  We derive the low-energy effective field theory of the extended t-J model in
the regime of light doping. The action includes a previously unknown Berry
phase term, which is discussed in detail. We use this effective field theory to
calculate spin-wave excitations in the disordered spin spiral state of
La_{2-x}Sr_xCuO_4 (the spin-glass phase). We predict an excitation spectrum
with two distinct branches: The in-plane mode has the usual linear spin-wave
dispersion and the out-of-plane mode shows non-trivial doping dependent
features. We also calculate the intensities for inelastic neutron scattering in
these modes.
