Progress on high-energy hundreds-ns all-fiber single-frequency fiber master-oscillator-power-amplifier (MOPA) is mainly constrained by the stimulated Brillouin scattering (SBS) effect. The SBS can be generally suppressed by using large-mode-area active fibers with high dopant concentration so that short fiber length is allowed for power scaling. Whereas the insufficient extraction of population inversion due to the limited seed power induces a significant part of amplified spontaneous emission (ASE), which degrades the spectral purity. To avoid this when increasing the SBS threshold of the pulsed single-frequency laser, a fiber amplification scheme based on a hybrid active fiber structure is proposed here to achieve a high energy output. A piece of heavily Yb³⁺ -doped phosphosilicate fiber with a large core diameter is spliced after a piece of Yb³⁺ -doped silica fiber with a smaller core diameter. The difference in active fiber size introduces the loss to the reversed Stokes light, which is similar to the operating principle of an outstanding tapered fiber, thereby increasing the threshold of SBS while avoiding the complicated fabrication process. The signal which is pre-amplified by the low Yb³⁺ -doped silica fiber can effectively extract the population inversion in the phosphosilicate fiber, which exhibits the benefits on ASE suppression and efficiency improvement.

The 1064.44 nm continuous-wave (CW) laser with a power of 76 mW is modulated by an electro-optic intensity modulator (EOIM) to become a pulsed laser source with a pulse duration of 100 ns. A single-frequency pulsed laser source with an average power of 1 mW is obtained after two-stage Yb³⁺ -doped fiber core-pumped pre-amplification and an acousto-optic modulator (AOM) synchronized with the EOIM. The average power of the pulsed laser source is increased to 120 mW corresponding to the pulse energy of 0.024 mJ after two-stage Yb³⁺ -doped fiber cladding-pumped pre-amplification. In the power amplifier, a hybrid active fiber structure, consisting of a 30 cm long piece of Yb³⁺ -doped silica fiber with a core/cladding diameter of 35 μm/250 μm (pump absorption of 12 dB/m @ 976 nm) and a 30 cm long piece of heavily Yb³⁺ -doped phosphosilicate fiber with a core/cladding diameter of 50 μm/400 μm (pump absorption of 27 dB/m @ 976 nm) spliced after it, is employed as the gain medium. Meanwhile, the coil diameters of the hybrid active fibers are controlled to ~12 cm and ~20 cm, respectively, in order to optimize the beam quality. For comparison, a 60-cm silica fiber with the same fiber length as that of the hybrid active fiber structure and the 40-cm phosphosilicate fiber which maintains almost the same pump absorption as that of the hybrid active fiber, are also employed as the gain medium in the power amplifier, respectively. The pump power in different cases is increased until the reversed Stokes light appears.

The maximum pulse energy of 0.59 mJ is obtained with the hybrid active fiber structure at the pump power of 13 W and the PRF of 5 kHz [Fig. 2 (a)], and the corresponding average power is 2.95 W with the conversion efficiency of 22.7%. The measured average power of the pulsed laser is 3.05 W, indicating that there is only 0.1 W CW component in the laser output. However, the maximum pulse energy of only 0.327 mJ is obtained at the pump power of 11 W [Fig. 2 (a)] when a 60-cm silica fiber is employed as the gain medium, which indicates that in the hybrid active fiber structure, the SBS threshold is effectively raised by increasing the mode field area of the active fiber and introducing the loss to the reversed Stokes light. When a 40-cm phosphosilicate fiber is employed as the gain medium, the maximum pulse energy is 0.187 mJ, which is corresponding to the average power of 0.935 W and the conversion efficiency of only 10.4% [Fig. 2 (a)]. The measured average power is 1.67 W, which indicates that plenty of ASE and CW components exist, which decreases the signal-to-noise ratio (SNR) and the conversion efficiency. Therefore, in the hybrid active fiber structure, with the benefit of the pre-amplification of the signal by the silica fiber, the population inversion in the phosphosilicate fiber is effectively extracted and the conversion efficiency is enhanced. The pulse shape is measured at the pulse energy of 0.59 mJ, and the pulse duration is broadened to 104 ns from 100 ns (the pulse duration of the seed laser) (Fig. 3). The corresponding maximum peak laser power is ~5.33 kW, the measured spectral linewidth is 40 MHz (Fig. 4), the center wavelength is 1064.44 nm, the SNR is 41 dB (Fig. 5), and the measured beam quality in the x and y directions are 1.48 and 1.50, respectively (Fig. 6).

A 1064-nm single-frequency Yb³⁺ -doped pulsed all-fiber laser based on a hybrid active fiber structure is demonstrated. It can achieve a 0.59-mJ laser output with spectral linewidth of 40 MHz and beam quality of 1.50. The hybrid Yb³⁺ -doped active fibers with different core diameters are served as the gain media of the power amplifier to enhance the threshold of the SBS and inhibit the ASE. The calculated conversion efficiency is 22.7% and the measured SNR is 41 dB at the highest pulse energy. The hybrid active fiber structure provides a beneficial reference for further pulsed fiber amplifiers.