Fig. 1.

System layout. The Yb-fiber chirped pulse amplifier is designed to operate with negligible nonlinear phase accumulation, such that the pulse duration at the output of the HC-PBF compressor is independent of the pump power of the second amplifier stage, and thus output pulse energy. The collimated output beam is guided to the laser scanning two-photon microscope (leftmost part of the image). The x-y-position of the focus in the sample plane of the microscope is controlled by two galvanometric mirrors in the beam path. PMT: photomultiplier tube. LD: laser diode. Dashed box inset: measured beam profile with x- and y-projections (full lines) and respective Gaussian fits (dashed lines).