Narrow-band Interference Filters Research Articles

Advances in optical coating uniformity of interference filters

We propose a method to obtain uniform multilayer coatings using vacuum assemblies with a linear ion source to manufacture narrow-band interference filters essential in astrophysical research for space object imaging in important spectral lines, and to provide multi-channel fiber-optic transmission of information. Interference filters and other optical systems, for example, high-reflective mirrors, steep-front beam splitters contain up to hundreds of film layers deposited onto a substrate. During the coating deposition, film thickness on the substrate must be strictly withstood and be uniform throughout the entire surface. This can be achieved by placing the linear source and target on the same platform and by conducting a test target sputtering on a fixed substrate. There is an inflection line on the coating thickness distribution on the substrate. By moving the platform, the inflection line midpoint is aligned with the center of the rotating working substrate, and in this position, the substrate is coated to a specified thickness, which is monitored during the deposition process.

Derivation of depolarization ratios of aerosol fluorescence and water vapor Raman backscatters from lidar measurements

Abstract. Polarization properties of the fluorescence induced by polarized laser radiation are widely considered in laboratory studies. In lidar observations, however, only the total backscattered power of fluorescence is analyzed. In this paper we present results obtained with a modified Mie–Raman–fluorescence lidar operated at the ATOLL observatory, Laboratoire d'Optique Atmosphérique, University of Lille, France, allowing us to measure depolarization ratios of fluorescence at 466 nm (δF) and of water vapor Raman backscatter. Measurements were performed in May–June 2023 during the Alberta forest fires season when smoke plumes were almost continuously transported over the Atlantic Ocean towards Europe. During the same period, smoke plumes from the same sources were also detected and analyzed in Moscow, at the General Physics Institute (GPI), with a five-channel fluorescence lidar able to measure fluorescence backscattering at 438, 472, 513, 560 and 614 nm. Results demonstrate that, inside the planetary boundary layer (PBL), the urban aerosol fluorescence is maximal at 438 nm, and then it gradually decreases with the increase in wavelength. The smoke layers observed within 4–6 km height present a maximum fluorescence at 513 nm, while in the upper troposphere, fluorescence maximum shifts to 560 nm. Regarding the fluorescence depolarization ratio, for smoke its value typically varies within the 45 %–55 % range. The depolarization ratio of the water vapor Raman backscattering at 408 nm is shown to be quite low (2±0.5 %) in the absence of fluorescence because the narrowband interference filter (0.3 nm) in the water vapor channel selects only the strongest vibrational lines of the Raman spectrum. As a result, the depolarization ratio at the water vapor Raman channel is sensitive to the presence of strongly depolarized fluorescence backscattering and can be used for the evaluation of the aerosol fluorescence contribution to measured water vapor mixing ratio.

Online Testing Method for the Fine Spectral Characteristics of Narrow-Band Interference Filters Based on a Narrow-Linewidth Tunable Laser.

The transmission spectrum of a narrow-band interference filter is crucial and highly influenced by factors such as the temperature and angle, thus requiring precise and online measurements. The traditional method of measuring the transmission spectrum of an interference filter involves the use of a spectrometer, but the accuracy of this method is limited. Moreover, placing a narrow-band interference filter inside a spectrometer hinders real-time online measurements. To address this issue, there is demand for high-precision online spectral testing methods. In response to this demand, we propose and experimentally validate a fine spectral characterization method for narrow-band interference filters. This method uses a narrow-linewidth tunable laser, achieving a spectral resolution in the MHz range for online testing. Two types of narrow-band interference filters were tested using the constructed laser spectroscopy experimental system, obtaining a transmission spectrum with a spectral resolution of 318 MHz. In comparison to spectrometer-based methods, our proposed method demonstrates higher spectral accuracy, enables online measurements, and provides more accurate measurements for special spectral interference filters. This approach has significant application value and promising development prospects.

