Spatial coherence analysis of intense ultra-flat white laser

<sec>White light is typically considered incoherent; however, the recently popular supercontinuum laser, also known as white laser, spans the visible spectrum and features high laser intensity and good coherence, challenging this traditional limitation. The white laser has a wide range of applications, including multi-channel confocal microscopy, color holography, and white light interferometric surface topography. Although white lasers have been proposed and developed extensively in terms of technology, specific analyses of their optical wave properties—especially spatial coherence—are still lacking. Since many applications impose certain requirements on the spatial coherence of white light, the lack of research into the spatial coherence of white lasers has, to some extent, limited their practical use.</sec><sec>This paper presents a detailed experimental study and analysis of the wavefront intensity, polarization characteristics, and spatial coherence of the high-intensity ultra-flat spectrum white laser that was independently developed by our research group in 2023. The laser is generated by broadening the spectrum of a high-intensity Ti:sapphire femtosecond laser through second- and third-order nonlinear effects.</sec><sec>A bandpass filter is used to extract eight components from the white laser, with a central wavelength range from 405 nm to 700 nm and a bandwidth of 10 nm for each component. By measuring the performance of these eight quasi-monochromatic lasers, the characteristics of the white laser of the entire visible spectrum can be evaluated.</sec><sec>The CCD imaging of the collimated quasi-monochromatic laser spots reveals that their wavefront intensities exhibit a quasi-Gaussian distribution with uniform beam profiles. Polarization measurements by using polarizers at various angles show that the white laser is linearly polarized. A Young’s double-slit interferometer is used to measure the interference fringe contrast of the eight quasi-monochromatic beams to assess their spatial coherence. The experimental results show that the average interference fringe contrast of the entire visible spectrum is 0.77, and the difference between different wavelengths is very small.This indicates that the white laser has excellent spatial coherence in the visible range.</sec><sec>The eight quasi-monochromatic lasers in the visible spectrum all exhibit quasi-Gaussian wavefront intensity distributions, linear polarization, and high spatial coherence. This indicates that the white laser inherits the excellent properties of the Ti:sapphire laser. All of these data provide valuable guidance for the application of white lasers in color holography, white light interferometric surface tomography, microscopic imaging, and other fields that require white light with a certain degree of coherence.</sec>