Fibre optic sensing apparatus for demodulating a first phase-modulated signal the modulation of which corresponds to a desired signal and noise, the apparatus comprising: a phase-demodulation unit arranged to receive the first phase-modulated signal and a second phase-modulated signal the modulation of which corresponds to substantially the noise only, said demodulation unit comprising a quadrature signal generator arranged to receive the second phase-modulated signal and to output corresponding I and Q signals, a quadrature phase-demodulator (DQPD) adapted to mix said I and Q signals with said first phase modulated signal to produce an output proportional to said desired signal, an ADC arranged to digitise the inputs to the DQPD and QSG; characterised in that the QSG and DQPD are implemented in digital electronics, and wherein the QSG comprises: (i) a first portion for generating a first intermediate signal in response to input of the second phase modulated signal, the first intermediate signal having an amplitude or magnitude proportional to the difference between the frequency of the second phase modulated signal and the frequency of the I and Q signals; (ii) a second portion for generating a second intermediate signal in response to input of the first intermediate signal, the second intermediate signal having a magnitude proportional to the rate of change of the magnitude of the first intermediate signal; and (iii) a signal generator for generating the I and Q signals in response to input of the second intermediate signal and adjusting the frequency of the I and Q signals such that said frequency tends towards that of the second phase modulated signal, the signal generator being arranged for feedback of the I and Q signals to the first portion; and further wherein the second phase modulated signal is generated by a reference sensor desensitised to not detect the desired signal but to still detect the noise. a phase-demodulation unit arranged to receive the first phase-modulated signal and a second phase-modulated signal the modulation of which corresponds to substantially the noise only, said demodulation unit comprising a quadrature signal generator arranged to receive the second phase-modulated signal and to output corresponding I and Q signals, a quadrature phase-demodulator (DQPD) adapted to mix said I and Q signals with said first phase modulated signal to produce an output proportional to said desired signal, an ADC arranged to digitise the inputs to the DQPD and QSG; characterised in that the QSG and DQPD are implemented in digital electronics, and wherein the QSG comprises: (i) a first portion for generating a first intermediate signal in response to input of the second phase modulated signal, the first intermediate signal having an amplitude or magnitude proportional to the difference between the frequency of the second phase modulated signal and the frequency of the I and Q signals; (ii) a second portion for generating a second intermediate signal in response to input of the first intermediate signal, the second intermediate signal having a magnitude proportional to the rate of change of the magnitude of the first intermediate signal; and (iii) a signal generator for generating the I and Q signals in response to input of the second intermediate signal and adjusting the frequency of the I and Q signals such that said frequency tends towards that of the second phase modulated signal, the signal generator being arranged for feedback of the I and Q signals to the first portion; (i) a first portion for generating a first intermediate signal in response to input of the second phase modulated signal, the first intermediate signal having an amplitude or magnitude proportional to the difference between the frequency of the second phase modulated signal and the frequency of the I and Q signals; (ii) a second portion for generating a second intermediate signal in response to input of the first intermediate signal, the second intermediate signal having a magnitude proportional to the rate of change of the magnitude of the first intermediate signal; and (iii) a signal generator for generating the I and Q signals in response to input of the second intermediate signal and adjusting the frequency of the I and Q signals such that said frequency tends towards that of the second phase modulated signal, the signal generator being arranged for feedback of the I and Q signals to the first portion; and further wherein the second phase modulated signal is generated by a reference sensor desensitised to not detect the desired signal but to still detect the noise.