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