Measurements In Spectral Range Research Articles

Disposable sheath that facilitates endoscopic Raman spectroscopy.

In vivo endoscopic Raman spectroscopy of human tissue using a fiber optic probe has been previously demonstrated. However, there remain several technical challenges, such as a robust control over the laser radiation dose and measurement repeatability during endoscopy. A decrease in the signal to noise was also observed due to aging of Raman probe after repeated cycles of harsh reprocessing procedures. To address these issues, we designed and tested a disposable, biocompatible, and sterile sheath for use with a fiber optic endoscopic Raman probe. The sheath effectively controls contamination of Raman probes between procedures, greatly reduces turnaround time, and slows down the aging of the Raman probes. A small optical window fitted at the sheath cap maintained the measurement distance between Raman probe end and tissue surface. To ensure that the sheath caused a minimal amount of fluorescence and Raman interference, the optical properties of materials for the sheath, optical window, and bonding agent were studied. The easy-to-use sheath can be manufactured at a moderate cost. The sheath strictly enforced a maximum permissible exposure standard of the tissue by the laser and reduced the spectral variability by 1.5 to 8.5 times within the spectral measurement range.

Design of a spectrum-folded Hadamard transform spectrometer in near-infrared band

A spectrum-folded Hadamard Transform spectrometer in near-infrared band has been proposed and designed to widen the spectral range of measurement as well as to improve the spectral resolution. The full spectral region of 800nm to 2000nm is divided into two bands of 800nm to 1400nm and 1400nm to 2000nm and dispersed by two sub-gratings. The optimal incident angles of 12.6° and 6.7° respectively for two sub-gratings are chosen to achieve the highest diffraction efficiency. The results show that the spectral resolution of the system is better than 6nm. The stray light from sub-gratings and DMD are suppressed to minimize the effect on the measurement results.

A micro-spectrometer with phase modulation array

A micro-spectrometer with phase modulation array is investigated in this paper. The vital component of this micro-spectrometer is a micro-interferometer array, which is built on a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Each element of micro-interferometer array is formed by polymethyl methacrylate (PMMA) grooves with different depth. When we illuminate the surface of the interferometer array, different interference intensity distribution would be formed at the bottom of each micro-interferometer. Optical power of this interferometer can be measured by the pixels of CCD or CMOS. The data can be substituted into a linear system. By solving the linear system with Tikhonov regularization method, spectrum of the incident beam can be reconstructed. Simulation results prove that the detection range of the spectrometer is a wide wavelength range covering from 300 to 1100 nm. Furthermore, the wavelength resolution of the device reaches picometer level. In comparison with conventional spectrometers, the novel spectrometer has distinct advantages of small size, low cost, high resolution, wide spectral measurement range, real-time measurement, and so on.

High-resolution broadband spectroscopy with a resonator-based phase modulator

A method for significant enhancement of the spectral resolution of a Fabry-Perot resonator in transmission and absorption measurements is proposed. In the method, a laser with ultrashort pulses is used as the optical source. A dispersive element is placed in front of the Fabry-Perot resonator and a phase modulator is incorporated into the resonator. The spectrum of the laser pulse transmitted through the system is approximately periodic with ultranarrow peaks. The sample transmission spectrum is measured by scanning the output pulse spectrum. It is demonstrated, in numerical simulations, that for realistic parameters of the phase modulator, the finesse of the Fabry-Perot resonator is increased from 72 to 1900 and a resolution of 1 MHz is achieved. A method for increasing the spectral range of measurements with scanning the periodic spectra is also proposed. The method is based on the use of a waveguide array of Mach-Zehnder interferometers or a single discretely tunable interferometer. The measurement of the sample transmission spectrum within 33 free spectral ranges of the resonator is numerically demonstrated. The spectral range of the measurement can be increased up to 10 THz resulting in the equivalent finesse of the system of 10(7) for a 100 fs laser pulse.

IR-spectroscopy of ethanol formed by recondensation from a nitrogen cryomatrix

The processes of formation and the properties of ethanol recondensates, formed from a nitrogen cryomatrix during its evaporation, are investigated. Ethanol molecules create matrix-isolated polyagregates due to co-condensation with nitrogen. The matrix evaporation at 35 K is accompanied by recondensation of ethanol from the matrix onto a substrate. This leads to a formation of a highly-dispersive film (recondensate) consisting of agregates of different size including dimers and monomers. IR-spectroscopic investigations of recondensed samples were performed. The condensation temperature was Tc = 12 K. The gas phase pressure in the cryodeposition process was P = 10–5 Torr. The ethanol concentration in nitrogen was from 0.5 to 10%. The film thickness was varied from 1 to 30 μm. The spectral range of measurements was 400–4200 cm–1. From a comparative analysis of oscillatory spectra of samples of ethanol cryocondensates, matrix-isolated ethanol in a nitrogen cryomatrix and ethanol recondensates, we concluded that both polyagregates located previously in a matrix and recondensed onto a substrate are in a glass state. A film heating near the glass point of ethanol (97 K) leads to transitions, which take place in a few steps at different temperatures. Such a character of a warming-up curve is explained by a highly-dispersive composition of recondensates and by a dependence of the glass point of polyagregates on their size. The character of warming-up curves of recondensates allows to speak about grouping of elements of recondensates by their size. In other words, in the process of recondensation and possible further coalescence, polyagregates with preferential sizes, which are energetically optimal for these conditions, are formed. In the considered case it is reasonable to speak about existence of three such sets, involved sequentially in a glass transition.

