Non-Hermitian mode-locking synthesized by parity-time and anti-parity-time symmetric modulations.

We study the pulse characteristics in a laser mode-locked by active modulators with non-Hermitian driven signals. The signal assembles a parity-time ( ) symmetric and an anti-parity-time ( ) symmetric function with fundamental and harmonic frequencies, respectively, inducing the complex coupling between modes in the frequency domain. A one-dimensional synthetic lattice is used to analyze the spectral mode coupling. By enlarging the weight and harmonic order of the part of the signal, the optical spectrum can be adjusted from redshift to blueshift. Simultaneously, the pulse duration and spectral width are shortened and broadened, respectively. The work explores the role of non-Hermitian modulation in the mode-locked laser area.