We present the results of the implementation of two VCSEL-based optoelectronic oscillators (VBO) using one on-chip 850-nm vertical-cavity surface-emitting laser (VCSEL). The reported VBOs are implemented at 10 and 12 GHz using a direct-modulated VCSEL. The laser direct modulation bandwidth is 13.3 GHz. The VBOs performance is described through the phase noise and time-domain stability.

This guest editorial summarizes the Special Section on Optical Fiber Sensor Technology.

A low cost and easily fabricated plastic optical fiber (POF) displacement sensor is presented. The sensor is based on the macrobending POF with a V-groove structure fabricated by a simple die-press-print method, which is easy to implement and effectively reduces the complexity of the fabrication process. The intensity modulation method is adopted for displacement sensing, which lowers the sensor system’s cost and complexity. Experiments are carried out to investigate the influence of the structural parameters on the displacement sensing performance and the proposed POF probe is optimized by changing the structure parameters. Results showed that when the V-groove structure depth is 200  μm, the length is 22 mm, the angle is 60 deg, the pitch is 2 mm, and the macrobending radius of the POF probe is 15 mm, the highest sensitivity could reach to 3.19  ×  10^−  2  /  mm with the measurement range of 18 mm.

For structural health monitoring applications, recent studies have demonstrated an increased ultrasonic detection sensitivity of fiber Bragg grating (FBG) sensors through remote bonding of the FBG to a structure. In this case, the S₀ or A₀ Lamb waves in a structure are coupled to guided L₀₁ waves in an optical fiber at the adhesive bond location, resulting in L₀₁ modes of equal amplitudes propagating in both directions along the optical fiber. We demonstrate that when the adhesive bond is replaced with adhesive tape the S₀ mode couples to the same L₀₁ modes, however, with a preferential direction. Therefore, our study investigates the mechanism causing the preferential direction. We first identify different ultrasonic coupling pathways from a plate to an FBG through adhesive tape, demonstrating that both direct and indirect pathways are active, but that the indirect pathway produces the directional coupling. Then we test different adhesive tape parameters through experiments and simulations, demonstrating that flexural rigidity and bond length are parameters that can be used to control the directionality. The results of this work could be applied to design multiplexed FBG sensor arrays with specified signal pathways through the optical fiber networks.

A distributed fiber optic strain sensor based on Rayleigh backscattering, embedded in a fiber-reinforced polymer composite, has been demonstrated. The optical frequency domain reflectometry technique is used to analyze the backscattered signal. The shift in the Rayleigh backscattered spectrum is observed to be linearly related to the change in strain of the composite material. The sensor (standard single-mode fiber) is embedded between the layers of the composite laminate. A series of tensile loads is applied to the laminate using an Instron testing machine, and the corresponding strain distribution of the laminate is measured. The results show a linear response indicating a seamless integration of the optical fiber in the composite material and a good correlation with the electrical-resistance strain gauge results. The sensor is also used to evaluate the strain response of a composite-laminate-based cantilever beam. Distributed strain measurements in a composite laminate are successfully obtained using an embedded fiber optic sensor.

An optical blade tip timing scheme is presented and demonstrated experimentally using an optical microstructure surface inscribed on the blade tip. The results show that the method is not sensitive to the fluctuations of light intensity introduced by the change of tip to sensor clearance, the light intensity of light source, and quality of the tip surface.

Fiber sensors were first proposed over 50 years ago, creating very significant enthusiasm and interest. In the interim, a few specialist sensor systems have evolved to occupy a little over 1% of the total sensor market. However, the enthusiasm and interest have most definitely continued! The somewhat subjective discussion presented here explores why these apparently conflicting observations coexist and also speculates—optimistically—on the future evolution of the technology.

Fiber-optic sensing technology is best adapted to health monitoring and evaluation of power grids because of its immunity of electromagnetic interference, capabilities of multiplexing and distributed sensing, and tolerance to harsh environments. We review key fiber-optic sensing technologies, including fiber Bragg gratings, fiber-optic interferometers, optical time domain reflectometries, and their applications in three main parts of power grids, transformers, power towers, and overhead transmission lines, during the past 20 years. In particular, optical fiber composite overhead ground wire and optical phase conductor applied in power grids are the areas of great potential to go further. The perspectives of an intelligent fault diagnosis subsystem for power grids based on a fiber-optic sensing network are discussed, and related on-going work is described. The review shall be of benefit to both engineers and researchers in power grids and fiber-optic sensing.

