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.
Laser Measurement Solutions for Industrial Applications
Industrial processes involving lasers (marking, cutting, etc.) present special challenges. A laser behaving even just a little differently than it should (for example, power a bit too low or high, or beam shape not quite what the process “expects”), can significantly impact the profitability of the process. The goal of this webinar is to provide you with tools to help you solve – or, better yet, prevent – these problems.
The webinar will take place on Wednesday March 20th details are below. We will host this webinar at two separate times, so that you can choose one according to your time zone:
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.
Purpose: To quantify the effect of distance on the irradiance and beam homogeneity from 4 curing lights.
Materials and Methods: Four light-emitting diode curing lights were evaluated: Fusion, Bluephase 16i, Demi and FlashLite Magna. The irradiance at the centre of the light beam (ICB) was measured at 1.0 to 9.0 mm from the emitting tip using a 3.9-mm diameter probe connected to a spectrometer. The uniformity of the beam from each curing light was characterized by means of the “top hat factor” at 2.0, 4.0, 6.0 and 8.0 mm from the emitting tip. The useful beam diameter, within which irradiance values were greater than 400 mW/cm2, was calculated. The ICB, top hat factor and useful beam diameter were compared by analysis of variance and Fisher’s protected least significant difference test at α = 0.01.
It’s time to buy another car. Like everyone else these days, you’re a bit cost-conscious, so you’re looking at getting the most for your money. You decide to see what the local used car lot has to offer. Before you get out of your car, the slick used car salesman approaches you, shoves a card into your hand and is a little too happy to help you find a new automobile. You approach a couple of cars that you think might fit your budget. The closer you get, you notice something that seems odd. You’re standing in front of two identical cars: same make, same model, same year, same color, even the same warranty. But you see that one is $5,000 less than the other. Hmm. The first, more expensive car seems to be in good order, looks nice, smells okay. You climb into the second car and it hits you why there is a difference in price: the second car has no dashboard instrumentation panel! No speedometer, no fuel gage, no warning lights.
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
Ophir Photonics’ BeamCube is becoming popular as a one-stop product for measuring the performance of laser welding systems used in the manufacture of medical devices. Low to medium power laser weld parameters can be simultaneously measured, recorded, and monitored for compliance to federal requirements and previously set manufacturing limits. BeamCube uses a CCD camera, Thermopile sensor, and fast photodiode to measure the beam profile, average power, and temporal pulse shape of the welding laser.
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).
- Define Concept of Mode Quality
- Show What Happens in Process When Mode Changes
- Detailed Introduction to Beam Profiling Instrumentation
- Examples of How to Diagnose Processing Problems
- New Profiling Techniques
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.
There are many applications of lasers in which the beam profile is of critical importance. When the beam profile is important, it is usually necessary to measure it to insure that the proper profile exists. For some lasers and applications this may only be necessary during the design or fabrication phase of the laser. In other cases it is necessary to monitor the laser profile continuously during the laser operation. For example scientific applications of lasers often push the laser to its operational limits and continuous or periodic measurement of the beam profile is necessary to insure that the laser is still operating as expected. Some industrial laser applications require periodic beam profile monitoring to eliminate scrap produced when the laser degrades. In other applications, such as some medical uses of lasers, the practitioner has no capability to tune the laser, and the manufacturers measure the beam profile in the design phase to ensure that the laser provides reliable performance at all times. However, there are medical uses of lasers, such as photo-refractive keratotomy, PRK, wherein periodic checking of the beam profile can considerably enhance the reliability of the operation. PRK is an example of laser beam shaping which is a process whereby the irradiance of the laser beam is changed along its cross section. In order for this laser beam shaping to be effective, it is necessary to be able to measure the degree to which the irradiance pattern or beam profile has been modified by the shaping medium. This paper describes the general state of the art of laser beam profile analysis.1-14 It introduces the general need for beam profile analysis, methods for measuring the laser beam profile, a description of instrumentation that is used in beam profile measurement, a discussion of the information that can be obtained simply by viewing the beam profile, and finally, how quantitative measurements are made on laser beam profiles, and the significance of those quantitative measurements.
The Hartmann sensor was invented a century ago to perform optical metrology. Subsequently these sensors have been adapted to a wide variety of applications including adaptive optics, ophthalmology, and laser wavefront characterization. In this document we will explain the operation of the Spiricon Hartmann Wavefront Analyzer (HWA), analyze the device limitations, and compare it to other similar technologies.