What is thermal lensing?

Thermal lensing causes the focus position of a lens to change as the optics are heated or cooled.

How to get rid of thermal lensing? 

By determining the thermal time constant of the optical system using the Haas BWA-MON or by using the Haas LTI TLC optics which passively mitigate thermal lensing.

Thermal lensing is a significant challenge in high-power laser applications, particularly for collimators and focusing lenses. This phenomenon arises due to the inherent thermal properties of optical materials, notably the coefficient of thermal expansion (α, measured in 10^-6 °C) and the temperature coefficient of refractive index (dn/dT, also in 10^-6 °C). As these materials absorb laser energy, temperature variations lead to changes in their refractive indices and physical dimensions, causing unintended alterations in the lens’ focusing behavior.

Traditionally, mitigating thermal lensing has involved the use of low-expansion glasses, such as fused silica. While fused silica offers a low coefficient of thermal expansion, it does not possess a correspondingly low dn/dT. Consequently, the dn/dT of the glass significantly impacts the thermal lensing effect. Thermal management of the lens mechanics has been one of the primary methods to address this issue. However, since optical glass generally exhibits poor thermal conductivity, substantial temperature gradients can develop across the lens, adversely affecting its performance. Even with thermal management, these gradients can lead to performance issues, necessitating several minutes for the optical system to stabilize thermally before the focus shift remains within the Rayleigh range.

In response to these challenges, Haas Laser Technologies has developed the TLC optics, a novel solution designed to passively compensate for thermal lensing in high-power laser systems. Protected under U.S. Patent No. 8,274,743, this innovative optic design employs a proprietary merit function that balances the dn/dT between two high-power laser optical materials, along with optimized thicknesses, air spacing, and curvatures. This approach minimizes thermal lensing across a broad temperature range, from 20°C to 200°C. As a result, applications utilizing the TLC optics remain in focus from the moment the laser is activated until it is turned off, eliminating the need for a warm-up period.

The effectiveness of this design is illustrated in Figure 2, which compares a simple fused silica best-form singlet at ambient temperature (depicted with blue rays) to the same lens subjected to a significant temperature gradient (shown with green rays). The shift in focus due to thermal effects is evident. In contrast, Figure 5 provides a close-up view of the Haas TLC doublet (as seen in Figure 1) with the same focal length and temperature gradient, demonstrating its superior thermal stability and consistent focusing performance under varying thermal conditions.

By addressing the root causes of thermal lensing through advanced optical design and material selection, the Haas TLC optics represents a significant advancement in high-power laser optics, ensuring reliable and precise performance in demanding applications.

Using the Haas LTI patented BWA-MON®, we measured a customer’s 10 kW, multimode fiber laser and their F125 mm, all-fused silica collimation lens for thermal lensing.  Figure 5 is a screenshot of the measurement that shows a focus shift of a 1 mm over 1 minute.

Figure 6 shows the same laser from Figure 5 but with a Haas LTI TLC collimator and the beam waist location over another minute.  The thermal shift was less than 100 microns over the same time frame.  Equally important to observe between Figure 5 and Figure 6 is that the all-fused silica lens is experiencing tremendous thermal stress to the point that severe astigmatism manifests.  At about 5 seconds, you can see the two axes cross, and the astigmatism gets worse as the thermal load increases.  Whereas the relative position of the beam axes does not change with the TLC optics.  This is due to the coefficient of refractive index of the glass and the thermal expansion are compensated by material choice and mechanical design.

Introduction

A laser beam delivery system is designed to transport the laser beam safely and accurately to the workpiece while maintaining stability and efficiency in an industrial environment. Several factors influence the design of these systems, including the type of laser process, material properties, processing speed, reliability, and beam characteristics such as wavelength, polarization, and pulse properties.

Manufacturers develop each component of a beam delivery system to maximize performance for specific applications. Haas Laser Technologies specializes in advanced beam delivery solutions designed for industrial laser processing. This paper explores the key components of laser beam delivery systems and their applications across different laser types.

A Custom Laser System with complex beam delivery internals

Understanding Beam Delivery Systems

Laser beam delivery systems are typically categorized into two main types, each tailored to enhance performance for specific applications.

