Dependence of internal differential quantum efficiency on cavity length and its implications in measuring optical losses of laser diodes

The conventional method of measuring the internal optical loss, using the dependence of the reciprocal external quantum efficiency on the cavity length in laser diodes, is examined. It is shown that the implicit assumption of constant internal differential quantum efficiency eta _id, which has been customarily misinterpreted as internal quantum efficiency eta _i, is not valid for devices with short cavity length. Therefore, for reliable measurements, long cavity data should be used.