The operation of a laser is influenced by its structure as well as the characteristics of the gain region. The simplest structure is a Fabry–Perot laser that embodies a slab waveguide and feedback by reflection at the end facets. Some lasers use optical feedback from Bragg gratings to improve control of the wavelength. General properties of gratings and specification of the Bragg wavelength are described. Devices with the axis of the cavity in the plane of the epitaxial layers are termed ‘in-plane lasers’ wherein the light propagates along the gain layer and is emitted from the edge of the chip; examples include ridge waveguide lasers, distributed feedback lasers and distributed Bragg reflector lasers. By contrast, in vertical-cavity surface-emitting lasers (VCSELs) the light propagates perpendicular to the plane of the gain layer between two closely spaced multilayer Bragg reflectors. The threshold condition for each of these structures is considered.
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