Transmissive Beam Delivery Systems (Fiber Delivery)

Also known as fiber-optic beam delivery, this system transmits the laser beam through a specialized fiber-optic cable, offering flexibility for high-power laser applications. It is commonly used with short-wavelength systems around 1um. The fiber's core diameter and material composition are carefully chosen to maintain stable and consistent beam quality.

Reflective Beam Delivery Systems (Mirror Optics)

Reflective beam delivery systems rely on high-quality mirrors to guide the laser beam to the workpiece. This approach is ideal for applications requiring high power and long-distance beam delivery while minimizing energy loss. Haas Laser Technologies provides engineered mirror-based systems that maintain beam integrity across various laser wavelengths.

Key Beam Delivery Components

A well-designed beam delivery system incorporates various components that contribute to accuracy, reliability, and efficiency. Haas Laser Technologies offers a comprehensive range of solutions designed for industrial applications.

Beam Directing Components

These components control the laser beam’s path from the source to the workpiece.

• High-Performance Mirrors – Redirect the beam with minimal loss.
• Beam Splitters – Divide the laser beam into multiple paths for parallel processing.
• Adjustable Kinematic Mounts – Fine-tune alignment for improved accuracy.

19mm Kinematic Beam Splitter: Used to split beam to different destinations	25mm Beam Bender Kinematic: Used to change direction of the beam

Beam Enhancement & Positioning Components

To maintain beam quality and adapt to various applications, these components adjust the beam’s size, shape, and position:

• Circular Polarizers – Convert linear polarization to circular polarization, improving energy distribution for cutting and drilling.
• Collimators & Spatial Filters – Control beam divergence and clean up stray energy.
• Autofocus Systems – Adjust focal point dynamically to maintain optimal performance.

25mm Circular Polarizers: Changer the polarization of the beam	19mm Beam Expander: Used to increase or decrease the size of the beam

Beam Position Viewing Components

These tools help monitor and verify beam alignment, ensuring proper positioning during operation.

• Closed-Circuit TV (CCTV) Systems – Provide a remote view of the laser beam and workpiece.
• Capacitive Sensors – Measure the distance between the laser head and the workpiece, providing real-time feedback for focus adjustments.

CCD Camera: Used to view the beam

Laser Process Heads & Accessories

The laser process head focuses and directs the laser beam onto the workpiece. Depending on the application, Haas Laser Technologies provides both transmissive and reflective process heads. These process heads may incorporate:

• Gas Jet Systems – Assist in material processing by directing gas flow thorough the cut kerf.
• Protective Windows & Lens Holders – Safeguard optics from contamination.
• Adaptive Mounting Systems – Allow for easy integration into automated and manual setups.

FCHM-30: A robust laser processing head for many different application	PHA-19: Universal process head

Adapters, Mounts, and Clamping Systems

To achieve stability and flexibility in laser processing, Haas Laser offers various mounting solutions:

• Beam Clamps & Adjustable Mounts – Secure laser components while allowing fine adjustments.
• Fixed and Articulated Arm Systems – Provide structured movement and stability for beam positioning.
• Beam Protection Shields – Prevent contamination and external interference.

25mm 'Clamp Mount' Beam Tubes & Couplers: Used to enclose the beam path

Why Partner with Haas Laser Technologies?

With decades of experience in laser beam delivery, Haas Laser Technologies provides advanced solutions designed for industrial applications. Our high-quality components help manufacturers achieve greater accuracy, efficiency, and reliability in laser processing.

Whether you need a complete beam delivery system or individual components, Haas Laser is your trusted partner in optimizing laser performance.

Conclusion

Selecting the right beam delivery system is critical for achieving consistent and high-quality laser processing results. Haas Laser Technologies provides industry-leading solutions to improve performance, increase reliability, and support long-term success in industrial laser applications.

To explore our beam delivery solutions, visit www.haaslti.com or contact our team today.

