We present highly miniaturized optical and electronic components for a two-channel fluorimeter with pin photodiodes with a very small detection limit (with an excitation of 100 μW and 590 nm, we detect one nW at 650 nm). One potential application lies in continuous monitoring of blood glucose levels. In a first step, novel dichroic beam splitter cubes made of fused silica with an edge length of 3 mm were developed. The diagonally dividing surface of the cubes was provided with a new dichroic layer system using the PARMS (Plasma Assisted Reactive Magnetron Sputtering) coating technology, which made it possible to achieve an angle of incidence of 45° (the current state of the art is more like 30°). The wavelength ranges are between 560 and 800 nm. In a second step, a special LED lighting unit with dimensions of 4 mm by 6 mm by 5 mm was developed. The angular distribution in the excitation beam that hits a dichroic beam splitter cube is limited to 3 degrees. In addition, very thin optically direction-selective filters have been developed to reduce stray light on the photodiodes. Here, glass substrates with a 100 μm thick Si-layer were used. This silicon layer was structured using inductively coupled plasma etching (ICP). Breakthroughs measuring 40 μm by 40 μm were created with inner walls made of black silicon. The transmission of these filters is 0.4 at angle of incidence at zero degrees and is less than 0.01 at angles of incidence greater than 13°.
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.
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%.
The Sentinel 2 mission shall ensure the continuity and enhancement of Landsat and SPOT data and sustain operational
land services in the frame of the Global Monitoring for Environment and Security (GMES) initiative. Sentinel-2 is
designed to image the Earth’s landmasses from its orbit for at least 7.25 years. The Multi-Spectral Instrument (MSI),
delivered by Astrium Toulouse, will provide high resolution imagery in 13 spectral channels extending from the Visible
Near Infrared (VNIR, 400-1100 nm) to the Short Wave Infra-Red (SWIR, 1100-2500 nm) range, down to a resolution of
10 meters with an image width of 290 kilometers. A dichroic splitter device is located in back-focal path of the telescope.
It allows splitting the incoming optical beam between VNIR and SWIR focal planes. It shall ensure an extremely high
rejection, better than 1:1000, between both ranges while introducing negligible aberrations in reflected (VNIR) and
transmitted (SWIR) paths. The splitter assembly consists of a wedged dichroic filter plate and a wedged compensator
plate mounted in a common frame. Both plates are made of fused silica (Infrasil) and polished to lambda/40. The major
challenges reside in the design complexity of the dichroic coating and in the deposition process control to ensure the
required high uniformity of performances through the large aperture. The paper presents the final spectral and optical
performances of this challenging sub-system. It also discusses the main difficulties that have been overpassed during the
development and qualification phase.
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 production of advanced optical coatings with complex spectral characteristics and high performance is directly
dependent on the stability of the deposition process and on the accuracy of the monitoring system employed for
controlling the thickness of the constituent single layers. The present contribution is concentrated on the current state in
deposition control and manufacturing of coatings with improved precision. As major topics the simulation of deposition
processes, modern monitoring concepts, and the handling of errors occurring during the deposition process will be discussed. For illustration of some recent developments, results on the deposition of rugate filters on the basis of an ion beam sputtering process will be presented.
Interference filters for spectroscopic purposes or sensor applications are characterized by strictly specified spectral
blocking and transmitting regions with intermediate steep edges. These steep edges must be positioned within nanometer
accuracy while the coating may consist of more than one hundred non-quarterwave layers. Though modern ion assisted
deposition processes in conjunction with quartz crystal control are well suited for the production of complex filters, an
optical monitoring device seems to be necessary to fulfill the demanding spectral requirements. Broad band optical
monitoring (BBM) directly on the calotte has been employed to control the production of this type of band stop filters.
For a large number of also different types of these coatings the BBM-technique demonstrated its capability to improve
the reliability and flexibility in industrial production. Within a stable well-characterized deposition process error self-compensation
effects allow for a fast realization of various designs within specified tolerances. Nevertheless, optical
broad band monitoring could not be applied to all types of these steep edge filters because error propagation leads to
unreachable solutions of the thickness tracing algorithm for specific cases. The given examples of complex steep filters
and the corresponding post analysis of stored online spectra as well as the simulation of the monitoring process reveal
the influence of the design itself to this occurrence. A suggestion for an identification of critical thickness values within
the layer sequence is discussed and solutions to the problems are presented.
