1.33 $\mu$m Single-Longitudinal-Mode Ultra-Narrow-Linewidth Fiber Laser by Bismuth-Doped Phosphosilicate Fiber

We report a high-performance single-longitudinal-mode (SLM) bismuth/phosphosilicate co-doped fiber laser (BPDFL) at 1.33 μm featuring an ultra-narrow linewidth of 465 Hz. This is enabled by combining a dual-coupler-ring compound-cavity (DCR-CC) filter and femtosecond-laser-inscribed high-scattering fiber (HSF). For the first time, a DCR-CC filter is implemented in a BPDFL providing a free spectral range of 6.82 GHz and a 4.73 MHz bandwidth ensuring stable SLM lasing. Meanwhile, the HSF effectively reduces the linewidth by nearly 50% from 917 Hz to 465 Hz. The laser achieves an optical signal-to-noise ratio above 68 dB, a relative intensity noise of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\leq$</tex-math></inline-formula><inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\--$</tex-math></inline-formula>153.4 dB/Hz<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\geq$</tex-math></inline-formula>3 MHz, output power fluctuation <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\leq$</tex-math></inline-formula>0.307 dB, and wavelength and power stability <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\leq$</tex-math></inline-formula>0.005 nm and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\leq$</tex-math></inline-formula>0.288 dB, respectively. This high-performance BPDFL offers significant potential for applications in coherent optical communications, high-resolution spectroscopy, fiber optic sensing, optical metrology, and LIDAR.