Fig. 1. Design and optical characteristics of an ultrafast fiber laser with GHz repetition rate and multi-wavelength operation. (a) Schematic diagram of the laser where the black arrow indicates the hybrid fiber resonance to be expanded, comprising a section of 2.8-cm-long SM YDF, a piece of 3.4-cm-long passive graded index MMF, and a section of 2.8-cm-long SM YDF. LD, laser diode; ISO, isolator; WDM, wavelength division multiplexer; DF, dielectric film; SESAM, semiconductor saturable absorber mirror. (b) Calculated electric field intensity of the transverse modes along the propagation direction z in the structure of the diagram of the SMF-MMF-SMF configuration. (c) Measured transmission spectra of the hybrid resonant structure between 1015 and 1040 nm, showing the spectral period of 1.4 nm.

Fig. 2. Representative mode-locked spectra of the GHz laser using the SMF-MMF-SMF hybrid resonant structure. (a) Mode-locked spectrum with four main peaks operated at the launched pump power of 430 mW. (b) In order to verify the stability of (a), several spectra were recorded for 1 h under the same measured conditions. (c) The second spectral shape of the mode-locked laser was then observed, at the pump power of 434 mW and the polarization position was appropriately adjusted relative to (a). (d) The third mode-locked spectral shape can be attached as the pump power increased to 479 mW and the polarization position was further adjusted.

Fig. 3. Mode-locked SMS laser characteristics at a pump power of 479 mW. (a) Measured laser waveform with a fundamental repetition rate of 1.11 GHz. (b) RF spectrum in the 1.11238–1.11738 GHz region with a resolution bandwidth (RBW) of 10 Hz. (c) Measured autocorrelation trace. (d) Measured variation of the output power with the pump power.

Fig. 4. Spectral evolution of the GHz laser using the hybrid fiber structure versus the launched pump power: the optical spectra in the 1021.28–1032.78 nm region were acquired for several levels of the launched pump power: 322 (a), 340 (b), 358 (c), 375 (d), 393 (e), and 456 mW (f).