To describe the elastic wave scattering, which reflects the performance of propagation control materials, the approximate directional cloaks of an elastic wave are designed using the zero- and first-order approximation coordinate transformation method. Because the convergence features of fractional Fourier transform (FRFT) are more acute and sensitive to the frequency change than those of short time Fourier transform, the spatial signals in the designed materials are transformed in the FRFT domain. The spatial frequency changes of elastic waves through inhomogeneous materials are quantitatively analyzed under several circumstances. The provided time-frequency analysis method with FRFT can support the design evaluation of the material parameters.

We demonstrated heterodyned optical Kerr gate (HOKG) ballistic imaging of an object hidden behind a turbid medium with two Kerr media. The experimental results showed that when compared with using HOKG of common Kerr medium CS₂, a higher spatial resolution of the imaging system can be obtained by using HOKG of tellurite glass, which more effectively compensates for the high spatial-frequency components of the objects.

On bandwidth-limited visible light communication (VLC) transmission systems, direct current (DC) component loss, DC-unbalance of code, and severe high-frequency attenuation cause baseline wander (BLW) and data-dependent jitter (DDJ) phenomena, which deteriorate signal quality and result in a higher bit error rate (BER). We present a scheme based on hybrid run length limited codes and pre-emphasis techniques to decrease the intersymbol interference caused by BLW and DDJ phenomena. We experimentally demonstrate, utilizing 1-binary-digit-into-2-binary-digits (1B2B) codes and postcursor pre-emphasis techniques, that the impacts of BLW and DDJ on on–off keying nonreturn-to-zero VLC systems are alleviated and a 130  Mb/s data transmission rate with a BER performance of <10⁻⁴ can be achieved.

Smartphones have been very versatile communication devices. Cameras fitted in smartphones are primarily used to capture photographs and for relevant multimedia functions. Recently, smartphone cameras have been studied for short-range wireless communications. Camera-based optical wireless communication, termed optical camera communication (OCC), is a form of visible light communication. In the OCC, a camera is employed as the receiver, while either a light emitting diode or a liquid crystal display (LCD) is used as the transmitter. This paper proposes a downlink OCC scheme with a dual camera (receiver) and an LCD (transmitter) is employed, using a unique capture method called split shutter method. The split shutter method combined with dual camera offers a significantly increased capture rate as well as increased coverage distance. Experiments were conducted to verify the proposed dual camera-based split shutter OCC scheme. Compared with a standard camera shutter, it is found that the proposed method effectively increases the transmission rate by a factor of two under identical experimental conditions, achieving a maximum of 11,520 bits per second and a distance of up to 2 m using a 1280×720  pixels camera resolution. When compared with conventional rolling shutter OCCs, the proposed method offers four times farther the coverage distance at an identical resolution. It is also demonstrated that the data rate can be further increased when a larger number of transmitters are employed.

In free-space optical (FSO) communications, signal beams suffer from atmospheric turbulence, transmission loss, and pointing errors. With different channel conditions, there exist different optimal divergence angles for the beams. Therefore, a high-performance adjustable collimator for the transmitter is an important component in an FSO system. An adjustable, high-sensitivity, optical collimating scheme is proposed. In the proposed scheme, a graded-index fiber is coated with palladium film and placed in a hydrogen environment and the graded-index fiber can be extended precisely while the palladium atoms react with the hydrogen molecules. Consequently, with different hydrogen pressures, the output beams present different divergence angles. Theoretical analysis and numerical calculation show that the output divergence angle can be precisely adjusted with the system design.

The use of the wide-field-stop and beam-expansion method for sensitivity enhancement of one-shot Fourier spectroscopy is proposed to realize health care sensors installed in smartphones for daily monitoring. When measuring the spectral components of human bodies noninvasively, diffuse reflected light from biological membranes is too weak for detection using conventional hyperspectral cameras. One-shot Fourier spectroscopy is a spatial phase-shift-type interferometer that can determine the one-dimensional spectral characteristics from a single frame. However, this method has low sensitivity, so that only the spectral characteristics of light sources with direct illumination can be obtained, because a single slit is used as a field stop. The sensitivity of the proposed spectroscopic method is improved by using the wide-field-stop and beam-expansion method. The use of a wider field stop slit width increases the detected light intensity; however, this simultaneously narrows the diffraction angle. The narrower collimated objective beam diameter degrades the visibility of interferograms. Therefore, a plane-concave cylindrical lens between the objective plane and the single slit is introduced to expand the beam diameter. The resulting sensitivity improvement achieved when using the wide-field-stop and beam-expansion method allows the spectral characteristics of hemoglobin to be obtained noninvasively from a human palm using a midget lamp.

This PDF file contains the editorial “Special Section Guest Editorial:Free-Space Laser Communication” for OE Vol. 55 Issue 11

In the charge-coupled device (CCD)-based tracking control system of fast steering mirrors (FSMs), high control bandwidth is the most effective method to enhance closed-loop performance, which, however, usually suffers a great deal from time delay induced by a low CCD sampling rate. Moreover, mechanical resonances also limit high control bandwidth. Therefore, a tentative approach to implementing a CCD-based tracking control system for an FSM with inertial sensor-based cascade feedback is proposed, which is made up of acceleration feedback, velocity feedback, and position feedback. Accelerometers and gyroscopes are all the inertial sensors, sensing vibrations induced by platforms, in turn, which can contribute to disturbance supersession. In theory, the acceleration open-loop frequency response of the FSM includes a quadratic differential, and it is very difficult to compensate a quadratic differential with a double-integral algorithm. A lag controller is used to solve this problem and accomplish acceleration closed-loop control. The disturbance suppression of the proposed method is the product of the error attenuation of the acceleration loop, the velocity loop, and the position loop. Extensive experimental results show that the improved control mode can effectively enhance the error attenuation performance of the line of sight (LOS) for the CCD-based tracking control system.