Introduction

Lasers are ubiquitous in industry today. Carbon Dioxide, Nd:YAG, Excimer and Fiber lasers are used in many industries and a myriad of applications. Each laser type has its own unique properties that make them more suitable than others. Nevertheless, at the end of the day, it comes down to what does the focused or imaged laser beam look like at the work piece? This is where beam profiling comes into play and is an important part of quality control to ensure that the laser is doing what it intended to.

In a vast majority of cases the laser beam is simply focused to a small spot with a simple focusing lens to cut, scribe, etch, mark, anneal, or drill a material. If there is a problem with the beam delivery optics, the laser or the alignment of the system, this problem will show up quite markedly in the beam profile at the work piece.

What is Laser Beam Profiling?

Beam profiling is a means to quantify the intensity profile of a laser beam at a particular point in space. In material processing, the "point in space" is at the work piece being treated or machined. Beam profiling is accomplished with a device referred to a beam profiler. A beam profiler can be based upon a CCD or CMOS camera, a scanning slit, pin hole or a knife edge. In all systems, the intensity profile of the beam is analyzed within a fixed or range of spatial position. The CCD/CMOS camera-based profilers provide a 3-D representation of the beam whereas the other methods provide a 2-D slice of the beam with the scanning of a slit, pin hole or knife edge across the beam while measuring the intensity with a photo diode.

Figure 1: Focused Laser Beam and a Blow Up of the Focal Spot Where
a Beam Profiler Would Measure the Profile.

Within the Gaussian profile as shown in the figure 2, what is the beam diameter? The answer to that question is dependent upon the statistical reference it is measured to. In the laser industry the diameter can be measured to the full width of the beam at half its maximum intensity (FWHM), 1/e² point (0.135 times the maximum intensity, D4σ or second moment (which is 4 times the standard deviation of the horizontal or marginal distribution), Knife-edge width (the width of the beam is defined as the distance between the points of the measured curve that are 10% and 90% (or 20% and 80%) of the maximum value), D86 width is defined as the diameter of the circle that is centered at the centroid of the beam profile and contains 86% of the beam power.ⁱ

Figure 2: Intensity Profile of Gaussian Laser Beam

One will find that laser manufacturers will provide the beam diameter as the 1/e²  point for Gaussian, or "Gaussian Like" beams and excimer lasers are generally specified at the FWHM point since the beam is generally rectangular and has asymmetric beam divergence in the X and Y axes. It is therefore important to confirm with the laser manufacturer what their beam diameter is based upon and at what location from the output port of the laser. The distance from the laser, as we will see shortly, plays and extremely important role in size of the beam diameter. Without getting into the complex math of some of the beam diameter measurement definitions, beam profile device manufacturers provide in their software the ability to select the type of beam diameter measurement, i.e., FWHM, 1/e²  point, D4σ, Knife-edge 90/10 or D86. This is convenient for the user as it avoids a high potential of calculation error.

Where and What to Measure for a Laser Beam?

Not only is the type of beam diameter measurement method important but so is the location in space it is measured as regards its propagation through space. All lasers have a "beam waist" the beam waist of a laser is the location along the propagation direction where the beam radius has a minimum.

Two characteristics of beam propagation of great importance are the beam radius, w(z), at any axial position, z, and the phase front radius R(z), given below in mathematical terms:

where λ is the wavelength of the laser.
Graphically this is depicted in figure 3.

Figure 3: Gaussian beam propagation beginning from a waist w₀

Referring now to figure 3 the beam expands as it propagates through space. The intensity distribution remains Gaussian at every beam cross section, with the width of the Gaussian profile changing along the axis. At the beam waist, where the phase front becomes plane, the beam contracts to a minimum diameter 2w₀ (z=0, R=∞). For most practical lasers, the location of the beam waist generally lies within the laser cavity itself.ⁱⁱ

For large values of z the beam expands linearly and has a far-field divergence angle (θ).

The focused spot size of a laser beam passed through a lens will then become:

This is simply the focal length of the lens times the measured, half angle beam divergence. More common, today, is the use of what is called M² factor. This is the beam parameter product dived by the corresponding product for a diffraction-limited Gaussian beam with the same wavelength. This is represented as follows:

Consequently, a more accurate calculation of the spot radius, taking into account the M² factor of a laser is as follows:

The laser beam profiler is therefore measuring the beam radiuses mentioned above and provide the numbers based upon the diameter method selected. It is therefore helpful to understand these basics to ensure an accurate measurement of a laser system under test.

