Research Progress in Fluoride Fiber Amplifiers (Invited)

2026-04-01

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Wenshuo, Li

Hao, Wu

Zihan, Sun

Ge, Pan

Changhui, Liu

Shunbin, Wang *

Pengfei, Wang *

DOI：10.3788/LOP252318

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Significance The exponential growth of global data traffic necessitates continuous advancements in optical communication systems, where optical fiber amplifiers serve as indispensable components for signal regeneration and power boosting. Silica-based fiber amplifiers dominate current networks due to their maturity and low cost. However, their inherent limitations, such as restricted rare-earth ion solubility and high phonon energy, confine their operational bandwidth and impede expansion into the mid-infrared (MIR) spectrum. The MIR region is of paramount importance, housing fundamental molecular absorption fingerprints for gases and biomolecules, which is critical for applications in environmental monitoring, medical diagnostics, industrial process control, and secure free-space communication. Consequently, developing high-performance optical amplifiers for this spectral region is a significant research frontier. Fluoride glasses emerge as a superior alternative to silica for infrared photonics. Their defining advantages include exceptionally low phonon energy, which minimizes non-radiative decay and enhances radiative efficiency of rare-earth ions; high solubility for rare-earth ions, enabling efficient doping and high gain per unit length; and a broad optical transmission window extending from the ultraviolet deep into the MIR. These intrinsic properties make fluoride fibers an outstanding gain medium for amplifying signals across the near-infrared to the MIR. Exploring and optimizing rare-earth-doped fluoride fiber amplifiers (FFAs) is therefore essential for unlocking the full potential of infrared photonic systems. This review aims to systematically chronicle the evolution, current state-of-the-art, and future directions of FFAs, providing a comprehensive resource for researchers in this field. Progress The development of FFAs is intrinsically linked to advances in fluoride glass science and fiber fabrication technology. Three primary glass systems have been explored: fluorozirconate (such as ZBLAN), fluoroaluminate, and fluoroindate glasses, each with distinct properties summarized in related studies. Fluorozirconate glasses, particularly ZBLAN, have been the most widely studied due to their excellent glass-forming ability and low phonon energy (--580 cm(-1)). Fluoroaluminate glasses offer higher thermal and mechanical stability, while fluoroindate glasses provide the widest transmission window but suffer from lower thermal damage thresholds. Fabricating low-loss fluoride fibers remains a formidable challenge, requiring ultra-high purity raw materials, meticulous preform fabrication techniques like the suction-casting method, and precisely controlled drawing processes in inert atmospheres to minimize scattering losses and hydroxyl contamination. The research progress of FFAs is categorized based on the active rare-earth ion and its target amplification band: Pr3+-doped fiber amplifiers (PDFAs). The O-band is crucial for metropolitan area networks due to lower fiber dispersion, PDFAs are the key technology for this window. Since the first demonstration in 1991, significant progress has been made. Early research established the feasibility using ZBLAN fibers. Subsequent optimizations involved shifting to lower phonon energy hosts like PbF2/InF2-based glasses to improve quantum efficiency, employing Yb3+ sensitization to enable pumping with more readily available--1 & micro;m laser diodes (though with trade-offs in efficiency), and refining pump schemes (such as bidirectional pumping). These efforts led to remarkable gains exceeding 40 dB and improved noise figures. Comparative studies between fluorozirconate and fluoroindate-based PDFAs have highlighted the performance benefits of lower phonon energy matrices. Tm3+-doped fiber amplifiers (TDFAs). Amplifying the S-band expands the usable fiber bandwidth. The main challenge for TDFAs is achieving population inversion due to the shorter lifetime of the upper laser level (H-3(4)) compared to the lower level (F-3(4)). Researchers have ingeniously overcome this using up-conversion pumping schemes, dual-wavelength pumping (such as 1.05 & micro;m and 1.56 & micro;m) to depopulate the lower level and shift the gain spectrum, and co-doping with ions like Ho3+ to facilitate energy transfer and enhance performance. Through high-concentration doping and advanced pumping configurations, TDFAs have achieved gains over 26 dB with high power conversion efficiencies exceeding 70 % in the S-band. Recent advancements have successfully extended efficient high-gain (over 24 dB) amplification into the short-wave mid infrared (MIR) region around 2.33 & micro;m using Tm3+-doped ZBLAN fibers. Er3+-doped fiber amplifiers (EDFAs). While silica-based EDFAs are mature for the C/L-band, fluoride-based EDFAs offer advantages for extended L-band operation and, more importantly, are the cornerstone for high-power MIR supercontinuum (SC) generation. Early work in the 1990s demonstrated gains around 30 dB at both 1.5 & micro;m and 2.7 & micro;m. A major breakthrough has been the use of Er3+-doped fluoride fiber amplifiers as ultra-bright, pulsed pump sources for SC generation. By amplifying and then coupling into a length of passive fluoride fiber (such as ZBLAN or InF3), researchers have generated octave-spanning SC spectra covering from--1.8 & micro;m to beyond 5 & micro;m. The evolution from free-space coupling to all-fiberized configurations has dramatically improved robustness and power handling, enabling record SC output powers with over 90% of the power concentrated beyond 3 & micro;m. Active spectral shaping techniques using cascaded gain fibers have further refined the SC flatness and spectral coverage. Conclusions and Prospects Fluoride fiber amplifiers have unequivocally demonstrated their superior performance potential across strategic spectral bands from the O-band to the MIR, filling critical gaps left by silica-based technology. They have enabled high-gain amplification in communication windows and empowered the generation of high-power, broadband MIR light sources for sensing and spectroscopy. However, the path to widespread commercialization and higher power scaling is obstructed by several persistent challenges. First, material stability. Issues with thermal damage, chemical durability (especially moisture resistance for ZBLAN), and end-face degradation under high power. Second, fiber loss. The practical transmission loss of fluoride fibers remains orders of magnitude above the theoretical limit, limiting device length and efficiency. Third, system integration. The lack of a mature ecosystem of compatible passive fiber components (couplers, isolators, WDMs) hinders the development of robust, fully integrated all-fiber systems. Future research must be directed towards: material innovation to discover new glass compositions or composites with enhanced stability, higher damage thresholds, and maintained low phonon energy; process revolution to achieve fibers with losses approaching the theoretical minimum through breakthroughs in purification, preform homogeneity, and drawing process control, potentially benefiting from novel approaches like microgravity fabrication; and device development to foster the design and fabrication of reliable, low-loss passive components specifically for fluoride fibers. Overcoming these hurdles will not only solidify the role of FFAs in next-generation multi-band communication networks but also propel their adoption as enabling technologies in transformative MIR applications across scientific research, healthcare, and industrial monitoring.

Keywords:

fluoride fiber

optical fiber amplifier

rare-earth doping

fluoride glass

Research Progress in Fluoride Fiber Amplifiers (Invited)

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