Recently, a research team from the Joint Laboratory of High Power Laser Physics at the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has made new progress in the measurement and control of inter-beam carrier-envelope phase (CEP) and temporal jitter in coherent combining of few-cycle femtosecond lasers. They proposed a measurement and control technique based on spectral interferometry. The related results 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 have important applications in intense field physics research, and coherent combining technology is a direct and efficient approach to increasing the energy of such lasers. The unique temporal characteristics of few-cycle femtosecond pulses make their coherent combining efficiency and stability highly susceptible to interference from inter-beam CEP differences and temporal synchronization jitter. Therefore, measuring and controlling these two factors is key to achieving stable and efficient coherent combining.
Figure 1 Optical path of a far-field coherent beam combining system for few-cycle femtosecond lasers
Figure 2 Time-synchronized measurement of inter-beam CEP difference and temporal jitter using quadratic function symmetry axis phase fitting based on spectral interferometry
To simultaneously measure the inter-beam temporal jitter and temporal jitter of two few-cycle femtosecond laser beams, the research team proposed a quadratic function symmetry axis phase fitting method based on spectral interferometry. This method rapidly calibrates the inter-beam temporal jitter of two few-cycle femtosecond laser beams and simultaneously obtains the CEP phase difference, enabling simultaneous measurement of both. Theoretical analysis demonstrates that this measurement method achieves a temporal resolution of tens of attoseconds and a CEP phase measurement accuracy of tens of milliradians. Based on this method, the research team constructed a few-cycle femtosecond laser coherent beam combining system using a Ti:sapphire mode-locked femtosecond laser. Closed-loop feedback control of the inter-beam CEP difference and temporal jitter of the two laser pulses was implemented. Using a tiled aperture structure, the team achieved far-field coherent combining of two few-cycle femtosecond laser beams. After enabling closed-loop control, the standard deviation of inter-beam temporal jitter was kept to 42 as. By adjusting the inter-beam CEP difference to 0 mrad, the far-field coherent combining efficiency reached 98.5%. During the experiment, the research team also demonstrated how the far-field post-combining interference fringes and combining efficiency change with continuous adjustment of the inter-beam CEP difference within a range of π, verifying the coherent combining system's ability to measure and control temporal synchronization and inter-beam CEP differences. Next, the research team will expand the number of few-cycle femtosecond laser beams and apply this measurement technique to a larger number of inter-beam CEP and temporal synchronization measurements, providing technical support for achieving high-energy few-cycle femtosecond lasers.
Figure 3 (a) After closed-loop feedback control, inter-beam temporal jitter is significantly suppressed; (b) Temporal jitter frequency spectra before and after closed-loop feedback control.
Figure 4 (a) Changes in far-field interference fringes with inter-beam CEP adjustment; (b) Changes in combining efficiency with inter-beam CEP adjustment. (c)-(e) Far-field interference fringes when the beam combining efficiency is 98%, 92%, and 85.6%, respectively
