Advances in Photonic Devices for Optical Computing

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Computers have enhanced our lives to a great extent and become integral to modern civilization. The speed of conventional computers is achieved by miniaturizing electronic components to a very small micron-size scale so that electrons need to travel only very short distances within a very short time. The goal of improving computer speed has resulted in the development of very-large-scale integration (VLSI) technology with smaller device dimensions and greater complexity.

Currently, companies such as Intel can mass-produce transistors (the miniature electronic components through which electrons travel in computers) that are 14 nm across, while UC Berkeley researchers recently used carbon nanotubes and molybdenum disulphide to create a transistor smaller than 7 nm. Whereas VLSI technology has revolutionized the electronics industry and established the 20^th century as the computer age, increasing usage of the Internet demands better accommodation of a 10 to 15% per month growth rate. Additionally, our daily lives demand solutions to increasingly sophisticated and complex problems, which requires more speed and better performance of computers.

For these reasons, it is unfortunate that VLSI technology is approaching its fundamental limits in the sub-micron miniaturization process. It is now possible to fit up to two trillion transistors on a single silicon chip, and commercially available single-chip microprocessors have a transistor count of 19.2 billion. Moore’s law observes that transistor count will double every year but, of course, the laws of physics will not allow this to continue indefinitely.

Further miniaturization of lithography introduces several problems such as dielectric breakdown, hot carriers, and short channel effects. All of these factors combine to seriously degrade device reliability. Even if developing technology succeeded in temporarily overcoming these physical problems, we will continue to face them as long as increasing demands for higher integration continues. Therefore, a dramatic solution to the problem is needed and, unless we gear our thoughts toward a totally different pathway, we will not be able to further improve our computer performance for the future.

Optical interconnections and optical integrated circuits will provide a way out of these limitations to computational speed and complexity inherent in conventional electronics. Optical computers will use photons traveling on optical fibers or thin films instead of electrons to perform the appropriate functions. In the optical computer of the future, electronic circuits and wires will be replaced by a few optical fibers and films, making the systems more efficient with no interference, more cost-effective, lighter and more compact.

Optical components would not require insulators like those needed between electronic components because they do not experience cross-talk. Indeed, multiple frequencies (or different colors) of light can travel through optical components without interfacing with others, allowing photonic devices to process multiple streams of data simultaneously.

Why Use Optics for Computing?

Optical interconnections and optical integrated circuits have several advantages over their electronic counterparts. They are immune to electromagnetic interference and free from electrical short circuits. They have low-loss transmission and provide large bandwidth; i.e. multiplexing capability, capable of communicating several channels in parallel without interference. They are capable of propagating signals within the same or adjacent fibers with essentially no interference or cross-talk. They are compact, lightweight and inexpensive to manufacture, and more facile with stored information than magnetic materials.

The business of photonics is booming in industry and universities worldwide. It is estimated that the global market for silicon photonic devices will reach $2.61 billion by 2024 due to an ever-increasing demand for data traffic, in particular video streaming, and the advent of the Internet of Things. According to the Cisco Visual Networking Index (VNI), data traffic is growing worldwide at a compound annual growth rate of 24%, while North American data traffic will reach 85 exabytes per month by 2021 (a compound annual growth rate of 20%). Ericsson estimates the compound annual growth rate of data traffic to be 39% between 2017 and 2023. In fact, SK Telecom announced this year it will install dense wavelength division multiplexing equipment on South Korean railways to improve the data transfer rate tenfold, from 10 Gbps to 100 Gbps. This will allow staff on even the fastest trains to communicate seamlessly with control.

Most of the components that are currently very much in demand are electro-optical (EO). Such hybrid components are limited by the speed of their electronic parts. All-optical components will have the advantage of speed over EO components. Unfortunately, there is an absence of known efficient nonlinear optical materials that can respond at low power levels, despite some recent developments in the field. Most all-optical components require a high level of laser power to function as required.

A group of researchers from the University of Southern California, jointly with a team from the University of California Los Angeles, have developed an organic polymer with a switching frequency of 60 GHz. This is three times faster than the current industry standard lithium niobate crystal-based devices. The California team has been working to incorporate their material into a working prototype. Development of such a device could revolutionize the information superhighway and speed data processing for optical computing.