Dynamics of the plasma induced by laser on a cryogenically cooled aluminum target for application in space propulsion

In this work, we investigate the properties of the plasma induced by focusing a high-power laser beam on an aluminum target that was cooled by a helium refrigerator from room temperature down to 20 K. Fast, streak photographs of the plasma were taken at different temperatures and laser energies. From the images obtained, position-vs-time plots were made for each experiment, and from them, the speed was calculated. Additionally, narrowband interference filters were employed to image the dynamics of ions and neutrals separately. It was found that the plasma plume has two distinct speeds: that of its center and that of the outer edge. For unfiltered images, the former has values within the interval 6.2 to 9.1 km/s, while the latter can reach speeds of the order of hundreds of km/s. It was found that the plume of a target cooled to 20 K has a length that is 8%–12% less than the corresponding size at room temperature. Chilling the target did not seem to affect significantly either the plume's speed of expansion or the size of the crater produced. Lower bounds were estimated for the momentum imparted to the ejecta and the specific impulse. The latter can reach a 920-s level, nearly twice as much the amount obtained with chemical rocket fuel.

Interference filter based external-cavity diode laser with combined dual interference filters and largely adjustable feedback range

External-cavity diode lasers (ECDL) are widely used as light sources in laser spectroscopy, atomic physics, and quantum optics. This study demonstrated a home-made 852–nm ECDL with variable feedback, using the combined dual narrow-band interference filters (IFs) as the laser longitudinal mode selection element. The combination of narrow-band IFs, mainly for the current commercially available narrow-band IFs with the full width at half maximum (FWHM) of approximately 0.5 nm for 780-895 nm. We designed different narrow-band IFs combinations to achieve a narrower FWHM, and applied them in the ECDL. The combination of the dual narrow-band IFs further reduces the laser linewidth. We measured the laser linewidth using a high-finesse Fabry-Perot cavity with a linewidth of approximately 10 kHz. The laser linewidth is approximately 176 kHz for the Single-IF-ECDL and 96 kHz for the Dual-IF-ECDL with the same feedback. In addition, we have experimentally verified the results of narrower laser linewidth and larger tuning range with increase in the feedback. The developed Dual-IF-ECDL has the capability of narrow linewidth and wavelength tunability and can be applied to precision spectroscopy, cooling and trapping of neutral atoms.

Study on the Technique for Wavelength Modulation Spectroscopy with Narrow Band Interference Filter Based External Cavity Diode Laser

In this study, an improved tunable external cavity diode laser (ECDL) is proposed. It is on the basis of narrow band interference filter, and its theoretical research is carried out as the light source of tunable diode laser absorption spectrum-wavelength modulation spectrum (TDLAS-WMS) system. The proposed model can use piezoelectric ceramic actuator to change and modulate its output central wavelength so that the laser central wavelength could periodically scan the absorption spectral band of the desired gas. Compared with the tunable monolithic diode laser, the narrow band interference filter based tunable ECDL might be able to accurately sense the type and concentration of a large variety of gases. Therefore, it can be used in the TDLAS-WMS system for high precision gas sensing and is expected to replace the tunable monolithic diode laser.

The Effect of Superpositions on the Planetary Nebula Luminosity Function

Planetary nebula (PN) surveys in systems beyond ∼10 Mpc often find high-excitation, point-like sources with [O iii] λ5007 fluxes greater than the apparent bright-end cutoff of the planetary nebula luminosity function (PNLF). Here we identify PN superpositions as one likely cause for the phenomenon and describe the proper procedures for deriving PNLF distances when object blends are a possibility. We apply our technique to two objects: a model Virgo-distance elliptical galaxy observed through a narrowband interference filter, and the Fornax lenticular galaxy NGC 1380 surveyed with the MUSE integral-field unit spectrograph. Our analyses show that even when the most likely distance to a galaxy is unaffected by the possible presence of PN superpositions, the resultant value will still be biased toward too small a distance due to the asymmetrical nature of the error bars. We discuss the future of the PNLF in an era where current ground-based instrumentation can push the technique to distances beyond ∼35 Mpc.

