I'll back this up, fibre is almost always speed specific. I fell foul of this a few years ago when I brought some new HP switch gear with gigabit fibre modules in, thinking I could continue to use the 100mbs converters at the other end until the school could afford to upgrade them to gigabit. Oh how wrong was I, found out the hard way it doesn't work!

However a patient exists for the technology to auto negotiate, http://www.patentstorm.us/patents/6504849.html so someone somewhere probably does make modules that can vary their speeds.