SOI-based 4×4 MMI device for high-contrast QPSK pattern recognition

Optical label pattern recognition is widely applied not only in packet switching but also in data security because of its broad range of uses. In particular, waveguide-based pattern recognition devices are highly valued for eliminating the need for encoder processing. However, conventional approaches face a significant challenge of reduced contrast ratio, defined as the ratio between the ON and OFF ports, since approximately half of the input power is lost through non-target waveguides. In this paper, a method was proposed to enhance the contrast ratio in a quadrature phase-shift keying (QPSK) pattern recognition device based on a 4×4 multi-mode interference (MMI) coupler. The proposed method employs a phase-conjugated QPSK signal in addition to the original QPSK signal and the reference signal. A QPSK pattern recognition device based on the MMI structure was fabricated on a silicon on insulator (SOI) platform. In the device, it was confirmed that the proposed method accurately identified each QPSK pattern under two-input and three-input configurations. The optical output from all ports closely matched theoretical values, and, as observed, increasing the number of input signals improved the contrast ratio from 3.0 dB to 9.5 dB.