Video-rate three-photon imaging in deep Drosophila brain based on a single Cr:forsterite oscillator

We have demonstrated 30-Hz three-photon imaging using a single 24-MHz mode-locked Cr:forsterite oscillator with a center wavelength at 1260 nm. By managing the dispersion distribution in the resonator using double-chirped mirrors, we have produced 32-fs pulses with 22-nJ pulse energy. Using the oscillator as a driving source, we have realized multi-color three-photon images using a GFP-labeled Drosophila brain and an AF647-labeled mouse brain. To demonstrate the capability of deep-tissue imaging, we have obtained a 10-times higher SBR from the three-photon images than the two-photon results at different depths in a GFP-labeled Drosophila brain dissection. Furthermore, we have shown the impact of excitation pulse width on three-photon deep-tissue imaging. Our results indicate the superiority of using shorter pulses for deeper-tissue imaging, especially in the Drosophila brain. In addition, we have recorded the three-photon calcium imaging in vivo from the Drosophila mushroom body in response to external electric shocks. We believe our demonstration provides a robust approach for high-speed three-photon microscopy applications, especially for intravital investigations in the Drosophila brain. ### Competing Interest Statement The authors have declared no competing interest.