Coexistence of noise-like pulse and dark pulse in an Er/Yb co-doped fiber laser

The coexistence of a noise-like pulse(NLP)and a dark pulse was experimentally demonstrated in a net-anomalous dispersion Er/Yb co-doped fiber(EYDF)laser,for the first time,to our knowledge.The cavity was mode-locked by nonlinear polarization rotation(NPR)technique.Meanwhile,a Sagnac loop with a section of polarization-maintaining fiber(PMF)was used as a comb filter to enable multiwavelength pulse operation.When the PMF length was 0.3 m,an asymmetric two-peak spectrum with central wavelengths of 1565.3 and 1594.2 nm was obtained by adjusting polarization controllers(PCs).It is a composite state of NLP and dark pulse due to the cross-phase modulation between the two different wavelength components along orthogonal polarization axes.The two pulses are synchronized with a repetition rate of 7.53 MHz.By adjusting the PC in the Sagnac loop,the spectral ranges of NLPs and dark pulses can be tuned from 1560 to 1577.8 nm and from 1581.8 to 1605.4 nm,respectively.In addition,the pulse characteristics were investigated by incorporating the PMF with different lengths,where the coexistence patterns can be generated when the PMF lengths were 0.2 and 0.3 m.A longer PMF can lead to a narrowband comb filtering,which causes a larger loss and is not favorable for stable operation of the coexistence regime.This fiber laser demonstrates an interesting operation regime and has significant potential for numerous practical applications.