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Transmission spectrum for optical waveguide

Posted Aug 8, 2012, 9:17 a.m. EDT RF & Microwave Engineering Version 4.2a 9 Replies

Frédéric Peyskens
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Hello,

I am currently trying to extract a transmission spectrum from a Si waveguide on top of SiO2 substrate. There are some gold particles on top of the Si (this configuration is taken from an article in which they simulate it with FDTD). I tried several things to extract the transmission spectrum:

1) S21 parameter in dB
2) ratio of the Poynting flux at the input and output (tried both with fields Ex,Ey,Ez, emw.Hx, emw.Hy and emw.Hz and with fields from the Boundary mode electric/magnetic field)

I am however not able to get the right profile (for some wavelenghts I even get quit high gains which is unphysical).

It seems that also reflections are taken into account, but in an experiment you will only take the ratio of the actual input power (which is forward propagating) and the forward propagating output power.

Does anybody know how to extract a sensible transmission spectrum (which you would obtain in experiments).

Thanks in advance.

F. Peyskens
9 Replies Last Post Mar 20, 2013, 8:17 a.m. EDT
Alexander Kuznetsov
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Posted: 1 decade ago Aug 9, 2012, 2:53 p.m. EDT
plasmonic wire grating example is a good start. In general, the approach shown there gives the right reflection/transmission.
plasmonic wire grating example is a good start. In general, the approach shown there gives the right reflection/transmission.
Frédéric Peyskens
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Posted: 1 decade ago Aug 10, 2012, 4:44 a.m. EDT
Thank you for the answer. I tried however a similar reasoning with the waveguide: input of the waveguide is port 1 and output is port 2. Then I extract the S21, but this gives me not the right transmission spectrum. I tried it through the z-component of the Poynting vector (as the waveguide propagates waves in the + z-direction), but this also does not give me the right results. In case of this 2-port system I would expect that the transmission is defined as

(Power propagating in the + z-direction at the output port)/(Power calculated in the Boundary Mode Analysis at the input port)

Or am I seeing this wrong? At least, this is what you would measure experimentally.

Kind regards,
F. Peyskens
Thank you for the answer. I tried however a similar reasoning with the waveguide: input of the waveguide is port 1 and output is port 2. Then I extract the S21, but this gives me not the right transmission spectrum. I tried it through the z-component of the Poynting vector (as the waveguide propagates waves in the + z-direction), but this also does not give me the right results. In case of this 2-port system I would expect that the transmission is defined as (Power propagating in the + z-direction at the output port)/(Power calculated in the Boundary Mode Analysis at the input port) Or am I seeing this wrong? At least, this is what you would measure experimentally. Kind regards, F. Peyskens
Alexander Kuznetsov
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Posted: 1 decade ago Aug 10, 2012, 1:14 p.m. EDT

input of the waveguide is port 1 and output is port 2.

port 1 is input as well as output for the reflected wave.
abs(S11)^2 would be reflectivity coefficient, and abs(S21)^2 - transmission.
To validate your model you can use it to calculate reflectivity (or transmission) of SiO2 film and compare to theoretical value.
[QUOTE] input of the waveguide is port 1 and output is port 2. [/QUOTE] port 1 is input as well as output for the reflected wave. abs(S11)^2 would be reflectivity coefficient, and abs(S21)^2 - transmission. To validate your model you can use it to calculate reflectivity (or transmission) of SiO2 film and compare to theoretical value.
Frédéric Peyskens
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Posted: 1 decade ago Aug 12, 2012, 6:17 a.m. EDT
Hello,

Thank you for your fast response. I have tried what you suggested, but the problem with the S-parameters is that in transmission it gives me large gains for some wavelengths, which is unphysical. I would try to extract transmission through the Poynting vector, but I only want the forward propagating power at the output (and not the reflected power going back to the input as you mention) and I guess part of the reflected power is taken into account too. Is there a way to extract:
1) the excitation power at the input (and not taking the reflective power into account)
2) the forward propagating output power

Thanks in advance.

Kind regards,
F. Peyskens
Hello, Thank you for your fast response. I have tried what you suggested, but the problem with the S-parameters is that in transmission it gives me large gains for some wavelengths, which is unphysical. I would try to extract transmission through the Poynting vector, but I only want the forward propagating power at the output (and not the reflected power going back to the input as you mention) and I guess part of the reflected power is taken into account too. Is there a way to extract: 1) the excitation power at the input (and not taking the reflective power into account) 2) the forward propagating output power Thanks in advance. Kind regards, F. Peyskens
Brian West
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Posted: 1 decade ago Aug 29, 2012, 2:45 p.m. EDT
COMSOL's sign convention for the imaginary part of dielectric constants is opposite to that of most other photonic modeling software. Your non-physical gain might be caused by this.
COMSOL's sign convention for the imaginary part of dielectric constants is opposite to that of most other photonic modeling software. Your non-physical gain might be caused by this.
Frédéric Peyskens
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Posted: 1 decade ago Sep 12, 2012, 4:29 a.m. EDT
Thank you for this response. We got the same answer from the comsol helpdesk and they now found indeed a loss instead of a gain.
Thank you for this response. We got the same answer from the comsol helpdesk and they now found indeed a loss instead of a gain.
Richie Guo
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Posted: 1 decade ago Jan 4, 2013, 11:18 p.m. EST

Thank you for this response. We got the same answer from the comsol helpdesk and they now found indeed a loss instead of a gain.


Hi, I have meet similar problem with you. Can you share your mph file? thank you.
[QUOTE] Thank you for this response. We got the same answer from the comsol helpdesk and they now found indeed a loss instead of a gain. [/QUOTE] Hi, I have meet similar problem with you. Can you share your mph file? thank you.
Richie Guo
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Posted: 1 decade ago Jan 10, 2013, 10:21 p.m. EST
Hi, thank you very much for your fast response.

How do you calculate the transmission spectrum, use S21 or output power/input power?

I attached my ring resonator and need to simulate the transmission spectrum, I try but always fail, would you please help me?
Hi, thank you very much for your fast response. How do you calculate the transmission spectrum, use S21 or output power/input power? I attached my ring resonator and need to simulate the transmission spectrum, I try but always fail, would you please help me?

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Sruthi R
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Posted: 1 decade ago Mar 20, 2013, 8:17 a.m. EDT
hi,
im also experiencing a similar problem like yours. can u pls share ur mph file so that i will come to know how u have taken the transmission spectrum.
hi, im also experiencing a similar problem like yours. can u pls share ur mph file so that i will come to know how u have taken the transmission spectrum.

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