Ring resonator features are of paramount importance in various optical systems. In this context, the enhancement of linewidth, and in turn the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i>-factor, stands as the pivotal achievement. However, a distinctive trait of ring resonators lies in the delicate balance between the linewidth and resonance depth; an improvement in the linewidth may entail a compromise in the resonance extinction ratio. In this context, here a silicon nitride ring resonator is presented, boosting a record-breaking linewidth for a single-mode ring resonator lower than 10 fm (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i>-factor > 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup>), alongside an extinction ratio of 5 dB. This significant improvement of linewidth is experimentally demonstrated by harnessing the optical bistability effect through the combination of low optical losses (in dB/m) and the Kerr effect, suppressing the detrimental impact of self-heating-induced dissipation. This breakthrough holds several advantages, primarily in the realm of selectivity and signal-to-noise ratio, thereby paving the way for its utilization for data storage as well as numerous sensing applications.

We investigate the potential to expand the trans mission bandwidth beyond the C-band to include the L- and U bands over an unrepeatered link. We characterize three fibers- a G.654 compliant Sumitomo Z+ fiber, a G.657 compliant Corning SMF28ULTRA fiber, and a G.652 compliant OFS AllWave Zero Water Peak (ZWP) fiber- for their attenuation profiles and Raman gain efficiencies in the L- and U-bands. We demonstrate the transmission of C+L+U bands over an unrepeatered link of 257.1 km. The transmission system includes band-specific erbium-doped fiber amplifiers for the C- and L- bands, lumped Raman amplifiers for the U-band, and bidirectional distributed Raman amplification for the three bands. We report a decoded data-rate of 84.5 Tb/s over a spectral bandwidth of 109.2 nm using 520 polarization-multiplexed 16-ary quadrature amplitude modulated channels across C+L+U bands. We investigate ways to manage the effects of stimulated Raman scattering while expanding the transmission window beyond 100 nm over an unrepeatered link.

We analyze the weather-related link availabilities of high-capacity THz-wireless transmission systems, focusing on fiber-integrated point-to-point links in the lower THz frequency range around 300 GHz. First, we discuss latest component technologies for electronic and photonic THz generation, showing that there is a good basis for the implementation of wideband THz-wireless links. Then, we review the application of a THz-wireless fiber extender and identify challenges for its integration into future 6G optical networks, being mainly the linear optical-to-THz conversion and the weather-dependent THz link loss. Both aspects will be addressed in the following: We describe the concept and implementation of a fiber-integrated THz outdoor unit prototype based on electronic THz components and provide link budget calculations. Then, we estimate the THz link loss from theoretical models and real weather data, showing that high link availabilities above 99.999% are possible for link distances of 500 m with state-of-the-art components. Estimates for a longer distance of 1 km are given as well. Finally, we describe our outdoor testbed built with our prototypes and present measured data on the link attenuation over 500 m in comparison to the theoretical expectations, achieving a good correlation.

The rapid emergence of large language model (LLM) enabled machine-learning applications is fueling an explosive growth in datacenter and campus scale network bandwidth demand. This tutorial examines the progress and scaling limitations of intensity modulation and direct detection (IM-DD) based optical technologies and explores how optimized coherent technology, such as the newly proposed 2-dimensional (2D) polarization-folding (PoFo) time-diversity approach, as well as a novel Heterodyne single sideband (H-SSB) coherent detection technology, can address some of these challenges

In this paper, we experimentally demonstrated 224 Gbit/s polarization-multiplexed (PM) PAM4 intensity modulation and direct detection (IM/DD) transmission system using a thin-film Lithium Niobate (TFLN) polarization controller. We proposed and implemented a pilot-tone-assisted and calibration-free polarization tracking algorithm in an FPGA, achieved a record convergence time of 0.2 ms and polarization tracking speed of 2 krad/s for PM-IM/DD systems. The proposed scheme enables stable dual-polarization demultiplexing under fast polarization perturbations and represents a step towards practical high-speed, low-complexity PM-IM/DD systems with robust polarization tracking performance.

