Smartphone-based interrogation of a fiber Bragg grating sensor is, to the best of our knowledge, reported for the first time. The smartphone flashlight LED was used as a light source and a transmissive diffraction grating projected the CFBG spectra on the smartphone camera. In order to efficiently couple light from the smartphone LED to the fiber with CFBG, multimode fiber was used for inscription. Interrogation setup consists of a smartphone and low-cost off-the-shelf available components. Measurement principle was illustrated through the fiber strain caused by applied longitudinal force. Attained measurement sensitivity and resolution were validated via comparison with commercial spectrometer and theoretical results based on Cramer-Rao approach. Also, to the best of our knowledge, the influence of modal noise on the smartphone-based fiber optic sensor interrogation system performance is considered for the first time.
Conjugate-impedance matched superabsorbers are metamaterial bodies whose effective absorption cross section greatly exceeds their physical dimension. Such objects are able to receive radiation when it is not directly incident on their surface. Here, we develop methods of physical modeling of such structures and investigate interactions of the superabsorbers with passing electromagnetic radiation. The particular superabsorbing structure under study is a wormhole comprised of meshes of loaded transmission lines. A theory of electromagnetic wave propagation and absorption in such metamaterial structures is developed. At the frequency of operation, the structure exhibits greatly enhanced absorption as compared to the black body-type absorber of the same size. Peculiar wave absorption effects such as trapping of nearby passing beams of electromagnetic radiation are demonstrated by numerical simulations. Possible modifications of the wormhole structure under the goal of optimizing absorption while minimizing complexity of the involved metamaterials are discussed. Conjugate-impedance matched superabsorbers may find applications as efficient harvesters of electromagnetic radiation, novel antennas, and sensors. I. INTRODUCTIONFrom wave optics, it is known that the scattering and absorption cross sections of resonant particles can be much greater than that of non-resonant bodies with the same dimensions. 1 For instance, the extinction cross-section in subwavelength particles exhibiting plasmonic or polaritonic resonances can be orders of magnitude greater than the same for a black-body type absorber of a comparable physical size. 2-9 Effectively, such resonant particles are able to collect the incident wave power from an area much bigger than their physical cross section.The same physical principle of optimal resonant absorption is used when designing compact receiving antennas. From the theory of wire antennas 10-12 it is known that a short wire dipole (with length much smaller than half wavelength) is a rather ineffective receiver unless it is loaded with complex impedance Z load (ω) = Z * dip (ω), where Z * dip (ω) is the complex conjugate of the input impedance of the dipole antenna at the frequency ω. Such a conjugateimpedance matched load compensates for the excess reactance of the short dipole antenna, tunes it in resonance with the incident field, and provides for the maximum of the received power. 10 The ultimate limit for the effective receiving area of a resonant dipole is (3/8π)λ 2 (e.g., Ref.2), where λ is the radiation wavelength. Note that this limit is determined by the wavelength rather than by the dimensions of the dipole. If a particle supports higher order multipolar resonances (besides the main electric dipolar mode) at the same frequency ω, its absorption cross section can be made larger than that of a resonant dipole. 8,9 In fact, it can be shown that there is no ultimate upper limit on the effective absorption cross section of a resonant object when more and more multipolar modes of the object pile ...
Smartphone-based optical spectrometers allow the development of a new generation of portable and cost-effective optical sensing solutions that can be easily integrated into sensor networks. However, most commonly the spectral calibration relies on the external reference light sources which have known narrow spectral lines. Such calibration must be repeated each time the fiber and diffraction grating holders are removed from the smartphone and reattached. Moreover, the spectrometer wavelength scale can drift during the measurement because of the smartphone temperature fluctuations. The present work reports on a novel spectral self-calibration approach, based on the correspondence between the light wavelength and the hue features of the spectrum measured using a color RGB camera. These features are caused by the nonuniformity of camera RGB filters’ responses and their finite overlap, which is a typical situation for RGB cameras. Thus, the wavelength scale should be externally calibrated only once for each smartphone spectrometer and can further be continuously verified and corrected using the proposed self-calibration approach. An ability of the plug-and play operation and the temperature drift elimination of the smartphone spectrometer was experimentally demonstrated. Conducted experiments involved interrogation of optical fiber Fabry-Perot interferometric sensor and demonstrated a nanometer-level optical path difference resolution.
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