Sciencemadness Discussion Board - Laser-Diode based Raman Spectroscopy

Laser-Diode based Raman Spectroscopy

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not_important - 3-11-2010 at 10:31

While it won't be the same wavelength, why would that bee a problem? The spatial location is what is important. The problem is that the fluorescent layer is not directly on the sensor, unless you coat the sensor surface it's done), but above it - meaning that a small spot will be visible to several adjacent sensors giving less resolution.

BTW - there are N-photo phosphors that absorb several photons and emit one or one in the 'visable' and some fairly deep in the IR. Then can fluoresce at wavelengths shorter than the exciting one, or at the same or nearly the same) wavelength, although there's little use for that as the intensity will be lower than the exciting intensity.

The lamp - standard RF plasma, an electrodeless arc. No electrodes means no electrode evaporation, in some cases lower power consumption as there's no electrodes for heat to follow out of the bulb.

aliced25 - 4-11-2010 at 22:45

I'm actually interested in trying to make one of these to get my eye in so to speak. I'm looking at the 640-660nm range of Laser Diode (with internal diode for feedback), with an external cavity. I would prefer to use an aspheric lens (as in the attached paper by Dunn). I honestly don't think the cooling should be a huge issue, the duty cycle is pretty low.

Now, with the PWM controllers on the various DSP's, is there any reason they cannot be utilized to control the laser? I mean, it is not like I am using it as a transmitter and need a multi-GBPS transmitter driver is it?

I'm thinking that if I line-lock the Diode with feedback, it may be possible to add in a <100l/mm laser line narrowing beamsplitter (it could even be ruled) as a component of the partial reflector, the wavelength that is in the center would be the one used, the others would be reflected back into the diode and used to line lock the diode (needs a bigger diode as it can only be run at part power or it will overload with the feedback - Here is the thesis from Dunn that I am referring to).

Anyone got any ideas on how many mW of power I'd need in order to get a decently bright (ie. measurable) response in the Stokes-shifted radiation? It will impinge upon the NIR end of the spectra ~850nm is the upper edge IIRC.

The sensor, a simple 2MP CMOS camera is way cheaper than the filters, the fiber, the lenses, etc. could be utilized then to convert the spectra into a digital signal which can then be processed (the signal processor can run algorithms to remove spectres/ghosts, dead pixels, hot pixels, etc. by averaging the entire column).

It looks a sight easier than the NMR as a first effort is all.

[Edited on 5-11-2010 by aliced25]

not_important - 5-11-2010 at 11:04

Raman light is 5 to 7 orders weaker than the exciting light, shouldn't be too difficult to put that against sensor data along with focusing optics to determine needed laser power.

Remember that colour camera sensors are less sensitive than grey scale ones, roughly 2/3 of the light of a given wavelength is wasted on the other two colour channels. They generally are noisier than CCDs intended for such work. And you've very little control over the operation of CCD and accompanying data collection; low end cameras are optimised to deliver acceptable picture capture using common light levels used by humans, generally much brighter than the light a Raman system will be presented with.

You are thinking of all the hardware as idealised, ignoring the problems of scatter and of higher order spectra from gratings; those will limit the sensitivity of your device and not unlikely create ghost peaks. You'll not get away with lenses, attempting to drag the Raman emissions out of the scattered but not shifted laser light is going to take a filter or really well constructed dispersive systems. Even cleaning up the laser light before sending it to the sample will take at least a well built double monochromator.

There are reasons that commercial and research instruments are expensive and complicated, it's not just for the hell of it. Demonstrating an effect often is not too difficult, having be an useful analytical tool is generally more difficult.

BTW

Quote:

. I honestly don't think the cooling should be a huge issue, the duty cycle is pretty low.

The duty cycle of the laser? Matching that to the timing of the camera should be fun, CW is more typically used unless you directly control the CCD.

aliced25 - 5-11-2010 at 17:19

The cycle is going to be pretty short, the camera and the laser are both controlled directly. Matching the two is a matter of turning both on. The duty cycle in this case will be the time it takes for the CMOS Image Sensor to collect a thousand frames, which is fuck all. 1,000 frames, each of which is effectively a statistically improved average of 1,000 single pixel high captures, will effectively give an average of 1M single pixel high captures. This will allow for the Constant Wave Laser output to be shortened dramatically, thus reducing heat and effectively removing it from the equation.

The Camera I am looking at is a 2 MP CMOS camera, the light captured by it and by CCD's designed to be monotone is really down to how the RAW format data is played around with. In terms of accuracy, the single line sensors cannot effectively remove the value of a hot/dead pixel without skewing the data in that space appreciably, it is illogical to suspect that they could. With a thousand pixels in height, there is the potential to remove a single/multiple dead/hot pixels by averaging or getting the mean and excluding any value outside a percentage of the median, etc then getting the mean of the values for that column.

I don't see your concern insofar as conversion of the RAW data into RGB then greyscale. Yes it does frig about a bit more than I'd like, but it is digitized from the camera, so the data shouldn't really be too much problem with the conversion. I've actually manually converted the RGB spectra I've gotten with a camera into greyscale, it works fairly well. I am of course in the hunt for a monotone camera module in the 2MP range, because I honestly believe that the capability to remove dead/hot pixels with less overall effect on the averaged values is a better way of collecting the spectra. Add to the that the fact that each and every value on the single pixel high result that comes from the averaging of the spectra down the height of every column is effectively an average of 1,000 (or more) image captures with the single pixel high units and the benefit in terms of accuracy should be obvious.

Put it this way, each column of pixels is at a calculated angle from the grating. Each column is 1,000+ pixels high. Each column will receive a spectral value (intensity) from the diffraction of the light through the grating at every pixel. The median of each column of pixels is calculated and everything outside say 1/4 of the value thereof is replaced with the median value (which gets rid of the hot/dead pixels). The mean is then taken of the pixel values, including the standardised deviation pixels, to give a representative value of the value over the entire column. This is kept as the mean value in terms of intensity of that pixel position. Each column is then treated the same way, to give a mean intensity per pixel position. The trigonometric conversion of the pixel position into Wavenumbers (cm-1 and Wavelengths (nm) will also take into account the known transmission curve values of the filter(s), equalizing the transmission. This gives us the data pairs for the mapping of the values, intensity v Wavenumber/Wavelength.

That will give a better result than the alternative single pixel high sensors, which try and approximate and/or ignore dead/hot pixels. It will also give a result which is effectively an average of some 1,000+ image captures with the single pixel high sensors, which are statistically improved/modified to give a better representative value. Statistical anomalies, such as higher order wavelengths & spectral ghosts, will be removed by the use of the median dispersion technique. All of these tools are built into a variety of C/C++/C# image processing libraries.

