Superposition- and interference-induced optical spectrum distortion in the figure-9 fiber laser

Kavli Affiliate: Xiang Zhang

| First 5 Authors: Xiang Zhang, Guochao Wang, Kangrui Chang, Haobin Zheng, Yongzhuang Zhou

| Summary:

The spectrum of the output pulses from the figure-9 laser typically exhibits
more distortion than the spectra from mode-locked lasers based on other
saturable absorbers and the spectrum of its intracavity pulses. Here, we
demonstrate two figure-9 lasers with repetition rates of 190.6 MHz and 92.4 MHz
and introduce the self-designed beam splitter with little spectral impact in
the fiber loop to output two interference-free pulses. By numerically
processing the spectra of these two pulses, the formation mechanisms of
specific spectral features are determined, and the features are consistent with
the experimental spectral features of the pulses from the other two ports.
Furthermore, by analyzing the pulse propagation of lasers through the
interference theory of the figure-9 laser, it is found that the superposition
and interference of spectra at the two output ports of the linear arm are the
reasons for the severe spectral distortion, rather than the commonly believed
nonlinear effects. On the beam splitter where interference occurs, the
$p$-components of the two intracavity light beams always interferes with equal
intensity, while the $s$-components usually interfere with non-equal intensity,
resulting in a large but stable spectral difference between the pulses inside
the cavity and the output pulses. These findings can provide new perspectives
for simulating spectra that closely resemble experimental results and deepen
our understanding of spectral evolution and pulse dynamics of the figure-9
laser.

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