Double-grating Monochromator Research Articles

Novel multi-function setup comprising ultra-bright uniform source for spectral and radiometric calibrations of digital imaging sensors and multiple spectrometric applications

Abstract A new setup is being developed for calibrating CCD/CMOS sensors and cameras, and measuring spectrophotometric quantities in both point-wise and imaging modes. The setup includes a laser-driven light source (LDLS), double-grating monochromator, and an integrating sphere with specific design for two-dimensional (2D) spectrophotometric measurements of complex samples and spectral responsivity calibration of digital imaging sensors and cameras in the visible to near infrared (VIS-NIR) region. The setup can cover a spectral range from 250 nm to 2000 nm with relatively high radiance. The study presents optimization and validation work in the visible through NIR spectral range, up to 1000 nm. Preliminary experiments suggest improved accuracy and lower uncertainties for better metrological performance and sophisticated applications. The developed instrument promises a variety of novel applications while suppressing systematic errors and inaccuracies. The presented measurements results include the validation of 2D spectral reflectance/transmittance measurements in the 360-1000 nm spectral region using selected homogenous standards. Additionally provided are the findings of demonstration measurements for 2D spectral transmittance and the CCD camera’s spectral responsivity within the same spectral range.

Novel Time-Saving Technique to Measure the 2D Complex Profile of Spectral Transmittance of Linearly Variable Filters and Assessing Homogeneity of Optically Transparent Materials

Emerging technologies for coating new types of filters, such as linearly variable bandpass filters, urges new kind of measurement systems requiring accurate and time-saving spatially resolved transmittance measurements. On the other hand, one of the main uses of coated filters is standardization use; e.g., as certified reference materials (CRMs) for transmittance and absorbance standards. In this case, the homogeneity assessment is of great interest. To do such measurements with low uncertainty, spatial scanning methods using reference double-beam spectrophotometers are routinely used. High time consumption and relatively high-cost are disadvantages of this traditional method. In this work, we propose a new system based on an imaging technique. The newly developed system comprises a quartz tungsten halogen (QTH)-based illuminator, with collimator and coupling optics, a double-grating monochromator, customized integrating sphere, and scientific highly digitized, high-resolution and dynamic range Si-based CCD (monochrome) camera. The system is time-saving: in one-shot we may measure the spatial transmittance distribution of the samples (e.g. filters) without separately measuring the reference (air), which has many instability issues. Validation measurements using standard filters are presented. Systematic errors may be studied and corrected for. Many types of filters and liquids, for instance chemical reference materials (RMs), may be measured and reported. Uncertainty of spatial, spectral and radiometric components are studied carefully and estimated. The combined uncertainty evaluation has shown that relatively low uncertainty levels may be achieved, specially in the visible range (U < 1 %, k=2 ). Spatially-resolved data with different curves at different positions (spatial points) are shown in the spectral range of 380 nm through 950 nm. The combined uncertainties at some spectral points are presented and proved to be sample- and wavelength-dependent at all the points in the field-of-view (FOV) of the CCD camera.

Realization of the Radiance Scale Using Transfer Function of the Laser-Based Optical System

This work aims to determine the radiance responsivity to be used in the calibration of polychromatic radiation sources with low uncertainty. To realize the radiance, Ar-ion, He-Ne, and Nd-YAG lasers as well as an integrating sphere with a 0.15 m diameter are used to obtain radiation sources having Lambertian distributions. Then, a silicon photodiode-based reflection-type trap detector with calibrated precision aperture, which is traceable to a liquid helium cooled laser-based cryogenic radiometer, is used to measure the photocurrent corresponding to each wavelength and thereby to obtain radiance. The proposed system, which measures the spectral current response of this laser-based radiance, is a double-grating monochromator with a 2 × 300 mm focal length and triple gratings in each of its turrets. First, the radiance of the laser beam that emerged from the integrating sphere is calculated, and then the radiance responsivity of the system is obtained by measuring the photocurrent outputted from the exit slit of the monochromator at each laser wavelength. Finally, the spectral radiance values of the polychromatic lamps are obtained using the radiance responsivity of the system. Consequently, the study aims to develop the derivation and better understand traceability of the other radiometric and photometric quantities with low uncertainty from the fundamental radiometric radiance unit. Measurement results obtained in the expanded measurement uncertainty scale are determined using both classical and Monte Carlo methods.

Miniature double-grating monochromator with Littman configuration for pure rotational Raman LIDAR

Conventional double-grating monochromators used in pure rotational Raman (PRR) temperature LIDAR usually adopt the Littrow configuration to separate backscattered PRR lines. A mirror is added to this configuration, thereby producing the Littman configuration. The dispersion of the modified configuration is greater than thrice that of the Littrow configuration for incident angles larger than 60 deg. A spectroscopy system with a linear dispersion better than 0.9 mm / nm is designed for the 532-nm excitation laser wavelength, and the selected N2 PRR lines are effectively separated. The overall system is simulated using only off-the-shelf optical components, and the results show that the volume is smaller than 240 × 200 × 80 mm3.

