Discussion Forum

Hi,

The Wavefront shape list is used to define the shape of the isosurfaces of constant phase within the beam, not surfaces of constant electric field amplitude. The wavefront shape must be known so that the curvature of these surfaces of constant amplitude can be described; this information can be used to identify, for example, caustic surfaces that are produced by the interaction of rays with lenses.

In contrast, the Gaussian function that defines a Gaussian beam concerns the isosurfaces of constant amplitude, which can be specified independently of the phase distribution. Thus it is possible for the correct Wavefront shape to be a Plane wave even if the intensity distribution within the beam is Gaussian.

To release a beam with a Gaussian intensity distribution, you can do one of the following:
1) If you're releasing rays from a boundary using the Inlet feature, you can use a Density-based release of rays, in which the density would be a Gaussian function. You may need to refine the mesh on the Inlet surface to get an accurate distribution. You would then see a larger quantity of rays in the center of the beam.
2) Alternatively, you could multiply the initial intensity by a Gaussian distribution function. You would then see a more uniform distribution of rays, but those at the center of the beam would have greater intensity.

Best Regards,
Christopher Boucher,
COMSOL

Hi Christopher
Thanks for the reply. The Gaussian function can be defined as the Initial intensity distribution. If I release rays from the Gaussian waist, I should choose the Plane wave as the Wavefront shape. Then the diameter of the Gaussian beam keeps a constant, while it’s not the fact. Usually the radius of Gaussian beam can be written as the following form when it propagates along z axis:
W(z)= sqrt(A+B*z+C*z^2)
How can I define a Gaussian laser with the beam radius W(z) when it propagates along z axis.
Thank you for attention to my problem!

Hello everyone
Can anybody help?

Hi,

To see a variation in beam radius of the form w(z)= sqrt(A+B*z+C*z^2), it would be necessary to solve Maxwell's Equations using the RF Module or the Wave Optics Module. The Wave Optics Module, in particular, includes an Electromagnetic Waves, Beam Envelopes interface that is useful for modeling beams when you know the direction of beam propagation. For example, see the tutorial "Self-Focusing": www.comsol.com/model/self-focusing-14639.

The Ray Optics Module, on the other hand, doesn't show this relationship between beam radius and position because it uses an approximation to Maxwell's Equations that is valid for optically large geometries.

Best Regards,
Christopher Boucher

Hi Christopher
Thank you very much. It's very helpful.

Hello Christopher

Can we simulate the focussing of gaussian beam by lens ??

For example gaussian beam is propagating in air and interacts with lens and focussed at focal plane. So can we model this focussing effect through "Wave optics module" ?