Development of a multichannel Fourier transform spectrometer

We devised a novel type of multichannel Fourier transform spectrometer (MCFTS) that incorporates a Wollaston prism, polarizing interferometer combined with two Savart plates and a phase-retarding plate. This original MCFTS produces a number of lines of folded interferograms recorded with a two-dimensional imaging detector such as a CCD detector. In the present type of MCFTS, the total incident light is available except for a small amount of reflection loss. It is possible to enhance the signal-to-noise ratio. The enhancement of the resolving power is also expected by the connection of the interferograms with a newly developed method.     

Fourier transform near-infrared spectrometer using a corner-cube integrated prism scanning interferometer

This paper describes a Fourier transform (FT) near-infrared spectrometer that uses an integrated prism scanning interferometer whose optical paths are stabilized by corner cubes. A combination of corner cubes and a retroreflection mirror, which is sometimes used in the conventional interferometer for FT spectrometers, is adopted and adapted to the integrated prism scanning interferometer through a special design. Without any degradation of spectroscopic properties, the optical path in the interferometer is highly stabilized and the moving distance of the stage is halved. These advantages provide a robust and portable FT spectrometer for field use.     

An enhanced Fourier transform spectrometer with a search algorithm

Interferometric instruments have the following serious weak points: (1) the necessity of doing a Fourier transform that involves a vast amount of calculation; (2) the lack of knowledge of suitable measuring conditions until the Fourier transform is finished; and (3) the spectral resolution of the conventional Fourier-based techniques is significantly affected by the sampling rate, data length, and noise in signal processing. In this paper, an enhanced spectrometer is proposed using the modified forward-backward linear prediction method (MFBLP) with a search algorithm. To document the advantage of the method presented, a computer simulation for multiple-wavenumber estimation is investigated. The MFBLP method is truly superior to the fast Fourier transform (FFT) method. In general, the spectral resolution using the FFT method is proportional to the data length. In this paper, however, it is shown that excellent results can also be obtained from only 60 sample points using the FFT method. Moreover, from experimental results, we also conclude that the sampling rate must be consistent with the condition 632p·t·D<3164, where f_p , represents the value of the pulse generator frequency in Hz, t the observation time, and D the decimation factor     

Field-of-view analysis of a polarization interference Fourier transform imaging spectrometer

The Fourier transform imaging spectrometer (FTIS) is an important tool for the measurement of spectral information in a scene. Advances in electro-optic crystal systems have led to the advent of the FTIS based on polarization interference filters. The operation of these devices as spectrometers has been well characterized, but the imaging capabilities have yet to be thoroughly explored. We explore the field-of-view limitations that occur when using this particular type of FTIS.     

Mid-infrared measurements of the atmospheric emission over the South Pole using a radiometrically calibrated Fourier transform spectrometer

We conducted year-round measurements of the downwelling atmospheric infrared emission over the South Pole in 1992. The instrument covered the 550-1600-wave-number region with 1-wave-number resolution. We calculated the water vapor content for clear-sky cases and found a good correlation with the surface temperature, with values ranging from 0.2 to 0.8 mm. Ozone-sonde profiles were compared with total column abundances of O(3) retrieved from the spectra. The experiment is explained in detail, including the instrumentation, calibration, and retrieval methods used. The calibrated spectra contain information about several trace gases, water, clouds, temperature profiles, and aerosols.     

Effects of reference laser intensity noise in a Fourier transform spectrometer

Many noise sources deteriorate the performance of a Fourier-transform spectrometer. When such an instrument utilizes a reference source such as an HeNe laser to sample the measurement data, it is desirable to determine how the intensity fluctuations of the reference source will affect the final spectrum. This text shows how the intensity noise of the reference laser is transformed at the output of the interferometer, how it translates into a sampling jitter by a simple linear relation under a small bandwidth approximation, and how it is manifested in the final spectrum as a noise floor. Analytical and simulation results show that for an instrument designed for optical communication applications, the intensity noise of the reference laser may be one of the dominant noise source limiting the overall performance of the instrument.     

