function [ w, s ] = gaussian_focus( w0, s0, f, lambda )
%Focusing of Gaussian beams based on Sidney A. Self's "Focusing of
%spherical Gaussian beams" Applied Optics, Vol. 22, Issue 5, pp. 658-661 (1983)
% Input: lambda = wavelength;
%         w0 = waist before the lens;
%         s0 = distance of waist before lens (s>0 before lens)
%         f = focal length of lens (f>0 for converging lens)
% Output: w = new waist;
%         s = position of waist from lens

zR = pi*w0^2/lambda; %Rayleigh Range
s = f*(1+(s0/f-1)/((s0/f-1)^2+(zR/f)^2)); %Eq. (9b)
w = w0/sqrt((1-s0/f)^2+(zR/f)^2);
end