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Overcoming Performance Limitations of Distributed Brillouin Fiber Laser Sensors

A technical analysis of the effectiveness of distributed Brillouin fiber laser sensing (DBFLS) in overcoming performance limitations of existing Brillouin sensors in structural health monitoring and environmental sensing applications.

The distributed Brillouin laser sensor is formed by periodically pumping a fiber ring cavity. The pump pulse repetition period is matched to roundtrip time in the cavity, ensuring that Brillouin scattered light will experience Brillouin amplification at the same position in the fiber after each round trip. (Image: Naval Research Laboratory)

Distributed fiber sensors are a powerful tool for structural health monitoring and environmental sensing due to their ability to remotely monitor the strain at 1,000s of locations using low-cost optical fiber. Sensors based on Brillouin scattering are uniquely suited to these tasks since they can make completely distributed, absolute measurements of strain, with a long range (>100 km), small sensing size (<1 cm), and a huge absolute dynamic range, all in standard off-the-shelf telecom fiber. These sensors function by measuring the resonance frequency of the non-linear Brillouin interaction in fiber which shifts linearly with strain and temperature.

However, existing Brillouin sensors are hampered by fundamentally poor response resulting in small frequency shifts compared to the linewidth of the interaction. To overcome this limitation we introduced a technique known as distributed Brillouin fiber laser sensing (DBFLS) which establishes a series of narrowband lasing modes that experience Brillouin gain at discrete locations.

Brillouin fiber sensors are able to make fully distributed absolute strain and temperature measurements using standard telecom fiber. Brillouin sensors have been demonstrated with large dynamic range and operating at long distances with high spatial resolution. These aspects have made Brillouin sensors well suited for a number of applications, particularly in structural health monitoring. However, these sensors exhibit poor sensitivity when compared to other fiber sensing modalities such as fiber Bragg gratings or Rayleigh scattering. Ultimately this weak sensitivity comes from the inherently low strain and temperature response of the Brillouin frequency shift compared to the linewidth of the interaction (despite the relatively narrow linewidth of the Brillouin resonance, ~30 MHz), resulting in poor signal to noise ratio (SNR).

To overcome this limitation we proposed to use the linewidth narrowing effects associated with the lasing transition to greatly improve the SNR. Brillouin fiber lasers have previously been demonstrated with very narrow linewidths by leveraging the already narrowband nature of the interaction. However these fiber cavities, while sensitive to strain and temperature, are sensitive to these changes anywhere in the cavity and could not be used to make distributed measures. Here we use a pulsed pump to excite a series of lasing modes in a fiber cavity. The pump pulse period is carefully tuned to match the round trip time in lasing cavity such that the lasing modes only experience Brillouin gain at distinct locations in the fiber under test (FUT). The frequency of the lasing modes then matches the Brillouin resonance frequency and is responsive to changes in temperature or strain at only one position each.

My Karle fellowship was proposed to create, demonstrate, and investigate the performance of a distributed Brillouin fiber laser sensor. During the fellowship I produced an initial prototype and presented the basic noise performance, bandwidth and dynamic range. This was followed up with investigations into noise scaling with sensor spatial resolution. This work was then extended to practical lengths and numbers of sensors while addressing a weakness in the initial design where the fiber was not contiguously sampled.

This work was performed by Joseph Murray, for the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) at mobilityengineeringtech.com/tsp under the Sensors category. NRL-5670

Topics:
Aerospace Fiber Optics Fiber optics Fibers Lasers & Laser Systems Optical Components Optics Sensors Sensors and actuators Telecommunications Vehicle health management (VHM)
This Brief includes a Technical Support Package (TSP).
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Distributed Brillouin Fiber Laser Sensor

(reference NRL-5670) is currently available for download from the TSP library.

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    Aerospace & Defense Technology Magazine

    This article first appeared in the April, 2023 issue of Aerospace & Defense Technology Magazine (Vol. 8 No. 2).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document is the final report of the Karle Fellowship conducted by Dr. Joseph B. Murray at the Naval Research Laboratory, focusing on the development and performance of a Distributed Brillouin Fiber Laser Sensor (DBFLS). This innovative sensor technology aims to enhance the capabilities of fiber optic sensors for measuring strain and temperature over long distances using standard unmodified telecom fiber.

    The research involved probing a 3.5 km length of fiber, utilizing 1000 modes, which allowed for the measurement of temperature and strain across a range of 18 to 40 °C and 0 to approximately 4 microstrain (mε). The findings indicated that while the increase in fiber length and sensor count introduced manageable mode competition, the impact of changes in one mode on others was significantly reduced due to the larger number of modes being controlled. The study successfully measured the expected Brillouin frequency shift, with cross-talk remaining below the noise level.

    A key aspect of the research was the investigation of sensor noise, which was measured at a minimum of 34 nε/√Hz, with an average noise of 52 nε/√Hz. The study explored how the strain noise varied with peak pump pulse power, revealing that noise decreased with increasing pump power until a minimum was reached at around 1 W. Beyond this point, further increases in pump power did not improve noise levels for sensors at the beginning of the fiber but resulted in increased noise for sensors located at the end. This phenomenon was attributed to modulation instability rather than pump depletion, highlighting a common limitation in Brillouin fiber sensor systems.

    The report concludes by emphasizing the successful development of a high-performance DBFLS that addresses the sensitivity limitations of traditional Brillouin sensing systems. The findings demonstrate the potential of this technology for distributed, absolute measurements of strain and temperature, paving the way for future applications in structural health monitoring and other fields. Overall, the work represents a significant advancement in fiber optic sensor technology, showcasing the trade-offs and limitations inherent in such systems while providing solutions to enhance their performance.

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