The longer the cavity and the narrower the beam waist, the better the beam quality of the laser emission, or <math> |\langle x | s \rangle|^2\ = \sum… - F. J. Duarte

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The longer the cavity and the narrower the beam waist, the better the beam quality of the laser emission, or
<math> |\langle x | s \rangle|^2\ = \sum_{j=1}^\N\,\Psi(r_j)^2\ +2 \sum_{j=1}^\N\,\Psi(r_j)\bigg(\sum_{m=j+1}^\N\,\Psi(r_m)cos(\Omega_m-\Omega_j)\bigg) </math>