Diffraction has been considered historically as a nuisance for the optics enthusiast, from Galileo to the designers of the Hubble telescope. Refractive and reflective optics have been used as the de facto tools in the optical designer’s toolbox for centuries, along with accurate modeling techniques using light as a particle such as Snell’s law of refraction. But light has a dual nature and can be also considered as a wave, which opens new design and modeling windows based on diffraction of light rather than refraction.
Diffractive and Holographic optical elements (respectively DOEs and HOEs) are used today in numerous consumer electronic devices such as CD/DVD reader heads, gesture sensors such as Kinect for Xbox, anti-counterfeiting tags, and even in augmented reality (AR) headsets such as Hololens.
The industry sector has also been embracing the use of diffractive optics such as in CWDM Mux/Demux for optical telecom, dispersion gratings in spectroscopy or high resolution optical encoders for motion control applications.
By structuring a substrate directly at the micrometric or even nanometric scale, through Integrated Circuits (IC) manufacturing process such as optical lithography and dry etching, one can produce very interesting diffractive elements. The picture below is an Atomic Force Microscope (AFM) scan of typical diffractive micro-structures. The lateral dimension of the AFM scan below is 10 microns.
To celebrate the IYL 2015, we have designed and fabricated different sets of microstructured fused silica plates as Optically Variable Imaging Devices (OVIDs) or as Computer Generated Holograms (CGHs) acting as diffractive laser pointer pattern generators, all incorporating different variations of the IYL 2015 pattern.
See below the reflective OVID arrays (left) and the transmission CGH arrays (right) etched on a 4 inch round fused silica wafer.
The picture below shows a larger set of OVIDs etched on a gold coated fused silica wafer for higher brightness and sparkle (see below).
Through white light illumination, – the California sun being very appropriate- , one can produce magnificent multicolor diffraction effects celebrating the IYL 2015 logo. Under a specific illumination angle, the etched plate would not produce any image, but as the illumination angle changes, all sort of colorful light patterns will appear and seem to hover over the substrate as it moves around.
The way we designed these OVIDs is quite simple: an original 256 grey scale image is converted into a GDSII mask layout with an equally large array of small square grating pixels, which periods and orientations are related to the particular grey scale of the considered pixel. Such layout is then patterned on a photomask by a laser patterning system and transferred via optical lithography onto several wafers which are then dry etched, and eventually coated with a reflective layer such as gold, and finally diced out via a diamond sawing process.
The set of CGHs we produced are very different from the set of OVIDs: they reconstruct by diffraction the IYL 2015 logo in the far field, when illuminated by a monochromatic collimated light source, such as a simple laser pointer. These CGHs have been designed by an iterative algorithm IFTA (Iterative Fourier Transform Algorithm), and fabricated as binary phase elements (one single etching step), yielding therefore two conjugate diffraction orders having equal intensity. As the IYL 2015 logo is a perfectly symmetric pattern, both fundamental diffracted orders could superimpose perfectly on the optical axis, and show a single resulting diffraction pattern (see below left), without higher orders appearing. For non-symmetric patterns (see below right), the two conjugate fundamental orders appear around the central diffraction order.
Armed with our small laser pointers on which we glued the CGHs, we decided to add some excitement to the otherwise quite boring Stanford University Clock Tower at night, by shining the IYL 2015 logo on it. The diffused full moon light also contributed that night to the light festivities (below).
Based on the local students amazement produced the unusual light show on campus that night, we wondered how the IYL 2015 logo would look like on the monumental Hoover tower erected nearby. So we walked down the alley with our laser pointers, aiming at the tower to brighten up its austere looks (see below). By the time the pattern reached the top part of the Hoover tower, it was already a few feet wide, adding some fun to its majestic stature, even with a small diffraction angle of about 5 degrees.
Now, imagine a crowd of IYL 2015 participants having each their own laser pointers projecting as many dancing IYL logos at night on monuments such as the pyramid of Teotihuacan or other landmarks, celebrating with their own hands on a grand scale the IYL 2015.
Bernard Kress has made significant scientific contributions as a researcher, associate professor, consultant, instructor, and author for over 20 years. He has been instrumental in developing numerous optical systems for consumer electronics, generating IP, teaching and transferring technological solutions to industry. Many industries have consulted with him on a wide range of applications such as: laser material processing, optical security and anti-counterfeiting, biotechnology sensors, optical telecom/datacom, optical data storage, optical computing, optical motion sensors, digital image projection, displays, back and front light illumination systems, optical gesture sensing, depth mapping, head-up and head mounted displays.
Bernard joined Google in Mountain View CA in 2011 as the Principal Optical Architect, working on Google Glass project and various stealth projects at Google [X] labs. Bernard has recently left Google to join the Hololens team at Microsoft in Redmond, WA. At Microsoft, he is now responsible for the development of next generation optical architectures for AR headsets such as Hololens.