Layer composition and mode structure analysis of heterojunction laser diodes by near field scanning optical microscopy

Reports mode structure and layer composition analysis of high power strained (In,Ga)As lasers using the super-resolution capabilities of near field scanning optical microscopy (NSOM). The lasers are designed to pump erbium doped fiber amplifiers in a configuration optimized for a single transverse laser mode. At high current levels, coupling efficiency decreases due to broadening of the spot size and the onset of multiple transverse modes. Sub-micron collection mode imaging and spectroscopic mapping of the emission mode structure as a function of laser pulse length and current easily identify a regime of operation where multiple transverse modes are observed. The evolution of multiple transverse modes coincides with a kink observed in the L-I curve. Near field microscopy enables the mode profile and spectral image to be correlated with the layer structure of the device with 100 nm resolution.