Temporal Response of Ultrafast Grating Monochromators

The temporal response of double-grating monochromators is analyzed considering two effects on the ultrafast pulse given by the configuration. The first effect is the compensation of the pulse-front tilt, i.e., all the rays emitted by the source in different directions at the same wavelength have to travel the same optical path. The second effect is the group delay introduced by the two gratings, i.e., different wavelengths within the bandwidth transmitted by the slit travel different paths. The methodology to calculate the group delay introduced by the double-grating configuration is presented. Some practical design examples are provided to show the design rules and the achieved performances.

Spectral responsivity calibration of the reference radiation thermometer at KRISS by using a super-continuum laser-based high-accuracy monochromatic source

We report on the calibration of the relative spectral responsivity of the reference radiation thermometer, model LP4, which is used for the experimental realisation of the international temperature scale of 1990 above 960 °C at the Korea Research Institute of Standards and Science. The relative spectral responsivity of LP4 is measured by using a monochromatic source consisting of a super-continuum laser and a double-grating monochromator. By monitoring the wavelength of the output beam directly with a calibrated wavelength-meter, we achieved a high-accuracy measurement of spectral responsivity with a maximum wavelength error of less than 3 pm, a narrow spectral bandwidth of less than 0.4 nm, and a high dynamic range over 8 decades. We evaluated the contributions of various uncertainty components of the spectral responsivity measurement to the uncertainty of the temperature scale based on a practical estimation approach, which numerically calculates the maximal effects of the variations of each component. As a result, we evaluate the uncertainty contribution from the spectral responsivity measurement to the temperature scale to be less than 64 mK (k = 1) in a range from 660 °C to 2749 °C for the LP4 with a filter at 650 nm.

Active-grating monochromator for the spectral selection of ultrashort pulses

Active gratings have been used to realize a grazing-incidence double-grating monochromator for the spectral selection of ultrashort pulses while preserving the temporal duration by compensating for the pulse-front tilt. The active grating consists of a bimorph deformable mirror on the top of which a diffraction grating with laminar profile is realized by UV lithography. The time-delay compensated configuration has been tested with ultrashort pulses at 800 nm. The feasibility of this configuration for the extreme-ultraviolet spectral region has been demonstrated by ray tracing studies.

Ultrafast Grating Instruments in the Extreme Ultraviolet

Optical grating instruments enjoy several advantages in the optimal conditioning of subpicosecond, or ultrafast, pulses in the extreme ultraviolet (XUV) spectral region. The main application of such instruments is the spectral selection of high-order laser harmonics and free-electron-laser pulses in the femtosecond time scale. Broadband XUV monochromators require the use of diffraction gratings at grazing incidence. We discuss here an innovative configuration, that is the off-plane geometry, to realize single-grating XUV monochromators with high efficiency and ultrafast time response. Furthermore, the off-plane design has been modified to realize a double-grating XUV monochromator with time-delay compensated (TDC) response. Two examples of realizations applied to the spectral selection of high-order laser harmonics are presented. In particular, XUV pulses as short as 8 fs have been measured at the output of the TDC monochromator. Finally, the problem of temporal compression of broadband XUV attosecond pulses by means of a double-grating compressor will be addressed. The TDC design has been modified to realize an XUV attosecond compressor that introduces a variable group-delay dispersion to compensate for the intrinsic chirp of the attosecond pulse.

A pure rotational Raman lidar using double-grating monochromator for temperature profile detection

A pure rotational Raman lidar is constructed for measurements of vertical temperature profiles of atmosphere. A fiber-coupled, double-grating monochromator is used as the filter system so that a high spectral resolution of <0.23 nm and an out-of-band rejection rate of 10 7 are achieved. Comparison with in-situ measurements indicates that the accuracy of atmospheric temperature measurements using this lidar system is better than 1.1 K up to an altitude of 10 km. Different effects on signal accuracy resulting from various noise sources are analyzed and uncertainties in temperature due to detection noises are estimated.

Evolution of Instrumentation for Detection of the Raman Effect as Driven by Available Technologies and by Developing Applications

The evolution of Raman instrumentation from the time of the initial report of the phenomenon in 1928 to the present will be reviewed. The earliest systems were prism spectrographs with photographic plates, and the spectrum was excited with a mercury arc lamp. Because samples were synthesized and then extensively purified to guarantee that the spectrum was representative of the sample and not impurities, problems of Rayleigh scattering and fluorescence that became important in the period between the 1960s and 1990s were not present. During the period between the mid-1950s to the late-1970s most systems were double grating monochromators, scanned with photomulplier detectors. During the mid-1970s the first microprobes were introduced on scanning instruments, but were then adapted to spectrographs after the multichannel detectors became the method of choice for detection. Initially these were triple spectrographs where the first two stages were used in subtractive mode to filter the laser line, but after the introduction of the holographic notch filters in 1990, a new generation of truly benchtop Raman systems were developed and saw increasing popularity.

