Effects of self-phase modulation (SPM) on femtosecond coherent anti-Stokes Raman scattering spectroscopy

The effects of self-phase modulation (SPM) on the power spectra of femtosecond (fs) pulses and the consequent impact on N 2 chirped-probe-pulse (CPP) fs coherent anti-Stokes Raman scattering (CARS) spectra are discussed in this paper. We investigated the pressure dependence of CPP fs CARS for N 2 in a room-temperature gas cell at pressures ranging from 1 to 10 bar, and in our initial experiments the CPP fs CARS spectrum changed drastically as the pressure increased. We found that the spectra of the near-Fourier-transform-limited, 60-fs pump and Stokes pulses at the exit of the gas cell changed drastically as the pressure increased due to self-phase-modulation (SPM). This effect was examined in detail in further experiments where the pulse energies of the pump and Stokes pulses were controlled using a combination of a half-wave plate and a linear polarizer. Along with the generated CARS spectrum, the spectra of pump and Stokes pulses were measured at the entrance and exit of the gas cell. The extent of SPM effects for a particular spectrum was characterized by the least squares difference between that spectrum and a spectrum recorded at low enough pressure and laser intensities that SPM was negligible. SPM effects were investigated for N 2 , O 2 , CO 2 , and CH 4 , for pressures ranging from 1 to 10 bar, and for pump and Stokes pulse energies ranging from 10 to 60 µJ. We found that SPM effects in N 2 were much weaker than for O 2 , CO 2 and CH 4 .