Scattering by random particles on optical surfaces

An analytical technique has been developed for calculating the angle-resolved light scattering by contaminant particles on smooth opaque surfaces. The analytical method was tested by a comparison of measured and calculated bidirectional reflectance distribution function (BRDF) values for contaminated surfaces. BRDF values were calculated from particle sizes, shapes, and areal densities obtained from scanning electron microscope images of the contaminated surfaces by a Princeton gamma-tech image analyzer. Measured and calculated BRDF values agreed to within the uncertainty associated with the particle characterization process for most scattering angles.     

Mullins effect characterization of elastomers by multi-axial cyclic tests and optical experimental methods

Besides the typical hyperelastic behaviour, large elastic deformations with non-linear stress–strain behaviour, rubber-like materials may also exhibit some inelastic effects, like hysteresis and permanent set. One of them is a particular damage phenomenon called Mullins effect. This is visible when cyclic tension tests are performed with increasing values of deformation. Material is deformed up to a fixed strain value and then unloaded. When a second load is applied it is possible to observe a stress softening effect. In the present work uniaxial and equibiaxial tension tests have been carried out by a standard tensile machine and by an hydraulic bulge test experimental rig, respectively. In both tests optical methods have been used for strain measurement. Experimental data have been successively introduced in a numerical procedure that permitted to extract the best material parameters for two of the most known pseudo-elastic models [Ogden, R.W., Roxburgh, D.G., 1999. A pseudo-elastic model for the Mullins effect in filled rubber. Proceedings of the Royal Society London A 455, 2861–2877; Dorfmann, A., Ogden, R.W., 2004. A constitutive model for the Mullins effect with permanent set in particle-reinforced rubber. International Journal of Solids and Structures 41, 1855–1878] accounting for both stress-softening behaviour and residual strain.     

Analysis of optical surfaces by means of surface plasmonspectroscopy

The sensitivity to material parameters of surface plasmon oscillations is explored for the design of a prototype system operating at a wavelength of 670 nm, for the simultaneous determination of thickness, complex permittivity and surface roughness of metal films. The apparatus was employed for characterization of gold and silver films with thicknesses within the 10-100 nm range. Thickness data measured with the prototype system were well correlated with those measured on a conventional surface profilometer. The Fourier spectrum of surface irregularities extracted from scattered light, enhanced by surface plasmon oscillations, was used to determine surface roughness parameters for the distinct samples. Those parameters were shown to be representative of typical surface structures observed with the atomic force microscope     

Photon-noise limits to the detection of the closure phase in optical interferometry

Michelson stellar interferometers with long baselines have been proposed as a means for obtaining high-resolution images of space objects. The fringes measured in such interferometers move randomly owing to atmospheric turbulence effects. For overcoming turbulence effects the fringe phase at any instant is summed around groups of three or more aperture pairs to create the so-called closure phase. The closure phase is insensitive to atmospheric turbulence effects; however, it is corrupted by photon-noise effects. The probability-density function of the error in the closure-phase estimate that is due to photon noise is derived as a function of the fringe visibility and is evaluated. It is shown that, for dim objects and low fringe visibility, several hundred to several thousand independent realizations of the closure phase must be averaged to obtain acceptable closure-phase variance.     

