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Sensitivity Comparison of Microwave-Frequency and Optical Fibre Interferometry Based on State-of-the-Art Components
Georgios Aias Karydis, Marco Fasano, Paola Parolari, Pierpaolo Boffi, Charis Mesaritakis, Adonis Bogris
TL;DR
Fibre interferometry requires sensitive vibration detection across long fibre paths, but microwave and optical interrogation offer different phase-signal and noise characteristics. The paper compares both modalities using controlled fibre-stretcher stimuli over 300 m and 70 km. Optical interferometry performs better above 10 Hz, while the systems become comparable below 10 Hz for 70-km paths, motivating hybrid operation.
Problem
Fibre interferometry needs sensitive vibration measurements over long distances, while microwave and optical interrogation differ in phase signal strength and low-frequency noise performance.
Method
The paper compares MFFI using 10-GHz microwave oscillators with OFI using a 100-Hz-linewidth fibre laser over 300-m and 70-km paths under controlled fibre-stretcher stimuli.
Results
Optical interferometry outperforms MFFI above 10 Hz, whereas the systems provide comparable performance below 10 Hz for 70-km interferometers.
Takeaways & Limitations
MFFI and OFI are complementary modalities, and their integration could provide high sensitivity and broad dynamic range across acoustic frequencies.
Abstract
from arXiv · showhide
We compare the sensitivity of fibre interferometers to vibrations using microwave oscillators and state of the art lasers over distances up to 70 km. High spectral purity lasers outperform only above 10 Hz, highlighting microwave advantages at low frequencies and the potential of hybrid microwave optical interferometry systems
Introduction
Fibre sensing commonly relies on stable laser oscillators, while microwave sources offer lower phase-noise performance at low frequencies despite weaker phase signals. This study compares microwave-frequency and optical fibre interferometry across paths up to 70 km and finds complementary frequency-dependent strengths.
- Introduction: Stable laser oscillators dominate fibre sensing implementations, including reflectometry and interferometry configurations.Sub-kHz linewidth fibre lasers are used in commercial DAS, while longhaul systems may use sub-Hz linewidth lasers.
- Introduction: Microwave interrogation produces smaller phase changes than optical measurements because phase changes are proportional to interrogation frequency.
- Introduction: State-of-the-art microwave oscillators provide better noise performance at low frequencies, an advantage relevant to geophysical monitoring.
- Introduction: The comparison covers microwave-frequency and optical fibre interferometers over path lengths from 300 m to 70 km.The systems use commercially available 10-GHz microwave generators and a 100-Hz-linewidth fibre laser.
Experimental setup
The experiment implements microwave-frequency and optical fibre interferometers, measures phase noise over 300 m and 70 km, and evaluates vibration sensitivity using controlled fibre-stretcher stimuli. Optical interferometry has stronger performance at higher frequencies and shorter paths, whereas the systems become comparable below 10 Hz for 70-km paths.
- Experimental setup: The MFFI uses a DFB laser, intensity MZM, fibre under test, photodiode, microwave oscillators, and I/Q down-conversion for 10-GHz interrogation.An EDFA compensates losses for longer fibres, and a fibre stretcher applies perturbations from 5 Hz to 10 kHz.
- Experimental setup: The OFI uses a 100-Hz-linewidth NKT laser, a 40-MHz AOM shift, coherent heterodyne detection, and offline I/Q phase extraction.
- Results and Discussion: At 10 Hz, MFFI phase-noise PSD is -100 dB/Hz versus -40 dB/Hz for OFI at 300 m, giving MFFI a 60 dB noise advantage.
- Results and Discussion: At 10 Hz and 70 km, the RS-oscillator MFFI noise advantage over OFI increases to around 80 dB.
- Results and Discussion: A 9 μm stretcher elongation produces approximately 55 rad phase variation in OFI and 3 mrad in MFFI.The tested stimulus frequencies are 5, 10, 70, 130, and 190 Hz.
- Results and Discussion: For short interferometers, OFI provides a 20 dB–30 dB C/N detectivity advantage over a microwave generator, increasing with vibration frequency.
- Results and Discussion: For 70-km interferometers, MFFI and OFI provide comparable performance below 10 Hz, with nearly identical 15 dB C/N at 5 Hz using the RS oscillator.
- Conclusions: The study concludes that MFFI and OFI become comparable below 10 Hz and may be combined in hybrid systems for wider sensitivity and dynamic range.