Laser induced breakdown of single-layer, ion-beam sputtered TixSi1-xO2 composite films was studied using single and
multiple pulses from a femtosecond Ti:sapphire laser. The bandgap of this coating material can be gradually adjusted
with the composition parameter x. A scaling law with respect to the bandgap energy and pulse duration dependence of
the single-pulse damage threshold that was observed previously for pure oxide films was found to apply to the composite
films as well. The dependence of the damage threshold as a function of pulse number F(N) was similar to the behavior
observed for pure oxide films. It was possible to explain the dependence as a function of pulse number using a
theoretical model based on the formation and accumulation of defects. The shape of F(N) can be used to estimate the role of shallow traps and deep traps on the multiple-pulse breakdown behavior.
The utilization of oxide mixtures as layer material for coating design has been transferred to the ion beam sputtering technology and was applied to high-reflecting as well as anti-reflecting components at the wavelength of 193nm. Exclusively, the oxides SiO2 and Al2O3 are candidates for appropriate thin film designs below 200nm. Experimental data received from laser-calorimetric measurements, spectroscopic investigations and laser-induced damage tests are presented for several specimens.
With respect to state-of-the-art thin film deposition in the DUV spectral range, conventional quarterwave designs have also been characterized and will be compared to the sputtered mixed oxide coatings.
A modified IBS-process was used to create mixtures of oxide coating materials. The process allows to manufacture new
designs, whereas the important optical and electronic properties of the material can be varied in a wide range. Especially
for ultra short pulse applications, higher damage thresholds can be achieved.
In this paper, LIDT measurements of mixed and pure single layers are presented. The coatings were investigated at
different wavelengths and in a wide pulse duration range. The results of the measurements confirm the empirical law of
the linear LIDT dependency on the absorption gap. Based on this empirical law, the Refractive Index StEps Down
(RISED) concept was developed. From the data of the single layer measurements, an optimization of RISED optical
components in the fs-regime will lead to even higher damage thresholds. Particularly, for high reflecting mirrors the
damage threshold could be doubled for different dielectric coating materials. Additionally, the paper presents a
theoretical analysis of the stack LIDT on the basis of the single layer properties.
The scaling law of subpicosecond laser induced damage (LID) with respect to pulse duration and band gap for TixSi1-xO2 composite films is studied. The band gap in these materials can be changed gradually by varying the composition pa-rameter x. Damage is very deterministic and scaling laws with respect to pulse duration and band gap energy derived previously for pure materials are found to apply to composite films. The scaling can be explained theoretically by using a modified Keldysh theory. The composite materials also show a dependence of the damage threshold as a function of pulse number F(N) (incubation) that is similar to observations in pure dielectric oxides. The measured F(N) is explained with a theoretical model that assumes the formation of an intermediate sample state that increases the absorption of sub-sequent pulses in the train.
Investigations in fs-laser damage mechanisms within the recent years indicate that damage mechanisms in the fs-range
are based on electronic interaction schemes in the material. Usually, a direct correlation of the power handling capability
to the band gap structure of the material and the field strength distribution in the optical system is observed.
The present work is focused on the optimization of high refractive index coating materials by mixing with silica. The
different compositions of mixed materials are manufactured with an IBS coating process using a zone target. This
technique allows for a continuous variation of the material composition.
In addition, new coating designs were developed to adapt the contents of silica within the layers to the high field
strengths. By combining these techniques a significant increase of the laser damage threshold could be accomplished.
Microscopic imaging methods are valuable tools to analyze damage morphologies of laser optics for ns and fs
applications. In the fs-regime, the morphology of TiO2/SiO2 coatings with modified field strength distributions were
investigated, whereby a characteristic morphology was caused by the special designed vertical field strength profile,
depending on the local power density. In the ns-regime, the morphology of the damage sites has shown significant
differences between the quarter wave stacks and the gradual index systems without abrupt interfaces in the functional
layers. Typically, these Rugate high reflectors did not show catastrophic damage. Rather the damage becomes apparent
by the creation of colour centres.
