Anomalous slowdown of pump light in Raman fiber lasers

Usually, the pump light in lasers should perform fast light owing to operating in the absorption band. In this study, we observe and demonstrate anomalous slowdown of the pump light in a Raman fiber laser. Experiments show that the pump light can be slowed down to sub-nanoseconds at a repetition rate of 50-500 MHz. Theoretical analysis shows that the hole-burning effect is formed at the Raman gain spectrum in the saturation regime, which imposes on the pump light by normal dispersion. Consequently, the pump light experiences an unusual slow light effect rather than the fast light effect after absorption. We believe it has promising potentials in the improvement of ultrashort pulse generation, and may have significant influence on improving the conversion efficiency in pulse-pumped laser systems.