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Optimal Coherence Length Control in Interferometric Fiber Optic Hydrophones via PRBS Modulation: Theory and
Wujie Wang1, Qihao Hu1, Lina Ma1
1College of Meteorology and Oceanology, National University of Defense Technology, Changsha 410073, China.
Sensors (Basel, Switzerland)
|August 14, 2025
Summary
This study introduces a novel pseudo-random binary sequence (PRBS) phase modulation to control laser coherence length in interferometric fiber optic hydrophones (IFOHs). This method effectively suppresses noise while optimizing performance for underwater acoustic detection.
Area of Science:
- Optoelectronics
- Acoustics
- Signal Processing
Background:
- Interferometric fiber optic hydrophones (IFOHs) offer high sensitivity for underwater acoustic detection.
- A key challenge is managing the trade-off between laser monochromaticity and coherence length, impacting performance.
- Parasitic interference noise and diminished visibility-induced background noise are significant issues.
Purpose of the Study:
- To propose and validate a pseudo-random binary sequence (PRBS) phase modulation method for controlling laser coherence length in IFOHs.
- To establish a theoretical model linking PRBS parameters to coherence length and noise suppression mechanisms.
- To determine an optimal coherence length for minimizing both parasitic and visibility-induced noise.
Main Methods:
- Development of a theoretical model for PRBS phase modulation and its effect on laser coherence length.
- Experimental validation of the theoretical model using IFOH systems with varying link fiber lengths (3.3 km and 10 km).
- Quantitative analysis of noise suppression and interference visibility at different coherence lengths.
Main Results:
- The PRBS method effectively controls laser coherence length, suppressing parasitic interference noise.
- Reduced coherence length, while mitigating parasitic noise, can increase background noise due to lower visibility.
- Optimal coherence lengths were determined: 0.93 m for 3.3 km and 0.78 m for 10 km link fibers.
- At optimal coherence length, a 4.5 dB noise suppression was achieved in the 3.3 km system, reaching -84.5 dB (re: rad/√Hz @1 kHz).
Conclusions:
- A balanced optimization of coherence length is crucial for IFOHs to suppress parasitic noise without increasing background noise.
- The proposed PRBS modulation offers a comprehensive solution to the laser monochromaticity-stability-coherence length contradiction.
- This framework provides a noise suppression strategy applicable to hydrophones, gyroscopes, and distributed acoustic sensing (DAS).
