Dispersive elements are in general the key components of spectrometers and define mainly their performance. Prisms and filters are typically used for lower resolution applications, e.g. color measurement for industrial applications. Many high performance spectrometer applications are using gratings as dispersive elements instead. Different spectrometer layouts require various grating approaches in order to maintain the optical imaging performance. A frequent aim is a progressively optimization of the optical performance in balance to the mechanical parameters like weight, volume or robustness against variations of environmental conditions of a spectrometer module as well. Thus, the optical designer has to draw on additional design degrees of freedom. This in turn results often in more and more complex grating types featuring curved substrates and/or variable and bended grating lines. Especially the trend toward hyperspectral imaging applications demands appropriate options for enhanced field correction. The main ZEISS technology chain for grating manufacturing includes holography and reactive ion etching is a flexible base for these special types of gratings. A close entanglement between holography and accurate test procedures for the optical functionality of the holographic grating is a pre-condition for the ability to meet the often challenging specifications. Therefore, beside the brief description of the manufacturing technology in this text we show a set of newly developed measuring procedures supporting the holographic surface patterning approach.
Scattered light level of optical components can severely impair SNR and overall performance of optical systems for imaging and spectrometry. It is therefore necessary to directly assess its angular distribution in terms of BRDF measurements which is, due to the extreme dynamics required for high quality optical surfaces, still a challenging task. In our contribution we will present a self-built scatterometer that is based on a Czerny-Turner geometry in conjunction with a CMOS-camera detector and a single mode fiber coupled 405 nm diode laser source. Our setup is, besides simple spherical mirrors, purely based on stock-components and both, cost-effective and simple to build. Considerations on system design for high resolution and minimized instrumental signature as well as a first breadboard experimental setup will be discussed. The scatterometer utilizes the sensor’s pixels for adaptive sub-slit resolution and 2d measurements in the close vicinity of the plane of incidence. It can cover BRDF-values of up to 14 orders of magnitude and reaches resolutions well below 0.01° which allows to gain useful insights about small-angle scattering that has in the past been difficult to experimentally address. First measurements of superpolished mirrors as well as holographic and mechanically ruled diffraction gratings will be presented. Simple formulae can be used to assign rotation angles to spatial frequencies and, for smooth surfaces with negligible particulate contribution of scattering, also to PSD values and band-limited RMS roughness.
Reflection losses due to refractive index mismatch limit the obtainable diffraction efficiencies for transmission gratings in the highly dispersive regime, i.e., with period to wavelength ratios smaller than 0.7. The design and fabrication of such gratings with high-diffraction efficiencies (≥94 % , Littrow configuration) will be discussed with an emphasis on process strategies to control the profiles in the reactive ion beam etching step. Experimental results from the manufacturing of monolithic fused silica pulse compression gratings with 3000 L / mm optimized for a center wavelength of 519 nm will be presented. The influence of different etching parameters such as etch gas mixture, ion incidence angle, and acceleration voltage of the ion source on profile depth, side-wall angle, duty cycle, and ultimately diffraction efficiencies will be discussed.
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.
There are several applications for diffraction gratings in laser physics like frequency stabilization, wavelength tuning and temporal pulse shaping. Especially the growing market for femtosecond lasers with increasing pulse energies and peak powers boosts the requirement for highly dispersive diffraction gratings with diffraction efficiencies close to unity and highest damage thresholds imposing the use of purely dielectric materials. These advanced requirements also give rise to new challenges for the grating design. Classical design approaches like gold-coated reflection gratings or monolithic transmission gratings are becoming insufficient. Different approaches utilize dielectric multilayer coatings in conjunction with gratings to achieve high transmission or reflection efficiencies together with high damage thresholds. However, to realize a reasonable and robust design, the optimization of the grating and the multilayer stack has to be completed in one step using rigorous methods because interference of multiply diffracted orders contributes to the overall diffraction efficiencies. Moreover, to make these designs feasible for manufacturing, also a tolerancing is necessary. In our contribution, we present self-developed design tools for multilayer gratings where the optimization of both, grating and multilayer stack are combined in one step using Rigorous Coupled Wave Analysis and standard local and global optimization methods like interior point and genetic algorithms. Moreover, a tolerancing routine is included. New designs are presented for multilayer dielectric reflection and transmission gratings based on our approach, including considerations on tolerancing. Gratings etched through multiple layers are proposed to achieve higher bandwidths with top hat diffraction efficiencies.
Reproducible manufacturing especially of large diffraction gratings using two-beam laser interference lithography gives rise to exceptional requirements on the stability of environmental conditions like temperature, air pressure, humidity, vibrations as well as a robust exposure setup using stable components, a highly coherent, frequency-stable laser and highquality optics. In our contribution, these requirements are reviewed systematically. The influences of atmospheric refractive index, laser frequency fluctuations, and thermomechanical drifts on the exposed dose contrast and hence on profile variations for surface-corrugated gratings are discussed. Moreover, mid-spatial frequency surface-errors of the used optical elements are identified as a main cause for local dose variations. Reasonable specifications for series manufacturing of grating masters are given and real-world measurement data from a holography laboratory is presented to illustrate the interplay between these different influences. This experimental data includes atomic force microscope scans of highgroove density resist gratings, spatially resolved diffraction efficiency measurements and moiré-interferometric measurements of the fringe stability. The results of our analysis are also useful for other holographic manufacturing facilities, including the manufacturing of surface and volume holographic optical elements of any kind.