With all of today’s technology, we are drowning in data. This is true in fields like marketing and running a business, and it rings true with measuring your laser, as well. There are all sorts of things you can measure, but it’s never worth measuring something unless you plan on doing something with that information.
If you’re running a business, there are all sorts of statistics and information you can find out about your market. But you have to consider what data will help you make educated business decisions. If you’re using a laser, there are many different ways to measure your laser and many different values you can check. The question is which measurements you should make to help optimize your laser efficiency: whether you measure in terms of power consumed, time to process parts, etc.
Having worked in laser marking and engraving for several years, I can say that there is a certain mindset when it comes to laser quality that, in some cases, could be a dangerous one, especially in industrial applications. The mindset is, “If the laser is making a mark, then why check the performance of the laser?” Let’s discuss this as it relates to several different laser applications, including why this mindset could be causing you to lose time and money.
In 1997, Dr. Carlos Roundy, founder and president of Spiricon Inc., presented a paper at the 4th International Workshop on Lasers and Optics Characterization in Munich Germany. This paper was based on work that was carried out at Spiricon in the mid 90’s. At the time new insights were being presented on how to characterize a laser beam. Previous definitions were somewhat simplistic and most often were driven by customers telling us how they wanted the beam measured.
Part 2 of this paper examines the effects of baseline caused errors in beam width measurements using the four most common methods for calculating a beam width:
- D4σ, second moment
- 13.5% of Peak, using the moving slit method
- 86.5% encircled power smallest aperture (Power in a Bucket)
- Knife edge 16/84
- Knife edge 10/90
A, B, C & D are all ISO recommended methods. Method E is one that we at Spiricon recommend over D as it tends to be more accurate for beams of higher modes, including Donut TEM01* .
In Part 2, we demonstrated the need for employing an accurately computed baseline and retaining both the positive and negative temporal noise inherent in the camera’s output data. We observed that for the most common beam width methods, preserving the noise moved each method closer to reality, even though many of these results are not always a very accurate second moment equivalent.
There is no doubt that the application of the fiber laser source has changed the way laser users look at the use of lasers. Fiber lasers are used in processes too numerous to count and people are discovering new uses every day.
We’ve heard for several years about the high quality beams that the fiber laser produces. Laser quality is usually defined by an M2 measurement. The closer the M2 measurement is to the value of 1, the better the “focusability” of the laser and, therefore, the higher the quality of the beam. Fiber lasers usually tout an M2 number of anywhere from 1.5 to 2.
A recent application called for the beam analysis of a 1550nm laser source with a challenging optical arrangement. Signal loss occurred at each beam transfer across multiple reflective surfaces. A more sensitive camera with Frame Summing was required to bring out the full beam pattern, size, and depth.
The 1550nm laser source was measured with an Ophir Laser Power Sensor, PD300- IR, at 1.5mW, with a beam diameter of 3.5mm. The lab resources were limited to using a phosphor coated CCD camera that is sensitive to the wavelengths between 1140nm and 1610nm. The requirement of the analysis necessitated that the beam be reflected off different surfaces to document the changes of the beam image relative to absorption and reflection.
W. Edwards Deming said, “If you can’t measure it, you can’t control it,” and “You cannot inspect quality into the product; it is already there.” Instead of trying to inspect-in quality, focus on adding value once you’re assured that the previous process was done right and to specification.
Since the mid-1980’s, tens of thousands of machine vision camera systems have been installed along manufacturing lines and in assembly stations. They can be found in every conceivable industry where there are volume users - automotive, pharmaceutical, medical devices, electronics, aerospace, and, at this point, almost every industry that makes something. Why? Because subsequently adding value to a good part reduces the cost of goods, making the manufacturer more money. In fact, machine vision is one of the reasons the U.S. regained its world position producing quality products at reasonable prices.
The New Year is a time to look back on the past year and contemplate how to do things better. It’s a time to make New Year’s resolutions in an effort to be a better person and do things better. One of the most common and popular New Year’s resolutions is the renewed commitment to losing weight. As many of us are all too familiar with, that means eating better and getting involved in a regular exercise routine. One of the advantages of losing weight in the 21st Century is the technology at our fingertips that helps us reach those goals. From the sciences behind how the body uses the food we eat, to the plethora of Smartphone apps that help us measure and track our results, it seems like losing weight would be an easy task. So why is it not?
A new instrument design allows the M2 beam propagation ratio to be measured in real time at the update rate of a standard CCD camera. This allows lasers from single shot to CW to be measured while the laser cavities are being adjusted. This drastically reduces the test time required for this operation. In this paper we will discuss the theory behind this innovative approach to the M2 measurement and the methods for the selection of the proper optical components for use of the system with various laser types and beam shapes. The authors will show results of numerous measurements of different lasers and laser types, including solid state diode, fiber and traditional gas lasers with M2 values from near 1 to considerably higher values, and show comparisons these results with other measurement methods.
