Monochromator Output Research Articles

Light-Emitting-Diode-Induced Fluorescence from Organic Dyes for Application in Excitation-Emission Fluorescence Spectroscopy for Food System Analysis.

An experimental study is presented on the possibility of using the fluorescence from organic dyes as a broadband light source together with a monochromator for applications in excitation-emission matrix (EEM) fluorescence spectroscopy. A high-power single-chip light-emitting diode (LED) was chosen as an excitation source with a central output wavelength at 365 nm to excite a fluorescent solution of Coumarin 1 dye dissolved in ethanol. Two excitation configurations were investigated: direct excitation from the LED and excitation through an optical-fiber-coupled LED. A Czerny-Turner monochromator with a diffraction grating was used for the spectral tuning of the fluorescence. A simple method was investigated for increasing the efficiency of the excitation as well as the fluorescence signal collection by using a diffuse reflector composed of barium sulfate (BaSO4) and polyvinyl alcohol (PVA). As research objects, extra-virgin olive oil (EVOO), Coumarin 6 dye, and Perylene, a polycyclic aromatic hydrocarbon (PAH), were used. The results showed that the light-emitting-diode-induced fluorescence was sufficient to cover the losses on the optical path to the monochromator output, where a detectable signal could be obtained. The obtained results reveal the practical possibility of applying the fluorescence from dyes as a light source for food system analysis by EEM fluorescence spectroscopy.

Measuring the spectral response of a division-of-focal-plane polarization imager using a grating monochromator.

Spectral characterizations are performed on imagers to obtain a relative spectral response (RSR) curve. This process often utilizes a grating monochromator with an output that changes polarization as a function of wavelength (our monochromator's degree of linear polarization was found to vary from less than 10% to more than 70%). When characterizing a polarization-sensitive imager, this introduces polarization artifacts into the RSR curve. We present a simple method to avoid these polarization artifacts for division-of-focal-plane polarization imagers by directly illuminating the camera with the monochromator output and calculating the S0 Stokes parameter at each super pixel, then we show consistent results from this method for two division-of-focal-plane polarization imagers. We also show that ignoring the monochromator polarization results in order-of-magnitude RSR errors. The recommended method uses an iris to limit the spatial extent of the monochromator output, which was found experimentally to increase the minimum signal-to-noise ratio by more than a factor of 2.

Influencing factors analysis of supercontinuum laser monochromator in calibration of polarization remote sensors

Relative spectral responsivity and the in-band spectral response non-consistency of polarization remote sensors are the basic parameters for laboratory calibration. In this paper, a spectrally-resolved calibration facility was assembled for the calibration of relative spectral responsivity by using novel supercontinuum laser and monochromator. Taking three polarization channels of 490 nm, 670 nm and 910 nm as examples, the relative spectral responsivity of the channel polarization remote sensors was obtained under the two states of the monochromator output light directly and the monochromatic light steered into an integrating sphere respectively, and the polarization state of the calibrated light source was measured under the two states. The experimental results show that there are obvious changes in degree of linear polarization (DOLP) and polarization orientation angle in the mode of direct output light of monochromator, and the non-consistency of in-band spectral response of the three polarization channels were 0.91%, 4.25% and 1.06%, respectively. When the monochromatic light steered into the integrating sphere, the DOLP of the light source decreases significantly, and the non-consistency of in-band spectral response were 0.15%, 0.47% and 0.57%, respectively, which indicates that depolarized light source can be effectively applied to the spectral responsivity calibration of polarization remote sensors. And the system-level calibration facility based on supercontinuum laser and monochromator has wide application prospects in the calibration of polarization remote sensors.

Two-dimensional spatial heterodyne spectrometer for atmospheric nitrogen dioxide observations.

A broadband monolithic spatial heterodyne spectrometer (SHS) system is built for measuring nitrogen dioxide in the atmosphere based on our newly developed fabrication technique. This system is calibrated and tested with Hg, Kr and Xe lamps, as well as monochromator output illuminated by a high-voltage Xe lamp (as a white light source). The obtained overall efficiency profile presents an effective spectral range of 425-495 nm (when the efficiency values are greater than 40%). The maximum fringe visibility is ~0.85. The measured instrumental line shape function gives an actual spectral resolution of ~0.073 nm. The effect of phase distortion of this 2-D SHS system can be neglected. Direct solar-irradiance spectra in the NO2 absorption band were measured with the SHS system. The measured spectra are consistent with the results simulated by Modtran6 within the SHS spectral range. The vertical column contents of NO2, VC(NO2), derived from the SHS data by the direct sun - differential optical absorption spectroscopy (DS-DOAS) method coincide closely with the simultaneously acquired (OMI) satellite data.

