Broadband grating waveguide mirror for high efficiency pulse compression and high-power high efficiency green laser

These broadband grating waveguide mirrors with a diffraction efficiency of 99.7 %, that are as well polarization and wavelength selective, can be realised with standard nano- and microfabrication processes. They can be used within the setup for frequency conversation in green lasers as well as for pulse compressors for high-energy lasers.

Processing highly reflective materials such as copper poses a particular challenge in laser processing, especially in terms of reproducibility and avoiding undesired spattering. On the other hand, this type of processing offers many advantages, particularly when it comes to joints that are difficult to access.

Green wavelength laser light is ideal for processing copper because the radiation is very well absorbed by the material – six times better than infrared radiation (at room T°). Green laser systems are already available, but usually their power is limited due to undesired effects (e.g. thermal lensing) occurring with polarization and wavelength selective optical components (e.g. Brewster windows, etalons, etc.) that are required and used for the second harmonic process. Often, several of these components are required in the cavity which leads to a cumbersome system and additional losses.

The principle of polarization and wavelength selective "grating waveguide mirrors" developed at the Institut für Strahlwerkzeuge (University of Stuttgart) allows an efficient and stable frequency conversion with simultaneous polarization and wavelength selection. Using a single highly efficient optical element can yield a very high output power. The new configuration allows to almost double the total efficiency having a diffraction efficiency higher than 99.8%. Such a mirror can be used as cavity end-mirror or output coupler. The developed components were demonstrated to sustain more than 125 kW/cm² (at an incident power of 40 KW) making them exceptionally robust and efficient at the same time.
The grating waveguides can also be used for pulse compression, having a diffraction efficiency of over 99 % for a spectral bandwidth of several nanometers. The device also has a high laser induced damage threshold of more than 3 J/cm2 for a nanosecond regime.

• minimum reflection loss at high damage threshold
• multi-kW power with green laser and thus automated processing of copper
• high efficiency with good beam quality thanks to a highly efficient optical element
• robust component
• easy to implement and cost-effective

The configuration of a grating mirror presented here enables a previously unattainable increase in power for green-wavelength lasers. Other applications of this technology are pulse compressors or "dense wavelength multiplexing" of high-power lasers (diodes, fibers or solid-state lasers).