7. An triangular laser cavity has 3 equal length sides of distance d. One mirror has a radius of curvature R; the other 2 are flat. a) Draw the cavity. b) Draw a lens-waveguide and identify a unit cell. c) Find the ABCD matrix for this cavity and simplify it. (Hint: you don't need to use anywhere near 6ABCD matrices.) d) For what values of d/R is the cavity stable? e) Where in the cavity is the beam the largest, and where is it the smallest? f) Where in the cavity are the wavefronts planar, and where are they most curved?