Measurements of cavity-length-dependent internal differential quantum efficiency and internal optical loss in laser diodes

We measured the cavity length dependence of the internal differential quantum efficiency η_int and the internal optical loss α_int in 1500-nm-wavelength laser diodes (LDs). By evaluating α_int directly from gain/absorption spectra for various injection current densities and measuring the external differential quantum efficiency η_ext, we obtained the η_int and α_int values of all the LDs with different cavity lengths. The obtained η_int and α_int respectively showed strong and weak cavity length dependences, and were both very different from those derived by the widely used method of plotting η_ext ^-1 against the cavity length.