Grating Random Fiber Laser (RFL) provides feedback by writing some randomly distributed Fiber Bragg Gratings (FBGs) on the single mode fiber. However, because there are many grating pairs that satisfy the feedback condition, the mode competition is fierce, and the center frequency drift and multi-spike problems caused by gain competition are more serious. To alleviate the modes competition to a certain extent, we can reduce the spacing of FBGs and decrease the length of random structure. In this experiment, we write a random weak grating array with a spacing of the order of micrometer by using the micro-nano processing technique of the femtosecond laser, and the array length is as short as 2.5 cm. At the same time, the reflectivity of the FBG is improved by increasing the energy of the femtosecond laser pulse. The random structure length is reduced while ensuring sufficient feedback efficiency, and the mode competition and multi-spike resonance are suppressed. At the same time, we use a FBG acts as a filter, the linewidth of the RFL measured by delayed heterodyne is 377 Hz. The frequency noise of the RFL is 8 Hz/Hz1/2 at 10 kHz, and the flatness is good when the frequency is larger than 10 kHz, which creates favorable conditions for improving the resolution of the fiber sensor.
The random fiber laser (RFL) combining a 3×3 coupler phase interrogation technique is demonstrated and applied to acoustic emission (AE) detection for damage detection of buoyant materials. The random chirped grating array (RCGA) is used to provide random distributed feedback and act as an AE sensing head. The random distributed feedback inside RCGA significantly extends the effective cavity length of the RFL, thus reduces the thermal frequency noise of the RFL and improves the resolution of AE signal. A π-phase-shifted fiber Bragg grating (π-FBG) loop mirror is used as a wavelength locking to ensure a stable and narrow lasing. A high AE signal resolution of 2.6×10-7 pm/√Hz@10 kHz is achieved.
In this paper, a real-time monitoring method of buoyancy material curing process based on cascaded fiber Bragg grating (FBG) sensors is proposed. The strain change of buoyancy material curing at different heights is monitored by embedding prestressed-cascaded FBGs in the mixture of hollow glass beads and epoxy resin. Meanwhile, the cascaded reference FBGs encapsulated in a capillary glass tube is used to monitor the temperature change of buoyancy material curing process. The experimental results show that the strain and temperature change trends at different heights are different during the entire process. The proposed method is practical in monitoring of the curing process of buoyancy materials.
The two-cavity FBG-FP is used to suppress random mode in random fiber laser (RFL). The relation between the total cavity length of two-cavity FBG-FP and mode suppression characteristics is given. By increasing the total cavity length to about 45.1 mm, only one random mode is allowed to pass. Compared with single-cavity FBG-FP, the two-cavity FBG-FP has better performance in suppressing random mode due to its narrowed band, flat response and steeper rolloff.
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