% 澇葷阿俊 蝶弗 祈堡家磊狼 弊貳橇
x_max=0.016
y_max=1

lamda =532* 10^-9 ;
d=38 * 10^-6 ;
theta_r=20* pi/180 ;  
theta_s=-20* pi/180 ; 
dn=0 : 0.00005 : x_max;

dn_desired=lamda*(cos(theta_r)*cos(theta_s))^0.5/(2*d);

Vs=pi*dn*d/(lamda*(cos(theta_r)*cos(theta_s))^0.5); %grating strength for s-polarization
%Vp=Vs*cos(theta_s-theta_r); %grating strength for p-polarization

plot (dn, (sin(Vs)).^2,'r');
hold on;
%plot (dn, (sin(Vp)).^2,'b');
%legend('s-polarization','p-polarization')

line ([dn_desired dn_desired], [0 y_max],'Linestyle',':','Color','m')
line ([0.05  0.05],[0 y_max],'Linestyle',':')
temp=mat2str(dn_desired,2)
text(dn_desired,y_max-.1,['\Deltan=',temp,'\rightarrow'],'HorizontalAlignment','right')
text(0.05, y_max-.1,'\leftarrow\Deltan=0.05')
axis([0 x_max 0 y_max])

xlabel(['Index modulation','(\Deltan)'])
ylabel(['diffraction Efficiency','(\eta)'])
grid
hold off;