Self-mode-locking of cw solid-state lasers with a nonlinear birefringent polarisation modulator

Abstract

A fluctuation model is used to show that effective self-mode-locking can be achieved in cw solid-state lasers with nonlinear birefringent polarisation modulators of two types. The discriminating action of modulators is based on the rotation and distortion of the polarisation ellipse, which depends on the intensity of the radiation propagating in the birefringent medium and is related to self-phase-modulation of two polarisation components. The linear phase shift of these components and the angular orientations of the modulator parts are determined to ensure that the modulator acts as a discriminator. An analysis is made of the regions of efficient self-mode-locking of cw lasers with nonlinear birefringent modulators as a function of the laser parameters.

New principle of formation of ultrashort pulses in solid-state lasers with self-phase-modulation and gain saturation

“…2 shows that a perturbation of the operator in square brackets in the second equation quadratic in respect of energy, has a critical influence on the branches of the solutions represented by the dashed curves. Hence, we can conclude that only the quasisoliton solutions, described by expression (2) and represented by the continuous curves, are stable against perturbations in the quasisoliton sector of the operator Q. Such solutions correspond to lower energies of the pulses described by expression (2).…”

“…Hence, we can conclude that only the quasisoliton solutions, described by expression (2) and represented by the continuous curves, are stable against perturbations in the quasisoliton sector of the operator Q. Such solutions correspond to lower energies of the pulses described by expression (2). The greatest interest from the point of view of achieving the conditions described above is the stability of expression (2) against laser noise (continuum).…”

“…Efficient generation of ultrashort pulses in solid-state lasers is possible by utilisation of a wide range of traditional resonant and nonresonant optical nonlinearities. The latter include self-phase-modulation (SPM) [1], the optical Kerr effect [2], and self-focusing [3]. Since the response time of third-order nonlinearities amounts to a few femtoseconds, extremely short pulses have been generated in solid-state laser systems.…”

“…It is obvious that the behaviour of a perturbation ahead of the leading edge of a pulse described by expression (2), where the gain is maximal, is of decisive importance from the point of view of stability. We shall therefore use the operator g{-oc|a}.…”

“…The smaller modulation depth for curve 2 can be explained by the less rapid depletion of the gain which occurs in this case and, consequently, by a smaller value of a(-oo). If the dynamics of the gain is taken into account rigorously, the boundary between the stable and unstable regions lies between curves 7 and 2, and if z ^ 1, a typical moderate modulation depth is sufficient for the stabilisation of a quasisoliton described by expression (2). In practice, the excitation of a pulse described by expression (2) may be the result not only of introduction of an active or passive modulator of the traditional type (including one based on the use of a nonresonant nonlinearity), but also of a self-induced Stokes shift resulting from SPM and gain saturation, and responsible for the formation of a pulse described by expression (2); this follows also from Ref.…”