We demonstrate two types of all-fiber speckle spectrometer. One of the spectrometers is composed by cascading a coreless fiber (CLF) and an all-solid photonic crystal fiber (PCF). Using a 10 cm-long fiber with 20-segment-PCF spliced elements, the spectrometer achieves a resolution of 0.03 nm over a bandwidth from 1540 to 1560 nm. The other spectrometer is realized by using the periodically tapered CLF. A remarkable spectral resolution of 0.03 nm in the near-infrared spectrum can be achieved with a 5-cm long fiber. Our compact spectrometers based on CLF promise picometer-resolution spectroscopy in portable applications, providing a new way for miniature spectrometer systems.
Photonic spin Hall effect can manipulate EM waves. One of the most representative manifestations of is spin-momentum locking. The spin changes only when the direction of EM wave propagation changes, offering many applications in photonic devices. Spin-momentum locking can be realized in spoof surface plasmon polariton (SSPP) waveguide. Spin-momentum locking demonstrates that photonic spin Hall effect (SHE) can manipulate electromagnetic (EM) waves, however, programmable EM wave manipulation is still challenging. In this work, the spin-momentum locking in programmable plasmonic metamaterial is demonstrated. Digital coding spoof surface plasmon polaritons (SSPPs) waves have two transverse spins. Moreover, photonic spin logic devices based on programmable plasmonic metamaterial are developed. By implanting spin degrees of freedom in digital coding SSPPs, both spin and coding can be used to control energy flow. Furthermore, SSPPs logic “AND” gate and “NIMPLY” gate (A AND NOT B) are designed and verified by simulation and experiment. The combination of digital coding technology with photonic SHE provides a more powerful and flexible platform for controlling EM waves.
The fiber Bragg grating (FBG) was written in hollow suspended-core fiber (HSCF) using femtosecond laser point-bypoint technology. The HSCF based-FBG has a higher refractive index (RI) sensitivity to liquid in the air hole because the core of HSCF is completely exposed to air. In the RI range of 1.35~1.45, the reflection peak of the FBG moves toward the longer wavelength with the increase of the external RI. The wavelength shift of FBG is greater for the higher RI, which means that the HSCF-FBG has a higher sensitivity for the high RI environment. The RI sensitivity of the HSCF-FBG is 14.24 nm /RIU in the range of 1.42~1.44. The axial strain sensitivity of the sensor is 1.02 pm /με in the range of 0~900 με, and the temperature sensitivity is 10.22 pm /℃ in the range of 30~150 ℃. The HSCF-FBG RI sensor has higher sensitivity and lower temperature and strain crosstalk, therefore, proposed HSCF-FBG has prospective applications in biological detection, medical and health fields.
Microfabrication of microstructures on the end of an optical fiber is a challenge. A double-interferometer cascaded sensor on the end face of a dual-core fiber is demonstrated to measure the refractive index and temperature. The Michelson interferometer consists of two cores of dual-core fiber, where one core is spliced with a flat-topped microtip, and the Fabry-Perot interferometer is constructed based on an epoxy resin microcap. The fabricated sample achieved a refractive index sensitivity of -14880 nm/RIU and a temperature sensitivity of -374.2 pm/℃. The optical fiber tip microlaboratory has a robust structure and low manufacturing cost.
A refractive index (RI) sensor manufactured on a tapered hole-assisted dual-core fiber (HADCF) is proposed. The sensor is manufactured by tapered HADCF between two single-mode optical fibers, and works based on the coupling between the fundamental mode (LP01) and the low-order mode (LP11) in two cores. HADCF is tapered to achieve the phase matching condition between the LP01 mode in the centric core and the LP11 mode in the suspended core. The air hole in HADCF forms microfluidic channels for injecting glycerol solution, and can be free from external interference. The shift of the coupling peak is linear with the RI and sensitivity is 924.57 nm/RIU. In addition, the proposed RI sensor has negligible temperature crosstalk.
