In this paper, we propose a new, to the best of our knowledge, saturable absorber (SA)
based on sodium carbonate (Na2CO3) for producing an ultrafast
mode-locked stretched pulse in a passively erbium-doped fiber laser at
near-zero dispersion. The solid film of a Na2CO3-SA was fabricated by the drop-casting
method using polyvinyl alcohol as a host polymer. The modulation depth
of the proposed SA, which was measured by a balanced twin detector
technique, was 2.3% with saturation intensity of 181MW/cm2. The mode-locking operation of the
EDFL-based Na2CO3-SA was observed at a pump power of
117 mW. A stable stretched pulse was generated by using the
proposed Na2CO3-SA. The laser can generate pulses
with a repetition rate and duration of 1.87 MHz and
820 fs, respectively, within a bandwidth of 6.6 nm. The
single pulse energy reaches up to 5 nJ, which is equivalent to
the average output power of 9.3 mW. Finally, to the best of our
knowledge, this is the first report on using the Na2CO3-SA for generating a stretched-pulse
mode-locked fiber laser.