From Optical Computers to Photonic Networks (The Conflict Between Linear and Nonlinear)

In the 1980s, extensive research was conducted to realize "optical computers," leading to the establishment of study groups and the organization of international conferences dedicated to the topic. The vision held by researchers at the time was that optical computers would perform various signal processing tasks in parallel at the speed of light, far surpassing electronic computers. By analogy with electronic circuits, optical computers require all-optical devices with nonlinear optical input/output characteristics, equivalent to diodes and transistors. During that era, I prototyped and presented an "Optical Triode." [1]

The figure of merit for materials used in nonlinear optical devices is given by (nonlinear refractive index) / (absorption coefficient). Since the 1980s, there has been little improvement in the figure of merit for these materials, and to this day, optical computers have not been realized.

On the other hand, in photonic networks, optical transmission systems using optical fibers as the transmission medium have made dramatic progress. The figure of merit for these systems is measured by transmission speed × transmission distance, and today it is approaching 10¹⁷[(bit/s)km]. In optical transmission systems, the disadvantages of optical nonlinearity in optical fibers (such as signal degradation due to self-phase/cross-phase modulation and four-wave mixing) outweigh the advantages (such as Raman amplification), creating a demand for optical fibers with low nonlinearity. Furthermore, various linear optical circuits, such as wavelength multiplexing/demultiplexing circuits and coherent receiver circuits, have been put into practical use and play a crucial role in optical transmission systems.

Our laboratory focuses on research essential for advancing photonic networks, such as optical switches, optical multiplexing/demultiplexing circuits, and optical devices for new wavelength bands. While the research and development of linear optical circuits are useful and promise reliable results, research on nonlinear optical circuits is challenging and intriguing. In particular, as shown in Figure 3, we have proposed an optical switch that uses a phase-change material with a large refractive index change and memory properties as a nonlinear optical device. [2]

References

[1] H. Tsuda and T. Kurokawa, “Optical Triode Switch Module with a Nonlinear Etalon,” IEEE Photon. Technol. Lett., 1 (12), 449(1989).

[2] Y. Ikuma, Y. Shoji, M. Kuwahara, X. Wang, K. Kintaka, H. Kawashima, D. Tanaka, and H. Tsuda, “Small-sized optical gate switch using Ge2Sb2Te5 phase-change material integrated with a silicon waveguide,” Electron. Lett., 46 (5), 368 (2010).