But bottom line, I personally dislike the single-pixel high sensor concept. It does not allow for the fact that if there is an actual problem pixel in the array, it cannot be effectively/accurately accounted for or removed from the image without building in an approximation/guess as to its spectral data. That cannot help but skew the data coming therefrom, there can be no meaningful connection between the approximation of that data and the reality of the spectra at that point.

PS Also, please check out the paper by Hong (attached) with a "claimed" linewidth of 0.06nm with a single Littrow-type setup with an external cavity. I am trying to think up a way to utilize the low-groove, high-dispersion gratings that are designed to effectively separate the laser-line to give narrow beamwidths (<0.5nm) by basic refraction through a transmission grating (<100l/mm). That idea with the concept suggested in Dunn's paper (cited above) should allow for the narrowing of the laser line as well as the locking of the laser (provided it has a feedback diode), while minimizing parts count and allowing for a smaller module to get the laser line from the diode into the fiber, with the narrowest possible laser-linewidth. The splitting of the beam, so that we could take the 0th order beam (straight-through), while allowing the others to be reflected back into the diode, should allow for a straight-line module from the diode to the fiber of minimal linewidth (of course, in terms of effectiveness it is quite probable that we would have to take a different order, in which case we would have to tilt the transmission grating, which would improve the effectiveness anyway, as the order taken would also have a reflected counterpart, the use of which as feedback would lock the diode onto the appropriate mode.

PPS As to the Raman Lightwave beings 7-8 orders lower than the exciting light, that is why I am looking at the DVD burner range of diodes as the incident light source (640-660nm, ~100-150mW). They are designed from the outset to be high-powered, feedback locked sources (that is why they incorporate the feedback diode). They are also mass produced, mature technology (read fairly accurate and cheap), as are the lenses and optics that go along with them.

Attachment: Hong.Design.and.Characterization.of.a.Littrow.Configuration.External.Cavity.Diode.Laser.pdf (234kB)
This file has been downloaded 4592 times

[Edited on 6-11-2010 by aliced25]

not_important - 5-11-2010 at 17:56

RAW in a colour camera is in effect RGB; it's not the data format but rather how the sensor is built - 3 cells per pixel, one per colour band meaning 2/3 (actually between 2/3 and 3/4) of the light of a given wavelength is wasted. A "B/W"monochromatic sensor has no such losses, and can be had in 2D form. It also has a simpler response curve, rather than the 3 curves of a colour sensor in which there is overlap between channels you can filter the unwanted ones out in software, true, but its extra work)

When an image sensor is built into a camera you lose full control of its operation, the control is busy being a camera. At low light level (ie Raman) you need to operate it differently that when taking photographs under lighting typically supported by at least lower end cameras.

Remember the need to run a transform on the data to compensate for the sensor column alignment vs spectral lines before doing any further signal processing.

Regrading duty cycle - I am doubtful you can push a low end camera faster than 10 frames a second, well over a minute for your thousand samples. In the real world, and given the small mass of the laser system, that's 100% duty cycle. If you can pop it on and off for each frame, another story. However powering up and down usually cause non-stable and off frequency operation for a period of time, in some cases for seconds or longer.

Calibrating the sensor may be more work that you expect, right now I'm assuming you've not done it before.

This all is pretty old hat to me, the concepts behind the bad cell removal and all that go back decades and are used in may other fields as well. I've worked with specialised hardware that did cross correlation between 5 scan lines data in real time to generate a 5 line time delayed noise filtered feed, and that was decades ago.

aliced25 - 6-11-2010 at 00:11

The camera is rated for 360 fps, so <3 seconds for the 1,000 samples (it is actually a high-ish resolution web camera), so that brings the cycle down to around 3 seconds and a laser diode shouldn't have too much opportunity to generate sufficient heat in that period.

I know what you are saying with the RGB/RAW/YUV image formats, but unless I can find a decent Monochromatic CMOS Module in the 2MP range, it is really Hobson's choice. I am still looking around for a decent supply of a monochromatic sensor (CCD/CMOS) in this size, they do exist, but it is either too difficult to source or the lack of documentation is sufficient to make me very wary of purchasing the same (I have tried to utilize other sensors that lacked documentation and it is just too hard to work out how to build it and utilize the components thereof). That said, I am still looking, if anyone knows of such a camera (there are several makes, Toshiba, Samsung, etc.), let me know (by PM if necessary).

When the image sensor and the image sensor controller are built into a normal camera you do lose control over the majority of the functions, but when you control the image sensor and the controller you should be able to alter the way the data is dealt with (which after all is the limitation on using standard cameras).

With the sensor calibration, I'm building a neon, xenon or krypton light source into the board that can be used directly for calibration. The high power torch bulbs are what I am looking at for that, minimal power, maximum result.

Given your experience I look forward to working on the project then, it is always nice to have someone to ask the questions that need answers. The alternative is to reinvent the wheel every time a problem is hit.

Once I have the rest of the pieces in place, I'll try it out with a DVD grating, or three gratings from around the outer edge of a DVD and a web camera. Not so much as an analytical instrument in its own right, but more to prove the concept. I'm actually acquiring bits and pieces at the moment so it is on its way. As the concept is proved, I'll gradually change one part (or more) at a time, in the proof of concept setup until we have a prototype.

Each step will be documented on here, that will allow people to pick it to bits (faster, less formal version of peer review in my book, hopefully it works better too). It is suprising how cheap the components are if you are willing to look around a bit. For starters the webcam based model will allow for us to actually get spectra to work on. When the basics are worked out, it'll be time for another step.

And you are right, I haven't calibrated the sensor before, but I'm more than happy to in the furtherance of an interesting part of my hobby. That said, there are an awful lot of fairly low-end articles where they have managed this with some very simple optics and even simpler light sources. Once I can replicate their results, I'll push on to more advanced optics and light sources (and algorithms and digital filters).

I personally believe that there has been insufficient attention paid by amateur's to instrumentation. Yeah, it costs money, but it costs less to design and build these than it does to refurbish a $10K spectrometer, and the understanding of how the instrument works has to be higher.

[Edited on 6-11-2010 by aliced25]

not_important - 6-11-2010 at 12:47

The noble gas lamp is good for wavelength calibration, and should be included a s part of a sample scan. But sensitivity calibration requires a known intensity and spectral curve such as a standardised tungsten lamp; the optics for that are simple. I need not be done very often, although some instruments calibrate each day against a tertiary standard bulb built into them.

I suspect that frame grabbing at that speed isn't going to yield high sensitivity; try running the camera where the only illumination is a wide band emission LED being feed with about a microwatt or two of power.

Three seconds power on should be sufficient for the LED to walk all over; you'll need to quantify it first to make sure it gives you useful output.