Many components for free-space optical (FSO) communication systems have shrunken in size over the last decade. However, the steering systems have remained large and power hungry. Nonmechanical beam steering offers a path to reducing the size of these systems. Optical phased arrays can allow integrated beam steering elements. One of the most important aspects of an optical phased array technology is its scalability to a large number of elements. Silicon photonics can potentially offer this scalability using CMOS foundry techniques. A phased array that can steer in two dimensions using the thermo-optic effect is demonstrated. No wavelength tuning of the input laser is needed and the design allows a simple control system with only two inputs. A benchtop FSO link with the phased array in both transmit and receive mode is demonstrated.

Free-space optical (FSO) communication system is vulnerable to link disruption due to the propagation loss and atmospheric turbulence. Cooperative FSO system has been proposed to mitigate the adverse effects of link disruption by the cooperative transmission in assistance of relay. However, most of the related works focus on the outage probability analysis and put less attention on the resource utilization. In fact, there exists a waste of bandwidth resource due to time slot waiting and more FSO transceivers are occupied. We mainly focus on the relay selection and link scheduling mechanisms of cooperative FSO system to improve not only network reliability but also resource utilization. We first propose a relay selection algorithm as a solution to combat channel fading. In the relay selection algorithm, two cooperative modes including intrarelay-set and inter-relay-set cooperative modes are proposed to enhance network capacity. Then, a link scheduling algorithm is further proposed to improve the bandwidth utilization and reduce the occupancy rate of transceivers by utilizing more sufficiently the idle FSO links. Simulation results show that the proposed algorithms can provide significant performance gain for cooperative FSO system in reliability and resource utilization and are highly suitable for the growing traffic demands.

The next generation free-space optical (FSO) communications infrastructure will need to support a wide range of links from space-based terminals in low Earth orbit, geosynchronous Earth orbit, and deep space to the ground. Efficiently enabling such a diverse mission set requires an optical communications system architecture capable of providing excellent sensitivity (i.e., few photons-per-bit) while allowing reductions in data rate for increased system margin. Specifically, coherent optical transmission systems have excellent sensitivity and can trade data rate for system margin by adjusting the modulation format, the forward error correction (FEC) code rate, or by repeating blocks of channel symbols. These techniques can be implemented on a common set of hardware at a fixed system baud rate. Experimental results show that changing modulation formats between quaternary phase-shifted keying and binary phase-shifted keying enables a 3-dB scaling in data rate and a 3.5-dB scaling in system margin. Experimental results of QPSK transmission show a 5.6-dB scaling of data rate and an 8.9-dB scaling in system margin by varying the FEC code rate from rate-9/10 to rate-1/4. Experimental results also show a 45.6-dB scaling in data rate over a 41.7-dB range of input powers by block-repeating and combining a pseudorandom binary sequence up to 36,017 times.

We report on the development, testing, and initial space qualification of a 1.5-μm, high-power (6 W), high wall-plug efficiency (∼15%), pulse-position-modulated (PPM), polarization-maintaining, fiber laser transmitter subsystem for deep-space laser communication links. Programmable high-order PPM modulation up to PPM-128 formats, with discrete pulse slots ranging from 0.5 to 8 ns, satisfies variety of link requirements for deep-space laser communication to Mars, asteroids, and other deep-space relay links, as per the National Aeronautics and Space Administration’s space laser communication roadmap. We also present initial space qualification results from thermal-vacuum tests, vibration testing, radiation testing, and an overall reliability assessment.

Multiple-input multiple-output (MIMO) techniques are employed in free-space optical (FSO) links to mitigate the degrading effects of atmospheric turbulence. We consider a MIMO FSO system, which consists of a radial laser array with partially coherent Gaussian beams at the transmitter and a detector array with Gaussian apertures at the receiver. The average power and the power correlation function at the finite-sized receiver apertures are formulated by using the extended Huygens–Fresnel principle in weak atmospheric turbulence. This let us further quantify the performance metrics such as the power scintillation index, the aperture averaging factor, and the average bit error rate (BER) as functions of system parameters. The derived power scintillation equation correctly reduces to the existing coherent and partially coherent Gaussian beam scintillation indices in the limiting cases. Using the performance metrics, we analyze the effect of various practical system parameters on the performance of a MIMO FSO system. Practical system parameters include the transmitter and receiver ring radius, number of beamlets, number of finite-aperture receivers, source size, degree of source coherence, receiver aperture radius, link distance, and the structure constant of atmosphere.

Part of the operational concept for laser communication networks such as the European data relay system is planning constraints. In addition to hard constraints such as angular range of the actuators, there is also the atmosphere, which has a gradual increasing impact as the distance between laser beam path and earth decreases. The shortest distance between the laser beam path and the surface of the earth is called “grazing altitude.” The atmosphere impacts spatial acquisition as well as communication performance. In-orbit measurement data of geostationary orbit–low-Earth orbit (GEO-LEO) links between two laser communication terminals (LCT) developed by TESAT-Spacecom, Germany, in the frame of Alphasat TDP1 optical GEO-relay demonstration program are presented. Planning constraint guidelines are formulated based on the observed influence. TESAT-Spacecom’s LCTs use a highly sensitive and high-performance coherent transmission technology and a beaconless spatial acquisition system.

High-sensitivity photodetectors serve two purposes in free-space optical communication: data reception and position sensing for pointing, tracking, and stabilization. Two separate detectors are traditionally utilized to perform these tasks, but recent advances in the fabrication and development of large area, low-noise avalanche photodiode (APD) arrays have enabled these devices to be used both as position-sensitive detectors and communications receivers because of the conflicting performance criteria. Combining these functionalities allows for more flexibility and simplicity in optical assembly design without sacrificing the sensitivity and bandwidth performance of smaller, single-element data receivers. Beyond eliminating the need to separate the return beam into two separate paths, these devices enable implementation of adaptive approaches to compensate for focal plane beam wander and breakup, which is often seen in highly scintillated terrestrial and maritime optical links. While the Naval Research Laboratory and Optogration, Inc. have recently demonstrated the performance of single period, InAlAs/InGaAs APD arrays as combined data reception and tracking sensors, an impact-ionization-engineered epilayer design achieves even lower carrier ionization ratios by incorporating multiple multiplication periods engineered to suppress lower ionization rate carriers while enhancing the higher ionization rate carriers of interest. This work presents a three-period I²E concentric, five-element APD array rated for bandwidths beyond 1 GHz with measured carrier ionization ratios of 0.05 to 0.1 at moderate APD gains. The epilayer design of the device will be discussed along with initial device characterization and high-speed performance measurements.

