Direct measurement of pump-induced phase modulation in erbium-doped fibres

ing spectrum with increasing of the pumping current. Within the limits of the free-running cw generation regime (I, = 120 mA), no evidence of chirping in the spectrum is observed. The modal chirp appears when passing to the mode-locking regime (I, = 130 mA), and its value increases with pumping current. As mode-locking develops, the laser spectrum becomes markedly asymmetric (I, = 150 mA). This effect can be explained by the fact that the dispersion of the laser active medium decreases with wavelength. At I, = 150 mA, the minimal value of time-bandwidth product ATAV = 0.52 was registrated. It should be noted that the optical pulses obtained are not transform-limited ones due to the chirp effect and it is possible to compress them in a medium with a proper dispersion. With further increase in pumping current (I, = 170 mA), the lasing spectrum broadens (with the corresponding increase in the time-bandwidth product) and becomes symmetric again, which is supposedly due to gain saturation for longer wavelength modes. An important advantage of the lasers investigated is that their entire spectrum lies in the amplification band of erbiumdoped fibers. Being combined with fiber amplifiers, these lasers could serve as a source of soliton pulses with a very high repetition rate. It should be also noted that the measurement of laser characteristics was being carried out during approximately 50 hours of operation under cw pumping and no degradation of spectra or pulse parameters was observed. CW passive mode-locking was observed for the first time in InGaAsP laser diodes with saturable absorber regions created by the ion implantation. Optical pulses of 650 fs width at 110-GHz repetition rate were obtained in the spectral range of 1.53-1.54 pm. No changes in spectra and autocorrelation traces were observed after 50 hours of cw operation. *Institute of Physics and Technology, Russian Academy of Sciences, 26 Politekhnicheskaya str., St. Petersburx 194021