State-of-the-Art Report: Laser Beam Measuring Instruments
By Allen Cary, Director of Marketing, Ophir Photonics Group (U.S.)
People working with lasers 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 characterization instruments provide the tools to know precisely what the laser 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 in many industries and applications. In this article, we will discuss the various types of measurements that can be made and how these can help to ensure that the laser is optimized for the application.
The most basic measurement of the laser beam is power or energy.¹ It is an important indicator of the laser's performance, but it does not tell the whole story. It indicates the overall impact that the laser is having on the material, but does not show how that power or energy is distributed.
Power/energy meters and sensors come in a variety of types; each suited to particular wavelengths, beam types, and power levels. Spatial characterization, or beam profiling, provides additional information about how the laser is going to perform its job. Because there are many different jobs that a laser might be performing, knowing the beam shape is very important.
Figure 1. Assorted power and energy sensors
Lasers come in many configurations with wavelengths from UV to far infrared, and each of these has its uses. They also can range in power from fractions of milliWatts to thousands of Watts, nanoJoules to kiloJoules. On the low end of the power ranges, applications can be for fiber optic telecommunication, laser scanning, or laser printing. Middle ranges, from a few hundred mW to tens of Watts, are used for surgery, eye repair, marking, LIDAR and range finding, plastic welding, and many other precision applications. At the high end, industrial welding and cutting, and military applications are predominant. Each of these requires different configurations of the laser beam, and profiling can help ensure that the laser matches the application's needs.
CCD Camera-Based Profilers
There are several different types of profiling instruments, CCD camera arrays, pyroelectric arrays, scanning slits, spinning reflectors, to name a few. The simplest to understand is the CCD camera, such as the Spiricon SP620. Combined with some attenuation optics, the CCD is placed in the beam path and a picture of the beam is captured. Beam profiling software, coupled to the CCD, provides an analysis of the beam profile. From CCD data it is possible to determine the size of the beam, the distribution of the power/energy in the beam, and, in general, whether the laser is performing as expected.
The silicon CCD is limited to the wavelength range from ~200nm to ~1100nm (UV to near infrared). Although this covers a lot of laser applications, there are wavelengths outside this range. The CCD sensor is very sensitive and delicate, which means that for even the low power lasers, substantial attenuation of the beam is required to avoid saturating or even damaging the array. For this reason, there is always the potential for the addition of error and distortions to the beam profile from these optics. It is also generally impossible to measure tightly focused laser beams with a CCD, because the attenuation optics require a fairly long beam path.
Figure 2. Ophir-Spiricon BeamGage® beam profiling software user interface
Scanning Slit Profilers Scanning slit beam profilers, such as the Photon NanoScan™, are available with different detectors. This means they can be used for any wavelength, from UV to far infrared. The scanning slit is also a natural attenuator; a very narrow slit is passed through the beam to measure it. Since this slit is only allowing a fraction of the beam to impinge on the detector, much higher powers can be measured without requiring additional attenuation. Because the slit plane is the measurement plane, it is also possible to measure the tightly focused beams and tiny diameters that cannot be directly measured with a CCD.
Figure 3. Photon NanoScan scanning slit profiler operation
The scanning slit profiler also provides instantaneous dynamic range. This means it can be used to measure a beam that is being focused. As a beam goes from a defocused to a focused condition, its power density increases exponentially. With a CCD, this increase needs to be compensated by adding more attenuation. With the scanning slit profiler, the defocused and focused beam can be viewed with direct feedback and no additional adjustment to the profiler. This greatly eases processes that require real-time adjustment of laser optical systems, such as when building a laser printer, marking system, or laser scanner.
Figure 4. Typical output from a slit scanning profiler
The scanning slit profiler only provides a measurement along the X and Y axis of the beam, and therefore lacks the detail information available from the CCD. Additionally, it can only measure CW and relatively high frequency pulsed beams. Beams with low pulse repetition rates, <1kHz, need to be measured with an array. CCDs can do this well for the wavelengths for which they are sensitive. Longer wavelengths require other types of systems. For the near infrared and "telecom" wavelengths from 1100nm-1800nm, InGaAs array cameras are available and can be used just like a CCD for measurement. Above this there is the Spiricon Pyrocam™ pyroelectric array camera. This system can measure laser beams with wavelengths all the way out to the terahertz range.
Preventing Damage from the Laser Measuring very high power lasers presents additional challenges, in particular how to place a measurement device, be it a profiler or a power meter, without the laser's damaging it. In order to do this, it is generally necessary to use a technique that samples only a small fraction of the beam. One way is to use some type of beam splitter, which steers a fractional portion of the beam into the profiler. Another method is to use a small spinning device, which can be a sensor or a reflector that samples the beam as it passes through the beam path.
¹Continuous wave (CW) beams are measured in units of power, Watts; pulsed lasers are measured in units of energy, Joules.