The nanosatellite optical downlink experiment (NODE) implements a free-space optical communications (lasercom) capability on a CubeSat platform that can support low earth orbit (LEO) to ground downlink rates>10  Mbps. A primary goal of NODE is to leverage commercially available technologies to provide a scalable and cost-effective alternative to radio-frequency-based communications. The NODE transmitter uses a 200-mW 1550-nm master-oscillator power-amplifier design using power-efficient M-ary pulse position modulation. To facilitate pointing the 0.12-deg downlink beam, NODE augments spacecraft body pointing with a microelectromechanical fast steering mirror (FSM) and uses an 850-nm uplink beacon to an onboard CCD camera. The 30-cm aperture ground telescope uses an infrared camera and FSM for tracking to an avalanche photodiode detector-based receiver. Here, we describe our approach to transition prototype transmitter and receiver designs to a full end-to-end CubeSat-scale system. This includes link budget refinement, drive electronics miniaturization, packaging reduction, improvements to pointing and attitude estimation, implementation of modulation, coding, and interleaving, and ground station receiver design. We capture trades and technology development needs and outline plans for integrated system ground testing.

In order to address the high throughput requested for both downlink and uplink satellite to ground laser links, adaptive optics (AO) has become a key technology. While maturing, application of this technology for satellite to ground telecommunication, however, faces difficulties, such as higher bandwidth and optimal operation for a wide variety of atmospheric conditions (daytime and nighttime) with potentially low elevations that might severely affect wavefront sensing because of scintillation. To address these specificities, an accurate understanding of the origin of the perturbations is required, as well as operational validation of AO on real laser links. We report here on a low Earth orbiting (LEO) microsatellite to ground downlink with AO correction. We discuss propagation channel characterization based on Shack–Hartmann wavefront sensor (WFS) measurements. Fine modeling of the propagation channel is proposed based on multi-Gaussian model of turbulence profile. This model is then used to estimate the AO performance and validate the experimental results. While AO performance is limited by the experimental set-up, it proves to comply with expected performance and further interesting information on propagation channel is extracted. These results shall help dimensioning and operating AO systems for LEO to ground downlinks.

The fluctuation in the intensity, which is quantified by the scintillation index, is evaluated for cross beams when such beams propagate in an underwater medium experiencing turbulence. The variations in the scintillation index are investigated against the changes in the size of the cross beams, the ratio of temperature to salinity contributions to the refractive index spectrum, the rate of dissipation of mean-squared temperature, and the rate of dissipation of kinetic energy per unit mass of fluid.

The performance of a free-space optical (FSO) communication system is strongly affected by optical scintillation. Scintillation fades can cause errors when the power on a detector falls below its noise floor while surges can overload a detector. The very long time scale of scintillation compared to a typical bit in an FSO link means that error-correcting protocols designed for fiber optic links are inappropriate for FSO links. Comparing the performance effects of different components, such as photodetectors or protocols, such as forward error correction, in the field is difficult because conditions are constantly changing. On the other hand, laboratory-based turbulence simulators may not really simulate the effects of long-range propagation through the atmosphere. We have investigated a different approach. Scintillation has been measured during field tests using FSO terminals by sending a continuous wave beam through the atmosphere. A high dynamic range photodetector was digitized at a 5-KHz rate and files of the intensity variations were saved. Many hours of scintillation data under different environmental conditions and at different sites have been combined into a library of data. A fiber-optic-based scintillation playback system was then used in the laboratory to test modems and protocols with the recorded irradiance files. This enabled comparisons using the same atmospheric conditions allowing optimization of such parameters as detector dynamic range. It also allowed comparison and optimization of different error correcting protocols.

We present simulative and experimental investigations of different coherent receiver designs for high-speed optical intersatellite links. We focus on frequency offset (FO) compensation in homodyne and intradyne detection systems. The considered laser communication terminal uses an optical phase-locked loop (OPLL), which ensures stable homodyne detection. However, the hardware complexity increases with the modulation order. Therefore, we show that software-based intradyne detection is an attractive alternative for OPLL-based homodyne systems. Our approach is based on digital FO and phase noise compensation, in order to achieve a more flexible coherent detection scheme. Analytic results will further show the theoretical impact of the different detection schemes on the receiver sensitivity. Finally, we compare the schemes in terms of bit error ratio measurements and optimal receiver design.

A framework is presented for the analysis of average symbol error rate (SER) for M-ary quadrature amplitude modulation in a free-space optical communication system. The standard probability density function (PDF)-based approach is extended to evaluate the average SER by representing the Q-function through its Meijer’s G-function equivalent. Specifically, a converging power series expression for the average SER is derived considering the zero-boresight misalignment errors in the receiver side. The analysis presented here assumes a unified expression for the PDF of channel coefficient which incorporates the M-distributed atmospheric turbulence and Rayleigh-distributed radial displacement for the misalignment errors. The analytical results are compared with the results obtained using Q-function approximation. Further, the presented results are supported by the Monte Carlo simulations.

