Hello, 

I am trying to simulate the Gaussian beam system with 405 nm wavelength of my experimental design. I would like to verify and confirm the beam waist of the Gaussian beam and the associated working distance. There are three main components of the system: 

• Optical fibre with MFD 2.4um and 0.091 for NA at 1/e^2. So I set for the aperture type as Entrance Pupil Diameter as the size of 0.0024 mm. The waist location between the fibre centre is calculated by taking account the NA at 1/e^2 as 0.013mm. 
• The Gaussian beam pass through Collimator and reached to the focusing lens put in the Black-Box by the manufactuer. Therefore, we are interested in determing the beam waist focused by the focusing lens and the associated working distance. 
• The system layout and lens data is shown below: 

I modelled the Gaussian beam as the paraxial ray system as shown below: 

I optimised the working distance of the beam waist as shown below: 

Additionally, I compared the results of X-position in FTT PSF cross section function at the 1/e^2 intensity (0.1387), I found the beam waist should be 5.347 um in the same working distance of 43.523mm given in PSF readout which is slightly different compared to the results of paraxial readout as 2.69 um in beam waist. 

Therefore, my questions is which method would be the best way to compute for the beam waist according to me situation? Are there any parameters I missed during setting up the system? 

Many thanks, 

Kevin