The bonded composite laser crystals are common solid active media in the field of high-power lasers and amplifiers, used to improve thermal effect and laser output. There is a potential risk of affecting optical transmission due to the imperfect bonding interface. To characterize the interface for crystal fabrication and laser design, we propose a distributed interferometer-based interfacial loss characterizing method. Based on the stable broad-band light source and photodetector, our Optical Low-Coherence Reflectometry (OLCR) is easier to obtain the low noise background and realize the high-accuracy measurement, which is important for detecting the weak reflection of the interface. Using a fiber probe, any local area on the interface can be detected individually. With scanning the optical delay line, the surficial reflection and interfacial reflection of one light spot can be monitored separately to enable the lateral and vertical distributed measurement. Specifically, a Yb:YAG/YAG crystal was measured and demonstrated as an example. To avoid absorption and realize high spatial resolution(40μm), a source located at 1535nm with a bandwidth of 58nm is used. Meanwhile, the reflection coupling loss (RCL) of the fiber probe is self-calibrated and eliminated by the simulation and calculation model. Moving the fiber probe, the vertical and lateral interfaces are scanned. The point cloud distribution testing of vertical bonding interfaces is accomplished, and a total internal reflection method is employed to detect the lateral interface. At last, the resolution (~0.5μm) of the probe scanning is validated and discussed using a photomask.
An ultra-long adiabatic tapered single-mode fiber is proposed to enhance the received scattered light in the fiber without additional loss. By controlling the radius and refractive index distribution on the length of the fiber, the backscattering coefficient of the scattered light is increased, and the signal-to-noise ratio of the scattered light is improved while the long distance sensing length is maintained. By analyzing and optimizing the length, radius and refractive index distribution of tapered optical fibers, scattering enhancement of 7dB can be achieved at 45km. Ultra-long adiabatic tapered single-mode fiber can realize Rayleigh backscattered light enhancement of equal signal-to-noise ratio at each position, which can be used to improve the signal-to-noise ratio and measurement range of fiber sensing system based on Rayleigh backscattered, and further promote the application of fiber sensing technology for ultra-long distance sensing.
Residual stress in a part is a key factor in its fatigue life, structural stability, corrosion resistance and other properties. Therefore, accurate measurement of residual stress is essential. The mechanical release method is widely used in residual stress measurement for its high accuracy and mature technology. However, the method assumes that the material is homogeneous, and its accuracy is greatly affected in non-uniform materials. Aiming at the problem of inaccurate residual stress measurement in non-uniform materials, a fiber-optic ring array residual stress measurement method based on high spatial resolution distributed optical fiber measurement technology is proposed. The drilling method is taken as an example. In this method, we arrange the fiber-optic ring array around the hole in the same center to measure the residual stress on the circumference with high spatial resolution. The finite element simulation is used to analyze the influence of the spatial resolution of the fiber sensor, the non-uniform size, position and Young's modulus on the strain distribution and residual stress measurement. It is proved that the method can accurately measure the residual stress and obtain the information of the non-uniform state of the material.
Fiber Bragg grating(FBG) written in rotated fused polarization maintaining(RF-PM) fiber is proposed. The fiber structure constructs two Fabry-Perot interferometers. The spectral characteristics is analyzed and simulated. The Bragg reflection spectrum of fiber subjected to different loading angles are measured as the rotated fused angle is 22.5°. The experimental results show that the asymmetrical fiber structure can measure transverse stress and discriminate its direction.
FBG inscribed on splice joint is proposed to be used as a sensor. It could realize temperature and strain measurement simultaneously because of the different strain responses of the different parts of the FBG inscribed on splice joint, while the temperature responses are same. The strain responses of four overlap values are discussed, which is used to optimize the discrimination capability of the strain and temperature response of the FBG device.
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