That simple Raman spectrometer integrates for many seconds, 10 to 60 are mentioned in the paper; far longer than the 3 seconds you're speaking of. Also note that it is simple in part because of the box labeled 'spectrometer', much complexity is hidden in that including the fact that it is not very sensitive to the lasers wavelengths.

aliced25 - 10-11-2010 at 02:27

Well I had another look around, there is a much better camera, around 30fps, @1.6MP & it is monochrome (about 5 times the price). It certainly does look like a better unit, although I'll have to go for cooling for the diode & the UV-NIR light (ah well), ~$25 for the sensor.

As to the UV - the xenon lamps go right through the UV-NIR (actually past the CCD/CMOS sensor's ability to collect it 185-~2000nm). Anyone got anything on the xenon lamps, I know the HID ones are filtered to remove UV, but what about the xenon bulbs for flashlights/downlights? Anybody in a position to find out whether any of them are UV friendly? The alternative is one of the major manufacturers and I'd expect them to run @ around $250-300/ea.

The laser diodes are not too bad, expect the 100mW end @ 635-650nm to be around $20 and I was right (the ones with the feedback diode).

Anyone got any idea of the lens setups needed to get the laser/UV-NIR light into the respective fiber, then into the sample(s)? Then collect it from the samples and into a multi-channel spectrometer?

aliced25 - 13-11-2010 at 14:36

OK, I'm going to build a simple testbed. I've got a 30mW Rated DPSS 532nm Module (Battery Powered), an Orange Plastic Filter (approx. same color as OG-550), that the Green won't go through, several plastic cuvettes and some mirrors, etc. I've also got a Microsoft LifeCam VX-1000 (with the infra-red filter removed from under the optics) and a blue plastic filter (from Edmund Optics), which will allow the green through, but will stop and stray light.

Presuming I shine the light in from the left side of the cuvette, and take the light from the front (ie. 90' to incident) through the orange plastic filter, then use a 45' mirror to direct that light onto a reflective holographic grating (I'm using one from a CD - so ~ 600 l/mm), and put a solid wall with a slit 45' to the mirror (extend the right-side of the cuvette holder), then the grating should be at what angle to the wall in order to collect the orange+ wavelengths?

I'm going to have to sit down and work this fuck out

not_important - 13-11-2010 at 16:02

Note that some XE HID, such as automotive headlamps, are more properly metal-halide lamps, with quite strong emission lines.

Flashlight bulbs with the 'xenon' tag are xenon-filled incandescent lamps, argon was used in the previous generation. The inert gas reduces the evaporation of tungsten from the hot filament, giving a long life for hotter (brighter) lamps. But they're still tungsten incandescents, with the near blackbody spectrum for their temperature which means a big hump in the yellowish region.

Pure Xe lamps have noticeable spikes in the VNIR, and fall off below 400 nm, but are fairly close to a flat spectrum between those regions, and from above 1000 out a bit past 2000 nm. They are expensive, as are their power supplies. Because of the spikes and fall-off a differential mode, with reference and sample beams and either 2 detectors or a rotating mirror style beam diverter, is often used. Comparing the reference beam and that passed through the sample allows nulling out of the deviations from a flat source. Note that there can be some shifting over fair short time scales, unless you know how your particular lamp acts you can't trust storing a reference for computational subtraction, you need to actually compare on a short term basis.

http://jp.hamamatsu.com/products/light-source/pd024/index_en...

XeLampSpec-s.png - 81kB

As for getting light into the fiber, you want input couplers. This can be simple lenses or a form of telescope. Lots of literature on them. Ditto for the output, although in some cases you don't need any lenses. Beams from the fiber should be small enough that you can simple locate both fibers in the same region. BTW - the fiber for the laser should be low in Raman generation, or you filter it through a narrow pass filter between the fiber and sample.

aliced25 - 13-11-2010 at 17:48

Yeah, the Short-Arc Xenon Lamps are pretty fucking spectacular actually - they kick shit out of Deuterium Lamps, less cost more light - Trouble is standard quartz fiber ain't gonna get you far (fluorescence @ 250??), which is why no-one has one for sale that gets much below 300nm. The price list at MS Scientific is a good read.

I'll have a fairly short distance between the laser & the sample when I do it properly.

At the moment I'm just frigging about proving it can be done

Codeine Phosphate + Paracetamol in the sample should have some effect.

PS I'm checking out some of the HID Lamps, I'm sure some of them get off cheap by including a glass sleeve (to absorb UV - the alternative is a special - read expensive - grade of quartz, for once the cheaper the better might be the option). Then again, I don't expect anything much lower than 300nm out of them.

PPS I'm Looking at GRIN Lenses (for the Laser) with a silica spacer (with as little air as possible), I'm thinking about using a glass prism to catch the narrowed line from a grating, then an aspheric lens straight into the GRIN Lens. I'm looking into the UV fiber as well, but if you want to go under 300nm, the only real answer is probably going to be the aluminium hollow core fiber, that or photonic crystal and you don't need much of that to raise the price through the fucking roof. Silver Hollow Core would probably work (vacuum inside), but I remember reading an online paper that it had appreciably higher losses than aluminium (then again, silver mirrors are a fucking lot easier to make than aluminium ones). I'm thinking that with a silver hollow core, you could probably get away with a hollow plastic tube & form a mirror on it. Alternatively, a plastic tube with a conductive plastic & plate it, although with a vacuum how would nickel work (I'm thinking it is unlikely to corrode)?

not_important - 14-11-2010 at 11:05

The special grade of quartz is to pass the short UV, not to filter it. Thus the HIDs you're talking about are going the opposite of what you need and certainly will be weak in the vacuum UV region; the ozone-free curve in blue is similar to the HIDs you're talking about. Also make sure they are pure Xe, with no Hg in them, else the mercury lines swamp everything else.

aliced25 - 14-11-2010 at 16:15

Go read up on HID Lamps - they have a special grade of quartz specifically to STOP UV. The Short-Arc Xenon Lamps have a different grade of Quartz (Ozone Free) to pass it, but there is a different additive in the UV Absorbing Quartz that stops everything under ~400nm so as to qualify for use as headlights. As the Low-Cost HID Lamps have a second sleeve that is essentially normal optical glass (that is cheaper than UV-Stop Quartz and every bit as effective at absorbing <400nm), that is why I am suggesting that the cheap HID Tubes might be worth a look, especially if anyone has the capacity to check the output. They don't use UV-Stop Quartz so they won't be using Ozone-Free Quartz either, so no, they won't output very well at all <300nm, but if they can go down to the 300nm mark, then they will perform every bit as well (given they go all the way to about 1600nm IIRC, although nowhere near as flat as the short-arc xenon) as the Deuterium/Tungsten Halide Lamps used in the Ocean Optics Fiber-Based UV Spectrometer (which is only rated to 300nm & given that neither Tungsten Halide or Deuterium are exactly flat either), for a lot less fucking money (and a hell of a lot cheaper to replace).