Degradation and destruction of optical surfaces by hypervelocity impact

This study investigates the damage caused by impacts of space debris and micrometeoroids (SD/MM) on mirrors of placed on spacecrafts in an orbit of 700 km and 1400 km altitude with an inclination of 48°. For the investigated orbits the maximum damage from impact degradation of the optical surface as well as the probability of total destruction by single particle hit has been calculated. Based on the NASA statistical standard model ORDEM 96, the calculation of SD particle fluxes shows that at 700 km altitude an average of 62000 impact damages per m² and per year are caused by particles with a diameter equal or larger than 1 μm. At 1400 km altitude the figure is reduced by 30%. The corresponding MM fluxes have been calculated with the Grün model and are 1.5 orders of magnitude smaller than the SD fluxes. For the generation of a damage law and for the determination of the total destruction limits, 50 impact damages were produced on coated and uncoated quartz glass samples, employing the impact facilities of the Ernst-Mach-Institute (EMI) and the Aerospace Department of the Technical University Munich (TUM/LRT). The particle sizes were varied between 3 μm and 1000 μm. The impact velocities were between 2.0 km/s and 16.1 km/s. Due to the irregular damages a clear correlation with the impact angle (0°, 30°, 60°) could not be proven. The diameter of the optically inactive surface after impact is proportional to E^0.458 (where E = kinetic energy of the impact). The experimental total destruction limit of 5 mm thick quartz glasses is reached with an impact energy of 13.5 J (Aluminum sphere, 0.9 mm diameter, 5 km/s impact velocity). The degradation analysis showed that 3.5 % of the optical surfaces of the mirrors in 700 km orbits (48 ° inclination) is destroyed within 10 years by space debris and micro-meteoroids. The probability of total destruction for the considered mirrors in 700 km altitude is in the percent range for an operational period of 10 years. Degradation damages and the probability of total destruction in an orbit in 1400 km is slightly below the values for the 700 km orbit.     

Threshold algorithm to recover the deformation of optical surfaces

The global optimization threshold algorithm is reported to obtain the deformations of an optical surface. The advantage of these types of algorithm is that they can be solved for the correlation problem presented in Seidel polynomials. We obtain the 2D deformations of a surface test with the transversal aberration along one direction only. In order to apply this algorithm we used exact ray tracing to simulate the transversal aberration adapting the same mathematical theory for the Ronchi test. The error obtained in sagitta recovering deformation was 1 microm.     

Role of light intensification by cracks in optical breakdown on surfaces

The intensity distribution of an initially plane light wave incident on planar and conical surface cracks is calculated numerically by using a wave propagation computer code. The results show that light intensity enhancements caused by interference of internal reflections at the crack and the surface are very sensitive to the light polarization, the beam angle of incidence, and the crack geometry (e.g., crack width and orientation with the surface). The light intensity enhancement factor (LIEF) can locally reach 2 orders of magnitude for conical cracks of ideal shape. The electric field direction relative to the crack surfaces determines the light intensity profile around the crack. For normal-incidence illumination on the output surface, total internal reflection at the crack and the surface can occur and leads to higher LIEFs. For identical geometry and illumination conditions, a crack located on the entrance surface of an optic generates electric field enhancements that are weaker than those on the exit surface. As cracks on polished surfaces are randomly oriented, the probability for large intensity enhancements to occur is high. The model is able to predict quantitatively the magnitude of surface laser-induced damage threshold drop and damage propagation enhancement in dielectric materials that are due to cracks.     

Information theoretic limits to the capacity of volume holographic optical memory

We derive the information theoretic limit to storage capacity in volume holographic optical memories for the limiting cases of dominant intensity noise (Gaussian noise) and dominant field noise (Rician noise). These capacity bounds are compared with the performance achievable using simple Reed-Solomon error-correcting codes.     

Numerical reconstruction of optical surfaces

There are several problems in optics that involve the reconstruction of surfaces such as wavefronts, reflectors, and lenses. The reconstruction problem often leads to a system of first-order differential equations for the unknown surface. We compare several numerical methods for integrating differential equations of this kind. One class of methods involves a direct integration. It is shown that such a technique often fails in practice. We thus consider one method that provides an approximate direct integration; we show that it is always converging and that it provides a stable, accurate solution even in the presence of measurement noise. In addition, we consider a number of methods that are based on converting the original equation into a minimization problem.     

Application of holographic interferometry to optical monitoring of solid surfaces

A new optical method is proposed for probing transition layers on solid surfaces, which combines the high precision inherent in holographic interferometry and the high sensitivity characteristic of optical microscopy using surface electromagnetic waves. The proposed technique has been numerically modeled for monochromatic radiation in the visible spectral range.     

