Laterally coupled laser-diode pairs, also called "twin stripe lasers", are coupled by means of the evanescent em-field in between two dielectric laser cavities. The coupling strength between the two lasers is determined by the decay length of this evanescent em-field.
This length depends on the indices of refraction of the material
which separates the two cavities compared to the cavity material. However the index of refraction of the cavity material also depends on the inversion density in the lasing material. Therefore the coupling coefficient in the rate equation for the twin stripe device can be written as κ0[1 + ½(αc(1) ρ1 + αc(2) ρ2)], where ρ1 and ρ2 are the inversion densities in the two stripes. We give an expression for αc>(i) in terms of the properties of the device.