Horst Schreiber

Extended lifetime of fluoride optics

Surface and coating technology plays an important role for extending lifetime of fluoride optics for ArF excimer laser applications. Optically finished CaF2 optics is characterized as top surface and subsurface by means of non-distractive quasi-Brewster angle technique. The subsurface is revealed by removing the top surface via distractive methods. Color centers on plasma ion and laser irradiated CaF2 optics are discussed. The results suggest that fluorine depletion is associated with laser damage, dense smooth coatings enable one to extend the lifetime of CaF2 optics.

Optical coherence tomography probe design for reduced artifact generation and manufacturability

Many fiber based probes used in Optical Coherence Tomography (OCT) are comprised of a spacer, GRIN lens, fiber, and a microprism. This design form suffers from many material interfaces, which induce back reflections into the sample arm of the interferometer. With so many interfaces, these probes can produce artifacts in the system’s imaging window. We present a design which has just two interfaces to minimize image artifacts. The two components of this design are the fiber endface and a reflective optic. With optimization, these two components can produce back reflections below -90dB which will minimize image artifacts. This will results in high fidelity imaging for medical diagnostics.

Novel light diffusing fiber for use in medical applications

Fiber-based cylindrical light diffusers are often used in photodynamic therapy to illuminate a luminal organ, such as the esophagus. The diffusers are often made of plastic and suffer from short diffusion lengths and low transmission efficiencies over a broad spectrum. We have developed FibranceTM, a glass-based fiber optic cylindrical diffuser which can illuminate a fiber from 0.5 cm to 10 meters over a broad wavelength range. With these longer illumination lengths, a variety of other medical applications are possible beyond photodynamic therapy. We present a number of applications for Fibrance ranging from in situ controllable illumination for Photodynamic Therapy to light guided anatomy highlighting for minimally invasive surgery to mitigating hospital acquired infections and more.

Low-loss dual-wave laser optics coatings at 1060 nm and 530 nm

Red-to-green laser conversion requires dual-wave laser optics coatings at 1060nm and 530nm. Loss analysis of the dual-wave coatings were presented for 3 types of coating designs with a coating material combination of HfO2/SiO2. Homogeneity and smoothness of the HfO2/SiO2 multilayers via standard plasma ion assisted deposition were evaluated. A modified plasma ion assisted deposition process with in-situ plasma smoothing was developed to deposit dense and smooth HfO2/SiO2 multilayers. Improved film microstructure was revealed on single layer and multilayer coated samples by means of atomic force microscopy and scanning electron microscopy. The improved film microstructure led to low loss and laser durable coating performance.

Wavefront control of UV narrow bandpass filters prepared by plasma ion-assisted deposition

Metal oxide layers produced by plasma ion-assisted deposition are extensively used for complex optical coatings due to the availability of materials, the high packing density of films, and the smooth surfaces. Stringent optical surface figure specifications necessary for both laser optics and precision optics require film stress to be well controlled and surface deformation to be corrected or compensated. In this paper, SiO2 based single cavity UV narrow bandpass filters were prepared by plasma ion-assisted deposition. The correlation between film stress, refractive index, deposition parameters, and post deposition annealing was established. The film stress was calculated based on interferometric surface deformation. The refractive index and film thickness were determined by means of variable angle spectroscopic ellipsometry. The center wavelength of the filters was obtained through spectral transmission measurement. The results suggest that the wavefront distortion of the multilayer coatings is dominated by the compressive stress of the SiO2 layers, and can be controlled and corrected by the amount of plasma ion momentum transfer, substrate temperature, post deposition annealing, and stress compensation via backside SiO2 coating. Based on the understanding of the mechanical and optical properties, the wavefront correction technique enables us to satisfy stringent surface figure specifications.

Nanostructure of GdF3 thin film evaluated by variable angle spectroscopic ellipsometry

As excimer lasers extend to deep-ultraviolet and vacuum-ultraviolet wavelengths at 193nm and 157nm, optical coatings experience the challenge of eliminating possible environmental contamination, reducing scattering loss, and increasing laser irradiation durability. Wide band-gap metal fluorides become the materials of choice for the laser optics applications. In order to understand the optical properties of nanostructured fluoride films, thin GdF3 films grown on CaF2 (111) substrates were evaluated by variable angle spectroscopic ellipsometry. An effective medium approximation model was used to determine both the film porosity and the surface roughness. Structural evolution of the GdF3 film was revealed with improved ellipsometric modeling, suggesting the existence of 3-layer structure, a densified bottom layer, a porous middle layer and a rough top surface. The nanostructure of the film and the surface roughness were confirmed by atomic force microscopy. The attraction of the nano-structure to environmental contamination was experimentally demonstrated.

System test for high-NA objectives at the 157-nm wavelength

Focusing on small features for optical inspection or defect repair, shorter wavelengths are used to increase resolution and energy density. Objectives designed for 157 nm using calcium fluoride are optimized and evaluated interferometrically at the wavelength of use to include all actinic effects. An image evaluation set-up is presented using a custom illuminator to image 130nm features, enlarged 500 times, onto a back-thinned CCD camera in real time.