A temperature-sensor is presented based on the solid core photonic crystal fiber (PCF) infiltrated with the magnetic fluid (MF) in all or some parts of the cladding air holes with the help of the syringe injection technique. The 8.5-cm length of the proposed PCF sensor is used. A Mach–Zehnder interferometer modal is used for the detection process. The applied temperature is given in a range from 25.6°C to 100.1°C. The calculated outcome justifies that a very clear interference spectrum is obtained between the change of the refractive index of the MF and the temperature. From the experimental outputs, the temperature sensitivity of the intended PCF is −0.38  nm  /    °  C, and at high temperature, it achieves 0.176  nm  /    °  C. The MF has great sensitivity at high temperature. This proposed sensing system may find potential applications in medical science and industry.

Needs for sensor miniaturization, versatile sensing solutions, and improved measurements’ performances in difficult operating environments have recently driven considerable research in optical fiber sensor for multiparameter measurements. Multiparameter sensors not only enable new sensors’ functionalities, but can also improve achievable measurement performances for some frequently measured parameters considerably. This study provides a review of work in the field of miniature fiber-optic sensors that allows independent and simultaneous measurements of two or more different physical or chemical parameters. Sensor designs and corresponding signal processing schemes are reviewed and compared.

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.

We demonstrate the potential of applying hollow core and negative curvature optical fibers (HC-NCF) as efficient sensors to monitor the concentration of three high-toxicity gases: methane (CH₄), carbon monoxide (CO), and nitrogen monoxide (NO). Numerical simulations demonstrate that the insertion of holes in such fibers guarantees the entry of these gases into their hollow core and allows strong interaction of these gases with the low-loss mode propagating in the HC-NCF. This interaction between light and gas in spectral regions with high gas absorption allows one to monitor reduced concentrations of these gases present in the environment simply by monitoring the optical power at the fiber output. The results show a linear behavior of propagation losses as a function of concentrations of 0% to 100% of CO and NO gas, and for concentrations of 0% to 5% of CH₄ gas. This linearity between the propagation losses and the variation of the concentrations of gases in the environment can promote its application in healthcare and environment, to monitor low concentrations of gases ensuring high speed and accuracy.

A technique for measurements of output power of fiber lasers using a metal-coated fiber sensor is proposed. Optical radiation transmitting through the core of a copper-coated silica fiber is partially scattered and, therefore, absorbed in the copper layer. The change of an electrical resistance of the metal coating induced by its heating is measured. This technique can be used for the real-time measurement of output radiation power of fiber laser sources. The measurement range of optical power can be controlled by changing the bend curve radius of the metal-coated fiber. Optical scattering coefficients of the metal-coated fiber core were determined using the proposed theoretical model of the fiber sensor heating.

Plasmonic devices using microstructured optical fibers are particularly well suited for the development of high sensitivity chemical and biochemical sensors, thanks to their intrinsic microfluidic properties. We review the most recent literature on such sensors, comparing different types and approaches. Then, we present some examples of the design/analysis using finite-element software tools. In particular, a sensor for liquid analytes based on a birefringent solid-core microstructured optical fiber is discussed in detail. This device has six holes, quite a limited number in comparison to other structures with similar performances, whose geometrical characteristics can be tailored to optimize the power fractions carried by the core and plasmon modes at the loss-matching points in the analyte layer depending on the analyte refractive index.

The quality assessment of stereoscopic images plays an important role in various three-dimensional (3-D) areas and encounters more problems than two-dimensional (2-D) image quality assessment. We propose a perceptual full-reference quality assessment model by considering human adaptable double visual channel. Human binocular combination characteristics and depth perception are both considered in this model, and an adaptable gain-control model is adopted to assign appropriate weights to the information of human visual channels. Experimental results indicate that the proposed algorithm can serve as an efficient predictive image quality feature, which delivers not only highly competitive prediction accuracy but also moderate computational complexity.