Vibration sensing based on forward transmission is an emerging topic for network protection and environmental monitoring, especially in long-haul submarine cables and urban fiber networks. However, previous field trials of this approach have mainly focused on localizing strong events under controlled or relatively quiet conditions. In this work, we investigate the capability of forward-transmission vibration sensing to detect weak signals in noisy environments. We demonstrate a highsensitivity vibration sensing system operating over an 80-km deployed live urban fiber loop without optical amplifiers. The system is enhanced by adaptive time-frequency masking and inband laser phase noise suppression techniques to improve sensitivity and noise robustness. It has successfully identified and localized weak real-world vibration events with peak-to-peak amplitude lower than 20 rad, such as construction activity near a manhole and even footsteps on handhole lids. Field trial results confirm its robust performance under dynamic environments, including road traffic-induced ground vibrations and aerial cable disturbances. To the best of our knowledge, this is the first demonstration of weak vibration event detection using forward transmission in urban fiber networks. It remarks a significant step towards practical distributed vibration sensing in smart city applications.

We present a 425-Gbps/λ dual-polarization (DP) IMDD pluggable transceiver in an OSFP form factor for intra- data center application scenario. Both DP transmitter (Tx) and receiver (Rx) are monolithically integrated in a silicon photonics (SiPh) chip. The Tx multiplexes two 212.5Gbps polarization tributaries onto a single wavelength optical carrier, forming a 425-Gbps/λ data stream into the transmission link. The Rx performs a robust polarization tracking based on a proposed fast converging eigenvector-decomposition-based polarization recovery algorithm. Aided by off-the-shelf digital signal processing (DSP) and transimpedance amplifier (TIA) chips, the transceiver demonstrates a real-time 2×212.5Gbps transmission over 1km single mode fiber (SMF). A stable pre-FEC bit-error ratio (BER) floor of 2×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> level is obtained in both dynamic polarization scrambling and fiber handling cases. The experiment proves the feasibility that the DP solution is capable of doubling system capacity for short-reach transmission with robust polarization tracking.

In this study, we propose an optical link on a silicon (Si) wafer that integrates a membrane laser, membrane electro-absorption modulator (EAM), and membrane photodiode (PD) connected via a low-loss silica-based (SiOx) waveguide for chip-chip optical interconnects. The membrane structure incorporates a lateral p-i-n diode encased in a low-refractive-index SiO2 layer, which facilitates strong optical confinement, reduces the active layer volume in the laser, and lowers the capacitance in the EAM. The laser features a compact 100-μm-long cavity utilizing detuned distributed feedback (DFB) gratings in which the rear grating is designed to select one of the side modes in the stop band of the front grating. This design achieves a low threshold current of 1.7 mA and asymmetric output, reducing the driving current. The EAM utilizes a Franz-Keldysh-effect-based modulator with an absorption length of 100 μm, featuring a low capacitance of 14.5 fF and a 3-dB bandwidth exceeding 67 GHz. The PD is fabricated with an absorption length of 120 μm to ensure high internal responsivity, providing a 3-dB bandwidth of 48 GHz. These membrane photonic devices are integrated onto a Si wafer using a combination of a wafer bonding method and epitaxial regrowth on the bonded InP layer formed on the Si wafer. Static measurements from the fabricated optical link, which connects the DFB laser, EAM, and PD via a 7.6-mm silica-based waveguide, show an output amplitude of 104 μA at the PD, achieved with a low laser input current of 10 mA and an EAM voltage swing of 1.5 V. Dynamic evaluations at laser currents of 5 and 10 mA revealed eye openings for 50- and 64-Gbit/s NRZ signals, achieving low energy costs of 0.14 and 0.26 pJ/bit, respectively. Eye diagrams for high-speed NRZ signals at data rates up to 96 Gbit/s were also observed.

Deployable quantum key distribution (QKD) for free-space link must be both reference frame tolerant and hardware compact, yet existing free-space implementations still rely on bulk optics and active alignment. To address this gap, we develop the first reference frame independent (RFI) QKD transmitter fully integrated on a photonic-integrated circuit (PIC) and packaged in a CFP2 optical-transceiver form factor. The PIC incorporates a six-channel laser diode array, a six-channel variable optical attenuator array, and a polarization beam combiner, providing wavelength indistinguishability and high polarization fidelity. Combined with a micro-optics based polarization decoder and a bidirectional two-stage pointing, acquisition, and tracking (PAT) system, the PIC transmitter forms a complete free-space RFI QKD system. Based on the system, RFI QKD is performed with 10 m free-space channel, providing that QBERs measured in the same basis are below 3.81 % and the security parameter is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1.77\pm 0.02$</tex-math></inline-formula>. This leads a stable secret key rate of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$77.33\pm 1.65 \text{kbps}$</tex-math></inline-formula> for 100 s. These results verify the feasibility of PIC-based RFI QKD transmitter and establish a clear pathway toward fully deployable PIC-based QKD systems.