Optical Computing Golden Age (1980–2004)
This period of time could be called the optical computing golden age. There was a lot of enthusiasm in the field, the future looked very bright, there was funding for the programs on the topic and the research effort was very intensive worldwide. Every years, several international conferences were organized by different international societies on subjects related to optical computing. The journals had frequently a special issue on the topics and Applied Optics had every 10th of month an issue entitled “Information Processing”. The research was very fruitful in all the domains of optical information processing including theoretical work on algorithms, analog and digital computing, linear and nonlinear computing. Optical correlators for real applications were even commercialized. However, around 2000, we could feel that the interest for the subject started to decline. The reasons are multiple, but the evolution of digital computers in term of performance, power and also flexibility can be pointed out. They are also very easy to use even for a non-specialist.
It is impossible to list here all the work carried out in the domain from 1980 to 2004. Several books give the state of the art of the domain at the time of their publication .
In the following, we will describe only some aspects of the research during this period, and we apologize for some important results that may be missing. The purpose is to give to reader an idea of the evolution of the domain during this quarter century.
CGHs are important components for optical processing since they can process the information. The first CGHs were mostly cell-oriented since these methods were well adapted to the power of the computers with a small memory capacity and to the technology of the printers of this time. In the eighties, the technological landscape has changed, more powerful computers with a larger memory capacity were available, e-beam writers were more commonly used. Therefore new encoding methods, the point-oriented methods, were developed in order to achieve high quality and high diffraction efficiency optical reconstructions of the CGHs. First, the error diffusion algorithm, used for printing applications, was adapted to encode CGHs where it was possible to separate the noise from the desired pattern in the reconstruction plane . Then, iterative algorithms were proposed and the best known are the Direct Binary Search (DBS) algorithm proposed by Seldowitz et al. in 1987 [61] and the Iterative Fourier Transform Algorithm (IFTA) proposed by Wyrowski and Bryngdahl in 1988 . The CGHs encoded with these algorithms produce a reconstruction with a high Signal to Noise Ratio and a high diffraction efficiency, especially in the case of pure phase CGHs. Later some refinements were proposed, for example the introduction of an optimal multicriteria approach . It should be noted that these iterative methods are still used.
In the nineties, the main progress concerns the fabrication methods with the use of lithographic techniques allowing the fabrication of high precision phase only components etched into quartz. The name Diffractive Optical Elements (DOEs) that includes the CGHs is now used and reflects this evolution.
Thank to the progress in lithography, submicron DOEs can be fabricated such as a polarization-selective CGH , artificial dielectrics , a spot generator . The state of the art of digital nano-optics can be found in Chapter 10 of the book written by Kress and Meyrueis . The nano structures fabrication required new studies of the diffraction based on the rigorous theory of diffraction instead of the scalar theory of diffraction .
Several books give a complete overview of the field of DOEs and their applications [68, 70] and a very complete paper on the evolution of diffractive optics was published in 2001 by Mait .
Since the availability of SLMs was an important issue for the success of optical information processing, a lot of effort has been invested after 1980 into the development of SLMs fulfilling the optical processors requirements in terms of speed, resolution, and size and modulation capability. A paper written by Fisher and Lee gives the status of the 2D SLM technology in 1987 and shows that, at this time, the best feasible SLM performance values are found to include: about resolution elements, 10-Hz framing rates, 1-s storage, less than 50 J/cm2 sensitivity, five-level dynamic range, and 10-percent spatial uniformity. Updated reviews of the state of the art of SLMs is given in a book edited by Efron in 1995 and in several special issues of “Applied Optics” .
More than 50 types of SLM have been introduced in the eighties and nineties . Many different SLMs have been proposed and many prototypes fabricated—for example, besides liquid crystal SLMs, magneto-optic SLMs , multiple quantum wells devices (MQW) [81], Si PLZT SLMs and Deformable Mirror Devices . However very few of these SLMs have survived. Therefore, today, among the SLMs commercially available, mostly for display purpose, two technologies prevail: liquid crystal technology and Digital Micromirrors Devices DMD (MEMS based technology).
There are different types of liquid crystal SLMs. Twisted nematic liquid crystal SLMs are commonly used and their theory and experimental characterization show an amplitude and phase coupled modulation as well as an operating speed limited to the video rate. Ferroelectric liquid crystal SLMs can reach a speed of several kilohertz, but most of the devices on the market are binary bistable devices that consequently limit the applications. Although it is not so commonly known, analog amplitude only modulation is possible with specific ferroelectric SLMs [86]. Nematic liquid crystal or Parallel Aligned liquid (PAL) crystal SLMs produce a pure phase modulation that can exceed They are particularly attractive for applications requiring a high light efficiency such as dynamic diffractive optical elements. Their speed can reach 500 Hz . The matrix electrically addressed SLMs using twisted nematic liquid crystal have progressed considerably. Around 1985, the small LC TV screens were extensively evaluated , but their poor performance (phase nonuniformity, limited resolution…) limited their use for optical computing; then VGA, SVGA, and XGA resolution SLMs were introduced in video projectors and these SLMs extracted from the video projectors were widely characterized and integrated into optical processors. During the same period, high performance optically addressed SLMs were fabricated, for example, the PAL SLM from Hamamatsu . Now high resolution Liquid Crystal on Silicon (LCoS) SLMs are commercially available, for example an pure phase LCoS SLM with pixel resolution is commercialized . All these SLMs must be characterized very precisely and numerous papers were published on the subject .
In conclusion, today, for the first time since the origin of the optical processors, commercially available SLMs are fulfilling the requirements in terms of speed, modulation capability, and resolution. The applications of SLMs are numerous, for example, recent papers have reported different applications of LCoS SLMs, such as pulse shaping , quantum key distribution , hologram reconstruction , computer generated holograms , DOEs , optical tweezers , optical metrology .
In a parallel optical computer, a parallel access optical memory is required in order to avoid the bottleneck between the parallel processor and the memory. Therefore the research for developing a 3D parallel access optical memory was very active in the last two decades of the last century. Different architectures using different technologies were proposed. For example, Marchand et al. constructed in 1992 a motionless-head parallel readout optical-disk system achieving a maximum data rate of 1.2 Gbyte/s. Psaltis from Caltech developed a complete program of research on 3D optical holographic memories using different materials such as photorefractive crystals. In the frame of this program, Mok et al. achieved to store 10000 holograms of 440 by 480 pixels [104] into a photorefractive crystal of 3 cm3 . IBM was also very active into the field of holographic memory and two important programs of the Darpa were carried out in the nineties: project PRISM (Photorefractive Information Storage Material), and project HDSS (Holographic Data Storage Systems). All the information on these holographic memories can be found in a book . Several start-up companies were created for developing holographic memories and most of them disappeared. However one of them, In Phase Technologies, is now commercializing a holographic WORM disk memory system using a photopolymer material . Other types of optical memories were investigated such as a two-photon memory , spectral hole burning [109].
Today the holographic memory is still seen as a candidate for the memory of the future, however the problem of the recording material is not yet solved; particularly there is no easy to use and cheap rewritable material. The photopolymers can only be written once and despite all the research effort photorefractive crystals are still very difficult to use and expensive.