Diffraction limitations of the lamellar grating interferometer in far-infrared fourier transform spectroscopy

The prediction of scalar diffraction theory that the effect of varying the groove depth of a lamellar grating illuminated by a plane wave at near-normal incidence is to modulate sinusoidally the intensity of specularly reflected flux is herein submitted to a critical re-evaluation.     

Early history of fourier transform spectroscopy

This is an attempt to explain how and why the stage was set for the appearance on the scene of Fourier transform spectroscopy (FTS) in the 1950s and not a whit before. The play begins 100 years earlier with Fizeau and Foucault who first produced high path-difference interference phenomena and used them for measuring solar spectrum wavelengths in the near IR. Next, the story unfolds with Michelson's contribution, which led to important discoveries around 1890: the hyperfine structures and widths of atomic lines. Somewhat less well known is the Rubens interferometric technique, presented in 1910, because no such striking results were ever collected; still, it represented a distinct advance over the Michelson one.What is the reason why Michelson, Rubens and Lord Rayleigh (who made no experiments himself but understood all about them) never managed to get together and propose the modern form of FTS? Part of the responsibility we must ascribe to chance; however, sufficient motivation could not be felt as long as basic noise limitations had not been understood and closely approached.     

Pulsed millimeter wave fourier transform microwave spectrometer

An improved pulsed microwave spectrometer for the detection of rotational transitions in gaseous molecules in the frequency range of 130–150 GHz is described. It incorporates a tunable Fabry-Perot cavity and a low noise superneterodyne receiver for the detection of the molecular emission signals. The molecules are excited by /2 pulses provided by a high efficiency frequency doubler which is pulse modulated at an IF frequency of 1.4 GHz.     

An infrared photothermal beam deflection fourier transform spectrometer

A photothermal Fourier transform spectrometer built to accommodate a beam deflection detection system is described. The instrument, which can also function as conventional absorption FT spectrometer, and also a conventional FT photoacoustic spectrometer, covers the 3950−450 cm⁻¹ range. With quantities of l mg of powdered organic or inorganic material, infrared spectra of high quality can be obtained in minutes. Sample cells are not needed, and sample preparation is not required. The surface of truly massive objects may be examined, and surface studies may be carried out under rigorously controlled conditions.     

An electric-field cross-correlation fourier transform millimeter wave spectrometer

The principle and fundamental structure of a millimeterwave electric-field cross-correlation Fourier transform spectrometer are presented. In the spctrometer, two pseudorandom binary sequences (PRBS) with controllable phase variation are used to modulate individually two wide bandwidth microwave signals with variable phase delay are obtained. And the Fourier transform of the crosscorrelation function of these microwave signals is the cross-power spectrum. Some experimental results about the spectrometer are given in 8mm-wave range.Supported by Foundation of Specialities Opened to Doctorate Study.     

Slow-light spatial solitons

We numerically and experimentally report the observation of slow-light spatial solitons in a Kerr medium owing to light amplification by stimulated Raman scattering. This was achieved in a CS2 nonlinear planar waveguide that possesses both a strong self-focusing nonlinearity to generate the spatial Raman soliton and a Raman susceptibility sharp enough to induce the slow-light process simultaneously. We show that the Raman Stokes component is optically delayed by more than 120 ps for a 140 ps Raman pulse duration and only 3 cm of propagation length, while propagating as a spatial soliton beam.     

Fully automatic software correction of fourier transform NMR spectra

A simple computational procedure is discussed by which it is possible to correct fourier NMR spectra automatically for all amplitude and phase distortions resulting from signal-conditioning circuits and from anomalies in quadrature phase detection.     

A fast-scanning Fourier transform 2D IR interferometer

Ultrafast two-dimensional infrared spectroscopy (2D IR) allows for the characterization of vibrational couplings and chemical dynamics. The fastest method of acquiring a Fourier transform 2D IR data set involves spectrally dispersing the signal field onto an infrared array detector. However, use of this method carries disadvantages, including the high cost of IR arrays and the decrease of signal intensity due to dispersion. As an alternative, we demonstrate a readily implemented full time-domain 2D IR detection method in which data from a pulsed laser source is rapidly acquired by scanning an interferometer delay at constant velocity. The stage's position is determined with high accuracy on a shot-to-shot basis by quadrature detection of HeNe tracer interference fringes.     

