高斯光束

The Gaussian beam is a solution to the paraxial Helmholtz equation, which is an approximation of Maxwell’s equations for beams that do not diverge rapidly. The intensity [latex]I(r, z)[/latex] of a Gaussian beam as a function of radial distance [latex]r[/latex] from the center of the beam and axial distance [latex]z[/latex] from its narrowest point (the ‘beam waist’) is given by [latex]I(r, z) = I_0 \left(\frac{w_0}{w(z)}\right)^2 \exp\left(\frac{-2r^2}{w(z)^2}\right)[/latex]. Here, [latex]I_0[/latex] is the peak intensity at the beam waist, [latex]w_0[/latex] is the beam waist radius (where the intensity drops to [latex]1/e^2[/latex] of its axial value), and [latex]w(z)[/latex] is the beam radius at distance [latex]z[/latex].

Key parameters describing a Gaussian beam include the beam waist ([latex]w_0[/latex]), the Rayleigh range ([latex]z_R[/latex]), which is the distance over which the beam remains relatively collimated, and the beam divergence angle ([latex]\theta[/latex]), which describes how fast the beam spreads out in the far field. These parameters are all interrelated. A smaller beam waist results in a larger divergence angle, a consequence of diffraction. The quality of a real laser beam is often described by the M-squared ([latex]M^2[/latex]) factor, which compares its beam parameter product (waist radius times far-field divergence) to that of an ideal Gaussian beam, for which [latex]M^2=1[/latex]. The Gaussian profile is desirable because it can be focused to the smallest possible spot size for a given wavelength, maximizing intensity.