Lars Erdmann

Coherence management for microlens laser beam homogenizers

We suggest an optical system for beam homogenization and speckle reduction of spatially highly coherent Laser beams. The new method is applicable to laser beams with moderate temporal coherence. Based on the finite temporal coherence the spatial coherence is reduced prior to the homogenizer component. The new design was experimentally tested for ArF - Excimer laser at 193nm. For this Deep UV application we used silicon micromirrors in combination with fused silica microlenses with a pitch of 150 μm . In contrast to former flys eye homogenizers for laser beams the new method employs a large number of sub-apertures even for lasers with high spatial coherence. This results in a speckle free and uniform intensity distribution in the target plane. Furthermore the pupil filling can be increased drastically. Experimental results for a DUV microscopy application are presented and discussed.

MEMS-based lithography for the fabrication of micro-optical components

We present a new method for the fabrication of diffractive and refractive microoptical components. The method is suitable for low-volume production, process development, high quality rapid prototyping of optical components and allows the fast experimental test of designs for a wide variety of different microoptical components e.g. computer generated holograms, blazed diffraction gratings or refrative microstructures. Our method is based on employing a computer-controlled digital-multi-micromirror device (DMD) as a switchable projection mask. The DMD is imaged into a photoresist layer using a Carl Zeiss lithography objective with a demagnification of 10:1 and a numerical aperture of 0.32 on the image side. The resulting pixel-size is 1.36 μm x 1.36μm. In comparison with laser direct writing with a single spot our method is a parallel processing of nearly 800000 pixels (1024 x 768).

Fabrication of low straylight holographic gratings for space applications

The main challenges of fabricating diffraction gratings for use in earth monitoring spectrometers are given by the requirements for low stray light, high diffraction efficiency and a low polarization sensitivity. Furthermore the use in space also requires a high environmental stability of these gratings. We found that holography in combination with ion beam plasma etching provides a way to obtain monolithic, robust fused silica gratings which are able to meet the above mentioned requirements for space applications. Holography accompanied by plasma etching allows the fabrication of a wide range of different grating profiles to optimize the efficiency including the polarization behavior according to a wealth of applications. Typical profile shapes feasible are blazed gratings, sinusoidal profiles and binary profiles and this allows to tailor the efficiency and polarization requirements exactly to the spectral range of the special application. Holographic gratings can be fabricated on plane and also on curved substrates as core components of imaging spectrometers. In this paper we present our grating fabrication flow for the example of plane blazed gratings and we relate the efficiency and stray light measurement results to certain steps of the process. The holographic setup was optimized to minimize stray light and ghosting recorded by the photoresist during the exposure. Low wave front deviations require the use of highly accurate grating substrates and high precision optics in the holographic exposure.

Microscope Illumination Systems for 157 nm

The image quality of an inspection microscope depends strongly on the performance of the illumination system. Especially in the case of laser-based illumination it is necessary to transform the original beam profile into a homogeneous light spot with a flat top field distribution. Simultaneously, speckles caused by the coherence of the laser have to be reduced. Here we discuss different ways to homogenize the multi mode beam profile of a pulsed compact 157 nm excimer laser. A variety of setups, combining dynamic acting diffusers, microlens arrays and primary lenses were realized and characterized in several geometrical arrangements. The homogenizers were evaluated and characterized especially with respect to the statistical behavior on the integrated pulse number.

Optical gratings with low wavefront aberrations and low straylight for enhanced spectroscopical applications

