A free space optical communication system at 2 μm band was preliminary demonstrated. The fiber laser source is a homemade continuous wave thulium-doped fiber laser with single-wavelength output. The center wavelength of the fiber laser is 2048.15 nm, and the fluctuation of the wavelength and optical power is less than 0.02 nm and 0.893 dB, respectively. By modulating and demodulating the radio frequency signal, the text and real-time video transmission was realized. The transmission distance in free space is ~1 m, and the transmission rate is less than 66 kbit/s. With further improvement, the system may be well suited in free space optical communication system.
Transverse Mode Instability (TMI) effect has become the biggest obstacle limiting the further improvement of output power and beam quality in high-power fiber lasers. For 2μm-band high-power thulium-doped fiber laser, the simulation is carried out using semi-analytical theoretical model. The results show that the TMI threshold is negatively correlated with the core radius, signal wavelength and positively correlated with the pump wavelength. On the other hand, reducing the relative intensity noise can slightly but not obviously improve the TMI threshold. The relationship curve between them is nearly logarithmic. The simulation provides an effective reference for further improving the TMI threshold.
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