Lowest-order paraxial-wave solution: w(z) = w_0√(1+(z/z_R)²) with Rayleigh range z_R = πw_0²/λ; phase has Gouy term ψ(z) = arctan(z/z_R). The fundamental output mode of a stable optical resonator.
Lowest-order paraxial-wave solution: w(z) = w_0√(1+(z/z_R)²) with Rayleigh range z_R = πw_0²/λ; phase has Gouy term ψ(z) = arctan(z/z_R). The fundamental output mode of a stable optical resonator.