Hello Jeeyoung Lee,

I would recommend using the Paraxial Gaussian beam calculation to get an idea of the Rayleigh length:

So if you are within the Rayleigh length in your system, a collimated beam can be a good approximation.

Then it depends what you need to study in your system. For further information, check our learning path: https://my.zemax.com/learning-paths/modeling-laser-beam-propagation-in-opticstudio/

Do not hesitate if you have any further questions.

Sandrine