THORLABS M2MS, Operating Manual

Page: 82 / 167

BP209 Series 7 Operation Instructions
Page 80
21424-D02
7.2.3 Beam Diameter Requirements
The Beam Quality measurement is based on the determination of the waist diameter of a fo-
cused beam and the convergence (divergence) of the beam before (after) the beam waist.
Ideal Gaussian beams have the best focusability, which results in the minimum possible beam
diameter and M²=1. The absolute minimum waist diameter increases as the focal length of the
lens increases. In other words, for a given focal length, the smaller the waist diameter the
closer M² is to 1.
The minimum waist diameter increases (at a constant M²) with the wavelength.
Limitations to the measurement of the beam diameter are given by the capabilities of the Beam
Profiler, see
Technical Data . The initial (unfocused) beam diameter must not exceed the
maximum measurable beam diameter, the waist diameter must be greater than the minimum
beam diameter. The following diagram illustrates the requirements for the initial beam diameter
depending on the wavelength and for M²=1 and M²=2 (lens with a focal length of 250 mm as
delivered with the M2MS system):
BP209 Series Beam Diameter Requirements
145