Rotating Diffraction Grating Research Articles

Visible to near-infrared single pixel microspectrometer using electrothermal MEMS grating.

Compact spectrometers facilitate non-destructive and point-of-care spectral analysis. Here we report a single-pixel microspectrometer (SPM) for visible to near-infrared (VIS-NIR) spectroscopy using MEMS diffraction grating. The SPM consists of slits, electrothermally rotating diffraction grating, spherical mirror, and photodiode. The spherical mirror collimates an incident beam and focuses the beam on the exit slit. The photodiode detects spectral signals dispersed by electrothermally rotating diffraction grating. The SPM was fully packaged within 1.7 cm3 and provides a spectral response range of 405 nm to 810 nm with an average 2.2 nm spectral resolution. This optical module provides an opportunity for diverse mobile spectroscopic applications such as healthcare monitoring, product screening, or non-destructive inspection.

Time-resolved continuous-filtering Vernier spectroscopy of H2O and OH radical in a flame.

We use broadband near-infrared continuous-filtering Vernier spectroscopy (CF-VS) for time-resolved detection of H2O and OH radical in a premixed CH4/air flat flame. The CF-VS spectrometer is based on a femtosecond Er:fiber laser, an external cavity that contains the flame, and a detection system comprising a rotating diffraction grating and photodetectors. Spectra of H2O and OH radical around 1570 nm are continuously recorded with 6.6 GHz spectral resolution, 4.0 × 10-7 cm-1 absorption sensitivity, and 25 ms time resolution, while the fuel-air equivalence ratio is periodically modulated with a square wave. The concentrations of the two analytes are retrieved with percent level precision by a fit of a Vernier model to each spectrum spanning 13 nm. The temporal profiles of both concentrations in each modulation cycle are repeatable and the steady-state concentration levels are in good agreement with predictions based on one-dimensional simulations of a static flat flame. The robust CF-VS spectrometer opens up for quantitative monitoring of multiple products of time-varying combustion processes with relatively simple data acquisition procedures.

Absolute distance measurement based on spectral interferometer using the effect of the FSR of a Fabry–Perot etalon

A method for absolute distance measurement is developed here that uses a Fizeau-type spectroscopic interferometer with a Fabry−Perot etalon and a rotating diffraction grating. The integral multiple of the free spectral range (FSR) can be measured using a fiber-type etalon. When the range of length is greater than half of the FSR, the absolute distance can be determined with arbitrary ranges. Moreover, the limited position can be measured over a long distance. Instead of calibrating the wavelength, we use the Fabry−Perot etalon as a standard to determine the absolute position and calibrate the system. Two diffraction gratings increase the spectroscopic resolution to considerably extend the effective range of measurement. Experiments to test the proposed method indicate that it can be used to measure absolute distance with arbitrary ranges of length of up to 120 mm, with repeatability between 0.23 μm (0.0272%) and 0.77 μm (0.0619%), and can measure distances up to 1200 mm.

Spectroscopic interferometer with a large length range by rotating diffraction grating.

A new method for spectroscopic interferometry using rotating diffraction grating was developed for industrial measurements. Two diffraction gratings increase the spectroscopic resolution, and the effective measuring range can be extended considerably. Instead of calibrating the wavelength, we used the Fabry-Perot Etalon (standard) to calibrate the system and determine the absolute position. The rotation diffraction gratings may also be used as a spectroscopic element over extensive ranges for low-cost and high-speed measurement. Our experiments indicate a length range of approximately 4.00 mm with repeatability of 0.17μm (0.0167%) for the narrow range and 3.84 μm (0.0955%) for the wide range.

A preliminary study on the correlation between TOMS aerosol index and ground-based measured aerosol optical depth

The aim of this work is to study the correlation between ground-based measured aerosol optical depth (AOD) and TOMS Aerosol Index. For this reason, two AOD data-sets have been analysed. The first set of measurements has been obtained in a desert plateau in Namibia during July 1998, while the second one has been collected in Tito Scalo (Italy), a very small industrial zone surrounded by a large rural area, in June–July 2000. The AOD has been computed in the spectral range 400–870 nm with a resolution of 3 nm by measuring the direct solar irradiance. The used spectroradiometer is an Optical Spectrum Analyser, equipped with a continuously rotating diffraction grating. Successively, a correlation between the Earth Probe TOMS Aerosol Index, whose definition uses backscattered radiances at 331 and 360 nm, and the AOD in the visible range was searched for. A satisfying correlation was found, whose Pearson correlation coefficient R 2 values range from 0.64 to 0.91.

