9.10.1 Beam Shape and Divergence

As you learned in Chapter 4, a beam emitted through a small aperture D has a relatively large divergence angle θ, because divergence is proportional to the wavelength divided by the aperture (i.e., to the number of wavelengths in the aperture). For circular apertures many wavelengths across, the divergence angle is

where K is a constant near 1 and depends on the beam profile.

Stripe-geometry, edge-emitting diode lasers emit from the edge of the active layer, a region a fraction of a micrometer high and several micrometers wide. This shape is quite different from the round emitting areas of most gas and solid-state lasers, so the formula for beam divergence is only an approximation. Nonetheless, measurements confirm that the beam spreads more rapidly in the vertical direction, where the emitting aperture is narrow, than in the horizontal direction, where the emitting aperture is wider, as shown in Figure 9-19. Typical values for beam divergence are 10 degrees (0.17 radian) in the direction parallel to the active layer and 40 degrees (0.70 radian) in the direction perpendicular to the active layer. This spreading angle is larger than the beam from a good flashlight, making the beam quite different from the tightly focused beam of a helium neon laser or a solid-state laser.

Fortunately, external optics can correct for this broad beam divergence. A cylindrical lens, which focuses light in one direction but not in the perpendicular direction, can make the beam circular in shape. Collimating lenses can focus the rapidly diverging beam from an edge emitter so it looks as narrow as the beam from a helium neon laser. Those lenses are hidden inside standard red semiconductor-laser pointers, so they produce tightly focused spots on the screen or wall.

VCSELs are a different matter because they emit from circular apertures generally at least 5 µm across, so their beams diverge symmetrically, with typical divergence of 10 degrees (0.17 radian). As for edge emitters, the addition of collimating optics can reduce beam divergence.