Fourier-transform phase-shifting interferometry

Phase-shifting interferometry is a preferred technique for high-precision surface form measurements, but the difficulty in handling the intensity distortions from multiple-surface interference has limited the general use of the technique to interferometer cavities producing strict two-beam interference. I show how the capabilities of phase-shifting interferometry can be extended to address this problem using wavelength tuning techniques. The basic theory behind the technique is reviewed and applied specifically to the measurement of parallel plates, where surfaces, optical and physical thickness, and homogeneity are simultaneously obtained. Basic system requirements are derived, common error sources are discussed, and the results of the measurements are compared with theory and alternative measurement methods.     

Performance analyses of an infrared single-mode all-fiber-optic Fourier-transform spectrometer

Stretching one of a pair of fiber arms can be done to realize optical phase modulation for an IR single-mode all-fiber-optic Fourier-transform spectrometer (FTS). But this operation will inevitably limit the physical performance of a FTS. We study these limits theoretically and experimentally. The optical path difference (OPD) will be dispersive. At the first-order approximation, this OPD dispersion will result in a shift in the recovered spectra. The spectral resolution and the sampling distance will also be dispersive. Linear birefringence introduced when a curved fiber is stretched will affect the final spectra. This effect can be eliminated by real-time compensation and (or) by system design. Errors encountered uniquely in the all-fiber-optic FTS in the optical phase domain, such as the fiber-parameter errors, nonlinearity of the piezoelectric cylinder, and their effects on the spectra are analyzed, from which we deduce the requirements for calibration. Finally, calibration methods for optical phase modulation are discussed.     

Infrared laser isolation of ions in Fourier transform mass spectrometry

A new method for isolating ions for tandem mass spectrometry analyses in Fourier transform mass spectrometry is illustrated. The method employs an infrared laser to dissociate completely the undesired ions. The selected ions are excited to an orbit away from the degradative portion of the laser beam. Ion isolation was accomplished and tandem mass spectrometry experiments were performed on model oligosaccharides and compounds from biological samples.     

Theoretical and experimental considerations for a single-mode fiber-optic bend-type sensor

A novel single-mode bend fiber-optic sensing principle is presented. The design makes use of the translucent protective sheath that encases a typical fiber as a means of locating the position of a small bend present on an otherwise straight fiber. We can simultaneously determine bend magnitude by measuring the reduction in the fiber's core light. The theoretical model presented and the experimental results obtained are in excellent agreement, suggesting that a single-point sensor system is feasible with the proposed technique.     

Wavenumber scanning-based Fourier transform white-light interferometry

Fourier transform white-light interferometry recovers the optical path difference of an interferometer by measuring the phase change caused by scanning wavelength. However, the optical spectrum, obtained by wavelength scanning method (λ-method), contains a chirp in period. The chirp would induce deviation and decrease the measurement accuracy. An improved method, the wavenumber scanning method (k-method), is proposed and experimentally demonstrated, in which there is no chirp in the optical spectrum. The measurement results using the k-method and the λ-method are compared experimentally. The experimental results show that the standard deviation of the measurement results decreases from 0.015 to 0.004?μm, when an extrinsic Fabry-Perot interferometer with a cavity length of 387?μm is interrogated.     

Hybrid sampling approach for imaging Fourier-transform spectrometry

A new approach to interferogram sampling is demonstrated for Fourier-transform spectrometry. Sampling of the infrared channel is triggered at equidistant optical path differences while samples are time-referenced with a high-resolution digital clock. This hybrid method exploits the advantages of both time and position-sampling techniques. It minimizes the dataload while allowing a postcorrection scheme to remove sampling errors. It is therefore highly adapted to imaging spectrometers designed for massively parallel spatial sampling. Also, this technique is particularly interesting for spectrometers equipped with an integrating detector, such as a CCD camera, since it can account for the inevitable delay caused by camera integration.     

Intrinsic fiber-optic ultrasonic sensor array using multiplexed two-wave mixing interferometry

An intrinsic multiplexed laser interferometer is presented that allows for the simultaneous detection of acoustic waves by an array of fiber-optic sensors. The phase-modulated signals from each sensor are demodulated by use of an adaptive two-wave mixing setup. The light from each sensing fiber in the array is mixed with a reference beam in a single photorefractive crystal (PRC), and the output beams from the PRC are imaged onto separate photodetectors to create a multiplexed two-wave mixing (MTWM) system. The sensing fibers are embedded in graphite-epoxy composite panels, and detection of both acoustic emission and ultrasonic signals in these materials is demonstrated. The intrinsic MTWM system is an effective tool for the simultaneous demodulation of signals from a large fiber sensor array. Also, the adaptive nature of the MTWM setup obviates the need for active stabilization against ambient noise.     

Fourier fringe processing by use of an interpolated Fourier-transform technique

Recently a powerful Fourier transform technique was introduced that was able to extract the phase from only one image. However, because the method is based on the two-dimensional Fourier transform, it inherently suffers from leakage effects. A novel procedure is proposed that does not exhibit this distortion. The procedure uses localized information and estimates both the unknown frequencies and the phases of the fringe pattern (using an interpolated fast Fourier transform method). This allows us to demodulate the fringe pattern without any distortion. The proposed technique is validated on both computer simulations and the profile measurements of a tube.     

