Spectroscopy is the branch of physics and analytical chemistry dealing with electro-magnetic spectra. In physics, spectroscopic methods are used to study the properties of any possible interaction of radiation with matter. In analytical chemistry, it is used for the detection and characterization of materials by measuring their characteristic spectra.
To begin with, there is a brief review how spectrometers are used and what characteristics they feature. Spectrometers measure the intensity of electromagnetic radiation as a function of frequency or wavelength. Spectrometers operating in the optical range (from ultraviolet (UV) to infrared (IR)) are referred to as spectrophotometers. One of the problems that spectrophotometers can solve is measuring transmittance, reflectance, and absorption of a sample as a function of frequency (wavelength) of light incident on it. Key parameters of a spectrophotometer are operating spectral range, spectral resolution, dynamic range, and the rate of spectrum acquisition.
Operating spectral range is the interval between the minimum and maximum light frequency (or corresponding wavelengths) within which the spectrometer measures reliable data.
Spectral resolution is the minimum frequency or wavelength difference that can be distinguished (resolved) in the spectrum. Usually spectrophotometers are designed so that the spectral interval corresponding to resolution coincides with the minimum scanning step. Spectral resolution is expressed either in terms of wavenumber - reciprocal centimeters (cm-1), or in wavelength units - nanometers (nm).
Dynamic range is the ratio of the maximum signal level of a radiation detector (at which the detector is still in linear mode) to the detector noise level. Dynamic range determines, for example, the interval between the minimum and maximum optical density recorded by the instrument. Under the condition of low optical density or high transmittance the dynamic range can be limited by photon noise. At high values of optical density (low transmittance) the photodetector dark noise becomes critical. In Fourier transform spectrometer due to interferogram averaging the noise level is reduced regardless of its nature.
Rate of spectrum acquisition is the number of spectra acquired in a unit of time for a given spectral resolution. In scanning monochromators the time of spectrum acquisition is determined by a scan rate (typically expressed in nm/min) and depends on the entire spectral range width; in Fourier transform spectrophotometer the spectrum is calculated based on interferogram once for the entire spectral range, so the registration speed is determined by number of acquired spectra per time unit, such as per second.
A modern dispersion spectrophotometer is typically based on a monochromator having one or several dispersion gratings. A light beam entering the input slit is collimated with a concave mirror and then goes to the diffraction grating. The grating reflects the light in some range of angles, different angles corresponding to different wavelengths. A narrow slit is set at the output, which cuts out certain spectral band out of the angle decomposition. It is the width of the spectral band defined by the size of the slit that determines spectral resolution of the instrument. The narrower the slit the higher spectral resolution, but at the same time the larger light loss. As the grating is rotated, different wavelengths can be “cut out”. The amount of light at each wavelength is measured with a detector, for example, silicon photodiode. Thus, in order to collect transmission spectrum of a sample, e.g., a glass plate with deposited film of substance under investigation, one should place the sample between the monochromator output slit and the photodetector. Sequential scanning through wavelengths by means of the grating rotation together with light intensity registration behind the sample provides a spectrum – light intensity vs. wavelength. The spectrum contains data on transmittance of the sample as a function of wavelength. In order to get these data one should also measure intensity spectrum at the monochromator entrance without a sample (so called "reference" spectrum). Having calculated the ratio of the intensities of the two spectra one gets the transmission spectrum of the sample.
Mechanical rotation of a diffraction grating itself takes time due to its being inertial. So, even rough registration of spectrum (with low spectral resolution and low signal-to-noise ratio) takes tens of seconds. Acquisition of a spectrum with high resolution and in a wide dynamic range, when each separate wavelength should be filtered with high accuracy, may take tens of minutes or even hours. This imposes very strict requirements for the light source stability in time. Moreover, the light scatters in a dispersion spectrometer on the grating and other inner parts. It means that in addition to useful light a scattered light with unknown spectral characteristics falls onto the output slit. The scattered light causes actual spectrum distortion and dynamic ratio decrease. This problem is treated with second grating installation, application of special coatings onto inner details of the spectrophotometer. One more problem is that the grating provides a multiplicity of diffraction orders that must be filtered (excluded). All these facts limit both spectral and dynamic range of the instrument and obviously increase the cost.
Fourier transform spectrophotometer is based on a Michelson interferometer. Collimated beam of light falls onto the interferometer entrance. Then the beam is split by a beam splitter plate into two beams having approximately equal intensity. After being reflected from mirrors these beams interfere at the interferometer output. At least one mirror in FTS is movable. Special mirror movement control systems provide its periodic movement, moving speed being maintained constant for maximal possible part of the mirror’s path. The resulting interferogram that is registered with the photodetector includes all information about irradiation spectrum. Spectral resolution in this case is controlled by movable mirror path length, whereas spectral range is limited only with working range of the light source, the beam splitter, the mirrors and the photo detector.
It is worth noting that in order to provide spectral resolution of 10 cm-1 (being moderate for IR range, this value corresponds to more than dignified resolution of 0.1 nm in UV range) mirror’s movement is required as small as 1 mm. This movement can be done very quickly. Thus, high spectra acquiring rate is an obvious advantage of Fourier transform spectrophotometer. FTS design does not imply any slits, which might considerably blind the light beam; accordingly, these instruments are considered as light efficient. Scattered light does not participate in interference, therefore non it affects both the interferogram and the resulted spectrum. Also Fourier transform spectrophotometer does not have the problem, which is typical for grating instruments and related to the necessity of higher order suppression. An interferogram accumulation, on the contrary to a spectrum accumulation, does allow to expand dynamic range considerably.
However, along with many clear advantages of Fourier transform spectrometers over diffraction spectrophotometers, they still have their weak points. FTS technology requires very accurate mirror movement that is achieved by using advanced mechanics and complicated mirror control system based on laser reference channel. In order to provide high accurate interferogram, all optical elements must be of ultimate quality. All factors mentioned lead to current realizations of FTS being high cost and covering mostly IR spectral range, in which they surpass any other device by spectral resolution and dynamic range.
Novelty of our Fourier transform spectrometer is elimination the idea of the mirror movement at a constant speed. Instead we have developed the principle of the mirror movement according to well-known harmonic law. This approach has allowed to simplify mechanical part, to exclude reference laser channel and, at the same time, considerably raise the upper frequency boundary of working spectral range.