1. Departamento de Física, Universidad del Valle, A.A 25360, Cali 760002, Colombia
2. Sao Carlos Institute of Physics, University of Sao Paulo, C.P. 369, Sao Carlos 13560-970, Sao Paulo, Brazil
3. National Institute of Telecommunications (Inatel), Santa Rita do Sapucaí 37540-000, Minas Gerais, Brazil