We present experiments on cell cultures and brain slices that demonstrate two‐photon optogenetic pH sensing and pH‐resolved brain imaging using a laser driver whose spectrum is carefully tailored to provide the maximum contrast of a ratiometric two‐photon fluorescence readout from a high‐brightness genetically encoded yellow‐fluorescent‐protein‐based sensor, SypHer3s. Two spectrally isolated components of this laser field are set to induce two‐photon‐excited fluorescence (2PEF) by driving SypHer3s through one of two excitation pathways – via either the protonated or deprotonated states of its chromophore. With the spectrum of the laser field accurately adjusted for a maximum contrast of these two 2PEF signals, the ratio of their intensities is shown to provide a remarkably broad dynamic range for pH measurements, enabling high‐contrast optogenetic deep‐brain pH sensing and pH‐resolved 2PEF imaging within a vast class of biological systems, ranging from cell cultures to the living brain.