Laser Beam Measurement, Rayleigh Scatter, and Adhering to ISO 11146
This document is intended to show how the process of obtaining data and providing measurements using Ophir-Spiricon’s Rayleigh-scatter beam profiling technology, BeamWatch®, aligns with standards established in ISO 11146.
New Rayleigh-Scatter Technology
All BeamWatch models apply the same approach to provide non-interfering real-time beam measurement for lasers that are typically too powerful for direct measurement or require constant monitoring while in use. A real-time non-contact measurement is obtained by relying on the Rayleigh-scattering properties of the most common air molecules: nitrogen and oxygen. This is a well-documented technique with consistent behavior utilized in everything from chemical solution analysis to atmospheric lidar studies. By capturing the proportionally scattered laser light with an appropriately designed optical system and camera placed transverse to the beam’s path, BeamWatch can provide the equivalent of simultaneous scanning slit measurement slices along the entire FOV of the camera.
The products have been designed to meet the requirements of ISO 11146-1 sections 5 (Test principles) and 6 (Measurement arrangement and test equipment), with the exception that the equipment is not coaxial with the laser beam (see section 6.2). This is a key technology difference between BeamWatch and the NanoScan 2, our ISO 11146 compliant full-contact scanning slit beam profiler. However, testing shows measurements between these two tools to be compatible. Since the equipment positioning difference is the only feature fully unanticipated by the ISO documentation, and Rayleigh-scatter measurement is a firmly established scientific technique for measuring light, we recommend an update to include this novel method of contactless measurement.
ISO Measurement Process and Compliance
For each device model the entire system has been extensively tested to ensure accuracy and reproducibility in a variety of conditions appropriate for intended applications and current laser technologies. To further verify BeamWatch’s ability to provide ISO measurements, testing has also included the aforementioned comparisons to measurements from the NanoScan 2.
Because the raw Rayleigh-scatter data is only proportionally representative of actual beam measurements, some processing is mandatory in order to improve signal-to-noise and reduce variance. As recommended by ISO, this process includes some basic data filters and a background subtraction to ensure that the data recorded by the detector provide a precise representation of the beam. We utilize a proprietary data model to account for the signal-to-noise differential between the edges and center of the beam section selected for study. This differential occurs due to a combination of factors including lens selection, beam divergence, and the design of the BeamWatch device itself. Most of this processing falls under the umbrella of ISO 11146-1 sections 6.4 and 6.5. The portion that does not, involves a Fourier transform method as recommended by ISO 11146-3 section 3.4, and is the largest component of the proprietary data model spoken of.
A significant advantage of BeamWatch over your typical scanning slit profiler is that it measures a substantial length of the beam all at once. This means that the initial determination of beam waist location, beam width, divergence angles, and beam propagation ratios are easily and almost instantly obtained from a hyperbolic fit along the propagation axis. This procedure, summarized in the next paragraph and detailed in ISO 11146-1 section 9, is functionally equivalent to taking measurements at hundreds of separate locations along the beam with a full-contact scanning slit profiler. Two items of note here are the positioning requirements of the fitting method, and the constraint that beams be stigmatic or simple astigmatic. Since each BeamWatch model is designed with specific applications in mind, it is largely the end user’s responsibility to determine if their beam is an appropriate candidate for use with the technology.
The hyperbolic fit is performed on at least 10 points at different z-positions, with the requirement that roughly half be distributed within one Rayleigh length of the waist, and the remainder spread beyond two Rayleigh lengths from the waist. Due to the nature of the technology the requisite number of data points is easily obtained, but depending on the BeamWatch model and laser in use, the beam waist may need to be positioned off-center from the camera FOV to achieve more than two Rayleigh lengths on one side of the beam. Although this is sometimes visually disconcerting, tests show that such a positioning does not negatively impact measurement accuracy. Initial beam width measurements are based off the moving-slit method. A correction for D4σ is then applied as per ISO 11146-3 section 4, equation #69. It follows that a corrected M2 and divergence can be reached by applying equation #63 and the definition of M2 as given in ISO 11146-1 section 3.16.2. The software highlights measurements as ISO whenever this data-fitting requirement is met, and falls back on a simple direct calculation of these variables if attempts to fit fail. This can help the user know if they need to adjust the relative position of their laser with respect to the BeamWatch unit.
The BeamWatch Rayleigh-scatter measurement technology can be used to analyze the properties of a laser beam without having the beam incident on the sensor. Despite the technology’s infancy and the lack of official standards for the approach to data acquisition, test comparisons of results with those from the widelyaccepted scanning-slit technology of the NanoScan 2 show that BeamWatch achieves ISO-compliant measurements. It has been demonstrated that the methods used to obtain and refine the BeamWatch data validates the claim that the technology meets ISO 11146 standards.