#In order to force a constant effective index, set the group index and effective index to the same value
#In order to enter optical length simply set effective index/group index to '1'

#Parameters to generate 
#Andrea Melloni, "Synthesis of a parallel-coupled ring-resonator filter" 
#Opt. Lett. 26, 917-919 (2001) 

#The center frequency for the waveguide propagation properties

f0 = getnamed("ONA1","center frequency");
ng = 1;
neff = ng;
lambda = c / f0;

#filter parameters
FSR = 100e9;	#free spectral range
B = 3.333e9;	#bandwidth
N = 4; 		#order of the filter
nr = 2;		#integer multiple of FSR (second periodicity)
oddm = 0;		#odd multiple of a quarter-wavelength
range = 0.5e12;	#frequency range analysis

#calculate coupling coefficient from Q factor for each resonator
cq = matrix(N);
for( q=1:N ) {
   gq = sqrt( 2 ) * sin( ( 2 * q - 1 ) / ( 2 * N ) * pi ); 	  	#equation (2)
   Qq = FSR / gq / B;					  	#equation (1)
   cq( q ) = pi ^ 2 / 2 / Qq^2 * ( sqrt( 1 + 4 * Qq^2 / pi^2 ) - 1 );  	#equation (3)		
}

#the circunference of the rings
lr = c / neff / FSR;

#the connecting waveguide lengths
nr = nr / 2;
lc = lr / nr;

#combined length of busbar arms must be an odd number of half guide wavelength at the design frequency
if( oddm ) {
   n = round ( lc * neff / lambda );
   lc = ( n + 1 / 2 ) * lambda / neff;
}

lc1 = lc / 2;
lc2 = lc / 2;

switchtodesign;

setnamed("ONA1","fwhm excursion",1);
setnamed("ONA1","frequency range",range);

setnamed("R11","frequency",f0);
setnamed("R11","length",lr);
setnamed("R11","group index 1",ng);
setnamed("R11","effective index 1",neff);
setnamed("R11","coupling coefficient 1 1",cq( 1 ));
setnamed("R11","coupling coefficient 1 2",cq( 1 ));

setnamed("L11","frequency",f0);
setnamed("L11","length",lc1);
setnamed("L11","group index 1",ng);
setnamed("L11","effective index 1",neff);

setnamed("L12","frequency",f0);
setnamed("L12","length",lc2);
setnamed("L12","group index 1",ng);
setnamed("L12","effective index 1",neff);

setnamed("R12","frequency",f0);
setnamed("R12","length",lr);
setnamed("R12","group index 1",ng);
setnamed("R12","effective index 1",neff);
setnamed("R12","coupling coefficient 1 1",cq( 2 ));
setnamed("R12","coupling coefficient 1 2",cq( 2 ));

if( N > 2 ) {
     setnamed("L21","frequency",f0);
     setnamed("L21","length",lc1);
     setnamed("L21","group index 1",ng);
     setnamed("L21","effective index 1",neff);

     setnamed("L22","frequency",f0);
     setnamed("L22","length",lc2);
     setnamed("L22","group index 1",ng);
     setnamed("L22","effective index 1",neff);

     setnamed("R13","frequency",f0);
     setnamed("R13","length",lr);
     setnamed("R13","group index 1",ng);
     setnamed("R13","effective index 1",neff);
     setnamed("R13","coupling coefficient 1 1",cq( 3 ));
     setnamed("R13","coupling coefficient 1 2",cq( 3 ));
}

if( N > 3 ) {
     setnamed("L31","frequency",f0);
     setnamed("L31","length",lc1);
     setnamed("L31","group index 1",ng);
     setnamed("L31","effective index 1",neff);

     setnamed("L32","frequency",f0);
     setnamed("L32","length",lc2);
     setnamed("L32","group index 1",ng);
     setnamed("L32","effective index 1",neff);

     setnamed("R14","frequency",f0);
     setnamed("R14","length",lr);
     setnamed("R14","group index 1",ng);
     setnamed("R14","effective index 1",neff);
     setnamed("R14","coupling coefficient 1 1",cq( 4 ));
     setnamed("R14","coupling coefficient 1 2",cq( 4 ));
}

if( N > 4 ) {
     setnamed("L41","frequency",f0);
     setnamed("L41","length",lc1);
     setnamed("L41","group index 1",ng);
     setnamed("L41","effective index 1",neff);

     setnamed("L42","frequency",f0);
     setnamed("L42","length",lc2);
     setnamed("L42","group index 1",ng);
     setnamed("L42","effective index 1",neff);

     setnamed("R15","frequency",f0);
     setnamed("R15","length",lr);
     setnamed("R15","group index 1",ng);
     setnamed("R15","effective index 1",neff);
     setnamed("R15","coupling coefficient 1 1",cq( 5 ));
     setnamed("R15","coupling coefficient 1 2",cq( 5 ));
}