Ground to satellite optical uplinks are a critical part of the infrastructure in the growing satellite network using freespace optical communications. Because of distortions to the free space optical beam caused by scattering, absorption and turbulence in the atmosphere, the optical power requirements for the uplink are higher than other parts of the system. Fiber lasers are expected to be a key component in the uplink transmitter, due to their robust nature, ability to operate at high average power with excellent beam quality, and high efficiency. This work will provide a brief overview of considerations for optical ground stations, and review of potential fiber laser sources for free space optical communications systems. Detailed results will be presented on a very-large mode area Er-doped fiber amplifier, core pumped by a 1480 nm Raman fiber laser, capable of operating at 100 W average power. Bit error rate testing results for the system at operating powers of up to 90 W will be presented.

Passive-Optical-Networks (PON) have emerged as a pivotal technology for broadband access network and are now expanding to wireless communication, supporting 5G and development of future 6G frameworks. PON systems are expected to find many new applications, including in electrical power grids, modern industrial factories, and smart city infrastructure. With the growing capabilities and increasing complexity and extent of the optical distribution network, effective surveillance of fiber infrastructure has become increasingly important to ensure long-term viability and dependability. Simultaneously, there is increasing demand for effective distributed monitoring systems for the power-grid elements and machinery in automated factories operating within PON environments. This paper discusses the challenges and potential solutions for implementing distributed acoustic sensing (DAS) within PON architecture. We will present design and experimental demonstrations of a co-existing DAS and 10G PON (XGS-PON) system with a 23.5km feeder fiber (FF) and a 1x16 splitter. A unique signature from each distributed fiber (DF) and optical network units (ONU) is detected by utilizing a “coded” Enhanced Scatter Fiber (ESF). Vibration events originating from up to three DF/ONUs are identified using a novel scheme using the “coded” ESFs in conjunction with fiber delay lines. We further investigated the sensing performance and potential crosstalk between XGS-PON and DAS signals within this co-existing DAS and XGS-PON system.

We demonstrate digital backpropagation-based compensation of fibre nonlinearities in the near-zero dispersion regime of the O-band. Single-step DBP effectively mitigates self-phase modulation, achieving SNR gains of up to 1.6 dB for 50 Gbaud PDM-256QAM transmission over a 2-span 151 km SMF-28 ULL fibre link.(c) 2025 The Author(s)

We report the first single-stage/single-pass bismuth-doped fiber amplifier (BDFA) achieving > 10 dB gain across the full S-band (1465-1530 nm), a critical step toward extending capacity in multi-band optical transmission systems. The proposed amplifier employs a 400 m-long germanosilicate fiber, bi-directionally pumped at 1425 nm, delivering 11.3-35.6 dB small-signal gain and a noise figure of 5.6-7.7 dB. Compared to thulium- and Raman-based S-band solutions, our BDFA offers a compact, efficient, and silica-compatible platform with improved performance in the lower S-band, where other technologies struggle. This demonstration paves the way for practical S-band amplification in future high-capacity, power-efficient optical networks without reliance on fluoride fibers or kilometer-scale nonlinear media.

We discuss recent advances in the use of engineered optical fibers for distributed sensing in telecom networks. Fibers with low attenuation and high back scatter can greatly increase acoustic sensing signals.

We demonstrate a wavelength tunable Distributed- Vibration-Sensing over PON scheme using low-cost ITLA and Enhanced-Scattering-Fibers. Vibrations at frequency grids of 193.40THz and 194.60THz in a PON with 1x16 splitter and 21 km feeder-fiber were successfully detected.

EDFA-BDFA cascaded S-band amplification is realized by 2.5m EDF with single 980nm pump and 400m BDF with single 1341nm pump. Flat gain >25dB from 1452nm to 1526nm and low NF from 3.6dB is demonstrated by placing 980nm pumped EDFA first with very high population inversion. (c) 2025 The Author(s)

Multi-band transmission is crucial for enhancing optical communication systems, enabling higher capacity and more efficient spectrum usage [1]. For applications such as free-space communication and frequency comb generation in unconventional telecom bands, high-power amplification is needed [2], [3]. In this work, we present the first watt-level O-band amplifier, designed to support these advanced optical communication systems and unlock new possibilities for both terrestrial and free-space networks.

