Fig. 1. The concept of phase summation in SFG for single-frequency upconverted laser generation. The spectral and phase evolutions at different stages of the laser are schematically illustrated.

Fig. 2. The experimental setup includes two phase-modulated seed lasers, a two-stage Raman fiber amplifier and a frequency mixing unit. EOM, electro-optic phase modulator; WDM, wavelength division multiplexer.

Fig. 3. Spectra of the fundamental lasers. (a) Spectra of two single-frequency and phase-modulated lasers measured with the FPI. (b) Optical spectra of the seed lasers and main amplifier.

Fig. 4. Power curves of the fundamental Raman amplifier and upconverted single-frequency laser by sum-frequency generation. (a) The output and backward light power of the Raman fiber amplifier as a function of 1120 nm pump power. (b) The power and conversion efficiency of the single-frequency upconverted laser as a function of the fundamental laser power. Insert: spectrum of the generated 590.3 nm laser.

Fig. 5. The spectral evolution of upconverted lasers at different phase offsets between sinusoidal signals. (a) The simulated spectra of upconverted lasers. (b) The measured spectra with the FPI. The intensity is normalized by the intensity of the single-frequency case.

Fig. 6. Spectra of upconverted output. (a) The fine spectrum and FPI scan signal of the single-frequency upconverted laser with phase-modulated fundamental lasers at output power of 17.3 W. (b) The fine spectrum and FPI scan signal of the upconverted single-frequency laser without phase modulation.