Whitepaper: Variable Beam Expander for High-Average-Power Ultrafast UV Laser Systems Design

By Adomas Charevičius, Ignas Pikas

In recent years, high-power ultraviolet (UV) laser systems have become increasingly prevalent across a wide range of industrial and scientific applications, driven by the demand for higher process throughput, efficiency, and the emergence of new technologies such as advanced material processing, wafer dicing, and quantum computing.

Operation in the UV spectral region imposes significantly stricter requirements on optomechanical components compared to visible or infrared systems. Achieving long operational lifetimes, low scattering losses, stable wavefront performance, and resistance to contamination remains a persistent technical challenge, particularly in ultrafast, high-average-power regimes.

At Altechna, these challenges are addressed through a combination of durable thin-film coating technologies and carefully optimized optomechanical designs. Leveraging long-term development in UV optics, manufacturing and assembly, Altechna has developed variable beam expander solutions tailored to the demanding requirements of high-power UV laser systems, offering stable beam delivery, high transmission, and a robust design.

Existing Requirements and Solutions

In precision manufacturing and inspection applications, merely guiding the laser beam is insufficient. The beam must be delivered with well-defined diameter, divergence, and stability, as variations in these parameters can directly affect process reproducibility and yield.

Even nominally identical laser sources may exhibit differences in output beam diameter and divergence, necessitating beam conditioning to achieve consistent beam delivery across systems. Variable beam expanders enable compensation for these variations, ensuring a stable and reproducible beam geometry at the target.

In addition to unit-to-unit variation, laser beam parameters may also evolve over time due to system aging, thermal or environmental effects, and component degradation—an effect particularly pronounced in UV laser systems. In such cases, minor adjustments of the beam expander magnification and divergence allow restoration of optimal beam conditions without modification of the laser source itself. This consistency is critical in manufacturing environments, where identical processes must be maintained across many laser systems.

Furthermore, increasing the beam diameter prior to focusing enables the formation of smaller focal spots, improving the achievable resolution and processing precision. As a result, beam expansion is often a functional requirement rather than an optional adjustment.

In UV material processing, applications such as laser lift-off, cutting, drilling and dicing continue to expand, driven by the growing demand for compact consumer electronics and advanced semiconductor devices. Increasing throughput typically requires higher laser power and repetition rates, which in turn place increasing demands on beam delivery optics and optomechanics. These requirements introduce additional challenges for system integrators, particularly in maintaining optical stability, lifetime, and cleanliness under high-power UV exposure.

Emerging UV laser applications place strict requirements on optical subsystems, particularly with respect to wavefront quality, pointing stability, and long-term reliability. Minimizing optical aberrations is critical, as even small distortions can negatively impact process accuracy and repeatability.

Many existing solutions on the market achieve acceptable initial performance but suffer from limited operational lifetime or unstable behavior under high-power UV exposure. While lifetime can be extended through multi-lens optical designs, nitrogen purging, and the use of low-outgassing mechanical components, these measures are not always implemented consistently or without compromise. Extended operational lifetime is particularly important at the system level, as it directly reduces maintenance frequency, associated downtime and service costs.

High transmission is another key requirement, as optical losses directly reduce delivered laser power and system efficiency, while insufficient anti-reflection performance can introduce unwanted back-reflections. At the same time, beam quality must be preserved across the full magnification range to achieve the desired processing precision.

Consequently, customers require a variable beam expander that combines exceptional transmitted wavefront quality, high pointing stability over the entire adjustment range, compact mechanical dimensions, and contamination-free operation, enabling straightforward system integration without sacrificing performance or lifetime.

Existing Technologies

A variety of variable beam expander architectures are available on the market, each optimized for a specific subset of performance parameters. In the context of ultrafast, high-average-power UV laser systems, these designs often involve unavoidable tradeoffs between wavefront quality, mechanical stability, contamination risk, footprint, and achievable lifetime. The most common solution categories are outlined below.