Another group at Brown University and the IBM Almaden Research Center (San Jose, CA) have used ultrafast laser pulses to build ultrafast data-storage devices. This group was able to achieve ultrafast switching down to 100ps. Their results are almost ten times faster than currently available “speed limits”. Optoelectronic technologies for optical computers and communication hold promise for transmitting data as short as the space between computer chips or as long as the orbital distance between satellites.

A European collaborative effort demonstrated a high-speed optical data input and output in free-space between IC chips in computers at a rate of more than 1 Tb/s. Astro Terra, in collaboration with Jet Propulsion Laboratory (Pasadena, CA), has built a 32-channel 1-Ggb/s earth–to–satellite link with a 2000 km range. Researchers at the US Department of Energy’s SLAC National Accelerator Laboratory have found that magnetite (a mineral that occurs naturally) electrical switches can be turned on or off in just 1 trillionth of a second, thousands of times faster than the transistors used now. Many more active devices are in development and some are likely to become crucial components in future optical computer and networks.

The race is on with foreign competitors. NEC (Tokyo, Japan) have developed a method for interconnecting circuit boards optically using Vertical Cavity Surface Emitting Laser arrays (VCSEL). Researchers at Osaka City University (Osaka, Japan) reported on a method for automatic alignment of a set of optical beams in space with a set of optical fibers. Researchers at NTT (Tokyo, Japan) have designed an optical backplane with free-space optical interconnects using tunable beam deflectors and a mirror. The project had achieved 1000 interconnections per printed-circuit board, with throughput ranging from 1 to 10 Tb/s. This year, the IBM Research Lab team in Zurich, Switzerland, alongside partners from Skolkovo Institute of Science and Technology, Russia, and Southampton University, UK, built the world’s first cascadable, all-optical transistor that can operate at room temperature. This advance was achieved by using an organic semiconducting polymer and enabled the team to measure the highest net optical gain for an optical transistor ever recorded (approximately 10dB/micrometer).

Optics has a higher bandwidth capacity over electronics, which enables more information to be carried and data to be processed, arises because electronic communication along wires requires charging of a capacitor that depends on length. In contrast, optical signals in optical fibers, optical integrated circuits and free space do not have to charge a capacitor and are therefore faster.

Another advantage of optical methods over electronic ones for computing is that optical data processing can be done in parallel much more easily and less expensively than in electronic data processing. Parallelism is the capability of the system to execute more than one operation simultaneously. Electronic computer architecture is, in general, sequential, where the instructions are implemented in sequence. This implies that parallelism with electronics is difficult to construct. Parallelism first appeared in Cray supercomputers in the early 1980s. Two processors were used in conjunction with the computer memory to achieve parallelism and to enhance the speed to more than 10 Gb/s. It was later realized that more processors were not necessary to increase computational speed, but could be detrimental. This is because as more processors are used, more time is lost in communication.

On the other hand, using a simple optical design, an array of pixels can be transferred simultaneously in parallel from one point to another. To appreciate the difference between both optical parallelism and electronic one can think of an imaging system of as many as 1000x1000 independent points per mm² in the object plane connected optically by a lens to a corresponding 1000x1000 points per mm² in the image plane. For this to be accomplished electrically, a million nonintersecting and properly isolated conduction channels per mm² would be required.

Parallelism, therefore, when associated with fast switching speeds, would result in staggering computational speeds. Assume, for example, there are only 100 million gates on a chip, much less than what was mentioned earlier (optical integration is still in its infancy compared to electronics). Further, conservatively assume that each gate operates with a switching time of only 1 nanosecond (organic optical switches can switch at sub-picosecond rates compared to maximum picosecond switching times for electronic switching). Such a system could perform more than 10¹⁷ bit operations per second. Compare this to the gigabits (10⁹) or terabits (10¹²) per second rates which electronics are either currently limited to, or hoping to achieve. In other words, a computation that might require one hundred thousand hours (more than 11 years) of a conventional computer could require less than one hour by an optical one.