As the majority of Quartz fibers (certainly all that I've found to date) absorb UV @~250-280nm (thus the need for photonic crystal / hollow core fiber), the makers of the various Fiber-Optic Spectrometers don't have anything I've seen that goes under 300nm (Kind of a waste of a Deuterium Lamp I'd have thought, but there you go). I'm wondering if anyone makes an Ozone-free Quartz fiber, if so that should be capable of carrying the UV from 185-2000nm (as it is transparent thereto), but I haven't seen any mention of it (which is kind of a shame - hardly seems worth buying a lamp that can go that far if the light won't be utilized).

I'm actually beginning to wonder if polished aluminium waveguides (with an internal vacuum, or can I fill it with argon/whatever?) might not be the way to go. Yes, I'd need mirrors and if it is going to be portable, they'll need to be set it in place absolutely (or be adjustable from outside). I'll also need lenses, but I'm sure I can find Ozone-Free Quartz lenses & Optics if I look about. A beamsplitter will also be required. It is just not really feasible to utilize fiber when it is not up to the task.

But the only reason I was thinking about the HID's is that if they do output in the 300nm-1600nm range (albeit up & down all over the place, like D2/Tungsten Halide really), the ones without the special UV-Stop Quartz sleeve (or the ones in which it can be removed), would truly be a decent replacement for the $1500 light-source in the majority of Fiber UV Spectrometers that do the same numbers. I'm serious about that, I personally am looking for something that will carry the 185-2000nm light generated by the Ozone-Free Xenon Short-Arc Lamps (I'm trying to get quotes, looks like there could be some decent buys ~$150/lamp). That saves fucking around with D2 Lamps (~$300ea) and then tungsten Halide as well, in order to get a smaller envelope.

PS One really has to be sure that everything is Hg free nowadays, otherwise I could not recoup my costs all that well, given that pretty much anything that contains Hg/Pb is verboten in Europe (especially with basic electronics, I suspect more will follow).

PPS I tried out the mini-Raman, but the **I^(*&*)(*)(*)(* Camera is playing up now (If it isn't one thing it is a fucking 'nother). If one more shitty little Laser Module lets go on the blue wire, I'm going to have to go and buy some bits and bobs to build a module out of the 200mW 650nm Burner-Diode Laser's I've got on my desk. I have a proper Filter specifically for that wavelength if that happens (but it will slow shit down).

[Edited on 15-11-2010 by aliced25]

not_important - 14-11-2010 at 21:14

From
http://www.tfhrc.gov/safety/hsis/pubs/04146/introduction.htm

HID lamps.gif - 8kB

Or just look at one through a prism or off a CD/DVD, the lines are obvious.

aliced25 - 14-11-2010 at 21:16

The alternative to taking the 0th Order and using the +/- 1st order for feedback control is to split the beam at the grating and utilize the +/- 1st Order beams as Equal outputs (which they are), line-narrowed laser light sources. If the 0th Order is coupled to a photodiode, which is then sent to a comparator which controls the power supplied to the diode, it will generate a positive/negative error signal. Altering the power-supply to minimize the error signal, will effectively modulate both frequency & temperature induced effects on the outgoing signal, allowing for the production of a very small, very accurate, temperature & frequency modulated, narrow line- dual-output laser.

Yes, but the majority of those lines are from the Tungsten-Halide component. As the alternative option is a Deuterium Coupled Tungsten Halide Lamp, I can't see that it would be much improved.

(From the Manufacturer, Ocean Optics, website, so take it with a grain of salt).

(From Zeiss Website, rather more believable & has many of the same/similar peaks as the HID).

I have to work out how I come up with the diffraction angle through a transmission grating (anyone got a formula? I'm busy trying to design the electronics), but if the two (+/-) 1st order signals are equal (and they are supposed to be), then I don't see the utility of trying to split the light later or even of trying to direct the light back to the diode (which while it apparently works is less effective than external photodiode based modulation). The papers are out there if you look, but if we use the 0th order, which should stay the same if that is the one we modulate from (and so will the others), then we avoid repetitive & unnecessary dithering/oscillation with the light output, while we output two signals which are collimated and can be output to fiber optics.

[Edited on 15-11-2010 by aliced25]

aliced25 - 15-11-2010 at 03:41

As it stands at present, I'm going to press on with the design of the Fiber-Raman Spectrometer, with a multi-channel Spectrometer Design and two Sample Holders (one for the Analyte + Solvent the other for the neat solvent). The third channel will be for the production and direction of the light from a neon/krypton torch bulb (2-3V) so the known spectra can be collected on the third channel. As the Spectrometer will be built from quality parts and will (for the moment) utilize only Image J (or similar) to determine the spectra (Until I can get around to programming OEM Software, or sourcing someone else to do so). The filters will be quality units, as will the holographic gratings in the spectrometer, the Optical Fiber, the connectors and the laser module. As I will have to recoup costs, please be aware that someone else will be offering these for sale, either complete or as a kit.

When I work out how to direct the UV Light properly within a portable, cigarette-packet sized unit, I'll add it. With UV-NIR and Raman, it should be quite a useful diagnostic/analytical tool. Be aware that due to the insane prices on the UV-NIR Light sources, that version will not be as cheap as I'd like, whether to design, to build or to onsell. I will sort this shit out then I'll start on the NMR again.

not_important - 15-11-2010 at 18:23

A torch bulb is an incandescent lamp, meaning typical black body curve, no lines. They can be useful for determining the sensitivity of sensor at given wavelengths - provided the bulb has been related to a standard lamp - by comparing the detector output to the calculated flux at each wavelength.

aliced25 - 16-11-2010 at 00:34

What I'm seriously considering is utilizing the fact that if we shine a Laser diode at a transmission grating (see the whitepaper attached) we can maximize the transmission of equivalent +1st & -1st Order wavelengths, while minimizing the loss as 0th order transmission (which we can pick up with a photodiode at the end of the cavity (avoiding having to fuck around with Littrow/Littman-Metcalf Configurations).

Getting the feedback from a photodiode mounted directly in-line with the diode, allows for feedback to the Comparator, Current Ouptut DAC Laser Driver & Controller, generating a modified +/- error signal. By minimizing the error signal, the 0th Order remains the same regardless of frequency/temperature changes (NB So too, the +1st & -1st Orders remain the same). The concept of utilizing external digital modulation is not new (http://pdfserv.maxim-ic.com/en/an/AN1811.pdf), the idea of utilizing the equal magnitude split laser linewidth appears to be.

