US 12,436,004 B2
Fibre optic sensing
Richard Hooper, Farnborough (GB); and Roger Crickmore, Farnborough (GB)
Assigned to Optasense Holdings Limited, Camberley (GB)
Appl. No. 18/556,061
Filed by Optasense Holdings Limited, Camberley (GB)
PCT Filed Apr. 25, 2022, PCT No. PCT/GB2022/051043
§ 371(c)(1), (2) Date Oct. 18, 2023,
PCT Pub. No. WO2022/229616, PCT Pub. Date Nov. 3, 2022.
Claims priority of application No. 2106046 (GB), filed on Apr. 28, 2021.
Prior Publication US 2024/0175727 A1, May 30, 2024
Int. Cl. G01D 5/353 (2006.01)
CPC G01D 5/35361 (2013.01) 20 Claims
OG exemplary drawing
 
1. A distributed fibre optic sensing apparatus, comprising:
an optical arrangement configured to generate a local oscillator signal and also to repeatedly interrogate a sensing optical fibre with interrogations of coherent optical radiation at an interrogation rate, wherein each interrogation comprises at least one optical pulse, the or each optical pulse being at a respective launch frequency which differs from a frequency of the local oscillator signal by an amount equal to a respective carrier frequency;
a mixer configured to receive a backscatter signal comprising radiation from the optical pulse which is backscattered from within the sensing optical fibre and to mix the backscatter signal with the local oscillator signal;
a detector arrangement configured to receive the backscatter signal mixed with the local oscillator signal and provide a first corresponding digital detector output signal; and
a processor configured to process the first digital detector output signal:
wherein the processor is configured, for the or each respective carrier frequency, to down-convert the digital detector output signal to a respective baseband signal based on that carrier frequency and convert the baseband signal to a phase signal; and
wherein the processor is operable to apply time-domain low pass filtering to the or each baseband signal with a first cut-off frequency which is less than half the interrogation rate.