Interferometric Spectral Modulation of sub-100-fs Pump Pulses for High Chemical Contrast, Background Free, Real Time CARS Imaging

20 MHz, sub-ps, tunable ti:sapphire laser system for real time, stain free, in vivo histology of the skin

“…In Fig. 2B), an IF-CARS histological image of murine skin (after post-processing) is shown, for which a spectrally modulated Mai Tai pump laser was used for recording "protein" and "lipid" CARS images with optical pulses with spectral maxima at 792 and 796 nm, respectively [3].…”

“…1. In our setup at the University Szeged (USZ) [3,5], we used a ~80 MHz, ~80 fs Ti:S laser (Mai Tai, Newport Spectra-Physics, USA) as a pump laser (for details, see Refs. 3 and 5).…”

“…B) Phase difference Δφ = 0 at the central frequency results in a narrower spectrum, whereas Δφ = π results in a double-peaked spectrum around the central wavelenght, when the path difference offset, ΔL is properly set. When the phase difference is π/2 or -π/2, two different spectrally shifted laser spectra is obtained with two maxima a few nm-s apart [3]…”

“…This fact leads to the appearance of an enhanced non-specific background and the decrease of spectral sensitivity in CARS imaging. Two years ago we proposed a fast spectral modulation technique for sub-100 fs pulse Ti:S lasers [3], which allowed us to modulate the laser spectrum on ms time scale with the use of a piezo-driven Michelson interferometer. In one of the settings we used, we modulated the laser spectrum of our laser in such a way, that CARS imaging for CH 2 bonds in "lipids" and CH 3 bonds "proteins" did not require any tuning of the pump (Ti:S) laser or any readjustment of the delay between the pump and Stokes (Yb-amplifier) pulses, which allowed us to record stain-free histological images [4] of brain slices.…”

“…In this paper we report on a newly developed, ~20 MHz, sub-ps Ti:sapphire laser system, which supports real time, in vivo, twochannel, high chemical contrast, DVRF CARS imaging, i.e. histology of the skin by a commercial LSM 7 MP microscope (Carl Zeiss, Jena, DE) without any modification of its ZEN software or post-processing of the images like in case of our previous CARS setups used for histology [3,4].…”