Fiber laser intracavity absorption spectroscopy for in situ multicomponent gas analysis in the atmosphere and combustion environments

Intracavity absorption spectroscopy with a broadband Er3+-doped fiber laser is applied for the measurements of several molecular species revealing quantitative information about the gas concentration, temperature and chemical reactions in flames. The spectral range of measurements extends from 6200 cm−1 to 6550 cm−1 with the proper choice of the fiber length and by moving an intracavity lens. With a pulsed laser applied in this experiment, the sensitivity to absorption corresponds to an effective absorption path length of 3 km assuming the cavity is completely filled with the sample. For a cw laser, the effective absorption path length is estimated to be 50 km. Absorption spectra of various molecules such as CO2, CO, H2O, H2S, C2H2 and OH were recorded separately in the cell and/or in low-pressure methane and propane flames. The presented measurements demonstrate simultaneous in situ detection of three molecular products of chemical reactions at different flame locations. Variation of the relative strengths of OH absorption lines with the temperature enables the estimation of the local flame temperature. The sensitivity of this laser does not depend on the broadband cavity losses and it can be used for in situ measurements of absorption spectra in hostile environments such as contaminated samples, flames or combustion engines. The presented technique can be applied for various diagnostic purposes, such as in environmental, combustion and plasma research, in medicine and in the determination of stable isotope ratios.

Spectral and temperature features of coherent picosecond nonlinear response of HTSCs at low excitation levels

It is shown that for the appropriate choice of the spectral measurement range (the choice of the coincidence point for the pumping component frequencies in the methods of biharmonic pumping and degenerate four-photon spectroscopy), the spectral and temperature features of the picosecond nonlinear response of high-temperature super-conductors (HTSCs) caused by interband transitions in the electronic spectrum with a metastable energy gap are stable with respect to the excitation level. The character of these features is determined by the resonance component of the total nonlinear response, which is formed at initial HTSC temperatures below the phase transition point (when the energy gap appears in the electronic spectrum).

Estimation of Forage Production of Nilegrass Using Vegetation Reflectance

Changes in reflectance spectrum of a crop are known to follow the morphological development of vegetation, and thus spectral models combining spectral characteristics correlated with biomass production may be used for yield estimation. Field experiments were conducted to validate use of reflectance spectra (350–2400 nm) to estimate forage production (i.e., aboveground fresh weight) of nilegrass (Acroceras macrum Stapf) vegetation from June 2002 to May 2004. Correlation coefficients (r) between spectral reflectance and forage production varied across the spectral range of measurements. A linear relationship (P < 0.010) was found for several wavebands, with the highest r value located at 891 nm (r = 0.671; P < 0.010). Of the examined spectral indices, forage production was found to be best correlated with RGREEN/RNIR ratio (R2 = 0.654, P < 0.001) where RGREEN was reflectance of green light (490–560 nm) maximum and RNIR was reflectance of the near‐infrared (740–1300 nm) peak. Assessment of forage production was further improved by using a multiple linear regression (MLR) model. The best five‐variable linear regression equation provided the best fit (R2 = 0.726, P < 0.001, Mallows' Cp criterion = 6.000). When validating the MLR model with other datasets from different growing seasons, the model gave reasonable prediction values (r = 0.833; P < 0.001) with a slope of 1.086 and root mean square error of 3.891 (N = 21).

Infrared temperature measurements on solar trough absorber tubes

The temperature distribution on solar trough absorber tubes determines thermal losses and hotspots can lead to material stress and limit absorber tube lifetime. The concentrated solar radiation, however, makes it difficult to determine the temperature on solar absorbers. Temperature sensors that require contact to the measurement object are not appropriate and even pyrometry fails, when external light sources interfere. Only solar-blind pyrometry offers reliable temperature readings without perturbation through reflected solar radiation. This paper presents two concepts for a pyrometric solar-blind measurement on solar trough absorber tubes. One solar-blind approach is a spectral measurement range in regions, where the solar spectrum shows gaps due to the discrete absorption of the atmosphere. Another possibility for a solar-blind pyrometric temperature measurement results from the optical behavior, i.e. the distinct angle dependence of the directional reflectance and emittance of a typical selective trough absorber coating. First experimental results are shown and the accuracy and performance advantages and disadvantages of the setups are reported and discussed.