Grease-Lubricated Variable Beam Expanders

Many commercially available variable beam expanders use grease or lubricant based mechanical stages to enable smooth axial motion of internal optical elements. This approach allows for fine adjustment resolution, low actuation forces, and long mechanical lifetimes under moderate operating conditions.

However, in high-power UV laser environments, lubricant-based designs introduce significant contamination risks. Outgassing of organic compounds is more dangerous under UV irradiation, increasing the probability of these contaminants depositing on optical surfaces. Such contamination can lead to increased absorption, wavefront perturbations, and localized laser-induced damage on thin-film coatings, resulting in transmission loss and performance degradation as well as reduced operational lifetime.

Additionally, grease-based mechanisms are generally incompatible with near-vacuum environments and may limit achievable laser power or repetition rates to maintain acceptable lifetime. These constraints make lubricant-based VBEs less suitable for demanding ultrafast UV applications despite their mechanical advantages.

Fixed Beam Expanders

Fixed (non-adjustable) beam expanders provide an alternative approach, typically offering robust optical performance with high transmission and excellent wavefront quality. Due to the absence of moving elements, these systems eliminate contamination risks associated with mechanical motion and achieve stable, well-controlled beam propagation. Their simplified optical design allows optimization of lens surface curvatures and spacing, enabling near diffraction-limited wavefront performance.

In addition, the absence of internal adjustment mechanisms results in a mechanically stable configuration that is less susceptible to changes in optical alignment over time, where the laser beam may drift away from the optimal lasers optical path. This makes fixed expanders well suited for applications with tightly controlled and stable laser sources.

However, the lack of adjustability significantly limits their applicability in practical systems. Variations between laser sources, as well as changes in beam parameters over time, cannot be compensated without replacing or redesigning the optical component. This reduces flexibility in system integration and may require maintaining multiple optical configurations to accommodate different operating conditions. As a result, fixed expanders are less suitable for manufacturing environments requiring adaptable and easily tunable beam control.

In ultrafast, high-power UV laser systems, beam expansion solutions must balance optical performance, mechanical reliability, contamination control, and operational flexibility. Grease-lubricated variable expanders offer adjustability but introduce contamination-related risks, while fixed expanders provide stable optical performance but lack tunability.

As a result, there is a clear need for variable beam expander designs that maintain high wavefront quality, pointing stability, and contamination-free operation without sacrificing flexibility or system compatibility.

Altechna’s Solution

Altechna has designed a variable beam expander solution specifically for ultrafast, high-average-power UV laser systems, where beam quality, pointing stability, high transmission and contamination control must be maintained simultaneously across a variable magnification range. The design addresses the well-known limitations of conventional beam expander architectures through a combination of optical layout optimization, contamination-free mechanical design, and durable UV-optimized thin-film coatings.

Optical Design Strategy

The optical layout is based on a design which delivers the laser energy homogenously over all lenses, eliminating any high peak intensity localized hotspots that could otherwise lead to plasma or radical formation, nonlinear effects, or immediate optical damage under ultrafast UV irradiation.

This configuration enables controlled beam expansion over a 1–4X magnification range while maintaining low transmitted wavefront distortion at infinite conjugation. Careful selection of lens curvatures, diameters, and inter-element spacing allows wavefront performance to be preserved across the full adjustment range without requiring excessive system length or restrictive input beam parameters.

Mechanical Architecture and Motion Control

To ensure long-term stability and cleanliness in high-power UV environments, the beam expander employs a greaseless mechanical architecture. All moving elements are guided by precision linear motion stages designed to provide reproducible axial movement without introducing angular deviations.

The mechanical layout constrains internal degrees of freedom to minimize beam pointing stability error during magnification and divergence adjustment. As a result, consistent beam alignment is maintained across the full operating range, supporting integration into precision laser processing systems.

Material selection, surface finishes, and tolerances were optimized to balance wear resistance with low particle generation, enabling reliable long-term operation without lubricants and with compatibility for controlled-atmosphere environments.

