[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

In the free space optical communication, the information can be encoded as the orbital angular momentum (OAM) state of light, which is called OAM shift keying (OAM-SK). This paper has proposed a communication system with OAM-SK, in which an image has been delivered from the transmitter to the receiver successfully in the simulation environment. Specifically, we have carefully designed and implemented the phase holograms used at the transmitter and the receiver for multiplexing and de-multiplexing the OAM states, respectively. At the transmitter, the multiplexing phase hologram designed by the modified Lin's algorithm is loaded on the spatial light modulator 1 (SLM1) to generate the multiplexing vortex beam, which is a superposition of multiple vortex beams with different OAM states. Correspondingly, at the receiver, a novel phase hologram is designed and loaded on the SLM2 to effectively de-multiplex the multiplexing vortex beam in different directions. In our phase hologram used at the receiver, the detected power of each OAM state can be controlled by adjusting the weight coefficient by the modified Lin's algorithm. This way, the incident power can be concentrated to the target OAM states, from which the target OAM states can be detected more effectively than conventional fork grating.

Orbital Angular Momentum Shift Keying Based Optical Communication System

A singlemode-multimode-singlemode (SMS) fiber structure consists of a short section of multimode fiber fusion- spliced between two SMS fibers. The mechanism underpinning the operation of an SMS fiber structure is multimode interference and associated self-imaging. SMS structures can be used in a variety of optical fiber systems but are most commonly used as sensors for a variety of parameters, ranging from macro-world measurands such as temperature, strain, vibration, flow rate, RI and humidity to the micro-world with measurands such as proteins, pathogens, DNA and specific molecules. While traditional SMS structures employ a short section of standard multimode fiber, a large number of structures have been investigated and demonstrated over the last decade involving the replacement of the multimode fiber section with alternatives such as a hollow core fiber or a tapered fiber. The objective of replacing the multimode fiber has most often been to allow sensing of different measurands or to improve sensitivity. In this paper, several different categories of SMS fiber structures, including traditional SMS, modified SMS and tapered SMS fiber structures are discussed with some theoretical underpinning and reviews of a wide variety of sensing examples and recent advances. The paper then summarizes and compares the performances of a variety of sensors which have been published under a number of headings. The paper concludes by considering the challenges faced by SMS based sensing schemes in terms of their deployment in real world applications and discusses possible future developments of SMS fiber sensors.

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Singlemode-Multimode-Singlemode Fiber Structures for Sensing Applications—A Review

In recent years, optical fiber sensors based on multimode interference (MMI) have attracted increasing interest and developed into various sensors used in many practical applications. This review presents MMI-based fiber sensors with a specific focus on the probe structures, measurement methods, and sensing properties of different structures. The fundamentals of MMI-based fiber sensors are briefly described. Further, five main categories of SMS structure-based fiber sensors are reviewed, including conventional SMS fiber sensors, MMI-based sensors with no-core fiber (NCF), MMI-based sensors with etched/polished fiber, MMI-based sensors with tapered fiber, and cascades of MMI-based fiber sensors. Besides, their probe structures, sensing properties, practical fields, and measurement sensitivities are summarized. Finally, the current challenges and technique outlook of the MMI-based fiber sensors are pointed out, which gives the potential solutions for the existing problems and future development. This work indicates that the MMI-based fiber sensors bring the possibilities of applying multi-mode fiber (MMF) in different measurement fields with high sensitivities, easy fabrication, and low cost. With the appearance and development of new SMS fiber sensors, it will contribute significant value to scientific research and industrial applications.

Advances in Optical Fiber Sensors Based on Multimode Interference (MMI): A Review

The metasurface concept is employed to planarize retroflectors by stacking two metasurfaces with separation that is two orders larger than the wavelength. Here, a retroreflective metasurface using subwavelength-thick reconfigurable C-shaped resonators (RCRs) is reported, which reduces the overall thickness from the previous record of 590 λ0 down to only 0.2 λ0 . The geometry of RCRs could be in situ controlled to realize equal amplitude and phase modulation onto transverse magnetic (TM)-polarized and transverse electric (TE)-polarized incidences. With the phase gradient being engineered, an in-plane momentum could be imparted to the incident wave, guaranteeing the spin state of the retro-reflected wave identical to that of the incident light. Such spin-locked metasurface is natively adaptive toward different incident angles to realize retroreflection by mechanically altering the geometry of RCRs. As a proof of concept, an ultrathin retroreflective metasurface is validated at 15 GHz, under various illumination angles at 10°, 12°, 15°, and 20°. Such adaptive spin-locked metasurface could find promising applications in spin-based optical devices, communication systems, remote sensing, RCS enhancement, and so on.

