Recently, the research team from the Joint Laboratory on High Power Laser Physics at the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, has made significant progress in the measurement and control of inter-beam carrier-envelope phase (CEP) and timing jitter for coherent beam combining of few-cycle femtosecond lasers. The team proposed a novel measurement and control technique based on spectral interference. The findings were published in High Power Laser Science and Engineering under the title "Simultaneous realization of time and carrier-envelope phase synchronization for an ultra-intense few-cycle laser pulse coherent combining system."
High-energy few-cycle femtosecond lasers hold substantial value in strong-field physics research, and coherent beam combining stands as a direct and efficient method to enhance the energy of such lasers. Due to the unique temporal characteristics of few-cycle femtosecond pulses, the efficiency and stability of coherent combining are highly susceptible to inter-beam CEP differences and timing jitter. Therefore, precise measurement and control of these two parameters are critical for achieving stable and efficient coherent combining.
To enable simultaneous measurement of inter-beam CEP difference and timing jitter, the team developed a spectral interference-based technique using quadratic function symmetry axis phase fitting. This method rapidly calibrates the timing jitter between two few-cycle femtosecond laser beams while simultaneously acquiring the CEP difference. Theoretical analysis indicates that this approach achieves a timing resolution on the order of tens of attoseconds and a CEP measurement precision of tens of milliradians.
Leveraging this technique, the team constructed a coherent beam combining system for few-cycle femtosecond lasers using a Ti:sapphire mode-locked femtosecond laser. They implemented closed-loop feedback control for inter-beam CEP difference and timing synchronization jitter between two laser pulses. Employing a tiled-aperture coherent beam combining architecture, they achieved far-field coherent combining of the two few-cycle femtosecond laser beams. With closed-loop control enabled, the standard deviation of inter-beam timing jitter was reduced to 42 attoseconds (as), while the CEP difference was adjusted to 0 mrad, resulting in a far-field coherent combining efficiency of 98.5%.
The team also demonstrated far-field interference fringes after beam combining and characterized the variation in combining efficiency as the inter-beam CEP difference was continuously tuned over a range of π radians,verifying the system’s precise control over timing synchronization and inter-beam CEP difference.
Moving forward,the team plans to scale the system to combine a larger number of few-cycle femtosecond laser beams,extending this measurement technique to simultaneously monitor CEP and timing synchronization across multiple channels. This work will provide crucial technical support for generating high-energy few-cycle femtosecond lasers.
This research received funding from the National Key R&D Program of China (International S&T Innovation Cooperation Project),the National Natural Science Foundation of China,and the Chinese Academy of Sciences’Strategic Priority Research Program (Category A).