Lateral scanning Raman scattering lidar for accurate measurement of atmospheric temperature and water vapor from ground to height of interest.

A novel lateral scanning Raman scattering lidar (LSRSL) system is proposed, aiming to realize the accurate measurement of atmospheric temperature and water vapor from the ground to a height of interest and to overcome the effect of a geometrical overlap function of backward Raman scattering lidar. A configuration of the bistatic lidar is employed in the design of the LSRSL system, in which four horizontally aligned telescopes mounted on a steerable frame to construct the lateral receiving system are spatially separated to look at a vertical laser beam at a certain distance. Each telescope, combined with a narrowband interference filter, is utilized to detect the lateral scattering signals of the low- and high-quantum-number transitions of the pure rotational Raman scattering spectra and vibrational Raman scattering spectra of N2 and H2O. The profiling of lidar returns in the LSRSL system is performed by the elevation angle scanning of the lateral receiving system, in which the intensities of the lateral Raman scattering signals at each setting of elevation angles are sampled and analyzed. Preliminary experiments are carried out after the construction of a LSRSL system in Xi'an city, whose retrieval results and statistical error analyses present a good performance in the detection of atmospheric temperature and water vapor from the ground to a height of 1.11 km and show the feasibility for combination with backward Raman scattering lidar in atmospheric measurement.

A Compact Full-disk Solar Magnetograph Based on Miniaturization of the GONG Instrument* *Submitted January XX, 2023.

The design of compact instruments is crucial for the scientific exploration by smaller spacecraft such as CubeSats and deep space missions, as these missions require minimal instrument mass. In this proof-of-concept study, we demonstrate the miniaturization of the Global Oscillation Network Group (GONG) instrument. GONG instruments routinely produce full-disk maps of the Sun’s photosphere using the Ni i 676 nm absorption line, measuring Doppler shifts and magnetic fields. To miniaturize the GONG optical design, we propose replacing the bulky Lyot filter with a narrow-band interference filter. We validate this concept through numerical modeling and proof-of-concept observations. Finally, we propose a simple optical design for building a compact version of GONG.

Experimental investigation of aluminum-air burning velocity at elevated pressure

Aluminum is an energy-dense metal that reacts exothermically with a range of oxidizers, making it a potentially useful fuel for certain thermal energy and power applications (e.g., boilers, swirl-stabilized burners, rockets, etc.). In addition to its reactive properties, aluminum is naturally abundant and commercially available in fine powders that are relatively inexpensive and chemically stable. Fine aluminum dusts, with particle diameters on the order of micrometers, can be aerosolized and mixed with a gaseous oxidizer such as air to create stable dust flames and heat for a thermodynamic cycle. In order to realize the potential utility of aluminum-air dust flames, the fundamental combustion behavior, such as the burning velocity, must be understood. In this work, an experimental system was developed that allows metal dust flames to be observed within an optically-accessible pressurized chamber. Using a high-speed camera, fuel-rich premixed aluminum-air jet flames were imaged through narrowband interference filters, and the data were used to calculate burning velocity. A single polydisperse size distribution of particles was tested at pressures ranging from 1 to 7.2 bar. Burning velocity was found to decrease with increased pressure (P), with an approximate proportionality to P−0.6 over the range of pressures tested (∼P−0.3 for only the laminar flow conditions). This reduction in burning velocity with increased pressure is hypothesized to occur due to increased oxidizer density along with a decrease in interparticle spacing, which may result in asymmetric particle heating and increased competition for oxidizer.

Feasibility study of a high spatial and time resolution beam emission spectroscopy diagnostic for localized density fluctuation measurements in Lithium Tokamak eXperiment-β (LTX-β).