A light-field camera combines optics and computation to provide the ability to perform ranging. Many methods and algorithms, which are time-consuming and noise-sensitive, have been proposed for per-pixel depth estimation. We present a Fourier domain ranging method for light fields (LFs). Instead of estimating the depth for every pixel, we attempt to detect the depths of the different planes at which objects are located. The method is somewhat like using the energy in the focal stack, but it is carried out in a more efficient way. Our method has the advantages of speediness and robustness compared with traditional per-pixel depth estimation methods. In addition to the ranging algorithm, we also demonstrate the idea and application of a region adaptive denoising filter, in which the depth parameters are tuned by the proposed method. We include results for synthetic LFs, the Stanford LF archives, and LFs captured with a Lytro camera, exploiting the algorithm complexity, accuracy, depth resolution, and noise performance of our method. Our method uses less computation and is more robust than per-pixel depth estimation, making it appropriate for many applications, such as autorefocusing and autodenoising.

Imaging systems can be used to obtain situational awareness in maritime situations. Important tools for these systems are automatic detection and tracking of objects in the acquired imagery, in which numerous methods are being developed. When designing a detection or tracking algorithm, its quality should be ensured by a comparison with existing algorithms and/or with a ground truth. Detection and tracking methods are often designed for a specific task, so evaluation with respect to this task is crucial, which demands for different evaluation measures for different tasks. We, therefore, propose a variety of quantitative measures for the performance evaluation of detectors and trackers for a variety of tasks. The proposed measures are a rich set from which an algorithm designer can choose in order to optimally design and assess a detection or tracking algorithm for a specific task. We compare these different evaluation measures by using them to assess detection and tracking quality in different maritime detection and tracking situations, obtained from three real-life infrared video data sets. With the proposed set of evaluation measures, a user is able to quantitatively assess the performance of a detector or tracker, which enables an optimal design for his approach.

We present an accurate and efficient transformation estimation algorithm called local reference frame-based maximum consensus statistic (LRF-MCS). Moreover, a local reference frame (LRF) is proposed for effectively implementing our LRF-MCS. The proposed LRF uses a small local region to calculate its z axis and employs a salient local point to determine its x axis for improving the robustness to incomplete region. The proposed LRF-MCS first efficiently calculates the consensus set connected to each input correspondence by an LRF-based transformation technique. Then a switch program is used to select an appropriate consensus set for calculating the optimal transformation, which ensures the strong robustness to the seriously corrupted input correspondences. Extensive experiments are implemented on four standard datasets for comprehensively evaluating the performance of the proposed LRF and LRF-MCS techniques. The experimental results show that our LRF and LRF-MCS techniques outperform the state-of-the-art algorithms in terms of accuracy, efficiency, and robustness, achieving the best overall performance.

The performance of an infrared search and track (IRST) sensor depends on a large number of variables that are important for determining systems performance. One of the variables is the pulse visibility factor (PVF). The PVF is linearly related to IRST performance metrics, such as signal-to-noise ratio (SNR) or signal-to-clutter ratio (SCR). Maximizing the performance of an IRST through a smart design of the sensor requires understanding and optimizing the PVF. The resulting peak, average, or worst case PVF may cause large variations in the sensor SNR or SCR as the target position varies in the sensor field of view (FOV) and corresponding position on the focal plane. As a result, the characteristics of the PVF are not straightforward. The definitions and characteristics for the PVF to include ensquared energy (best case PVF), worst case PVF, and average PVF are provided as a function of F  *  Lambda  /  d_cc (d_cc is the center-to-center distance between pixels, i.e., pixel pitch). F  *  Lambda  /  d_cc is a generalized figure of merit that permits broad analysis of the PVF. We show the PVF trends when the target has a finite size but is still unresolved on the focal plane [smaller than an instantaneous field of view (IFOV)]. The target size was constrained to be no less than 2% of the IFOV but also no greater than 100  %   to study the effects on the PVF as a function of target size. Finally, we describe the characteristics of the PVF when optical degradations, such as aberrations, are inherent in the sensor transfer function. The results have illustrated that small F  *  Lambda  /  d_cc with large fill factor maximized the PVF at the expense of greater variability. Larger F  *  Lambda  /  d_cc can reduce the PVF variations but results in a decreased PVF. Finite target sizes and additional optical degradation decreased the PVF compared to diffraction-limited systems.

Extending existing scalar Schell-model source work, we derive the M² factor for a general electromagnetic or vector Schell-model source to assess beam quality. In particular, we compute the M² factors for two vector Schell-model sources found in the literature. We then describe how to synthesize vector Schell-model beams in terms of specified, desired M² and present Monte Carlo simulation results to validate our analysis.

