I realise that there are two factors that must affect the directivity of a laser. 1, The length of the cavity and number of reflections and 2. The aperture of the exit or the subsequent optics. This must be related somewhat to the respective directivities of Endfire and Broadside RF transmitting antennae. The difference being in the number of wavelengths for the dimensions. The effect of the aperture is easy to estimate but where does the length of the cavity come into it? It will clearly affect the coherence but is that just a secondary effect on the emerging wave as it goes through the exit lenses? I could see it relating to the bandwidth of the signals fed to the various elements of an RF array - although it is normal to feed them all with the same signal. I did the Google thing about this but it seems to be either not basic enough or too basic to trigger any hits that I could spot.