A simplified femtosecond laser repetition frequency divider for two-photon imaging

Commercial Ti:Sapphire femtosecond lasers used for conventional two-photon microscopy typically operate at a [~]80 MHz repetition rate. However, this frequency is often suboptimal for cortical tissue imaging, where lower rates of 20-40 MHz are considered ideal. However, achieving these lower frequencies has remained a significant technical and financial challenge. Here, we present a compact, cost-effective resonant electro-optic modulator that halves the laser repetition rate to 40 MHz. When imaging neurons in the macaque visual cortex in vivo, this 40 MHz configuration yielded a >2-fold increase in fluorescence intensity and a [~]3 dB improvement in signal-to-noise ratio (SNR) for both green (GCaMP5G) and red (mScarlet) indicators. This enhancement proved particularly pronounced in deep-tissue imaging. Furthermore, the system demonstrated excellent long-term stability and induced no detectable phototoxicity. This simple and robust device represents a powerful upgrade for conventional two-photon microscopes, significantly enhancing imaging quality for the investigation of neural circuits within scattering brain tissue.