A research organization was developing a critical procedure that required a pulsed DUV laser beam at 193nm. The current equipment only produced a gray-scale image that told little about the distribution of the energy intensity across the beam profile. The solution was a CCD camera-based profiler.
Measuring the propagation parameters of a laser beam is an important method of understanding the quality of the laser beam and predicting its performance for various laser applications. For this reason, it is one of the major specifications required by laser users and reported by laser manufacturers. Ophir-Spiricon has been a leader in providing instruments dedicated to this important measurement. There are currently three different instruments available under the Spiricon and Photon brands, and in this article we will explain the differences between them and the reasons for these different approaches to making this measurement. Hopefully, this will assist you in deciding which approach is best for your laser and laser application.
One of the laser’s most useful properties is that it can propagate over great distances defining a straight line, and many optical systems are designed to exploit this property. Therefore, beam profilers are often used to verify the performance of lasers and optical systems to avoid problems caused by bad alignment or to streamline the manufacturing processes of these devices. This is often a simple measurement of where in space a laser spot is focused or aimed, which is relevant to a number of laser applications from laser range finding to optical scanning, laser marking to building laser printers. Beam profilers have the capacity to tell where the beam is located on the detector, and different types of profilers can do this with varying degrees of accuracy and precision. Most of the time the measurement is made by placing the beam profiler at some distance from the laser source and aligning the beam until it points to the ideal target. For laser scanning applications the measurement may be made in different locations across the scan plane to ensure proper linearity of the scan. But there is more to this measurement and more that can be learned about the laser’s performance.
The need to profile lasers with powers in the 10mW to 1W range are becoming more common. Many of these lasers are in the visible spectrum, allowing them to be measured with CCD and CMOS camera systems. As with any laser that is being measured with a camera array, the beam needs to be attenuated, but there are some cautions to be observed. These beams are not so powerful that they will damage or destroy typical absorptive filters. In fact, it is possible to stack up a sufficient optical density to reduce the power of a 1W laser to the nW levels that will not saturate the detector. Unfortunately, doing this will more than likely result in erroneous measurements. This is due to a phenomenon called thermal lensing, or thermal blooming. The laser’s energy heats the local area of the absorptive filter, changing its optical properties. These changes often result in changes to the refractive index of the substrate, forming a lens that may either focus or expand the beam. At lower powers, this phenomenon can be observed over discernable timeframe, hence the term “blooming.” However, it may also occur almost instantly, giving the illusion of stability and accurate measurements.
There is a fair amount of confusion caused by the reporting of dynamic range of beam profilers. The purpose of this applications note is to explain some of the terminology used in the discussion of this parameter by both Ophir- Spiricon and other suppliers of beam profilers.
Dynamic Range is the ratio of the largest measurable signal to the smallest measurable signal. The smallest measurable signal is typically defined as that equal to the noise level, or alternatively the “Noise Equivalent Exposure” or that point where the Signal-to-noise ratio (SNR) is 1.
To measure a beam profile we should have a SNR of at least 10 to obtain a minimally useful result. It requires more like SNR of 100:1 to achieve the 2% accuracy. However, in instrument specifications Ophir and its competitors use the Digitization Dynamic Range. The discussion below will follow this approach.
Ophir Photonics uses pyroelectric detectors in a number of their products, both for beam profiling and for laser power measurement. The Photon and Spiricon brands are laser beam profilers based on scanning slit or array technologies; Ophir brand products are laser power measurement instruments.
Spiricon Pyrocam III
The Pyrocam™ III is a pyroelectric array camera that can be used to profile lasers of very short wavelength UV light or Infrared from the near IR wavelengths to the very far IR and even Terahertz wavelengths.
Abstract: The Mode-Field Diameter (MFD) and “spot size” of an assortment of lensed and tapered specialty fibers were determined from far-field and near-field measurements. In the far field, measurements were made using a 3D-scanning goniometric radiometer that provides a complete hemispherical profile. Indirect measures of the near field derived from these data are reported, including the Petermann II MFD, the 1/e2 spot size using the farfield Gaussian approximation, and a measure obtained from 2D Fourier transform inversion of the far field using phase retrieval techniques. In the near field, direct profile measurements were made using an IR Vidicon camera and magnifying objective lenses, with the spot size reported as the 1/e2 diameter of the imaged profile.
Laser beams are like light bulbs, they change in output over time. When laser manufacturing high precision, high reliability parts, this presents a challenge. Because of this, there is a need to regularly measure and manage the characteristics that determine a beam’s quality and consistency.
There are several steps involved in maintaining a laser’s efficiency over time. This maintenance is important to help prevent laser output variation from affecting the end product being manufactured.
