Digital OPLL-based distributed Brillouin sensing system in optical fibers

Area of Science:

• Optical Fiber Sensing
• Photonics
• Signal Processing

Background:

• Distributed Brillouin sensing systems are crucial for monitoring long-haul optical fibers.
• Traditional Brillouin Optical Time Domain Reflectometry (BOTDR) systems can be complex and costly.
• Accurate measurement of Brillouin frequency shift distribution is essential for performance.

Purpose of the Study:

• To implement a digital optical phase-locked loop (OPLL) for a distributed Brillouin sensing system.
• To develop a simple, low-cost BOTDR system without compromising sensing performance.
• To measure the Brillouin frequency shift distribution over an extended fiber length.

Main Methods:

• Implemented a digital OPLL using field-programmable gate array (FPGA) electronics.
• Phase-locked two commercial semiconductor lasers with a flexible offset frequency.
• Stabilized and scanned the difference frequency near the Brillouin frequency of standard single-mode optical fibers.

Main Results:

• Successfully measured the Brillouin frequency shift distribution over a 50 km sensing fiber.
• Achieved a measurement repeatability of 4.5 MHz at 50 km with 5 m spatial resolution.
• Demonstrated a figure-of-merit of 3.0 under fast measurement conditions.

Conclusions:

• The developed digital OPLL-based BOTDR system is simple, low-cost, and effective.
• The system achieves high performance in terms of accuracy and repeatability for long-distance fiber sensing.
• This approach offers a viable alternative for distributed Brillouin sensing applications.