High-power single-frequency fiber lasers with exceptional properties, such as mode-hop-free and narrower linewidths, lower noise, and compact all-fiber designs, have been attractive in many applications, including high-resolution sensing, coherent telecommunications, optical frequency domain reflectometry, and as a seed laser for light detection and ranging (LIDAR). However, the development of high-power single-frequency fiber lasers is constrained greatly by the lack of high-gain optical fiber and components. In this review, we discuss the basic considerations of advanced techniques for soft glass fiber and fiber preform as well as three key issues among them. Then, the recent advances in RE-doped soft glass and fiber lasers operating at near-infrared (NIR) and mid infrared (MIR) wavelengths are evaluated in detail. Finally, prospects, applications and challenges for the realization of high-efficiency soft optical fibers in single-frequency fiber laser applications are analyzed and discussed. The review is organized as follows: Sections 1-3 introduce the basic requirements for the rare-earth (RE) ions and host matrix material for obtaining efficient NIR-MIR laser output, a review of the background of processing fundamentals on the fabrication and characterization for newly developed soft optical glass fibers, an outline of the key issues of platinum removal and the dehydration technique, effective doping of RE, and low splicing loss together with the recent theoretical and experimental results. Section 4 summaries the design and development of RE-doped soft optical glass and fiber lasers operating at 1, 1.5, 2, and 3 gm with emphatical discussions on the related luminescence mechanism and laser foundation as well as single-frequency lasers. Section 5 reviews the recent progress of various novel glass fibers and fiber lasers with unique photoelectricity properties, such as transparent glass-ceramic fiber and fiber lasers, quantum-dot fiber and fiber lasers, Bi-doped fiber and fiber lasers, as well as other novel glass fibers and devices, and outlines their prospects, applications, and challenges. Furthermore, Section 6 presents the conclusions of this review, which include the following: (1) From the single-component silica fiber to the multicomponent soft optical glass fiber, the glass compositions become more and more complex and the corresponding preparation method is constantly developing and improving. Various new types of optical fiber materials and components greatly expand their applications in the fields of fiber amplifiers, fiber lasers, and nonlinear optics, etc. (2) High-gain optical fiber is the key to the development of high-power single-frequency fiber lasers. To achieve intense and highly-efficient luminescence and lasers at the NIR and MIR regions from soft glass and optical fiber, the following methods were adopted, including using RE sensitization, uniform doping with high-concentration RE, hydroxyl and impurity-removing, new optical fiber and laser cavity design, low fusing loss, and the high-efficiency pumping method, etc. These novel glass systems and luminescence mechanisms enrich the glass systems of the past and provide a wealth of valuable data for optoelectronic materials and devices.