Here we will mainly investigate the coupling mechanism of bilayered optical metamaterials with a standing wave and the selective coherent perfect absorption in bilayered optical metamaterials by the two coherent beams. The metamaterials consist of bilayered asymmetrically split rings (ASR) with different twist angles 0° and 180°, spatially separated by a very thin dielectric substrate. Electric and magnetic dipolar excitations can be selectively enhanced or eliminated when the ASR is placed at the antinode and the node of the standing waves, meaning that the phase difference of the signal and control beams is 0° and 180‡°. The simulated results show that coherent perfect absorption can be realized at different frequencies and in particular each one can be switched on/off depending on the phase difference between the signal and control beams. In comparison with 35% absorption in the single-layer ASR metamaterial, the coherent perfect absorption occurs with larger than 95% absorption. In summary, we realize an ultra-thin subwavelength coherent absorber that holds much more flexibility operating at any frequency ranging from microwave to optical regimes. The response of metamaterials can be coherently tailored by easily changing their positions in the standing wave.
In this work, a metamaterial with 90°-twisted nanorods is presented and numerically investigated. Numerical simulated results demonstrate that our scheme can realize strong chiroptical response of the metamaterial to circularly polarized waves. We investigate circular dichroism in a bilayer chiral metamaterial in the optical range. The circular dichroism is very strong up to 75%. We also consider how the incident angle of the circular polarized wave affects the circular dichroism. We can achieve strong circular dichroism by changing the incident angle. The proposed metamaterial will be a good candidate for biosensing applications.
In this work, two split U-shaped metamaterials with photoconductive silicon layer are presented and numerically investigated. The electromagnetically induced transparency can be manipulated by changing angles of the incident wave and the conductivity of silicon. At oblique incidence of 50°, the metamaterials exhibit an obvious photoswitching effect. The transmittance peaks of both two structures are as high as about 0.7, respectively. The fishscale metamaterials with integrated silicon layer show a very high modulation depth. Based on the modulation of the conductivity of the silicon, the scheme can be readily applied to realize THz metamaterial switch.
In this work, we proposed a dual-core fiber coupler. Identical long-period fiber grating is written in each core of the dual-core fiber by using point-by-point high-frequency CO2 laser irradiation. The peak attenuations of the two similar LPGs are -19.8 dB and 13.2 dB, respectively. And the proposed coupler can achieve a peak coupling efficiency of 48%.
The directions of refracted and reflected beams in gradient metasurfaces are governed by the generalized Snell’s law. The metasurface response can also be controlled dynamically by a standing wave formed by two coherent counterpropagating beams. In this work, we study sensing properties of two types of optically controlled metasurfaces. With different ambient dielectrics, the metasurface reveals a shift of coherent absorption spectrum. The thicker the dielectric layer is and the higher its refractive index ise, the more the absorption spectrum moves. Moreover, the intensities of the anomalous scattering light change with the refractive index and thickness of the dielectric layer while the radiation direction is kept unchanged.
As new artificial electromagnetic materials, metamaterials have unique properties compared to natural materials, such as negative refraction, invisibility, and perfect imaging. Surface plasmon-polaritons (SPPs) are electromagnetic waves travelling along the surface of the polar material SiC. Spectral selectivity, high intensity, and unidirectional emission of thermal radiation into a single desired angle have been investigated, which promises important applications such as thermophotovoltaic devices, mid-infrared light sources and thermal spectroscopy. In this work, a kind of silicon carbide (SiC) structure is designed and the corresponding thermal radiation is simulated. SiC, as a polar material, exhibits a metal-like characteristic at about 10.6 to 12.6 μm, supporting a surface phonon polariton wave in this band. Due to high melting point of silicon carbide, it can be used for thermal source design to control direction and efficiency of thermal radiation.
