Wavelength dependent characterization of a multimode fibre endoscope

Multimode fibres have recently shown promise as miniature endoscopic probes. When used for non-linear microscopy, the bandwidth of the imaging system limits the ability to focus light from broadband pulsed lasers as well as the possibility of wavelength tuning during the imaging. We demonstrate that the bandwidth is limited by the dispersion of the off-axis hologram displayed on the SLM, which can be corrected for, and by the limited bandwidth of the fibre itself. The selection of the fibre is therefore crucial for these experiments. In addition, we show that a standard prism pulse compressor is sufficient for material dispersion compensation for multi-photon imaging with a fibre endoscope

Two-color, two-photon imaging at long excitation wavelengths using a diamond Raman laser

We demonstrate that the second-Stokes output from a diamond Raman laser, pumped by a femtosecond Ti:Sapphire laser, can be used to efficiently excite red-emitting dyes by two-photon excitation at 1080 nm and beyond. We image red fluorescent protein (RFP) expressing HeLa cells, as well as dyes such as Texas Red and Mitotracker Red. We demonstrate the potential for simultaneous two-color, two-photon imaging with this laser by using the residual pump beam for excitation of a green-emitting dye. We demonstrate this for the combination of AlexaFluor 488 and AlexaFluor 568. Because the Raman laser extends the wavelength range of the Ti:Sapphire laser, resulting in a laser system tunable 680 nm-1200 nm it can be used for two-photon excitation of a large variety and combination of dyes

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