Another advantage of light is that photons are uncharged and do not interact with one another as readily as electrons. Consequently, light beams may pass through one another in full-duplex operation, for example, without distorting the information carried. In the case of electronics, loops usually generate noise voltage spikes whenever the electromagnetic fields through the loop changes. Furthermore, high frequency or fast switching pulses will cause interference in neighboring wires. Signals in adjacent fibers or optical integrated channels do not affect one another nor do they pick up noise due to loops. Finally, optical materials possess superior storage density and accessibility over magnetic materials.

Obviously, the field of optical computing is progressing rapidly and shows many dramatic opportunities for overcoming the limitations described earlier for current electronic computers. The process is already underway whereby optical devices have been incorporated into many computing systems. Laser diodes as sources of coherent light have dropped rapidly in price due to mass production

The Role of NLO in Optical Computing and the Need for New Materials

The field of optical computing is considered to be the most multidisciplinary field, and for its success requires collaborative efforts of many disciplines, ranging from device and optical engineers to computer architects, chemists, material scientists and optical physicists. On the materials side, the role of nonlinear materials in optical computing has become extremely significant. Nonlinear materials are those which interact with light and modulate its properties. For example, such materials can change the color of light from being unseen in the infrared region of the color spectrum to a green color where it is easily seen in the visible region of the spectrum.

All optical computer components require efficient nonlinear materials for their operation. In fact, what restrains the widespread use of all optical devices is the inefficiency of currently available nonlinear optical materials, which require large amounts of energy for responding or switching . In spite of new developments in materials, presented in the literature daily, a great deal of research by chemists and material scientists is still required to enable better and more efficient optical materials. Although organic materials have many features that make them desirable for use in optical devices (such as high nonlinearities, flexibility of molecular design and damage resistance to optical radiation) their use in devices has been hindered by processing difficulties for crystals and thin films.

A team of NASA researchers at the Marshall Space Flight Center (MSFC) focuses on a couple of these materials, which were also processed in space either by the MSFC group or others. These materials belong to the classes of phthalocyanines and polydiacetylenes. These classes of organic compounds are promising for optical thin films and waveguides. Phthalocyanines are large ring-structured porphyrins in which large and ultrafast nonlinearities have been observed. These compounds exhibit strong electronic transitions in the visible region and have high chemical and thermal stability up to 400^oC. The NASA team measured the third-order susceptibility of phthalocyanine, which is a measure of its nonlinear efficiency to be more than a million times larger than that of the standard material, carbon disulfide. This class of materials has good potential for commercial device applications and has been used as a photosensitive organic material, and for photovoltaic, photoconductive and photoelectrochemical applications.

Polydiacetylenes are zigzag polymers having conjugated (alternating) mobile ð-electrons for which the largest reported nonresonant (purely electronic) susceptibility for switching have been reported. Subsequently, polydiacetylenes are among the most widely investigated classes of polymers for nonlinear optical applications. Their sub-picosecond time response to laser signals makes them candidates for high-speed optoelectronics and information processing.

The NASA researchers have chosen to study these classes of compounds because the growth of these films on ordered organic and inorganic substrates under various processing conditions promises to be useful for preparing highly oriented films. One such processing condition of interest to NASA is the effect of microgravity on the structures and properties of thin films and crystals.

The potential benefits of processing optical materials in space were demonstrated by the deposition of copper phthalocyanine in microgravity by physical vapor transport by the applied science and innovation company, 3M (figure 1). Analyses of these films revealed that microgravity grown films are more highly uniaxially oriented than earth-grown films. From figure 1, it is clear that the molecules are stacked toward one direction (uniaxially oriented) for microgravity processed films, and are randomly oriented for ground processed films. The group at MSFC observed intrinsic optical bistability and demonstrated an all-optical AND-logic gate (as will be explained later) in vapor-deposited thin films of metal-free phthalocyanine. Optical bistable devices and logic gates are the equivalents of electronic transistors. They switch light ON and OFF. They are also useful as optical cells for information storage. This nonlinear effect could improve dramatically in highly oriented microgravity processed films.

