Optical Digital Decoder Consisting of 4×4 Multi-Mode Interferometer Coupler

Photonic computing in the digital domain is gaining attention because of its potential ability to address issues in complementary metal-oxide-semiconductor technology. Although many optical technologies have been reported, the majority rely on nonlinear effects, which present challenges in terms of both power consumption and processing delays. Consequently, additional technologies using linear effects have been proposed in recent years. However, use of these technologies has primarily been limited to simple structures such as logic gates, with little research being performed on combinational circuits. In this paper, we propose a 2-bit optical decoder based on linear effects. This decoder consists of a 4×4 multi-mode interferometer coupler and targets decoding of binary phase-shift keying signals. It requires two signals and one probe light, and it activates different output ports depending on the pattern of the two signals. When the intensity ratio of the signal to the probe is 0.8, the loss is minimized, resulting in an extinction ratio of 5.1 dB. We fabricated the decoder on a silicon-on-insulator platform and configured it to have a rectangular shape with an area of 3.6 × 23.0 $\mu$m$^{2}$. Additionally, we confirmed operation of the 2-bit decoder under the conditions where the phases between the signals were 0-0, $\pi$-$\pi$, 0-$\pi$, and $\pi$-0 when using continuous light at 1550 nm. The obtained extinction ratios were approximately 4$\sim$5 dB, which matched the theoretical value closely. As it does not depend on nonlinear effects, this technology provides a low-power and compact optical digital device.