Monochromator-based cryogenic radiometry between 1 µm and 20 µm

The spectral responsivity of Ge photodiodes and thin-film thermopiles has been measured between 1 µm and 20 µm. These detectors are used as transfer standards for the spectral responsivity measurements made at the Nederlands Meetinstituut-Van Swinden Laboratorium (NMi-VSL) monochromator-based cryogenic radiometer facility. For measurements beyond 2.5 µm we used an Ar maxi-arc as infrared source in combination with a single-grating monochromator. Below 2.5 µm a quartz-tungsten-halogen lamp is used as light source in conjunction with a double-grating monochromator. An optical system based on three mirrors is used to match the f-numbers of the monochromator and cryogenic radiometer with negligible residual astigmatism. In this paper we discuss the design of the facility, the characteristics of the transfer standards used and the estimated uncertainty budget of the infrared measurements.

Monochromator Wavelength Calibration Standards Extending into the Near-Infrared Using Second- and Third-Order Emission Lines from Mercury Vapor Lamps

The emission spectra of four mercury vapor lamps are used to obtain wavelength calibration curves for the double-grating emission monochromator of a spectrofluorimeter. The use of second- and third-order diffraction lines and emission lines from the argon carrier gas provides a rich spectrum, which extends well into the near-infrared spectral region and produces an improved calibration curve. More than 60 emission lines are listed between 250 and 900 nm, which is sufficient to produce an extremely accurate monochromator calibration. Additional second- and third-order lines can be used to even longer wavelengths (>1200 nm). The effectiveness of three scattering surfaces is compared.

Etude de l'influence d'un monochromateur sur la cohérence de la lumière

The effect of a single- and a double-grating monochromator on the coherence properties of light fields is studied in detail experimentally by measuring the degree of coherence of the field at the entrance and exit slits of a monochromator. It is found that the state of spatial (spectral) coherence of the light field incident at the entrance slit of a monochromator is modified considerably at the exit slits of a single and a double monochromator. The spectral degree of coherence (SDC) at the exit slit of a double monochromator is found to be more than the SDC at the exit slit of a single monochromator. It is therefore necessary that, while synthesizing a secondary source at the exit slit of a monochromator, the coherence properties of the secondary source must be carefully studied to analyse the far-field spectrum of such a source.

New type of CD spectropolarimeter with LD option

A new type of circular dichroism (CD) spectrometer has been designed and constructed based on a principle which is entirely different from that of commercial CD spectrometers. The Mueller matrix approach is used to analyze and evaluate important factors determining its performance, which is revealed to be dependent upon (1) the quality of a depolarizer, (2) the residual static birefringence inherent in a collimator lens, a photoelastic modulator, and an analyzer. Furthermore, the polarization characteristics of both a monochromator and a detector are found to have no effect on CD signal in the new CD spectrometer. Thus, a double-grating monochromator can be used instead of a double-prism monochromator. The total performance of the new CD spectrometer is ascertained to be equivalent to that of commercial CD instruments. With using a lock-in amplifier tuned to 100 kHz, the new CD spectrometer can be easily converted into a linear dichroism spectrometer.

Wavelength Calibration of Optical Multichannel Detectors in Combination with Single- and Double-Grating Monochromators

For the measurement of a spectrum with an optical multichannel detector (OMCD), the relation N = N(λ) or λ = λ( N) between the channel number N and the wavelength λ must be known. For an ideal OMCD, N(λ) and λ( N) are completely defined by the physical parameters of the monochromator and the channel spacing of the OMCD. Series expansions N = N0 + Σ kA k(λ – λ0) k and λ = λ0 + Σ kBk( N – N0) k are given for single-grating monochromators ( k = 1, 2, 3) and double-grating monochromators ( k = 1, 2). The validity of the series expansions has been checked experimentally and numerically for a double monochromator.