An image contrast correction method is proposed for en face images obtained with full-field optical coherence tomography (FFOCT). First, the mechanism of image contrast decrease in FFOCT is considered and the theoretical models of main parameters that degrade image contrast are analyzed. Second, changes of contrast with depth are calculated under various conditions, from which the main parameters that affect contrast in tissue are identified. Then based on the analysis, the methods are proposed for correcting image contrast. Finally, the en face tomographic images of human liver tissue at different depths with and without contrast correction are presented to demonstrate the capability of the method. The results are very helpful for correctly interpreting FFOCT images for applications in disease diagnosis.

Resonators with an equidistant spectrum are often required in optics. One of the ways to obtain it is the frequency degeneracy of the resonator mode, which is observed, for example, in the well-known classical (linear) confocal resonator. At the same time, there exist configurations of ring resonators similar to it in terms of their properties—ring confocal resonators. In the first approximation, they can be obtained using several reflecting surfaces in the cavity, at least one of which is concave toroid with radii of curvature in the two main meridional sections ensuring the confocal condition and the degeneracy of the spectrum. Such resonators can be used, in particular, as sensitive elements of miniature optical gyroscopes. We consider the specifics and properties of ring confocal resonators and the conditions necessary to obtain them. Using the modified Fox and Li method to calculate open-ring resonators with astigmatic reflective surfaces, the field of the ring confocal resonator is simulated.

A basic concept for detection and identification of liquid phase chemicals that may harm the environment is presented. Reliable detection is shown to be possible using a simplified model describing a uniform layer as well as realistic dispersal, shown to be in nonuniform irregular surface coverage. The results are dependent upon the effect of droplet size and spectral scattering from liquid–air interface on the measured scattered light. This information is crucial for accurate identification for on-site real-time sensing.

The tilted wave interferometer has been developed as a fast and accurate instrument for the measurement of aspheric and freeform surfaces. We present a method for increasing its robustness and flexibility. Tilted wave interferometry crucially depends on accurate calibration and any changes to a calibrated setup require, in general, a recalibration. Therefore, we propose a method for simultaneous topography reconstruction and elimination of errors arising from such changes. An approach to identify trends in systematic errors for the complex non-null setup with a large number of blackbox model parameters is worked out. The procedure allows deriving an error removal scheme for nonrotationally symmetric components based on measurements in different rotational positions. The feasibility and benefit of the error elimination method are shown both by simulation methods and dedicated experiments. A significant reduction of systematic errors even in a miscalibrated state is achieved. Hence, recalibrations are avoided and measurement time and flexibility are improved.

An achromatic objective lens with the object-space numerical aperture (NA) of 0.7 is designed for endomicroscopy applications for in vivo diagnosis of cancer. For these applications, it is required that the size of the optical components be no more than the diameter of a biopsy needle. The challenges associated with designing such an optical system are discussed. The clear aperture size for the lens is set to 1.5 mm. The lens is achromatic for the wavelength range from 452 to 623 nm. The system was designed and optimized using CODE V optical design software. The performance of the design is analyzed and found to be within the design requirements. Tolerance analysis is also performed to check the robustness of the design.

The fluorescence imaging spectrometer (FLORIS) is the payload of the fluorescence explorer mission (FLEX) of the European Space Agency. The mission objective is to perform quantitative measurements of the solar-induced vegetation fluorescence aiming at monitoring photosynthetic activity. FLORIS works in a push-broom configuration, and it is designed to acquire data in the 500 to 780 nm spectral range with a sampling of 0.1 nm in the oxygen bands (759 to 769 nm and 686 to 697 nm) and 0.5 to 2.0 nm in the red edge, chlorophyll absorption, and photochemical reflectance index bands. FLEX will fly in formation with Sentinel-3 to benefit from the measurements made by Sentinel-3 instruments, OLCI, and SLSTR, particularly concerning the cloud screening, the proper characterization of the atmospheric state, and the determination of the surface temperature. The instrument concept is based on a common telescope and two modified Offner spectrometers with reflective concave gratings both for the high resolution (HR) and low resolution (LR) spectrometers. In the frame of the instrument predevelopment, Leonardo Company (Italy) has built and tested an elegant breadboard of the instrument consisting of the telescope and the HR spectrometer. OHB System AG (Germany) is in charge of the development of the LR spectrometer. The main objectives of the activity are to anticipate the development of the instrument and provide early risk retirement of the critical components; evaluate the system performances such as imaging quality parameters, straylight, ghost, polarization sensitivity, and environmental influences; verify the adequacy of critical tests such as spectral characterization and straylight; and define and optimize instrument alignment procedures. Following a brief overview of the FLEX mission, we will cover the design and the development of the optics breadboard with emphasis on the results obtained during the tests and the lessons learned for the flight unit.

