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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.

Optical comparators have the potential to improve the operational speed and power consumption of optical communication systems, as they directly perform the required operations in the optical domain to avoid any optic-electro conversions. Various comparators are optically implemented; however, most studies have been confined to an on-off keying modulation. Although there is a comparator available for the phase-shift keying (PSK) modulation, the device has a limitation that requires integration into an actual system. In this paper, the author provides an optical comparator based on a silicon waveguide for a quadrature PSK (QPSK) signal. The proposed comparator is a simple 2.0 × 0.1 mm2 rectangle, which is approximately 1/7500 the size of comparators from previous studies, and the device mainly comprises a delay line interferometer, a variable splitting ratio optical coupler, and a phase shifter. The performance of the comparator designed for 00 00 and 11 00 codewords was experimentally evaluated. The comparator generates the coupled symbol with the comparison result between the target code and the input signal. The paper demonstrates the feasibility of the ultracompact optical comparator for a 4-bit QPSK-modulated signal at 10 Gbaud.

In this paper, a novel optical comparison operation for phase-shift keying (PSK) modulated signal has been proposed by using optical delayed interferometer to realize an advanced functional component for future optical network. The feasibility of the optical comparators designed for 4-bit and 6-bit codewords was experimentally demonstrated. It has been shown that the constellations obtained from each comparator are located in conformity with a $Hamming$ distance between a comparator code and a given codeword, which is comprised of a successive two and three QPSK-modulated RZ symbols at 10.72 Gbaud/s. It was concluded that the proposed scheme offers a comparison operation for 4-bit and 6-bit codewords at 10 Gbaud/s.

In this paper, we propose an optical forward-error-correction (FEC) coding scheme with convolutional code using four-wave mixing (FWM) in a highly nonlinear fiber (HNLF) to realize an adaptive coding scheme corresponding to the signal-to-noise ratio (SNR) between nodes in photonic networks. The performance of the proposed scheme was numerically investigated for 211-1 pseudorandom bit sequence (PRBS) differential phase-shift keying (DPSK) modulated return-to-zero (RZ) format signals at 10 Gb/s. The optimized condition of optical XOR operations, which are the main components of the target coding scheme, is obtained for fiber length, signal wavelengths, and signal powers. Based on the optimized condition, the power penalties of the FEC coding scheme achieved are approximately 0.5 and -0.8 dB at BER 1/4 10-9 in the cases of twoand three-input optical XOR operations, respectively, indicating that the proposed scheme obtains a net coding gain. The high-quality coded signals, which keep 5-dB degradation of the Q-factor from its maximum value of 40, are achieved with the 6.1and 6.2-dB input power tolerances for coded signals 1 and 2, respectively.

An all-optical comparator is desirable to realize large-capacity, fully-transparent, and energy-efficient communication systems, as it is considered to be a fundamental component to perform most of the operations, including packet switching, label recognition, error detection and correction, and so on. However, most of the previous studies have been confined to the on–off keying (OOK) modulation format, not phase-shift keying (PSK) modulation. In this paper, the author provides a novel optical comparator designed for quadrature PSK (QPSK)-modulated signal, which comprises a code word with 8-bit length, using a serially-cascaded delay line interferometer. The proposed comparator yields constellations having the information of a Hamming distance based on the designed code, when several patterns of QPSK signal are injected into the comparator. The paper experimentally demonstrates the feasibility of the optical comparison operation for 8-bit QPSK-modulated return-to-zero (RZ) signal at 10 Gbaud.