function [w0, z0] = cavity_optics2its_waist_and_position( cavity_optics, lambda )

abcd_cavity = optics2abcd( cavity_optics );
cavity_length = cavity_optics{end}.x;
N=length(cavity_optics);

if ~isCavityStable(abcd_cavity)
    display('Cavity is unstable !');
    display('We should stop now. There is no stable mode in this cavity');
    error('Not possible to calculate a cavity mode in the unstable cavity');
end

zstart = 0;
qstart = abcd2self_repeating_q( abcd_cavity );

% waist location of this particular Gaussian beam
% which disregards other optical elements
z0 = - real (qstart); 

i = 1;
if ( (z0 > cavity_optics{1}.x) || (z0 < 0) )
    % waist is to the left of the cavity or beyond first reflective/refractive element
    % we need to swap cavity elements by one 
    % in attempt to find waist in between optics elements
    x1 = cavity_optics{i}.x;
    for i=2:N
        cavity_optics_transpose{i-1}   = cavity_optics{ i };
        cavity_optics_transpose{i-1}.x = cavity_optics{ i }.x - x1;
    end
    cavity_optics_transpose{N}   = cavity_optics{1};
    cavity_optics_transpose{N}.x = cavity_length;
    [w0, z0] = cavity_optics2its_waist_and_position( cavity_optics_transpose, lambda );
    z0 = cavity_optics{1}.x + z0;
else
    q0 = gbeam_propagation ( z0, qstart, zstart, cavity_optics );
    [w0, r0 ]  = q2wr( q0, lambda );
end


end