Modified Calibration Procedures for a Yankee Environmental System UVB‐1 Biometer Based on Spectral Measurements with a Brewer Spectrophotometer

The calibration of the erythemal irradiance measured by a Yankee Environmental System (YES) UVB-1 biometer is presented using two methods of calibration with a wide range of experimental solar zenith angles (SZAs) and ozone values. The calibration is performed through simultaneous spectral measurements by a calibrated double-monochromator Brewer MK-III spectrophotometer at "El Arenosillo" station, located in southwestern Spain. Because the range of spectral measurements of the Brewer spectrophotometer is 290-363 nm, a previously validated radiative transfer model was used to account for the erythemal contribution between 363 and 400 nm. Both methods are recommended by the World Meteorological Organization and we present and discuss here a wide range of results and features given by modified procedures applied to these two general methods. As is well established, the calibration factor for this type of radiometric system is dependent on atmospheric conditions, the most important of which are the ozone content and the SZA. Although the first method is insensitive to these two factors, we analyze this behavior in terms of the range used for the SZA and the use of two different mathematical approaches for its determination. The second method shows the dependence on SZA and ozone content and, thus, a polynomial as a function of SZA or a matrix including SZA and ozone content were determined as general calibration factors for the UV radiometric system. We must note that the angular responses of the YES radiometer and Brewer spectroradiometer have not been considered, because of the difficulty in correcting them. The results show in detail the advantages and drawbacks (and the corresponding associated error) given by the different approaches used for the determination of these calibration coefficients.

In situ measurements of sensor film dynamics by spectroscopic ellipsometry. Demonstration of back-side measurements and the etching of indium tin oxide

A new liquid flow cell design for in situ ellipsometric measurements on transparent multilayer samples using variable angle spectroscopic ellipsometry is presented. In this cell, films made on transparent substrates are in direct contact with liquid solution. Ellipsometry measurements are made through the transparent substrate, that is, from the back-side relative to the incident light so that films are in continuous contact with the liquid. This cell is not limited to just one angle of incidence of light allowing the films to be characterized at several angles before, during and after liquid contact. The spectral range of measurements is limited only by absorption of light in the underlying transparent substrate and not by the liquid solution that the film is in contact with. As a demonstration, we have measured and analyzed the dynamics of an indium tin oxide film on glass undergoing acid etching. Data from this in situ experiment were successfully modeled and the ITO layer thickness decreased uniformly during the etching process with an average etch rate of 0.23 nm/min.

Emissivity of condensed substances from their thermal radiation spectra

A technique for measuring the temperature and spectral emissivity of condensed substances is described. With this technique, measurements can be made in any spectral range, rather than only at short wavelengths, where the Wien approximation for the Planck formula holds. In particular, in the spectral range around the peak-emission wavelength, the highest signal-to-noise ratio can typically be attained, which raises the accuracy in temperature and emissivity determination. The proposed approach to processing the experimental emissivity curves offers the possibility of analyzing the effects of the major factors (signal-to-noise ratio, spectral range of measurements, type of the model function representing the spectral emissivity, and others) on the accuracy in temperature measurements. The potentialities of the technique are demonstrated by measuring the temperatures and emissivities of W, Re, and Ta strip lamps in the spectral range 0.9 to 2.1 Μm.

Ultrawide spectral range group-velocity dispersion measurement utilizing supercontinuum in an optical fiber pumped by a 1.5 μm compact laser source

Group velocity dispersion (GVD) measurement is presented utilizing supercontinuum (SC) white pulses generated in an optical fiber by 15 /spl mu/m compact laser sources. This provides 1) ultrawide continuous spectral measurement range >600 nm from a single optical source without the use of interpolation formulae and 2) stable far-end measurements by the simultaneous multi-wavelength nature of the SC pulses. A novel method that is independent of the detector bandwidth is proposed which measures /spl lambda/-dependent phase shifts of one of the Fourier components of a short pulse train. Fiber GVD's of unusual dispersion characteristics were measured using SC pulses extended over the spectral range of 1150-1770 nm. It is shown that fiber lengths of up to 130 km can be measured with a group delay resolution of 0.01 ps/km.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Characterization of Quantum well Structures Using Microscope-Spectrophotometry

AbstractIn this paper the application of microscope-spectrophotometry to the nondestructive characterization of a variety of multi-layer GaAs/A1GaAs structures, is described. Spectral reflectance results are used to indirectly determine variations in aluminium content, and the interdependency of aluminium content with layer thicknesses. The penetration depth of light from the visible spectrum is assessed from the correlation between spectral reflectance measurements and fitted optical models. Finally, a series of single quantum wells are investigated, and it is concluded that a significant improvement in the characterization of these materials will be achieved with an extension of the spectral measurement range into the ultra violet.