Various aspects of the rate equation model of a diode laser are discussed in light of the dielectric waveguide model of the laser. Precise definitions of various quantities used in the rate equation model are given. Emphasis is on analysis of wavelength modulation or chirp. It is shown that the so-called linewidth enhancement factor relevant for predicting wavelength modulation must be defined in a way that depends on laser structure and modulation frequency. A number of proposed mechanisms leading to wavelength modulation are compared directly within the framework of the waveguide model.