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function q = gbeam_propagation_froward_only(x_pos, q_in, x_in, optics_elements)
% calculate the 'q' parameter of the Gaussian beam propagating through optical
% 'optics_elements' only in the positive direction along 'x' axis at points 'x_pos'
% takes the gaussian beam with initial q_in parameter at x_in
% Note!: if optics element position coinside with a x_pos at some place
% the reported value will be the one calculated after the element
% so the resulting value will coinside with submitted q_in
%
% all x_pos must be to the right of x_in
% x_pos must be monotonic!
if (any(x_pos < x_in))
error('all beam positions must be to the right of the x_in');
end
optics_elements=arrange_optics_along_x(optics_elements);
% Forward propagation to the right of x_in
Np=length(x_pos); % number of 'x' points
Nel=length(optics_elements) ;
q=0*x_pos; % q vector initialization
q_last_calc=q_in;
x_last_calc=x_in; % the furthest calculated point
for i=1:Np
x_pos_i=x_pos(i);
for k=1:length(optics_elements)
% iterates through optics_elements to make sure
% we take them in account for the beam propagation
el=optics_elements{k};
if ( (x_last_calc < el.x) && (el.x <= x_pos_i) )
abcd=abcd_free_space(el.x-x_last_calc);
q_last_calc=q_after_abcd(q_last_calc,abcd);
q_last_calc=q_after_abcd(q_last_calc,el.abcd);
x_last_calc=el.x;
endif
endfor
if (x_pos_i > x_last_calc);
abcd=abcd_free_space(x_pos_i-x_last_calc);
q_last_calc=q_after_abcd(q_last_calc,abcd);
x_last_calc=x_pos_i;
endif
q(i)=q_last_calc;
endfor
end
%!test
%! lambda= 1.064E-6 ;
%! f1=0.1526;
%! lns1.abcd=abcd_lens( f1) ;
%! lns1.x= 0.2;
%! f2=0.019;
%! lns2.abcd=abcd_lens( f2) ;
%! lns2.x= lns1.x+ f1 + f2;
%! lns3.abcd=abcd_lens( 0.0629) ;
%! lns3.x= 0.500;
%! q0=1.8098e-96 + 1.3453e+02i;
%! x0=0;
%! optics={lns1,lns2,lns3};
%! x=[0, 0.1, 0.2, 0.3, 0,4, 0.5, 0.6, 0.7];
%! q_test = [1.8098e-96 + 1.3453e+02i, 1.0000e-01 + 1.3453e+02i, -1.5260e-01 + 1.7310e-04i, -5.2600e-02 + 1.7310e-04i, -5.2600e-02 + 1.7310e-04i, 3.4371e+00 + 1.8961e-03i, 3.4371e+00 + 1.8961e-03i, 3.4371e+00 + 1.8961e-03i, 3.4371e+00 + 1.8961e-03i];
%!
%! q=gbeam_propagation(x,q0,x0,optics);
%! assert(q,q_test, -1e-4)
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