A focus error method photothermal microscope was designed for the simultaneous annealing and characterization of defects in thin film multilayer coatings for high power lasers. The technique relies in the detection of the thermal lens induced by the local absorption of a light power focused laser. A 10W CW laser at 1.06μm wavelength was used as a pump and a HeNe laser at 632.8nm as a probe. A 4 quadrant detector and specifically designed astigmatic optic is used to determine the defocusing of the transmitted probe beam at the modulation frequency of the pump. The instrument scans the surface and detects the evolution of the absorptance with time with sensitivity below 0.1ppm. The pump beam focus determines the spatial resolution of the instrument and the probe beam size, much larger than the pump, has to match the modulation frequency that yields a thermal diffusion distance of the order of the probe beam in one modulation period. The detailed design of the instrument will be presented showing the design parameters that should be considered for an adequate sensitivity. The sensitivity of the system is better than 0.1ppm and allows the realization of spatial sweeps and even measurements of the evolution of absorption as a function of time. These capabilities allow the location of defects and their characterization.
The design of a scanning photothermal accessory is presented, which can be attached to the camera port of commercial microscopes to measure thermal diffusivity maps with micrometer resolution. The device is based on the thermal expansion recovery technique, which measures the defocusing of a probe beam due to the curvature induced by the local heat delivered by a focused pump beam. The beam delivery and collecting optics are built using optical fiber technology, resulting in a robust optical system that provides collinear pump and probe beams without any alignment adjustment necessary. The quasiconfocal configuration for the signal collection using the same optical fiber sets very restrictive conditions on the positioning and alignment of the optical components of the scanning unit, and a detailed discussion of the design equations is presented. The alignment procedure is carefully described, resulting in a system so robust and stable that no further alignment is necessary for the day-to-day use, becoming a tool that can be used for routine quality control, operated by a trained technician.