Investigation of ammonium chloride crystal formation on end faces of chlorine-rich fused silica hollow core fibers

The transmission of light through a fiber air core has revolutionized the world of optical fiber technology. Recent advancements in antiresonant hollow core fibers have significantly pushed the field forward over the past decade, achieving lower attenuation than all-solid fused silica fibers. With light propagating primarily in air, these fibers offer higher transmission speeds, reduced latency, lower nonlinear effects, and enhanced radiation resistance. They are particularly well suited for high-power laser delivery, including ultrashort pulsed lasers or high power cw lasers. Despite of these advantages, challenges remain for the specialty fiber applications, especially regarding system integration and operation under demanding environmental conditions. One critical issue is the formation of ammonium chloride (NH4Cl) crystals on the end faces fabricated from chlorine-rich fused silica (e.g., F300). These crystals cause absorption, scattering, beam degradation, and ultimately fiber failure. This work presents a systematic investigation of NH₄Cl crystal growth, addressing three key aspects: the fiber manufacturing process, the handling and storage history of the drawn fiber, and the influence of environmental factors such as humidity, fiber length, fiber purging with N₂ and glass composition. A simplified capillary fiber model without an inner antiresonant structure was employed to accelerate testing while maintaining relevance to complex HCF designs. Based on these studies, several mitigation strategies were tested, including optimized handling procedures and the use of alternative fused silica materials. These approaches aim to enable reliable deployment of hollow core fibers in specialty applications by reducing or eliminating NH₄Cl contamination.