Saturation caused phase jump avoidance in DAS
Aspects of the present disclosure describe systems methods and structures for avoiding saturation caused phase jump in systems that extract information from the phase of a complex sequence and exhibit an overflow or “spike” in the output of a high-pass filter. Operationally, during phase unwrapping—when an output signal exceeds a supported range—it is adjusted to be back in range by adding N·2π, to a phase where N is negative or positive integer, depending on the direction to be adjusted.
1. A distributed optical fiber sensing (DOFS)/distributed acoustic sensing (DAS) method for a DOFS/DAS system having
a length of optical fiber; and
an optical interrogator unit that generates optical pulses, introduces the generated optical pulses into the optical fiber and receives a sequence of Rayleigh reflected signals from the optical fiber; and
a coherent receiver unit configured to extract information from the sequence of Rayleigh reflected signals;
the method comprising:
operating the DOFS/DAS system to obtain a sequence of beating products for a plurality of locations along the length of the optical fiber;
determining if a phase difference between every two samples in the sequence results in an overflow condition; and
adjusting any determined overflow phase difference to a normal value by increasing or decreasing any determined overflow phase difference by a fixed value;
outputting a flag to indicate the overflow condition and a direction of the overflow condition; and
providing the adjusted phase difference and flag to a filter for DC removal;
wherein the fixed value is one selected from a group consisting of 2π and 2Nπ, where 2Nπ≤a supported range of normal values and 2(N+1)π≥a supported range of normal values and N is an integer number.
2. The method of claim 1 wherein the filter is a high-pass filter having infinite impulse response (IIR) that is implemented using cascaded 2 nd order IIR sub-filters.
3. The method of claim 2 wherein a first 2 nd order IIR filter is a direct-form expressed as:
Y ( n )=Σ m=−N N a m ·x ( n+m )=Σ m=−N −1 a m ·x ( n+m )+ a 0 ·x ( n )+Σ m=1 N a m ·x ( n+m ),
such that if x(n) overflows, then for m∈[−N, −1], x(n+m) is replaced by x′(n+m)=x(n+m)−Δ(n), and
Y ( n )=Σ m=−N −1 a m ·x ′( n+m )+Σ m=0 N a m ·x ( n+m ).
4. The method of claim 1 wherein the filter is a high-pass filter having a finite impulse response (FIR) and any samples prior to x(n) in the sequence are all adjusted by a same value as phase unwrapping.