Spatially Resolved Droplet Size Measurements

This paper encompasses initial tests of a droplet sizing technique, providing spatially and temporally resolved measurements within a liquid spray from an air assist nozzle. The method utilizes laser interferometry. The primary size measurement is based upon the signal visibility; it is validated by the peak intensity of the scattered light. As the system is based on the Doppler effect, a single component of velocity is extracted concurrent with the size information. The size of the measurement volume is controlled with off-axis light collection. Beam splitting is accomplished with a rotating diffraction grating, allowing the signal to be frequency shifted. Measurements are compared directly to those of a standard diffraction based sizing system (i.e., Malvern). The diffraction information is processed via Rosin-Rammler (2 parameter) and Model Independent (15 parameter) distribution algorithms. Correspondence between diffraction and interferometric results is satisfactory for the Model Independent algorithm. Differences observed using the Rosin-Rammler distribution are attributed to an inadequate representation of the spray by the two parameter model.

On combustion generated noise from turbulent diffusion gaseous flames

A study is made into the mechanism of noise emission process from an ‘inverse’ open turbulent diffusion gaseous double-concentric jet flame. Correlations are presented between the mean temperatures, mean and fluctuating velocity, and the fluctuating pressure on one hand and the overall noise emission on the other from the natural gas-air flames and the He-air jets. Time mean and temporal component of velocity were measured using a laser doppler anemometry system. The LDA was operated in the fringe mode with a radial rotating diffraction grating. Signal processing was carried out by a counter linked to an on-line mini computer. A comparison is presented between the experimental results and the theoretical predictions based on a rational correlation and shows a good agreement for the peak frequency of the radiated sound power.

Rotating Diffraction Grating Laser Beam Scanner

The resolution, bandwidth, and duty cycle of a laser beam scanner make it useful as an image recorder. Laser image recorders can have either a flatfield or a cylindrical recording surface, the flatfield being preferable in applications where dimensional stability is extremely important . A problem with flat field recording systems is the difficulty in obtaining a constant scan rate. This difficulty is a result of the image position for a distortion-free lens being proportional to the tangent of the field angle, rather than the field angle itself. Hence, if the field angle changes at a constant rate (such as the case for rotating mirror scanners), the scan rate is not a constant. One method of eliminating this problem is to use a so-called f-0 lens, which produces a distorted image by the conventional definition, so the image height is proportional to the field angle. 2 This Letter describes a second method of obtaining a linear scan rate for a flat field recorder. It is shown that by using two counterrotating diffraction gratings a constant angular mechanical motion can be used to give a nonuniform angular scan of a light beam. Under appropriate conditions, the angular scan is sufficiently nonlinear that when used with a distortion-free focusing lens a substantially linear scan rate results. The counterrotating diffraction grating scan system has been described previously by Brameley in U.S. Patent 3,721,486; however, it was not pointed out that for appropriate grating parameters a substantially linear scan rate is obtained over a reasonable scan angle. Also, this paper demonstrates that similar results can be obtained using a single rotating diffraction grating folded back onto itself. The rotating diffraction grating scanner is capable of scanning more than one line at a time without line curvature. A 100% duty cycle can be obtained. The rotating diffraction grating scanner is illustrated in Fig. 1. If two identical blazed diffraction gratings are illuminated with a quasi-monochromatic plane wavefront, as illustrated in the figure, and the gratings are counterrotated, the focus of the principal diffraction order will scan a vertical straight line in the focal plane of the focusing lens. If in the zero angle position the lines in the two gratings are parallel, when one grating rotates an angle α in the clockwise direction and the other grating rotates an angle α in the counterclockwise direction, the beam resulting from the +1 diffraction order of the first grating and the -1 diffracted order of this beam produced by the second grating is deviated by an angle θ, given by the equation where d is the spacing of the grating lines and l is the wavelength. If the lens is distortion-free and has focal length f, the focused spot will move a distance l, given by the equation

Laser anemometer measurements in flames with swirl

An experimental study has been made of flow fields in turbulent swirling jets under flame and no-flame conditions. Natural gas was supplied separately to a burner with a divergent exit of 20°. The recirculation zone penetrated into, the diffuser at a swirl number of 0.3. Time mean axial, radial and circumferential components of velocity and rms velocity fluctuations were measured. The laser anemometer operated in the double Doppler mode and frequency shifting was obtained with a rotating diffraction grating. Signal processing was carried out by an electronic single particle pulse counter. Substantial changes in flow patterns were detected as a consequence of combustion, and the kinetic energy of turbulence per unit mass under flame conditions was higher than in the corresponding cold conditions in almost all regions of the flame.

Optical Frequency Shifting by Means of a Rotating Diffraction Grating

The doppler frequency shift produced in the various diffraction orders of a rotating radial diffraction grating allows such a device to be used as an optical modulator. The theory and performance of such devices is presented.