Evaluating the beam width of a laser is an important step in characterizing its performance and determining its suitability for a particular application. There are several different methods and parameters that can be used to evaluate the beam width of a laser, including:
Full-width-at-half-maximum (FWHM): This is the width of the beam at a point where the intensity is half the peak intensity. It is commonly used as a measure of the beam width for laser beams with Gaussian intensity distributions.
1/e² radius: This is the radial distance from the center of the beam at which the intensity has dropped to 1/e² (about 13.5%) of the peak intensity. The 1/e² radius can be used as a measure of the width of the beam at a particular point and is commonly used to calculate the M² parameter.
Beam diameter: This is a measure of the width of the laser beam at a particular point, and can be defined in many ways, such as the D4σ, D9σ, D15σ etc.
Especially, for the beam with irregular shape, a statistical approach is preferred. The most popular being: D4σ, or simply: 4σ meaning: 4 times standard deviation of the Gaussian statistical distribution.
Gaussian fit: This method consists in fitting the measured beam profile to a Gaussian function, and extracting the parameters of the fit such as the beam waist and the divergence.
Top-hat fit: This method consists in fitting the measured beam profile to a Top-hat function, and extracting the parameters of the fit such as the beam diameter and the flat-top radius.
The definition of the beam width parameters of the Gaussian distribution of the intensity distribution across the beam is shown in the graph below:
The method used to evaluate the beam width will depend on the type of laser and the characteristics of the beam, as well as the specific requirements of the application. For example, a Gaussian fit may be more appropriate for a laser with a Gaussian intensity distribution, while a Top-hat fit may be more appropriate for a laser with a non-Gaussian intensity distribution. Additionally, a well-calibrated and well-designed system is needed to accurately measure these parameters.
Please mind, that beam width parameter is probably the most common metrics used to characterize the beam of a laser. For this reason it has been standardized and described in ISO 11146 norm.
In the mentioned standard the measurement of the elliptical beams has been also defined. The methodology of measuring such beams used in the Huaris software has been directly implemented according to this definition.
Beam width monitoring is a critical aspect to control the quality of the process conducted by the laser.
Methods to measure beam width
There are several different methods that can be used to measure the beam width of a laser, including:
1. Knife-edge scan: This method consists of moving a knife edge across the beam and measuring the intensity of the light transmitted through the edge. This can be done by using a photodiode or a camera. The data obtained from the knife-edge scan can be used to calculate the beam width by analyzing the intensity profile of the beam.
2. Beam profiler: A beam profiler is a device that captures an image of the beam profile and then analyzes the image to determine the beam’s characteristics. Beam profilers can be used to measure the beam width by analyzing the intensity distribution of the beam. They can be used to measure both the spatial and temporal profile of the beam.
3. Power meter: A power meter is a device that measures the power of a laser beam. It can be used to measure the beam width by measuring the power of the beam at different points along the beam axis. The data obtained from the power meter can be used to calculate the beam width by analyzing the power distribution of the beam.
4. Interferometry: This method consists in using an interferometer to split the laser beam into two beams and then recombining them to create an interference pattern. The interference pattern can be used to determine the phase and amplitude of the two beams, and from that, the beam width can be inferred.
5. Far-field measurement: It consists in measuring the beam’s intensity distribution in the far field. The far-field measurement can be done by using a camera or a detector array, and it can provide information about the beam’s divergence and other parameters that can be used to infer the beam width. In the far-field measurement the profiler is used. To achieve far-field image of the beam most commonly an additional focusing lens is used. An example of a measurement setup is shown in the graph below:
In such a setup a detector array of a profiler is positioned in the beam waist.
Each method has its own advantages and limitations. For example, knife-edge scan and beam profiler are easy to use and can provide a lot of information about the beam profile, but they can be affected by the alignment of the system. Interferometry is a precise method but is more complex to set up and use.
Why array-based detectors are the best for the laser beam characterization?
Array-based detectors are considered as one of the best options for laser beam characterization because they offer several advantages over other types of detectors:
High spatial resolution: Array-based detectors, such as CCD or CMOS cameras, have a large number of individual detector elements that are closely spaced together. This allows for a high spatial resolution, which can be useful for measuring small features or variations in the beam profile.For example, Huaris Five profiler has a pixel size of only 2.2 micrometer.
High dynamic range: Array-based detectors can measure a wide range of intensities, from very low levels to very high levels. This makes them well-suited for measuring laser beams with a wide range of power levels or for measuring beams with both high and low intensity regions.
High speed: Array-based detectors can acquire images at high speeds, which can be useful for measuring rapidly changing beams or for measuring the beam’s temporal characteristics. Nowadays CMOS and CCD cameras are able to acquire the intensity map much faster than typically the changes in its distribution in the beam happen allowing real-time monitoring of the beam quality.
High signal-to-noise ratio: Array-based detectors typically have a low noise floor, which allows them to measure weak signals with a high degree of accuracy.
Versatility: Array-based detectors can be used in a wide range of applications, from simple measurements of the beam profile to more advanced measurements of the beam’s temporal and spatial characteristics.
Cost-effective: Array-based detectors, such as CCD or CMOS cameras, can be less expensive than other types of detectors and they are widely available.
It’s worth noting that while array-based detectors are widely considered as one of the best options for laser beam characterization, other types of detectors can also be used, depending on the specific requirements of the application. Additionally, the performance of array-based detector can be affected by the optics, electronic noise, and the detector’s sensitivity.
When discussing array detectors it is necessary to mention the electronics and software that work with it. CMOS and CCD cameras due to their technological maturity are capable of working with high-level and very advanced software. As a result, a lot of new metrological functionalities can be implemented which is very often not possible or extremely difficult with other methods and equipment. As an example Huaris architecture can be presented: a local detector with electronics is physically connected with a local computer which hosts a local application allowing monitoring of the beam parameters at site. The local application also works as a communication hub feeding the data to the remote cloud server. The Cloud stores the data in the long term, analyzes the measurement results using artificial intelligence and helps to interpret them.