Currently, the 1 μm waveband laser is one of the most widely used ultrafast laser sources. In general, traditional lasers suffer from poor stability and high noise levels. Exploring the regulation mechanism for laser performance in this waveband and developing low-noise and high-stability seed sources that meet practical application requirements have become urgent needs in this field. In this work, we construct a dispersion-managed all-polarization-maintaining (PM) figure-9 Yb-doped fiber laser based on a nonlinear amplifying loop mirror (NALM). A tunable grating pair is incorporated into the cavity to enable dynamic dispersion control. The influences of three different coupling ratios (50:50, 40:60, and 30:70) on the laser performance are comparatively investigated. Compared with 50:50 and 40:60 configurations, the case with a 30:70 coupling ratio achieves more stable self-starting mode-locking at a pump power of 210 mW. Furthermore, by optimizing net cavity dispersion to the near-zero dispersion region, stable pulses centered at 1030 nm are obtained with a pump power of 64 mW. This state exhibits the optimal laser performance. The pulses have a repetition rate of 34.782 MHz and a pulse duration of 1.908 ps. The measured signal-to-noise ratio (SNR) is as high as 81 dB. More importantly, as the asymmetric coupling ratio increases, the nonlinear effects within the cavity are effectively suppressed, and the spectrum gradually becomes smoother. In chirped pulse amplification systems, spectral smoothness is crucial. Smooth spectra can effectively suppress the accumulation of nonlinear phase shifts during the amplification process, thereby reducing spectral broadening and distortion. This helps to obtain much better pulse quality. This work also reveals that a high coupling ratio can optimize the dynamic balance between cavity power cycle efficiency and nonlinear effects, thereby effectively reducing the threshold of the NALM mode-locking and enhancing the stability of pulses. As a low-threshold, high-stability seed source, this laser has broad application prospects in terahertz generation, optical frequency combs, and ultrafast spectroscopy.