UV-Optimized Coatings and Contamination Control

All optical elements are equipped with Altechna’s in-house engineered anti-reflection coatings, specifically optimized for UV wavelengths around 343 nm. These coatings are designed to provide high transmission while maintaining durability under prolonged UV exposure and high-repetition-rate ultrafast operation.

To further mitigate contamination-related degradation, the beam expander incorporates nitrogen purging, enabling the removal of residual organic compounds and limiting oxidative or photochemical processes on optical surfaces. Combined with a lubricant-free mechanical design, this approach significantly reduces the risk of UV-induced outgassing and subsequent contamination-driven damage.

In high-power UV environments, particularly around 343 nm, interaction of the laser radiation with atmospheric oxygen can lead to the formation of ozone and other reactive oxidative species. These species can interact with contaminants or the anti-reflection coatings, promoting the formation of absorbing layers and accelerating coating degradation under irradiation, in essence, rapidly decreasing the laser damage threshold and operational lifetime. By maintaining an inert nitrogen environment inside the variable beam expander, these reactions are suppressed, reducing the likelihood of contamination buildup and improving long-term optical stability.

Assembly and Quality Assurance

Assembly of the variable beam expander is performed in a contamination-controlled cleanroom environment, following defined assembly instructions and a multi-stage cleanliness protocol. Optical metrology and quality assurance procedures are applied to verify VBE’s compliance.

This controlled manufacturing process allows consistent performance to be achieved while preserving traceability and ensuring that each beam expander meets the strict requirements of ultrafast high-power UV applications.

Performance Balance and System Integration

By combining an optical layout that distributes the laser energy uniformly across all optical surfaces, a contamination-free mechanical design, and durable UV coatings, Altechna’s variable beam expander achieves a balanced performance profile. High wavefront quality, stable beam pointing, high transmission and resistance to laser damage with a compact footprint without imposing restrictive limits on laser power or system architecture.

This integrated approach enables reliable beam conditioning for high power UV laser systems, addressing the key limitations identified in conventional variable beam expander designs.

Quality Assurance and Validation

The performance of each variable beam expander is experimentally validated by using optical metrology techniques commonly applied in high-precision laser optics. Measurements focused on transmitted wavefront quality, beam pointing stability, transmission and LIDT performance of optical components are critical for reliable integration into ultrafast high-power UV laser systems.

Transmitted Wavefront Error Measurement

Transmitted wavefront error (TWE) is evaluated using Fizeau interferometry in a double-pass autocollimation configuration. This measurement method allows precise assessment of wavefront distortions introduced by the optical assembly without alignment error influence.

Transmitted wavefront error measurement results demonstrate near diffraction-limited performance of 0.062λ peak-to-valley at 633 nm for the required entrance beam diameter.

While optical design allows for even lower theoretical wavefront distortion, the measured performance is primarily limited by achievable lens dimensional and form manufacturing tolerances as well as mechanical assembly part manufacturing tolerances. Each test is conducted to confirm that the VBE’s optics and mechanics are manufactured with correct tolerance ranges and do not introduce significant additional aberrations.

Figure 1. Transmitted wavefront error phase data. Peak-to-Valley error demonstrating 0.062λ result at 633 nm.

Beam Pointing Stability Measurement

Beam pointing stability is assessed by monitoring the angular displacement of a highly stable He-Ne laser beam passing through the VBE as the beam expander’s magnification and divergence adjustments were varied. The output beam position is recorded using a CMOS sensor positioned at a known and measured distance from the beam expander.

This measurement directly reflects mechanical precision, internal alignment accuracy, and repeatability of the linearly moved optical components.

Across the full adjustment range, the measured angular beam displacement magnitude must fall under 1 mrad. In Figure 2, Effective Value defines this displacement value as the averaged X and Y value of the laser beam centroid, defined as per ISO 11146. The results of <0.3 mrad demonstrate that the greaseless linear motion architecture maintains stable beam propagation without inducing unintended angular deflections during adjustment.