Hybrid optical (freeform) components--functionalization of nonplanar optical surfaces by direct picosecond laser ablation

The performance of optical systems is typically improved by increasing the number of conventionally fabricated optical components (spheres, aspheres, and gratings). This approach is automatically connected to a system enlargement, as well as potentially higher assembly and maintenance costs. Hybrid optical freeform components can help to overcome this trade-off. They merge several optical functions within fewer but more complex optical surfaces, e.g., elements comprising shallow refractive/reflective and high-frequency diffractive structures. However, providing the flexibility and precision essential for their realization is one of the major challenges in the field of optical component fabrication. In this article we present tailored integrated machining techniques suitable for rapid prototyping as well as the fabrication of molding tools for low-cost mass replication of hybrid optical freeform components. To produce the different feature sizes with optical surface quality, we successively combine mechanical machining modes (ultraprecision micromilling and fly cutting) with precisely aligned direct picosecond laser ablation in an integrated fabrication approach. The fabrication accuracy and surface quality achieved by our integrated fabrication approach are demonstrated with profilometric measurements and experimental investigations of the optical performance.     

光学加工中真空夹具设计及分析

为了在光学加工中快速、可靠地装夹光学元件,设计了真空夹具,对该真空夹具引起光学元件变形量进行了分析.首先,根据光学加工对夹具的要求,应用手动静压快速夹头以及传统真空吸盘结构,设计了真空夹具;接着,对真空夹具引起光学元件变形量进行了仿真分析,并根据分析结果改进了真空夹具结构;最后,针对改进后的真空夹具进行了仿真验证.仿真结果表明:有防护层的真空夹具引起光学元件表面最大变形量在0.1 μm～0.32 μm之间,是没有防护层的真空夹具引起光学元件表面最大变形量的1/14.该种真空夹具可以应用于高精度光学加工中.     

卫星真空热试验星内污染检测分析

本文利用石英晶体微量天平(QCM)监测某卫星内部真空热试验过程中污染物的沉积量;并用气相色质谱联用仪分析星内污染物的成分,为邻苯二甲酸酯类和硅氧烷类物质,主要来源于星用电缆和导热硅脂等粘结剂.结合污染物的成分及卫星污染物来源,提出了在真空热试验中减少污染物的方法.     

Polarimetric optical imaging of scattering surfaces

A polarimetric optical specular event detector (OSED) has been developed to provide spatially and temporally resolved polarimetric data of backscattering in the visible from water wave surfaces. The OSED acquires simultaneous, two-dimensionally resolved images of the remote target in two orthogonal planes of polarization. With the use of plane-polarized illumination the OSED presently can measure, in an ensemble of breaking waves, the equivalent four-element polarization matrix common to polarimetric radars. Upgrade to full Stokes parameter state of polarization measurements is straightforward with the use of present single-aperture, multi-imager CCD camera technology. The OSED is used in conjunction with a coherent pulse-chirped radar (PCR), which also measures the four-element polarization matrix, to provide direct time-correlated identification of backscattering mechanisms operative during wave-breaking events which heretofore have not been described theoretically. We describe the instrument and its implementation, and examples of spatially resolved polarimetric data are displayed as correlated with the PCR backscatter cross section and polarization ratio records.     

Fluid jet polishing of optical surfaces

We present a new finishing process that is capable of locally shaping and polishing optical surfaces of complex shapes. A fluid jet system is used to guide a premixed slurry at pressures less than 6 bars to the optical surface. We used a slurry comprising water and 10% #800 SiC abrasives (21.8 mum) to reduce the surface roughness of a BK7 sample from 350 to 25 nm rms and to vary the shape of a polished sample BK7, maintaining its surface roughness of 1.6 nm rms, thereby proving both the shaping and polishing possibilities of the presented method.