A novel anti-bending long period grating (LPG) in an embedded-core hollow optical fiber (ECOF) is proposed and experimentally demonstrated. A piece of ECOF was rotated by 180° around the geometrical center of the fiber, and then it and other ECOF were aligned along the fiber core and spliced. And a LPG, the center of which is the fusion splicing point, was fabricated in the ECOF by using a high-frequency CO2 laser to form an anti-bending sensor. The dependence of the resonant peak on the bending was studied. Experimental results show that the maximum sensitivity of bending is only 0.47 nm/m-1.
In the present work, a compact all-fiber plasmonic focusing beam generator with single and multiple spots is proposed and demonstrated in a conventional multimode fiber. Here, the focusing beam generator is composed of air slit arrays perforated through the gold films deposited on the end facet of a multimode fiber. The array of nanoscale slits with varying widths is used to modulate phase distribution of the focused light. An all-fiber focusing beam generator provides many advantages, such as self-alignment, high flexibility, lower insert loss, and easy portability, which is of importance to realize optical trapping, micromanipulation, beam shaping, and fiber integrated devices.
Polarization properties provide a controlled degree of freedom in the process of light propagation. Efficiently manipulating polarization states plays a pivotal role in the areas of electromagnetic wave detection and information communication. In this work, we propose a chiral metamaterial that is comprised of an array of 90°-twisted E-shaped resonators with incorporated vanadium dioxide films. The hybridized chiral metamaterial allows us to effectively modify the conductivity of vanadium dioxide utilizing a thermal trigger. A thermo-controlled cross-polarization conversion can be realized. The phase transition metamaterials may open an opportunity in the THz regime to acquire a variety of functionalities, such as tunable filters, modulators and switches.
A linear 5-core fiber was sandwiched in between two single mode fibers (SMF) to construct an all fiber Mach-Zehnder interferometer (MZI). The interferences between the fundamental supermodes, between the fundamental supermodes and the high order core modes, and between the fundamental supermodes and the low order cladding mode are investigated. The experimental results show both the interference between the core modes and the interference between the core modes and the cladding modes have approximately equal temperature sensitivity. The interference between the core modes is insensitive to the axial strain.
A novel Bragg fiber grating (FBG) in an embedded-core hollow optical fiber (ECHOF) has been proposed and experimentally demonstrated. The high-quality FBG fabricated with phase-mask technique by using 248 nm ultraviolet laser, has a resonant wavelength of ~943.1 nm and a dip of ~24.2 dB. Subsequently, the dependences of the resonant peak on the temperature and the axial strain were studied. Experimental results show that the temperature and axial stain sensitivity are 6.5 pm/°С and 1.1 pm/με, respectively. In addition, a 0.03 nm shift of the transmission dip can be obtained when the polarization state changes from X polarization to Y polarization.
We demonstrate selective coherent perfect absorption based on interaction between bilayered asymmetrically split rings (ASRs) metamaterials and a standing wave formed by two coherent counter propagating beams. The selective coherent perfect absorbers with high absorption have been achieved depending on the phase difference between two coherent beams. The selective coherent control absorbers can be well designed by changing the thickness of the dielectric layer and the asymmetry of the ASRs. The coherently controlled metamaterials provide an opportunity to realize selective multiband absorption and ultrafast information processing.
We propose an ultrathin planar metamaterial with an abrupt phase change along its surface for beam manipulation. The metamaterial is composed of bilayered asymmetrical split ring apertures (ASRAs) on either side of a dielectric substrate. The proposed metamaterial relies on eight variable ASRAs in a super cell to modulate the phase of transmitted wave. Efficient beam direction manipulation for cross-polarization transmission has been achieved and co-polarization transmission has been completely suppressed. Numerical simulation results show that the linearly polarized incident wave can deflect in a designated direction passing through the ultrathin metamaterial. An intensity efficiency of 70% and a deflection angle of 24° at 6.2GHz have been verified.
We present an optically controlled terahertz (THz) switch to tune the state of polarization based on single-layer chiral metamaterial. The chiral metamaterial consists of an array of perforated S-shaped slits with incorporated photoactive silicon, which allows us to control dynamically cross-polarization transmission. The switch state can be efficiently controlled by external optical stimuli. The realization of cross-polarization THz switch in a single-layer metamaterial has simple structure design and easy fabrication and therefore the S-shaped metamaterial will be a promising candidate for polarization control devices.