UV LED, which can be used as the illumination source in the direct imaging (DI) exposure equipment, has the advantages of rich wavelengths and low cost. It has a good application prospect in the field of printed circuit board (PCB) manufacturing. In practical application, there are high requirements for output power density, stray light control, and luminance uniformity of DI system to improve the productivity and exposure performance. We present an optical structure of the illumination system for DI lithography to provide uniform illumination low stray light for digital micromirror device (DMD), which combines Köhler illumination and double telecentric imaging. In this design, the high luminance illumination can be obtained by LED étendue analysis, fly’s eye condenser is not needed, and the stray light on the DMD caused by light outside the effective angle of the LED can be mostly filtered out. Based on this idea, a lithographic illumination system with the numerical aperture of 0.1 is designed and fabricated for 0.95-in. DMD. According to experimental measurements, the effective illumination power is up to 10 W, the stray light accounts for 10.7%, and the illuminance uniformity is about 90%, which meet the requirements of DI lithography equipment used in PCB manufacturing.

We explore methods that efficiently replicate arbitrary spectra with both high precision and accuracy using multichannel light-emitting diode (LED) lighting systems. It is well known that LED-based light sources deteriorate over time and change their spectral output with varying operating junction temperatures. A simple open-loop approach to the spectral matching problem would bring about unbearable spectral and color inaccuracies. In the literature, different solutions have been studied that make use of integrated spectrometers as closed-loop feedback elements that warrant spectral awareness and self-correction. However, the prohibitive cost of small spectrometers (that generally involve CMOS-based gratings) constitutes a high barrier that prevents their integration into final lighting products. We demonstrate how a cost-effective colorimeter can be used not only to preserve the color point of the target spectrum but also to keep the spectral matching error extremely low (relative spectral error <10  %  ). With the proposed system and methods, we obtain relative color differences between target and emitted spectra below Δu^′v^′  <  0.002, always with spectral shape preservation.

The performance of discrete Fourier transform-spread (DFT-spread) discrete multitone (DMT) transmission systems based on four high-order quadrature amplitude modulation (QAM) formats (4/16/32/64QAM) for optical interconnection is investigated and experimentally compared on the same platform. We first theoretically study and analyze the benefits of the implemented DFT spread for the peak-to-average power ratio (PAPR) deduction in DMT transmission systems, and then experimentally compare the receiver optical power sensitivities and transmission performance using flexible transceiver configuration in standard single-mode fiber (SSMF) link. Experimental results show that the power penalties are 1.2, 1.8, 2.2, and 2.7 dB for 10 Gbaud DFT-spread DMT signal employing 4/16/32/64QAM over 10-km SSMF transmission. There are, respectively, 0.3, 0.7, 0.9, and 1.1 dB transmission power penalty improvements by the implementation of the DFT-spread scheme, compared with the conventional DMT transmission systems. Furthermore, we experimentally compare the improved fiber nonlinear effect tolerance for the proposed DFT-spread DMT system, which shows the potential reach improvement and thus can provide an abundant system loss budget for leveraging legacy optical access networks. In addition, a comparison of the hardware implementation complexity of the DFT-spread scheme with several typical PAPR deduction schemes is also presented.

A nonstandard type of Yb:YAG laser intracavity pumped by an all-solid-state Nd  :  LuVO₄ laser at the wavelength of 916 nm around the 915-nm absorption peak of Yb^3  + is presented. A model is derived to improve the 1030-nm output power of the Yb:YAG laser by optimizing the absorption ratio of Yb:YAG at the 916-nm pump wavelength. Both the continuous wave (CW) and pulsed performances of the presented laser are experimentally investigated. At the maximal diode pump level of 18 W, a 5.63-W output power (CW) and a 3.71-W average output power (pulsed) at 1030 nm are achieved, corresponding to optical conversion efficiencies of 31.3% and 20.6%, respectively.

