A pulse-train laser refers to a laser that has a “continuous” pulse output over a period of time with multiple sub-pulses included in a pulse envelope. The characteristics of intermittent operation can, to some extent, alleviate the thermal effect of continuous laser output. However, in the development process of pulsed lasers, the background noise between pulses and the gain saturation effect of the amplification stage restrict the further development of pulsed lasers toward high peak power. Currently, pulse trains are mostly generated using electro-optical modulators (EOMs) and free-space acousto-optic modulators (AOMs) to chop the continuous laser output. This out-of-cavity modulation method can directly control the amplitude and interval of sub-pulses in the pulse-train envelope. However, background noise between sub-pulses remains a problem. Simultaneously, the obtained single-pulse energy requires a system gain of 10⁷?10⁹ levels to generate high-energy output at the nanojunction level. Therefore, investigating high extinction ratio pulse-train-laser generation technology with single-pulse energy above the μJ level, along with flexible and adjustable time-domain parameters, is of great significance for the development of pulse-train lasers in terms of high repetition rate and high peak power.

In this study, a semiconductor optical amplifier (SOA) is used to modulate a 1064-nm continuous laser source outside the cavity to produce a pulse-train laser. We use the SOA as a modulation device to chop 1064-nm continuous light from distributed feedback (DFB) lasers under amplification and absorption with and without trigger signal excitation to generate optical pulses. Through measurements of the output spectrum of the SOA, the signal-to-noise ratio (SNR), extinction ratio, and other SOA performances are analyzed. The repetition frequency, pulse width, and single-pulse energy of the output pulse-train laser are controlled by designing the time-domain parameters of the trigger signal and the power of the input light. By pre-amplifying the seed light through an ytterbium-doped fiber amplifier (YDFA), we can obtain sufficient energy for the nanosecond pulse-train seed laser.

Results show that when the input power and center wavelength are 10.8 mW and 1064.15 nm, respectively, the SNR of the SOA output is 13.02 dB. Approximately 95.25% of the SOA output power derives from signal light amplification power, and the extinction ratio is 41.29 dB (Fig. 4). The pulse-train laser beam generated through SOA modulation has good quality, with beam quality factor in horizontal direction (Mx2) of 1.11 and beam quality factor in vertical direction (My2) of 1.15, and no significant spectral broadening is observed (Fig. 7). When the triggering signal is adjusted, a high-beam-quality 1064-nm pulse-train laser output is achieved with a sub-pulse repetition frequency of 10?100 kHz (Fig. 6), sub-pulse width of 8.6?88.2 ns, and single-pulse energy of 0.892?6.124 μJ (Fig. 9). The maximum average power is 86.4 mW, and the output instability is 1.66% (Figs. 10 and 11). The Mx2 and My2 are 1.12 and 1.16, respectively (Fig. 13). These results show that this study achieves a nanosecond pulse-train laser using an SOA with high beam quality, extinction ratio above 40 dB, and high power.

Using an SOA with a high extinction ratio to modulate a continuous laser source outside the cavity and generate a pulse-train laser, this study realizes a 1064-nm pulse-train seed laser with an extinction ratio of over 40 dB. Flexible adjustable time-domain parameters are obtained by adjusting the trigger signal amplitude and time-domain parameters of the SOA. The pulse-train sub-pulse width is 8.6?88.2 ns, and the single-pulse energy is 0.892?6.124 μJ. The repetition frequency is 10?100 kHz, and the duration of the pulse train reaches the millisecond level. These results verify the effectiveness of using an SOA as a modulator to modulate the continuous laser source outside the cavity and in turn produce a pulse-train laser. When the trigger signal has a pulse width of 100 ns, repetition frequency of 100 kHz, and amplitude of 1.8 V, the average output power of the pulse-train laser reaches 86.4 mW with an energy instability of 1.66%. Research has shown that pulse-train lasers using an SOA as a modulation device can achieve higher single-pulse energy, stable output, and flexible and tunable time-domain parameters. This method of generating pulse-train lasers using modulation devices with a high extinction ratio provides a new research approach for the design of high average power, adjustable time-domain parameters, and high SNR pulse-train lasers.