It is challenging to capture a high-dynamic range (HDR) scene using a low-dynamic range camera. A weighted sum-based image fusion (IF) algorithm is proposed so as to express an HDR scene with a high-quality image. This method mainly includes three parts. First, two image features, i.e., gradients and well-exposedness are measured to estimate the initial weight maps. Second, the initial weight maps are refined by a guided filter, in which the source image is considered as the guidance image. This process could reduce the noise in initial weight maps and preserve more texture consistent with the original images. Finally, the fused image is constructed by a weighted sum of source images in the spatial domain. The main contributions of this method are the estimation of the initial weight maps and the appropriate use of the guided filter-based weight maps refinement. It provides accurate weight maps for IF. Compared to traditional IF methods, this algorithm avoids image segmentation, combination, and the camera response curve calibration. Furthermore, experimental results demonstrate the superiority of the proposed method in both subjective and objective evaluations.

This paper presents a highly distinctive and robust local three-dimensional (3-D) feature descriptor named longitude and latitude spin image (LLSI). The whole procedure has two modules: local reference frame (LRF) definition and LLSI feature description. We employ the same technique as Tombari to define the LRF. The LLSI feature descriptor is obtained by stitching the longitude and latitude (LL) image to the original spin image vertically, where the LL image was generated similarly with the spin image by mapping a two-tuple (θ,φ) into a discrete two-dimensional histogram. The performance of the proposed LLSI descriptor was rigorously tested on a number of popular and publicly available datasets. The results showed that our method is more robust with respect to noise and varying mesh resolution than existing techniques. Finally, we tested our LLSI-based algorithm for 3-D object recognition on two popular datasets. Our LLSI-based algorithm achieved recognition rates of 100%, 98.2%, and 96.2%, respectively, when tested on the Bologna, University of Western Australia (UWA) (up to 84% occlusion), UWA datasets (all). Moreover, our LLSI-based algorithm achieved 100% recognition rate on the whole UWA dataset when generating the LLSI descriptor with the LRF proposed by Guo.

The micromotion dynamics and geometrical shape are considered to be essential characteristics for exoatmospheric targets discrimination. Many methods have been investigated to retrieve the micromotion features using radar signals returned from targets of a given shape. We explore a way of jointly estimating micromotion dynamics and geometrical shape parameters from the infrared (IR) signals of targets in remote detection distance. It is found that the micromotion dynamics of the target would induce a periodic fluctuating variation on the IR irradiance intensity signature. In addition to the micromotion characteristics, the fluctuation could also reflect target structure properties, which offer a possible clue in extracting the features of micromotion dynamics and geometrical shape. Thus, the data model of target IR irradiance intensity signatures induced by micromotion patterns including spinning, coning, and tumbling is developed, and a method of parameters estimation based on joint optimization analysis techniques is proposed. Experimental results demonstrated that the parameters of target micromotion dynamics and geometrical shape can be effectively estimated using the proposed method, if the input signature contains multiple dominant frequency components.

In the applications of computer graphics, bidirectional texture function (BTF) is used for realistic and predictive rendering. The goal of current research is to get a surface representation indistinguishable from the real world. We developed, built, and tested a portable instrument for BTF acquisition based on kaleidoscopic imaging. We discuss the color issues we experienced after the initial tests. We show that the same color balance cannot be applied to the whole image as the spectral response of the instrument varies depending on the position within the image. All optical elements were inspected for their contributions to the spectral behavior of the instrument. A software simulator of a mathematical model of the device was implemented. We found a way to implement all these contributions into the image processing pipeline. In this way, the correct white balance for each individual pixel in the image is found and applied, allowing for a more faithful color representation. Also proposed is an optimized dielectric protective layer for the kaleidoscope’s mirrors causing the least possible color aberration.

In order to obtain quantitative phase distributions from interferograms, phase retrieval composed of phase extracting and unwrapping is adopted in quantitative interferometric microscopy. However, phase unwrapping often requires a long time, limiting applications such as high-speed phase observations and measurements. In order to accelerate the processing speed, a phase unwrapping free Hilbert transform (HT)-based phase retrieval method is proposed. Though another background interferogram without a sample is needed, phase unwrapping can be omitted, saving a large amount of time for phase recovery. Additionally, the proposed HT-based method can maintain more sample details, thus providing high-accurate quantitative phase imaging. Considering its fast speed and high accuracy in phase retrieval, it is believed that the unwrapping free HT-based phase retrieval method can be potentially applied in high throughput cellular observations and measurements.

We present a technique for determining the perceived relative clutter among different images. This experiment involves participants ranking different sets of images in terms of clutter. The law of comparative judgment is then used to determine the relative levels of clutter on the physiological continuum. Also introduced are two metrics for predicting the level of clutter in an image. The first of these metrics uses a graph-based image segmentation algorithm and the second uses the change in gradients across the image. We present how these two metrics along with an existing metric based on wavelets can successfully predict the perceived clutter in an image.

To achieve better designs for spacecraft heat shields for missions requiring atmospheric aero-capture or entry/reentry, reliable thermal protection system (TPS) sensors are needed. Such sensors will provide both risk reduction and heat-shield mass minimization, which will facilitate more missions and enable increased payloads and returns. This paper discusses TPS thermal measurements provided by a temperature monitoring system involving lightweight, electromagnetic interference-immune, high-temperature resistant fiber Bragg grating (FBG) sensors with a thermal mass near that of TPS materials together with fast FBG sensor interrogation. Such fiber-optic sensing technology is highly sensitive and accurate, as well as suitable for high-volume production. Multiple sensing FBGs can be fabricated as arrays on a single fiber for simplified design and reduced cost. Experimental results are provided to demonstrate the temperature monitoring system using multisensor FBG arrays embedded in a small-size super-light ablator (SLA) coupon which was thermally loaded to temperatures in the vicinity of the SLA charring temperature. In addition, a high-temperature FBG array was fabricated and tested for 1000°C operation, and the temperature dependence considered over the full range (cryogenic to high temperature) for which silica fiber FBGs have been subjected.

We experimentally subject a fiber Bragg grating to an unknown, variable temperature gradient. We use the full-spectral response of the grating to determine the magnitude of the gradient over the length of the grating via the full width at quarter maximum bandwidth. The experimental bandwidth and spectrum deformation were compared with a numerical model consisting of an analytical heat transfer model, a finite element analysis model, and the transfer matrix (T-matrix) method. The numerical model showed excellent agreement with the experimental results when the T-matrix method was modified to include the slope of the gradient in addition to the magnitude of the gradient.