We investigate the relationship between an optical pulse shape and a time lens implemented by means of sinusoidal phase modulation. Based on this investigation, two schemes are proposed to obtain an optical frequency comb (OFC) with exceptionally high flatness and a large number of spectral lines by carving an optical pulse shape to result in a quasilinear chirp via a simple sinusoidal phase modulation technique. The first scheme utilizes an intensity modulator with a single-drive port or with dual-drive ports to carve a narrow pulse. The experimental results show very good spectral profiles with 38 OFC lines at 1.2-dB flatness and 53 lines at 1.5-dB flatness when the intensity modulator is combined with two and three phase modulators for sinusoidal phase modulation, respectively. The second scheme is implemented by replacing the intensity modulator by a dual-parallel Mach–Zehnder modulator (DP-MZM). In this case, we obtain 35 OFC lines at nearly perfect flatness of less than 1 dB and 53 lines at 1.5-dB flatness after combining the DP-MZM with two and three phase modulators, respectively.

To date, free-space optical (FSO) networks play an important role in current network construction to support large-capacity transmission, where randomly distributed FSO terminals desire to exchange a tremendous amount of information over atmospheric turbulence channels. However, in the presence of atmospheric turbulence and misalignment fading channels, FSO network topology can be dynamic and disconnected. To mitigate the impact of dynamic network environments, appropriate higher-layer protocols should be designed. We explore a practical terrestrial mobile ad-hoc FSO network based on the bundle protocol of disruption-tolerant network, and the theoretical cross-layer system model between physical layer and network layer is derived. To design the topology, at the bundle layer, the distributed routing scheme centrality and probability (CAP) is proposed, where contact probability, sociocentric measure, and message replication strategy are considered simultaneously, and the joint forwarding decision rule is given. Simulation results on the opportunistic networking environment simulator are presented, which show that CAP can be better compared with the conventional end-to-end protocol-based routing scheme.

For future elastic optical networks, the narrow filtering effect induced by cascaded reconfigurable optical add–drop multiplexers (ROADMs) is one of the major impairments. It is essential to accurately estimate the filtering penalty to minimize network margins and optimize resource utilization. We present a method for estimating filtering penalty using machine learning (ML). First, we investigate the impact of ROADM location distribution and bandwidth allocation on the narrow filtering effect. Afterward, an ML-aided approach is proposed to estimate the filtering penalty under various link conditions. Extensive simulations with 9600 links are implemented to demonstrate the superior performance of the proposed scheme.

High peak-to-average power ratio (PAPR) is the biggest problem of orthogonal frequency division multiplexing (OFDM) systems. To compensate this problem, we suggested a technique that is based on the combination of discrete Fourier transform (DFT) and root-based nonlinear companding. Since DFT precoding is a linear technique, it reduces PAPR by distorting the phase with minimum system complexity. Moreover, root-based μ-law companding (RMC) is also adopted to further minimize PAPR by changing probability density function from Rayleigh distribution that OFDM signals follow. By knowing that standard μ-law companding (MC) only enlarges the low amplitude of the signals and does not influence high peaks. However, RMC not only expands low-power signals but also compresses higher amplitude of the OFDM signals simultaneously unlike standard MC. In comparison to other mentioned techniques, the proposed technique (DFT-precoded RMC) accomplishes significant PAPR minimization of the OFDM systems. When complementary cumulative distribution function  =  10^−  4, the proposed technique reduces PAPR up to 1.98 and 1.99 dB for subcarriers N  =  128 and subcarriers N  =  256 quadrature phase shift keying (QPSK)-based OFDM system, respectively. At an error probability of ^{10  −  4}, the bit error rate (BER) performance of the proposed technique improves by 0.23 and 0.18 dB over Anoh et al.’s technique for subcarriers N  =  128 and N  =  256, QPSK, and additive white Gaussian noise channel, respectively. Also, for multipath Rayleigh fading channel, the BER performance of the proposed technique is improved over Anoh et al.’s and RMC techniques at the same BER.

