Novel spatial-mode selection using distributed feedback gratings in single-mode resonant antiguided diode laser arrays

A distributed feedback (DFB) grating located below the active region in resonant antiguided diode laser arrays (ROW-LDFB arrays) provides not only frequency discrimination, but also serves as a novel in-phase spatial-mode selector that remains effective to high drive levels. Previously, spatial-mode discrimination has been accomplished by placing loss in the high effective-index interelement regions, suppressing modes with significant interelement field. However, the presence of interelement loss could lead to self-pulsations due to saturable absorption at high drive levels.;The DFB grating is typically located above the active region in single-frequency semiconductor laser arrays due to ease of fabrication. In this configuration, however, the optical feedback as a result of the grating is virtually equal for all array modes. As a result, grating-based array-mode selection is not possible. When positioned below the active region in antiguided arrays, however, the amount of optical field overlap with the lower grating layer becomes array-mode-dependent, The array mode with the highest coupling to the grating experiences the greatest amount of optical feedback. That is, the grating simultaneously operates as a longitudinal-mode (frequency) discriminator and as a spatial-mode (array-mode) selector. As will be discussed, this method of spatial-mode selection has attractive advantages over more common techniques such as the incorporation of interelement loss or intracavity spatial filters.;The theory of ROW-LDFB operation is described in detail and design considerations for in-phase array-mode operation are addressed. The effects of various AR/HR coatings on device behavior are presented. In addition, the statistical implications of random cleave locations relative to the DFB grating phase are included in the analysis. 20-element facet-coated ROW-LDFB arrays operate up to 0.45 W in a single in-phase array mode and at a single frequency. Experimental results confirm theoretical predictions for resonant in-phase array mode operation to high output powers.