I see little point in splitting the line later on when it can be accomplished in such an effective manner using the same parts as are needed to narrow the line to <100kHz. Dual-Narrow Linewidth Laser Line Source sounds good doesn't it? Especially given the cost of 650nm Laser Diodes ($10-40 depending on where you source them from). That would enable me to build a connector for the + & - line on the front of the Laser generator and expect to get ~40mW on each (from a 100mW (standard)/150mW(pulse) diode). That should be ample for my application I'd have thought (anything higher than that stands a real risk of burning the sample & the sample container).

With the sensor, I've found a 5MP Aptina LLC Monochrome Camera Module (MT9P031I12STM - getting the app.notes etc. out of Aptina is a bit of a fight).

The Grating I'm sourcing now, actually I have sourced several I just have to pay for them. But the use of the external photodiode and digital modulation to compensate for both frequency & temperature, seems like a saner choice than trying to keep the diode cool, etc.

Being able to take-off both laser-lines from the unit in simple patch-cords (Single-mode) and take them to the samples (Analytic = Analyte + Solvent & Reference = Solvent) will allow for the rapid collection of Raman Spectra from both, comparison of the two will give the Raman Spectra of the Analyte itself. Comparison of both with the Neon/Krypton Spectra will allow for alignment.

The electronics are still being designed, the laser is just about finished being designed. The Camera is waiting on App. Notes from Aptina LLC, but it should be able to be interfaced with the Mixed Signal Microcontroller. The Dual Current Output Dacs & the Comparator on the same MSP, will allow for the efficient external digital modulation of the Laser Diode, while the USB 2.0 Connection (and the requisite multi-stage LDO, which can charge a Li-Ion Battery and run the Laser, the Camera, etc.), in addition to the 12V DC Jack (also run to the same LDO) will ensure that the entire unit has sufficient power.

The problem is now the design of the 3-channel Spectrometer, how to split the one grating into 3 compartments without stray light? Also, do I need optics on the Aptina CMOS Module? I've seen some articles where the bare CMOS Chip is used, but I'm not sure.

Attachment: Laurila.etal.Tuneable.External.Cavity.Diode.Laser.at.650nm.Based.on.a.Transmission.Diffraction.Grating.pdf (152kB)
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Attachment: Ibsen.Photonics.White.Paper.Fused.Silica.Transmission.Gratings.pdf (499kB)
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Sorry, had an absolute bastard of a time trying to remember what I'd read that makes the above make sense. It is the utilization of the Pound-Drever-Hall Modulation Technique, which while it looks like a conceptual nightmare to put into effect, is basically the use of a photodiode to measure the output of the Laser Diode, and utilizing the analogue signal therefrom, use a comparator to digitize and compare the laser-feed with the optical output and modify the laser feed in order to stabilize the output. This paper makes it seem even more of a nightmare than the Wikipedia article, but this paper actually helps to describe the procedure. This paper describes the use of a Fiber Fabret-Perot sensor, whereas it seems complex, it literally isn't. The current from the photodiode is amplified to allow for its comparison with the current fed to the Laser diode (remember if we are using the 0th Order as the order coupled to the photodiode and taking the two equal +/-1st order outputs as the narrowed line outputs, then the photodiode is only getting from 4-5% of the total light from the Laser-Diode).

As the amount of light incident upon the photodiode alters with frequency and temperature, by altering the amount of electrical current fed to the Laser Diode to equalize the output to make up for the alteration, we are modulating the output to be equivalent regardless of frequency and temperature fluctuation. That is the power of the concept. As the external photodiode response is significantly faster than the internal photodiode (if it even exists), the modulation is faster and prevents mode-/frequency-/temperature-fluctuation. Thus we get a truly Laboratory Class, Temperature, Frequency and Mode Stabilised/Modulated, Narrowed Linewidth, Dual Output Laser using a Laser Diode, a short external cavity (and a collimating tube (ie. a tube with the laser diode at one end and an aspheric lens at the other) and a transmission grating (plus a photodiode & a MCU with at least one current output DAC & a comparator). Determine the angles at which the +/-1st order outputs will be diffracted, and utilize two equivalent prisms to straighten the beams and feed them directly into the Aspheric Optics & the Grin Lens connected to the Fiber Output.

The whole sounds complex, but try drawing it - it is straight through the grating to the photodiode (0th Order), the +/-1st Order (40-45% each) both leave the grating at the same angle. The modulation is built in using the Laser Driver IC and there is no need for multiple geometry mirrors, holographic gratings, Littrow/Littman Metcalfe Configuration or the rest... Instead we have a collimation tube - a simple tube with the Laser Diode pressed into one end and an Aspheric Lens pressed into the other, a transmission grating 90-110 lines (depending), which is standing absolutely vertically (which makes design simple), with no need for reflection into cavities, or the rest. In terms of manufacture, it is quite probably the easiest configuration. The prisms are placed where they are intersected by the +/-1st Order beams on either side of the module and the ST-Connector (with the Aspheric Lens built-in) collects the light from the prisms and passes it into the Connector, ready for it to be passed to the GRIN Lens in the other Connector. No need for beamsplitting or anything else, we have two equal beams.

EDIT 2

Here as good a paper as I've yet seen explaining Laser Control. Notice it discusses the importance of Pound-Drever-Hall Modulation (via an external PID).

[Edited on 17-11-2010 by aliced25]

aliced25 - 20-11-2010 at 03:56

Have a good look at that paper on "Pound-Drever-Hall Modulation", in particular check out the circuit diagram for what they utilize in order to Modulate the Laser at the Laser Peak and then check out these two little items (about $5.00):

Here is the paper I got the numbers from: Intersil: Temperature Compensated Laser Diode Controller

X96012 Universal Sensor-Conditioner with Dual Look-up-Table and Dual DAC's (it has an onboard ADC too, so you need an analogue Photodiode)

ISL29009 Non-Linear Output Current, Low Power Ambient Light Photo Detect IC

The photodiode detects the light and sends the voltage from the conversion thereof to the Laser Diode Controller (which is driving the Diode with one/both DAC's), is converted to a digital signal - the digital part of the chip compares the photodiode signal with the current it is sending and moderates it to minimize the error signal. Pretty much the entirety of the required modulation needed in two very small IC's, not bad. Couple that with the narrow-linewidth after passing through the splitter and that is going to be one top-notch laser, based on a diode, some electronics, a grating & a couple of lenses.