The development of image sensors and optics lenses has contributed to the rapidly increasing use of vision-based methods as noncontact measurement methods in many areas. A high-speed camera system is developed to realize the displacement measurement in real time. Conventional visual measurement algorithms are commonly subjected to various shortcomings, such as complex processes, multiparameter adjustments, or integer-pixel accuracy. Inspired from the combination of block-matching algorithm and simplified optical flow, a motion estimation algorithm that uses modified Taylor approximation is proposed and applied to the vision sensor system. Simplifying integer-pixel searching with a rounding-iterative operation enables the modified algorithm to rapidly accomplish one displacement extraction within 1 ms and yield satisfactory subpixel accuracy. The performance of the vision sensor is evaluated through a simulation test and two experiments on a grating ruler motion platform and a steering wheel system of a forklift. Experimental results show that the developed vision sensor can extract accurate displacement signals and accomplish the vibration measurement of engineering structures.

An absolute testing method for cylindrical surfaces is presented in a null test setup with a computer-generated hologram. The absolute test exploits the symmetry of cylinders, which allows us to introduce a certain shift of the test surface both parallel to and rotated about the centerline while the null test condition is still maintained. With two shifts of the cylindrical surface, four measurements belonging to two groups in conjugate positions can be accomplished to obtain the absolute differential map with the interferometer and null optics errors removed. The absolute surface can be obtained by wavefront reconstruction from local differential data. A simulation of the method is presented to estimate the error propagation. Experimental absolute test results of a concave cylindrical surface with 100-mm radius are given. The measured profiles are compared with those obtained from a commercial profiler, showing a difference of less than 15 nm (root-mean-square).

A small, lightweight, and inexpensive hyperspectral camera based on a linear variable filter close to the focal plane array (FPA) is described. The use of a full-frame sensor allows large coverage with high spatial resolution at moderate spectral resolution. The spatial resolution has been maintained using a tilt/shift lens for chromatic focusing corrections. The trade-offs of positioning the filter relative to the FPA and varying the f-number have been studied. Calibration can correct for artifacts such as spectral filter variability. Reference spectra can be obtained using the same camera system by imaging targets over homogeneous areas. For textured surfaces, the different materials can be separated by using statistical methods. Accurate reconstruction of the sparse spectral image data is demonstrated.

With the demand of ultrahigh-speed logic, there has been an emphasis on low-power design techniques. Reversible computing has been proposed as a possible alternative to address the energy dissipation problem. Thus, the reversible circuit implementation in optical domain gives a new dimension in ultrahigh-speed, low-power consumption of quantum computing. In this study, a design of reversible multiplexer using electro-optic effect of lithium niobate-based Mach–Zehnder interferometer is proposed. It is verified using a beam propagation method along with MATLAB simulation.

An easily implementable illumination optical element enabling collection and redirection of multiple precollimated laser beams to be used as a laser spot illuminator (LSI) is proposed. Unlike the lenses, this element is free from any aberration and focusing/defocusing effects which may arise due to curved surfaces. The nonfocusing nature of beams reduces risk of the thermal instability at the target spot if compared with focused beam, especially for high brightness applications where multiple laser beams are used. Further, the illuminating spot is more uniform and pupil filling is higher which can help in the reduction of speckle contrast for imaging applications. In this research, we present two designs of LSI named biprism and sleek design, suitable for conventional grinding-polishing and molding methods of fabrication, respectively. It is shown that the proposed spot illuminator is suitable for conversion of lamp/LED-based projection engine design into a laser projector engine without any modification in its relay and imaging optics. Display performance parameters such as efficiency and uniformity across the modulator plane are reported and found acceptable for the projection engine designs using light tunnels as well as lenslet arrays for homogenization. Further, a prototype of a biprism laser illuminator for 2×4 laser arrays has been fabricated and evaluated to demonstrate its manufacturability.

Although remote phosphor technology outperforms conformal phosphor technology for midpower applications, one of the limiting factors due to its impact on the total cost is the amount of phosphor required. Furthermore, an important loss mechanism in remote phosphor light-emitting diode (LED) technology is the reabsorption of recycled, downconverted light by the phosphor. An obvious solution to this issue is to enable a light path for the converted light, such that further interactions with the phosphor element are avoided. We propose a spot phosphor concept to achieve this goal. To explore this configuration, a simulation model of a phosphor element is devised and validated. The optical input parameters are based on experimental data and the application of the inverse adding-doubling method. The resulting configuration, along with a long-pass filter, is shown to be a potential solution for reduction of phosphor usage. The moderate decrease in the light extraction ratio (LER) when applying the spot concept is partly attributed to the losses in the secondary optics needed to narrow the LED beam; the combination of the spot concept configuration with a directional light source such as a laser diode is shown to be a powerful combination for the enhancement of the LER.

We present an optical surface in closed form that can be used to design lenses for controlling relative illumination on a target surface. The optical surface is constructed by rotation of the pedal curve to the ellipse about its minor axis. Three renditions of the surface are provided, namely as an expansion of a base surface, and as combinations of several base surfaces. Examples of the performance of the surfaces are presented for the case of a point light source.

Several factors impact the light irradiance and relative illumination produced by a lens system at its image plane. In addition to cosine-fourth-power radiometric law, image and pupil aberrations and light vignetting also count. We use an irradiance transport equation to derive a closed form solution that provides insight into how individual aberration terms affect the light irradiance and relative illumination. The theoretical results are in agreement with real ray tracing.

