1. Department of Photonics Engineering; Technical University of Denmark; DK-2800 Kongens Lyngby Denmark
2. Institute of High-Frequency and Communication Technology; University of Wuppertal; 42119 Wuppertal Germany
3. Current address: I. Physikalisches Institut; RWTH Aachen; D-52056 Aachen Germany
4. Department of Theoretical Physics and Astrophysics; Belarusian State University; 220030 Minsk Belarus
5. IPU; Technology Development; Produktionstorvet 425 DK-2800 Kongens Lyngby Denmark