The cavitation gain refers to the phenomenon of increasing the number and intensity of cavitation by lowering the threshold of cavitation. Microparticles play a direct and critical role in the cavitation process. Laser cavitation gain based on microparticles has been found. To study the mechanism of cavitation gain, the finite-difference time-domain simulation model is established to analyze the field enhancement effect around the microparticle under laser irradiation. A laser-induced cavitation experiment platform is built, and the pulsation process of cavitation is recorded by a high-speed camera. Combined with FLUENT bubble collapse vector model, the impact of the cavitation water jet on the wall is analyzed. The result shows that microparticles reduce the cavitation threshold of liquid. Owing to the field enhancement effect at the bottom of microparticles, the number of primary cavitation bubbles in the mixed solution is greater than that in distilled water under the same experimental conditions, and the dominant cavitation bubble has a larger radius and a longer pulsation period. The bubble collapse near the wall produces an obvious water jet and wall-leaning effect, and the pressure on the wall in the microparticles mixture is higher than that in distilled water. Cavitation gain of microparticles in the laser-induced cavitation processing is helpful in revealing the mechanism of cavitation erosion and solving the problem of cavitation damage. The behavior of cavitation gain can help us rationally utilize water jet force produced by laser-induced cavitation in microparticle mixtures, which has a profound influence on the innovative development of material surface micromachining.

A conical hybrid plasmonic probe (CHPP) for ultrahigh field enhanced nanofocusing with lower loss is demonstrated. The CHPP consists of two different low-index dielectric layers sandwiched between a high-index conical core and a silver cladding. Properties of nanofocusing are analyzed by finite element method, under illumination of a radially polarized beam at a wavelength of 632.8 nm. The numerical results prove that, by introducing the additional lower-index layer, the whole low-index dielectric region is broadened to collect more energy efficiently, and the energy is converged on the apex of the CHPP to form the ultrahigh field enhancement. Compared with the traditional hybrid plasmonic probe, the optimized CHPP exhibits lower loss and higher field enhancement of 1771 times. The thickness and refractive index of the lower-index layer are discussed for optimizing the structure. The results indicate that the CHPP has a simple structure with excellent performance, which has important potential applications in relevant fields, particularly in nanotechnology of field enhancement. This work also provides a convenient way for designing and optimizing hybrid plasmonic structure.

An intensity-interrogated fiber-optic magnetic field sensor functionalized with magnetic fluids is proposed and experimentally demonstrated by encapsulating an S-taper concatenated with down-fusion taper in a silica capillary. The down-fusion taper serves as a higher order mode exciter, whereas the S-taper is employed to couple the cladding modes back into the fiber core, and so an interferometric spectrum could be acquired for sensing interrogation. Spectral characteristics of the proposed sensor dependent on magnetic field intensity and environmental temperature are investigated in detail. Experiment results show that the maximum magnetic field sensitivity reaches −0.02336  dB  /  Oe within a specific range of 25 to 200 Oe, and the temperature sensitivity reaches 0.07116  dB  /    °  C within a temperature range of 26°C to 46°C. By exploiting the distinction between magnetic field intensity and temperature sensitivities of peak 1 and dip 1 that are selected for intensity interrogation, temperature cross-sensitivity issue can be resolved.

The applications for optical resonators are manifold, ranging from laser resonators and filters to optical sensors based on resonators, such as optical gyroscopes. A concept of a miniaturized optical gyroscope has been developed. To realize a gyroscope of this sort, the assembly and precision alignment of the optical microcomponents to form a triangular cavity is crucial. In order to detect the rotation rate, the laser beam must circulate many times and not leave the resonator due to small misalignments. Therefore, the assembly of a miniaturized passive free space triangular ring resonator, in which the light can circulate by reflections by three mirrors, is investigated. To utilize the inherent alignments of crystal planes, two of the mirrors are realized by micromanufacturing within the same silicon crystal using wet etching, resulting in very perfect {111} facets. To further increase the reflectivity of the mirrors, different kinds of coatings are tested. With these two perfectly aligned mirrors, the resonator assembly challenge reduces to a three-degree of freedom alignment of a third mirror, in which a well-designed adjustable spacer is developed. Resonances with the etched {111} micromirrors in a linear cavity setup as well as in a triangular ring cavity setup have already been demonstrated.

