Comparison of modal gain and material gain for strong guiding slab waveguides

The relations between the gain factor, defined as the ratio of modal gain to material gain, and the optical confinement factor are discussed for the TE and TM modes in slab waveguides. For the TE modes, the gain factor is larger than the optical confinement factor, due to the zigzag propagation of the modal light ray in the core layers. For the TM modes, the existence of a nonzero electric field in the propagation direction results in a more complicated relation of the gain factor and the confinement factor. For an air-Si-SiO₂ strong slab waveguide, the numerical results show that the modal gain can be larger than the material gain and the higher-order transverse mode can have an even larger modal gain than the fundamental mode. The efficiency of waveguiding photodetectors can be improved by applying the modal gain or loss characteristics in strong waveguides