Figure 2. Pointing stability measurement report. At a measurement distance of 1 m, a lateral beam displacement of 1 mm corresponds to an angular deviation of 1 mrad, displayed as Effective Value.

Coating Reflectivity Measurement

Spectral performance of the anti-reflection coatings is verified for each batch using witness samples processed together with the optical components during thin-film deposition. This ensures compliance with the specified coating performance prior to assembly, while transmission performance of the assembled variable beam expander is confirmed using a dedicated laser and power meter setup.

Figure 3. Reflectivity curve of the lenses thin-film anti-reflective coating.

Laser-Induced Damage Threshold (LIDT) measurement

Laser-induced damage threshold (LIDT) measurements were performed to evaluate the resistance of the thin-film anti-reflection coatings to cumulative laser damage under ultrafast, high-average-power UV irradiation. Testing was conducted using the S-on-1 protocol in accordance with ISO 21254-2, which is utilized for assessing coating durability and long-term performance.

Measurements were carried out using a 343 nm ultrafast laser source with a pulse duration of 1 ps (FWHM). The laser beam was linearly polarized and focused to a 23.5 µm beam diameter at the test site. A repetition rate of 500 kHz was employed, with up to 10⁷ pulses applied per site.

Under these conditions, no laser-induced damage to the thin-film coatings was observed up to a fluence level of 0.473 J/cm², demonstrating resistance to cumulative UV exposure and supporting suitability for variable beam expander operation for ultrafast high-average-power systems.

Figure 4. S-on-1 test results. Characteristic damage curve of anti-reflection coating. Tested by Lidaris.

Product Specifications

Optical and Operational Parameters

• Wavelength range: 343+355 nm
• Magnification range: 1–4X variable beam expansion
• Optical conjugation: Infinite (collimated for all magnification ranges)

Optical Performance

• Transmitted wavefront error: < λ/10 peak-to-valley at 633 nm
• Transmission: ≥ 97% at 343 nm and 355 nm

Mechanical and Stability Characteristics

• Beam pointing stability: < 1 mrad over the full magnification and divergence adjustment range
• Mechanical architecture: Greaseless linear motion design
• Assembly cleanliness: Assembly in contamination-controlled manufacturing processes

Environmental Compatibility

• Atmosphere: Nitrogen or dry air purge capable
• Operating conditions: Suitable for controlled-atmosphere and ambient environments
• Angular sensitivity: Optimized for normal beam incidence

Integration Considerations

• Each variable beam expander requires individual alignment within the laser beam path, as plug-and-play installation is not feasible for adjustable optical systems of this type
• Upon request Altechna can provide technical consultation and support on VBE alignment and integration procedure

Roadmap

While this whitepaper focuses on a variable beam expander designed for ultrafast high-power UV laser systems, it represents only one element within Altechna’s broader beam delivery and optical subsystem development activities.

In parallel with variable beam expanders, Altechna designs and manufactures complementary optomechanical components for high-power laser applications, including turning mirrors, polarizers, objectives, attenuators and others. These components are developed using the same in-house optical coating technologies and contamination-controlled optomechanical manufacturing processes described in this work.

Building upon its experience in UV optics and ultrafast laser systems, Altechna is also extending its research and development efforts toward shorter-wavelength applications, including ultrafast high-power fourth-harmonic generation around 257 nm. This development area imposes even stricter requirements on optical materials, coatings, and contamination control and benefits directly from the design principles discussed in this paper.

Future development efforts continue to focus on close collaboration with system integrators and end users, enabling tailored optical solutions that balance performance, lifetime, and system-level constraints while maintaining full traceability and quality assurance.

Contact Altechna

Altechna develops custom optical and optomechanical solutions for high-power laser systems, working closely with customers to address application-specific requirements.

For technical consultation or collaboration for high-power ultrafast laser system components, please contact our team at [email protected].