Trapped electron mode (TEM) is the main source of turbulence predicted for the unique operation regime of a flat temperature profile under low-recycling conditions in the LTX-β tokamak, while ion temperature gradient driven turbulence may also occur with gas fueling from the edge. To investigate mainly TEM scale density fluctuations, a high spatial and time resolution 2D beam emission spectroscopy (BES) diagnostic is being developed. Apart from spatially localized density turbulence measurement, BES can provide turbulence flow and flow shear dynamics. This BES system will be realized using an avalanche photodiode-based camera and narrow band interference filter. The system can acquire data at 2MHz. Simulations with the Simulation of Spectra (SOS) code indicate that a high signal to noise ratio can be achieved with the proposed system. This will enable sampling the density fluctuations at this high time resolution. The design considerations and system optimization using the SOS code are presented.

RESULTS OF SPECTROSCOPIC MEASUREMENTS OF PLASMA DISCHARGES AT THE KTM TOKAMAK

The paper presents the results of spectroscopic measurements of plasma discharges at the KTM tokamak. The measurements were carried out during the June 2021 campaign, which plasma discharges were obtained with a plasma current from 120 kA to 150 kA and a duration of ~250 ms.In connection with the design changes inside the vacuum chamber of the tokamak, special attention is paid to the study of the elemental composition of the plasma and the analysis of impurities and the ways of their entry is carried out. Impurities in plasma directly affect radiation losses. Optical spectroscopy (OS) is one of the main methods for studying the penetration of impurities into plasma.An Avaspec-ULS2048CL-EVO-RS-BB spectrometer was used to determine the emission spectrum of the KTM tokamak plasma. The time evolution of the hydrogen lines was measured using the diagnostics of a Hα-Dα monitor equipped with narrow-band interference filters (FWHM 1 nm).Based on the measured emission spectrum, it was found that in the hydrogen plasma of the KTM tokamak, in addition to the presence of a working gas, there were also impurities of carbon, oxygen, nitrogen and argon (C, O, N, Ar) in various ionization states.

Calculation of the electric field in interference coatings

A technique for calculating the electric field in interference coatings is described. The technique is used to find areas in the coating structure with electric field minima. Thin absorbing metal films are introduced into these areas during design. The results of calculating the transmission spectra of narrow-band interference filters with absorbing metal films are presented. It is shown that such metal-dielectric thin-film structures block radiation in the long-wavelength region of the spectrum. Keywords: electromagnetic field amplitude, narrow-band interference filters, spectra, films, absorption, transmission.

Расчет напряженности электрического поля в интерференционных покрытиях

A technique for calculating the electric field in interference coatings is described. The technique is used to find areas in the coating structure with electric field minima. Thin absorbing metal films are introduced into these areas during design. The results of calculating the transmission spectra of narrow-band interference filters with absorbing metal films are presented. It is shown that such metal-dielectric thin-film structures block radiation in the long-wavelength region of the spectrum.

Realisation of the ITS-90 and thermodynamic temperature measurements above 960 °C using a monochromator-based radiance comparator

At LNE-Cnam, the international temperature scale of 1990 (ITS-90) and thermodynamic temperature measurements above the silver point, are carried out with a radiance comparator. This instrument is, more generally, devoted to any radiance comparison in temperature range from the ambient to 3000 °C. The instrument developed in the early 1990s at LNE-Cnam has the advantage of being completely adjustable. Compared to compact radiation thermometers based on lenses and a narrow-band interference filter, the radiance comparator is only made of gold coated mirrors and a Czerny–Turner monochromator to select the spectral bandwidth. The instrument offers the possibility to tune the geometric extent and the slit scattering function. In return, the radiance comparator is a complex instrument that requires a complete and a regular characterisation at the highest level of accuracy. In the first part, this paper describes the instrument and its operating principle. In a second part, a complete study of the wavelength calibration, the slit scattering function, size of source effect, out-of-band transmittance, linearity and other main sources of uncertainty are presented and discussed. Their associated uncertainties are estimated separately and are grouped together to give an example of propagation of uncertainties when realising the ITS-90.