Laser Wavelength & Power Meter Design

I've been looking at this, it seems to me that it would be terribly difficult to get a narrow enough signal return with a grating-spectrometer, whereas if the basic wavelength is known - ie. within 10nm of 405, 532, 632, 650, 808, etc. it would surely be possible to revert to a fast-monochromator with a calorimeter at the monochromator output to measure intensity at the wavelength. That would give a rapid, even digital (possibly even a GUI) based Wavelength/Power Meter which would be as accurate as one wanted to pay for.

aliced25 - 21-11-2010 at 17:39

I'm wondering with a small Photodiode IC that is able to digitize the incident light directly and a rotating stage whether it wouldn't be better to go that way with a Wavelength/Power Meter, the conversion of the power from Lumen to Watt's shouldn't be too difficult (provided we are staying within the "normal visual" range) and the monochromator would only have to account for small variations in wavelength, some 10nm around a central range (which could be easily enough adapted for - set it to scan the wavelengths between say 400-410, 527-537, 645-655 and it would be able to return a profile of power-wavelength (simply by mounting a photodiode where the exit slit would normally sit. As it would be designed for lasers that are at least "partially" collimated, then the majority of the optical components could be safely left out, also provide a kit where people can mount uncollimated laser-diodes, which would include the collimation optics (basic collimation tube) with the necessary drive electronics (powered by the PC, while the results would be read directly by the PC and displayed as a graph). That would be the best, conceptual, model I could think of in terms of determining the power/performance of laser diodes and diode lasers in one unit. It would provide a useful chart of the output and allow for people to experiment with line narrowing.

aliced25 - 24-11-2010 at 16:13

The other issue is the design of the circuit for the Xenon Short-Arc Lamp (150-350W), which I don't expect will be a doddle and whether I can find a decent Liquid Light Guide (220nm should be low enough shouldn't it?)...

Anyone got any ideas for either?

not_important - 24-11-2010 at 19:12

Xenon lamps are inherently unstable devices, operated with current-limiting power supplies. The operating voltage typically is between 15 and 30 V, with a current level to match the rated power divided by voltage; thus a 150 W lamp dropping 19 V would run at 7.9 A. The lamps take a starting pulse of some 10s of KVs.

The stability of the power supply is important, as the current affects the lamp output - often in a nonlinear fashion. Welding power supplies can drive a xenon lamp being used as simply are illumination, but spectroscopy takes something better. Expect to drop US$ 150 to 300 for parts.

The lamps run hot, the envelope can hit 400 to 700 C. Most commercial power supplies for the lamps include safety interlocks to insure any needed cooling fan is in operation, else they cut power to the lamp.

Xenon arc lamps take several minutes to reach operating pressure. That pressure makes them an explosion hazard, be sure to include safety shielding.

As for the wavelength range you appear to be seeking, I suggest you brows the following site, starting with http://www.horiba.com/scientific/products/optics-tutorial/di...

As for the CCD, several folks I've communicated with have used the one I'm attaching the data sheet for. They use the standard support chips, but as opposed to using the modes set by the camera they can control scan acquisition times for better performance at low light levels (some have used cooling of the CCD to get lower dark current and noise)

Attachment: a6811244.pdf (1.1MB)
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aliced25 - 26-11-2010 at 03:24

What strength light would be suitable for both spectroscopy (in terms of half-the light being sufficient for the job, remember we are splitting it) and for not shorting out 10A 240V AC? I'm obviously looking in the 100-350W range, but how big is just big enough...

I'm also looking closely at the various Deep-UV Fibers, they use standard connectors, a beamsplitter could presumably be designed into the Integrating Rod Lens(s) from a Xenon Light in a Ellipsoidal Housing... There are some interesting articles online for anyone interested in finding them on the importance of reflector design and lamp orientation, etc.

I was actually considering a Power Supply similar to the one Len1 used for the Sodium, or a Computer power supply, they can be purchased as discrete components CW/Fan. Then again, so can the ones used in those little stick welders, they retail complete (little welders) for ~$100, so the parts can't be too hard to get. Anyway, that would just mean the UV Component needed a wall-plug power source, the laser can be pretty much run from the USB (as could the camera).

If one were to find a decent (read affordable/cheap) source of Quartz cuvettes & that Lumogen coating, the way might be open for both in one unit, one after another with no mirrors in sight (outside the spectrometer & the light housing that is). Of course, separate spectrometers would have to be used, the Raman would be specifically tuned to catch the smallest changes in a fairly narrow bandwidth, while the UV-NIR (what, just on one thousand nm?) would be geared to detecting spikes & changes, but wouldn't be that accurate, it would only have, given 2,000 pixels and absolutely brilliant alignment, 2 pixels per nm, whereas with a line-narrowed laser and the Raman would preferably give about 10 pixels per nm.

not_important - 26-11-2010 at 10:05

As small of a lamp as you can get, I'd say. Eas to calculate, the intensity of the lamp is generally given over some range of wavelengths, for a pure Xe lamp the mild-UV & vis output is flat enough that simple math will tell you the energy in a given width slice, and thus the intensity reaching a cell of the CCD. The absorption spectroscopy isn't that demanding as the transitions involved are pretty wide.

The power supplies you're looking at won't cut it for the applications your want to do. Some will drive the lamp, but with poor stability and shortened life, there's a chance of over-driving the lamp and destroying it (with a small but non-zero chance of a cloud of glowing hot quartz fragments). Computer power supplies are constant voltage, and too low at that. Plus you need the 10s of KV igniition pulse ridding on that.

aliced25 - 27-11-2010 at 17:23

I'm looking at the various xenon arc lamps that have been built, including the idiot with the parabolic mirror in a rubbish bin & a [rul=http://benkrasnow.blogspot.com/2010/10/diy-searchlight-housing-for-1000w-xenon.html]1KW lamp[/url]. The power supply is from a modified arc welder.

Granted it isn't going to give long life, but changing the voltage to modulate the lamp doesn't work from all reports, changing the amps can, but it is going to have to be carefully controlled. I'm looking at the MW Xenon Arc concept, if it can be done with sulfur & Deuterium and Argon & here, it should work with xenon too (well, it does, just have to get the details).

As MW Ovens work on 2.45GHz - the same wavelength as the miniature electronics in the wireless equipment - is it possible to boost the wattage of the signal to the 100W mark? Getting the xenon isn't too hard (welding gas apparently), building the housing out of reflective metal (like a waveguide) with a sapphire window should do it. No tungsten, no quartz blowing, greatly moderated explosion hazard. Even the pressures are better (under atmospheric to atmospheric). Glove box time of course, evacuate the whole thing, fill it with xenon and assemble the lamp in a xenon atmosphere. Not as easy as it sounds said quick, but it would be a hell of a nice light source.

not_important - 28-11-2010 at 11:43

I already said that the Xe lamp needed controlled current, they are negative impedance devices as are many gas discharge/arc. Poor regulation of the current will cause various problems, and can make it not too useful for your intended application (although it may be successful at quick scarification)

You're proposing replacing a lamp manufactured by a knowledgeable outfit with a home fabbed device; and then instead of using a DC supply with current control you want to drive a variable load with a high power RF amp - I suggest you go study the term "impedance matching" for awhile.

