Intracavity femtosecond-pulse compression with the addition of highly nonlinear organic materials

Abstract

It is experimentally shown that using organic materials of high nonlinear refractive index dissolved in a saturableabsorber solution permits femtosecond-pulse compression in a simple colliding-pulse mode-locked laser employing cavity-mirror dispersion adjustment.Fast self-phase modulation 1 ' 2 (FSPM) arising from the nonlinear refractive index (NRI) of a solvent of a saturable absorber, ethylene glycol (EG), restricts direct generation of pulses shorter than 100 fsec from a colliding-pulse mode-locked (CPM) cw dye laser.3 - 6For shorter-pulse generation, therefore, compensation for the upchirp caused by the FSPM is always needed, and lossless dispersive elements are used. Furthermore, in order to compress pulses external to the laser, a dispersive NRI medium such as an optical fiber is used in conjunction with dispersive elements such as a grating pair. 7 ' 8 The NRI medium broadens the pulse spectrum without disturbing relative phases between modes, and the dispersive elements remove the chirp to generate transform-limited pulses.Recently Martinez et al. 9 theoretically showed that short, stable, solitonlike pulses are generated by. a CPM laser operating in the balance between the FSPM that is due to EG and cavity dispersion. They further concluded that the dependence of the pulse duration on its dispersion is remarkably asymmetric. More recently, Haus and Silberberg' 0 pointed out that a NRI medium introduced into a CPM laser with adjustable cavity dispersion leads to shorter, though non-soliton-like, pulses. In this case, the dependence of the pulse duration on the dispersion is symmetric.However, to the authors' knowledge, no one has experimentally investigated how the introduction of a NRI medium into a practical CPM laser influences the generation of femtosecond pulses. This may be because the additional insertion of the usual NRI medium, such as an optical fiber or a Selfoc lens, 10 gives rise to high optical losses, dispersion broadening, and further complication of the cavity configuration. In this Letter we demonstrate that the addition of high-NRI organic materials to a solution of the saturable absorber permits intracavity pulse compression in a simple CPM laser employing cavity-mirror dispersion adjustment.The NRI organic materials used have the following properties: (1) a higher NRI than that of the EG solution (n 2 = 3.0 X 10-16 cm 2 /W), 4 ' 9 (2) an ultrafast time-dependent index change by electronic hyperpolarizability, (3) high solubility for EG or a mixture of EG and benzylalcohol, (4) no absorption around the lasing wavelength region, and (5) good photochemical stability in the solution. Recent studies of high-NRI organic materials enabled us to find suitable materials that satisfy these properties."1 The third-order molecular hyperpolarizabilities, y, of paradimethylamino-3 nitrostyrenel 2 (DMA-NS) and 2'-4'-methyl nitroanilinell (MNA) are much larger than that of usual solvents (by a factor of -104).4,9 13 Therefore, from the NRI equation, 14 n 2 = 37rNy (n 2 + 2)4/81, where n is the lin...

Theoretical analysis of pulse development in a colliding pulse mode-locked dye laser

“…Self-phase modulation occurs due to laser induced refractive index changes [66][67][68][69] in the gain and absorber jets. They are due to population changes [4, 40, 70 73] in the saturable absorber (absorber saturation modulation [29][30][31]) and the gain medium (gain depletion modulation) as well as due to the optical Kerr effect in the solvents [4,40,[70][71][72][73][74][75][76][77][78][79][80][81][82].…”

“…n L i, Q t 0 For ethylene glycol a value of n 2 = 8 x 10 2 3 m 2 V 2 is given in [7,48,72,78,79] for the field coefficient of the nonlinear refractive index. n L is the linear refractive index at the laser frequency, e 0 is the permittivity, and c 0 is the vacuum light velocity.…”

Theoretical analysis of contributions of self-phase modulation and group-velocity dispersion to femtosecond pulse generation in passive mode-locked dye lasers

“…It may be enlarged by extending the jet thicknesses, by increasing the nonlinear refractiveindex coefficient by adding a liquid of high optical Kerr coefficient to the dye solutions, 54 or by inserting an additional nonlinear sample in a third resonator cavity focus.…”