Application Cases

Experiment Name: Experimental Study on the Response Characteristics of Remote FBG Sensors for Longitudinal Waves in Optical Fibers

Research Direction: Communication Engineering, Optical Fiber Communication and Optoelectronics Technology, Structural Health Monitoring

Experiment Objective: This experiment aims to clarify the defining conditions for remote FBG as a longitudinal wave resonance sensor versus a longitudinal wave standing wave sensor; to investigate the influence of the pigtail fiber length (first pigtail L1, second pigtail L3) on the longitudinal wave response characteristics of remote FBG; and to verify the consistency between the theoretical model of longitudinal wave propagation resonance frequency in optical fibers and the experimental results.

Testing Equipment:

1. Acoustic excitation equipment: Function signal generator, high-voltage amplifier (Aigtek ATA-2021H), PZT5-3300, piezoelectric fiber patch

2. FBG sensing and demodulation equipment: FBG, narrowband laser source, optical circulator, isolator

3. Signal receiving and processing equipment: Photodetector, data acquisition card, computer (LabVIEW software)

4. Auxiliary materials: Epoxy resin, couplant, polyimide tape

   
   

Figure 1: Schematic Diagram of the Sensor Experimental System

Figure 2: Physical Diagram of the Sensor Experimental System

Experimental Procedure:
The experiment first constructed a system consisting of three parts: acoustic excitation (PZT generates longitudinal waves), FBG demodulation (edge filtering method), and signal processing (LabVIEW acquires time-domain/frequency-domain signals), ensuring that the optical fiber axis was aligned with the PZT motion direction. Next, Condition 1 was carried out: fixing L3 = 5 mm, varying L1 (0-110 mm), applying low-frequency sinusoidal excitation from 2000 to 20000 Hz, and recording the response amplitude under different L1 values. Subsequently, Condition 2 was performed: three sets of experiments were set up (L1 = 725 mm and L3 = 1080 mm, varying L1 with fixed L3, varying L3 with fixed L1). Excitation at frequencies such as 7000 Hz was applied, and the FBG response at antinode/node positions was compared.

Experimental Results:
When the FBG was located at the fiber end and L1 was within the resonance region of 65-100 mm, it exhibited elastic body vibration characteristics with a response amplitude. When L1 was less than 60 mm or greater than 100 mm, forced vibration occurred with a relatively small response. Under long pigtail fibers (L1 = 725 mm, L3 = 1080 mm), the FBG response exhibited standing wave characteristics, with the response amplitude at the antinode being 1.5 to 2 times that at the node. After the couplant absorbed the elastic wave, the standing wave disappeared. When the pigtail fiber length was an integer multiple of half the wavelength (e.g., L1 = 270 mm, 540 mm), the FBG response exhibited peaks, validating the assumption of "uniform strain distribution" in the theoretical model.

Figure 3: Influence of L1 Length on Remote FBG Response with FBG at the Fiber End

Figure 4: Three Experimental Configurations for the Influence of Pigtail Fiber Length on Remote FBG Response

Product Recommendation: ATA-2021B

Figure: ATA-2021B High-Voltage Amplifier Specifications and Parameters

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