Why Modulated signal after EAM do not start from level 0

[Ibn Aziz]

Hi every one !
Hope u all are fine.

i am trying to modulate a signal using Electro-absorption modulator but after modulation the modulated signal always start form a power level much above then 0 zero. As u can see in below attachment (in red colour).

I sweep bias point at many different values but the modulated signal never started from zero and i have also checkd with many other settings like changing modulating voltage of EAM etc.

So i want to ask forum members that how can i make my modulated signal able to start from level zero ??
where i am getting wrong or just give me a hint so that i could get my required signal.

I am also attaching file and pic of scenario as below.
(I am using opti sys 7)

i will very thankful to you.
Best Regards

I am sharing picture of a setup

[Jaffar Emad Kadum]

Try to make the laser power equals to zero. and then check the diagram.

[Ibn Aziz]

thanks Alot Jaffar !

i have made many calculations my minimizing laser power but response is still similar i.e. the modulated signal never started from zero.

Only thing that happens by minimizing laser power is that the modulated signal start him self from lower values in same proportion as i am minimizing laser power.

I am attaching some the graphs of modulated signal at different laser powers

fig A with laser power = -100 dBm
fig B with laser power = -20 dBm
fig C with laser power = 0 dBm
fig D with laser power = 10 dBm

Basically i want modulated signal to start from level zero without changing laser power (because minimizing my laser power will degrade my system performance very badly) .

So there is any way to get modulated signal from level zero is by using any procedure/technique on EAM. So any one know any technique on EAM to get required signal kindly share.

OR

if any one has any clue regarding this problem kindly share.

Best Regards

[Jaffar Emad Kadum]

You can try signal processing component to remove the DC value of the power.

[Ravil]

Hi Ibn Aziz,

As it was correctly noticed by Jafar, your modulated power spectral signal has DC (constant component) in it. Because of this reason, your modulated signal never comes to zero. Not sure what is the final goal of your project. If you are designing transmitter and want to send this signal through the channel, and your signal-to-noise ratio is within the requirements (i.e. your DC component is at the channel noise level), you can correct it at the receiver which usually has photodetector, amplifier and the correction circuit.

[Ravil]

Otherwise you will need to add an optical filter at the receiver, if I am not mistaken…

[Ibn Aziz]

Thanks a lot Ravil !

Yes my modulated signal has DC component and I want to remove it but could not find any way to get rid of it.

If it would be an electrical signal than I can use clipper/ clamper to remove Dc component but I don’t know how to remove Dc component in Optical signal.

Dear Ravil after modulation from EAM, I am going to contract this signal but due to DC component I am getting a contracted modulated signal like shown in below attached pic.

If I would transmit such a signal over fiber it will degrade my sys overall performance severely due to collision( as u can see from red portion in graph) and many other factors.
So to get better performance I have to get rid of this DC component. I hope u get my point.

If anyone has solution to this problem kindly share.
Best Regards

[Damian Marek]

This DC component comes from the properties of the Measured Electroabsorption modulator. Double click the component and use the Help to find the technical reference and see how you can edit the text file to provide Total absorption at a 0 Voltage signal.

[Author]

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