For maximum fibre-coupled power, high power broad area diode lasers must operate with small lateral far field angles at high continuous wave (CW) powers. However, these structures are laterally multi-moded, with low beam quality and wide emission angles. In order to experimentally determine the origin of the low beam quality, spectrally resolved near and far field measurements were performed for a diode laser with 50 μm stripe width. Within the range measured (CW optical output powers to 1.5 W) the laser is shown to operate in just six stable lateral modes, with spatially periodic profiles. Comparisons of the measured profiles with the results of two-dimensional modal simulation demonstrate that current-induced thermal lensing dominates the lateral waveguiding, in spite of the presence of both strong built-in index guiding and gain guiding. No evidence is seen for filamentation. Building on the diagnosis, proposals are presented for improvements to beam quality.

For maximum fiber-coupled power, broad-area (BA) diode lasers must operate with small lateral far field angles. However, these structures are laterally multi-moded, with low beam quality and wide emission angles. We use a combination of device simulation and diagnostic measurements to determine the physical factors limiting the lateral far field angle in state of the art BA lasers emitting at 975 nm. Two-dimensional simulations of the optical field enable the dominant lateral waveguiding mechanisms to be diagnosed. Spectrally resolved near and far field measurements will allow us to determine the nature of the lateral modes.

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