[IEEE 2008 Joint Conference of the Opto-Electronics and Communications Conference (OECC) and the Australian Conference on Optical Fibre Technology (ACOFT) - Sydney, Australia (2008.07.7-2008.07.10)] OECC/ACOFT 2008 - Joint Conference of the Opto-Electronics and Communications Conference and the Australian Conference on Optical Fibre Technology - Accurate measurement of the Brillouin frequency shift in optical fibers using a doubly phase-modulated probe light