I have found a shitload of references on this, the use of birefringent, uniaxial Wollason Prisms for Michelson-interferometer-less FT-spectroscopy with NO moving parts (see Padgett, et al, 'Fourier-Transform Spectrometer
Utilizing a Birefringent Optical Component' (1998) US Patent No.5,781,293 & Sietz, et al, 'Wollaston Prism and Use of it in a Fourier
Transform Spectrometer' (1999) European Patent Application No.0 939 323 A1), the benefits of which are obvious - no moving parts = no mechanical
breakdowns, wear, etc. and also seriously reduces the complexity of the entire device, which is extremely useful for remote instruments (such as those
used on spacecraft, see Hayden Smith & Hammer, 'Digital Array Scanned Interferometer: Sensors and Results', Applied Optics, Vol.35(16)
1996, pp.2902-2909).
Funny thing is, it might actually be rather easier to work out how to achieve this than the Raman Spectroscopy, given that the light-source is OTC and
that there are numerous online variants of Fast-Fourier-Transform (FFT) and the associated code... In fact there is even at least one paper on how the use of high-computing power in the display computer, to both
display the spectrum, but also to compute it directly in minimal time (in comparison to the old punch-card based mainframes) has changed the FT-IR
Spectrometer completely (previously the mathematics involved would have been prohibitively slow - see Balashov, et al, 'Modern Fourier Transform
Spectrometers - A New Branch of Computer Optical Instrumentation', Computer Optics, Vol.2(2) 1990 pp.173-180).
There is even at least one reference to utilizing a photodiode-array (Okamoto, et al, 'A Photodiode Array Fourier Transform Spectrometer Based on
a Birefringent Interferometer ', Applied Spectroscopy, Vol.40(5) 1986 pp.691-695) to collect the beams so they can be evaluated (also Hayden
Smith & Hammer, op. cit.). While at least one paper uses CCD/CMOS sensors (ie. flat panel arrays) to demonstrate the utility (obviously in the
UV-NIR range - see Luet, et al, 'Imaging Polarization Interferometer for Flat Panel Display Characterization', Soc. Information Display
(SID), Digest O4, pp.1-4). Indeed, one extremely interesting paper describes the use of Light-Emitting Diodes as a way of measuring femtosecond and
picosecond laser pulses through a Wollaston prism and also though a Michelson Interferometer (Reid, et al, 'Light-Emitting Diodes as Measurement
Devices for Femtosecond Laser Pulses, Optics Leters, Vol.22(4) 1997 pp.233-235), how they'd stand up to longer exposure would be interesting to
find out (given the enormous disparity in price between LEDs and Photodiodes).
There is also a range of papers on how to build static (ie. no moving parts) spectrometers, and even how to widen the field of view of the same
(Courtial, et al, 'Design of a Static Fourier-Transform Spectrometer with Increased Field of View', Applied Optics, Vol. 35(34) 1996
pp.6698-6702 - attached), while other researchers have taken the opportunity to examine the alternative Birefringent, Uniaxial Crystals available and
discovered ways that this technology can be taken from the mid-UV (~200nm) all the way to the upper edge of the NIR region (1400nm), (see Harvey & Fletcher-Holmes, 'Birefringent Fourier-Transform Imaging Spectrometer' Optics Express, Vol.12(22)
2004 pp.5368-5374), while another group(s) has published an article on the use of the same in the Vacuum UV-Region (the <200nm region, where the UV
light is absorbed by the atmosphere - see de Olievera, et al, 'A Fourier Transform Spectrometer without a Beamsplitter for the Vacuum UV Range:
From the Optical Design to the first UV Spectrum, Review of Scientific Instruments, Vol.80(4), 2009 pp. 043101.1-043101.13).
These looked interesting when they first appeared. One thing you must remember is that both the optics and the detectors need to have fairly flat
characteristics across the wavelengths of interest; this limits the overall bandwidth.
The need for arrays of detectors restricts it to roughly the same range as standard dispersive UV-Vis spectrometers, although it does have advantages
over those.
Quote:
Funny thing is, it might actually be rather easier to work out how to achieve this than the Raman Spectroscopy, given that the light-source is OTC...
But that is operating in the same mode as vanilla UV-Vis instruments, giving you standard absorption and fluorescence data in that range. It does not
give you the structural information provided by IR/Raman spectroscopy. So your comparison is a prototypical hammer-to-saw, two different categories of
tools. Now you could use this technique in place of the grating based dispersive section of a Raman spectroscope, but you still need the same light
source and filtering - you're just displacing gratings or prisms.
You should note that Light-emitting diodes as measurement devices for femtosecond laser pulses has very little to do with what your are
interested in. The lasers have pulse rates around 80 MHz, call it 10 nsec between pulses `cause I'l lazy, and pulse widths of 1 ps or less. That means
the average power is no more that 1 / 10000 of the pulse power, and they are measuring average power levels in the 5 to 100 mw range. Plus they are
not doing spectrum analysis, but rather auto-correlation of the pulse to determine its width, they only need one detector for this.
unome - 25-3-2010 at 13:52
I was wondering about using them as the beamsplitter for the Raman Spectroscopy as a matter of fact... On top of which, how in god's name do we carry
out FT-Raman? I mean, it sounds like a good idea, but how does it work?not_important - 25-3-2010 at 14:11
Just use a Wollaston-Prism based FT spectroscope in place of the more traditional prism or grating based dispersive spectroscope for analysing the
Raman radiation. It work well when the exciting light is of a wavelength that the Raman radiation lies within the range a CCD detects well, same as
with using a grating. You still need the same sort of filtration for either method.
However the Wollaston prism(s) and other optics are likely to cost more than a decent grade grating. There may be slightly reduced optical quality
requirements, but scattered light and detector noise & resolution still limit the system performance.
As for beamsplitters in Raman, if you mean for the laser it's not too difficult as the requirements aren't too bad - you're dealing with one
wavelength after all. You can even use fiber optics, a pair of fibers have the filtered laser focused on them with one fiber illuminating the sample
and the other the reference. BTW - polystyrene or toluene in CHCl3 is a decent reference, having bands at both ends of the shift range as well as
points in the mid-range.
unome - 25-3-2010 at 18:19
Yeah, I was mainly after a beamsplitter - optical grade Calcite crystals are cheap as shit (AU$2/ea), cutting & shaping them may be interesting,
but the means of splitting the 532nm beam is what I was after - that would allow me to separate it and get both the reference solvent as well as the
reference solvent + analyte. That allows me to use maths to remove the solvent from the analyte spectra
Does the fact one beam is going to be polarized going to make a difference?
