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Abstract Pattern recognition of optical label is utilized not only in packet switches but also in the field of data security due to its wide range of applications. In particular, pattern recognition using waveguide devices is highly valued because it eliminates the need for encoder processing. However, existing methods suffer from the problem of degraded contrast ratio because half of the input power are emitted from non‐target waveguides. In this article, a method is proposed to improve the contrast ratio employing a 4×4 multi‐mode interference (MMI) coupler with multiple input signals. The theoretical results reveal that the proposed method improves the contrast ratio from 3.0 dB to 9.5 dB and ∞ dB for the three‐input and four‐input cases, respectively, for quadrature phase‐shift keying (QPSK)‐modulated signal. Furthermore, numerical simulation was performed through the three dimension finite‐difference time‐domain (3D‐FDTD) method, and the proposed scheme successfully discriminated each QPSK pattern. The power value |E|2 at all ports closely matches the theoretical value, and the ratios of 9.5 dB and >30.0 dB are numerically obtained.

Abstract Optical digital‐to‐analog converters (DACs) are key to overcoming the enormous power consumption caused by the slowdown of Moore's Law. In previous work, an optical DAC consisting of a signal generator and an intensity converter was presented. The generating component was integrated into the device, but a higher level of device integration is desirable. An optical DAC device with a built‐in intensity converter for a 2‐bit binary phase‐shift keying (BPSK) signal is demonstrated. The integrated device consists of two delay line interferometers (DLIs) with silicon waveguides. The first is used to generate a quadrature PSK (QPSK) signal from two BPSK signals, and the second is used to convert the QPSK constellation into light intensities. Experimental results show that the 2‐bit digital codes 00, 01, 10, and 11 can be successfully converted into four different light intensities depending on their pattern at 10 Gbps.

Abstract All‐optical likelihood calculation has the potential to improve the performance of operating speed and power consumption in future communication systems. The author previously proposed a novel scheme of likelihood calculation, which is capable of applying multi‐value modulation. However, these studies have a limitation that requires higher integration into an actual system. In this study, an optical device of likelihood calculation for a 4‐bit quadrature phase‐shift keying (QPSK) modulated signal (i.e. two sequential QPSK symbols) is demonstrated. The integrated device consists of two delayed interferometers with silicon waveguide. The device is designed according to the types of 4‐bit code string as follows (00 00), (00 11), (11 00), and (11 11), and the output light waveform from the device is observed by an oscilloscope for a plurality of 4‐bit QPSK signals. The light intensity obtained from the device accurately corresponds to the Hamming distance between the code string and the input signal. The results indicate that the proposed scheme correctly calculates a likelihood for a 4‐bit QPSK signal at 10 Gbaud.