We study various properties of solutions of an extended nonlinear Schrödinger (ENLS) equation, which arises in the context of geometric evolution problems -including vortex filament dynamicsand governs propagation of short pulses in optical fibers and nonlinear metamaterials. For the periodic initial-boundary value problem, we derive conservation laws satisfied by local in time, weak H 2 (distributional) solutions, and establish global existence of such weak solutions. The derivation is obtained by a regularization scheme under a balance condition on the coefficients of the linear and nonlinear terms -namely, the Hirota limit of the considered ENLS model. Next, we investigate conditions for the existence of traveling wave solutions, focusing on the case of bright and dark solitons. The balance condition on the coefficients is found to be essential for the existence of exact analytical soliton solutions; furthermore, we obtain conditions which define parameter regimes for the existence of traveling solitons for various linear dispersion strengths. Finally, we study the modulational instability of plane waves of the ENLS equation, and identify important differences between the ENLS case and the corresponding NLS counterpart. The analytical results are corroborated by numerical simulations, which reveal notable differences between the bright and the dark soliton propagation dynamics, and are in excellent agreement with the analytical predictions of the modulation instability analysis. 1991 Mathematics Subject Classification. 35Q53, 35Q55, 35B45, 35B65, 37K40. 1 Sec. 3]. Briefly speaking, this derivation begins from a natural generalization of the localized induction equation (LIE) which governs the velocity of the vortex filament. The generalization takes account of the axial-flow effect up to the second-order (see [27, Eqs. (3.1)-(3.2)]). Then, the Hirota equation is derived by repeating the original Hasimoto's procedure [33], which proved the equivalence between LIE and the integrable NLS equation iφ t + φ xx + 1 2 |φ| 2 φ = 0 (see also [47,48]). This equivalence implies that LIE is completely integrable. Thus, since the generalized LIE of [27] preserves integrability, the equivalent evolution equation which is in this case ENLS equation (1.1), should be also integrable. This is the reason why condition (1.2) is essential for the association of Eq. (1.1) with the vortex filament dynamics.On the other hand, non-integrable versions of (1.1), i.e., when the balance condition (1.2) is violated, can be derived in the context of geometric evolution equations. In particular, it was shown in [53] that certain differential equations of one-dimensional dispersive flows into compact Riemann surfaces, may be reduced by a definition of a generalized Hasimoto transform, to ENLS type equations. See also [54,55], and references therein.Another important physical context, where certain versions of the ENLS equation have important applications, is that of nonlinear optics: such models have been used to describe femtosecond pulse pr...