High-power O-band (1260–1360 nm) lasers with fine wavelength tunability are essential for advancing photonic technologies. Fine tunability is particularly crucial for applications such as optical frequency comb (OFC) generation and optical coherence tomography [1]. In OFCs, precise wavelength control ensures stable and coherent comb lines for high-resolution metrology, spectroscopy, and high-capacity telecommunication networks [2]. Despite their importance, the development of high-power, finely tunable O-band lasers has faced significant challenges, including the lack of effective active media and temporal stability. Overcoming these limitations is vital for unlocking the full potential of O-band systems in diverse applications. In this work, we present cutting-edge advancements in photonic integrated circuits (PICs) and fiber optics to develop a compact hybrid laser with Watt-level output power and fine wavelength tunability across a 35 nm range.

Transmission matrix measurements of multimode fibers are now routinely performed in numerous laboratories, enabling control of the electric field at the distal end of the fiber and paving the way for the potential application to ultrathin medical endoscopes with high resolution. The same concepts are applicable to other areas, such as space division multiplexing, targeted power delivery, fiber laser performance, and the general study of the mode coupling properties of the fiber. However, the process of building an experimental setup and developing the supporting code to measure the fiber's transmission matrix remains challenging and time consuming, with full details on experimental design, data collection, and supporting algorithms spread over multiple papers or lacking in detail. Here, we outline a complete and self-contained description of the specific experiment we use to measure fully polarization-resolved transmission matrices, which enable full control of the electric field, in contrast to the more common scalar setups. Our exact implementation of the full polarization experiment is new and is easy to align while providing flexibility to switch between full-polarization and scalar measurements if desired. We utilize a spatial light modulator to measure the transmission matrix using linear phase gratings to generate the basis functions and measure the distal electric field using phase-shifting interferometry with an independent reference beam derived from the same laser. We introduce a new method to measure and account for the phase and amplitude drift during the measurement using a Levenberg-Marquardt nonlinear fitting algorithm. Finally, we describe creating distal images through the multimode fiber using phase-to-amplitude shaping techniques to construct the correct input electric field through a superposition of the basis functions with the phase-only spatial light modulator. We show that results are insensitive to the choice of phase-to-amplitude shaping technique as quantified by measuring the contrast of a razor blade at the distal end of the fiber, indicating that the simplest but most power efficient method may be the best choice for many applications. We also discuss some of the possible variations on the setup and techniques presented here and highlight the details that we have found key in achieving high fidelity distal control. Throughout the paper, we discuss applications of our setup and measurement process to a variety of specialty fibers, including fibers with harsh environment coatings, coreless fibers, rectangular core fibers, pedestal fibers, and a pump-signal combiner based on a tapered fiber bundle. This demonstrates the usefulness of these techniques across a variety of application areas and shows the flexibility of our setup in studying various fiber types.

Established techniques for measuring the transmission matrix (TM) of a multimode fiber (MMF) allow for spot scanning at the distal end of the fiber through phase control at the proximal end, enabling ultrathin medical endoscopes and other applications that benefit from controllable light fields in MMF. Adding this capability to fibers utilized for other applications allows imaging to be performed within these areas. One outstanding limitation of this technology is the need to re-calibrate the fiber upon bending or other environmental perturbation. Here, we demonstrate a modified shape-sensor fiber that allows both shape sensing and imaging within the same fiber. In addition to permitting an image at the end of a shape sensor probe, the unification of these two technologies opens up the possibility of using the reconstructed fiber shape to mathematically update the calibration of the imaging waveguide in a dynamic environment, as has been proposed in the prior literature. Creating a robust method for maintaining knowledge of the fiber's TM as the fiber is manipulated is critical for clinical deployment of this technology.

We review recent progresses of advanced ultra-low-loss (ULL) fibers, introducing an 85μm2 effective-area fiber with record-low-attenuation of 0.1474 dB/km at 1550 nm. We also highlight system demonstrations using ULL fibers and their relevance to DCI/metro and undersea network.

We demonstrate a TMI-free 5.2 kW single-mode output from a fiber amplifier using Yb 20/400 fibers with reduced core thermo-optic coefficient. The TMI threshold is increased by 50% compared to that of commercial Yb-doped fibers.

Bismuth doped fiber amplifiers (BDFA) have been pumped with single mode 1190-1260 nm lasers to amplify signals over the O-band. We report here a BDFA using 915 nm multimode laser via an ytterbium fiber conversation stage. The conversion stage transforms 915 nm pump to single mode 1150 nm light used to pump BDF. This pump provides up to 3W of 1150 nm power over 20-70 ˚C with 0.25-0.2 electrical to optical conversion (wall-plug) efficiency. Compared to directly pumped BDFAs with 1195 nm single mode semiconductor laser(s) the 915/1150 nm pumping has superior optical performance and lower power consumption. The resultant BDFA provides >20 dB gain over 1255 to 1355 nm (17.6 THz) with a maximum gain of 29.3 dB and corresponding noise figure (NF) of 4.6 dB (λ = 1300 nm, Pin = -20 dBm). The BDFA has electrical power consumption of 8.1-9.6 W over 20-70 ˚C respectively. We also show the amplifier is suitable for high-speed data transmission by amplification of 50 Gbaud/s PAM-4 WDM signals over 30 km of G.652 fiber.

