How to Overcome the Challenges to UV Optics used in Industrial Lasers14th Feb 2019
Engineering and manufacturing have become reliant on the many advantages offered by industrial lasers. Enhancements in quality, accuracy and cost and energy efficiency have resulted in significant advances in manufacturing production and control processes, leading to sweeping changes in quality and safety.
Superior quality results courtesy of a clean, precision process; uninterrupted consistency and the absence of damage to surrounding materials are all laser-related features that have transformed the manufacturing landscape considerably.
For this reason, lasers have widely replaced traditional methods of production and we now see cutting, drilling and welding processes mostly handled by laser instruments. Many sectors now place great reliance on the likes of excimer and Nd:YAG lasers.
Common industrial laser applications
Nd:YAG lasers are solid state lasers. They operate at high power levels and are used for etching, marking and engraving an array of materials. The automotive industry is a heavy user of this type of laser due to its efficiency in cutting and welding steel. Superalloy drilling to produce the likes of gas turbine parts generally uses pulsed Nd:YAG lasers. In addition, these lasers are used in aerospace applications for drilling cooling holes to boost heat exhaust and airflow efficiency, and also in a process known as laser peening; a flashed laser system that generates very high levels of power on the surfaces of parts without producing heat or adding material. Laser peening is used extensively in gas-fired turbine engines in the aerospace and power generation sectors.
Excimer lasers, also known as exciplex lasers, are gas-powered ultraviolet lasers widely used in the production of semiconductor integrated circuits (chips) via photolithography, micromachining and microelectronic devices. These lasers are considered the most powerful in the ultraviolet region.
A very important consideration…
It is true that lasers have enabled and automated processes that were previously impossible, or at very least incredibly risky. Extreme environments, challenging access criteria and complex processes pose little or no contest for the industrial laser application which offers exceptional power and capability.
Of course with power and capability come very high-level demands on the components of the laser applications. So there is one important condition to be met if a laser application is going to deliver in demanding situations: its optical components must be capable of delivering at the highest possible performance.
The challenges faced by UV optics
Optical components used in modern laser systems and applications need to endure exceptionally high fluxes of short wavelength radiation involving billions of intense laser pulses, without sustaining damage. They also need to be chemically resistant; have superior thermal tolerance and be incredibly robust.
Laser peening for example typically uses a very high energy level of 10 to 40 Joules alongside a 10-30 nanosecond pulse to generate several gigawatts of power.
Industrial strength, high power grade optical components are essential. Windows, lenses, mirrors and other optical components must be capable of the highest possible performance. There are however various challenges to be overcome in achieving this.
With industrial laser optics, there is no room for deviation from specification. Surface tolerances are tighter and applying coatings with precision accuracy can prove challenging. Imperfections in optical components used in UV applications are magnified which means there is a higher chance of multiphoton absorption, an issue which could damage the optical component.
Overcoming UV optics challenges
Various factors must be considered in order to ensure high performance and longevity of UV optics.
High grade fused silica is one of the most stable substrates available and used widely in UV laser components such as high power grade ND:YAG laser mirrors and high energy grade excimer laser mirrors as well as aspheric windows. The benefit of using fused silica is that it offers a very high surface quality and high surface accuracy, which means the components can withstand excessive damage thresholds.
Sapphire is another widely used material in UV applications. Sapphire components such as sapphire windows are extremely robust as the material is second only in hardness to diamond and can withstand temperatures up to 2030 Celsius, as well as resist almost all chemicals and abrasion.
The application of antireflective coatings to both surfaces of UV laser components can reduce surface losses to almost zero for the most efficient transmission possible. Antireflective coatings have exceptionally high damage thresholds and work to increase durability.
Precision front surface mirrors, for example, are used in a range of high demand manufacturing control applications. The mirrors are coated so that the specific region of the spectrum in which they are being used is optimised. As a coating, enhanced aluminium protects the optic, whilst the enhancement, a dielectric over-coating, boosts durability, eases handling and protects from oxidation. The over-coating can also be customised further to boost the reflectivity of the metal coating. Customisations such as these make precision front surface mirrors the ideal option for a variety of laser and other manufacturing applications.
The diameter of the optical lenses used in UV applications is an important consideration for two reasons. Firstly, higher power lasers call for larger lens diameters in order to avoid thermal overload. Secondly, a larger diameter will, at any given focal length, yield a more concentrated focal spot if the incoming beam expands to fill the larger sized lens.
Most Nd:YAG laser applications tend to use bi-convex lenses. These have a convex curve on both the incoming and outgoing surfaces which is designed to yield the optimum focused spot, generally smaller than that which can be achieved using a plano-convex lens. If achieving the smallest spot size is not a critical consideration, then plano-convex lenses are generally the more cost-effective option.
Customisation of UV optical components, such as edging or designing with square or rectangular apertures, fire polishing, diamond turning or thermal toughening can ready them for a range of manufacturing and engineering environments and applications.
The key consideration is that customisation should meet specific requirements and challenges. Working with an optical industry consultant to determine fully individualised precision solutions is vital.
Meticulous measurement and testing of optical components is absolutely crucial. Precision compliance with the specifications that have been determined as critical for your UV application optics comes through expert metrology. A combination of the very best, top grade, state of the art metrology instruments, top class industry expertise and a thorough understanding of the specific needs, objectives and challenges of the optical application will ensure exceptional quality and compliance.
Optical component specialists for UV laser applications
Knight Optical has been specialising in the custom design of precision optical components for almost 30 years. Our technical expertise spans every aspect of custom optical component design and manufacture.
Our state-of-the-art metrology instruments are some of the very best in the industry and are designed to elevate optical testing to the next level with devices capable of measuring the most complex materials and precision optical surfaces. Furthermore, our metrology experts are able to share a wealth of in-depth knowledge.
Should you wish to discuss your individual optical component requirements for any industrial laser applications, our experienced, multilingual sales team and industry trained technical experts are at your full disposal.
Laser technology has become a ground-breaking and highly efficient production process, which is why there is a consistent demand for the most sophisticated optical components. We welcome you to get in touch.