Optical logic gates in future computers

A new publication from Opto-Electronic Science; DOI 10.29026/oes.2022.220010 considers optical logic gates in future computers.

If you’re reading this on your smartphone, its CPU (central processing unit) is running at full speed. It is a tiny chip containing billions of more basic units called logic gates. In the world of electronic computers, logic gates are implemented in the form of transistors, the invention of which led to several Nobel Prizes in Physics in 1956. However, various types of electronic computers, including smartphones, computers laptops and huge server systems, face great challenges and bottlenecks. For example, the well-known curve of Moore’s Law is getting flatter and flatter. It is increasingly difficult to realize the task that “the number of transistors an integrated circuit can accommodate will double approximately every 18 months”. The classic von Neumann architecture, which requires the transfer of a large amount of data between the processor and the memory, becomes more and more exposed. The high power consumption of countless computing devices around the world is also a major concern.

In addition to electronic computers, scientists searched for new alternative computer schemes. Quantum computers, biological DNA computers, molecular computers and other ideas are constantly being tried. Among them, the optical computer is certainly a possible option by replacing electronic devices with photonic devices and electrical signals with optical signals. The light field has amplitude, phase, time, space, frequency, polarization and other dimensions, which can be used to represent information, while reflection, refraction, interference, diffraction and other optical effects can be used to process the information. It seems natural to use light for the calculation.

In the novel “The Three-body Problem”, by famous science fiction writer Cixin Liu, the emperor on a planet in a three-body system, builds a “human computer” with thousands of soldiers, each waving a flag black or white to indicate a binary signal, which is used as a logic gate. This indicates that logic gates do not necessarily need to be implemented electronically. So how can we build different types of logic gates using optics?

As the simplest way to implement optical logic gates, spatial coding of the optical field was proposed very early in the 1970s and 1980s. Its operating principle is simple and can be realized even without special optical experimental conditions in a laboratory. It can be made using small pieces of ordinary transparent thin film or perforated paper cards.

Later, many researchers turned to an optical device called a semiconductor optical amplifier (SOA), which to some extent resembles an optical version of the electronic transistor, but with many differences. SOA enables a variety of nonlinear effects between input and output light signals of different frequencies, which can be used to build mathematical relationships of logic gates. Moreover, SOA also has the effect of four-wave mixing. With appropriate settings, two light signals of different frequencies can be used as an input to generate a third frequency of light as an output, and the phase of the output signal is the sum of the phases of the two input signals. Other types of logic gates can also be implemented by SOA with proper design. Highly nonlinear fiber (HNLF) is another type of optical device that has many properties similar to SOA and can also be used in the realization of optical logic gates.

In more recent studies of this century, with the development of integrated photonics and micro-nanofabrication technology, waveguides and other optical devices can be implemented on optical chips and crystal structures. photonics at the micrometer or even nanometer scale, and “miniature versions” of optical logic gates are appearing. Despite the small size, the basic optical principles are still at work, including the interference of two light waves. In the waveguide, it is easier to adjust the phase, as long as the light propagation length can be changed. The light intensity after interference of these two light waves can be used as logic gate output. Depending on the difference in amplitude and phase of the input signal, the light intensity after constructive or destructive interference can be equivalent to the target output value of the logic gate (close to 0 or 1).

The combination of a linear waveguide and a micro-ring waveguide is another common method. It is worth mentioning that in this design, we can combine line waveguides and micro-rings in a system where light waves can propagate through multiple coupling resonances, such as multiple mechanical gears, to achieve a some type of logic gate operation. In addition, surface plasmons, nanowires, periodically polarized lithium niobate (PPLN) and other technologies are also widely used in micro/nano optical logic gates.

While all roads lead to Rome, designing a qualified optical logic gate for practical applications is quite difficult, and getting the output that follows the rules of the logic gate deserves only a “Pass” score. Professor David Miller of Stanford University has pointed out that optical logic gates must meet six necessary criteria simultaneously, including cascading capacitance between the input of the next logic gate and the output of the previous logic gate, the output of the logic gate, recovery from signal power loss, etc. However, after satisfying these six criteria, optical logic gates can only achieve a “Good” rating. To truly earn an “Excellent” score, optical logic gates must outperform electronic logic gates with obvious advantages, such as faster computation speed and lower power consumption. To be a good logic gate with an “A+” rating, the challenges are many. Most existing optical logic gates cannot meet all requirements. In state-of-the-art research works, a single logic gate or simple logic circuits consisting of a small number of logic gates are usually studied. The development of true optical digital computers with a large number of optical logic gates is still at the initial research stage. Of course, photons and electrons are inherently different, and it is also open to debate whether the “bottom-up” computing framework in the electronic world, built from the basic units of single logic gates, is still applicable in the photonic world. The research of the optical logic gate still has a very large space for further exploration and development.

Article reference Jiao SM, Liu JW, Zhang LW, Yu FH, Zuo GM et al. All-optical logic gate computation for high-speed parallel information processing. Opto-Electron Sci 1, 220010 (2022). doi: 10.29026/oes.2022.220010

Keywords: logic gate / optical computing / artificial intelligence / waveguide / crystal structure

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