Design and analysis of a worm gear turntable off-axis assembly method in a three-grating monochromator.

To solve the problem where the actual grating aperture decreases with an increasing scanning angle during the scanning of a three-grating monochromator, we propose an off-axis assembly method for the worm gear turntable that makes it possible to suppress this aperture reduction. We simulated and compared the traditional assembly method with the off-axis assembly method in the three-grating monochromator. Results show that the actual grating aperture can be improved by the off-axis assembly method. In fact, for any one of the three gratings, when the monochromator outputs the longest wavelength in the corresponding wavelength band, the actual grating aperture increases by 45.93%. Over the entire monochromator output band, the actual grating aperture increased by an average of 32.56% and can thus improve the monochromator's output energy. Improvement of the actual grating aperture can also reduce the stray light intensity in the monochromator and improve its output signal-to-noise ratio.

Single crystal CVD diamond membranes as Position Sensitive X-ray Detector

Transparent X-ray Beam Position Monitor (XBPM) has been specifically developed for low energy X-ray beamlines (1.4 keV < E < 5 keV) allowing to transmit more than 80% of 2 keV energy beam. The detector is based on a free-standing single crystal CVD diamond membrane of 4 μm thickness with position-sensitive DLC (Diamond-Like Carbon) resistive electrodes in duo-lateral configuration. The measured X-ray beam induced current (XBIC) due to the interaction of X-rays with diamond membrane allows precise monitoring of the absolute beam flux and the beam position (by the reconstruction of its center-of-gravity) at beam transmissions reaching 95%. This detector has been installed at SOLEIL synchrotron on the SIRIUS beamline monochromator output and it has shown charge collection efficiency (CCE) reaching 100% with no lag-effects and excellent beam intensity sensitivity monitoring. X-ray beam mapping of the detector showed an XBIC response inhomogeneity of less than 10% across the membrane, corresponding mainly to the measured variation of the diamond plate thickness. The measured beam position resolution is at sub-micron level depending on the beam flux and the readout electronics bandwidth.

Effects of temperature, mechanical motion and source positional jitter on the resolving power of beamline 02B at the SSRF.

A detailed analysis of the effects of temperature excursions, instrumental mechanical motion and source position jitter on the energy-resolving power of beamline 02B at the Shanghai Synchrotron Radiation Facility (SSRF) is presented in this study. This beamline uses a bending-magnet-based source and includes a variable-line-spacing grating monochromator with additional optics. Expressions are derived for the monochromator output photon energy shifts for each of the performance challenges considered. The calculated results indicate that measured temperature excursions of ±1 K produce an energy shift of less than 11% of the system's energy resolution. Mechanical displacements and vibrations measured at amplitudes of less than 0.5 µm produce changes of less than 5%, while measured source location jitter results in a change of less than 10%. Spectroscopic test experiments at 250 and 400 eV provide energy resolutions of over 104. This analysis, combined with the measured results, confirms the operational stability of the beamline, indicating that it meets the performance requirements for experimental use.

Position and flux stabilization of X-ray beams produced by double-crystal monochromators for EXAFS scans at the titaniumK-edge

The simultaneous and active feedback stabilization of X-ray beam position and monochromatic beam flux during EXAFS scans at the titanium K-edge as produced by a double-crystal monochromator beamline is reported. The feedback is generated using two independent feedback loops using separate beam flux and position measurements. The flux is stabilized using a fast extremum-searching algorithm that is insensitive to changes in the synchrotron ring current and energy-dependent monochromator output. Corrections of beam height are made using an innovative transmissive beam position monitor instrument. The efficacy of the feedback stabilization method is demonstrated by comparing the measurements of EXAFS spectra on inhomogeneous diluted Ti-containing samples with and without feedback applied.

A vacuum ultraviolet filtering monochromator for synchrotron-based spectroscopy

We describe the design, characterization, and implementation of a vacuum ultraviolet (VUV) monochromator for use in filtering stray and scattered light from the principal monochromator output of the Stainless Steel Seya VUV synchrotron beam line at the Synchrotron Radiation Center, University of Wisconsin—Madison. We demonstrate a reduction of three orders of magnitude of stray and scattered light over the wavelength range 1400–2000Å with minimal loss of light intensity, allowing for over six orders of magnitude of dynamic range in light detection. We suggest that a similar filtering scheme can be utilized in any variety of spectroscopic applications where a large dynamic range and low amount of background signal are of import, such as in transmittance experiments with very high optical density.

