The ability to focus the laser beam to the smallest waist size, or diffraction limited spot size, is a function of M² times the wavelength constant 4λ/π and the divergence angle θ. When the M² value approaches 1, then only the wavelength and divergence angle affect this waist size. For this reason many laser makers strive to build lasers and laser systems with M² value as close to 1 as possible. M² has thus become a specification of laser quality.
The ability to make real-time measurements of this parameter provides the fastest possible measurement and allows for the measurement of a single beam pulse in low repetition rate lasers and even some high repetition rate lasers with rates to 50kHz.¹
We will show the results of this new measurement technique for a number of lasers and compare these results, where possible, with those obtained by other recognized methods and instruments.
The instrument that we will describe uses a patented method to focus the entire caustic of the beam on a CCD camera at once. Ten reflective surfaces created by five uncoated quartz plates are positioned to reflect ten beams onto the camera chip. With the proper selection and positioning of a test lens, ten beam profiles from the waist region and several Rayleigh ranges either side of the waist are focused on ten defined regions of the camera chip. These beam profiles are analyzed simultaneously and a fit of the beam caustic is made to the M² curve. The M² is calculated for each frame captured on the camera, providing a real-time analysis of the M² value. In addition other ISO parameters of the beam propagation are also reported, including beam waist size and location, Rayleigh range and beam divergence angle.