Bernard C. Kress

Micro-step localization using double charge optical vortex interferometer

We investigate the diffraction effects of focused Gaussian beams yielding a double optical vortex by a nano-step structure fabricated in a transparent media. When approaching such a step the double vortex splits into single ones which move in a characteristic way. By observing this movement we can determine the position of the step with high resolution. Our theoretical predictions were verified experimentally.

Modeling of the angular tolerancing of an effective medium diffractive lens using combined finite difference time domain and radiation spectrum method algorithms

A new rigorous vector-based design and analysis approach of diffractive lenses is presented. It combines the use of two methods: the Finite-Difference Time-Domain for the study in the near field, and the Radiation Spectrum Method for the propagation in the far field. This approach is proposed to design and optimize effective medium cylindrical diffractive lenses for high efficiency structured light illumination systems. These lenses are realised with binary subwavelength features that cannot be designed using the standard scalar theory. Furthermore, because of their finite and high frequencies characteristics, such devices prevent the use of coupled wave theory. The proposed approach is presented to determine the angular tolerance in the cases of binary subwavelength cylindrical lenses by calculating the diffraction efficiency as a function of the incidence angle.

Low-cost replicable plastic HUD combiner element

We present a novel technique to fabricate low cost mass replicable plastic HUDs for the transportation industry. HUD are implemented in numerous sectors today (in avionics, automobile, military, machinery,...). Typical implementations include an optical combiner which produces the desired virtual image while leaving the field mostly unaffected by the optics. Such combiners optics are usually implemented as cumbersome catadioptric devices in automobile, dichroic coated curved plates, or expensive volume holograms in commercial and military aviation. We propose a novel way to design, model and fabricate combiner masters which can be replicated in mass by UV casting in plastic. We review the various design techniques required for such elements and the novel mastering technology.

Improvement of the signal integrity in diffractive optical encoders.

We are presenting several techniques to improve the quality of the signals in diffractive optics encoders, for either linear and rotational encoders. We have developed previously various hybrid incremental/absolute disk based rotary diffractive encoders architectures. While the binary signals for absolute encoding were usually of sufficiently good quality to retrieve the entire Gray code signal over the desired resolutions (10, 12 or 14 bits), the quality and integrity of the sinusoidal signals for the incremental part of the encoder needed to be improved, since these are the signals allowing the encoder to go to much higher interpolated resolutions (20 bits or over). A good precision over the interpolated signals assumes very accurate sinusoidal profiles form the raw signals. Strong interpolation can only be done on high quality sinusoidal native signals (also referred to as pulses per revolution or PPR). A typical high resolution incremental encoder might provide 12 to 16 native sinusoidal PPRs, but the interpolation over these signals can reach way over 20 bits of resolution if the signals are of good quality.

Exit pupil expander for wearable see-through displays

We present in this paper a novel implementation of a wearable see through display including a diffractive exit pupil expander element. The optical system is a pupil forming system which integrates also a diffractive beam combiner.

Novel solar cogeneration trough system based on stretched microstructured mylar film

Hybrid CSP / CPV (Concentrating Solar Power / Concentration Photovoltaic) systems provide a good alternative to traditional CPV systems or CSP trough architectures. Such systems are often described as solar cogeneration systems. Trough systems use mainly the IR portion of the spectrum in order to heat up a pipe in which water is circulating. CPV systems use only the visible portion of the spectrum to produce the photo-voltaic conversion. Due to the achromatic nature of traditional thermal trough CSP systems, it is very unlikely that a CPV system can be integrated with a CSP system, even a low concentration CPV system (LCPV). We propose a novel technique to implement a low concentration CSP/LCPV system which relies on commercially available solar trough concentrators / trackers that use reflective stretched Mylar membranes. However, here the Mylar is embossed with microstructures that act only on the visible portion of the spectrum, leaving the infrared part of the solar spectrum unperturbed. This architecture has many advantages, such as: the existing Mylar-based thermal trough architecture is left unperturbed for optimal thermal conversion, with linear strips of PV cells located a few inches away from the central water pipe; the infrared radiation is focused on the central pipe, away from the PV cells, which remain relatively cool compared to conventional LCPV designs (only visible light (the PV convertible part of the solar spectrum) is diffracted onto the PV cell strips); and the Mylar sheets can be embossed by conventional roll-to-roll processes, with a one-dimensional symmetric micro-structured pattern. We show how the positive master elements are designed and fabricated over a small area (using traditional IC wafer fabrication techniques), and how the Mylar sheets are embossed by a recombined negative nickel shim. We also show that such a system can efficiently filter the visible spectrum and divert it onto the linear strips of PV cells, while leaving the infrared part of the spectrum un-perturbed, heating up the water pipe.

High security optical tags for automotive/avionics parts anticounterfeiting

Horus technologies develops novel optical tags based on planar digital micro-optics. These tags are calculated by computer, fabricated as masters via optical microlithography and replicated in mass by plastic embossing. Such tags are composed of several different levels of anti-counterfeating features, ranging from traditional holographic patterns, to OVIDs, to micro-holograms, to machine readable digital holograms storing holographic 1D and 2D bar codes. These tags have a double aim: anti-counterfeating of automotive/avionic parts and providing the engineer using an appropriate tag reader with all the technical information referring to these parts.

Design of a laser cutting system at large distance by a synthetic aperture diffractive optical system in reflection mode

Laser material processing by the mean of Diffractive Optical Elements (DOEs) is a promising field, that has not yet entered the industrial world, except for hybrid on-axis spherical or aspherical lenses fabricated by diamond turning. Laser cutting, engraving, welding, and heat treatment are several applications among the potential of numeric-type beam shaping DOEs, designed and optimized by iterative algorithms and fabricated by microlithographic techniques. However, these DOEs have to be etched within materials which should withstand very high energy distributions (either from YAG, CO2 or Excimer lasers). Typical physical DOE configurations in transmission mode are etched ZnSe, ZnS, Ge substrates or even Quartz substrates, and in reflection mode, Au or Al coated etched substrates, blank etched Quartz substrates (YAG), or even SiC etched substrates.

Parity-time symmetry diffractives implementing unidirectional diffraction–application to optical combiners

We review our work regarding a new class of couplers, resonators and free space gratings that employ the concept of Parity-Time (PT) symmetry in optics. PT structures can be implemented as diffractive gratings having complex refractive index profiles. The complex index profile integrates both phase and loss modulations as in conventional gratings, but also gain modulation. We review our work on integrated waveguide grating structures (Bragg regime) and free space structures (Raman Nath regime). Uni-directionality in free space can be applied to the development of novel optical combiners for the HMD/HUD fields. Such elements can be replicated in mass via lithography/embossing, with similar efficiency as conventional volume HOEs (Holographic Optical Elements) such as photopolymers.

Novel optical encoder for harsh environments

We are presenting a new optical encoder architecture for shaft encoding, both in incremental and absolute modes. This encoder is based on a diffractive optics technology platform. We have developed various disk based rotary diffractive encoders previously. This encoder is different in the way it is not a disk composed of successive gratings or computer generated holograms, but rather composed of a single element placed on the shaft. It is thus best suited for hollow shaft or end of shaft applications such as in encoder controlled electrical motors. This new architecture aims at solving some of the problems encountered with previous implementations of diffractive encoders such as disk wobble, disk to shaft centering and also encoding in harsh environments.