The invention provides a low repetition frequency
erbium-doped
femtosecond fiber laser, which comprises an NALM ring, a main ring and a first coupler, and is characterized in that the NALM ring comprises a pumping source input end, a
wavelength division
multiplexer, a polarization-maintaining
erbium-doped
fiber and a first polarization-maintaining passive
fiber, and the first polarization-maintaining passive fiber is an annular main body of the NALM ring; the pumping source input end, the
wavelength division
multiplexer and the polarization-maintaining
erbium-doped
optical fiber are all connected to the first polarization-maintaining passive
optical fiber; the main ring comprises a polarization-maintaining
isolator, a second coupler, an output end and a second polarization-maintaining passive
optical fiber, the second polarization-maintaining passive optical fiber is an annular main body of the main ring, the polarization-maintaining
isolator and the second coupler are connected to thesecond polarization-maintaining passive optical fiber, and the output end is connected with the output end of the second coupler; and the first coupler connects the NALM ring and the main ring. According to the
laser, the NALM ring is fully utilized for
mode locking, the repetition frequency is reduced, the stability is high, the structure is simple, and the cost is reduced.