0.2 λ0 Thick Adaptive Retroreflector Made of Spin-Locked Metasurface

Simultaneous measurement of refractive index and temperature based on half-tapered SMS fiber structure with fringe-visibility difference demodulation method

We experimentally demonstrated a high sensitive gas pressure sensor, which also has more concise processing steps and lower cost. The sensor was fabricated by splicing the single-mode fiber (SMF) and a short section of hollow-core fiber (HCF) which was filled with a Polydimethylsiloxane (PDMS) film to form an enclosed air microcavity with the length of 55 μm. The thin PDMS film also plays a role in reflecting the beam. The thermo-optical effect and the thickness of PDMS film can affect the cavity that is sensitive to external pressure. When the thickness of the PDMS membrane is 3 μm, the measured gas pressure sensitivity reaches to 52.143 nm/Mpa, the linearity is 99.86%, and the cross-sensitivity of the gas pressure over temperature is lower than 2.51 × 10−3 Mpa/°C. Moreover, the compact structure, high sensitivity, high mechanical strength, and convenient fabrication make it suitable for measuring gas pressure in harsh conditions.

Optical Fiber Gas Pressure Sensor Based on Polydimethylsiloxane Microcavity

When linearly polarized light illuminates a plasmonic lens, it is challenging to realize centrally symmetric focal spot in the near field since there is mismatch between the symmetries of the plasmonic structures and the polarization spatial distribution of the incident light. In this paper, we propose a plasmonic lens that can focus surface plasmon to a centrally symmetric field when illuminated by light with linear polarization. Remarkably, the intensity distribution of the focal field is insensitive to the polarization direction, which makes it more reliable in optical applications. Numerical simulations are used to check the performance of the proposed structure. Comparisons with two conventional plasmonic lenses are made to demonstrate its unique superiority in polarization-insensitive focusing.

Focusing surface plasmon and constructing central symmetry of focal field with linearly polarized light

An in-fiber hybrid modal interferometer (HMI) is theoretically and experimentally demonstrated in a section of bias-tapered multimode fiber only fabricated by arc-discharge technique, which consists of multimode interferometer (MMI) and Mach-Zehnder interferometer (MZI). The MZI has the sensitivity as high as −31.08 nm/RIU, but the MMI presents the immunity of both wavelength and intensity to ambient refractive index (RI). In strain test, MMI has a 9.8-dB intensity deduction and MZI shows high wavelength sensitivity (−6.31 pm/ $\mu \varepsilon$ ). Thus, our HMI shows a strong multi-parameter discriminative proficiency and simultaneous measurement of RI, strain and temperature is achieved by the means of joint wavelength and intensity demodulation-based inversion-matrix method. Such a compact and low-cost sensor, without the crosstalk from the temperature and the light source, is very promising for the applications of biochemical sensing and structural health monitoring.

Bias-Taper-Based Hybrid Modal Interferometer for Simultaneous Triple-Parameter Measurement With Joint Wavelength and Intensity Demodulation

In this paper, a novel in-line modal interferometer for refractive index (RI) sensing is proposed and experimentally fabricated by cascading single-taper and multimode-double- cladding-multimode (MDM) fiber structure. Owing to evanescent field in taper area, the ultra-sensitive and linear intensity-responses to the varied surrounding RI are gained in both single- and double-pass structures. Moreover, the crosstalk from temperature can be effectively discriminated and compensated by means of the RI-free nature of MDM. The experimental results show that the RI sensitivities in single- and double-pass structures, respectively, reach 516.02 and 965.46 dB/RIU (RIU: refractive index unit), both with the slight wavelength shift (~0.2 nm). The temperature responses with respect to wavelength and intensity are 68.9 pm°C-1/0.103 dB°C-1 (single-pass structure) and 103 pm°C-1/0.082 dB·°C-1 (double-pass structure). So the calculated cross-sensitivity of intensity is constrained within 8.49 × 10-5 RIU/°C. In addition, our sensor presents high measurement-stability (~0.99) and low repeatability error (<4.8‱). On account of the ~620 μm size of taper, this compact sensor is cost-efficient, easy to fabricate, and very promising for the applications of biochemistry and biomedicine.

https://www.mdpi.com/1424-8220/19/18/3820/pdf

Lingling Ran

Papers

Simultaneous measurement of refractive index and temperature based on half-tapered SMS fiber structure with fringe-visibility difference demodulation method

Optical Fiber Gas Pressure Sensor Based on Polydimethylsiloxane Microcavity

Focusing surface plasmon and constructing central symmetry of focal field with linearly polarized light

Bias-Taper-Based Hybrid Modal Interferometer for Simultaneous Triple-Parameter Measurement With Joint Wavelength and Intensity Demodulation

Ultra-Sensitive Fiber Refractive Index Sensor with Intensity Modulation and Self-Temperature Compensation