Ring modulator: built-in element and compound structure give different results

ring
modulator

#1

I am simulating a microring modulator using either the built-in Optical Ring Modulator element or a compound modulator using waveguides, directional coupler and the Optical Modulator Measured, both with the same parameters.

I would expect the modulation response to be the same using both approaches but it turns out I am getting different results.
Also, the Q is about 10,000 and therefore the 3dB bandwidth should be approx. 19 GHz @ 1550 nm, but the simulation yileds a value 10 times smaller.

I would really appreciate if someone could clarify what is wrong in my simulation.

Here is a program in which both modulators are compared
single_ring_5um.icp (2.5 MB)

Thanks!


#2

Hi @m2souza,

The problem you are facing is due to the artificial delay added in the time domain simulation for the waveguide element. This problem is fixed and will be available in the next release (targeted at this May, so will be available very soon). Corresponding documentation will also be online together with the release, more explanation will be in there.

The primitive ring modulator element has build in delay compensation and build in FIR filter while the compound built by couplers and waveguides does not, so the time domain simulation results are different.

The following figures are the waveform measured by the oscilloscope for the primitive ring modulator, compound ring modulator without delay compensation and with delay compensation, respectively, with same settings (2 Gbits/s, 64 bits, 4096 samples/bit). You can see the result of the compound with delay compensation is very similar to the result of the primitive one.


For the ring modulators (generally for all resonance structures), the sampling rate should be large enough so that the time domain simulation matches the frequency domain characterization (impulse response matches the scattering analysis). Another trick, add a low-pass filter after the PD will help to eliminate the over/under-shoot in the waveform. The modified file may help a little bit with a more reasonable global setting: single_ring_5um_modified.icp (1.6 MB)

For the FWHM bandwith, the Q-factor measured at 193.333 THz is about 9664.73 (~10000). So the 3dB BW should be ~20 GHz, which is correct according to the gain curve measured by the ONA:

I hope this could help to some extent :slight_smile: The new version of IC will be available soon to solve this problem.


#3

Could you please show us an example of modulation response characterization using the impulse versus frequency domain? That sounds like a useful comparison.


#4

Hi @lukasc,

I did a simple comparison test for the compound ring modulator, the structure is shown below:

The gain curves of the scattering data analysis (blue), impulse response without delay compensation (green) and with compensation (red) are plotted below. You can hardly see the blue curve since it is overlapped with the red curve, the green curve only lines up with the blue one in the center, and starts to go off as the frequency deviates from the center.

I hope this is useful, more information will be online together with the new release :slight_smile:


#5

Hi all,

The new release 2016b is out now, and please check out the KB page Fractional delay compensation for more information on this newly added property :slight_smile:


#6

Why is there “distortion” for frequencies away from the centre of the simulation, when the “fractional delay compensation” is turned on? Any way to fix this?


#7

The distortion is caused by the fractional delay filter model that we use for the waveguide element, and it can be reduced by increasing the simulation bandwidth/sample rate. Another option is to disable the fractional delay, this will eliminate the distortion. The drawback of disabling the fractional delay is that INTERCONNECT will force the delay (caused by the group delay) to be an integer multiple of the sample period and there will be an small error in the free spectral range away from the center frequency (also minimized by increasing the simulation bandwidth/sample rate).