Beam quality improvement of broad area laser diode with external cavity feedback

A broad area laser diode (BAL) external cavity is experimentally investigated and analyzed using ray transfer matrices. In the experiment, a grating that is placed at the image plane of the output facet of the BAL is used as an external cavity mirror for the slow axis and as a wavelength selective component for the fast axis at the same time. By tilting the grating with respect to the slow and the fast axis direction, respectively, the number of transverse modes oscillating in the cavity can be limited and the spectral line width of the laser diode can be reduced. With this setup, a laser beam with an improved beam quality, an output power of 230 mW and a spectral line width of 0.6 nm (300 GHz) is obtained when operating the BAL at 2.7 times the threshold drive current. We have also investigated a different cavity configuration to achieve alignment insensitivity and stable operation in which the grating was replaced by a flat HR mirror. With a similar beam quality, at the same operation current of the BAL a laser beam with a power of 320 mW and a spectral fine width of 1.5 nm (750 GHz) is obtained with this setup.