With the development of space technology, spatial data transmission rate is becoming a bottleneck associated with its application. Spatial coherent optical communication technology has become a popular research topic in many countries because of its high communication sensitivity, anti-interference ability, and high confidentiality. In this technology, the homodyne coherent optical communication system theoretically exhibits optimal sensitivity and anti-interference ability, but simultaneously requires a complex phase-locked closed-loop system and a local oscillator laser with large linewidth and laser-frequency-tuning bandwidth. In the 1550-nm band, it is difficult for the conventional lasers to simultaneously satisfy the narrow linewidth and high tuning bandwidth requirements. Herein, a narrow linewidth seed source combined with external electro-optic modulation and narrowband grating filtering is used to create a laser source exhibiting a spectral signal-to-noise ratio of approximately 28 dB, a linewidth of approximately 5 kHz, and a laser-frequency-tuning bandwidth of approximately 1.5 MHz.