The evolution properties and the spectral degree of coherence of a partially coherent multi-Gaussian Schell-model vortex (MGSMv) beam propagating in uniaxial crystals orthogonal to the optical axis are studied. Propagation expression for the cross-spectral density and the average intensity of the partially coherent MGSMv beam propagating in uniaxial crystals are derived. Some analyses are illustrated by numerical examples related to the propagation properties and coherence vortices of the partially coherent MGSMv beam in uniaxial crystals. Numerical results show that, upon propagating in uniaxial crystals, the partially coherent MGSMv beam loses its initial dark center distribution. After propagating for sufficient distances, the partially coherent MGSMv beam will also evolve into a flat-topped beam. The evolution behavior of average intensity and coherent vortices depends on the crystal including the ratio of extraordinary and ordinary refractive indices, and beam parameters including the spatial correlation length and topological charge of the beams, as well as the propagation distance.

We report on the temperature characteristics of edge-emitting tilted wave lasers (TWL) composed of a thin active waveguide (0.7  μm) optically coupled to a thick passive waveguide (26  μm). The active region is based on four 1040-nm InGaAs/GaAs quantum wells. The 2-mm-long and 50-μm-wide broad area lasers show characteristic temperature of 115 K and lasing up to 120°C in the continuous-wave mode. The maximal pulsed output power as high as 40 W was achieved being limited by the available current source. Vertical far-fields of the TWLs consist of two tilted narrow lobs (2.4 deg full width at half maximum each), which contain >70% of the total output power. The tilt angle of the lobes slightly increases with the temperature (by 1 deg/40°C) due to the transverse mode hopping caused by the temperature-induced changes of the waveguide refractive indices.

A type of an interleaver with tunable bandwidth ratio for a dense wavelength division multiplexing (DWDM) system is proposed, which integrates a nested ring resonator with a Mach–Zehnder interferometer. The merits of the interleaver are its structure simplicity and the ultralarge tunability on bandwidth ratio. Based on the coupled-mode theory, the effects of essential parameter changes on the output waveforms have been analyzed. According to the theoretical analysis and simulation results, the bandwidth ratio is more determined by the coupling efficiency while the bandwidth is controlled by the optical length of the ring resonator. It was found that the bandwidth ratio is adjustable and varies between 1.5 and 10.5. Therefore, the interleaver can provide important implications for the DWDM system.

A high-temperature sensor using fiber Bragg grating (FBG) integrated with a transducer has been designed, developed, and tested. The transducing element furnishes temperature-dependent tensile strain on FBG by means of differential linear thermal expansion of two different materials—mild steel and stainless steel. The designed sensor is compact, cost effective, and measures temperatures between 20°C and 1000°C. The electrical means output of the sensor against applied temperature is found to be fitted with a second-degree polynomial. The achieved resolution at room temperature is 2°C and is found to be better when moving toward higher temperatures. However, by optimizing the materials and physical dimensions of the transducer, it is possible to improve the temperature range and performance of the sensor.

We propose a multiwaveguide cavity which can significantly enlarge the mode volume of distributed feedback (DFB) cavity without increasing the cavity length. The cavity is constructed by combining λ/4 shifted DFB cavity with contra-directional couplers. Different from single-waveguide DFB cavity, the resonant mode is oscillating in multiple waveguides. A smaller threshold gain density is also obtained compared to single-waveguide DFB cavity. Theory model of the multiwaveguide cavity is established and finite-difference time-domain simulation is performed to verify the model.

We present measurements of the dispersion of two negative curvature fibers at and around 1  μm. These fibers are suitable for beam delivery of both high power and ultrashort pulse lasers with one designed to operate at 1030 nm and the other designed for dual wavelength delivery at 515 and 1030 nm. Both fibers were found to exhibit anomalous dispersion measured to be 2.17±0.25  ps nm⁻¹ km⁻¹ for the fundamental mode rising to 4.69±0.17  ps nm⁻¹ km⁻¹ for the first-order mode in the 1030-nm fiber. Dispersion in the double wavelength fiber was found to be higher at 3.81±0.03  ps nm⁻¹ km⁻¹ for the fundamental mode guiding at 1030 nm.

A miniaturized diode-pumped solid-state laser (DPSSL) designed as part of the Raman laser spectrometer (RLS) instrument for the European Space Agency (ESA) Exomars mission 2020 is assembled and tested for the mission purpose and requirements. Two different processes were tried for the laser assembling: one based on adhesives, following traditional laser manufacturing processes; another based on a low-stress and organic-free soldering technique called solderjet bumping technology. The manufactured devices were tested for the processes validation by passing mechanical, thermal cycles, radiation, and optical functional tests. The comparison analysis showed a device improvement in terms of reliability of the optical performances from the soldered to the assembled by adhesive-based means.

A simple but accurate fiber-optic refractometer based on Fresnel reflection in the tip of a single-mode optical fiber is reported. The refractometer can be easily assembled using a distributed feedback laser as a light source and standard low-cost fiber-optic components. The short section of an optical fiber cleaved at right-angle in both tips is used as a disposable needle-type sensor head. The sensing fiber being connected with the angle-cleaved lead fiber forms a stable reflector used for light intensity reference. Signals from the sensing and reference fiber tips, generated by modulating the laser optical frequency with a sawtooth-wave form, are demultiplexed using methods of optical frequency-modulated continuous-wave interferometry. Despite the simplicity of the device, a repeatability and reproducibility of 6×10⁻⁴  RIU and 3×10⁻⁴  RIU, respectively, were demonstrated in dip-and-read experiments using replaceable disposable fiber tips.

In order to mitigate bandwidth attenuation of diffusion link visible light communication systems caused by multipath effects, we present an optical orthogonal frequency division multiplexing channel estimation scheme based on compressed sensing (CS) and estimation of signal parameters via rotational invariance techniques (ESPRIT). First, we derived a parametric channel model. Then, we used ESPRIT to obtain multipath channel parameters. After that, we built a dynamic over-complete dictionary that can be used in CS processing. Finally, we reconstructed the channel response by using a basis pursuit denoising algorithm to equalize the received signal in frequency domain. Compared with traditional schemes, the proposed scheme can improve channel estimation accuracy without increasing dictionary size. A set of computer simulations demonstrated the effectiveness of the proposed scheme.

