1. Hokkaido Institute of Technology, Maeda, Teine‐ku, Sapporo 006‐8585, Japan
2. Okayama University of Science, Ridaichou, Okayama 700‐0005, Japan
3. IPG Photonics Corporation, Old Webster Road, Oxford, MA 01540, USA
4. P.N. Lebedev Physical Institute, RAS, 53 Leninsky Ave, Moscow 117756, Russia