We report on environmentally stable long-cavity ultrashort erbium-doped fiber lasers, which self-start mode-locking at quite low thresholds by using spectrally filtered and phase-biased nonlinear amplifying long-loop mirrors. By employing 100-m polarization-maintaining fiber (PMF) in the nonlinear loop, the fundamental repetition rate reaches 1.84 MHz and no practical limitation is found to further decrease the repetition rate. The filter used in the long loop not only suppresses Kelly sidebands of the solitons, but also eliminates the amplified spontaneous emission which exists widely in low-repetition-rate ultrafast fiber lasers. The bandwidth of the filter is optimized by using a numerical model. The laser emits approximately 3-ps pulses with an energy of 17.4 pJ, which is further boosted to $1.5~\unicode[STIX]{x03BC}\text{J}$ by using a fiber amplifier.

We present a technique allowing the stabilization and tuning of a modulation sideband in the presence of high-carrier frequency jitter and increased carrier phase noise. This technique is of particular interest in communication systems where oscillators providing the carrier signal cannot be stabilized by a conventional phase-locked loop, such as systems relying on low-cost optical LO generation techniques. The results obtained in simulation are validated by measurements carried out on a modular system demonstrator.