High-Energy light in the UV-visible regime has seen an amazing boom of R&D primarily in the communities of the micro-machining and welding of highly reflective metals such as copper and aluminum, as well as in the areas of medical lasers and underwater sensing. Transport in the UV-visible range is extremely affected by the wavelength-dependent material dispersion and absorption of the transport optics. By utilizing the air core in a nested anti-resonant hollow core fiber (NANF) one can enable high confinement of the fundamental mode to achieve low-loss transmission in many environments. This paper presents a numerical study following the development process and initial experiments of a novel NANF designed for UV-vis transmission.
Anti-Resonant Hollow Core Fibers (AR-HCFs) are an emerging technology with a number of applications in low-latency communications, mid-IR transmission, and power delivery. Current R&D applications of AR-HCFs utilize free-space optics to obtain results in a timely and cost-effective manner. However, this approach exposes the HCFs to contaminants entering the HCF microstructures and if unmitigated can cause loss and potentially damage to the fiber. End-capping HCFs can protect the interfaces from damage, dust, and debris while simultaneously increasing their robustness and allowing for long-term use. Here, we present a novel endcap technique that allows low-loss transmission through a robust end-capped AR-HCF.
Multimode (MM) laser light has a vast application history spanning from laser pump sources, to high-speed optical links, to imaging systems but can suffer enormous inefficiencies when coupled through a solid core optical fiber for long transmission path lengths. One way to improve the MM transmission is by replacing the traditional solid-core fibers with uniquely tailored nested antiresonant hollow-core fibers (NANFs). By improving upon previous design methods, one can extend the application of the HCF to 100s of modes and beyond while maintaining low loss thus enabling novel concepts such as power beaming through fiber and the transmission of spatiotemporal tailored ultrafast wavepackets. We report a uniquely designed, fabricated, and tested MM NANF that enables low-loss transmission of 100s of modes.
Hollow-core core fibers with an anti-resonant nodeless structure are showing an incredible ability for mode confinement within the realm of optical transport. A subset of this application, high-energy laser beam delivery, requires extremely finite tolerances and precise design constraints in what are the traditional solid-core fiber solutions whereas nested antiresonant hollow-core fibers (NANF) allow for significantly more flexibility in both design and application. Increasing the number of capillaries in a NANF increases the number of allowed optical modes to propagate through the fiber thus reaching the few-mode and multi-mode regime, however increasing the modal count in propagation is not always desired. This study presents a comparison between experimental and simulated performance in the kW regime of a NANF with an internal structure consisting of five nested capillaries.
We demonstrate a fast and versatile approach to analyze the modal content of a high power fiber amplifier using a low-loss photonic lantern. By monitoring the first three modes of the photonic lantern on a photodetector we can directly determine the modal content of a laser beam, enabling real time diagnostics of the output and its corresponding beam quality factor, M2. We first investigate the beam quality and modal content of the output of a passive LMA double clad fiber commonly used as a delivery fiber in high power fiber laser amplifiers. The output of the fiber is analyzed by both a 6-mode mode-selective photonic lantern and a conventional M2 setup utilizing a translation stage and beam profiler. The modal content and beam quality measurements produced in real-time by the photonic lantern are compared to the M2 measurements resulting in an RMS error less than 0.098 across M2 values between 1.020 to 2.260. We then conduct a follow on experiment using the same photonic lantern to monitor modal instability in a large mode area fiber laser amplifier. In this case, we compare our photonic lantern mode analysis approach versus the commonly used RIN/pinhole method evaluating modal instabilities. Not only does the photonic lantern estimate the modal content and beam quality in real-time but the modal content trends with the RIN metric as the fiber laser amplifier progresses from stable regime below 300W through the chaotic transverse modal instability regime above 400W.
Advancements in high-power delivery of narrow linewidth single mode fiber lasers have garnered significant amount of recent interest with the advancement of hollow-core fibers. It has been shown that by propagating high intensity light through an air core as opposed to a solid glass core one can significantly delay the onset of nonlinear effects deleterious to laser performance. Precisely designed anti-resonant hollow-core fibers have been shown to handle both 100s of watts of average power and low-loss propagation while simultaneously resisting bending losses and discouraging the propagation of any higher order modes. This paper presents the recent progress in surpassing more the 1 kW of single mode 1070nm optical power through one such fiber at a length of more than 10m coiled on a 30cm mandrel.
Hollow core fibers have been investigated for several use cases relating to both single mode and multi-mode operation. Single mode, low-loss operation is a desired commodity in telecommunications and high-power delivery applications. Hollow core fibers can be designed with a structure that guides multiple modes in the core at low loss while also exhibiting strong stress sensitivity. In these anti-resonant hollow core fibers, perturbations to the structure such as micro-bending can efficiently couple core guided modes in short sections of fiber. This high sensitivity to structural distortion can be exploited for higher order mode generation, sensing, and for developing multimode nonlinear light sources. This work presents an investigation on using anti-resonant hollow-core fibers as a higher-order mode converter by inducing mechanical stress on the structure of the fiber.
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