High-power wideband radio-frequency intensity modulated continuous wave laser

A high-power wideband radio-frequency (RF) intensity modulated continuous wave light source is demonstrated. The high-power dual-frequency light source is obtained via a dual-frequency laser signal seeding fiber power amplifier. A diode laser pumped dual-frequency laser is built as the seed and a diode laser pumped three-stage Yb³⁺ doped large mode area fiber power amplifier is used to enhance the output power to 50 W. In the dual-frequency seed laser, a coupled cavity composed of the Nd:YAG gain crystal and output coupler is used as the mode selector and enforces single longitude mode to oscillate. Two quarter wave plates are inserted in the laser cavity to lift the frequency degeneration of the two orthogonally polarized modes. By changing the angle between the fast axes of the two quarter wave plates, the frequency difference between the two orthogonally polarized modes can be tuned from 30 MHz to 1.5 GHz. The standard difference of beat frequency is 1.6144 MHz and stability is 1.52% when a frequency difference of output dual-frequency laser is 250 MHz. This stable dual-frequency seed signal is amplified via a diode pumped Yb³⁺-doped fiber power amplifier. In order to suppress amplified spontaneous emission and other nonlinear effects, a three-stage fiber amplification system is used. The first stage is a diode pumped fiber (5 m, 6/125 μm, NA = 0:13) power amplifier. The pump power is fixed at 600 mW. The input dual frequency signal is 3.2 mW, and it is amplified to several hundred mW by the first fiber power amplifier. The second fiber amplifier is a diode laser pumped fiber (5 m, 10/125 μm, NA = 0:075/0:46) amplifier. The pump power is fixed at 10 W, and the dual frequency signal is amplified to sub watts after the second fiber amplifier. A 5 m large mode area fiber (25/250 μm, NA = 0:065/0.46) is used in the final amplification. A maximum amplified power of 50.2 W is obtained when the pump power is 70 W in the experiment. The signal-to-noise ratio of the beat note increases from 25 dB to 40 dB via amplification. The output power fluctuation of the amplified signal at 50 W is smaller than 0.1 W during 30 min. The RF frequency stability is well maintained during the amplification, and the beat-note frequency instability is 1.777 MHz. This high-power dual-frequency light source with wide beat note frequency bandwidth has potential applications in dual-frequency coherent lidar system for long distance ranging and imaging or underwater detections after the frequency has been doubled to 532 nm.