Abstract
A normal dispersion fiber oscillator with 40-m-long erbium-doped gain fiber, 1.5 W pump power and intracavity liquid-crystal modulator (LCM) providing extra 2nd-order and 3rd-order spectral phase was demonstrated in different operation modes. The highest pulse energy and the broadest -20 dB spectral width were 58 nJ and 54 nm (the spectra of both operation modes are centered at 1560 nm), respectively. We also discovered significant Raman signal at 1669 nm (red shifted by 13 THz) at strong pump power and excessively long fiber in the cavity. The production of Raman signal also implies there is a risk of soliton explosion (causing abrupt collapse of mode-locked spectrum) and energy leakage. By a series of experiments, such as long-range intensity autocorrelation (IA), and radio-frequency spectrum measurement, the probable pulse shape was determined. Based on the estimated pulse shape, we estimate there is over 89 % of pulse energy leaking from the main pulse, substantially reducing the peak power.