Effect of temporal partial coherence of flat-topped Gauss ultrashort-pulse lasers on coherent combination

Using quasi-monochromatic light interference theory, the far-field interference pattern is calculated for the combination of 2 × 2 flat-topped Gauss ultrashort-pulse lasers, and the effect of phase difference(PD) and temporal partial coherence on coherent combination is discussed. The results show that when the line-width of the beams is less than 100 nm, the far-field pattern and peak light intensity hardly change for ideal wavefront; when the line-width is more than 100 nm, the interference effect weakens gradually. With phase difference, the periodic variation of far-field pattern will be destroyed by temporal partial coherence, and the interference effect gradually becomes weak as the increment of PD. Incoherent combination appears for PD beyond coherent length, while the combination can still be viewed as coherent for PD within 1/6 times the coherent length, when the Strehl ratio is more than 0.9, the effect of temporal partial coherence on coherent combination can be neglected.