The 15th International Conference on Electronics, Communications and Networks (CECNet 2025)

Abstract: I report on recent advances in integrated-optic signal processing devices using a slab star coupler-type optical discrete Fourier transform (DFT) circuit [1], [2].
First, I explain the application of the optical DFT circuit to a demultiplexer of an optical orthogonal frequency division multiplexing (OFDM) signal [3]. Spectrally efficient optical communication with flexible capacity is required in the prospective adaptive photonic network [4]. Optical OFDM is one of candidates to contribute to this type of photonic network. In this talk, I demonstrate our gate-free integrated-optic tunable filter that can demultiplex optical OFDM signals with various number and symbol rate sub-carrier channels [5]. The filter consists of tunable couplers, an array of delay lines, and the optical DFT circuit. We can tune the number and/or the symbol rate of demultiplexed optical OFDM channels by selecting the delay lines with the tunable couplers. Optical timing gates were normally attached to the OFDM filter output ports with a view to extracting the effective time of the filtered channels [1], [2]. In [5], we removed the optical gates and substituted a high-speed photodetector for the gate with a view to achieving completely passive demultiplexing of the various capacity optical OFDM signals.
Optical logical gate circuits are regarded as significant components of photonic accelerators [6]. The photonic accelerators are used for increasing the processing speed and complementing the functions of existing electronic circuit-based computer systems by utilizing superior properties of lightwaves including coherence, high-speed processing, broadband performance, multiplexing capability, and low power consumption. In this talk, I report on our integrated-optic gates including a coherent exclusive OR (XOR) gate composed of the optical DFT circuit [7], [8]. The use of the DFT circuit enables us to realize processing of an arbitrary number of input bits, which cascaded coherent two-bit gates with the directional coupler configuration are weak in [9]. I explain the configuration, operating principle, and preliminary experimental results regarding the optical XOR gate. The optical XOR gate is useful for encryption/decryption and error detection of optical signals.

[1] K. Takiguchi et al., Opt. Lett., 36, 1140, 2011.
[2] K. Takiguchi et al., Photonics Technol. Lett., 31, 1327, 2019.
[3] A. J. Lowery et al., Opt. Fiber Technol., 17, 421, 2011.
[4] O. Gerstel et al., IEEE Commun. Mag., 50, s12, 2012.
[5] K. Takiguchi et al., OSA Continuum, 4, 2319, 2021.
[6] K. Kitayama et al., APL Photonics, 4, 090901, 2019.
[7] K. Takiguchi et al., in Proc. Optica Advanced Photonics Congress 2024, ITu2B.3, Quebec City, Canada.
[8] K. Takiguchi et al., in Proc. FiO/LS 2024, JW4A, Denver, USA.
[9] C. Peng et al., ACS Photonics, 5, 1137, 2018.
Keywords: optical signal processing, integrated-optics, optical Fourier transform, optical OFDM, optical logical gate circuit
Acknowledgements: This work was supported in part by JSPS KAKENHI Grant Number 23H04811.

Biography: Koichi Takiguchi received a Ph.D. degree in electrical engineering from the University of Tokyo in 1992. He then joined NTT Laboratories where he engaged in the research and development of integrated-optic functional devices including first tunable optical chromatic dispersion compensator (TODC) and an optical orthogonal frequency division multiplexing (OFDM) demultiplexer. In 2012, he joined Ritsumeikan University as a professor. He is now interested in optical signal processing technology and its application to next-generation optical and terahertz (THz)-wave communication,　optical and THz-wave sensing, and optical computing.