Lumerical FDTD has a very complete Knowledge Base to answer your question. You can refer to:

https://kb.lumerical.com/ref_sim_obj_sources.html

Dear Akila, you may check here for your doubts:

https://optiwave.com/forums/

Dear Akila Murugesan,

Research Article and Software indicating difference between point source, line source and plane wave excitation in FDTD.

I hope the information will help you.

With Best Wishes,

Samir G. Pandya

Please read the description of sources as the link below https://kb.lumerical.com/ref_sim_obj_sources.html

The following types of sources are available in FDTD and MODE' 2.5D FDTD solver:

📷 Point sources (dipole)

Oscillating dipoles act as sources in Maxwell's equation to produce electromagnetic fields. Their position and direction are specified in terms of the center position and their orientation through angles theta, phi.

In MODE, for the 2.5D FDTD solver, the orientation of the dipole source partially depends on whether the polarization of the propagator simulation is set to TE or TM.  Depending on the simulation polarization and dipole type, the theta, phi values may be locked.

📷 Gaussian and Cauchy/Lorentzian beam sources

A Gaussian source defines a beam of electromagnetic radiation propagating in a specific direction, with the amplitude defined by a Gaussian cross-section of a given width. By default, the Gaussian sources use a scalar beam approximation for the electric field which is valid as long as the waist beam diameter is much larger than the diffraction limit. The scalar approximation assumes that the fields in the direction of propagation are zero. For a highly focused beam, there is also a thin lens source that will inject a fully vectorial beam. The cross section of this beam will be a Gaussian if the lens is not filled, and will be a sinc function if the lens is filled. In each case, the beams are injected along a line perpendicular to the propagation direction, and are clipped at the edges of the source. For more information on the usage of these source, visit the planewave and beam sources page.

📷 Plane wave sources

Plane wave sources are used to inject laterally-uniform electromagnetic energy from one side of the source region. In two-dimensional simulations, the plane wave source injects along a line, while in three-dimensional simulations the plane wave source injects along a plane. It is also possible to inject a plane wave at an angle. The plane wave source is actually the same object as the Gaussian source, with the only difference being the SOURCE SHAPE setting. For more information on the usage of these source, visit the planewave and beam sources page as well as BFAST page for angled incidence.

📷 Total-field scattered-field sources

Total-field scattered-field sources are used to separate the computation region into two distinct regions – one contains the total field (i.e. the sum of the incident field and the scattered field), while the second region contains only the scattered field. The incident field is a plane wave with a wavevector normal to injection surface. This source type is particularly useful to study the scattering behavior of objects, as the scattered field can be isolated from the incident field.

TIP: More information

Some care is needed when using the TFSF source. The concept of total/scattered field is complex and can lead to misinterpretation of results, particularly with regards to energy conservation. See the TFSF section for more information.

📷 Mode sources

The mode source is used to inject a guided mode into the simulation region. The geometry of the mode source (i.e. center location, and span) is used to compute the guided modes for the structure. In three-dimensional simulations, the modes are computed across a plane, while in two-dimensions they are computed across a line. From a list of possible modes, a single mode is selected for injection into the simulation region. For additional details on the operation of the mode solver, consult the integrated mode source section.

📷 Import sources

The import source allows the user to specify a custom field profile for the source injection plane. The custom field profile can be calculated from an analytic formula, imported from another FDTD simulation, or imported from other simulation tools such as Breault Research Organization’s ASAP ray tracer.

📷 Global source options

The global source options window adjusts default frequency-domain parameters. These settings can be used by any of the frequency domain sources by unchecking the 'override global source settings' checkbox in the source's Frequency/Wavelength tab.