Single-beam CARS spectroscopy using a soliton Stokes pulse and a spectrally-compressed pump pulse from a PCF

Abstract

Coherent anti-Stokes Raman scattering (CARS) spectroscopy is one of the nonlinear Raman spectroscopy and is attracting attention recently because in CARS spectroscopy, fluorescence can be avoided, the spatial resolution is high, and the exposure time is short. Generally, two separate beams, a pump beam and a Stokes beam, have to be collinearly overlapped and focused on a sample using an objective lens in CARS. However, the adjustments of these two beams, necessary to generate strong CARS signals, are not easy in general. In this paper, we demonstrate a novel setup for single oscillator CARS spectroscopy using a single beam, where a Stokes pulse and a pump pulse are generated by the combination of a pulse shaper and a photonic crystal fiber (PCF). In this setup, two pulses, one for generating a wavelength-tunable fundamental soliton pulse [1], and the other for generating a narrowband pump pulse, are shaped by a pulse shaper and are inputted in a PCF. Especially, the pulse for a pump pulse is negatively-chirped by a pulse shaper for the spectral compression in a PCF [2], which is important to obtain a narrowband pump pulse to obtain the high spectral resolution while retaining most of the pulse energy. In this setup, the pulse is shaped before it is inputted in a PCF to create Stokes and pump pulses suitable for CARS spectroscopy, and is different from previous studies for single-beam CARS spectroscopy, where a broadband pulse that contains both Stokes and pump components is shaped to obtain CARS signals [3,4].