I have read with great interest the threads on "Cheap, Low-Resolution, Raman Spectroscopy", "Laser-Diode based Raman Spectroscopy", and "Home made
spectrometer".
I have a project that would require me to be able to do spectroscopy of aqueous solutions in the NIR range from 1100 to 1800 nm. The spectrometers
that are commercially available in this spectral range are generally prohibitively expensive. I need to DIY one as cheaply as possible, while still
maintaining sufficient resolution and S/N ratio.
As a starting off point, my thoughts are to attempt to eliminate optics to save on cost and reduce complexity. In order to do so, I would use either
a Rowland circle configuration of some sort, or would use a Holographic Abberation-Corrected Reflectance grating configured for a Constant Deviation
Monochromator. If I go with the Rowland circle configuration, then just maybe I could get away with flexing CD-ROM material into a circular curve
e.g., by sandwiching it between two pieces of some pipe material (ABS or whatever) of an appropriate diameter and cutting a "window" out of the
innermost concave piece to expose the CD as a grating.
In either case, I would need to avoid the use of the typically very expensive InGaAs diode array, and use a monochromator configuration, and use a
relatively inexpensive InGaAs photodiode like those found here: Thorlabs Unmounted Photodiodes.
The grating would be mounted on as simple a configuration of toothed pulleys, a timing belt, a tensioner, and a stepper motor as possible to achieve
the required angular resolution.
I would use an Arduino to handle the A/D conversion of the signal from the photodiode, and also to control the stepper motor.
I realize that in order to achieve the required resolution (perhaps as low as 2nm), I might have to build the thing rather large in size, but at this
phase of the project, that is not a problem.
I'm interested in any constructive criticism, helpful suggestions, identification of fatal flaws, etc., that forum members may have.Chemosynthesis - 17-4-2014 at 10:21
Hopefully you'll make some productive friendships there.DistractionGrating - 17-4-2014 at 11:03
Thanks for that link, Chemosynthesis! I think I'll go plug my new thread over there, as well as contribute to that one where I can.aga - 17-4-2014 at 12:52
Yep. This ties in nicely.
The Key is the spectrum analysis rig (unless i am mistaken - which could be the case).
Currently i'm waiting for the bits to make a monochromater of sorts, and until the bits arrive, i have no idea if it will work.
The idea i have in mind should handle NIR detection, and only experimentation ( hopefully 1 week ?) will tell.DistractionGrating - 17-4-2014 at 21:32
Assuming I go with the low-end DIY CD-ROM concept, then I anticipate a two-stage prototyping methodology. First stage is to implement the
spectrometer in the Visual range, using a DVD instead of a CD-ROM, and more or less using a factor of two (since CD grooves per mm = 625 and DVD
grooves per mm = 1350, and 1350 / 625 = 2.16) to troubleshoot and debug the basic design using the visual range, where everything is much easier and
less expensive. Once I get everything working in the Visual range, then I hope to be able to reproduce that success in the NIR range.
Pros and Cons of the various basic designs I am considering, based on the homework I've done recently:
Rowland Circle based design using a constant distance between grating and exit slit equal to the diameter of the Rowland Circle such as can be seen
here ("Rowland Mounting"): http://www.physics.arizona.edu/~haar/ADV_LAB/Rowland_Cenco.p... PROS: Constant focal distance. The "rigid support rod R" is the only thing that
moves, and constructing the device is relatively straightforward. CONS: Relatively severe constraint on the upper end of the wavelength that can be
achieved with this design using a CD-ROM. For a 625 groove/mm grating, the theoretical maximum wavelength is 1600 nm.
Rowland Circle not constrained to the "Rowland Mounting" design. PROS: Increased upper wavelength that can be achieved with a given grating groove
density. Grating does not move. CONS: Requires that the exit slit be rotated around the perimeter of the Rowland Circle with the axis at the center
of the Rowland Circle, while (optimally) being continuously oriented towards the intersection of the Rowland Circle and the center of the grating (not
the axis of rotation). More complex geometry than the "Rowland Mounting" design.
Basic Constant-Deviation Monochromator design, using a DIY curved CD-ROM based grating. PROS: Not many that I can identify. CONS: Need to both
rotate the grating *AND* also simultaneously adjust the distance between the grating and the exit slit to maintain optimal focus. Doable, but a pain.
Usage of a commercial Concave Holographic grating designed for a Constant-Deviation Monochromator. PROS: If they work the way I think they do, then
they address the "adjust the distance" issue of the option described in the previous paragraph. You only have to rotate the grating. CONS: Relative
expense.
One common "PRO" to all of these approaches is that none of them require additional optics -- no collimating lens/mirror, and no focusing lens/mirror.
The curved grating both disperses and focuses at the same time. One common "CON" to all of these approaches is that the lack of said additional
optics prevents the construction of relatively compact designs, such as the crossed Czerny-Turner design, so the overall design for a given resolution
and exit slit size tends to be rather large and bulky.
These are just a few thoughts of mine expressed "out loud" here on this forum for others to review and correct me if I'm missing any important points.unionised - 18-4-2014 at 02:54
Won't the CD/DVD absorb NIR?
Certainly the greenish stuff in the recordable disks will absorb it like crazy.DistractionGrating - 18-4-2014 at 06:32
Frankly, I don't know, unionised. Perhaps it will, and if so, then perhaps I'll just have to go with a commercial grating.
It does make sense that something green is absorbing the visible red spectrum, but when I use one of those green CD-R media as a diffraction grating,
in a darkened room, I sure do get strong bands in the red, orange and yellow when using a broadband source. I also get very nice dispersion in all
modes from a red laser.
Come to think of it, the CD-ROM media is made of polycarbonate. A quick Google search turns up this graph of the Transmission spectrum of Polycarbonate in the NIR range which shows that I do have some interference around 1660 nm, but for my project, that
might not be a problem.
Again, if I end up having to resort to commercial gratings, it will increase the cost of my DIY NIR spec, but that isn't necessarily a deal-breaker.
It sure would be nice to be able to cobble one together from inexpensive, ubiquitous materials, though!
Thanks, unionised, for the concern, and please everybody, keep this kind of feedback coming. That is the reason I'm posting here -- so that I'm not
working in a vacuum of my own empty head!
[Edited on 18-4-2014 by DistractionGrating]
[Edited on 18-4-2014 by DistractionGrating]IrC - 30-4-2014 at 20:36
For a 625 groove/mm grating, the theoretical maximum wavelength is 1600 nm.
Can you post the math used to figure this out with various gratings? DistractionGrating - 1-5-2014 at 13:01
The math is simple: λ = 10^6/G, where G is grating density in lines / mm.
Bear in mind this limitation specifically applies to the "Rowland Mounting" configuration, where the angle of diffraction (β) remains constant at
0 degrees at the center of the grating. This is a theoretical limit where the angle of incidence (α) reaches 90 degrees; the practical
limitation of this configuration is actually at a wavelength somewhat lower than λ.
Please note that this is a different limit than the longest wavelength that a given grating will diffract, where λ = 2d, where d = the groove
spacing in nm = 10^6/G. In other words, the longest wavelength a grating will diffract is twice as long as the limit imposed by the "Rowland
Mounting".
[Edited on 1-5-2014 by DistractionGrating]
[Edited on 1-5-2014 by DistractionGrating]IrC - 1-5-2014 at 14:11
