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Semiconductor Optical Amplifier (SOA) utilizes semiconductor materials as the gain medium, enabling direct optical signal amplification without prior conversion to electrical signals. This unique property not only simplifies the optical amplification process but also lays the foundation for more efficient and compact optical systems.

Directly generating visible light from compact all-fiber lasers while maintaining high output characteristics has always been a research topic in laser technology. Here, Ji et al. proposed a method to develop dual-wavelength lasers using the excitation mechanism in holmium-doped ZBLAN fluoride glass fibers, and experimentally achieved high output performance of all-fiber lasers, especially operating in the deep red band under 640 nm pumping. Notably, a maximum continuous wave output power of 271 mW was achieved at 750 nm with a slope efficiency of 45.1%, which is the highest direct output power recorded in all-fiber lasers with a core diameter of less than 10 μm in the deep red band.

Single-frequency lasers based on Er3+-doped or Er3+/Yb3+ co-doped gain fibers mainly work in the 1.5 μm band (C-band: 1530-1565 nm) and part of the L-band (1565-1625 nm). Its wavelength is in the C window of optical fiber communication, which makes the 1.5 μm band single-frequency fiber laser with narrow linewidth and low noise characteristics very important in coherent optical communication. It is used in high-resolution sensing, optical frequency domain reflectometer , laser radar and other fields also have a wide range of applications.

The linewidth of a laser, especially a single-frequency laser, refers to the width of its spectrum (typically full width at half maximum, FWHM). More precisely, it is the width of the radiated electric field power spectral density, expressed in terms of frequency, wavenumber, or wavelength. The linewidth of a laser is closely related to temporal coherence and is characterized by coherence time and coherence length. If the phase undergoes an unbounded shift, phase noise contributes to the linewidth; this is the case with free oscillators. (Phase fluctuations confined to a very small phase interval produce zero linewidth and some noise sidebands.) Shifts in the resonant cavity length also contribute to the linewidth and make it dependent on the measurement time. This indicates that the linewidth alone, or even a desirable spectral shape (lineform), cannot provide the full information about the laser spectrum.