Study of bandwidth effects in monochromator-based spectral responsivity measurements.

The errors caused by monochromator bandwidth in spectral responsivity measurements with a monochromator-based apparatus are discussed. Bandwidth effects are not negligible in high-accuracy cryogenic radiometer-based calibrations. A simple numerical method is used to calculate bandwidth effects for different types of detectors, monochromator slit scattering functions, and monochromator output spectral distributions. The method uses low-order Lagrange polynomials fitted segmentwise to measured spectral responsivity and monochromator spectral distribution data in order to make the calculations. It is shown that the shape of the slit function has only a small influence on the bandwidth errors, whereas the output spectral distribution of the monochromator can strongly affect bandwidth errors. It is also shown that in most cases the magnitude of bandwidth effects will vary as the square of the bandwidth. Bandwidth error calculations are presented for various types of detectors (silicon, silicon trap, germanium, InGaAs), for a V(X) detector, and for a typical filter radiometer. A comparison is made between calculated and measured bandwidth effects to validate the method used. In general, calculations of bandwidth effects will be mostly useful for determining uncertainties associated with monochromator bandwidth in spectral responsivity measurements; however, in certain cases the calculations can be used to apply corrections for such effects.

Chirp measurement of multimode Q-switched laser diode pulses by use of a streak camera and a grating monochromator

We discuss spectrotemporal measurements of laser diode pulses performed with a streak camera and a grating monochromator to yield a precise evaluation of chirping effects in Q-switched multimode emissions. We experimentally illustrate several causes of errors, depending on the grating size and period as well as on the adjustment of the collimating lens at the monochromator output. An analytical formula is derived that allows us to relate the chirp amplitude to the inclination of the modal structures in the streak image. Two configurations are proposed for a practical determination of the chirp amplitude in multimode emissions. Illustrations are provided with Q-switched AlGaAs laser diodes that exhibit chirp amplitudes larger than the mode spacing.

A soft x-ray/EUV reflectometer based on a laser produced plasma source

A soft x-ray reflectometer is described which is based on a laser-produced plasma source and is continuously tunable over the range 40 Å < λ < 400 Å. The source is produced by focusing 0.532-μm light from a Q-switched Nd:YAG laser on a solid target. The x-ray wavelength is defined using a high throughput spherical grating monochromator with moderate resolving power ( λ Δλ ≈ 100 to 500 ). A time-averaged monochromatized flux of more than 10 9 photons/s in a 1% bandwidth at 100 eV is obtained. Photon “shot noise” limited measurements are obtained by the use of an I 0 detector to normalize out the shot-to-shot variations in source intensity. Measurements with submillimeter spot sizes are readily obtainable. Various detectors have been used and the advantages and disadvantages of each are discussed. The higher order contamination of the monochromator output has been analyzed using a second grating for the purpose of making measurement corrections. The reflectometer thus provides the capability for precision absolute measurements of the reflectance of gratings and multilayer mirrors, the transmittance of thin film filters, or other properties of x-ray optical elements.

A Soft X-Ray/EUV Reflectometer Based on a Laser Produced Plasma Source

A soft x-ray reflectometer is described which is based on a laser-produced plasma source and is continuously tunable over the range 40 Å < λ < 400 Å. The source is produced by focusing 0.532-μm light from a Q-switched Nd:YAG laser on a solid target. The x-ray wavelength is defined using a high throughput spherical grating monochromator with moderate resolving power (λ/Δλ ≈ 100 to 500). A time-averaged monochromatized flux of more than 109 photons/s in a 1% bandwidth at 100 eV is obtained. Photon "shot noise" limited measurements are obtained by the use of an I0 detector to normalize out the shot-to-shot variations in source intensity. Measurements with submillimeter spot sizes are readily obtainable. Various detectors have been used and the advantages and disadvantages of each are discussed. The higher order contamination of the monochromator output has been analyzed using a second grating for the purpose of making measurement corrections. The reflectometer thus provides the capability for precision absolute measurements of the reflectance of gratings and multilayer mirrors, the transmittance of thin film filters, or other properties of x-ray optical elements.

