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.
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.
The scattering and absorption behavior of arbitrarily shaped metallic particles placed inside cylindrical holes in metallic layers is investigated numerically, including material dispersion properties and using rigorous coupled wave analysis (RCWA). Design parameters resulting in strong scattering are identified for some special geometries. For dielectric and metallic spheres, where analytical solutions of single scattering of a single sphere are known ("MIE" scattering), the numerical methods are verified to yield correct results.
We report on the realization of an diffractive optical isolator for use at 543 nm by the combination of two binary high frequency gratings, corrugated into the surface of a quartz substrate. A single-order grating acts as a polarizing beam splitter with a measured diffraction efficiency of greater 95%. The other grating is a zero-order diffraction with 290 nm period and 1300 nm depth, acting as a quarterwave plate for conical incidence. A good correlation between theoretical and experimental results is demonstrated.
We report on the simulation and experimental realization of two consecutive binary high spatial frequency gratings with high aspect ratios on the front and back faces of one fused silica substrate as polarization elements for visible laser light. The combination of rigorous coupled wave analysis and scalar decomposition of the incident Gaussian beam into a spectrum of plane waves results in good agreement between calculations and measured polarization properties.
We will discuss the design of surface structured optical elements (corrugated gratings) in fused quartz for application in the short wavelength range, namely as antireflection surface, as (lambda) /4-phase-plate, as polarization beam splitter and as highly efficient phase mask for writing fiber Bragg gratings in monomode fibers. Experimental results will be presented and are found to be in good agreement with theory.