KEYWORDS: Mode locking, Microwave radiation, Microsoft Foundation Class Library, Optoelectronics, Harmonic generation, Oscillators, Signal generators, Tunable filters, Pulse signals, Photonics
The optoelectronic oscillator (OEO) is a typical time-delay system with rich nonlinear dynamical characteristics. Most of the previous research on OEOs has been focused on analyzing the properties of OEOs with a long time delay, which makes it difficult to realize mode locking without additional phase-locking mechanisms. We have achieved, for the first time to our knowledge, a self-mode-locking OEO and generated stable microwave frequency combs by analyzing the characteristics of OEOs with an ultrashort time scale. In the experiment, the self-mode-locking OEOs with fundamental mode, second-order harmonic, and sixth-order harmonic were realized by adjusting the system parameters, all of which produced uniform square wave signals with tunable duty cycles, steep rising and falling edges, and periods of less than 20 ns. The self-fundamental-mode-locking OEOs with different time delays were also implemented and experimentally realized. Furthermore, the experiment revealed the self-hybrid mode-locking OEO, which is the coexistence and synchronization of the three measured self-locking modes in one OEO cavity, demonstrating the complex nonlinear dynamical behaviors of the OEO system and enabling the generation of periodic nonuniform hybrid square wave signals. The realization of the self-mode-locking OEO and the generation of flexible and stable square wave signals at ultrashort time scales enrich the study of OEO nonlinear dynamics in the realm of complex microwave waveform generation, offering promising applications in areas such as atomic clocks, radars, communications, and optoelectronic neural networks.
We propose and demonstrate a high-sensitivity phase-measurement-based interrogation technique enabled by microwave photonics for optical fiber Fabry-Perot interferometer (FPI) sensors. A low-coherent broadband light source is employed as the light source, and the reflection spectrum of the FPI device is modulated by microwave signals output from Port 1 of a vector network analyzer via an electro-optic modulator. The intensity-modulated optical signal goes through a long length of dispersion compensating fiber and is then detected by a highspeed photodetector. The electrical signal is sent to Port 2 of the vector network analyzer for synchronized detection at each modulation frequency to obtain the frequency response of the FPI sensor. The frequency passband of the FPI sensor in the frequency response is then located and an operating frequency within the passband is designated. By monitoring changes in the phase information of the operating frequency, changes in the optical path difference of the FPI sensor can be determined with high sensitivity. As a proof of concept, an FPI device constructed by fusion splicing a section of photonic crystal fiber (PCF) between two SMFs is employed for tensile strain sensing. Due to the low Young’s modulus of the PCF and the high sensitivity provided by the proposed phase-measurement-based interrogation approach, a high-performance strain sensor with a resolution of 60 nanostrains is revealed
This paper studies the use of Microsphere Photolithography (MPL) as an alternative to Focused Ion Beam milling or e-beam lithography to pattern plasmonic fiber-optic based sensors. In the MPL approach, silica microspheres are self-assembled to form a Hexagonal Close-Packed (HCP) array on top of a layer of photoresist. The microspheres serve as an optical element and focus collimated UV radiation to an array of photonic jets inside the photoresist layer. The exposed region is dependent on the angle of incidence of the UV radiation which facilitates hierarchical patterning. Pattern transfer can be accomplished using either etching or lift-off with the size of the features dependent on the exposure dose. While low-cost and very scalable, the use of MPL influences the design and performance of the fiber probe in several ways. Specifically, the entire cleaved face is patterned without alignment to the fiber core and any defects in the self-assembled microsphere lattice are transferred to the surface. This paper presents the low-cost fabrication of Extraordinary Optical Transmission (EOT) type plasmonic fiber-optic based sensors. The sensors consist of a thin aluminum film on the cleaved face of single mode optical fiber, perforated with a HCP hole-array. At resonance, Extraordinary Optical Transmission (EOT), decreases the reflection from the fiber tip. The conditions for resonance are dependent on the local environment surrounding the fiber tip and the resonant wavelength can be used to measure the index of refraction of a liquid. Experiments show that viable sensors can be created with MPL. The reflection spectra of the sensors was measured in various concentrations of sugar water with a measured sensitivity of 8.33 mg/mL/nm. These results are compared to simulation results which provides a sensitivity analysis of the sensors.
