
[Edited on 15-3-2010 by not_important]
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