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In order to bring frequency combs out of well-controlled laboratory environments, a robust mode-locked fiber laser is preferable with enhanced self-starting capability, and reduced size, weight, and power (SWaP). We try to reduce cavity loss as much as possible to lower the self-starting threshold and operation power. A lower concentration doped-fiber could restrain non-radiative transitions and scattering from color centers, which are two main loss sources of active fibers. We performed comparison experiments using two fibers with different rare-earth doping concentrations for better SWaP performance. Experimental results match with our predictions. Pumped by a 980nm semiconductor external-cavity stabilized diode laser, a mode-locking self-starts at about 60mW. The net-dispersion is managed by balancing the lengths of the positively dispersive Er-doped fiber with the negatively dispersive PM1550 fiber. It is worth noting that the passive mode-locking can be maintained with pump power as low as 30mW. The laser central wavelength is 1560nm and 3db bandwidth is about 42nm, corresponding to a transform-limited pulse duration of 61.43fs assuming sech2 pulse shape. The repetition frequency is 47.41MHz with more than 90dB signal-to-noise ration at 1KHz resolution bandwidth. At the minimum pump power, the laser output is at 4.2mW average power.
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