傅里叶变换红外光谱仪干涉仪系统的设计

设计了一种基于立体角镜与固定平面镜组合的干涉仪系统,以解决传统傅里叶变换红外光谱仪中干涉仪系统由于动镜的倾斜与横移影响光谱质量的问题。系统包含一对立体角镜,两面相互垂直的固定平面镜,以及分束器。通过对立体角镜的倾斜与横移、立体角镜垂直度误差以及两固定平面镜的倾斜与垂直度误差对干涉信号初相位与调制度的影响进行分析,表明立体角镜的倾斜与横移、固定平面镜的垂直度误差等因素不会影响干涉信号的初相位与调制度;通过对干涉仪进行实际测试表明,仪器具有结构简单紧凑、密封性好、分辨率高以及抗震性强等优点。     

Edge-enhancement fourier transform X-ray microscopy using a Laguerre-Gaussian zoneplate

Efficient techniques to achieve isotropic edge enhancement in optics involve applying a radial Hilbert, Laguerre-Gaussian, or differentiating filter on the object spectrum. We demonstrate a simple setup for isotropic edge-enhancement in soft-X-ray microscopy, using a single Laguerre-Gaussian zone plate (LGZP). Since the LGZP acts as a beam-splitter, diffraction efficiency problems usually associated with X-ray microscopy optics are not present in this system. As numerically demonstrated, the setup can detect optical path differences as small as λ/50 with high contrast.     

Three-dimensional fourier transform NMR imaging. High-resolution chemical-shift-resolved planar imaging

A conventional high-resolution NMR spectrometer has been adapted for a three-dimensional imaging experiment which involves two spatial coordinates plus chemical shift. One dimension has far greater digital resolution than the other two, and when it is used to encode the chemical shifts it is possible to obtain separate slice images showing the distribution of each chemical species within the imaging plane. The method is illustrated using a tube phantom containing ethanol and water, in this case the ethanol gives sufficiently narrow finewidths that it is possible to obtain a separate image from the individual transitions of each multiplet.     

High-resolution infrared spectra of air pollutants with fourier transform spectroscopy

A Fourier transform spectrometer coupled with a long-path cell was used to record high resolution infrared spectra of air pollutants at very low concentrations. The spectral transmittance curves of CO were measured at room temperature, at a constant resolution of 0·5 cm^-1, at partial pressures from 2·32 to 0·004 torr, N₂ being the diluent. Apparent absorption coefficients were tabulated at 0·2-cm^-1 intervals. The techniques and data are applicable to studies of our contaminated atmosphere.     

Non-unitary representations and fourier transform on the poincaré group

Two families of representations of the Poincaré group $\text{P}$ associated to complex orbits (with complex linear momenta) are introduced. The conditions for equivalence and irreducibility are stated. Fourier transforms related to them are defined and inversion formulas obtained for functions infinitely differentiable and of compact support on $\text{P}$.     

Fabry-Perot etalons as prefilters for astronomical far-infrared fourier transform spectroscopy

We show that a Fourier transform spectrometer (FTS) can be made competitive in sensitivity (system noise equivalent power) with scanning single detector spectrometers (SSDS) such as scanning Fabry-Perot spectrometers or grating instruments for work on specific lines in FIR astronomy by prefiltering the radiation so that the broadband background on the detector(s) of the FTS is reduced. Fabry-Perot etalons made with electroformed metal mesh reflectors are suitable prefilters. A Fabry-Perot prefiltered FTS has a capability for simultaneous observation of lines at substantially different wavelengths in the same beam on the sky. The lines are passed by different orders of the Fabry-Perot, and their separation and resolution is accomplished by the FTS. We discuss the sensitivity comparison between the prefiltered FTS and an SSDS, identify several suitable pairs of astrophysically interesting lines, discuss some of the technical aspects of building very compact etalons designed for use at LHe temperatures, report new measurements of properties of commercially available metal mesh, and describe the performance of etalons built for line pairs near 50 μm.     

A new scheme for resolution enhancement in fourier transform NMR

A new method for resolution enhancement in FT NMR is described, the physical principle of which is that of digital filtering by a sinusoidal window. The practical aspect of the method is developed, showing that the result can be obtained on a classical spectrometer without additional programming and with great versatility.     

IR spectral depth profiling using fourier transform photothermal beam deflection

FTIR photothermal beam-deflection spectroscopy (PBDS) was used to make spectral depth-profiling measurements with synthetic bilayer samples of polyethylene/nitrocellulose, with a commercial plastic having surface printing and with a single human hair. A Digilab interferometer modified to operate at several scan speeds was used to record the spectra, without the cell-resonance problems found with photoacoustic spectroscopy (PAS). The utility of spectral depth profiling is discussed; significant S/N improvements seem to be needed and, with either PBDS or PAS, a wider range of modulation frequencies is required for the methods to be useful.     

High resolution fourier transform spectroscopy of shallow acceptors in ultra-pure germanium

The excitation spectra of shallow acceptors in ultra-pure germanium (10⁹ cm^-3 < N_A ? N_D < 10¹² cm^-3 were studied using Fourier Transform Spectroscopy and photothermal excitation. Using samples with a total number of less than 10⁸ acceptor atoms, the signal to noise ratio was better than 100. In the purest samples the natural width of some excited states was found to be below the instrumental resolution of 0.03 meV (= 0.25 cm^-1). Some new observations involve the resolution of lines of states near the top of the valence band and the discovery of two unknown shallow acceptors.