Over 30 W single-frequency all-fiber amplifier at 1120 nm with high ASE suppression

A 1120 nm linearly polarized all-fiber single-frequency amplifier based on Yb-doped fiber (YDF) is reported. A maximum output power of 30 W is achieved by 976 nm clad-pumping and 1018 nm core-pumping, with corresponding slope efficiencies of 51.9% and 70.0%, respectively. With the 976 nm clad-pumping configuration, a signal to amplified spontaneous emission (ASE) ratio of more than 45 dB is realized at maximum output power. The polarization extinction ratio of the amplifier is measured to be more than 21 dB in the whole power scaling process. In addition, the intensity noise of the amplifier at maximum output power is also characterized to be around -130dBc/Hz at frequencies above 2 kHz and is comparable to that of the seed laser. Moreover, the deterioration of frequency noise and linewidth of the amplified laser is negligible. To the best of our knowledge, this is the highest output power of a single-frequency all-fiber amplifier that exploits the fundamental transition of YDF at 1120 nm, even without any external intervention for suppressing ASE in the short wavelength range.