Multispectral earth observation is done using selected discrete filter channels. The optical filtering is typically accomplished using optical thin film filters. These filters can be placed in filter wheels or as an array directly in front of the detector. For compact system designs filter arrays are preferred. The manufacturing of filter arrays can be done by two different approaches called monolithic array and butcher block. Typical optical requirements for such filters are for example a filter transmission of above 90% and an out-of-band blocking in OD6 range.
The Copernicus Land Surface Temperature Monitoring (LSTM) mission is part of the Copernicus Sentinel Expansion Missions. It will carry a high spatial-temporal resolution thermal infrared sensor to provide observations of the land-surface temperature. The mission responds to priority requirements of the agricultural user community for improving sustainable monitoring requirements to better manage water resources and learn about yield, vegetation and crop growth. The spectral coverage in the multiple bands spans from 490nm to 1610nm for the VNIR/SWIR part of the instrument. Materion Optics Balzers was selected as responsible supplier for the VNIR/SWIR filter assemblies. This contribution addresses the design, manufacturing and characterization of the demanding dielectric optical coatings for the sophisticated band pass filters and dichroic by PARMS technology for the LSTM project by Materion Optics Balzers.
Within the Copernicus program, the Sentinel-5/UVNS instrument is dedicated to the monitoring of air quality, trace gases and aerosols. The instrument consists of two co-aligned telescopes and five spectrometers in the spectral channels named UV1, UV2VIS, NIR, SWIR1, and SWIR3. The spectral band of UV1 spectrometer is defined from 270 nm to 310 nm. To distribute incoming light and eliminate false light into the channels and within the UV1 channel dedicated coatings for UV spectral range are needed. OBJ was selected for development and application of these coatings.
Andreas Rahm, Marc Lappschies, Stefan Jakobs, David Blum, Federico La China, Carmine Alessio Mastrandrea, Barbara Grandclaude, Marie-Noëlle Langevin, Massimiliano Porciani
The Multi-viewing, Multi-channel, Multi-polarisation Imager (3MI) is one of the instruments of the “Satellite A” payload of MetOp-SG, developed to provide information on atmospheric aerosols. 3MI is a space based, wide-field-ofview spectroradiometer that is designed to acquire sequential images of the same ground target which are combined with multiple spectral views in both un-polarized and polarized channels. The spectral coverage in the multiple bands from 410 nm to 910 nm and from 910 nm to 2130 nm shall be done using a Filter Wheel Assembly which included the Filter Wheel Disk (FWD). OBJ was selected for development and production of these optical elements. This contribution addresses the manufacturing and characterization of the demanding dielectric optical coatings for the sophisticated BPFs developed by Optics Balzers.
Beside homogeneous filter coatings a coating can also be applied with a linear gradient. Linear gradient or linear variable filters show a gradient of a band edge or central wavelength depending on the filter type in spectral direction and they are homogeneous in spatial direction. In this paper, we present a linear variable narrow band pass filter with full width half maximum of about 8 nm and a transmittance of more than 98% in the wavelength range of 670 nm to 780 nm. The target for the gradient is 3.3. nm/mm. Due to the need of transmittance filter and AR coating are manufactured by means of Plasma Enhanced Magnetron Sputtering (PARMS). Additionally, the linear variable filter for FLEX mission requires a black mask to separate between HR1 and HR2 channel. This mask is also applied by OBJ by means of the PARMS process and a Ti based layer stack. Here, a reflectance of <1.5% in the range of 400 nm to 800 nm can be demonstrated. The definition of black mask was done by means of photolithography.
Beam splitters separating visible (VIS) and near-infrared (NIR) light are an important component of many optical systems such as spectrometers or telescopes. Here, one part of the spectrum is transmitted while the other is reflected. Typical goal requirements are broadband high transmittance and high reflectance without local minima combined with a steep transition zone. These requirements drive the complexity of the coating design. Beside the coating design also the deposition technology has an impact on the feasibility of the coating. In this contribution, we address manufacturing challenges for manufacturing of an ideal beam splitter and compare Ion Assisted Deposition (IAD) and Plasma Enhanced Magnetron Sputtering (PARMS) technology by presenting examples of VIS-NIR beam splitters manufactured at Optics Balzers Jena GmbH (OBJ). These examples reach from beam splitters manufactured by IAD with a total coating thickness of about 3.5 μm to the beam splitter of Sentinel 2 multi-spectral instrument with more than 100 layers a and a coating thickness of about 13 μm. An example which overcomes the limitations of the IAD process is the dichroic plate of ESAs Euclid telescope manufactured by OBJ by means of PARMS process. This dichroic plate shows a reflectance of over 99% in the VIS spectral range and a transmittance of more than 98% in the NIR spectral range. Both mentioned manufacturing technologies are discussed in terms of process stability, coating homogeneity, and straylight limitations.