We analyze the performance of a coherent orthogonal frequency division multiplexing (OFDM) system and a serial decode and forward relay transmission multihop coherent free-space optical OFDM system using an exponential distribution atmospheric turbulence model under the circumstance of strong atmospheric turbulence. The attenuation of the atmospheric channel fading model mainly considers the light intensity scintillation caused by atmospheric turbulence and interaction between the path consumption, the transmitter and the receiver. The OFDM signal mapping method uses quadrature amplitude modulation. We also derive the formulas of the outage probability and symbol error rate of the coherent OFDM and multihop system, respectively, under the conditions described above. In addition, a simulation is performed, which is essential to evaluate the influence of key factors including coherent detection in a number of relay nodes, the mapping orders, and the number of subcarriers, which have a significant effect on the outage performance and the bit error performance of the OFDM-FSO system under the strong atmospheric turbulence.

The properties of noise induced by stimulated Brillouin scattering (SBS) in long-range interferometers and their influences on the positioning accuracy of dual Mach-Zehnder interferometric (DMZI) vibration sensing systems are studied. The SBS noise is found to be white and incoherent between the two arms of the interferometer in a 1-MHz bandwidth range. Experiments on 25-km long fibers show that the root mean square error (RMSE) of the positioning accuracy is consistent with the additive noise model for the time delay estimation theory. A low-pass filter can be properly designed to suppress the SBS noise and further achieve a maximum RMSE reduction of 6.7 dB.

In the free-space optical (FSO) channel, the optical signal suffers from irradiance and phase fluctuations due to the atmospheric turbulence that reduces the performance of the overall FSO system. In this paper, we have applied the wavelength diversity technique over the exponentiated Weibull channel to mitigate turbulence-induced fading. The source information was transmitted onto three carrier wavelengths of 1.55, 1.31, and 0.85  μm. At the receiver side, an optical combining scheme has been deployed to combine the multiple carriers. Mathematical expressions are derived for the calculation of the outage probability and bit error rate (BER) of the wavelength diversity system. Results are presented for the link lengths of 1.5 and 2.5 km with two different atmospheric turbulence conditions: moderate and strong. An overall improvement of 40 dB in outage probability and 30 dB in BER is achieved.

Efforts have been put into investigating a variable-coefficient coherently coupled nonlinear Schrödinger system with the alternate signs of nonlinearities, describing the propagation of the waves in the nonlinear birefringent optical fiber. Via the Lax pair, Darboux transformation for the system is derived. Then, we derive the vector one- and two-soliton solutions. Figures are displayed to help us study the properties of the vector solitons: with the strength of the four-wave mixing terms γ(t) as a constant, the vector soliton propagates with the unvarying velocity and amplitude; with γ(t) being a time-dependent function, amplitude and velocity of the vector soliton keep varying during the propagation; bell- and M-shaped solitons can both be observed in q₂ mode, while we just observe the bell-shaped soliton in q₁ mode, where q₁ and q₂ are the two slowly varying envelopes of the propagating waves; head-on and overtaking interactions between the vector two solitons are both presented.

Indoor positioning or localization based on visible light communications (VLC) is an emerging technology with wide applications. In conventional localization schemes, the trilateration technique is widely used with at least three separate lighting sources to determine the user’s location. An indoor VLC positioning scheme based on the optical power distributions of only two light emitting diodes (LEDs) is reported for different environments. We have used two received signal strength indications to determine the user’s position based on the LEDs configuration offering less complexity. We propose comprehensive mathematical models for the VLC localization system considering the noise and its impact on the user’s location, and numerically evaluated it over a range of signal-to-noise ratios (SNRs). In addition, it is compared to the results with the exiting trilateration technique. The performance of the proposed system is evaluated with a reported accuracy of <20  cm for an SNR of >13  dB.

Liquid crystal optical phased array (LC-OPA) has been reported to be one of the promising methods to realize nonmechanical laser beam steering in free-space laser communication. The nonmechanical beam steering mechanism of LC-OPA in a free-space laser communication process is demonstrated. To analyze the steering performance of LC-OPA for a beam arriving from arbitrary directions, a theoretical model based on two-dimensional liquid crystal orient model and backward propagation method is proposed. In addition the phase characteristics are discussed both in normal and oblique incident cases, and a linear relationship between angle-of-arrival and detected incident angle is also derived. All of the theoretical results are verified by the following experiments with a good agreement.

A repetition-frequency-controllable double Q-switched (DQS) Ho:LuAG laser at 2098.34 nm with acousto-optic modulator (AOM) and Cr²⁺:ZnS saturable absorber (SA) was investigated. The maximum output power of 5.3 W was obtained at the AOM repetition frequency (RF) of 3.72 kHz, corresponding to the slope efficiency of 29.8%. The largest pulse energy of 2.8 mJ with the shortest pulse width of 54.2 ns was observed at the AOM RF of 2.64 kHz. Compared with the passively Q-switched Ho:LuAG laser, the DQS Ho:LuAG laser could generate shorter pulse width and larger pulse energy, and its RF could be controlled by the AOM RF.

We demonstrate a chip-scale tunable optofluidic dye laser with Au-coated fibers as microcavity. The chip is fabricated by soft lithography. When the active region is pumped, a relatively low threshold of 6.7  μJ/mm² is realized with multimode emission due to good confinement of the cavity mirrors, long active region, as well as total reflectivity. It is easy to tune the lasing emission wavelength by changing the solvent of laser dye. In addition, the various intensity ratios of multicolor lasing can be achieved by controlling flow rates of two fluid streams carried with different dye molecules. Furthermore, the convenience in fabrication and directional lasing emission outcoupled by the fiber make the tunable optofluidic dye laser a promising underlying coherent light source in the integrated optofluidic systems.

