  We theoretically investigate the quantum uncertainty in the beam width of
transverse optical modes and, for this purpose, define a corresponding quantum
operator. Single mode states are studied as well as multimode states with small
quantum noise. General relations are derived, and specific examples of
different modes and quantum states are examined. For the multimode case, we
show that the quantum uncertainty in the beam width can be completely
attributed to the amplitude quadrature uncertainty of one specific mode, which
is uniquely determined by the field under investigation. This discovery
provides us with a strategy for the reduction of the beam width noise by an
appropriate choice of the quantum state.