This paper reports the design, fabrication, characterization, and noise reduction of metasurface based uncooled infrared microbolometers with focus on device architecture. Two designs are investigated. In the first design, the devices are fabricated with the legs positioned underneath the microbolometer pixel. This is facilitated by the use of the metasurface which removes the need for a Fabry-Perot 1/4 cavity. Placing the legs underneath the pixel permits longer legs without sacrificing fill factor and raises the thermal resistance between the microbolometer and the substrate. The metasurface potentially allows spectrally dependent IR absorption. The second design extends this architecture to include a second microbolometer suspended above the first microbolometer to form a single pixel. Metasurfaces on each microbolometer can be designed to capture a portion of the spectrum with the combined structure maximizing the total absorptance across the Long Wave Infrared (LWIR) band. The TCR and resistivity are measured on the fabricated devices with and without the addition of the metasurface for both designs. The metasurface produces a slight increase in the TCR 5% to 12% and a dramatic reduction in the resistivity (>5×) which leads to a two order of magnitude reduction in the microbolometer noise voltage Power Spectral Density (PSD) after annealing in vacuum. The measured single cavity microbolometer has a voltage responsivity of 4.1×104 V/W and detectivity of 3.57×108 cm·Hz1/2/W.
This paper presents a study of metasurface integrated microbolometers. The semiconductor absorber is sandwiched between a metal Frequency-Selective Surface (FSS) and ground plane. When the semiconductor absorber is electrically isolated from the ground plane by a thin dielectric it can be used to measure the temperature of the pixel. The integration with the FSS removes the need for a Fabry-Perot cavity. The FSS allows control the attributes of radiation absorbed by the microbolometer on a pixel-by-pixel basis which provides the potential for spectral or polarimetric imaging. The FSS also affects he electrical performance of the semiconductor absorber and the thermal performance of the microbolometer. In addition, the complex permittivity of the semiconductor affects the optimal design of the FSS. The Si/Ge/O system is selected because it allows the properties of the absorber to be engineered (e.g., less oxygen gives lower absorptance and higher resistivity). This paper explores the absorber/FSS parameter space with an emphasis on the electrical and noise properties of the integrated system. Models are developed to explain results. Preliminary results show that the addition of the FSS improves TCR of the microbolometer by 10% while dramatically lowering its resistivity (factor of 5×). The resistivity reduction leads to a dramatic reduction of the noise power spectral density with the addition of FSS improving the measured 1/f noise by two orders of magnitude over an identical sample without the FSS. In addition, this paper will present the microbolometer figures of merits including voltage responsivity, detectivity, and thermal response time.
Inspired by Rayleigh backscattering based sensing methodology on an optical fiber, we present a novel sensing concept based on the random inhomogeneities on a coaxial cable. As an analogy of Rayleigh backscattering along an optical fiber length, “backscattering” also exists from a commercial cable due to its inherent defects along a cable length which induce a local variation. The accumulated back-scattered signals along the cable can be obtained using frequency domain reflectometry. By analyzing the shift in the local back-scattered signal, the local perturbations can be determined, so that truly distributed sensing capability using a coaxial cable can be achieved.
A hollow coaxial cable Fabry-Perot resonator for displacement and strain measurement up to 1000 °C is presented here. Inspired by optical fiber Fabry-Perot interferometers, a Fabry-Perot resonator is implemented on a homemade hollow coaxial cable by introducing two highly-reflective reflectors along the cable. By tracking the shift of the amplitude reflection spectrum of the microwave resonator, the displacement and strain can be determined. The displacement measurement experiment showed the sensor could function properly up to 1000 °C. The sensor was also employed to measure the thermal strain of a steel plate during the heating process from 100 to 900 °C.