Figure (1). A comparison of a scanning electron micrographs of 1 ìm thick films of copper phthalocyanine deposited by physical vapor transport in the 3M PVTOS flight (STS-20) and ground control experiments. In microgravity the film’s microstructure is very dense compared to that produced in unit gravity in the presence of convection. This difference in microstructure has a significant affect on the macroscopic film optical properties.

Figure (2). A comparison of a ground-grown polydiacetylene film with a microgravity-grown one. The aggregates are impeded into the film by the fluid convection on the ground, while the microgravity film is almost free of these aggregates where convection is almost absent.

A novel photodeposition process for film photodeposition onto quartz or glass surfaces, developed by members of the NASA group, enabled deposition of polydiacetylene (PDAMNA) films derived from 2-methyl-4-nitroaniline, a well-known organic NLO material, by irradiation of monomer (the building block of a polymer) solutions with UV light. Polydiacetylenes are highly conjugated polymers, i.e. the electrons in the polymer backbone are delocalized and can move freely along the backbone capable of exhibiting very large optical nonlinearities with fast response times (less than 120fs: 1fs = 10^-15 s). These response times are faster than they are for the fastest electronic switching by more than a hundred times. High-quality films that have potential application in integrated optical circuits were produced. Films of PDAMNA that were processed in space on space shuttle flight STS-69 had superior optical quality (i.e. greater homogeneity, fewer defects). This experiment also demonstrates that processing in microgravity offers an opportunity to study certain parameters affecting the production of higher quality materials.

Recent Advances in Photonic Switches at nasa/MSFC

Logic gates are the building blocks of any digital system. An optical logic gate is a switch that controls one light beam by another; it is “ON” when the device transmits light and it is “OFF” when it blocks the light. Researchers demonstrated in a laboratory at NASA MSFC two fast all-optical switches using phthalocyanine thin films and polydiacetylene fiber. The phthalocyanine switch is in the nanosecond regime and functions as an all-optical AND logic gate, while the polydiacetylene one is in the picosecond regime and exhibits a partial all-optical NAND logic gate.

To demonstrate the AND gate in the phthalocyanine film, the researchers waveguided two focused collinear beams through a thin film of metal-free phthalocyanine film. The film thickness was ~ 1 ìm and a few millimeters in length. The team used the second harmonic at 532 nm from a pulsed Nd:YAG laser with a pulse duration of 8 ns along with a cw He-Ne beam at 632.8 nm. The two collinear beams were then focused by a microscopic objective and sent through the phthalocyanine film. At the output, a narrow band filter was set to block the 532 nm beam and allow only the He-Ne beam. The transmitted beam was then focused on a fast photo-detector and to a 500 MHz oscilloscope. It was found that the transmitted He-Ne cw beam was pulsating with a nanosecond duration and in synchronous with the input Nd:YAG nanosecond pulse. The setup described above demonstrated the characteristic table of an AND logic gate. A schematic of the setup is shown in figure 3.

Figure (3). A schematic of the nanosecond all-optical AND logic gate setup.

The setup for the picosecond switch was very similar to the setup in figure 3, except that the phthalocyanine film was replaced by a hollow fiber filled with a polydiacetylene. The polydiacetylene fiber was prepared by injecting a diacetylene monomer into the hollow fiber and polymerizing it by UV lamps. The UV irradiation induces a thin film of the polymer on the interior of the hollow fiber with a refractive index of 1.7 and the hollow fiber is of refractive index 1.2. In the experiment, the 532 nm from a mode-locked picosecond laser was sent collinearly with a cw He-Ne laser and both were focused onto one end of the fiber. At the other end of the fiber a lens was focusing the output onto the narrow slit of a monochrometer with its grating set at 632.8 nm. A fast detector was attached to the monochrometer and sending the signal to a 20 GHz digital oscilloscope. It was found that with the He-Ne beam OFF, the Nd:YAG pulse is inducing a week fluorescent picosecond signal (40 ps) at 632.8 nm that is shown as a picosecond pulse on the oscilloscope. This signal disappears each time the He-Ne beam is turned on. These results exhibit a picosecond respond in the system and demonstrated three of the four characteristics of a NAND logic gate as shown in figure (4).

Figure (4). A schematic of the all-optical NAND logic gate setup.

Source: science.nasa.gov/