Chaotic lasers have a strong anti-interference ability, wide bandwidth, and good randomness. They are widely used in chaotic secure communication, random number generation, and optical fiber sensing. A fiber laser can generate a chaotic laser by the nonlinear Kerr effect of the fiber. Visible-wavelength chaotic lasers can be used in detecting hemoglobin concentrations, blood flow velocity, and fat foreign bodies, and in underwater ranging and other fields. In this study, a green chaotic laser with a wavelength of 535 nm is developed using a frequency-doubled ytterbium-doped fiber chaotic laser and nonlinear crystal. The output of the green chaotic laser at different powers is achieved by adjusting the pump current. The randomness of the green chaotic laser is analyzed using permutation entropy, skewness, and kurtosis.

Figure 1 shows the experimental setup of the 535 nm green chaotic laser. The setup is composed of an ytterbium-doped fiber chaotic laser, fiber amplifier, and frequency-doubling crystal. The 1070 nm chaotic laser is generated by adjusting the polarization controller (PC) and pump current. The ytterbium-doped fiber chaotic laser is composed of a 980 nm pump source 1 (LD 1), wavelength division multiplexer 1 (WDM 1), ytterbium-doped fiber 1 (YDF 1), single-mode fiber (SMF), polarization-independent isolator 1 (ISO 1), and coupler (OC). The YDF 1 with a length of 4 m is used as the gain medium to produce a chaotic laser with a wavelength of 1070 nm. The SMF with a length of 500 m is used to enhance the nonlinear effect in the cavity. The chaotic fiber amplifier is composed of isolators 2 and 3 (ISO 2 and ISO 3), 980 nm pump sources 2 and 3 (LD 2 and LD 3), wavelength division multiplexers 2 and 3 (WDM 2 and WDM 3), a ytterbium-doped fiber 2 (YDF 2), a double-cladding ytterbium-doped fiber (DCYDF) with a cladding pump absorption of 4.95 dB/m @975 nm, and a cladding power stripper (CPS). The YDF 2 with a length of 4 m is used for pre-amplification. The DCYDF with a length of 5 m is used to amplify the chaotic laser. Frequency doubling of the chaotic laser is realized using a barium metaborate (BBO) crystal, harmonic beam splitter (M), and two focusing lenses (L) with focal lengths of 150 mm.

The output power of the chaotic laser with a wavelength of 1070 nm is analyzed. Results show that the power output of the 1070 nm chaotic laser with pump currents has a good linear relationship. In the amplification process, the 1070 nm chaotic laser exhibits obvious spectral broadening (Fig. 3). The randomness of the 1070 nm chaotic laser is then analyzed. Results show that the permutation entropy is stable at 0.997881 as the pump current changes, indicating that the chaotic laser has good randomness. The randomness of the chaotic laser is improved after amplification (Figs. 2 and 4). The skewness and kurtosis of the 1070 nm chaotic laser fluctuate randomly with changes in the pump current. The output characteristics of the 535 nm chaotic laser are then analyzed. Results show that the power output of the 535 nm chaotic laser with the pump current has a good linear relationship, and the maximum output power is 2.5 mW (Fig. 5). The randomness of the 535 nm chaotic laser is then studied by permutation entropy. When the pump current is 4.00?4.75 A, the permutation entropy is found to be stable at 0.988579. The permutation entropy first decreases and then is stable when the pump current is 5?10 A (Fig. 6). For the 535 nm chaotic laser, the skewness and kurtosis at the pump current of 4–10 A fluctuate with the pump current (Fig. 7).

In this study, a green chaotic laser with a wavelength of 535 nm is realized using a frequency-doubled ytterbium-doped fiber chaotic laser and nonlinear crystal. The randomness of the chaotic laser before and after frequency doubling is analyzed by permutation entropy, skewness, and kurtosis. Fundamental-frequency light with a stable randomness is experimentally obtained. The 535 nm chaotic laser with a permutation entropy of 0.985150 is realized by frequency doubling, and the skewness kurtosis tends to be normally distributed. The 535 nm chaotic laser exhibits good randomness and conforms to a normal distribution. The research results are significant for the study of the randomness of chaotic lasers in the visible wavelength range and can be used for applications in sensing and biomedicine.