Optical remote sensing of the earth from air and space typically utilizes several channels in the visible and near infrared spectrum. Thin-film optical interference filters, mostly of narrow bandpass type, are applied to select these channels. The filters are arranged in filter wheels, arrays of discrete stripe filters mounted in frames, or patterned arrays on a monolithic substrate. Such multi-channel filter assemblies can be mounted close to the detector, which allows a compact and lightweight camera design. Recent progress in image resolution and sensor sensitivity requires improvements of the optical filter performance. Higher demands placed on blocking in the UV and NIR and in between the spectral channels, in-band transmission and filter edge steepness as well as scattering lead to more complex filter coatings with thicknesses in the range of 10 - 25μm. Technological limits of the conventionally used ion-assisted evaporation process (IAD) can be overcome only by more precise and higher-energetic coating technologies like plasma-assisted reactive magnetron sputtering (PARMS) in combination with optical broadband monitoring. Optics Balzers has developed a photolithographic patterning process for coating thicknesses up to 15μm that is fully compatible with the advanced PARMS coating technology. This provides the possibility of depositing multiple complex high-performance filters on a monolithic substrate. We present an overview of the performance of recently developed filters with improved spectral performance designed for both monolithic filter-arrays and stripe filters mounted in frames. The pros and cons as well as the resulting limits of the filter designs for both configurations are discussed.
Scattering in thin-film filters is mainly driven by the substrate roughness and the deposition technology but also by the coating design as well as by the filter orientation. A high-energy, low-loss coating technology (plasma-assisted reactive magnetron sputtering, PARMS) was used to deposit an advanced thin-film filter design on substrates with low micro roughness. This approach resulted in a significantly reduced level of scattered light as well as an excellent spectral performance of the produced filters. Compared to coatings deposited by ion-assisted evaporation (IAD), the level of out-of-band scattering could be reduced by more than 2 orders of magnitude to about 0.001%.
Besides the typical channels in the visible and near infrared spectrum, optical remote sensing of the earth from air and
space utilizes also several channels in the short-wave infrared spectrum from 1000 nm to 3000 nm. Thin-film optical
filters are applied to select these channels, but the application of classical multiple-cavity band-pass filters is impossible.
Because of their additional blocking elements they are disallowed due to geometrical or other non-optical reasons.
Within the sensitivity region of an MCT detector as typical detector device, the selection and blocking of radiation by the
filter has to be provided by a single multilayer system. The spectral region of the SWIR as well as blocking width and
depth require necessarily designs with overall thicknesses of more than 20 μm, with layer numbers up to 100. SiO2 and
TiO2 were used as thin-film materials deposited with reactive e-beam evaporation under ion assistance in a Leybold
SyrusPro box coater. A special challenge was the thickness measurement of the thin films by an optical broadband
monitoring device in the visible range. The results of manufacturing and characterizing of such filters are presented by
three examples for the center wavelengths of 1375 nm, 1610 nm, and 2190 nm.
For more than four decades band-pass filters are important components of microscopes used for the fluorescence
spectroscopy. During all the time this special field of application has been one of the main drivers for research and
development in thin-film optics, particularly for the thin-film design software and the coating technology. With a
shortwave pass filter, a multi-notch filter, and a classical band-pass filter as examples of such filters provided for the
latest generation of fluorescence microscopes we present the state-of-the-art in coating design and technology.
Manufacturing these filters is a great challenge because the required spectral characteristics need necessarily multilayers
with up to 300 layers and overall thicknesses up to 30 μm. In addition, the designs require also 3 to 5 nm as thinnest
layers and all the layers are completely of non-quarterwave type. The filters were manufactured in a rapid-prototyping
regime by a Leybold Helios plant using plasma-assisted reactive magnetron sputtering of thin films of different metal
oxides. Designed and real spectra are compared and differences are discussed. Measurement results of other optical and
non-optical characteristics as film stress, total integrated scattering, and micro roughness are presented.
Rugate designs for the realization of notch filters are well known in the literature. The required deposition of gradient
index layers is difficult to manufacture. In our approach we apply the equivalent index theory to replace the gradient
index profile of a notch filter design. We produce single and multiple notch filters with plasma ion-assisted deposition
and broad-band optical monitoring. As examples, a 500nm notch filter for the GREGOR telescope and a 589nm notch
filter for the GALACSI instrument of the VLT are discussed. Additionally, a 4-line multiple notch filter and a 218nm
notch filter made for fluorescence spectroscopy applications are presented.
The presented work is embedded in the research network “Integrative Ion Processes for Modern Optics”, called IntIon, consisting of 12 partners from the German optics industry and two research institutes. The main target of the IntIon network is the development of new process concepts on the basis of ion assisted deposition (IAD) for the industrial production of optical thin film components. Besides an improvement in efficiency, a major aim is concentrated on the optical characteristics for selected application fields with high economical potential. In this network, different ion and plasma sources are compared with regard to their qualification for ion assisted deposition processes. This work includes the characterization of the ion energy and ion current using Faraday-cup measurements. The selection of investigated coating materials includes a broad variety of standard and non-standard oxides. First results of the network will be presented for adapted deposition materials and different operation characteristics of ion sources.