Why Measure Laser Beam Quality?

In most laser applications some care has been taken to establish the best laser type and focusing optics to achieve a particular cutting speed, or drilling condition or whatever. Why worry after all this is done? Well, on day one everything is in good working condition: new laser, fresh and clean optics, good alignment, etc. In many applications there is a debris shield or other protective optic following a possibly expensive focusing lens. This debris shield serves as means to stop slag and other debris from getting on the lens. Debris on any optic will cause the wave front of the light to degrade which causes the M² factor of the laser system to decline as well. By monitoring the M² factor during a process, can greatly aid the performance of a system and its quality control.

If the M² factor is monitored, and limits set, a system can be stopped before laser machined parts become out of spec due to laser system degradation. There are many laser beam profilers on the market and many of them do not lend themselves to real time monitoring of the laser system. The Haas Laser Beam Analyzer is a compact unit that can be quickly inserted into a laser system between the focusing lens and work piece either before or after a product run to determine if the M² value is within a specified limit.

Introduction 

Galvanometer scanners have been used for nearly three (3) decades for laser material processing. They are most commonly used for laser marking and have somewhat less utility in fine machining applications, in particular drilling precision holes and features below 250 microns. This limitation is due to their positional accuracy which is in the range of 2 to 10s of microns, depending upon the galvanometer and the F-theta lens used. Galvanometer-based systems are the simplest and least expensive way to direct a focused laser beam over a wide area. Nevertheless, they lack the “localized” precision for finite features over a large field.

A conventional multi-mirror galvometric system positions a focused laser beam by moving the beam in vectors. There are no “true arcs” generated for circular features. Instead, a circle is approximated by a series of short vectors. It is very difficult to form precision holes or any arc feature below 100 micron radius. Moreover, the angular resolution of the galvo motors is a further hindrance to the problem of small features and the attainment of high repeatability.

Galvo scanners are also subject to limited angular resolution and thermal drift which further restricts the ability of the device to machine precision features over a long period of time, e.g., a single production shift in manufacturing.

Non-galvo-based methods such as rotating, offset, wedge pairs (Risley Prisms) allow good precision below 250 microns, but only permit circular features and have a limited dynamic range and tend to be electro-mechanically complex. 

In the Risley prism design, the offset of the matched wedges causes an angular displacement of the laser beam from the optical axis. This angular deviation causes a lateral displacement of the focal spot when the angularly displaced beam is passed through a focus lens. The difficulty with this technique is that it is hard to coordinate the two wedges precisely at the high rotational speeds or to rapidly change the desired angle of deviation while the wedges are rotating. This approach usually requires a multitude of wedge pairs to cover a wide diameter range. The requirement to change wedge pairs adds significant time to replace and align; it is therefore unsatisfactory for most production processes.

Linear stages, in particular air bearing stages, offer a means of high precision. The linear or air bearing X-Y stage moves under a fixed focused laser beam, providing precision and accuracy. However, both air bearing and linear stages devices are expensive and have high inertia arising from moving such stages and the part supported by the stage so the speed of drilling precision features is limited.

Yet another approach used over the years is the focus lens itself can be placed offset from the optical axis and rotated or even placed in an open frame X-Y stage used to make all conceivable geometries. Mounting a lens in such a way is bulky and limited over the area that can be machined due to common lens aberrations.

Given the limitations of existing galvo scanners, the expense and inertia of linear stages, complexity and aberrations of other optical methods, there is clearly a need to have a device which offers the convenience and simplicity of a galvo scanner coupled with the precision of a linear stage.

New Method for Precision Scanning

A new, patent pending optical device (NeoScan™ Scanner) offers a simple optical, electro-mechanical and software approach to directing a focused laser beam onto materials to machine simple and complex geometries. This novel structure provides the ease of use and simplicity of a galvo system but adds the “localized” precision lacking heretofore.

