High Power Requires State-of-the-Art Coating Methods

When the first lasers were developed almost 60 years ago, the power limits did not exceed milliwatts. Today, it is common for industry to use continuous wave lasers with powers in the region of several kilowatts; whereas research centres can use enormous pulsed laser devices that emit several hundred terawatts. In contrast to the general trend towards miniaturisation, laser optics are getting larger and larger and are expected to accommodate for ever increasing laser power levels. For LASER COMPONENTS, precision optics with high damage thresholds has been a key area of development.

Low absorption laser optics for high power lasers
Many industries use continuous wave (CW) lasers for welding and cutting. Producing the optics required for this purpose is a complex process because the high output power requires particularly robust components.

Compared to other areas of application, absorption in the ppm range has serious consequences in high-power lasers: The absorbed radiation produces heat in the substrate and coating. Damage can occur because optical components do not conduct heat well.

Temperature fluctuations can lead to the focal point “wandering” and no longer remaining in the processing plane. This is referred to as thermal lensing.

How the absorption affects the laser beam can be determined via the calorimetric measurement of the surface temperature or via a reference beam. The BWA-MON from our partner, Haas Laser Technologies, provides another possibility of monitoring the beam shape during operation. The use of so-called TLC ­optics™ (thermal lensing compensation) is relatively new. A method that has been used successfully for decades in infrared optics is here transferred to laser light.

What can you do to solve the problem of absorption? With Substrates that are low in OH and using a coating optimised for high power, optics with very low absorption rates can be produced.

Optics for the largest lasers in the world
Nuclear fusion and cancer research among other cutting edge applications require high-energy lasers in the megawatt and petawatt region. There are a number of very large facilities in use. The most well-known facility in Europe is most likely the Laser Mégajoule near Bordeaux. In 2014, the first of 22 beamlines went into operation. Another one will be added each year until 2025. The optics required by these facilities can be several times larger than even the largest optics familiar to industry. This becomes apparent from the size of the building alone. Each of the four laser halls is 100m long and 30m wide. The number of components used is also immense: for complex beam guidance, for example, 10,000 optics are required in various sizes [1].

LASER COMPONENTS manufactures optics with diametres of up to 390 mm for these scientific institutions. To carry this out successfully, it is first necessary to select the right substrate and coating material.

Not all substrates are low in absorption, suited for the desired sizes, and at the same time smooth enough. The surface roughness may only be a few Ångström at the most, and the surface figure must be in the region of  λ/10.

Furthermore, a chamber is required that can accommodate large substrates and ensure highly homogeneous coatings even over large areas. LASER COMPONENTS ensures a homogenous coating, even over a 390 mm diametre, using state of the art online coating monitoring techniques.

In good shape, even under pressure
In plasma-assisted processes, the packing density of the vapour-deposited coatings is particularly high. Depending on the diametre-to-thickness ratio of the substrates, this can lead to slight deformations in the substrates. It is possible to correct this effect by taking the proper measures: either by using an appropriately pre-curved substrate or by applying another coating to the back of the substrate that reverses this effect. Our production team shares a wealth of combined knowledge and experience in this area and knows how to obtain the best results after coating.

Experience and scientific curiosity
Alongside delivering practice-based solutions for the challenges of the present. LASER COMPONENTS always has the future in mind. Together with industrial partners and renowned research institutes, we collaborate worldwide to develop the technologies and processes of tomorrow.

This combination of development, production, experience, and research curiosity is the secret of our success at LASER COMPONENTS. It allows us to meet even the most complex technical requirements.

[1] http://www-lmj.cea.fr/fr/installation/index.htm

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Further product information:
Coated Laser Optics

Manufacturer:
LASER COMPONENTS Germany / Laser Optics