978 Nm All-Polarization-Maintaining Mode-Locked Fiber Laser Based on Phase-Biased Nonlinear Amplifying Loop Mirror

Terahertz (THz) wave generation based on ultrafast laser is one of the most predominant and popular technologies. For THz generation by photoconductor antenna and optical rectification, higher optical to THz conversion efficiency usually requires ultrafast laser excitation into semiconductor or nonlinear crystal, therefore it's of great desire to develop ultrafast laser with high performance. CW Yb-doped lasers at 980 nm have been applied to generate 0.5 to 6 THz wave by DFG technique. To meet the demand of some THz wave generation (e.g. BNA crystal, gold nanoplasma), 980 nm ultrafast fiber lasers are ideal pump sources and in desire to be developed. Here we demonstrate a 978 nm robust all-polarization-maintaining (PM) picosecond fiber laser mode-locked by phase-biased nonlinear amplifying loop mirror (PBNALM). By introducing a nonreciprocal $-\pi/2$ phase shifter into the fiber loop, a self-starting stable mode-locking is obtained with the center wavelength of 978.14 nm and a 3-dB bandwidth of 0.43 nm. The maximum average output power is 14.5 mW with pulse duration of 8.1 ps and repetition rate of 45.438 MHz. After a master oscillator power amplification system, the average output power reaches 2.12 W, which is injected into a PPLN crystal for second harmonic generation and 52 mW 489 nm ultrashort laser is obtained. This is, to the best of our knowledge, the first report of PB-NALM-based mode-locked fiber laser at 978 nm. The 978 nm ultrafast fiber laser and SHG to 489 nm have great potential in terahertz emission by shorter NIR and blue light.