Arrays of longpass filter coatings for high order suppression in miniature spectrometers were produced by plasma-ion assisted deposition and photolithogaphy. The filter edges were imaged by optical microscopy, scanning electron microscopy, and scanning force microscopy. Whereas a positioning accuracy of about 2 microns was achieved, the width of the filter edges varied between 2 and 10 microns.
The manufacturing processes for spectral-sensitive on-chip masking of Si-PIN-diodes using thin-film optical filters are described. As two examples, a diode array with red, green, and blue filters (RGB) and a UV-sensitive diode are explained in detail. The RGB-filters are made of TiO2SiO2 thin-films and the UV-filter is a metal/dielectric multilayer using HfO2, SiO2, and Al thin-films. Both filter types are self-blocked over a wavelength range from 200 to 1100nm. The optical coatings on the diodes are arranged as pixels with rhombic or rectangular shapes and with a later dimensions of about 20 microns as minimum. The used lift-off technique for patterning the coatings is described briefly. Reactive e-beam evaporation with ion-assistance is used to deposit the optical coatings.
Normal-incidence multicycle broadband AR coatings on glass with low residual over the wavelength region 400 nm to 800 nm are theoretically discussed. Using the method of effective interfaces and the concept of equivalent index layers a 4-layer design sub(AHBL)air is expanded into the designs sub[AH(C2LC)2BL]air where A, B, C, H, and L are quarter-wave layers with refractive indices nH equals 1.38, nL equals 2.35, and nL < n(Lambda ), bB, nC, < nH. These designs represent 1-cycle, 2- cycle, and 3-cycle broadband AR coatings and perform a residual reflectance of 0.20 percent, 0.14 percent, and 0.10 percent, respectively. Four other designs demonstrate the approach at other wavelength regions. In one example, all quarter-wave layers with refractive indices in-between nL and nH are analytically substituted using equivalent index layers to get two-material multilayers suitable for practical applications. After this, the design is refined with respect to the optical thickness whereby the layer number is fixed. The resulting design consists of 18 layers and performs an average residual reflectance 0.085 percent over the region 400 nm to 800 nm.
CaF2 has received increasing attention as a promising substrate for coatings in the VUV range. Optimization of the optical properties of these optical components requires the study of basic characteristics of the coated and uncoated CaF2 substrates such as surface roughness, optical performance, absorption and scatter losses, and laser induced damage threshold. The investigations reveal the influence of different substrate polishing grades on the quality of the coated components.
Effects of defect propagation from the substrate throughout thin film single layer and multilayer coatings are investigated on fluoride and oxide films evaporated onto different types of substrates. Atomic force microscopy, light scattering methods, and transmission electron microscopy are used to study the surface morphology of the coated and uncoated substrates. With fluoride films, propagation of rough substrate structures is observed for both single layer films and multilayers. Oxide coatings replicate the microtopography even of well polished surfaces and throughout thick multilayer stacks.
Plasma ion assisted deposition was used to deposit SiO2 coatings on polycarbonate and on silicon. To study the influence of ion bombardment during the deposition process the bias voltage as an equivalent for ion energy was varied. At a definite bias value surface microtopography is changed abruptly. Coatings deposited at higher bias voltages show a stable chemical composition and therefore constant optical properties can be expected. It was also found that the barrier effect of coatings against moisture uptake, measured for polycarbonate substrates, increases with bias voltage.
Substrate properties, coating design, and deposition process determine the surface morphology of optical coatings. The contribution of each factor can be estimated by measuring the surface topography with a scanning force microscope (SFM) and calculating the power spectral density functions. We present results for oxide and fluoride coatings on well polished glass and silicon substrates. The scattering of the coatings is predicted by calculations based on SFM data and compared with results from angle resolved scattering measurements.
A commonly used estimator for the microtopography of an optical surface is its rms-roughness. Raw surface profile data may contain trending components. Therefore they should be subjected to a detrending procedure before estimating the rms value. This procedure is limited in most cases to the removal of piston, slope, and curvature. Consequently, undesired artifacts may arise, which negatively influence the precision of rms-roughness estimation. In scanning surface metrology, the eigenvalues and eigenvectors of the covariance matrix of the surface can be used for a robust and precise multivariate estimation of rms-roughness.
The surface microstructure of evaporated single layer and multilayer fluoride coatings for KrF lasers as well as the topography of uncoated fused silica substrates have been investigated with an atomic force microscope (AFM). The fluoride films exhibit a pronounced columnar microstructure that accounts for the typical surface morphology and causes surface roughness the magnitude of which depends on film thickness and substrate temperature. Well polished fused silica substrates show low surface roughness, which has been determined from both AFM and light scattering measurements.
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