TABLE OF CONTENTS The DFB structures so far discussed emit from their ends (edge emitters) rather than from their top or bottom surfaces, but there is a steadily increasing motivation to design lasers which emit from the chip's surface [69 71] for the following reasons:
Distributing the optical output over a larger area of surface can reduce the peak power density at the emission facets and so facilitate yet higher-power lasers without the present difficulties of facet damage. With appropriate designs, one can also envisage improved heat sinking for high-power lasers leading to yet higher power outputs.
The divergence of a laser beam is inversely proportional to the emission area (solid angle of emission ~ ? = 4 ? 
/ A) so that an increased emission area A can give narrow beams which are useful for applications such as free-space optical communication. Appropriate circular far-field patterns from surface emitters, as compared with asymmetric patterns from edge emitters, should also increase the coupling efficiency to circular optical fibres.
Surface emitters can readily be arranged into two-dimensional (N X M) arrays (Figure 1.23) which could be useful for optical data switching and processing, whereas edge emitters can easily form only linear (I X N) arrays.
Figure 8.25 shows two main different approaches to forming surface emitters. The first approach (Figure 8.25a and b) uses one form or another of edge emitter to feed either a second-order grating (Sections 1.6.1 and 4.4.3) or a 45 ...