The beam profiler magnification calibration involves measuring spot centroids for known beam position translations. This can be done either by moving the profiler or moving the spot. The former method is preferred since the profiler with magnification is usually mounted to a high quality 3-axis translation stage. For a 25x or greater magnification it is recommended to use a stage equipped with a differential micrometer capable of producing accurate and repeatable 1m steps.
M², or Beam Propagation Ratio, is a value that indicates how close a laser is to being a single mode TEM00 beam, which in turn determines how small a beam waist can be focused. For the perfect Gaussian TEM00 condition the M² equals 1.
|Watch the beam profiling video|
|Benefits of Beam Profiling|
|You can get more out of your laser
Choosing the best profiler for a laser is a complex process. There is no one profiler available that works with all lasers because of all the factors involved. Here we’d like to help you begin figuring out what to focus on when doing laser profiler shopping (window or otherwise).
Most people working with lasers today are trying to do something with the light beam, either as the raw beam or, more commonly, modified with optics. Whether it is printing a label on a part, welding a precision joint or repairing a retina, it is important to understand the nature of the laser beam and its performance. Laser beam profiling provides the tools to characterize the laser and know precisely what the beam is doing at the point of the work and if the optics are having the desired effect. Lasers and laser applications come in many varieties, varying in power density, wavelength, depth-of-focus, beam size, pulse duration and myriad other parameters. It is this variety that makes lasers so useful for interacting with and manipulating many different materials and media. But, it is also this variety that adds complexity to the beam profiling process.
Whether you’re new to lasers or you’ve been working with them for some time, you may be wondering what all the fuss is over laser beam profiling. Why worry about the quality of the laser beam that you’ve just put into production? Or, if you think your process is humming along nicely, why fix what isn’t broken? You might think that laser beam quality has more than likely been addressed at the research and development stage of the laser you have, or even at the manufacturing or integration stages of the system that you’ve received. You might be correct about that and hopefully you are. However, you might be surprised (as I was) to learn that this is not always the case.
Question: How can I be certain that my Beam Profiler is measuring accurately? Is there a standard calibration methodology?
Answer: There is no calibration standard from which one can verify their camera based beam profiling measurement accuracy. Spiricon has done the next best thing to provide customer confidence in reliable and consistent results from its camera based profilers. The issue can be broken down into two major areas; 1) the input (camera), and 2) the output (from software algorithms).
Acrylic Mode Burns have historically been universally used because they are the lowest cost alternative to the budget-minded end user. This method, however has been cited for producing dangerous, cancer-causing airborne materials, and is not a real time technology. On the other hand, Real-time beam profiling has been historically too expensive for the average user to acquire.
We discuss a new technology that enables the design of a new low-cost real time CO2 high power beam profiler that is within reach of almost all end-users. This design does not produce dangerous fumes, and runs in real time. We will provide examples and results of testing with this device, and compare some of them to traditional camera based methods.
- THz Range available to the user – Wavelengths and Frequencies
- Tools to Image THz beams
- Optics- Type and Sources
- Cameras and Other Sensors
CCD cameras are commonly used for many imaging applications, as well as in optical instrumentation applications. These cameras have many excellent characteristics for both scene imaging and laser beam analysis. However, CCD cameras have two characteristics that limit their potential performance. The first limiting factor is the baseline drift of the camera. If the baseline drifts below the digitizer zero, data in the background is lost, and is uncorrectable. If the baseline drifts above the digitizer zero, then a false background is introduced into the scene. This false background is partially correctable by taking a background frame with no input image, and then subtracting that from each imaged frame. ("Partially correctable" will be explained in detail later.)
The second characteristic that inhibits CCD cameras is their high level of random noise. A typical CCD camera used with an 8-bit digitizer yielding 256 counts, has 2 to 6 counts of random noise in the baseline. The noise is typically Gaussian, and goes both positive and negative about a mean or average baseline level. When normal baseline subtraction occurs, the negative noise components are truncated, leaving only the positive components. These lost negative noise components can distort measurements that rely on low intensity background.
Situations exist in which the baseline offset and lost negative noise components are very significant. For example, in image processing, when attempting to distinguish data with a very low contrast between objects, the contrast is compromised by the loss of the negative noise. Secondly the measurement of laser beam widths requires analysis of very low intensity signals far out into the wings of the beam. The intensity is low, but the area is large, and so even small distortion can create significant errors in measuring beam width.
The effect of baseline error is particularly significant on the measurement of a laser beam width. This measurement is very important because it gives the size of the beam at the measurement point, it is used in laser divergence measurement, and it is critical for realistic measurement of M2, the ultimate criterion for the quality of a laser beam. One measurement of laser beam width, called second moment, or D4, which is the ISO definition of a true laser beam width, is especially sensitive to noise in the baseline. The D4 measurement method integrates all signals far out into the wings of the beam, and gives particular weight to the noise and signal in the wings. It is impossible to make this measurement without the negative noise components, and without other special algorithms to limit the effect of noise in the wings.