The source of FTIR contains a range of frequency so it should not be monochromatic. IR sources are usually lamps in far, mid and near IR.
An FTIR is a spectrometer, so a tool to measure the spectral power density of radiation. The source for an FTIR is whatever you choose as the source. If you want to measure absorption/reflection/transmission spectra, you would need a source that meets your requirements.
Additionally, lasers need not necessarily be monochromatic. Think of modelocked lasers, or supercontinuum sources, etc.
Any broadband laser source that spectrally covers region of interest can be used for FTIR. One just has to normalize measured absorption/transmission spectrum to the incident spectrum (spectral density). In commercial FTIR's that use lamps, this is done automatically.
As said Henning Höpfner and Lukasz Piatkowski, commercial FTIR's used lamps or Nernst pin as a IR source. But, manufacturers often install laser in their spectrometers for self calibration of interferometer (not as IR source).
You may use suitable diode lasers or gas/ solid state lasers as IR sources with a relatively definite narrower band width respect to IR lamps. In this case, you obtain fine spectra within the laser gain profile. The main reason that most of manufacturers use IR lamps in FTIR arises from the IR caharacteristics of molecular bases which are distributed over a wide spectral range where most of the laser bandwidths fail to fulfill. The tunable broad band lasers such as dye or Ti:sa lasers are available, however those can not cover whole NIR-MIR and FIR spectral ranges needed for the efficient FTIR spectroscopy. On the other hand, the tunable lasers are separately exploited in various laser spctrocopic instuments in order to obtain sharp and high resovlved caharacteristic peaks. Laser based instuments are employed in Lidars such as DIAL or LIBS/Raman lidars where no other IR sources can compete mainly due to the small divergence and long range characteristics of lasers.
Laser (CW or pulsed) can be used for FTIR-spectrometers in case of luminescence experiments. Laser excites luminescence in the sample, placed in position of the source, and spectrometer measure light from this sample.
The same method is used in LIBS . You may induce microplsma by intense pulsed laser and then use the spectometer for the measurement of IR spectra. However, it is not commercial instrument to my knowledge.
The idea of FTIR is to acquire simultaneously spectral information on a large spectral range. The three well-know advantages of this setup are :
1) The multiplex or Fellgett's advantage. This arises from the fact that information from all wavelengths is collected simultaneously. It results in a higher Signal-to-noise ratio for a given scan-time or a shorter scan-time for a given resolution.
2) The throughput or Jacquinot's advantage. This results from the fact that, in a dispersive instrument, the monochromator has entrance and exit slits which restrict the amount of light that passes through it. The interferometer throughput is determined only by the diameter of the collimated beam coming from the source.
3) The cones advantages. The use a of small HeNe laser (not as a source) allows a very good reproducibility of the wavelength or wavenumbers.
For sure, you can use a laser for IR experiments but you will have to forget your interferometer and main advantages.
Another important laser based source for FTIR is the frequency comb (regular comb of optical frequencies generated by a stabilized pulsed laser). It keeps all advantages of FTIR and brings more (high brightness/small focus, high speed especially in dual comb experiment) see e.g.
Optics Express, Vol. 13, Issue 22, pp. 9029-9038 (2005)
http://dx.doi.org/10.1364/OPEX.13.009029,
cavity enhanced version is very sensitive (but spectrally limited by cavity used) Nature Photonics 4, 55 - 57 (2010) doi:10.1038/nphoton.2009.217
Resolving the femtosecond com teeths brings another possibilities, some we demonstrated (in VIS-NIR) in Meas. Sci. Technol. 23 094001 doi:10.1088/0957-0233/23/9/094001
Thanks for the question and Thanks for all valuable comments. I wish all of you a Happy New Year 2014.
We generally used IR source is a Globar source (silicon carbide rod), which is electronically heated emits radiation of wavelength falls under IR region. We could used laser too as generally using in pump-probe method or 2D IR. Generally in FTIR laser is used to to measure mirror movements and hence for interferometer.
In general. even broadband laser does not acheive to cover a wide spectra ranging 1000-10000 cm-1 needed for FTIR. If it was technically possible, then that would be very expensive instrument and I am not assured the coherence and brightness property of laser can improve overall FTIR performance.
Hi Soheila, in an FTIR spectrometer a laser is normally used to align and calibrate the spectrometer while for IR source various kinds of lamps (like Globar) are used. The source has a wide spectral range (not monochromatic).
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