Design and preparation of low-dispersion mirrors.

We present a high-threshold, ultra-broadband, back-side coated low-dispersion mirror (BASIC LDM) based on SiO₂/Nb₂O₅ thin films operating in the 650–1050 nm wavelength range. The BASIC LDM consists of three main components: a front-side anti-reflection coating, a 1 mm quartz substrate, and a back-side coated dispersion compensation layer. Light first passes through the anti-reflection film and the quartz substrate before reflecting in the dispersion compensation layer. The substrate introduces positive dispersion, while the dispersion compensation layer provides equal negative dispersion, achieving zero net dispersion across an ultra-wide bandwidth. The positive dispersion provided by the substrate alleviates the stringent requirement for identical dispersion across all wavelengths in the design of the dispersion compensation layer, which offers greater optimization flexibility and reduces the limitations imposed by the refractive index ratio of the materials on the bandwidth, achieving zero net dispersion across an ultra-wide bandwidth. The anti-reflection coating on the front surface of the substrate enhances the overall reflectivity of the mirror, and cooperates with the substrate to improve the impedance matching between the back dispersion compensation layer and the incident medium, reducing the oscillation of group delay dispersion (GDD).