Soft x-ray absorption cross section of argon determined by a variable absorber technique

The x-ray absorption cross section of argon was measured in the 1.3-3.8-nm wavelength region. To correct for the presence of higher orders in the monochromator output, a variable pressure technique is used. The results are in good agreement with semiempirical calculations and earlier measurements.

Dispersive characteristics of one-dimensional Fresnel zone plates measured using synchrotron radiation

This paper presents the dispersive characteristics of a Ta film one-dimensional Fresnel zone plate (FZP) measured using synchrotron radiation as a light source. Spectral spatial distributions of the FZP linear monochromator output beam are recorded on an X-ray film. The observed distributions are well explained by the FZP's chromatic aberration. The spectral bandwidth of the monochromator output beam decreases proportional to the slit width and saturates at approximately the value limited by the size of the focused image of the synchrotron radiation source.

Programmable output intensity controller for monochromators

The design of a progammable monochromator output controller is presented. It can be programmed to produce a constant intensity light beam as a function of wavelength from 300 to 1100 nm. Alternately, it may be programmed to produce any desired intensity vs wavelength output. This feature has been applied to simulate the solar spectrum between 300 and 1100 nm. The latter capability may find an important application in the testing and precise characterization of solar cells.

Recent European advances in monochromator design for use with synchrotron radiation sources

The monochromators initially installed on synchrotron radiation sources throughout the world were not, generally speaking, designed to take full account of the characteristics of the electron beam cross section of the accelerator or the unique angular distribution of the synchrotron radiation. In the recent generation of monochromators, more care is being taken to match the characteristics of the monochromator with those of the source, in an attemps to optimise the product of throughout and resolution. One must, however, take account of the high photon flux in the new generation of high current storage rings in designing an integrated optical system. Moreover, there is a range of experiments where it is crucial to remove unwanted high spectral orders from the monochromator output. Such considerations may have a considerable bearing on the final design of a monochromator. The present report will outline the approach presently being taken at the European facilities at Bonn, Daresbury, DESY (Hamburg), FRASCATI and LURE (Paris).

Symmetrical wavelength modulation spectrometer

A new way of realising a double-beam wide-range wavelength-modulation spectrometer is described. A transmission chopper is used instead of a reflecting one. This divides the monochromator output slit image in two identical parts. This provides an optical balance between the two optical beams which are very symmetrical and use the same mirrors. The geometry of the apparatus is suitable for two different samples studied simultaneously. It is the only published way for obtaining the difference between two derivative spectra. The logarithmic derivative of the reflectivity is obtained with one sample. For two samples differing by a small amount in a parameter a double derivation is possible. The symmetry of the optics is completed by the symmetric two-way electronics. The servomechanisms use a large dynamic photocoupler and are adaptable to any kind of detector. An application of the apparatus is described for the determination of small composition variation of a ternary semiconductor alloy.

The Calibration of Scanning Monochromators for the Measurement of Absolute Irradiance

Certain approximations commonly used in the absolute calibration of a scanning monochromator are examined in terms of the response of the instrument to a monochromatic input. The absolute irradiance due to any spectral feature in the neighborhood of lambda(0) is commonly computed from the expression W = AB(lambda(0))/H(lambda(0)) where A is the area of the spectral feature as recorded by the monochromator output trace, B(lambda) is the spectral irradiance of a standard source, and H(lambda) is the response of the monochromator to B(lambda) when the monochromator corresponds to wavelength lambda. As an example, approximations used in justifying such calculations are examined and applied to an Ebert 0.5-m monochromator. For the case chosen, the approximation is shown to be valid to an accuracy of 1.5% to 2%, depending upon assumptions made in the calculation. It is found that the most serious error for this example is introduced by changes in the sensitivity of the monochromator over a wavelength interval comparable with that of the spectral feature under investigation. A second source of error is found to be the change in the irradiance of the standard source over a wavelength interval comparable to the instrument resolving power.

Quantum Yield of Gold-Cathode Photomultipliers

Two gold-cathode EMI 6255G tubes have been investigated for their quantum yield between 3100 and 1900 A. The tubes had cathodes of different appearances. One of these, numbered 3012, had a slight bluish tinge and was very transparent to visible light; the other, numbered 3021, had a definite gold coloration. The relative quantum yield of each tube was determined with the aid of a Cary model 14 recording spectrophotometer used as a monochromator. The monochromator relative-energy output was determined from the current output of a sodium-salicylate-coated RCA 1P21 photomultiplier. Each gold-cathode tube was then operated at 3000 v, and the central 1.8 cm cube of the cathode was exposed to the monochromator output.