Question

3 micron Gaussian spot setup - non-diverging?

  • 26 September 2021
  • 1 reply
  • 96 views

Badge

Hi all,

I hope you are all still well. Apologies for another query on my part!

I have been aiming to simulate a system that consists of a 5x Galilean collimating beam expander consisting of a -50mm plano-concave lens (LC1715-B, Thorlabs) and 250 mm plano-convex lens (LA1461-B-ML, Thorlabs), prior to a 25.4 mm plano-convex focussing lens (LA1951-B, Thorlabs).

The beam input to the beam expander is defined as having a 1/e^2 diameter of 2.6 mm, with a peak irradiance of 5.01 x 10^6 W/mm^2 and a divergence of 0.143 degrees (derived from an M^2 of 1.3). This theoretically gives an input beam of 13 mm 1/e^2 diameter into the 25.4 mm focusing lens, with this producing a focussed spot size of 3 micron (albeit with considerable spherical aberration). I wish to plot the axial peak intensity distribution of this rapidly diverging Gaussian spot using POP.

I have been able to successfully collimate the beam expander setup using the REAY AND RAED operands within the Merit Function Editor. I did this in a separate .ZMX file to avoid potential sampling issues, with it being a relatviely long propagation distance and tight focus. This provided the expected beam output of ~ 13 mm 1/e^2 diameter. It has a peak irradiance of around 2x10^5 W/mm^2.

5x beam expander setup
5x beam expander lens data
Input beam definition for 5x beam expander setup
Beam expander output beam

Using this beam diameter and peak irradiance for the input beam definition in the separate focusing lens .ZMX file, I have been able to resolve the expected beam shape of the tightly focussed 3 micron Gaussian spot with surrounding rings (from the spherical aberration).

3 micron Gaussian focusing spot setup
3 micron Gaussian focusing spot setup - lens data

 

Input beam definition for 3 micron Gaussian focusing spot setup
Beam profile at surface 9
Beam profile at surface 16

However, the beam is not highly divergent as I would expect, instead maintaining its 3 micron diameter across an axial range of over 1 mm? I recall previously being told that if the spherical aberration is visible on the layout plot (which it is here) that you should use ray tracing, but I am unsure how to (or if it is possible to) extract peak irradiance information using this simulation method?

I have attached the .ZMX and .ZOS files for the focusing lens setups for reference.

Thanks for any help you can provide!


1 reply

Badge

Hi all,

I hope everyone is still keeping well!

I have been working on this issue but unfortunately despite having sorted the sampling and frame widths to be adequate enough to avoid the “insufficient guard band” issue, the extended focus problem/consistent spot size still persists.

Any advice or suggestions would be greatly appreciated.

Thanks,

Donald

Reply