We propose an all-fiber configuration of tunable bandpass filter (TBPF), consisting of a mode selective coupler (MSC) and an acoustically induced fiber grating (AIFG). Either the MSC or the AIFG can act as high-order mode converters in a few-mode fiber. The working wavelengths of the MSC and the AIFG are chosen to achieve complete conversion cycle between the core-mode linearly polarized (LP₀₁) mode and the LP₁₁ mode. The tunability of the TBPF is achieved by changing the frequency of the radio frequency signal. An all-fiber tunable ring laser based on the TBPF is used to generate a stable continuous-wave laser with 3-dB bandwidth of around 0.8 nm. The tunable wavelength ranges from 1530 to 1580 nm. Furthermore, the TBPF is introduced into the cavity of a mode-locked fiber laser. Wavelength-tunable mode locking pulses are obtained at central wavelengths from 1530 to 1560 nm.