New approach to high-precision Fourier transform spectrometer design

Laser fringes have long been used to establish the x axis in interferometric spectrometry, but solutions for the intensity axis have been less satisfactory. Now we are seeing the rapid commercial development of low-cost, medium-speed, sigma-delta analog-to-digital converters developed for stereo audio applications. A single chip provides two channels of 20-bit precision at 50 kHz, a significant improvement over many current systems of much greater cost and complexity. But while the laser works in the spatial domain, this converter operates strictly in the time domain; it cannot be triggered. I have developed a bridge between these two domains, the adaptive digital filter, which not only permits us to use this converter to obtain measurements at arbitrary times but as a bonus shows us how to move much of the complexity of an interferometric-control and data-acquisition system from hardware to software. For example, flexible fringe subdivision (to increase the free spectral range) is easily obtained with a simple and efficient algorithm, completely free of laser ghosts. Compensation for drive velocity variation is also possible, requiring only a modest increase in computer memory.     

Fourier transform Raman spectroscopy of thin films

Conventional Raman measurements of dyes in the visible region exhibit resonant effects which enhance the Raman scattering cross-section of the chromophores by several orders of magnitude but make scattering from other parts of the molecules, such as the hydrocarbon chains, unobservable. Taking advantage of the benefits inherent to Fourier transform (FT) spectroscopy, Raman spectra can now be measured routinely with an FT IR spectrometer and a continuous wave Nd:YAG laser. By coupling the laser excitation into a thin film waveguide, we have recently observed FT Raman spectra of a thin film of polystyrene. The advantages of using integrated optics with FT Raman spectroscopy for Langmuir-Blodgett films of dye molecules are also discussed.     

Phase determination for a Fourier transform infrared spectrometer in emission mode

Beam-splitter emission strongly influences the spectra measured with a Fourier transform spectrometer (FTS) as it affects the entire phase behavior, in particular in emission spectroscopy. The various radiation contributions of the scene and the FTS itself have different phases in the complex spectrum. As a specific feature, the radiation of the beam splitter is rotated by approximately pi/2 relative to the scene effective radiation. By classical methods of phase correction, the radiation components of different phases are mixed in the complex plane, which may lead to serious errors in the calibrated spectra. For this reason, the nature of the FTS phase has been studied, and a statistical phase determination method has been developed. It allows us to determine the phase function of the scene by minimizing the correlation between the imaginary and the real parts of the complex spectrum and by reducing the variance of the imaginary part. Thus phase accuracies of 10 to 30 mrad can be achieved. In addition, the remaining error of the phase can be calculated for each individual spectrum. The total phase error and its effect on the spectra are discussed.     

Design and optimization of the calibration procedure for a miniaturized Fourier transform spectrometer

This paper presents a Fourier transform infrared (FT-IR) spectrometer calibration procedure based on an unusual source made from a spectrally selective surface. An alternative solution to the usual calibrators has been developed to cope with the tight mass budget of an instrument devoted to Mars surface exploration. The designed system has proved effective, in terms of achievable radiometric accuracy, despite the drawbacks due to the significant reflectivity of the sources. The proposed procedure is a standard "two-source" approach in which both cold and hot sources are thermally controlled surfaces, similar to an optical solar reflector, associated to a filament lamp. Such a system allows the required signal to be achieved in the 2-25 l m instrument wavelength range. Source optimization was performed using, as a cost function, the computed radiometric uncertainty, while the required absolute accuracy of the instrument was imposed as the optimization constraint.     

Instrumental phase-based method for Fourier transform spectrometer measurements processing

Phase correction is a critical procedure for most space-borne Fourier transform spectrometers (FTSs) whose accuracy (owing to often poor signal-to-noise ratio, SNR) can be jeopardized from many uncontrollable environmental conditions. This work considers the phase correction in an FTS working under significant temperature change during the measurement and affected by mechanical disturbances. The implemented method is based on the identification of an instrumental phase that is dependent on the interferometer temperature and on the extraction of a linear phase component through a least-squares approach. The use of an instrumental phase parameterized with the interferometer temperature eases the determination of the linear phase that can be extracted using only a narrow spectral region selected to be immune from disturbances. The procedure, in this way, is made robust against phase errors arising from instrumental effects, a key feature to reduce the disturbances through spectra averaging. The method was specifically developed for the Mars IR Mapper spectrometer, that was designed for operation onboard a rover on the Mars surface; the validation was performed using ground and in-flight measurements of the Fourier transform IR spectrometer planetary Fourier spectrometer, onboard the MarsExpress mission. The symmetrization has been exploited also for the spectra calibration, highlighting the issues deriving from the cases of relevant beamsplitter emission. The applicability of this procedure to other instruments is conditional to the presence in the spectra of at least one spectral region with a large SNR along with a negligible (or known) beamsplitter emission. For the PFS instrument, the processing of data with relevant beamsplitter emission has been performed exploiting the absorption carbon dioxide bands present in Martian spectra.