A new spectroscopic system for the measurement of hyper-Raman and Raman signals

A new and highly sensitive spectroscopic system which allows one to observe Raman and hyper-Raman scattering signals in a successive or simultaneous way has been developed for the first time. The present apparatus consists of a repetitively Q-switched Nd-yttrium aluminum garnet (YAG) laser, a single-grating monochromator supplemented with a premonochromator which is a double-grating monochromator of zero-dispersive type, and a gatable optical multichannel detector cooled below −40 °C. With this, a greatly improved sensitivity for hyper-Raman signals has been accomplished. For exciting hyper-Raman signals the original beam of the Nd-YAG laser was used, whereas the second-harmonic light of the laser was used for Raman signals. The second-harmonic light beam, collimated to the original one, was obtained from a phase-matched LiNbO3 crystal mounted on a kinematic mount. Thereby, either the original beam of the Nd-YAG laser or the second-harmonic one could excite the same portion of the sample, so that both hyper-Raman and Raman signals appeared in the same spectral region. Thus, it was demonstrated that the measurement of Raman and hyper-Raman scattering signals could be easily made by setting and unsetting the kinematic mount.

Determination of lithium in blood serum — a comparison between molecular absorption spectrophotometry and emission flame spectrometry

Emission flame spectrometry in acetylene-nitrous oxide and acetylene-air flames enables rapid, sensitive and precise determination of lithium in blood serum at 670.8 nm [l]. No deproteination of serum is necessary. Calibration plots are prepared for serum samples diluted 10-l 000 times with twice distilled water or sodium chloride solution to resulting 6-150 mmol/l concentration. Sample viscosity does not interfere with the measurements for samples diluted to 40% of serum and more. Insufficiently diluted serum samples (1: 2-l : 5) may cause partial blocking of the nebulizer capillary and the emission values decrease. A procedure for 100 ~1 serum containing 10 pmol/l-50 mmol/l Li and diluted with 10 ml of physiologic solution (1: 10) is recommended. The emission value of the diluted physiologic solution represents the blank measurement. One to ten microlitres of serum may also be sampled and diluted 1: 1000 for lithium concentrations higher than 100 nmol/l. If a highly efficient double grating monochromator producing monochromatic bandpasses of l-10 pm (picometer) is used with a low-noise photomultiplier of high quantum efficiency giving linear response in a five orders interval, 0.01-500 pmol/l of Li may be determined with a precision of f1.5% rel. The sensitivity falls by several orders and the precision of the method considerably decreases if current flame photometers containing interference filters or simple monochromator and phototube or selenium barriere cell as detector are used. We have, however, shown that the spectrophotometric determination of lithium with the common analytical reagent, thoron (2-arsonobenzene)-1-azo-1’-(2’-hydroxynaphthalene-3’,6’-disulphonic acid), [2,3] gives similar precision and accuracy in a concentration range of 0.25-1.5 mmol/l lithium in serum as the precise variant of the flame emission spectrometry. One hundred microlitres of serum containing 2.0 mmol/l Li is sampled into a 1.5 ml test tube of the Eppendorf type, 100 yl of 1.0 mol/l trichloroacetic acid is added, stirred and allowed to stand for 10 min at room temperature. The plugged test tube is then centrifugated for 2 min at 12000 rpm. A 100~~1 portion of the supernatant is

A Diffusive Mode in KH2PO4 Studied with a Tandem System Using a Fabry-Perot Interferometer and a Double-Grating Monochromator

A Fabry-Perot interferometer and a double grating monochromator were used in tandem to construct a new spectroscopic system with high resolution over a wide frecquency range. The system was used to measure the low-frequency spectra of KH2PO4 in the x(yx)y configuration near Tc without distortion. The diffusive central component of the spectrum was successfully analyzed using a Debye-type susceptibility function with a polarization relaxation time τ=τ0T0/(T-T0), where τ0=1.57×10-13 s and T0=118.7 K.

Atmospheric temperature measurements using a pure rotational Raman lidar

A Raman lidar technique for measuring atmospheric temperature using pure rotational Raman spectra of N2 and O2 is discussed in detail. The use of a double-grating monochromator in the lidar for isolating two portions of the pure rotational Raman spectrum (PRRS) of N2 and O2 and suppressing the line of aerosol light scattering is experimentally shown to be very efficient. The feasibility of the method is convincingly illustrated by the results of laboratory experiments, as well as with measurements of air temperature carried out in the atmosphere. The accuracy of temperature measurements using the ratio of intensities of two portions of PRRS of N2 and O2 was ∼±0.74 K in laboratory experiments. The accuracy of the atmospheric temperature profile measurements using the lidar varied from 0.8 K at altitudes up to 300–400 mto, and slightly exceeded, ±1.5 K at 1-km height. Lidar temperature data are in good agreement with radiosonde data.

Design of a vacuum ultraviolet spectrofluorimeter

The design of a vacuum ultraviolet spectrofluorimeter for the measurement of luminescence from thin films is described. The instrument uses a pulsed continuum light source and a specially built detection system. The wavelength of excitation is selected by a double-grating monochromator of high spectral purity and large aperture. The excitation bandwidth employed may be adjusted without change of central wavelength or of sample illumination. Spectra are shown for thin films of anthracene and results are also cited for polyolefins and DNA films.