Prime(s)Time 01/17
Convenient diagnosis of raw laser beam with the LaserQualityMonitor
Beam quality analysis of collimated laser radiation is a challenge. In principle, a caustic analysis according to ISO 11146 with measurements over up to 6 Rayleigh lengths is difficult because the beam waist location can lie almost at infinity. However, ISO 11146 allows focussing the collimated laser beam with lenses and using this caustic for the quality analysis. The measurement system LaserQualityMonitor (LQM) uses this principle to measure the quality of fiber coupled and collimated laser beam sources. As a camera-based measuring device, the LQM is designed for several wavelengths from NIR-, through VIS- to the UV-range with laser powers of up to 10 kW.
How the LaserQualityMonitor works
The used collimator can be connected to the LQM using one of the provided adapters. Depending on the maximum laser power, the basic LQM will be equipped with additional attenuation modules to reduce the average power. The collimated laser beam passes the focusing optics and is focused within the device. After passing additional attenuators it is finally enlarged and imaged onto a CCD sensor using an objective. A slot for optical density fillers (OD0 to OD5) is placed in front of the CCD sensor to allow a fine adjustment of the laser power to the dynamic range of the CCD.
A controlled and precise variation of the internal beam path length enables the detection of beam parameters in front of, in and behind the focal plane. The LaserDiagnosticsSoftware LDS calculates the laser beam caustic from the measured data and derives additional parameters like Rayleigh length or beam quality factor M². Data is transferred between the LQM and the PC via fast Ethernet connection.
Screenshot of a caustic analysis
Modular design for adaption to the laser power
The LaserQualityMonitor can be modularly adapted to the used laser power. The basic module with internal beam attenuation is designed for a laser power of up to 20 W. A first beam attenuator module increases the possible laser power to 200 W with air-cooled beam absorbers and to 500 W with water cooled absorbers. A second water cooled attenuation module extends the maximum laser power to 10 kW.
The optical components of the LQM are equipped with high-performance anti-reflection coatings. The coatings are designed according to the spectral working range of 1030 - 1090 nm, 515 - 545 nm and 340 - 360 nm respectively. Optionally, the absorber of the high-power attenuation module can be supplemented with an additional system for power measurement of the laser radiation. Thanks to this modular design, the LQM can be adapted to a wide range of measurement tasks and is unique on the market due to the integrated camera, optics and attenuation modules, allowing for direct high power measurements without the need for external attenuation.
Short Laser-ABC: The Rayleigh length
Rayleigh length: what is this?
In classical photography, the term "focus depth" (often also referred to as depth of field) describes the distance in which a lens images an object sharply on the film or sensor. The length of the focus depth depends on the aperture set on the lens as well as the focal length. The analogy to this in laser technology is the Rayleigh length, it also indicates a "sharp" range of the laser beam and is linked to the optical transformation of the laser beam by means of lenses or mirrors.
Figure 1: Determination of the Rayleigh length zr
What influences the Rayleigh length?
The Rayleigh-length is defined as the distance in front of or behind the focus of a laser beam, at which the cross-sectional area of the beam has doubled in size with respect to the focus site. The mathematical description of Rayleigh length is given by:
Accordingly, the Rayleigh length depends on the beam radius at the focus ([ω0] beam waist), the wavelength [λ] of the radiation and, of course, the beam quality [M2]:
- The smaller the waist radius, the smaller is the depth of focus. Since the relationship here is squared a change of the waist radius is correspondingly intensive.
- The wavelength is inversely proportional to the Rayleigh length. As an example, the Rayleigh length of a CO2 laser beam (λ = 10.6 μm) is 10 × shorter than that of a Nd: YAG laser (λ=1.064 μm) with the same beam quality and the same waist radius.
- The influence of the beam quality M² is also inversely proportional. A poorer beam quality means a larger value of M² and thus a shorter Rayleigh length. The ideal case is the Gaussian ray with M² = 1.
As the waist radius of the focused laser beam is influenced by the imaging ratio of the focusing optics (the focusing number F), the focusing of the laser radiation has a direct influence on the Rayleigh length:
The focusing number F is defined by the focal length of the optics divided by the beam radius on the optics (illumination of the optics). This means, that the smaller/larger the focal length of the focusing optics with the same illumination, the smaller/larger the Rayleigh length of the focused radiation will be.
Effects of beam quality, focus and Rayleigh length
References: Rolf Klein, Laser Welding Plastics, Wiley Verlag, ISBN 978-3-527-40972-3, 2011
Importance of Rayleigh length for the laser beam propagation
The Rayleigh length is of crucial importance for laser material processing. Within one Rayleigh length, the energy density (intensity) of a gaussian shaped laser beam varies by a factor of 2. Outside the Rayleigh length, the mathematical description of the beam propagation changes from wave optics to geometrical optics. The magnification of the beam radius depends on the divergence angle and the distance to the focus with a significantly stronger influence on the cross-sectional area and power density of the laser beam.
This implies that the Rayleigh length of the focused laser beam provides a measure for the process window during laser material processing. As an example, in laser beam cutting, the sheet thickness should correspond to a maximum of twice the Rayleigh length of the focused laser beam in order to produce straight cutting edges. If the sheet thickness is significantly greater than the Rayleigh length of the focused radiation, the cutting edges will become round and the relative focal position to the sheet will have a noticeable effect on the cutting result.
In general, a larger Rayleigh length results in a larger tolerance range for the height positioning of the focus during processing. For example, fluctuations in the material thickness or the surface position can be compensated better by a large Rayleigh length than in the case of a short Rayleigh length.