The development of advanced and reliable techniques for the production of optical coating systems with a
continuous variation of the refractive index opens the way towards a new generation of optical components in
laser technology and modern optics.
The present paper is dedicated to an Ion Beam Sputtering (IBS) concept for the production of coatings with
gradual index layers and Rugate filters. On the basis of a spectrophotometric online-control system, Rugate filter
coatings were produced with high precision and reliability. In addition to the optical performance, especially the
laser damage properties of the coating systems were investigated with respect to defined mixtures of two coating
materials and the influence of gradual index layer designs. A dramatic increase of the laser induced damage
threshold was observed for the produced Rugate coatings. The experimental results are discussed considering the
special properties of gradual coating systems.
The power handling capability of optical components is still one of the most important limitations for the further improvement of ultra-short pulse lasers in respect of average power and pulse energy. Laser-induced damage of functional dielectric coatings on laser crystals, pockels cells, out-coupling polarizers and compressor gratings is severely inhibiting the wide dispersion of ultra-short pulse laser systems especially in industrial production environments. Since the underlying physical causes for laser-induced damage with ultra-short pulses are distinctly differing from those in the nanosecond time scale, novel approaches must be found for an unambiguous improvement in damage resistance of optical coatings.
In previous investigations, the band-gap of the coating material and the maximum field strength in the layer stack were identified as most important influences on the laser-induced damage with ultra-short pulses. Furthermore, a significant nonlinear increase of absorptance in dielectric coatings was found to be strongly related to the band-gap of the material. These effects were traced back to the multi-photon and avalanche-ionization as driving mechanisms for producing a critical conduction band population. In the current investigations, numerous model layer systems were investigated concerning laser-induced damage and non-linear absorptance. Adapting the ion beam sputtering coating process for achieving co-deposition of high and low index materials, coatings with continuously tunable refractive indices were produced. The results of the experiments exhibit a strong correlation of the damage threshold to the controllable shifting band-gaps of the coating materials.
Although the measurement of the laser-induced damage threshold is a field of permanent research effort since the late 1960s, the optimization of the damage handling capability is still a key issue for the development of high performance laser systems. In conjunction with the ever increasing demand for lasers with high average power, energy, extreme wavelengths or short pulses, the resistance to laser damage has to be optimized with a special regard to the different
damage mechanisms. Therefore, a report of the current status of the laser-induced damage threshold is given for the most interesting components and laser systems applied in science and industry. Further, several results of recently performed damage investigations in the NIR spectral range and for ultra short pulses are presented in this paper. The reliability of damage threshold measurements is crucially depending on the chosen test parameters. The importance
of the different parameter values were investigated carefully during several Round-Robin experiments. These investigations can be regarded as the basis of the standardization process leading to the International Standard ISO 11254. In this paper, selected results of the comparative campaigns in damage testing are described, especially in the field of ns and fs pulses.
Rugate structures, as well as gradient refractive index films in general, attract a lot of interest. The gradient index systems may provide advantages in both, optical performance and mechanical properties of the optical coatings. Rugates have shown to be especially interesting for design of notch filters. A lot of theoretical work on design of rugate filters has been done in the last decades. However, only few of the designs could be deposited, which is often caused by practical problems, e.g. preparing materials with the desired refractive index values. In this paper two different gradient refractive index designs are compared to a classical high-low stack. One gradient design is synthesized by an apodized sinusoidal structure that is approximated by homogeneous sublayers. The other one is based on an apodized sinusoidal structure as well, but it is approximated by a hybrid structure, i.e. a combination of linear gradient index ramps between the lowest and the highest refractive index applicable and homogeneous layers of high index values. The two gradient designs take into account the constraints posed by limitations of the real deposition systems. Both designs are compared to a classical high-low stack and the advantages and drawbacks of each approach are commented.