We report the performance of an LMA Yb-doped fiber, designed for increasing the transverse mode instability threshold and minimizing nonlinear effects in multi-kilowatt class fiber lasers, by reducing the thermo-optic coefficient of the fiber core, compared with that of standard aluminophosphosilicate Yb-doped fibers. A TMI-free 5.2 kW single-mode output power from a Yb 20/400 fiber with a 17.5 mu m mode-field diameter was achieved in a broad bandwidth, co-pumped amplifier with 78% optical-to-optical efficiency, while a 4 kW signal output was attained in a 26 GHz linewidth amplifier. Negligible photodarkening loss was observed during 150 hour laser operation at 2 kW.

Minimally invasive and robotic surgeries are growing areas that benefit patients through reduced recovery time. Medical fiber optics play an important role in these procedures by enabling instrument navigation, imaging, sensing, power delivery, and diagnostics in a small form factor. One route to further miniaturization is to combine these functions, or a subset of these functions, into a single strand of optical fiber. In this work, we present a fiber and fan-in device that enables shape sensing, imaging, power delivery, and potentially additional sensing capabilities, such as temperature and/or pressure, in the same waveguide. The refractive index profile of the multimode waveguide in our fiber is similar to step index fibers used in laser delivery and is suitable for imaging applications; however, it also contains seven single mode cores twisted in a helix and with quasi-continuous Bragg gratings along their entire length, such as are used in fiber shape sensing. We first calibrate the transmission matrix of the multimode waveguide to enable the formation of a focused spot at the distal end of the fiber with a spatial light modulator. A second calibration allows us to reconstruct the shape of the fiber using optical frequency domain reflectometry in the twisted shape sensing cores. We show that these multiple functions can be performed simultaneously with our device and that changes in the curvature of the fiber correlate with the quality of the distal spot produced through the fiber, which is an important step towards maintaining the imaging calibration as the fiber is manipulated. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Simultaneous fiber-optic sensing and NG-PON data transmissions over a 1x16 splitter is demonstrated by enhanced scatter fiber. Acoustic signals from a single distribution fiber are identified. The crosstalk between sensing and data channels is studied.

Bismuth-doped phosphosilicate fibers have become the most promising gain medium for O-band amplifiers. Yet scientific challenges on understanding the nature of bismuth active centers (BACs), mechanisms of bismuth cluster formation in the phosphosilicate glass network still exist. It is likely that multiple BACs with different oxidation states in different structural sites all contribute to the broad, nonsymmetric luminescence and gain spectra. Due to the progress in the fundamental understanding of bismuth-doped phosphosilicate glass, various designs of optical amplifiers with decent performances have been demonstrated.

We report the performance of new LMA Yb fibers with increased cladding absorption for pumping in the 915 nm absorption band. A 0.5 dB/m cladding-absorption Yb20/400 fiber showed negligible photodarkening loss in 400-hour laser operation at 3 kW, with 77% optical-to-optical efficiency. Low-SRS and TMI-free operation at 3.5 kW signal power was achieved with a 0.65 dB/m cladding-absorption and 20.2 μm mode-field diameter Yb fiber, tested in a co-pumped amplifier. The Raman peak was 31 dB below the signal peak at the maximum power.

Multimode fibers (MMFs) have a very large number of propagating modes per unit area and therefore allow for imaging with a very large number of pixels relative to their diameter. This makes MMFs perfect candidates for ultrathin endoscopes in applications such as deep brain imaging. However, the accuracy of the input-output relation that is needed, e.g., for distal spot scanning without moving parts, requires a new calibration after the fiber position or temperature has been significantly altered. While neural networks have been used before to attempt to solve these challenges, we present an MMF-based imaging method that tolerates and classifies different fiber positions, using two single-layer fully-connected neural networks that only require the optical intensity without measuring the optical phase. One network learns the nonlinear relation between the input and output intensities and allows for image reconstruction in the presence of position changes, while the other network classifies that position change for different images. We show that our method is superior to memory-effect-based position sensing, both for small position changes where the relation between position change and output specklegram rotation angle is linear, as well as for larger position changes where this linearity and uniqueness break down. We also show that the position classification results are robust to temperature and polarization perturbations, and that our position classifier is able to effectively generalize. Likewise, we show that our imaging network also is robust to 30°C perturbations in temperature and 10° in polarization.