3.2 Laser Pretreatment Technology

Laser pretreatment is a technology that uses sub-threshold laser energy flow to process components before they are practically used. It can effectively improve the ability of some optical components to resist laser damage. The main function of laser pretreatment is to remove defects or reduce thermodynamic response under laser irradiation. Laser pretreatment is not a non-destructive process, and a certain degree of local influence is produced on the surface or body of the optical component during the treatment, but the degree of this influence needs to be controlled within the allowable range.

Due to the randomness of the defect distribution, the laser pretreatment technology is a process of scanning the surface of the optical component with a small spot, so that the surface of the component is irradiated with an approximately uniform energy flow, as shown in Figure 13. By improving the overlapping area of adjacent light spots, the laser beam can be controlled to irradiate the component surface with ideal uniformity. The spatial distribution of the output pulses of common lasers is mostly Gaussian, and the energy density varies within the irradiation range of the same pulse. In order to take into account the uniformity of the energy flow on the surface and the processing efficiency, the distance between adjacent spots is set to be the spot diameter corresponding to 90% peak energy of the laser pulse.

As a treatment method to remove defects or reduce the thermodynamic response of defects, the corresponding laser pretreatment technology should be designed according to the specific type of optical component and the defect characteristics. At present, in the field of ICF research, laser pretreatment has begun to be applied to multilayer dielectric films and KDP/DKDP crystals (www.wisoptic.com). We have designed and established nanosecond and sub-nanosecond laser pretreatment systems. The former uses Nd:YAG laser as the light source with the following specifications: pulse width ~ 10 ns, repetition rate 30 Hz, capable of frequency doubling and tripling output, target surface spot ~ 1.5 mm, maximum energy density 100 J/cm² (@1064 nm). This system is mainly used for laser pretreatment of multilayer dielectric films and KDP crystals (www.wisoptic.com). To treat DKDP crystals (www.wisoptic.com), we use frequency tripling laser with adjustable pulse width (300~800 ps) , repetition rate 100 Hz, maximum energy density on the target surface 10 J/cm² (@355 nm). The system can treat a maximum size of 1000 mm × 400 mm, and capable of on-line monitoring and automatic control (shown in Figure 14).