Beam Quality (M²) Measurement; General - THORLABS BC106-VIS Operation Manual

Beam Quality (M²) Measurement
3.5
3.5.1

General

In laser science, the parameter M² is the ratio of the beam parameter product (BPP)
of an actual beam to that of an ideal Gaussian beam at the same wavelength. It is
often referred to as the beam quality factor, since its value can be used to quantify
the degree of variation the actual beam is from such an ideal beam. M² is a better
guide to beam quality than Gaussian appearance, however, since there are many
cases in which the beam can look Gaussian, yet have an M² value far from unity. As
well, a beam can appear very "un-Gaussian", yet have an M² value close to unity. For
a single mode TEM00 Gaussian Laser beam, M² is exactly one.
The value of M² can be determined by measuring 4σ diameter or "second moment"
width. Unlike the beam parameter product, M² is unitless and has no variance with
wavelength.
In laser science, the beam parameter product (BPP) is the product of a laser beam's
divergence angle (half-angle) and the radius of the beam at its narrowest point (the
beam waist).
The M² value is an important measure of beam quality. It is widely used in the laser
industry as a specification, and its method of measurement is defined in ISO 11146
standard. It is especially useful for determining the degree of beam divergence of
real laser beams and the minimum focussed spot size.
The Camera Beam Profilers Series BC100 are camera-based instruments to
measure the beam quality factor M². For the measurement, the beam profiler is
mounted to a software controlled translation stage, moved step by step along the
propagation axis of a focussed beam and the beam diameter is measured at each
position in order to find the beam waist.
The associated extension set for the Camera Beam Profilers Series is the
xxx consisting of a linear translation stage of two selectable lengths and a
89
mounting adapter kit for BC100 beam profiler.
BC1M2 extension set features:
Accurate M² measurements
Measures divergence, waist diameter, Rayleigh range and astigmatism
Flexible design
Compatible with CW and Quasi-CW pulsed laser sources
Short measurement cycles
fully ISO11146 compliant
Motivation
Many laser applications require maximum optical power density at minimum beam
diameter. Not only the focusing optics but mainly the quality of the light source itself
has influence to the focusability. A high beam quality is a pre-condition for optimal
focusability.
What is Beam Quality
Beam Quality and its direct influence to focusability is a very important feature of
lasers.
© 2011 Thorlabs
Operating the Beam Profiler 81
BC1M2-