taper example results inconsistent

[lukasc-ubc]

Hello,

I am following your example: https://flaport.github.io/meow/examples/02_taper_length_sweep.html

and obtain strange results that don't make sense:

1. if I run the two lines in order, 10.0, then 5.0:

S1, _ = find_s_matrix(length=10.0)
S2, _ = find_s_matrix(length=5.0)
print(' - %s' % abs(S1[len(S1)//2][0]))
print(' - %s' % abs(S2[len(S2)//2][0]))

I get:

• 0.94347876
• 0.9601483

whereas if I run the two lines in reversed order, 5.0, then 10.0:

S2, _ = find_s_matrix(length=5.0)
S1, _ = find_s_matrix(length=10.0)
print(' - %s' % abs(S1[len(S1)//2][0]))
print(' - %s' % abs(S2[len(S2)//2][0]))

I get:

• 4.663316e-06
• 0.9602049

2. For a taper, I expect that a short taper would have higher loss, whereas a longer one would have lower loss. However, the results are opposite. For a 2 micron taper, the simulation predicts 0.98, but 10 µm predicts 0.94. The example you have also shows numbers where the shorter taper is lower loss. Doesn't make sense.

3. Performing a sweep of the length can be done by changing the cell_length. But when I change it, the results don't change at all.

def cell_length_sweep(length=10.0, length_sweep = np.linspace(2, 20, 10), cell_length=1.0, wl=1.55, Temp=25.0, num_modes=10):
    '''
    Perform a sweep of the taper length.
    Calculate the modes once, and only change the cell length to stretch the structure.
    '''
    modes = find_modes(
        length=length, cell_length=cell_length, wl=wl, T=Temp, num_modes=num_modes
    )
    lengths, T = [], []
    for length1 in tqdm(length_sweep):
        cell_length1 = length1/length*cell_length
        S, port_map = mw.compute_s_matrix(modes, cell_lengths=[cell_length1 for _ in modes])
        T.append ( abs(S[num_modes][0])**2 )
        lengths.append ( length*cell_length1/cell_length )
        print(' length %s, T %s' % (lengths[-1], T[-1]))
        print(' length %s, T %s' % (length1, T[-1]))

    plt.plot(lengths, 10*np.log10(T), '--pb', label='model', linewidth=1.5)
    SizeFont = 19
    plt.xlabel('Taper length (microns)', fontsize=SizeFont)
    plt.ylabel('Transmission (dB)', fontsize=SizeFont)
    plt.show()

4. Performing simulations on the same total taper length, but only changing the cell_length (hence number of cells) -- I would expect the loss to be lower for smaller cell sizes. But I get higher loss for more cells. Since this is a convergence test, it isn't conclusive if these numbers are not correct.

thank you
Lukas