The group velocity is the velocity of a wave packet (i.e. the velocity of energy) that is centred on a central carrier frequency f = ω/2π. The group velocity is different from the phase velocity for two reasons: (i) the waveguide changes the propagation coefficient as a function of frequency; and (ii) the material permittivity changes with frequency so that the propagation coefficient in the material changes with frequency. This appendix illustrates these two roles using a symmetric three-layer waveguide so that one can appreciate the physics through putting numbers into an analytic solution.

Inspec keywords: permittivity; laser beams; light propagation; waveguide lasers; refractive index; semiconductor lasers

Other keywords: group refractive index; carrier frequency; propagation coefficient; material permittivity; symmetric three-layer waveguide; wave packet velocity; laser waveguides

Subjects: Laser beam characteristics and interactions