Coherent supercontinuum generation by the high-order soliton molecules amplification

Abstract

We present an experimental investigation of the amplification of soliton molecules (SM) in an erbium-doped all-fiber amplifier. The amplification of a 10-pulse regime (with 509 fs pulse duration, 2.64 ps pulses separation, and low repetition rate Allan deviation of $1.98·{10}^{-9}$ at a 1 s averaging interval of 10-hour free-running measurement), generated by an erbium-doped fiber ring laser mode-locked through nonlinear polarization evolution (NPE), achieved an average power of 152 mW for stable amplified soliton molecules (SM). This process resulted in high coupling states with the same number of pulses and spectrum broadening leading to coherent supercontinuum generation. A wide spectrum with a comb-teeth structure in the range [1400 nm – 1700 nm] was obtained without pulse fission (10 pulses remains coherently in time domain with increased pulse duration up to 1.129 ps and without changing in the time separation between pulses), achieved through precise amplification of the SM without the use of high-nonlinearity fibers (HNLFs), and with low relative intensity noise of 0.0108 % in the range of 10 kHz – 1 MHz. Additionally, we demonstrate the characteristics of amplified SM as a function of the number of pulses, which was varied by adjusting the pump power of the master oscillator. The number of solitons remained constant during extra-cavity amplification; however, the results for SM amplification with 3, 4, 5, 7, 9, and 10 pulses indicate tunability in both the average and peak power of the amplified pulses. These findings position soliton molecules as a novel tool for optical frequency comb spectroscopy and with spectrum broadening suitable for potential application in f-2f interferometry.