An optical cavity is formed by two aligned concave mirrors of curvature R facing each other, as is most commonly used in gas lasers. The radiation inside such a cavity has a Gaussian beam profile. The actual beam that fits into the cavity is one whose wave fronts at the mirrors match the curvature of the mirrors. The radius of curvature R of a Gaussian beam wave front at a distance z along its axis is given by: Rz = z[1 + zo/z]. Where zo = 2w^2/λ is the Rayleigh range, the wavelength, and w the beam waist. Now consider a confocal symmetric optical cavity in which the mirrors are separated by L = R.

(a) Draw a sketch of such a cavity including the optical beam and indicate all relevant parameters. [3 marks]