We theoretically investigate electromagnetic responses of a bilayered metamaterial in the optical frequency range. The
metamaterial consists of two stacked split ring resonators with a twist angle spatially separated by a dielectric layer. The
simulated results show that the bilayered metamaterial exhibits multiple transmission windows based on mutually
coupling between two twisted resonators at normal incidence and the manifested transmission properties can be
controlled efficiently by adjusting structural parameters of a unit cell. The coupling in metamaterials offers an effective
way to manipulate the electromagnetic properties of metamaterial-based devices.
We propose a novel embedded-core hollow optical fiber composed of a central circular air hole and a semi-elliptical core
embedded in an annular cladding. Both the phase birefringence and group birefringence are investigated based on the
finite element method (FEM). The embedded-core hollow optical fiber has polarization-preserving properties. The
birefringence magnitude of the proposed fibers is the same order as that of the side-hole optical fiber. The theoretical
results reveal that the birefringence of embedded-core hollow optical fibers can reach the order of 10-4. The group
birefringence of the fiber is obtained by using the wavelength scanning technique and can reach 4.7×10-5. The
measurement results are basically consistent with the theoretical simulations.
We study local field energy density enhancement in planar metamaterials at normal incidence based on the finite element
method. The microwave metamaterials composed of asymmetric resonators with/without quartz substrates are utilized to
investigate the resonant response to incident electromagnetic waves. The trapped-mode resonant feature results from the
excitation of an antisymmetric current mode due to the broken symmetry between two resonators and the quality factor
and local field energy density enhancement strongly depend on the asymmetry. The proposed metamaterial on glass
substrate shows the high possible quality factor of about 1000 and energy density enhance factor of up to 150000. To
reduce losses of metamaterials further, freestanding metallic structure is considered being treated as perfect electric
conductor and real-loss metal respectively. Real metallic metamaterial provides a very sharp trapped-mode resonance
having the quality factor of up to 1500.
We study theoretically that a resonant response with a high quality factor could be achieved in a planar positive or
negative metamaterials by weakly breaking the geometrical symmetry of its unit cells. The trapped mode resulting from
broken structures is investigated using the finite element method and the dependence of the quality factor on structural
parameters is also discussed.
The reflectance and transmittance of thin films at oblique incident angles exhibit strong polarization effects, particularly
for the films inside a glass cube. However, the polarization effects are undesirable in many applications. To solve this
problem, non-polarizing beam splitters with unique optical thin films have been achieved employing a method of
combination of interference and frustrated total internal reflection, the non-polarizing condition expressions based on
frustrated total internal reflection has been derived, and the design examples of non-polarizing beam splitters with an
optimization technique have been also presented. The results of Rp=(50±0.5)%, Rs=(50±0.5)% andΔr=(0±0.3) degree in
the wavelength range of 400-700nm have been obtained. The thickness sensitivity of NPBSs is also analyzed.
When used at oblique angles of incidence, the reflectance and transmittance of thin films exhibit strong polarization
effects, particularly for the films inside a glass cube. However, the polarization effects are undesirable in many
applications. Novel non-polarizing beam splitter designs are shown, non-polarizing beam splitters with unique optical
thin films are achieved through combination of interference and frustrated total internal reflection, the non-polarizing
condition expressions based on frustrated total internal reflection is derived, and applied examples of the non-polarizing
beam splitters are also presented with the optimization technique and the results of Rp=(50±0.4)% and Rs=(50±0.4)% in
the wavelength range of 500-600nm are obtained.
