Hologram Production
Europeans find techniques for mass production of CGHs.
By Christophe Gorecki and Ignacio Moreno
More than four decades after the first computer-generated holograms (CGHs) were introduced by Adolf Lohmann and his coworkers 1. CGHs continue to play an important role in modern optics for generating specific optical functions. The CGHs can transform incident light beams into arbitrary wavefronts. This is possible because we can now create micro- and nano-size structures, designed to generate specific diffraction patterns.
CHGs are used for interferometric testing, wavefront shaping, pulse shaping of femtosecond lasers, and for highly efficient coupling of light onto optical fibers. Security is another big application for CGHs because they can authenticate products and prevent counterfeiting and falsification.
Numerous studies have been conducted to improve the hologram fabrication process and design, resulting in advances such as powerful numerical calculation techniques to efficiently calculate complex diffractive structures. Scientists have also developed cost-effective fabrication processes and effective tools for the quality evaluation of the fabricated elements.
Creating CGHs requires precise generation of the microstructures. Many different technologies have been demonstrated over a variety of substrates, including spatial light modulators (SLMs) capable of reproducing programmable CGHs.
Despite the progress, fabrication of these optical micro-structures remains a technically complex process 2. Manufacturing technologies adapted from the semiconductor industry, such as laser or e-beam lithography, have been demonstrated to be the most competitive ones. Such modern lithography techniques can expose large substrates to create diffractive patterns with high enough resolution and accuracy to efficiently reproduce CGHs.
Research teams at the Institut FEMTO, from Université de Franche-Comté (France), and the Departamento de Ciencia de Materiales, Óptica y Tecnología Electrónica, from Universidad Miguel Hernández (Spain), have been collaborating on solutions to this problem. Our 2010 paper, “Low cost production of computer-generated holograms: From design to optical evaluation ,” which received the 2012 EOS Prize from the European Optical Society, describes some useful techniques for the mass production of CGHs.
Our teams applied standard silicon micromachining techniques to fabricate CGHs and developed characterization techniques for their optical inspection 3. These techniques represent a useful guide for the production and inspection of CGH at a relatively low cost.
Master substrate on silicon
In spite of its relatively poor optical properties, silicon offers very attractive possibilities as a substrate for CGHs in combination with high-quality materials. In addition, the inclusion of micromachining steps is an accessible mass-production method that minimizes fabrication complexity, component turnaround time, and cost.
CGHs with a continuous phase profile (kinoforms) provide higher diffraction efficiencies. However, elements with multiple phase levels usually require a multistep fabrication process, with the consequent disadvantage in terms of time and the strict requirements on multimask alignment and etching accuracy.
Therefore, binary-phase holograms would be better in terms of fabrication simplicity and reduced cost.
We developed binary-phase CGHs by selectively etching a SiO 2 layer grown onto a silicon substrate. Although reflections on Si cause important losses that affect the overall diffraction efficiency, these are excellent candidates to be used as master CGHs for replication onto other types of substrates 4. This can be done directly with polymer or onto Ni molds by electroplating as inserts for mass production replication tools, such as hot embossing or micro-injection molding.
The phase difference between the two levels of binary CGHs is the key parameter to achieve a good diffraction efficiency. The optimal diffraction efficiency is obtained when this phase difference is of π radians (modulo 2 π ). In other words, the optical path difference between the two levels must be /2, with being the wavelength of the CGH.
Inspection of production process
The development of inspection tools to easily characterize this phase shift is obviously important for quality control of CGH production processes.
We applied various optical inspection tools such as spectral ellipsometry to measure the reflection coefficients and derive efficiency. Interference microscopy is another useful technique to measure depth changes as shifts in the interference fringes 5 .
We used a Nikon LV100-Pol microscope to directly visualize the phase shift between the two levels of the fabricated holograms. Figure 1 below illustrates this, here applied to a binary diffraction grating fabricated onto the same wafer with the same conditions as the CGH. Fig. 1(a) shows the image captured with broadband illumination and a 20x Mirau-type interference objective.
The microscope platform tilt is adjusted to obtain parallel interference fringes perpendicular to the border between the uniform region and the grating region. The uniform region corresponds to the area where SiO 2 was not removed. A shift in the polychromatic interferogram on the top of the image is clear in comparison with the one on the bottom, which corresponds to the area where SiO 2 is not removed. This lateral shift is greater than two fringes and is clearly visible for the whole broadband spectrum.
Fig. 1. Microscope images of the binary diffraction grating captured with the interferometric objective for broadband illumination and monochromatic illumination at 488 nm.
Figure 1(b) shows the corresponding image when a blue interference narrowband filter centered at 488 nm is introduced in the microscope. The blue image shows a shift of approximately 2.5 fringes, denoting a phase shift of 5 radians, thus providing a good diffraction efficiency for this hologram illuminated with this wavelength.
The application of powerful numerical tools such as iterative inverse Fourier transform algorithms (IFTA) permits us to calculate phase patterns that very efficiently reproduce a desired pattern.
Additional terms, or orders, are also observed in the diffraction pattern, like the zero order (DC) peak present on axis, and the inverted reconstruction corresponding to the negative first order.
