A femtosecond-pulsed tunable optical parametric generator at 1530-1790 nm for label-free third harmonic generation imaging

We have developed a simple tunable optical parametric generator (OPG), emitting broad band ultrashort pulses at around 1550 nm, that is suitable for nonlinear microscopy. The OPG consists of a periodically poled lithium niobate (PPLN) crystal, pumped at 1064 nm by a high pulse energy ultrafast fiber laser (Fianium HE1060-1uJ-fs). Because of the high pulse energy of the pump laser (1 J), it is not necessary to build a cavity around the PPLN crystal, and the long wavelength output is generated simply by focusing the pump light into the crystal. The output pulses have a band-width of about 130 nm and a pulse width of 300 fs. The OPG is tunable from 1530 nm to 1790 nm by using the multiple regions with different poling periodicity in the PPLN crystal, and can be fine-tuned by changing the temperature of the crystal. The output power at 1550 nm is 100 mW, which is sufficient for non-linear microscopy, even when taking into account the reduced transmission of the microscope optics at these long wavelengths. In addition, light throughout the visible spectrum and into the NIR is also generated in the PPLN crystal and could potentially be used for (simultaneous) single and two-photon imaging.We demonstrate the use of this OPG for label-free THG imaging of a 200 m thick fixed section of mouse brain. The output pulses from the OPG have high enough peak power that we can generate THG using moderate excitation powers at the specimen plane (< 15 mW), due to the low repetition rate of the laser (1 MHz). A higher 80 MHz repetition rate laser, such as commercially available OPOs, would, because of their lower peak power, require a much higher average power to generate the signal, likely causing sample damage. We observe very little change in sample morphology over 30 minutes of continuous imaging, which suggests that this approach does not cause significant photo-damage.