Logarithmic detectors (LogDs) have been used in coherent Brillouin optical time-domain analysis (BOTDA) sensors to reduce the effect of phase fluctuation, demodulation complexities, and measurement time. However, because of the inherent properties of LogDs, a DC component at the level of hundreds of millivolts that prohibits high-gain signal amplification (SA) could be generated, resulting in unacceptable data acquisition (DAQ) inaccuracies and decoding errors in the process of prototype integration. By generating a reference light at a level similar to the probe light, differential detection can be applied to remove the DC component automatically using a differential amplifier before the DAQ process. Therefore, high-gain SA can be employed to reduce quantization errors. The signal-to-noise ratio of the weak Brillouin gain signal is improved from ∼11.5 to ∼21.8 dB. A BOTDA prototype is implemented based on the proposed scheme. The experimental results show that the measurement accuracy of the Brillouin frequency shift (BFS) is improved from ±1.9 to ±0.8  MHz at the end of a 40-km sensing fiber.

A polymer-based multimode interference optical splitter chip has been designed and fabricated. Fiber-waveguide coupling loss as well as the structure of the multimode waveguide are optimized in the design to achieve higher performance. A simple UV-based soft nanoimprint lithography (Soft UV-NIL) technique is adopted in the fabrication. Fluorinated acrylate resins, LFR, with different refractive indices are used in this work. Both the residual layer and waveguide deformation are improved by controlling the fabrication processes. An average of 12.98 dB insertion loss is obtained from 1×4 splitters with 1.08 dB uniformity and 0.05 dB polarization-dependent loss. The validity of the polymer optical splitters fabricated through Soft UV-NIL technique is demonstrated by software simulation as well as experimental works.

Polychromatic tunable visible light emission in the region of the low correlated color temperature range using Dy³⁺/Tb³⁺ codoped PbGeO₃:PbF₂:CdF₂ glass phosphor under UV-blue LED light excitation is presented. The glass phosphor was synthesized and the light emission feature was examined under UV-blue (353, 375, 385, and 405 nm) excitation. Emission around 484, 573, 663, and 754 nm due to dysprosium, and 488, 545, 585, 620, and 690 nm owing to terbium ions, was observed and analyzed as a function of the dysprosium and terbium contents and excitation wavelength. The excitation spectrum was examined and showed resonance peaks around 385 nm for the 573-nm emission of Dy³⁺, and 375 nm for the 545 nm of Tb³⁺. Energy-transfer process from Dy³⁺(⁴F_9/2) to Tb³⁺(⁵D₄) was also observed. Results indicated that the polychromatic visible light emitter herein reported produced light possessing tunable color tone via excitation wavelength and ions’ mass ratio. The tint of the tunable overall emission resided in the warm region of the white-light boundary of the CIE-1931 chromaticity diagram.

A numerical study of a surface plasmon resonance (SPR) sensor based on photonic crystal fiber (PCF) filled with core-shell Ag-Au nanocomposite materials for measurement of refractive index of the analyte is presented. Considering the design flexibility and improved optical properties of the core-shell bimetallic nanocomposite materials, the proposed sensor is demonstrated to achieve high spectral sensitivity, high detection accuracy, and sensing spectra tuning ability. Meanwhile, the existence of the gold shell can protect the silver core from oxidation, which means high system stability and long lifetime. Based on numerical simulations, the influences of structural parameters of the sensor on sensing properties are discussed. Furthermore, the parameters are optimized by analyzing the simulation results to achieve better performance. It is observed that a high average spectral sensitivity of 5940  nm/RIU with the resolution of 1.68×10⁻⁵  RIU in the sensing range of 1.33 to 1.38 can be achieved.

The spectral discrimination function of uncooled infrared (IR) sensors has significant advantages for applications such as fire detection, gas analysis, and biological analysis. We have previously demonstrated wavelength-selective uncooled IR sensors using two-dimensional plasmonic absorbers (2-D PLAs) over a wide range spanning the middle- and long-wavelength IR regions. 2-D PLAs are highly promising in terms of practical application due to the ease of fabrication and robustness for structural fluctuations. However, dual-band operation based on this concept has not yet been investigated, even though the ability to absorb in two different wavelength bands is extremely important for object recognition. Thus, a dual-band uncooled IR sensor was developed that employs Fano resonance in the plasmonic structures. To achieve dual-band detection, asymmetric periods in the orthogonal x- and y-directions were introduced into 2-D PLAs. Theoretical investigations predicted an asymmetric absorbance line shape dependent on the polarization attributed to Fano resonance. The spectral responsivity of the developed sensor demonstrated that selective detection occurred in two different wavelength bands due to polarization-dependent Fano resonance. The results obtained in this study will be applicable to the development of advanced sensors capable of multiband detection in the IR region.

In this work, we develop a wet-processable scattering layer exhibiting a high refractive index that can be used in organic light-emitting diodes for light outcoupling purposes. The composite layers contain an acrylate casting resin, benzylmethacrylate, and phenanthrene, which is employed to increase the refractive index. The mixtures are first rheologically characterized and then polymerized with heat and UV radiation. For the refractive index measurements, the polymerized samples require a planar surface without air bubbles. To produce flat samples, a special construction consisting of a glass plate, a teflon sheet, a silicone ring (PDMS mold), another teflon sheet, and another glass plate is developed. Glue clamps are used to hold the construction together. The refractive index of the samples can be increased from 1.565 to 1.585 at 20°C at a wavelength of 589 nm following the addition of 20 wt% phenanthrene. A master mixture with a high refractive index is taken for further experiments. Nanoscaled titanium dioxide is added and dispersed into the master mixture and then spin coated on a glass substrate. These layers are optically characterized. Most of the presented layers present the expected haze of over 50%.

This PDF file contains the errata for “OE Vol. 55 Issue 11 Paper OE-2016-1110-ERR” for OE Vol. 55 Issue 11

This PDF file contains the errata for “OE Vol. 55 Issue 11 Paper OE-2016-1111-ERR” for OE Vol. 55 Issue 11

This PDF file contains the errata for “OE Vol. 55 Issue 11 Paper OE-2016-1121-ERR” for OE Vol. 55 Issue 11