A systematic investigation of shock wave-induced defect engineering on optical transport properties of triglycine sulfate (TGS) crystalline material is carried out under shock waves of Mach number 1.7 with a different number of shock pulses. Surface morphological changes and defect concentration are evaluated by the optical microscopic technique. Optical transmission of pre- and postshock wave-treated TGS crystal is analyzed using UV–visible spectrometer over the range between 200 and 800 nm. Unexpectedly, during the shock wave impact conditions, the test crystal exhibits vulnerability due to defects on its surface and it is confirmed by optical micrographs. Optical transmission is continually reduced while the number of shock pulses is increased due to the formation of defects on the surface of the test sample. Followed by the observation of optical transmission, optical constants and band gap energies are also calculated. The obtained results clearly show that surface morphology and optical transport properties of TGS crystal are greatly affected by the impact of shock waves.

We present the study of ZnO-Al₂O₃ thin coatings and nanocomposites prepared by a polymer-salt method. The coatings demonstrate high transparency in UV-A (wavelength is about 300 to 400 nm) and visible spectral ranges and the ability to generate singlet oxygen under UV irradiation. The materials were studied by spectroscopic methods, scanning electron microscopy, and x-ray diffraction analysis. Obtained ZnO-Al₂O₃ films are thin (about 250 to 300 nm) and fully cover the glass surface. They contain oriented ZnO nanocrystals 23 to 35 nm in size. Coatings chemical compositions strongly affect their structure and spectral properties. Al₂O₃ additions change the coatings' crystal structure by decreasing ZnO crystal size and making their spatial orientation more random and enhance the transparency in near UV due to significant increase of bandgap values.

Efforts to extend speckle-based focal plane array modulation transfer function measurements beyond the detector Nyquist frequency have unearthed challenging spectral estimation issues. In an attempt to better understand the task of speckle imagery spectral estimation, we explore the nuances of various estimation techniques, making comparisons using both real speckle imagery and simulated data. Parameters and features of the techniques investigated include number of image realizations, the size of image realizations, and applications of windows to speckle imagery spectral estimation. Real-world testing considerations such as laser stability and the challenge of collecting significant numbers of independent image realizations are addressed in the analysis. Results from this research show the advantage increasing the number of realizations has on estimation variance, the robustness of smaller realization segments when battling speckle field imagery spatial nonuniformities, the benefits of windowing image segments with regard to power spectral density estimation accuracy, and the impact that the increasing aperture area has on system signal-to-noise ratio.

We present a study of the microbubble formation using thinned optical fiber tips. Our study is carried out in an absorbent medium provided by a multiwall carbon nanotubes (MWCNT) solution. We focus on the radius of microbubble formation and its dependence on pumping power (pp) from an external light source, time of the pp, diameters of the tips, and concentration of MWCNT. We observe that using tapered single-mode optical fiber tips minor pp is needed for the microbubble formation, compared to a conventional optical fiber (not tapered). We observe that the experimental behavior of growth of generated microbubbles is still in agreement with the previously established Plesset–Zwick theory.

We have proposed and demonstrated position-sensitive detectors based on the spectral changes in fluorescent waveguides. The first prototype is a transparent heat-shrink tubing containing an organic luminescent dye at its core. With a laser beam incident on this linear fluorescent tubing, the redshift in the photoluminescence (PL) spectrum observed at its edge increases with the distance from the incident point. The range for position sensing is 2 cm. It is extended to 280 cm by adopting a scintillating fiber in our second experiment. Two-stage conversion enables two-dimensional position detection. We have attached two linear fluorescent tubing to a planar 50  mm  ×  50  mm  ×  8  mm fluorescent waveguide. When a laser beam excites the first luminescent material at a single spot in the planer waveguide, PL photons propagate to its edges and excite the second luminescent material in the two linear waveguides. Photon division between these linear waveguides gives the first coordinate. The second coordinate is given by the redshift in the linear waveguides. We have observed that the maximum error in position estimation is 1.5 mm. Unlike the conventional semiconductor technologies, no electronic components are required for the sensor head. This robust technology might be suited for deployment in large-scale harsh environments.

The dielectric properties of 0.55SrTiO₃-0.45NdAlO₃ ceramics under external optical fields were investigated by terahertz time-domain spectroscopy at room temperature. From the experimental results, it could be found that the tunability of permittivity with the external optical pump was reached up to 16% at 0.6 THz. And the change of refractive index has a linear relationship on scale with the applied external light power. These results could be explained by a built-in electric field caused by the excited free carriers in the ceramics.