Proceedings of SPIE - The International Society for Optical Engineering

This review covers photonic crystal cavities (PCCs) and their applications in optical sensors, with a particular focus on the structures of different PCCs. For each kind of optical sensor, the specific measurement principle, structure of PCC, and the corresponding sensing properties are all presented in detail. The summary of the reported works and the corresponding results demonstrate that it is possible to realize miniature and high-sensitive optical sensors due to the ultra-compact size, excellent resonant properties, and flexibility in structural design of PCCs. Finally, the key problems and new directions of PCCs for sensing applications are discussed.

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A review for optical sensors based on photonic crystal cavities

In recent years, with the development of materials science and architectural art, ensuring the safety of modern buildings is the top priority while they are developing toward higher, lighter, and more unique trends. Structural health monitoring (SHM) is currently an extremely effective and vital safeguard measure. Because of the fiber-optic sensor's (FOS) inherent distinctive advantages (such as small size, lightweight, immunity to electromagnetic interference (EMI) and corrosion, and embedding capability), a significant number of innovative sensing systems have been exploited in the civil engineering for SHM used in projects (including buildings, bridges, tunnels, etc.). The purpose of this review article is devoted to presenting a summary of the basic principles of various fiber-optic sensors, classification and principles of FOS, typical and functional fiber-optic sensors (FOSs), and the practical application status of the FOS technology in SHM of civil infrastructure.

https://www.mdpi.com/1424-8220/20/16/4517/pdf

Recent Progress of Fiber-Optic Sensors for the Structural Health Monitoring of Civil Infrastructure.

The distribution of interface stresses between the residual limb and prosthetic socket of a transtibial amputee has been considered as a direct indicator of the socket quality fit and comfort. Therefore, researchers have been very interested in quantifying these interface stresses in order to evaluate the extent of any potential damage caused by the socket to the residual limb tissues. During the past 50 years a variety of measurement techniques have been employed in an effort to identify sites of excessive stresses which may lead to skin breakdown, compare stress distributions in various socket designs, and evaluate interface cushioning and suspension systems, among others. The outcomes of such measurement techniques have contributed to improving the design and fitting of transtibial sockets. This article aims to review the operating principles, advantages, and disadvantages of conventional and emerging techniques used for interface stress measurements inside transtibial sockets. It also reviews and discusses the evolution of different socket concepts and interface stress investigations conducted in the past five decades, providing valuable insights into the latest trends in socket designs and the crucial considerations for effective stress measurement tools that lead to a functional prosthetic socket.

Techniques for Interface Stress Measurements within Prosthetic Sockets of Transtibial Amputees: A Review of the Past 50 Years of Research

A kind of compact and highly sensitive refractive-index sensor based on reflective tapered fiber coupler is proposed. The sensing structure is fabricated by fusing and tapering a twisted optical fiber. The achieved maximum sensitivity is 3617 nm/RIU for measuring refractive index ranging from 1.33 to 1.41. Comparing with other similar reflective devices, the transmission loss is relatively low ( $\sim 2$ dB), which is assigned to the employed Sagnac loop mirror to enhance the reflection of the system. Besides the intrinsic advantages of the reflective configuration, the proposed sensing structure has the additional advantages of compactness, low-cost, and ultrahigh sensitivity. It is also promising and cost effective for other all-fiber device applications.

Ultrasensitive Refractive-Index Sensors Based on Tapered Fiber Coupler With Sagnac Loop

A filmed extrinsic Fabry–Perot interferometric (FEFPI) sensor for the measurement of arbitrary refractive index (RI) is experimentally demonstrated. The FEFPI sensor was constructed by aligning two fiber endfaces, on which the multilayer dielectric mirrors are deposited. There is a fixed reflection at the filmed fiber endface no matter how much the RI is, and we can obtain a clear interferogram with a high visibility. A resolution of up to 1.64 × 10 −5 refractive index unit is achieved in our experiment.

Filmed extrinsic Fabry–Perot interferometric sensors for the measurement of arbitrary refractive index of liquid

A fiber optic white-light interferometry based on cross-correlation calculation is presented. The detected white-light spectrum signal of fiber optic extrinsic Fabry-Perot interferometric (EFPI) sensor is firstly decomposed by discrete wavelet transform for denoising before interrogating the cavity length of the EFPI sensor. In measurement experiment, the cross-correlation algorithm with multiple-level calculations is performed both for achieving the high measurement resolution and for improving the efficiency of the measurement. The experimental results show that the variation range of the measurement results was 1.265 nm, and the standard deviation of the measurement results can reach 0.375 nm when an EFPI sensor with cavity length of 1500 μm was interrogated.

A cross-correlation based fiber optic white-light interferometry with wavelet transform denoising

A white-light interferometry based on the Fourier transform method is proposed to measure the optical path difference (OPD) of the high-finesse fiber optic extrinsic Fabry–Perot interferometric (EFPI) sensor. The Fourier spectrum of the transmission spectrum signal consists of multiple frequency components because of the multiple-beam interference occurring in the high-finesse EFPI sensor. The high-order frequency component of the Fourier spectrum can be extracted by the Fourier transform method to recover the OPD in order to overcome the spectrum overlapping, which happens when the OPD of the EFPI sensor is short. In the experiment, a high-finesse fiber optic EFPI sensor with the cavity length of 120 $\mu $ 
m is measured, and the second-order frequency component was extracted to recover the cavity length. The standard deviation of the measurement results was 9.132 nm. The measurement range of the Fourier transform method was effectively extended to the short OPD.

A White-Light Interferometry for the Measurement of High-Finesse Fiber Optic EFPI Sensors

The measurement of the diameter change of a piezoelectric transducer cylinder with the white-light interferometry

Large optical flats play a remarkable role in advanced large-aperture optical systems and the testing of the surface shape error is indispensable for the fabrication. The widely adopted Ritchey-Common test for large optical flats will fail without the rigorous test configurations including a large F/# prerequisition and a flat-to-interferometer distance invariance. A virtual-real combination Ritchey-Common interferometry is proposed to avoid the large F/# prerequisition by accurately modelling the optical path in a virtual interferometer. Furthermore, a virtual-real combination iterative algorithm is proposed in this method to break the flat-to-interferometer distance invariance. Measurement experiments for 100 mm and 422 mm aperture flats were performed to demonstrate the feasibility of this method. Compared with a direct testing in a standard Zygo interferometer, the peak to valley (PV) and root mean square (RMS) errors were less than 0.1 λ and 0.01 λ (λ=632.8 nm), respectively, in different Ritchey angles and flat-to-interferometer distances. Further numerical simulations demonstrate that RMS errors for various Zernike aberrations in arbitrary F/# are less than 0.01 λ. This method can break the distance invariance restriction and achieve high accuracy with an arbitrary F/#, thus providing substantial freedom in the design of test configurations to accommodate various test scenarios.

Virtual-real combination Ritchey-Common interferometry.

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