function ret=isCavityStable (abcd_cavity) 
    % finds if a cavity is stable baced on its abcd matrix
    % see eq.4 for stability condition
    % doi:10.1088/1464-4258/7/1/002
    % http://dx.doi.org/10.1088/1464-4258/7/1/002

    A=abcd_cavity(1,1);
    B=abcd_cavity(1,2);
    C=abcd_cavity(2,1);
    D=abcd_cavity(2,2);

    u= (A+D+2)/4;
    if ( (0 < u) && (u < 1) )
        ret = true;
    else
        ret = false;
    end

end