Frequency Hopping DAS Using AOM-Gated Re-Circulating Loop

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Solution Overview

Problem

Conventional distributed acoustic sensing (DAS) systems face limitations in maximum acoustic frequency due to fiber length, which restricts their applicability in long-distance sensing applications, and rely on costly arbitrary waveform generators (AWGs) with memory length restrictions, limiting frequency mixing sequences for sensing beyond 100 km.

Innovation Solution

A re-circulating optical fiber loop with a gated acousto-optic modulator (AOM) generates frequency-hopping optical pulses, increasing acoustic sampling rate and bandwidth without AWG limitations, and employs in-band coherent detection with an offset local oscillator to reduce receiver bandwidth and improve signal-to-noise ratio.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If conventional DAS systems use fixed pulse repetition rates, then the system structure is simple, but the maximum acoustic frequency is limited by fiber length

Engineering Contradiction:

Improvemaximum acoustic frequencyVSAvoidsystem structure

Core Design Contradiction:

SpeedVSDevice complexity

Solution Approach 1:

The patent implements dynamic pulse repetition rates by having the pulse source operate at different repetition rates for different optical frequencies. The pulse repetition rate is adjusted based on the optical frequency being transmitted, allowing the system to adapt the sampling rate to match the acoustic frequency requirements while managing fiber length constraints.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The system changes the pulse repetition rate parameter dynamically according to the optical frequency. By varying the pulse repetition rate across different optical frequencies, the system can sense higher acoustic frequencies without being constrained by the fiber length limitation that applies to single-frequency systems.

Inventive Principle:

Principle #35Parameter changes

2Length of stationary object

If arbitrary waveform generators are used to generate frequency mixing sequences, then frequency hopping can be achieved, but memory length restrictions limit sensing distance beyond 100 km

Engineering Contradiction:

Improvesensing distanceVSAvoidmemory length capacity

Core Design Contradiction:

Length of stationary objectVSQuantity of substance

Solution Approach 1:

The patent transitions from generating frequency sequences in the time domain using AWG memory to generating them in the frequency domain using multiple optical frequencies. By utilizing the frequency dimension of light, the system can create long frequency mixing sequences without being constrained by temporal memory capacity, enabling sensing distances beyond 100 km.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Solution Approach 2:

The system replaces the electronic AWG-based frequency sequence generation with an optical-based approach using multiple frequency lasers. This substitution eliminates the memory length bottleneck of electronic systems by leveraging the inherent frequency diversity of optical sources, allowing for extended sensing distances.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Area of stationary object

If out-of-band coherent detection is used, then detection simplicity is maintained, but receiver bandwidth requirements are excessive

Engineering Contradiction:

Improvereceiver bandwidthVSAvoiddetection simplicity

Core Design Contradiction:

Area of stationary objectVSEase of operation

Solution Approach 1:

The patent applies local quality by performing coherent detection at each individual optical frequency rather than requiring the receiver to handle the entire frequency hopping bandwidth simultaneously. The receiver is tuned to detect signals at specific frequency points, reducing the instantaneous bandwidth requirement while maintaining detection capability across the full frequency range through sequential measurement.

Inventive Principle:

Principle #3Local quality

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach enhances acoustic sensing bandwidth, reduces costs by allowing longer sensing distances without AWG memory constraints, and achieves higher signal-to-noise ratios, enabling detection of frequencies up to N×f, where N is the number of frequencies used, and supports longer fiber lengths with fewer interrogators.

Implementation Method 1

a re-circulating optical fiber loop including a gated acousto-optic modulator (AOM) to generate frequency-hopping optical pulses

Methodology Applied

Scientific EffectAcousto-optic effect: Acousto-optic Effect

Implementation Method 2

systems, methods, and structures according to aspects of the present disclosure use an optical frequency shifter to perform 'in-band' coherent detection instead of 'out-of-band' detection

Methodology Applied

Scientific EffectOptical frequency shifting:

Data Source

PatentUS11159200B2High speed frequency hopping DAS interrogation using AOM-gated re-circulating loop and frequency-shifted receiver LO

Publication Date: 2021.10.26 NEC CORP

AI summary

Aspects of the present disclosure describe systems, methods, and structures for high speed frequency hopping distributed acoustic sensing using an acousto-optic modulated (AOM), gated re-circulating loop and a frequency shifted receiver local oscillator. Using the re-circulating loop controlled by the AOM to generate frequency-hopping pulse(s) increases DAS acoustic bandwidth overcomes infirmities exhibited in the art that generate multiple frequency patterns that are not suitable for long-distance DAS. Additionally, by employing frequency shifted local oscillator (LO) with asymmetric in band detection, bandwidth requirements are reduced by one half.