Tutorials - Slit-Based Profilers

Introduction
Problems that one might encounter with a NanoScan scanning-slit beam profiler are due to either scanhead damage, or out-of-tolerance conditions.

Scanhead damage can be categorized into two main types; Laser and Mechanical. Laser damage is the most prevalent, and results from exposure to lasers with excessive laser power/energy density, and or high average power. The damage can be classified into 2 categories, designated “Instantaneous” and “Long-Term”.

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“High Power” Defined

Photon’s High Power NanoScan is designed to measure ―high power‖ laser beams that were previously impossible to measure with standard BeamScan or NanoScan products. High power is a fairly indistinct term that means different things in different contexts. For our purposes, ―high power‖ is defined as between 100W and 5000W, however the High Power NanoScan will not be able to measure this power range for all wavelengths. High power laser beams are handled by using reflective materials, and the level of reflectivity, and thus its inverse, absorption, are dependent on the wavelength of the laser light. In general the long infrared wavelengths, such as that of the carbon dioxide laser at 10.6microns, are highly reflective. These allow for the highest power measurements up to the maximum levels of several kilowatts. When measuring these lasers and power levels the principle concern is the heat buildup in the scan head. The surfaces of the measurement drum and slits are better than 98% reflective to this wavelength, and thus only 2% of the incident power will be absorbed by the scan head and heat it up. Nonetheless, at 5000W this represents a heat load of 100W that will raise the temperature of the internal components, and may cause damage to the detector and encoder electronics. The High Power NanoScan is designed to be used for short-term measurements at these power levels. The beam should only be incident on the scan head for a few seconds. The software is equipped with a record mode that makes it easy to make a short measurement and then review the data while the scan head is allowed to cool down.

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Allen M. Cary, Jeffery L. Guttman Ph.D.,

Introduction

Profiling lasers with powers in the 10mW to 1W range is 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 pW 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.

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Measuring lasers can be a daunting task, particularly as the power or energy levels get higher. Many applications require that precise information about laser beam sizes be known, but these lasers may be capable of damaging the profilers. The slit-based profiler, such as the Photon NanoScan is ideally suited for measuring these higher power lasers, because it can accept a relatively high powered beam directly, without attenuation. Nonetheless there are limits. Determining these limits can be complicated, especially if the laser is operated in the pulsed mode, since both power and energy can contribute to the damage threshold.

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Device Alignment

Misleading results can be obtained when performing single axis or perpendicular axes scans if the device under test is either not centered in the entrance aperture or if it is not pointed reasonably well into the instrument. Some users assume the measurement is made through the device optical centroid axis and it may not be so. Only when the instrument optical axis and the device optical centroid axes are coincident will this be the case.

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The attached plot (next page) illustrate results obtained by two instruments for the same diode operating under the same conditions. The first instrument was a goniometer designed and built by our customer for their in-house testing, which they believe is very accurate. The second instrument was the Model LD 8900 Goniometric Radiometer from Photon.

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Using a NanoScan and standard optical accessories, acceptance testing and final system performance of a lens, lens assembly, optical subsystem, or overall system test may be measured. The NanoScan was developed to quickly and accurately evaluate a real-time measurement of an spatial image. This is done by evaluating the energy distribution of the spatial profile and measuring the beam size.

An image is considered in focus when you have a concentration of the largest amount of energy within the smallest spot size. Although there are many techniques for evaluating optical sub-assemblies such as a knife edge, Ronchi rulings, and interferometers, they tend to be time consuming or costly. The NanoScan has been engineered so that the non-optical engineer or technician can use it to its maximum potential with little training.

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Introduction
The Goniometric Radiometer Models LD 8900 and LD 8900R enable the user to characterize the angular radiation intensity of a wide range of light-emitting sources, including VCSELs, laser diodes, optical fibers and optical waveguides. In order to achieve accurate characterization, the light source in question must be positioned in a way that is both measured and repeatable. This application note suggests methods for adapting the LD 8900 and LD 8900R to accomodate the positioning of the light source in your application.

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Photon has a number of solutions for the measurement of M² ranging from simple manual processes to fully automated dedicated instruments. Depending on the frequency of the need to measure M² of laser and laser systems, Photon has a system that will meet most needs, whether for research and development of new laser systems, manufacturing quality assurance, or maintenance and service of existing systems.

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Astigmatism measurements of laser diodes, optical elements, and/or other sources can be made quickly and easily using a NanoScan/BeamScan. For example, by simply focusing the beam from a laser diode onto the profiler and measuring the distance between primary and secondary foci, an indication of source astigmatism is readily obtained. However, since this distance is on the order of 10 microns (μm), longitudinal magnification can be used to accommodate this measurement and reduce potential errors. This method has been used successfully for several laser diode astigmatism measurements.
The beam from the laser diode should be collimated with a lens of rather short focal length in order to reduce unwanted beam-truncation effects. The beam should then be focused with a rather long focallength lens. This will provide a long depth of focus and thus increase the distance between primary and secondary foci to the millimeter range—well within the range of most laboratory X-Y-Z stages. Figure 1 depicts the basic experimental setup.

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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.

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Although the NanoScan was designed originally to measure continuous wave (CW) laser beams, many lasers are operated in the pulsed mode. Measuring these pulsed beams has generally required the use of a CCD array profiler. This is a reasonable solution for low power lasers in the UV and visible wavelength range, but these will require external attenuation. Once the lasers leave the UV-VIS range, array cameras become extremely expensive. Although low frequency pulsed lasers operating in the 1Hz to 1000Hz range have no real alternative to the array profiler, the NanoScan can measure kHz frequency lasers. The NanoScan profiler incorporates the “peak connect” algorithm and softwarecontrolled variable scan speed on all scanheads to enable the measurement of these pulsed lasers. The NanoScan is ideal for measuring Q-switched lasers and lasers operating with pulse width modulation power (PWM) control. In the past few years, lasers with pico- and femtosecond pulse durations have begun to be used in many applications. Although these lasers add some additional complication to the measurement techniques, the NanoScan is well suited to measure them, too. We will discuss the measurement of all these types of pulsed lasers below.

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When profiling a coherent laser beam with a NanoScan with non-blackened, reflective
apertures (slit or pinhole), it is possible that interference may be observed on the beam
trace. (see Figure 1b).

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NanoScan applications are normally processes and the problems that are solved by them are usually one of the processes, such as an alignment, collimation, or a precise focusing process taking too long, not being accurate enough, or requiring too much high level intervention to accomplish by using other methods.

Many applications of lasers require that the laser beam be adjusted to meet some parameter, such as the beam size at the point of work, maintaining a collimated beam over a range of operation, or precisely aligning a beam on a target. Photon’s NanoScan makes these adjustments fast and easily. Its dynamic range allows both focused and unfocused beams to be measured without changes to attenuation, giving instantaneous feedback during the focus adjustment.

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Operation of the Goniometric Radiometer with Pulsed Sources

Photon has discovered that measuring pulsed sources with the goniometric radiometer is not straightforward. Although pulsed operation is possible, there are some parameter combinations that can cause inaccurate measurements. Because of the auto-ranging of the preamplifiers in the systems, some frequencies, pulse widths, duty cycles, and power levels will work fine and others that are scarcely different will create erroneous results. InGaAs and Silicon detector systems have different responses and operating spaces, but they both can generate erroneous results under some conditions.

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There is a fair amount of confusion in the reporting of dynamic range of beam profilers. The purpose of this technical note is to explain some of the terminology used in the discussion of this parameter by both Photon and our competitors.

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