Multi-band DAS with enhanced SNR, increased sampling rate and minimized inter-location interference
A distributed acoustic sensing (DAS) system that collects many frequency bands within the system (in particular, analog to digital converter, or ADC) bandwidth. The bands are divided into multiple groups, and within each group, the inter-band interference from band i to band j is at least l 1 dB lower than the power of band j. For different groups, the inter-band interference is at least l 2 dB lower than the power of band j, that l 2 ≥l 1 +C l , where C l is a value related to round-trip fiber loss. The bands belonging to the same group are cascaded one after another, with little guard time in between, or packed back-to-back. This arrangement advantageously increases the overall signal power, resulting in an improved SNR.
1 . A multi-band distributed fiber optic sensing (DFOS)/distributed acoustic sensing (DAS) method exhibiting enhanced signal-to-noise, increased sampling rate, and minimized inter-location interference, the method comprising:
generating a multi-band signal;
driving an I-Q modulator (IQM) followed by an acousto-optic modulator (AOM) with the multi-band signal to generate multi-frequency optical interrogation pulses, wherein a window function is applied to the multiband signal to minimize inter-location interference caused by harmonic frequencies or spectrum leakage;
introducing the multi-frequency optical interrogation pulses into a length of optical sensing fiber; and
receiving Rayleigh reflected signals from the optical sensing fiber with a coherent receiver configured to extract information from the reflected signals;
wherein each RF band of the multi-band signal is either a single tone pulse or a code having a certain frequency shift;
wherein a signal driving the AOM is synchronized with the multi-band signal.
2 . The method of claim 1 wherein RF bands are divided into multiple groups and within each group, inter-band interference from a band i to a band j is at least l 1 dB lower than the power of band j.
3 . The method of claim 2 wherein the different groups are interleaved equally in a frame, thereby creating a plurality of subframes.
4 . The method of claim 3 wherein a signal driving an acousto-optic modulator (AOM) is synchronized with each RF signal of the multi-band signal, either using one pulse to cover signals in each subframe, or multiple pulses each covering one band.
5 . The method of claim 4 wherein each RF band is reshaped by the window function such that it extends a smooth transition from zero to peak power to minimize spectrum leakage caused by non-smooth transitions in a time domain.