Paraxial-misalignment insensitive external-cavity semiconductor-laser array emitting near-diffraction limited single-lobed beam

We present a novel easy-to-align external cavity configuration for efficiently obtaining a near-diffraction limited single-lobed beam from gain-guided laser diode arrays. The external cavity has an external mirror (EM) in the image plane of the array. Two bulk-optics experimental setups are evaluated and a compact GRIN lens design is discussed. There are several advantages for the present geometry, including orders of magnitude lower misalignment-sensitivity and superior mode discrimination. Both experimental setups deliver near-diffraction limited single-lobed beams (of width ~0.8° at the rated output power of 100 mW of the array) in the entire current range. The light-current (L-I) characteristic is linear and kink-free and one of the setups shows higher efficiency than the free-running array. The broad-area laser (BAL) mode theory is discussed, introducing radiation losses to the analysis. Several aspects of amplification in laser arrays are theoretically explained, such as why the optimum angle of the far-field lobe is always greater than the tenth mode angle (for a uniform 10-stripe array). It is shown that the gain of the amplifying medium broadens the far-field profile. The width of the far-field lobe increases with gain and decreases with the lobe angle. An analytical expression for the broadened far-field profile of BAL modes is presented. The theory compares very well with the experiments