The sensing performance of spectroscopic systems can be enhanced by improving their optical core-element: the optical grating. in particular for imaging spectrometers - especially Hyper-Spectral Imagers - beside the polarization sensitivity and efficiency the imaging quality of the diffraction grating is an important parameter. Optical elements within the spectrometer are manufactured while aiming on lowest wave front aberrations. Thus, least imaging aberration quality of the grating is required not to limit the overall imaging quality of the instrument. Different types of spectrometers (Offner, Czerny Turner) lead to different requirements for the grating surface figure. Beside wavefront aberrations the straylight of gratings will impact the optical performance of spectrometers too. Both parameters are crucially influenced by the manufacturing processes. During the manufacturing process of the grating substrate, a sequence of polishing steps can be applied in order to minimize the wavefront aberrations and roughness. Chemical assisted polishing in combination with classical techniques lead to least surface roughness. A good practice for the manufacturing of aspheres and freeform substrates is the generation of an initial figure close to the final shape only by a classical process, followed by a careful applied aspherization. The imaging performance (wavefront and straylight) of the grating is also optimized due to the recording setup of the holography - including all employed optics for the wave forming. Holographically manufactured gratings with adapted wave forming functions are used for transmission or reflection gratings on different types of substrates like prisms, convex and concave spherical and aspherical surface shapes, up to free-form elements. Numerous spectrometer setups (e.g. Offner, Rowland circle, Czerny-Turner system layout) work on the optical design principles of reflection gratings. All those manufactured gratings can be coated with adapted coatings to support their reflection or transmission operation. The present approach can be applied to manufacture high quality reflection gratings for the EUV to the IR. In this paper we report our results on designing and manufacturing high quality gratings based on holographic processes in order to enable diffraction limited complex spectrometric setups over certain wavelength ranges. Most beneficial is an optimization of the grating during spectrometer design phase while regarding the manufacturing as well. However, the initial optical design approach will show that gratings can be tailored to the specific requirements of the spectrometer (in order to enhance the imaging quality). The enhancement of the optical performance may lead to a specific wavefront shape after the grating element. this special capability for aberration reduction can be defined to the grating during the holographic process. In general, holography enables to manufacture gratings with a specific and adapted wavefront error compensation functions. Beside the results of low aberration gratings the results on straylight measurements will be presented. Recent results and optimization will be shown.

Instrument pre-development activities for FLEX

The FLuorescence Imaging Spectrometer (FLORIS) is the payload of the FLuorescence Explorer Mission (FLEX) of the European Space Agency. The mission objective is to perform quantitative measurements of the solar induced vegetation fluorescence to monitor photosynthetic activity. FLORIS works in a push-broom configuration and it is designed to acquire data in the 500–780 nm spectral range, with a sampling of 0.1 nm in the oxygen bands (759–769 nm and 686- 697 nm) and 0.5–2.0 nm in the red edge, chlorophyll absorption and Photochemical Reflectance Index bands. FLEX will fly in formation with Sentinel-3 to benefit of the measurements made by the Sentinel-3 instruments OLCI and SLSTR, particularly for cloud screening, proper characterization of the atmospheric state and determination of the surface temperature. The instrument concept is based on a common telescope and two modified Offner spectrometers with reflective concave gratings both for the High Resolution (HR) and Low Resolution (LR) spectrometers. In the frame of the instrument pre-development Leonardo Company (I) has built and tested an elegant breadboard of the instrument consisting of the telescope and the HR spectrometer. The development of the LR spectrometer is in charge of OHB System AG (D) and is currently in the manufacturing phase. The main objectives of the activity are: anticipate the development of the instrument and provide early risk retirement of critical components, evaluate the system performances such as imaging quality parameters, straylight, ghost, polarization sensitivity and environmental influences, verify the adequacy of critical tests such as spectral characterization and straylight, define and optimize instrument alignment procedures. Following a brief overview of the FLEX mission, the paper will cover the design and the development of the optics breadboard with emphasis on the results obtained during the tests and the lessons learned for the flight unit.

Space applications: monolithic diffraction grating elements from EUV to NIR spectral range

Monolithic diffraction gratings are one of the key components of high sensitive spectral imaging systems including spectrometer used in space instruments. These gratings are optimized for high efficiency, lowest line spacing errors and low scattering values to improve the performance of a spectral imaging system. Spectral imaging systems lead to enhanced remote sensing properties when the sensing system provides sufficient spectral resolution to identify materials from its spectral reflectance signature comprising low signal-to-noise ratios.

Imaging gratings: Technology and applications for spectrometers (Conference Presentation).

For imaging spectrometers beside the polarization sensitivity and efficiency the imaging quality of the diffraction grating is essential. Low aberration imaging quality of the grating is required not to limit the overall imaging quality of the instrument. The wavefront aberration of an optical grating is a combination of the substrate wavefront and the grating wavefront. During the manufacturing process of the grating substrate different processes can be applied in order to minimize the wavefront aberrations. The imaging performance of the grating is also optimized due to the recording setup of the holography and a special technique to apply blazed profiles also in photoresist of curved substrates. This technology of holographically manufactured gratings is used for transmission and reflection gratings on different types of substrates like prisms, convex and concave spherical and aspherical surface shapes, free-form elements. All the manufactured gratings are monolithic and can be coated with high reflection and anti-reflection coatings. Prism substrates were used to manufacture monolithic GRISM elements for the UV to IR spectral range preferably working in transmission. Besides of transmission gratings, numerous spectrometer setups (e.g. Offner, Rowland circle, Czerny-Turner system layout) working on the optical design principles of reflection gratings. The present approach can be applied to manufacture high quality reflection gratings for the EUV to the IR. In this paper we report our latest results on manufacturing lowest wavefront aberration gratings based on holographic processes in order to enable at least diffraction limited complex spectrometric setups over certain wavelength ranges. Beside the results of low aberration gratings the latest achievements on improving efficiency together with less polarization sensitivity and multi-band performance of diffractive gratings will be shown.

