In the pulsed single-frequency fiber master oscillator power amplifier (MOPA), the stimulated Brillouin scattering (SBS) effect severely limits the increase of peak power. Although both soft glass large-mode-area fibers and tapered fibers can effectively suppress SBS, the complex manufacturing process and the relatively high requirements for use restrict their applications to some extent. Actually, large-mode-area silica fibers have been widely used in the preparation of fiber lasers due to their excellent compatibility. However, in previous work, the low doping concentration of rare-earth ions in silica fibers leads to low SBS thresholds of laser systems. In this work, a commercial silica fiber with high doping concentration is used as the gain medium, and the trade-off between SBS threshold and laser efficiency is investigated by optimizing fiber length. As a result, a single-frequency laser output with a peak power of 91 kW at 1064.4 nm is realized.

A pulsed single-frequency MOPA system based on a silica fiber is built. Firstly, an electro-optic intensity modulator (EOIM) is used to modulate a continuous-wave (CW) single-frequency Yb³⁺-doped fiber laser, so as to generate a pulse train with a pulse width of 2.4 ns and a pulse repetition frequency of 20 kHz. The CW single-frequency fiber laser with a central wavelength of 1064.4 nm consists of a single-frequency laser seed with an output power of 30 mW and a core-pumped pre-amplifier which can boost the power of the CW laser seed to 70 mW. Due to the insertion loss of the EOIM and the low duty cycle of the modulated laser, the weak signal pulses are pre-amplified using two stage core-pumped Yb³⁺-doped pre-amplifiers with a 0.5-m Yb³⁺-doped fiber and a 0.6-m Yb³⁺-doped fiber (LIEKKI, Yb-300-6/125) as the gain medium, respectively. The pre-amplified laser is then modulated by an acousto-optic modulator (AOM) which is synchronized to the EOIM to remove the amplified spontaneous emission (ASE) component for higher signal-to-noise ratio (SNR) in the time domain. The laser output with an average power of 0.16 mW, corresponding to 3.3 kW peak power, is obtained after the AOM, which is further amplified by two stage cladding-pumped pre-amplifiers. The first stage cladding-pumped pre-amplifier and the second stage cladding-pumped pre-amplifier use a piece of 1.5-m Yb³⁺-doped fiber (Nufern, LMA-YDF-10/130-M) and a piece of 2-m Yb³⁺-doped fiber (Nufern, LMA-YDF-20/130-VIII) as the gain medium, respectively. The average power of the pulsed single-frequency laser seed is boosted to 45 mW and 120 mW under a pump power of 4.6 W and 2.6 W in these two stage cladding-pumped pre-amplifiers, respectively. Before being injected into the main amplifier, the single-frequency laser reaches a pulse energy of 6 μJ, with the pulse width remaining 2.4 ns and the peak power being 2.5 kW. In the main amplifier, a piece of Yb³⁺-doped silica fiber (Liekki, Yb-1200-30/250) with a core diameter of 30 μm, a cladding diameter of 250 μm, and an absorption coefficient of 14 dB/m at 976 nm is used as the gain medium. The coiling diameter of the active fiber is controlled to 14 cm in order to optimize the beam quality.

With a 0.9-m active fiber used in the main amplifier, an average power of 4.37 W (Fig. 3) is obtained under a pump power of 21 W, which corresponds to an optical-to-optical efficiency of 21% [Fig. 2(c)]. The maximum pulse energy of 0.22 mJ is achieved with a pulse repetition frequency of 20 kHz, which corresponds to a pulse peak power of 91 kW (Fig. 3). The pulse width of the pulsed single-frequency laser seed modulated by the AOM and the main amplifier output with a peak power of 91 kW are both 2.4 ns, which manifests that there is no obvious distortion of the pulse waveform during the amplification process [Figs. 4(a) and 4(b)]. The measured spectral linewidth of the laser seed, namely, the full width at half maximum (FWHM), is 201 MHz [Fig. 5(a)], which is consistent with its theoretical transform-limited level. However, the spectral linewidth after amplification broadens to 279 MHz due to the self-phase modulation (SPM) effect [Fig. 5(b)]. The SNR of the laser at the maximum output power is 45 dB, and the central wavelength of the signal is 1064.4 nm [Fig. 6(a)]. Before the main amplification, the beam quality factors Mx2 and My2 in the x and y directions are 1.31 and 1.33 [Fig. 6(b)], respectively, while the beam quality factors Mx2 and My2 in the x and y directions are both 1.44 at a 91-kW peak power laser output of the main amplifier [Fig. 6(c)], which indicates that the pulsed single-frequency laser undergoes no obvious degradation in beam quality and maintains the near-diffraction-limited laser output.

In this work, a high-peak-power pulsed single-frequency fiber MOPA based on Yb³⁺-doped silica fiber is demonstrated. The influence of the active fiber length in the main amplifier on the peak power, the threshold of SBS, and the optical-to-optical efficiency of the pulsed single-frequency fiber laser is investigated experimentally. With a 0.9-m Yb³⁺-doped silica fiber used in the main amplifier, the pulsed single-frequency laser with an average power of 4.37 W is obtained at a central wavelength of 1064.4 nm under a launched pump power of 21 W with a pulse width of 2.4 ns and a pulse repetition frequency of 20 kHz. The maximum pulse energy is 0.22 mJ, which manifests that there is no obvious CW ASE component, and the corresponding peak power is 91 kW. The spectral linewidth is 279 MHz, and the SNR is 45 dB, with the beam quality factor M² being 1.44 at the maximum output power.