We report on an all–fiber polarization maintained (PM) erbium-based figure-9 structured nonlinear amplifying loop mirror mode-locked laser where a stable carrier-envelope offset frequency (fCEO) signal was detected via a fiber-packaged f-2f interferometer. At the pump power of 145 mW at 976 nm, the oscillator delivers an average output power of 6.13 mW with a repetition of 88.6 MHz and a signal-to-noise ratio of 79 dB. The spectrum of 3 dB bandwidth was measured to be 54.3 nm. Furthermore, an average power of 270 mW was obtained by a two-stage cascaded amplifier, corresponding to a single pulse energy of 3.05 nJ. Then, the lasing was injected into a 40-cm-long PM highly nonlinear fiber after optimizing the length of dispersion compensated PM single-mode fiber. At last, an fCEO beat note signal exceeding 34 dB was obtained, which implies the system can be potentially used in all–fiber optical frequency comb.