Reply To: Arbitrary sign for eigen mode

[Guilin Sun]

Here is the result I got directly from your script (except line 34 is modified):
?abs(Smat);
result: 
0.000439021  0.0166555  0.017211  0.000397579  
0.0028741  0.00273113  0.017198  0.000322681  
0.00300699  0.00305221  0.00322448  0.000252065  
0.000378088  0.00033553  0.000237014  0.000287072  

After the straight waveguide is added, I use the following script:

## solver

addfdtd;

set("x min",-x_shift/2);set("x max",(K+0.5)*x_shift);

set("y",0);set("y span",K*y_shift+2e-6);

set("z",0);set("z span",1e-6);

set("mesh accuracy", 4);

setresource("FDTD", "GPU", true);

setnamed("FDTD", "express mode", true);

## ports in the middle of each block

for (k=0:K){

addport();

set("x", k*x_shift);

}

selectpartial("FDTD::ports::port");

set("auto update", false);

set("frequency dependent profile", false);

set("mode selection", "user select");

set("selected mode numbers", 2); # 2nd mode is antisymmetric and will show the issue

clearportmodedata;

seteigensolver("correct backward propagating modes", false);

seteigensolver("correct backward propagating modes when pml is present", false);

 

wavelength = 1560e-9;

setglobalsource("set frequency", true);

setglobalsource("center wavelength", wavelength);

setglobalsource("wavelength span", 0);

setglobalmonitor("wavelength center", wavelength);

setglobalmonitor("frequency points", 1);

setglobalmonitor("wavelength span", 0);

setglobalmonitor("use source limits", 1);

##

save("sign_arb.fsp");

 

## s-parameter sweep

addsweep(3);

runsweep;

sweepdata = getsweepresult("s-parameter sweep","S matrix");

Smat = pinch(sweepdata.S); # scattering matrix

 

### see the issue when propagating from port 3 to 4 vs from

?conj(Smat(3,1))*Smat(4,1); # excite port 1, and look at the propagating from port 3 to 4

?conj(Smat(3,2))*Smat(4,2); # excite port 2, and look at the propagating from port 3 to 4

?conj(Smat(2,4))*Smat(1,4); # excite port 4, and look at the propagating from port 2 to 1

?conj(Smat(2,3))*Smat(1,3); # excite port 3, and look at the propagating from port 2 to 1

 

##

neff_list = matrix(K+1);

S0 = matrix(K+1,K+1);

for (k=1:K+1){

NEFF = getresult("FDTD::ports::port "+num2str(k), "neff");

neff_list(k) = NEFF.neff;

mode1 = getresult("FDTD::ports::port "+num2str(k), "mode profiles");

for (j=1:K+1){

mode2 = getresult("FDTD::ports::port "+num2str(j), "mode profiles");

cr12 = pinch(cross(mode1.E2, mode2.H2));

cr11 = pinch(cross(mode1.E2, mode1.H2));

cr22 = pinch(cross(mode2.E2, mode2.H2));

cr12 = integrate(pinch(cr12(:,:,1)), [1,2], pinch(mode1.y), pinch(mode1.z));

cr11 = integrate(pinch(cr11(:,:,1)), [1,2], pinch(mode1.y), pinch(mode1.z));

cr22 = integrate(pinch(cr22(:,:,1)), [1,2], pinch(mode1.y), pinch(mode1.z));

S0(j,k) = cr12/sqrt(cr11*cr22);

}

}

## modes have minus signs

image(real(S0));

I got the result:

Please use ONE staright waveguide for testing.