The innovative concept provides the precision and accuracy comparable to an air bearing X-Y stage that moves under a fixed focused laser beam but without the high cost and higher inertia of moving such a stage and the part. The device in its simplest description demagnifies the scan field by more than two (2) orders of magnitude and likewise the reputability and resolution.

A conventional scanner with fair resolution may have a scan field of 50 mm x 50 mm with an F-Theta lens having a focal length of 100 mm. This same scanner has great difficulty providing high accuracy of geometries below 250 micron, due to the angular resolution of the system and the fact that any curved features include a large number of short vectors

In a typical precision a laser galvo scanner that reflects a laser beam over an angular range of plus or minus (+/-) twelve to twenty degrees (12-20º) as the beam passes through a focusing lens, typically an F-theta lens. The angular repeatability of such a galvo is on the order of < +/- 22 μrad, which represents a resolution of ~ +/- 2.2 μm for a scan lens having a 100 mm focal length. The field of such a system will be f*(Tan
), where f is the focal length of the lens and theta is the angle the beam is reflected before the lens. A laser scanner operating then over a range of plus or minus twelve degree (+/- 12º) with a f=100 mm lens will therefore cover a distance of +/- 21.3 mm.

The optical deviation of a beam refracted through a thin optic is determined by the index of refraction of the material and the angle the optic is tilted. Tilting a two millimeter (2 mm) thick optical plate that has an index of refraction of 1.796 over a range of plus or minus twelve degrees (+/- 12º) degrees will cause a laser beam traveling on axis through said plate to deviate from the optical axis by plus or minus 0.188 mm. This increases the resolution of the same galvo by the ratio of 21.2mm/0.188 mm (113:1) which is better than two orders of magnitude. 

Figure 1: NeoScan refractive scanner with a
pair of meniscus lenses mounted to galvos. 

The NeoScan (a refractive scanner) uses the same control of the galvanometer but adds an optical demagnification that essentially maps, for example, a 50 mm x 50 mm field into a 0.2 mm x 0.2 mm field.

Figure 1 is the NeoScan in its simplest form and is comprised of a pair of inverted positive meniscus lenses. Each lens is mounted to a galvanometer to tilt each lens orthogonally to one another to displace the focus laser spot from the optical axis. The preferred optical material is the highest possible index material for the desired laser wavelength. Having a high index allows the thickness of the lenses to be as thin as possible to minimize optical aberrations and minimize the inertia of the galvo.

The focal length of the lens combination of the two lens system is defined by 1/f = 1/f1 + 1/f2 – t/f1f2, where f1 is the focal length of the first lens, f2 is the focal length of the second lens and t is the separation between lens 1 and lens 2 where it is assumed for simplicity of description that the lenses are thin. The two lenses are tilted and naturally introduce coma, astigmatism and spherical aberration. Accordingly, the design of the lens curvatures, thickness and material are optimized to minimize the lens aberrations at the designed radial displacement from the optical axis.

An inverted positive meniscus lens pair produces the fewest aberrations for the optical design as mentioned earlier. As well it is desired to have a high index optical material to facilitate longer radius of curvature surfaces and keep the optical elements as thin as possible to further minimize the aberrations and reduce inertia. Other lens curvatures can be used, e.g., a pair of plano-convex lenses; pair of double convex lenses, etc. Optical modeling has determined that the inverted, positive meniscus lens pair provides minimal aberrations and best optical performance.

The tilting of each of the two meniscus lenses causes the laser beam to be displaced in a controlled way from the optical axis. The amount of displacement is dependent upon the power of the designed lenses and the angle that the lenses are tilted about the optical axis and orthogonally to one another. In the system shown in figure 1 where the lens material is sapphire with an index of refraction of 1.796, a combined lens pair focal length of approximately 200 mm and a tilt angle of ten degrees (10º) of each lens, orthogonally, cause a radial shift of the focused spot by > 170 microns. Tilting the lenses beyond ten degrees causes the coma to become too great for usefulness.

Figure 2: Enlarge view of ray trace of laser spot
refracted away from the optical axis (dimensions in mm)

Fig. 2 depicts an enlarged ray trace at the focal point of the lens system on how the light is deviated from the optical axis from the corresponding tilt of first meniscus lens in the X plane in Fig. 1.