Optical gratings and grisms: developments on straylight and polarization sensitivity improved microstructures

Spectral imaging systems lead to enhanced sensing properties when the sensing system provides sufficient spectral resolution to identify materials from its spectral reflectance signature. The performance of diffraction gratings provides an initial way to improve instrumental resolution. Thus, subsequent manufacturing techniques of high quality gratings are essential to significantly improve the spectral performance. The ZEISS unique technology of manufacturing real-blazed profiles comprising transparent substrates is well suited for the production of transmission gratings. In order to reduce high order aberrations, aspherical and free-form surfaces can be alternatively processed to allow more degrees of freedom in the optical design of spectroscopic instruments with less optical elements and therefore size and weight advantages. Prism substrates were used to manufacture monolithic GRISM elements for UV to IR spectral range. Many years of expertise in the research and development of optical coatings enable high transmission anti-reflection coatings from the DUV to the NIR. ZEISS has developed specially adapted coating processes (Ion beam sputtering, ion-assisted deposition and so on) for maintaining the micro-structure of blazed gratings in particular. Besides of transmission gratings, numerous spectrometer setups (e.g. Offner, Rowland circle, Czerny-Turner system layout) working on the optical design principles of reflection gratings. This technology steps can be applied to manufacture high quality reflection gratings from the EUV to the IR applications with an outstanding level of low stray light and ghost diffraction order by employing a combination of holography and reactive ion beam etching together with the in-house coating capabilities. We report on results of transmission, reflection gratings on plane and curved substrates and GRISM elements with enhanced efficiency of the grating itself combined with low scattered light in the angular distribution. Focusing on the straylight characteristic a measurement of the actual straylight level, preferably with extremely high precision, was performed and will be discussed in this paper. Beside of the results of straylight measurement the actual results on improving efficiency for transmission and reflection gratings will be discussed on theoretical simulations compared to measured data over the entire wavelength range.

Enhanced monolithic diffraction gratings with high efficiency and reduced polarization sensitivity for remote sensing applications

Spectral imaging systems lead to enhanced sensing properties when the sensing system provides sufficient spectral resolution to identify materials from its spectral reflectance signature. The performance of diffraction gratings provides an initial way to improve instrumental resolution. Thus, subsequent manufacturing techniques of high quality gratings are essential to significantly improve the spectral performance. The ZEISS unique technology of manufacturing real-blazed profiles and as well as lamellar profiles comprising transparent substrates is well suited for the production of transmission gratings. In order to reduce high order aberrations, aspherical and free-form surfaces can be alternatively processed to allow more degrees of freedom in the optical design of spectroscopic instruments with less optical elements and therefore size and weight advantages. Prism substrates were used to manufacture monolithic GRISM elements for UV to IR spectral range. Many years of expertise in the research and development of optical coatings enable high transmission anti-reflection coatings from the DUV to the NIR. ZEISS has developed specially adapted coating processes (Ion beam sputtering, ion-assisted deposition and so on) for maintaining the micro-structure of blazed gratings in particular. Besides of transmission gratings, numerous spectrometer setups (e.g. Offner, Rowland circle, Czerny-Turner system layout) working on the optical design principles of reflection gratings. This technology steps can be applied to manufacture high quality reflection gratings from the EUV to the IR applications with an outstanding level of low stray light and ghost diffraction order by employing a combination of holography and reactive ion beam etching together with the in-house coating capabilities. We report on results of transmission gratings on plane and curved substrates and GRISM elements with enhanced efficiency of the grating itself combined with low scattered light in the angular distribution. Beside of the results of straylight measurement the actual results on improving efficiency and lowering the polarization sensitivity for transmission gratings will be discussed on theoretical simulations compared to measured data over the entire wavelength range.