Figure 2. Experimental optical spectra and fundamental radio frequency (RF) spectra of the laser operating on four typical wavelengths.

(a,b) The four typical laser outputs at the pump power P ≈ 13 mW, achieved through the appropriate adjustment of the polarisation controller, for λ₁ (black), λ₂ (red), λ₃ (blue), and λ₄ (dark yellow). The in-line polarisation controller with low insertion loss can adjust the pulse wavelength by means of the polarisation dependent loss. (a) Optical spectra of the laser at four different wavelengths λ_1–4. The central wavelengths of λ_1–4 are 1556.36, 1558.25, 1561.45, and 1564.25 nm, respectively. (b) Fundamental RF spectra with the resolution of 1 Hz and the span of 100 Hz for the corresponding λ_1–4. The fundamental repetition rates of λ_1–4 are 5.733487, 5.733062, 5.732286, and 5.731641 MHz, respectively.