Quantum mechanical selection rules dictate that laser light generated by a QCL must be polarized along the growth direction of the superlattice in the laser core. This arrangement makes a vertical emitting device, like a VCSEL, impossible.

Laser light in a QCL can be coaxed into vertical emission with diffraction gratings. The periodic gratings cause the backward and forward propagating waves inside the cavity to be able to couple into diffraction orders that propagate at an angle from the cavity.

A ring cavity can be made that causes in-cavity light to loop and reflect on an effectively unending track. With a surface emitting diffraction grating, this means that the only means of escape for the light is either re-absorption inside the device, or being emitted from the surface via diffraction.

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