An embeddable, robust and cost-effective optical interferometric strain sensor with nano-scale strain resolution is reported in this paper. The principal structure of the sensor consists of an optical fiber, a quartz rod coated with a thin gold layer, and two metal shells employed to transfer the strain, orient and protect the optical fiber and quartz rod. The optical fiber endface, combining with the gold-coated surface forms an extrinsic Fabry-Perot interferometer. When the sensor is subjected to an external compressive/tensile stress, slide between the two metal shells will occur, resulting in a cavity length variation of the interferometer. A temperature compensation design is employed in the structure to minimize the temperature crosstalk of the sensor. The sensor was firstly calibrated, and the result showed that our prototype sensor can realize a measurement resolution of 30 nanostrain (nε) and a sensitivity of 10.01 με/μm over a range of 1000 με. After calibration of the sensor, monitoring the shrinkage strain of a cubic brick of mortar in real time during the drying process was conducted. The strain sensor was compared with a commercial linear variable displacement transducer, and the comparison results in four weeks demonstrated that our sensor had much higher measurement resolution and gained more detailed and useful information. Due to the advantages of the extremely simple, robust and cost-effective configuration, it is believed that the sensor is significantly beneficial to practical applications, especially in structural health monitoring.
A miniaturized optical fiber tip Fabry-Perot interferometer for high-temperature measurement is presented in this paper. The fabrication process of the diaphragm-free Fabry-Perot cavity is quite simple, involving only two steps: fusion splicing and cleaving. By adjusting the arc power during fusion splicing, a concave-shaped structure is obtained, through which the light is coupled/split into the wall of the spliced hollow core fiber. By cleaving the end-face of the hollow core fiber, a concave-shaped diaphragm-free Fabry-Perot interferometer is formed. The temperature response of the sensor was demonstrated, showing a high-temperature tolerance up to 1000 °C and a sensitivity of 0.01226 nm/°C.
We report a distributed fiber-optic pressure sensor based on Bourdon tubes using Rayleigh backscattering metered by optical frequency-domain reflectometry (OFDR). In the proposed sensor, a piece of single-mode fiber (SMF) is attached to the concave surfaces of Bourdon tubes using a thin layer of epoxy. The strain profiles along the concave surface of the Bourdon tube vary with applied pressure, and the strain variations are transferred to the attached SMF through the epoxy layer, resulting in spectral shifts in the local Rayleigh backscattering signals. By monitoring the local spectral shifts of the OFDR system, the pressure applied to the Bourdon tube can be determined. By cascading multiple Bourdon tubes and correspondingly attaching SMF sections (i.e., a series of SMF-modified Bourdon tubes), distributed pressure measurements can be realized. Three Bourdon tubes are employed to demonstrate the proposed spatially distributed sensing scheme. The experimental results showed that linear relationships between spectral shift and pressure were obtained in all three SMF-Bourdon tubes (i.e., at three spatial locations). It is expected that the proposed sensing device, the SMF-Bourdon tube, can be used in applications where distributed/multipoint pressure measurements are needed.
Additive Manufacturing (AM) of low-profile 2.5D glass structures is demonstrated using a fiber-fed laser-heated process. In this process, glass single mode optical fibers with diameters 90-125 μm are fed into the intersection of a workpiece and CO2 laser beam. The workpiece is positioned by a four-axis CNC stage. Issues unique to the process are discussed, including the thermal breakdown of the glass and index inhomogeneity. Scanning electron microscopy reveals that the core/cladding structure of the fiber remains intact during printing and can be used to guide light for photonic applications.
A novel optical fiber extrinsic Fabry-Perot interferometer pressure sensor with low temperature cross-sensitivity is presented. The Fabry-Perot is formed by the endface of an optical fiber and the center part of a buckled beam. The working principle of the sensor involves a pressure transfer and displacement transfer and amplification mechanism based on a buckled beam. When the sensor is subjected to an external pressure, the sheet steel will deflect, resulting in an axial displacement of the buckled beam. The buckled beam will experience a deflection, and the deflection at the midpoint of the beam is one order of magnitude larger than the deflection of the sheet steel. So, a relatively large change in the cavity length can be obtained. Compared with the traditional diaphragm based pressure sensors, the sensitivity of our sensor was improved by one order of magnitude, and the temperature-pressure cross-sensitivity was found to be as low as 22 Pa/℃.
