The saturable absorption dynamics of DODCI in a femtosecond dye laser is studied theoretically. The N-isomer and P-isomer photoisomerization dynamics is included. The wavelength region between 570 nm and 650 nm is considered where the N-isomer and P-isomer absorption dynamics changes from short-wavelength to longwavelength So-Si excitation. The slow saturable absorber DODCI shortens a circulating pulse in a laser oscillator down to femtosecond duration if the gain medium compensates the absorber losses. Fast local relaxation in the S 1 -state in the case of short-wavelength excitation and fast level refilling in the S^state in the case of longwavelength excitation facilitate the pulse shortening and the background signal suppression. In some papers [3, 11, 15, 16] the bleaching of the ground-state P-isomer absorption is thought to be responsible for the picosecond and femtosecond pulse generation in passively mode-locked rhodamine 6G dye lasers in the wavelength region above 600 nm.
IntroductionIn this paper a detailed numerical analysis of the saturable absorption dynamics of DODCI in the wavelength region between 570 nm and 650 nm is given. The single transit of a pulse through the absorber and the repetitive passage of a pulse circulating in an oscillator are studied. The N-isomer-P-isomer photoisomerization dynamics is included [13,14]. In the long-wavelength absorption region the bleaching dynamics of the fraction of molecules interacting with the laser light is considered [17,18]. It is found that the slow saturable absorber is able to shorten nanosecond Gaussian pulses down to femtosecond pulses in multiple passages if the gain medium compensates the absorber losses. The fast relaxation of the excited Franck-Condon level in the S,-state in the case of short-wavelength excitation, and the fast spectral cross-relaxation in the S 0 -ground state in the case of