# Fabry Perot laser add grating

TWLMexampleLaserFP300umCF353THzR3R3nel100 (4).icp (253.2 KB) Thank you for contacting Lumerical support. Before proceeding, please note that this is a public forum and anything you post will be visible to the general public. For users residing in the United States, please ensure that the files and information contained within your posts are not International Traffic in Arms Regulations (ITAR) or export controlled.

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1. What device are you trying to simulate? Include diagrams if available.
I want to simulate add grating on FP laser.FP laser which is based on traveling wave laser,lumerical posted on this website:https://support.lumerical.com/hc/en-us/articles/360042326134-Fabry-Perot-laser-using-travelling-wave-laser-model-TWLM-

2. What results are you trying to obtain? Be as specific as possible.
2. What results are you trying to obtain? Be as specific as possible.*
I try to add grating on FP laser in order to make the spectrum look like DFB laser and optimize the difference of all kind of grating period

3. Description of the problem or issue.
I use this equation about Bragg grating on lumerical website to calculated grating period
λ_B=2n_eff Ʌ
Lambda B I use is 850 nm.Effective index is 3.5

So I use the equation above I get grating period is 121nm.But in my simulation result,adding 121nm grating doesn’t make the performance better.The side mode does not lower than without grating.Do I choose the wrong grating period?Do I need to add something such as quantum well makes the performance is similar as DFB?
(Sorry,by virtue of upload issue.I can’t upload the file I changed the grating period.The file which I am upload is example file.I only change TWLM to 121nm)

4. Lumerical product and software version.

R2.1

TWLMexampleLaserFP300umCF353THzR3R3nel100 (4).icp (253.2 KB)

Hello @blake6337685,

I believe the grating period you calculated should be correct. It would be very helpful to see your simulation file, could you please try uploading it again? You may have to put the file in design mode.

It might also be helpful to use this example as a starting point for your simulation instead of the Fabry Perot example:

TWLMexampleLaserFP300umCF353THzR3R3nel100 (4).icp (650.3 KB)
Hi,johnson.My file is upload on above.

Hello @blake6337685,

You will have to define a grating coupling coefficient property (under Waveguide > Mode 1) for your TWLM element for the grating to create a cavity. For an example, you can see the DFB page I posted above.

My problem is solved.Thank you Johnson.

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