A second variation of the NeoScan refractive scanner (Figure 3) incorporates a conventional galvo. The beam exits the F-theta lens of a conventional galvo scanner and passes through a pair of parallel plates, each of which is mounted to a galvanometer. The parallel plate galvanometers are orientated orthogonally to one another so that the beam can be offset from the optical axis in a controlled way. The offset of the beam is determined by the angle of the plate, its thickness and the index of refraction of the plate

The parallel plates permit the controlled shift of the laser beam passing through the scanner system and F-Theta lens. The plates are “thin” so the introduced aberration is minor spherical aberration and allows accurate machining of finite features over the large area of the scanner/F-Theta system which can range from a few millimeters to hundreds of millimeters, depending upon the rotation angle of the galvo scan mirror and the focal length of the F-theta lens. Through software control of the galvo pairs, features can be accurately machined over a field range limited only by the scanner/F-Theta system used. 

Figure 4: NeoScan Drilled Hole Array in Stainless Stee

The three ray traces (red, green & blue) in figure 3 represent the extremes of beam positioning by the “standard” galvo/F-Theta combination and at each extreme point the parallel plates mounted to galvos provides a higher degree of precision in the smaller field. Clearly a high degree of software synchronization between the mirror galvos and the refractive galvos is needed but would none the less provide a very versatile machining system. 

Figure 5: Conventional Scanner Drilled Hole Array in SS

Another optical configuration of the NeoScan has a laser beam which pass through a simple, positive lens and then through a pair of plane, parallel windows. Each of the plane, parallel windows is mounted to a galvanometer motor and positioned orthogonally to one another. The only difference between this scenario and the previous one mentioned is that the second has a pair of galvanometer mirrors to deflect a laser beam through a scan lens (F-Theta type).

In all three scenarios of the NeoScan, the resulting focused light is directed onto a material such as a metal, plastic, glass or ceramic for machining; with the NeoScan refractive optical elements mounted to galvanometers and oriented orthogonally to one another. 

The NeoScan has a limited field, generally < 500 μm, that is determined by the index of refraction of the optical material and the angle of rotation, but nonetheless provides very high precision capability of features below 500 microns in size in a very simple optomechanical configuration. 

Figure 6: Conventional & NeoScan
Scanner Machined Geometrical
Shapes in SS 

Figure 4 exhibits an array of nominally 155
m diameter holes with a corresponding sigma of 0.049 machined with NeoScan refractive scanner in Stainless Steel using a 20 watt fiber laser.

In contrast figure 5 depicts nominally 155 μum diameter with a sigma 2.3 using a conventional galvo scanner (F-Theta Lens = 100 mm) drilled in stainless steel.

A comparison of Figs. 4 and 5 indicates that the refractive scanner consistently produces highly regular circular and repeatable holes over that of a conventional scanner.

Figure 6 shows a circle, square, and triangle machined in 80 um thick stainless steel by a conventional and the NeoScan refractive scanner; each having a nominal feature size of 150
m.

A comparison of the results in figure 6 indicates that the geometrical shapes formed in stainless steel by the refractive scanner are substantially true to idealized shapes and that the geometrical shapes formed by the conventional scanner are not. The reason for the variance between the scanners is the acceleration and deceleration time in relation to the time to form the feature. In the conventional scanner case the time to form the feature is too close to the acceleration/deceleration time where as in the refractive scanner it is considerably less and therefore not an overriding factor in the formation of the features.

Summary

The primary purpose of the refractive scanner is to create an optical system that precisely and repeatedly locates a concentrated laser beam and to manipulate the laser beam in such a way as to remove a wide variety of materials in a controlled way to generate complex geometries with excellent precision and repeatability. This is achieved through three different galvo-based refractive optical configurations. The best configuration is dependent upon they type of machining being done and how large a field is required.

It is important to note as well that the refractive scanner can compensate for irregularities in the focused laser beam. If a focal spot of the focused laser beam is elliptical, for example, the scanner can be programmed to move in an opposing elliptical manner to compensate and achieve a perfectly round hole despite imperfections in the laser beam.