By taking multiple measurements of transmittance before and after laser conditioning in DKDP crystals, we found that the transmittance was increased by 0.05%~0.4% through laser conditioning with maximum fluence 6J/cm 2 , and then decreased by about 0.1% after subsequent higher fluence conditioning. Variation of scattering intensity and absorber density, the two major factors leading to transmittance change, was monitored by on-line and off-line detection systems. The transmittance decrease was

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... ttributed to laser damage scattering, and the increase was derived from reduction of absorbers. Moreover, the absorption was reduced further at higher conditioning fluence. Based on the above analysis, the heating process during laser exposure was analyzed in the time domain, and a local rapid-rising and slow-cooling process was confirmed to reduce defect concentration, which can improve laser damage resistance and increase the transmittance.