Diamond lenses make materials-processing laser optics lighter
The first laser-cutting system with diamond lenses is being tested.
|Diamond optics are characterized by significantly greater heat conductivity and a higher refractive index than lenses of other materials, while also having outstanding mechanical properties. (Image: Photo Fraunhofer ILT, Aachen / Volker Lannert)|
Diamonds have some striking properties: for example, their refractive index is 2.4, which is extremely high and allows optics to be made that are thinner for the same optical power. Their thermal conductivity is 2000 W/m*K, which is more than 1400 times higher than that of optical glass. Along with their high damage threshold, these properties make diamonds highly interesting for high-power optics.
To date, polycrystalline diamond substrates have only been used as windows for CO2 lasers. Due to impurities and imperfections, they absorb and scatter laser radiation at emission wavelengths about 1 μm, making them unsuitable for fiber lasers. Although single-crystal diamonds do not have this problem, they are more difficult to manufacture.
The Fraunhofer Institute for Applied Solid State Physics (IAF; Freiburg, Germany) has been researching the production of monocrystalline diamonds for years. The continuous vapor deposition (CVD) reactors developed at the IAF have stable plasma conditions that enable the manufacture of substrates up to several millimeters thick.
A maximum of 60 diamonds can be produced simultaneously. At build rates of up to 30 μm per hour, the reactors can produce optics with an aperture of approx. 10 mm.
90% weight reduction for laser heads with diamond optics
Lenses from these synthetic single-crystal diamonds show low absorption and also low birefringence. Currently, a few specimens have been provided with antireflection coatings and incorporated into a fiber-laser cutting head.
"We have optimized a complete laser optical system for the diamond lenses for the first time," says Martin Traub from the Fraunhofer Institute for Laser Technology (ILT; Aachen, Germany). "Thanks to this, the cutting head is more than 90% lighter."
The lenses with 7 mm diameter had previously passed tests with 2 kW laser power without problems. Now, the partners have built a system for cutting tests with a 1 kW fiber laser. Integrated in the cutting head are water cooling and the shielding gas supply. Process monitoring has not yet been planned. First tests are currently being carried out with the compact cutting head.
The new optical system should significantly increase the flexibility in laser cutting. The small size enables the system to process difficult-to-access areas, and the low weight facilitates highly dynamic movements during 3D processing.
The optical system will also be presented at LASER World of PHOTONICS 2017 (Munich, Germany; June 26 to 29) at the joint Fraunhofer stand A2.431.