This paper reports a novel hollow coaxial cable Fabry-Perot resonator for wide-range displacement measurements, up to meter range. The sensor is based on a novel and homemade sensing platform, hollow coaxial cable, in which the traditional insulating layer is replaced by air. The diameter of the inner conductor and outer conductor of the hollow coaxial cable are designed to be 6 mm and 14 mm respectively, to achieve an impedance of 50 Ohm. Two highly reflective reflectors, including a fixed one and a movable one, are engineered on a hollow coaxial cable to form a Fabry-Perot resonator (HCC-FPR). A measurement handgrip is permanently connected to the second reflector. Based on this design, the displacement of the measurement handgrip can be transferred to the cavity length change of the HCC-FPR. The change in the cavity length can be determined by tracking the shift in the reflection spectrum. The physics of the sensor was described, and the displacement response of the sensor was tested.
We report a low-cost and compact extrinsic Fabry-Perot interferometer-(EFPI) based optical fiber sensor for measuring three-dimensional (3D) displacements, including interfacial sliding and debonding during a delamination process. The idea is to use three spatially-arranged EFPIs as the triaxial displacement sensing elements. The sensor consists of an optical fiber component and a mirror component. The fiber component includes three optical fibers, and their corresponding mirrors form the mirror component. Two coincident roof-like metallic structures are used to support the two components, making sure that the endfaces of the fibers and corresponding mirrors maintain a parallel relationship during measurements. As a result, three EFPIs are formed by the endfaces of the optical fibers and their corresponding mirrors. The prototype sensor was first calibrated, and then an experiment monitoring the interfacial sliding and debonding between a long square brick of mortar and its steel base plate support during the drying/curing process was conducted to demonstrate the practicability of the sensor. The experimental results show that our sensor can function continuously for a long period of time. The details obtained from the measured data were also discussed. The robust and low-cost three-dimensional sliding and debonding sensor has a high potential in various applications, especially in structural health monitoring.
A low-cost and robust extrinsic Fabry-Perot interferometer (EFPI) based optical fiber displacement sensor with a wide measurement range, up to cm level, is reported in this paper. A gold-coated reflective mirror fixed inside the metal shell of the sensor, together with the endface of a single mode fiber form a Fabry-Perot cavity. The optical fiber is supported and oriented by a fiber ceramic ferrule, and the sensor is packaged and protected by a metal shell. A triangle geometry based displacement transfer mechanism is employed in the constructed sensor which makes the sensor capable of measuring wide-range displacement, and this triangle geometry based structure allows the measurement range of the sensor to be flexibly adjusted. When the measurement handgrip of the sensor experiences a displacement, the cavity length will change due to the triangle geometry design. By tracking the change in the reflection spectrum, the displacement can be determined. The experimental results show that a displacement measurement sensitivity of 42.68 nm/μm (change in EFPI cavity length/displacement magnitude) over a measurement range of 2.0 cm was achieved by the constructed prototype sensor. The present wide-range displacement sensor with low cost, high robustness has a great potential in the chemical-oil industry, the construction industry.
This paper describes the low-cost, scalable fabrication of 2D metasurface LWIR broadband polarized emitter/absorber. A Frequency Selective Surface (FSS) type design consisting of dipole antenna elements is designed for resonance in the 7.5-13 μm band. Frequency-domain Finite Element Method (FEM) is used to optimize the design with ellipsometrically measured properties. The design is synthesized to be broadband by creating a multiple cavities and by hybridizing the dipole modes with phonon resonances in a germanium/silica dielectric which separates metallic elements from a continuous ground plane. While IR metasurfaces can be readily realized using direct-write nanofabrication techniques such as E-Beam Lithography, or Focus-Ion Beam milling, or two-photon lithography, these technologies are cost-prohibitive for large areas. This paper explores the Microsphere Photolithography (MPL) technique to fabricate these devices. MPL uses arrays of self-assembled microspheres as optical elements, with each sphere focusing flood illumination to a sub-wavelength photonic jet in the photoresist. Because the illumination can be controlled over larger scales (several μm resolutions) using a conventional mask, the technique facilitates very low cost hierarchical patterning with sub-400 nm feature sizes. The paper demonstrates the fabrication of metasurfaces over 15 cm2 and are measured using FTIR and imaged with a thermal camera.
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