Multimode coherent states and the Lorentz-invariant mass of light pulses
  The total mass of noncollinear photons forming diverging light pulses is defined and found explicitly. Both classical and quantum derivations are presented. The quantum derivation is based on the use of multimode coherent states, and links between parameters of these states and those of the classical field strength are established. 
M. V. Fedorov and S. V Vintskevich
quant-ph