Objective High- average- power and high- repetition- rate femtosecond fiber lasers are widely used in industrial and scientific domains. However, the presence of excessive nonlinearity and transverse mode instability poses a constraint further scaling of energy and power within the fiber lasers. Coherent beam combination (CBC) has risen as a viable solution to the constraint, enabling the extension of average power and pulse energy limits while preserving beam quality. Under ideal conditions, the laser power and energy of the combined beam from N channels can reach N times that of a single amplifier. However, in real- world applications, variations in beam quality and discrepancies in the spatiotemporal properties of the beams result in power losses during the combining process. These losses are quantified by the combining efficiency. Recently, an average power of 1 kW with a pulse energy of 1 mJ and an average power of 1 kW with a single pulse energy of 10 mJ are achieved through 8- channel and 16- channel fiber laser CBC, respectively. Yet, the proliferation of combination paths not only increases system complexity but also affects stability. Moreover, due to gain narrowing and incomplete dispersion compensation, compressed pulses typically exceed 300 fs, presenting hurdles in achieving Fourier limit pulse duration. Although methods such as spectral shaping and post- compression can further shorten the pulse duration, these methods undoubtedly further increase the complexity and operational difficulty of the system. Hence, we focus on reducing the number of combination paths and improving dispersion compensation to achieve clean ultrashort femtosecond pulses while maintaining the existing power level. Methods To streamline the number of combination paths, enhancing the power of a single amplifier is crucial. By boosting the power of a single amplifier beyond 200 W, the number of required combination channels can be reduced by a factor of 2 to 3. An ultrafast femtosecond fiber system comprising four coherently combined large mode- area rod- type photonic crystal fibers as the main amplifier is constructed. To ensure high beam combining efficiency and subsequent high- quality pulse compression, it is necessary to strictly control the power of each amplifier to ensure the same B- integral. Meanwhile, due to the effect of nonlinear polarization rotation, a large amount of laser power cannot participate in beam combination. Circularly polarized amplification is an effective method to minimize nonlinear phase accumulation and ensure high beam combining power. The phase stabilization is achieved using H & auml;nsch-Couillaud (HC) detectors after beam combination. While spectral pre- shaping of seed light is effective for optimizing the duration of compressed pulses, its practical operation is cumbersome. Hence, we use the tunable pulse stretcher (TPSR) to pre- compensate the dispersion of the seed light. This matches the pre- compensate dispersion with the accumulated dispersion during subsequent amplification and compression processes, further optimizing the pulse duration. Results and Discussions The average output power of each channel ranges from 215 to 222 W, with a fitted slope efficiency exceeding 65%. The maximum discrepancy between the channels is no greater than 3%. This precision enables our four- channel CBC system to achieve an output power of 776 W with a pulse energy of 0.97 mJ, and a combination efficiency of 89%. The active phase stabilization system ensures excellent power stability for the four- channel CBC fiber lasers, with a root mean square of 0.59 degrees o. Despite the differences in the output beam profiles of different channels, the combined beam still exhibits a circular Gaussian profile. The beam quality is analyzed by M-2 measurement using the 4 sigma-method showing an almost diffraction limit beam quality of M-2<1.25 on both axes. Remarkably, we accomplish these results using only four amplifiers, whereas previous researchers required eight or more, effectively reducing system complexity. After compression by gratings, the combined beam exhibits a pulse duration of 445 fs and significant high- order dispersion residue, as shown in Fig. 6. By optimizing dispersion, particularly high- order dispersion, using TPSR, we reduce the pulse duration to 227 fs and significantly increase the proportion of main pulse energy, as illustrated in Fig. 7. Spectral phase analysis shows a significant reduction in second to fourth order dispersion. The compression efficiency reaches 93.3 %, with compressed power at 724 W and pulse energy at 0.9 mJ. Conclusions We present an ultrafast femtosecond laser system based on CBC. The system achieves an average power of 724 W and a pulse energy of 0.9 mJ through CBC of four channels. This approach effectively overcomes the power limitation of a single rod- type photonic crystal fiber. By employing an active phase stabilization system, the combination efficiency of the system reaches 89 degrees o, while the combined beam maintains good beam quality with M-2<1.25. Furthermore, the use of TPSR for pre- management of pulse dispersion enables precise compensation of the residual dispersion after pulse compression, successfully optimizing the pulse duration full width at half maximum (FWHM) from 445 to 227 fs. The system demonstrates the effective potential of coherent beam combining in achieving high average power and large pulse energy femtosecond lasers. In the future, it is expected that by increasing the chirp broadening of pulses, reducing the repetition rate, and incorporating spatial- temporal coherent beam combining, the peak power and energy of laser pulses can be further enhanced.