When used at oblique angles of incidence, the reflectance and transmittance of thin films exhibit strong polarization effects, particularly for the films inside a glass cube, which result from the fact that the tangential components of the electric and magnetic fields are continuous across each layer interface. However, for many applications, the polarization effects are undesirable and should be reduced. Therefore, the concept of non-polarizing beam splitter is proposed. Up to now, however, most of the reports of non-polarizing beam splitters are suitable for visible light. Therefore, it is necessary to find out some methods to reduce the polarization effects for infrared applications. A design method of infrared non-polarizing beam splitter in a cube is proposed, the theoretical analysis is given, designs for different substrates are demonstrated and the simulations of their optical properties are presented in this paper.
The dispersion properties of the reflection-induced retardance of the polarization-preserving reflecting medium layer and its effect upon the sensitivity of an optical current sensor are theoretically analyzed taking the Jones' Matrix as a mathematical tool and simulated with a computer. The investigation results show that the dispersion will obviously affect the scale factor of the system. Therefore, it is necessary to stabilize the driving current and the ambient temperature of the optical source by some technical means. These results might be useful for the researchers and designers working in the optical current sensing techniques area.
The wavelength dependence of the sensitivity of a bulk-glass optical current transformer is theoretically analyzed taking the Jones’ Matrix as a mathematical tool, simulated with a computer and verified with an experiment. The results show that the wavelength dependence will cause obvious sensitivity fluctuation of the system because of the existing of the dispersions of the reflection-induced retardance, Verdet constant and the linear birefringence inside the bulk-glass current transducing head. Therefore, it is necessary to stabilize the driving current and the ambient temperature of the optical source used by some technical means, because these two factors can result in wavelength variation. These results might be useful for the researchers and designers working in the optical current transducing techniques area.
The effects of the linear birefringence inside a bulk glass current sensing element and the incident polarizing angle upon the performance of a bulk glass optical current sensor are derived and analyzed theoretically. The investigation results show that the linear birefringence will modify the scale factor of the system with a sample function; it can also affect the extent of the influence of the incident polarizing angle, at the same time. When the incident polarizing angle has some special values such as 0, 45, or 90 degree, its effect to the system will be zero. These results might provide some useful reference to the researchers and designers of bulk glass optical current sensors.
Linear birefringence inside the sensing head can obviously affect the performances of optical (fiber-optic) current transformers. Therefore, it is important to measure the linear birefringence for the design and the property enhancement of optical current transformers. A method employing a retarder to measure the linear birefringence inside the bulk glass current sensing head is reported. The theoretical analyses of the principle and the measurement uncertainty of the method are given using Jones Matrix as a mathematical tool. An applied example is also given. The main advantage of this method is that it can uniquely determine the value of the linear birefringence inside the sensing head, which overcomes the shortcoming of that only a trigonometric function value of the birefringence can be determined by the method reported before. Furthermore, there are some other advantages such as simple structured, practical and that the devices needed are easy to obtain.
A novel method used for the enhancement of the accuracy of the measurement of the retardance of a quarter-wave plate employing two polaroids and a prism is reported under the condition of knowing the direction of the fast axis of the plate. The theoretical analysis of the principle and the uncertainty formula are given. An application example is also demonstrated. The measured result of the example is verified with an experiment. The main advantages of this method include the accuracy enhancement, simple measuring setup and easy operation.
In this paper, a novel approach to measure linear birefringence in bulk optical glass sensing heads of current sensors is reported. The theoretical analysis based on Jones matrix, the formula of uncertainty and applied examples are also given. The main advantages of this method are simple structured, practical and that the devices needed are easy to obtain.
The measurement of reflection-induced retardance of optical devices is the usual case encountered in optical research, development and engineering. This kind of measurement usually can be completed using a linear polarizer and a retarder. A novel method used for the measurement of reflection-induced retardance employing a polarizer and an analyzer is proposed in this paper, the theoretical analysis of the method and an application example are given.
Extinction ratio is an important parameter of a polarizer, and the bias angle of the polarizer used in optical current sensing systems is also an important adjustable parameter. Both them can certainly affect the performance of the sensing systems. In this paper, the effects of the extinction ratio and bias angle error of polarizers upon the output characteristics of an optical current sensor are investigated theoretically and experimentally, some useful results are given for the optical current sensor designers.
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