Synthesis of narrow-band interference filters with absorbing metal films

Designs of narrow-band infrared metal–dielectric interference filters are proposed. The unique feature of the developed filters is the fact that the metal films are positioned within the interference coating structure to suppress the unwanted transmission of long-wavelength radiation with wavelengths exceeding 3.5 µm. The peculiarities of the design and spectral properties of such filters are demonstrated using a narrow-band filter with a transmission maximum at 3.43 µm, which is used in infrared nondispersive absorption gas sensors.

Narrow-band interference filters with absorbing films

This paper describes narrow-band interference filters based on a Fabry–Pérot interferometer. They are characterized by absorbing metallic films incorporated into the structure of the interference filter. Such a design has virtually the same transmission spectrum in the required region as those with no absorbing films but at the same time suppresses all radiation in the long-wavelength region. An example is given of the synthesis of an interference filter based on Ge, SiO, and Ni films that operates in the mid-IR region for the wavelength of carbon dioxide CO2 at 4.25 µm.

Time-resolved optical characterization of the interaction between a laser produced plasma and a spark discharge

The interaction between a nanosecond laser produced plasma and a unipolar high voltage discharge was analyzed using time-resolved spectroscopy, fast photography with narrow band interference filters and profilometry. Here the laser ablation fluence was varied in the 60–1000 J/cm2 range. The 600-kW electrical discharge was triggered by an ablation plasma on the aluminum target at time 700 ns in order to enhance the laser-induced breakdown spectroscopy (LIBS) emission. Most of the spectra were obtained from a spot 1 mm above the target and these were: time-integrated to gauge overall enhancement and time-resolved to investigate emission enhancement, temperature and density as a function of time. It was found that the time-and-space integrated LIBS signal was enhanced by up to one order of magnitude at the lowest laser fluence employed when compared with a conventional LIBS experiment. Also, that this emission increment could not be mainly attributed to an increased mass removal, but rather to an enhancement of plasma ionization degree as was observed by means of fast photography with filters. This causes longer plasma duration and more enhancement of ionic species than neutrals. It was also found that both the electronic density and the temperature increase when the spark discharge is applied, in comparison with the ablation plasma. Emission enhancement is inversely proportional to laser fluence due to the fact that for higher fluences, by the time the discharge is applied, the laser-produced plasma is already highly excited.

STUDIES ON THE DETERMINATION OF THE TOTAL HARDNESS OF THE DRINKING WATER COMING FROM THE "FÂNTÂNILE RECI" SPRING IN MARAMUREŞ

The paper showcases research conducted in order to determine the total hardness of the drinking water coming from the "Fântânile Reci" spring, located at the edge of the DN18 road, at the end of the Mara town, in Maramures, Romania. A HI97735 advanced portable photometer was used for this experiment. The photometer can perform low, medium and high range measurements of the total hardness of the water samples. The counter comes equipped with a superior optical system which uses a reference detector and a narrowband interference filter used for extremely quick and repeatable measurements. The values obtained as the result of the experiment are: 204 mg/l CaCO 3 (Total Hardness MR), 15,2 °E, 12,2°dH and 21,7°f. The LED light sources are superior compared to the tungsten lamps. LED lights are more efficient, providing more light while using less energy, and they produce little heat, which could otherwise affect the electrical balance.

MANTIS: A real-time quantitative multispectral imaging system for fusion plasmas.

This work presents a novel, real-time capable, 10-channel Multispectral Advanced Narrowband Tokamak Imaging System installed on the TCV tokamak, MANTIS. Software and hardware requirements are presented together with the complete system architecture. The image quality of the system is assessed with emphasis on effects resulting from the narrowband interference filters. Some filters are found to create internal reflection images that are correlated with the filters' reflection coefficient. This was measured for selected filters where significant absorption (up to 65% within ∼70 nm of the filter center) was measured. The majority of this was attributed to the filter's design, and several filters' performance is compared. Tailored real-time algorithms exploiting the system's capabilities are presented together with benchmarks comparing polling and event based synchronization. The real-time performance is demonstrated with a density ramp discharge performed on TCV. The behavior of spectral lines' emission from different plasma species and their interpretation are qualitatively described.