Sources - how to create a flat spectrum

Hello everyone, does anyone knows how to create a flat spectrum source like this:

but lumerical always give me a gaussian source:

or is it theoretically not possible for flat spectrum source?

also, when say i define a broadband source to be startwavelength = 1um, stop wavelength=2um, the shape seems not very symmetrical. does anyone knows why? or is it just a graphic problem?

Thanks a lot.

Hi @geor0005,

By default, the source spectrum is created automatically with the pulse following the setting used to define the source. The spectrum is actually symmetric in the frequency domain. Since the wavelength is c/f, you lose this symmetry in the wavelength domain.

You can create a source with a custom spectrum, however, for linear system, you can simply run the simulation with the default settings and spectrum, and post process the results using the spectrum of interest, as used in the solar cell methodology. This is possible as the simulation frequency domain results are normalized with the source spectrum, so we get the impulse response of the system.

I hope this will help!