Rugate filters have a high potential for solving specific design problems in many applications of modern optics and lighting technology. However, the exact manufacture of these gradual layer systems is still a challenge which could not be solved completely until today. One of the prominent approaches for the production of rugate filters is based on independent quartz crystal devices measuring the rate of the different coating materials. As an alternative, optical broadband monitoring has been already qualified for controlling the deposition of complicated non quarterwave stacks. In the present study, promising results of this deposition control concept as a direct monitoring of rugate filters will be presented. In a first attempt, the continuous change of refractive indices in the graded layers was transformed to a set of discrete homogeneous sub-layers with thicknesses values of around 5 nm. These discrete layers are realized by defined mixtures of two materials. A data base for the dispersion behavior was created for the different mixing ratios and is employed for the production of such quasi-rugate filters. The optical monitor is operated in the routine mode determining the switching points of the layers. Selected examples will be presented for quasi rugate coatings produced by ion beam sputtering from a movable zone target. Different designs will be discussed considering production problems as well as achievable optical properties.
Since first publications on optical broadband monitoring 25 years ago, the technological progress in computational speed and hardware made it possible to expand this basic approach to a versatile tool for thin film thickness control. During the last few years, essential progress could be achieved for optical monitoring systems operating in the visible and near infrared spectral range. In conjunction with powerful computer algorithms these systems allow a precise determination of the thickness of a growing layer during deposition and the reliable production of very complex nonquarterwave interference filters. It was shown earlier that the application of such a monitoring system in an IBS-(ion beam sputtering)-process enables a fully automated rapid prototyping of optical coatings with very complicated designs. Using the ion beam sputter process with its immanent reproducibility of the optical coating properties, even the precise control of layers with a thickness of around a few nanometers is possible. Offering the capability of producing designs containing such delicate thin layers, this process concept opens new prospects for a new class of thin film systems. Recently, the high potentiality of this monitoring concept in applications to ion-assisted coating processes (IAD) could be demonstrated. This paper is concentrated on an examination of error sources, which brought a further improvement in reliability of the monitoring system.
As a consequence of the ever increasing application field of modern optical technologies, new demands for the optimization of deposition processes for high quality optical coatings with increased environmental stability and power handling capability are imposed on thin film manufacturers. Starting from this challenge, the presented work is focused on the development of an ion assisted deposition (IAD) process using a cold cathode ion source. Especially in the mid
infrared wavelength region (MIR) with its water absorption bands, the ion assisted deposition process leads to many practical advantages, e.g. for medical laser applications.
In the present study, a cold cathode ion source was operated with pure oxygen for the deposition of different oxide materials. Besides the determination of the optical properties, the characterization of the thin films included the first application of an in situ optical broadband monitoring system during the IAD process.
The produced single layers and MIR coatings are thermally stable, shift-free, and exhibit lower absorption compared to conventionally deposited coatings. In contrast to the conventional coatings, also no vacuum-to-air shift is observed for the realized MIR coatings. Therefore, the stable and reproducible IAD process in combination with the new process control strategies using the broadband transmittance measurements on the moving substrates allows an advanced process control and a precise determination of the layer thickness.
In the course of the rapid development of laser technology and modern optics, an ever increasing demand for optical coatings with extraordinary specifications can be observed. In practice, the production of such high quality optics with special requirements in respect to bandwidth, edge steepness or wavelength accuracy regularly requires an extended optimization of the coating process. In many cases, the resulting high production cost delays the development of new promising concepts in laser and optics technology. For the realization of new optical designs, generally two difficulties occur: At first, the physical properties of the coating materials change after completion of the coating process due to environmental influences. Furthermore, the accuracy of the commonly utilized methods for thin films thickness monitoring is not sufficient for a reliable thickness control. In this paper, an ion beam sputtering (IBS) coating process is described for the completely automated fabrication of optical coatings with extremely stable characteristics. In contrast to conventional arrangements with witness glasses, the presented thickness monitoring during the coating process can be directly performed for the optics. The precise transmittance measurement over a bandwidth of one octave is achieved by a fiber-coupled multi-channel spectrophotometer. With this arrangement also very small layer thickness errors are detected and may be compensated by optimizing the subsequent layers in the stack in order to meet the specifications. The combination of the innovative IBS- process with the broad-band spectrophotometric thickness monitoring is the key for new laser applications, e.g. low loss edge filters for high power diode laser wavelength multiplexing or phase-optimized mirrors for ultrashort pulse laser systems.
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