Vertical cavity surface-emitting lasers (VCSELs), by contrast, use more complicated-to-grow structures than edge-emitters, and incorporate multilayer high reflectivity mirrors, emitting perpendicular to the growth plane. They produce good quality symmetric beams and tend to be spectrally pure. Their geometry allows easy on-wafer testing and they are therefore inexpensive to manufacture, but their power outputs are generally very small, usually ~1 mW. They have dominated short-range optical interconnection applications but have insufficient powers for telecommunication, data storage and other mainstream applications. The NECSEL laser's emitting area is hundreds of times larger than conventional vertical cavity lasers, thereby allowing scaling to high power levels. Compared with conventional VCSELs, the NECSEL laser requires similar growth complexity and uniformity, has similar electrical and thermal characteristics yet produces higher output powers by virtue of its larger emitting aperture and superior mode control due to the external cavity. The NECSEL laser has patented technology to minimize current crowding, resulting in a flat and uniform gain distribution which produces a high quality beam. The difference is especially marked at high powers. While the VCSEL is inherently incapable of producing high power in a single-lobed, highly symmetric, nearly-diffraction-limited beam, the NECSEL diode does so with ease, generating ~1 W multimode and ~500 mW single mode in continuous operation and several W in pulsed operation. Another key advantage is in spectral purity: while both edge- and surface-emitting lasers generate broad and unstable spectra, the NECSEL laser can produce a single narrow spectral peak. In communication or pulsed-mode applications, the modulation-induced spectral shift, or chirp, is much lower in a NECSEL laser than in conventional VCSELs.
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