Low pressure Xe discharge lamps do not give the nice 'white' spectrum of the high arc lamps, they give the xenon line spectrum, the term to study is "spectral line broadening". Xenon has a lot of lines, but at lower pressures they still are discrete lines. As you raise the pressure the continuous emission rise in intensity, but around 1 atmosphere you'll still see strong lines; BTW a lamp that is at 1 bar when cold will be above that when running, your homebrew lamp will need to handle that while being a rather toasty temperature.

Yes, you can buy RF power amps, for much more money than the DC supply would cost. A microwave oven is already putting out more than 100 watts, wireless communications devices are piss poor sources of RF for boosting up for gas discharge drivers. Leaking microwaves are a fun way to fry bits of yourself, in some cases without knowing it right off as the nerves can be killed before they report getting hot.

All in all you've shown enough lack of comprehension of this general topic that I suspect your attempting to mach such a device could make it as a Darwin Awards nominee. You __really__ need to do more study of these things your wish to build, if you want to have a reasonable chance of success.

aliced25 - 28-11-2010 at 14:24

Well I'm waiting to see the data on that, according to several Russian Papers, Excimer-type lamps with Xe/Kr with a halide are the strongest sources available for the 200-400nm region. The problem they had was the attack of the halides on the electrodes (which were inside the vial), which made it interesting I'm sure and short-lived.

Then there is this, they dismantle a 12V, 2.5A supply and use it as a 5kV, 20mA supply in order to produce an Excimer-type lamp by simply wrapping the wire around the vial (which is the normal way of doing things now I believe?).

That said, if it makes Xenon give plasma I really don't give a rats, it shows it is possible from a regulated power supply, now the only question is how to translate that to a better quality light, and I'm not talking homemade.

Oh yeah, while I'm here, here is a fucking incredible site I wandered across, Luhs Laser, it is bloody incredible work, check out the various educational kits, the drawings & the pictures. It really is worth a look.

not_important - 28-11-2010 at 21:14

Your detector and dispersive elements, at least those you've discussed in the past, aren't going to work well below about 350 nm, from there down to 200 nm wouldn't seem to be of concern.

Indeed, they make some low intensity glow lamps; this does not mean that the method is suitable for what you want to do - you'll either need to try it, or find more details.

I myself would be leery of basing too much planning on research articles that include statements such as

Quote:

Computer codes do not calculate the discharge stability. Therefore, it is possible that the discharge with a positive column radius r = 4.5 cm and/or [Xe], [Kr] ~ 4 × 1019 cm–3 does not exist. Nevertheless, we present the results of calculations for these values.

It is also possible that there are fairies in the bottom of the garden, but I'd not take that to the bank.

You've now jumped from using HID lamps to homebrew (and yes you seemed to be talking about homemade back there) Xe lamps with microwave excitation to rare gas-halogen lamps with lash-up glow discharge drive, all in as many posts. You've also gone from light sources that give fairly flat output from the deep red down into the quartz UV region, to light sources intended for in a fairly narrow ban in the VUV. Again, I suggest picking one choice and studying it so you really truly understand it, remembering that the same overarching term may apply to a fair wide range of devices, conditions, and results. Example, nuclear magnetic resonance is used to measure degree of cure of a glue or the liquid water content of ice, for imaging living things, and for doing molecular structure determination, however those classes of applications differ by about an order of magnitude of needed accuracy for each step as you go from the first 2 to the mid to the last. A device can work wonderfully for the 1st two applications, and fail miserably for the last.

BTW - the reversed transformer is current limited, I'd bet, although not very well. And a current controlling supply is as much a regulated power supply as a voltage controlling one.

You may find this of interest http://www.opus.ub.uni-erlangen.de/opus/volltexte/2007/711/p...

aliced25 - 28-11-2010 at 22:01

This is one of the papers on the microdischarge arrays, pretty much the same principle, though much neater. I'm reading as much as I can, very quickly, but the glow-discharge lowers the spark-voltage of the Short-Arc by /2. It also produces light by itself, though mainly in the VUV - although with either sapphire/F-Doped Fiber (both commercially available, though not cheap), that might yet be overcome (Lumogen is the answer to the first question, really doesn't need discussing does it?). That microdischarge-microhollow plasma - given it does the 200-400 UV quite well, might be "AN" answer, there are plenty of lights that will do the remainder of the spectrum on fuck all power. In fact, those Microhollow plasma emmitters, anode and cathode about 1/2 mm apart, with a dielectric in the middle, with 1/2mm dia hole, enough of them should give pretty much the same spectra at STP (see p17/28) and high kV/ low mA as a D2 Lamp. I suspect the price would be lower though...

The barrier discharge and the microhollow-microdischarge all seem to be utilizing the tiny plasma formed in the presence of a high voltage - low current electrical source (or RF/Electrical). I'm actually wondering why a short arc wouldn't work with, essentially, a surface spark plug-type cathode in the center of the anode? It arcs in cylinder heads, so why wouldn't it arc there? It would certainly simplify the design issues of how to get the cathode in the vicinity of the anode without blocking the light-path.

[Edited on 29-11-2010 by aliced25]

[Edited on 29-11-2010 by aliced25]

[Edited on 29-11-2010 by aliced25]

not_important - 30-11-2010 at 15:23

There aren't many lights that give reasonably uniform 'white' light across the quartz-UV through red. Nor are there common analysers, prism or grating or fixed FT or whatever, that covers from the short QUV into VNIR; that's more than 2 octaves and SFAIK gets tricky. Lumogen is only a solution, really part of a solution, if other factors are taken into consideration, show me your proposed design and then it can be commented on if it is a solution.

Your speculation

Quote:

. I'm actually wondering why a short arc wouldn't work with, essentially, a surface spark plug-type cathode in the center of the anode? It arcs in cylinder heads, so why wouldn't it arc there?

is to a degree answered in the slides you referenced:

Quote:

For stable operation the product of pressure, p, times distance between cathodes, D, needs to be in the range between approximately 0.1 Torr cm and 10 Torr cm (depending on gas, electrodes, and geometry).
...
Electrode materials: molybdenum, semi-insulating silicon
Dielectrics: mica, ceramics
Dimensions (hole diameter): 30 µm to several 100 µm

The distance between electrodes appears to be on the same order of magnitude as the hole size.

Dimensions are important, the semi-insulating Si works as a ballast and to suppress the formation of current-hogging arcs, more evenly distributing the discharge across the electrode surface.

