Disclosed are ideas to produce an add-on device which turns widely used high repetition rate lasers used for 2-photon microscopy into a light source which can be used for 3-photon microscopy. The add-on encompasses a device to reduce the pulse repetition rate of the high repetition rate (>50 MHz) laser source (laser or OPO) to less than 10 MHz which allows for higher pulse energies while maintaining reasonable average powers. If the high repetition sources operate below 1250 nm the add-on shifts or broadens the seed light to cover 1.3 μm to 1.8 μm before amplification. If the high repetition rate source operates at or around 1.3 μm the add-on only needs to amplify the pulse after downshifting the repetition rate. In another implementation the add-on shifts or broadens the 1.3 μm light to cover the spectral range out to 1.8 μm before amplification.