Fig. 1. Schematic of two different NOGM structures. (a) Integrated NOGM Yb-Raman fiber amplifier. (b) Traditional NOGM Raman fiber amplifier.

Fig. 2. Pulse evolution at the repetition rate of 10 MHz. (a) Spectral and (b) autocorrelation trace evolution versus the pulse energy. (c) Schematic of temporal evolution of the pump and Raman pulses. (d) Ratio of the pump, first-order Stokes and second-order Stokes pulse energy versus the total energy.

Fig. 3. The first-order Stokes pulse characteristics with and without SF-CW seed input. With the SF-CW seed input: (a) spectrum; (b) autocorrelation trace of the compressed pulse (inset: before compression); (c) pulse train; (d) radio-frequency spectrum with a resolution of 10 Hz (inset: zoom-out spectrum). Without the SF-CW seed input: (e) spectrum; (f) autocorrelation trace; (g) pulse train; (h) radio-frequency spectrum with a resolution of 10 Hz (inset: zoom-out spectrum).

Fig. 4. Evolution of pulse energy in the 3 m YDF. The solid line (S1) represents spectral components over 1100 nm with gain from both the YDF and SRS, while the dashed line (S2) indicates spectral components over 1100 nm with gain only from SRS. The blue line corresponds to the 1065 nm pump pulse, the green line represents the 1121 nm Stokes pulse and the orange line denotes the 1178 nm Stokes pulse.

Fig. 5. Temporal evolution of the pump pulse and the first-order Stokes pulse (left-hand column: YDF gain was present for wavelengths over 1100 nm; right-hand column: YDF gain was absent for wavelengths over 1100 nm). (a), (b) Evolution of the pump pulse. (c), (d) Evolution of the first-order Stokes pulse. (e), (f) Temporal profile after compression and before compression (the inset figure) when achieving 529 nJ Raman pulse energy.

Fig. 6. Characteristics of the first-order Stokes pulse with 11.6 μJ pulse energy. (a) Spectral evolution in the 2-m-long YDF. (b) Pulse spectrum and (c) temporal profile after compression and before compression (the inset figure) at the length of 1.66 m.