In this work we present a novel technique for optical signal to noise ratio (OSNR) monitoring based on the application of a high birefringent (Hi-Bi) fibre Bragg grating (FBG). The Hi-Bi FBG reflects two different spectral bands, due to the different effective refractive indexes for the optical field components along the two optical polarization axes. The OSNR can be determined using this technique by decomposing, for a data channel, the polarized signal information and the depolarized noise. This technique was used to retrieve the OSNR of an intensity-modulated signal at 10 Gb/s.