Coupling of large telescopes and single-mode waveguides: application to stellar interferometry

The coupling between a turbulence-distorted optical beam and a single-mode waveguide is addressed. The coupling efficiency and the coupled phase are derived, both without aberrations and with small aberrations. These analytical expressions are validated by numerical simulations. Correction with adaptive optics is investigated. In the general case, the Strehl ratio is a pessimistic estimator, and the coupled phase is different and has a smaller variance than the classical phase averaged over the pupil. Application fields are heterodyne detection and stellar interferometry, for which spatial and modal filtering are distinguished.     

Infrared intracavity laser absorption spectroscopy with a continuous-scan Fourier-transform interferometer

High-quality broadband infrared high-resolution spectra were obtained by use of the intracavity laser absorption spectroscopy technique with a Ti:sapphire laser in combination with a continuous-scan Fourier-transform (FT) interferometer. With electronic filtering used to smooth out the fluctuations of the laser power, the absorption of atmospheric water vapor in the range of 12,450-12,700 cm(-1) was recorded at a resolution of 0.05 cm(-1). A signal-to-noise ratio of greater than 300 was observed in this spectrum, corresponding to a minimum detectable absorption of approximately 2 x 10(-9) cm(-1). Comparison with previous measurements by use of a conventional FT technique shows that this method gives absorption spectra with highly accurate line positions along with reasonable line intensities. Investigation of the evolution of intracavity laser absorption spectra with the generation time is also shown to be possible with a continuous-scan FT spectrometer by use of the interleave rapid-scan method.     

Application of a nonuniform spectral resampling transform in Fourier-transform spectrometry

We describe a nonuniform spectral resampling transform (NUSRT) that resamples a frequency-scaled spectrum that has been measured by a Fourier-transform spectrometer (FTS). Frequency scaling of a spectrum can arise from measurements made with off-axis detectors and Doppler shift induced by motion of a spaceborne FTS relative to an input radiation source. In addition, a spectrum may need to be rescaled in frequency to match spectral lines for applications such as the retrieval of atmospheric state parameters. The NUSRT is cast as a linear algebraic expression that relates a nonuniformly sampled interferogram to an input spectrum. A polynomial approximation is applied to this expression that reduces the inverse of the NUSRT to a series of Fourier transforms that can be implemented as fast Fourier transforms (FFTs). We show that this NUSRT algorithm requires on the order of 6N log N flops, which reduces the computational cost of rescaling by more than 1 order of magnitude compared with conventional FFT-based Shannon interpolation techniques while comparable accuracy is maintained.     

Microbubble based fiber-optic Fabry-Perot interferometer formed by fusion splicing single-mode fibers for strain measurement

We demonstrate an all-fiber optical Fabry-Perot interferometer (FPI) strain sensor whose cavity is a microscopic air bubble. The bubble is formed by fusion splicing together two sections of single-mode fibers (SMFs) with cleaved flat tip and arc fusion induced hemispherical tip, respectively. The fabricated interferometers are with bubble diameters of typically ~100 μm. Strain and temperature sensitivities of fabricated interferometers are studied experimentally; a strain sensitivity of over 4 Pm/με and a thermal sensitivity of less than 0.9 Pm/°C is obtained.     

傅立叶变换红外光谱仪在宝石鉴定中的应用

文章介绍了用傅立叶变换红外光谱仪对宝玉石进行快速、准确的鉴定。     

Surface-induced dissociation by Fourier transform mass spectrometry

A detailed procedure for performing surface-induced dissociation (SID) of ions in a dual-cell Fourier transform mass spectrometer is described. It is shown that the technique is applicable to both electron ionization and laser desorption measurements. SID spectra of perfluorotri-n-butylamine, anthracene, (5,10,15,20-tetraphenyl-21H,23H-prophinato)-iron(III) chloride, and [5,10,15,20-tetrakis(2,6-dibromo-phenyl)-21H,23H-prophina to]iron(III) chloride are presented. Conversion efficiencies of molecular ions between 1% and 30% are obtained. It is concluded the method holds promise for dissociation of high mass laser-desorbed ions.     

Radiometric errors in complex Fourier transform spectrometry

A complex spectrum arises from the Fourier transform of an asymmetric interferogram. A rigorous derivation shows that the rms noise in the real part of that spectrum is indeed given by the commonly used relation sigmaR = 2X x NEP/(etaAomega square root(tauN)), where NEP is the delay-independent and uncorrelated detector noise-equivalent power per unit bandwidth, +/- X is the delay range measured with N samples averaging for a time tau per sample, eta is the system optical efficiency, and Aomega is the system throughput. A real spectrum produced by complex calibration with two complex reference spectra [Appl. Opt. 27, 3210 (1988)] has a variance sigmaL2 = sigmaR2 + sigma(c)2 (Lh - Ls)2/(Lh - Lc)2 + sigma(h)2 (Ls - Lc)2/(Lh - Lc)2, valid for sigmaR, sigma(c), and sigma(h) small compared with Lh - Lc, where Ls, Lh, and Lc are scene, hot reference, and cold reference spectra, respectively, and sigma(c) and sigma(h) are the respective combined uncertainties in knowledge and measurement of the hot and cold reference spectra.