High-power ultrafast lasers are critical in frontier science, industrial manufacturing, and medical applications. Coherent beam combination (CBC) is a leading approach to overcoming the limitations of single-channel amplifiers, such as nonlinear and thermal effects, enabling further power and energy scaling. Among CBC methods, coherent polarization beam combination offers flexibility in handling varying input beam power ratios by utilizing polarization beam splitters. Unlike the conventional H?nsch?Couillaud (HC) detector-based phase control approach, the stochastic parallel gradient descent (SPGD) algorithm simplifies the system design by requiring only a single detector, providing enhanced scalability and stability.

The seed signal, centered at a wavelength of 1040 nm, is temporally stretched using a chirped fiber Bragg grating (CFBG) and reduces the repetition rate of 2 MHz with an acousto-optic modulator. After power amplification by a single-mode fiber pre-amplifier, the signal is split into 16 channels. Each channel undergoes two-stage amplification using polarization-maintaining fiber amplifiers. Precise optical path compensation is achieved by introducing a spatial delay line before the main amplifier of each channel. The 16 polarized beams are combined in a binary tree configuration using polarization beam splitters and half-wave plates, sequentially merging s- and p-polarized beams into a single beam. The combined beam is subsequently de-chirped using a folded Treacy grating compressor. A portion of the signal is detected by a photodetector, and feedback is provided to fiber stretchers for phase synchronization across all channels using the SPGD algorithm (Fig. 1).

The combined system achieves an average output power of 15 W, with a high combination efficiency of 94%. In open-loop mode, the combined beam intensity fluctuates with power changes [Figs. 2(b)?(d)], while in closed-loop mode, it stabilizes into a Gaussian profile [Fig. 2(a)]. The phase control system demonstrates long-term stability, as evidenced by the normalized temporal intensity profile in close-loop operation [Fig. 2(e)], with the calculated phase residual error being λ/21. The combined beam has a central wavelength of 1036 nm with a 3 dB bandwidth of 8.9 nm [Fig. 3 (a)]. After compression, the pulse duration is reduced to 633 fs [Fig. 3(b)].

In this paper, we achieve an efficient coherent polarization beam combining of 16-channel femtosecond fiber lasers. The results confirm the feasibility of a multi-channel filled-aperture coherent polarization beam combination for ultrafast lasers based on the SPGD algorithm. It is anticipated that by further optimizing the pointing error of each channel, higher power output and improved efficiency can be realized.