Quote:

should give pretty much the same spectra at STP (see p17/28) and high kV/ low mA as a D2 Lamp

Whatever. Xe Microdischarge, Deutarium, conventional Xe short arc :

aliced25 - 1-12-2010 at 08:18

Sorry, lost my reply

I'm actually contemplating utilizing an almost 3K pixel long sensor (monochromatic cmos), to collect the spectra - I'm wondering, there are a number of options regarding the light source, the DUV-VIS Fiber, etc. But with the width of the spectral bandwidth, it might be worthwhile looking at using 3 separate small gratings, having a 3 channel instrument, 150-450nm (I'm hoping the phosphor can cope), 400-800nm & 800-1150nm. There could be some overlap, but I'll try and work out a way to minimize it, realistically though, the NIR blocking optics should do the job for me @~800nm and the insensitivity of the sensor should do it for me @~400, so there is a bright side.

If high l/mm gratings were used, it should be possible to get resolution down to almost the tenth of a nm in some areas, which would be pretty fucking spectacular.

With a grating optimized for 400-800 it might be worthwhile going for the blue/green lasers after all. The main problem with them originally was the resolution, if the resolution is not a problem and the spectrometer could be realised for dual use, then a highly accurate Raman + UV-VIS-NIR Spectrum might actually be worthwhile taking.

I'm just re-reading on the optics, I'm thinking maybe going with aberration corrected, flat-field gratings (cheaper than you'd think) specifically optimized for each of the three regions. If it could also be used for Raman, the central channel with either the blue/green laser (or stick with the red and go for the Anti-Stokes?), I'm thinking the DPSS is the best choice by far, it is way cheaper than the respective high-power blue's...

3 gratings + 3 sensors, 2 or more light sources and the associated optics. It won't be cheap, but it will still be a fucking lot less than purchasing a basic spectrometer from the major players.

Damn, I'm sitting here going cross-eyed trying to work out the relative etenue of the spectrometer and the fiber. Might have another go in the morning.

aliced25 - 3-1-2011 at 04:35

The design will be based upon "hopefully" something like Ocean Optics is currently using - a three tier arrangement, which allows the full spectra to be broken into its constituent parts 200-500nm, 500-800nm & 800-1,100nm, or 300nm/grating with the blaze at the central wavelength. As the majority of reflective gratings are based upon aluminium coated epoxy, it should "technically" be possible to design a concave, aberration corrected, 3-level grating substrate that can be mounted so that the only light incident upon each section of the grating is the designed for part of the spectra (ie. physical isolation - separate chambers within the same spectrometer). The first one would be a design challenge and a half, the replicas not so much.*

The concave, aberration corrected grating is the key here, without it getting a flat-field image is going to be a fucking bitch. However, I'm not too concerned, given the price of gratings (if one is willing to try and separate the wheat from the chaff amongst the manufacturer's directly - no easy fucking task), the prices fall dramatically. Even to the point that it 'might' be feasible to actually utilize 3 separate gratings & 3 sensors to do the job, instead of trying to compromise. In any event, I'll still be sticking to the CAC-Holographic Grating. It goes without saying that designing the optics to cope with the etendue caused by using fiber optics to carry the light are going to be a major fuck around.

As to the use of Lumogen, there are other luminophor's/phosphor's that can and will allow for the detection of UV light - I'm actually wondering whether it is possible to mount these on/in a thin film, directly in front of the sensor (AR Coating on both) to eleminate stray light. If the phosphor is emitting above 500nm even stray light would not be a problem, the relevant grating is incapable of using it.

* I'm still considering how exactly to construct something like this, a single concave surface, with three replicated gratings molded onto it in strips, separated by flat surfaces allowing for the optical separation of the three channels... Given that replica gratings aren't precisely new, surely it is possible (if somewhat out of the square) to form multiple gratings on a single surface which is engineered to allow for 0-stray light between the chambers/channels. The concave grating spectrometer, with its minimalistic optics and flat-field output seems well suited to this design.

PS With those separations, the options for a combination Raman-UV/VIS/NIR Unit would require either a 780nm Stokes-Shift setup, with the Raman Spectrum being in the >800nm region, or an 808nm Anti-Stokes setup, with the Raman spectra in the <800nm region. In either event, the relevant diode lasers are fairly cheap.

[Edited on 3-1-2011 by aliced25]

aliced25 - 14-6-2011 at 22:06

Ok, having a look around - there are long, narrow, back illuminated Hamatsu photonics image sensors - they are fairly flat from 200-1,000nm. Now, if we used a quartz xenon flash-lamp, and a reflective grating, that should allow us to collect the entire UV-VIS-NIR spectrum in one hit, then run it 100 times and average the lot. I'm also thinking that with a decent filtered laser diode, it should also be possible to use the same optoelectronics (and grating) to grab the Raman Spectra using the same instrument & the same sample.

I'm putting some thought into this as I think there is a need for home-instrumentation that is cheaper than that currently available but which will do the job. That said, the Hamatsu sensors aren't cheap.

bquirky - 15-6-2011 at 11:43

if your not chasing high speed you can rotate your grating and use a single photo detector (then you can use several photo detectors with different spectral responses for different wavelength ranges) and dont have to worry about exotic hamatsu sensors (which often have a low pixel count) as an added bonus it gets you off the hook somewhat asfar as putting a flat field on your sensor. achieving that can be a real PITA

alot of high end OSA (optical spectrum analisers) do exactly this and can acquire very wide spectrums the only drawback is speed.

bquirky - 15-6-2011 at 11:47

oh and you can allso replace the photo detector with a photo multiplier tube and do single photon counting..

im sure you can find some reason to want to do that

not_important - 15-6-2011 at 12:56

Quote: Originally posted by bquirky

if your not chasing high speed you can rotate your grating and use a single photo detector (then you can use several photo detectors with different spectral responses for different wavelength ranges) and dont have to worry about exotic hamatsu sensors (which often have a low pixel count) as an added bonus it gets you off the hook somewhat as far as putting a flat field on your sensor. ...

The original idea was to use a monolithic interferometer based on designs mentioned in the thread http://www.sciencemadness.org/talk/viewthread.php?tid=13554

They have no moving parts, not even a piezo-driven mirror, and if properly designed can handle quite wide wavelength ranges. They produce an interference pattern on the detector array, which is resolved using Fourier transform processing. The Hamamatsu back thinned NMOS sensors would theoretically be able to resolve around 1/2 to 1 nm in this application.

Quote: Originally posted by aliced25

... Now, if we used a quartz xenon flash-lamp, and a reflective grating,....

Note that Xe flash lamps run at roughly a tenth or less the pressure as do continuous Xe arc lamps, as a result the flash lamps spectrum has noticeably more prominent lines. The energy distribution of the output also drifts towards the red end over the course of a single flash, not important unless you're taking really fast samples.

Also you should look at the specs on gratings. Most don't cover all that wide of a wavelength band very well. Some examples here http://www.coseti.org/specgrat.htm

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