All-fiber laser source for CARS-microscopy

Abstract

Summary form only given. CARS microscopy is a highly desirable tool to provide real-time information e.g. in brain-cancer surgery [1] due to its label-free chemical imaging capabilities of living tissue. Still a widespread use in real-world applications has not been achieved due to the lack of suitable laser sources. Today widely used CARS laser sources based on Ti:Sapphire and parametric frequency conversion in bulk crystals are expensive and large while their operation requires technical staff devoted to alignment and maintenance. This restricts the application of CARS microscopy to specialized research laboratories only.In this contribution an all-fiber, hence alignment-free, CARS laser source and its application to CARS microscopy is presented. Key components in our setup are a passively mode-locked fiber oscillator delivering nearly transform-limited 50ps pulses at an optimized repetition rate of 2MHz, a fiber integrated amplification system and a fiber optical parametric frequency conversion stage using degenerate four-wave-mixing (FWM) in an endlessly single-mode fiber [2]. In addition we present a seeded FWM CARS source with an extremely high spectral resolution. The cw seed enhances the resolution more than one order of magnitude and allows for distinguishing between CH2 and CH3 resonances. Due to the long pulse durations of the generated signal and pump pulses the temporal walk-off is negligible which makes them ideal for fiber delivery, e.g. to the microscope, without complex delay lines. This enhances the simplicity greatly and makes this source particularly easy to use. 2. Experiment and results The mode-locked 50ps all-fiber oscillator was optimized to run at 2 MHz to avoid photo damage and provide enough peak power for the CARS process (fig.1a). An Yb-doped single-clad fiber amplifier boosts the average power up to 0.5W. The active fiber is spliced to the conversion fiber with a transmission of 74%. A conversion to 798nm, corresponding to a frequency separation of 2845cm-1 was realized. All components are fusion spliced and polarization maintaining to obtain alignment-free and robust operation.Ultrahigh spectral resolution of 1cm-1 was realized by a seeded FWM scheme (Fig. 1b). A laser scanning microscope was used to scan at Raman shifts of 2850cm-1 and 2930cm-1 mapping lipid and protein in a human aorta sample with artheriosclerotic plaque deposition (Fig. 1c). In conclusion, a novel approach for an all-fiber mono-laser source for CARS microscopy is presented. The frequency conversion is optimized to access a frequency shift of around 2850cm-1. Cw-seeding enhances the resolution greatly. Inherently synchronized pump and Stokes pulses are available at one fiber end, readily overlapped in space and time. The source is applied to CARS spectroscopy and microscopy experiments in the CH-stretching band. Due to its simplicity and maintenance-free operation, the laser scheme holds great potential for bio-medical applications outside laser laboratories.