Linear fiber lasers can have single-mode operations especially if their cavities are short. The smallest length achieved is 100 micrometers . Such lasers have low thresholds, slope efficiencies and output powers. Ring lasers have much higher slope efficiencies and output powers, and narrow linewidths (a few kHz), but suffer from instabilities such as mode-hoppings. If double- ring resonators are used, the mode-hoppings can be eliminated without associated frequency shifts by dithering the two ring cavities together with a feedback mechanism. Double-clad fibers can be effectively used to increase the coupling of pump light into the core, while the absorption of the pump light can be greatly increased by codoping an Er3+-doped fiber with ytterbium.