It will be revealed that a myth of deploying low bit-rate uplink fiber-to-the-home (FTTH) services while providing a high bit-rate downlink is wrong. Therefore, for the future broadband FTTH services, the focus should be on the capability to provide gigabit- or even multigigabits-per-second both in up- and downlinks, namely gigabit symmetric systems. Optical code-division multiple access (OCDMA) now deserves a revisit as a powerful alternative to time-division multiple access and wavelength-division multiple (WDM) access in FTTH systems. In this paper, the authors will first highlight the OCDMA systems. The system architecture and its operation principle, code design, optical en/decoding, using a long superstructured fiber Bragg grating (SSFBG) en/decoder, and its system performance will be described. Next, an OCDMA over WDM passive optical network (PON) as a solution for the gigabit-symmetric FTTH systems will be proposed. The system architecture and the WDM interchannel crosstalk will be studied. It will be shown that by taking advantage of reflection spectrum notches of the SSFBG en/decoder, the WDM interchannel crosstalk can be suppressed and can enable OCDMA over WDM PON to simultaneously provide multigigabit-per-second up- and downlinks to a large number of users.

Sensing technology plays an important role in enabling innovation and efficiency in diverse industries, particularly in harsh and emerging environments where conventional sensing approaches exhibit limitations in meeting the industry’s standards and requirements. Fibre Bragg Grating (FBG) sensors have emerged as efficient sensing devices in various fields due to their unique properties. This paper provides a comprehensive review of FBG technological advancements since the exploration phase. The research highlights discuss applications of FBG sensors in harsh conditions, such as wide range and rapid perturbations of temperatures, extreme pressures under water or in the downhole region, and long-term radiation exposure. The paper also explores the integration of FBG sensors in emerging technologies, including smart monitoring and tactile sensing. Additionally, the review examines the role of sensing materials and the Internet of Things (IoT) in enhancing FBG sensing capabilities. By leveraging novel materials, IoT connectivity, and advanced interrogation methods in future development, enhanced sensitivity, selectivity, real-time monitoring, and remote accessibility can be achieved.

Fiber-induced intra- and interchannel nonlinearities are experimentally tackled using blind nonlinear equalization (NLE) by unsupervised machine-learning-based clustering (MLC) in ~46-Gb/s single-channel and ~20-Gb/s (middle-channel) multichannel coherent multicarrier signals (orthogonal frequency-division multiplexing (OFDM) based). To that end, we introduce, for the first time, hierarchical and fuzzy-logic C-means (FLC)-based clustering in optical communications. It is shown that among the two proposed MLC algorithms, FLC reveals the highest performance at optimum launched optical powers (LOPs), while at very high LOPs, hierarchical can compensate more effectively nonlinearities only for low-level modulation formats. When employing binary phase-shift keying and quaternary phase-shift keying, FLC outperforms K-means, fast-Newton support vector machines, supervised artificial neural networks, and NLE with deterministic Volterra analysis. In particular, for the middle channel of a QPSK wavelength-division multiplexing coherent optical OFDM system at optimum -5 dBm of LOP and 3200 km of transmission, FLC outperforms Volterra-NLE by 2.5 dB in Q-factor. However, for a 16-QAM single-channel system at 2000 km, the performance benefit of FLC over inverse Volterra-series transfer function reduces to ~0.4 dB at a LOP of 2 dBm (optimum). Even when using novel sophisticated clustering designs in 16 clusters, no more than additional ~0.3-dB Q-factor enhancement is observed. Finally, in contrast to the deterministic Volterra-NLE, MLC algorithms can partially tackle the stochastic parametric noise amplification .

The performance of the phase-shifted superstructured fiber Bragg grating (SSFBG) for optical code (OC) recognition was investigated with different reflectivity as well as input pulse width. The auto-correlation peak (P_A) and the ratios of P_A to the maximum wing level (P/W) and cross-correlation level (P/C) were used to quantitatively evaluate the OC recognition performance. There is a conflict between obtaining high P_A and high P/W and P/C ratios in high reflectivity regime. The approach of applying apodization technique to improve the performance in high reflectivity regime is proposed. The comparative experimental investigations with 127-chip 160-Gchip/s SSFBG are carried out to confirm the effectiveness of the proposed approach. Error-free transmission with multiplexing of two active users has been successfully achieved by the apodized SSFBG at a data rate of 1.25 Gbit/s.

The generation and recognition of a record-length 511-chip optical code is experimentally demonstrated by use of a superstructured fiber Bragg grating (SSFBG) with a chip rate of $640 \mathrm{Gchips}/\mathrm{s}$. Very high reflectivity (92%) is achieved with high-quality correlation properties. The temperature deviation tolerance is approximately $\pm 0.3 {}^{°}\mathrm{C}$, which is within the package’s temperature stability range $\left(\pm 0.1 {}^{°}\mathrm{C}\right)$. Experimental results show good agreement with the theory. They indicate the SSFBG’s potential for processing a long optical code with an ultrahigh chip rate, which could significantly improve the system’s performance.