I hope everyone is having a good New Year so far. For Optical Engineering, 2014 is going to be the continuation of some dramatic changes that we implemented in 2013. We added five major categories overseen by senior editors in the areas of: • Imaging Components, Systems, and Processing • Instrumentation, Techniques, and Measurement • Optical Design and Engineering • Fiber Optics, Communications, and Lasers • Materials, Devices, and Sensors.

Although the use of poisons and lethal chemicals in warfare perhaps predates recorded history, this autumn marks the 100th anniversary of the use of industrialized chemical gases in World War I. Shortly following the introduction of xylyl bromide (T-Stoff) in 1914 as a lacrimating agent, the first large-scale attack with chlorine gas occurred 22 April 1915 at Ypres, Belgium. Since the advent of chemical weapons, the world has added biological, radiological, nuclear, and most recently explosive (CBRNE) hazards to the list of threats expected by military forces on the battlefield, as well as to civilians in the homeland.

Analytic solutions for finding the achromatic triplet in the midwave and longwave infrared spectra simultaneously are explored. The relationship between the combination of promising refractive materials and the system’s optical power is also formulated. The principles for stabilizing the effective focal length of an air-spaced lens group with respect to temperature are explored, and the thermal properties of the optical component and mechanical elements mutually counterbalanced. An optical design based on these achromatic and athermal theories is demonstrated, and the image quality of the lens assembly seems to approach the diffractive limitation.

Lincoln Laboratory of Massachusetts Institute of Technology has developed a technique known as dynamic photoacoustic spectroscopy (DPAS) that could enable remote detection of trace gases via a field-portable laser-based system. A fielded DPAS system has the potential to enable rapid, early warning of airborne chemical threats. DPAS is a new form of photoacoustic spectroscopy that relies on a laser beam swept at the speed of sound to amplify an otherwise weak photoacoustic signal. We experimentally determine the sensitivity of this technique using trace quantities of SF ₆ gas. A clutter-limited sensitivity of ∼100  ppt is estimated for an integration path of 0.43 m. Additionally, detection at ranges over 5 m using two different detection modalities is demonstrated: a parabolic microphone and a laser vibrometer. Its utility in detecting ammonia emanating from solid samples in an ambient environment is also demonstrated.

A cascaded long-period fiber grating (CLPFG) sensor with film coating is presented. Two long-period fiber gratings (LPFGs) are cascaded to form a Mach-Zehnder interferometer. The optical transmission spectrum and the sensing characteristics for ambient refractive index (RI) measurement of the CLPFG sensor are analyzed in a form of transfer matrix based on rigorous coupled-mode theory. The results indicate that it is highly sensitive to the film RI and surrounding RI, so it can be used as a gas sensor or a solution sensor. Data simulation shows that the resolution of the RI of the films is predicted to be 10 ⁻⁷ . Further, a cascaded chirped LPFG is introduced to study its sensing performance. The influence of the chirp coefficient and the grating structural parameters on the transmission spectrum of the cascaded chirped LPFG is analyzed. Data simulation shows that the resolution of the RI of the films is predicted to be 10 ⁻⁹ . In contrast to the conventional measurement method based on interrogating the wavelength change, intensity detection of this cascaded chirped LPFG sensor means that no optical spectrum analyzer is required in the measuring system, which is favorable for practical applications, especially for in situ environmental motoring.

Multiplex coherent anti-Stokes Raman scattering (MCARS) is used to detect several chemical warfare simulants, such as dimethyl methylphosphonate and 2-chloroethyl ethyl sulfide, with high specificity. The spectral bandwidth of the femtosecond laser pulse used in these studies is sufficient to coherently and simultaneously drive all the vibrational modes in the molecule of interest. Evidence shows that MCARS is capable of overcoming common sensitivity limitations of spontaneous Raman scattering, thus allowing for the detection of the target material in milliseconds with standard, uncooled universal serial bus spectrometers as opposed to seconds with cooled, intensified CCD-based spectrometers. In addition, the obtained MCARS spectrum of the investigated sample provides multiple unique signatures. These signatures are used in an off-line multivariate statistical analysis allowing for the material’s discrimination with high fidelity.

The remote sensing and classification of battlespace explosions might be improved by developing optical signatures for the heat released during secondary combustion. To explore this possibility, mid-wave infrared spectra (1800 to 10,000  cm ⁻¹ ) from the detonation of aluminized cyclotrimethylenetrinitramine (RDX) have been observed and analyzed. The field detonations of 12 high-explosive compositions of RDX with varying aluminum and liner volumes were remotely observed with a suite of imagers, spectrometers, and radiometers. The evolution of spatially averaged fireball temperatures has been estimated from the infrared spectra. The effective temperatures decay from initial values of 1290 to 1850 K to less than 1000 K during a 1-s interval. Secondary maxima are observed in the temperature profiles, indicating delayed combustion resulting from the mixing of atmospheric oxygen with the detonation products. The heat released in the secondary combustion is well correlated with the high explosive and liner theoretical heats of secondary combustion and exhibits an average efficiency of about 50%. Fireball lofting rates increase by more than 50% for events where the combustion heat release increases by a factor of 2.

The proposed colorimetric sensor of ammonia for the confined animal feeding industry uses the method of optoelectronic spectroscopic measurement of the reversible change of the color of a nanocomposite reagent film in response to ammonia. The film is made of a gold nanocolloid in a polymer matrix with an ammonia-sensitive indicator dye additive. The response of the indicator dye (increase of the optical absorption between 550 and 650 nm) is enhanced by the nanoparticles (∼8  nm in size) in two ways: (a) concentration of the optical field near the nanoparticle due to the plasmon resonance and (b) catalytic acceleration of the chemical reaction of deprotonization of the indicator dye in the presence of ammonia and water vapor. This enhancement helps to miniaturize the sensing element without compromising its sensitivity of <1 parts per million (ppm) for the range 0 to 100 ppm. The sensor underwent field tests in commercial poultry farms in Georgia and Arkansas and was compared against a scientific-grade photoacoustic gas analyzer. The coefficient of correlation between the sensor and the photoacoustic data for several weeks of continuous side-by-side operation in a commercial poultry house was ∼0.9 and the linear regression slope was 1.0. The conclusions on the necessary improvements were made.

Performance of a new modification of xenon gamma-ray detector (XGD) is presented. This detector differs from the previous ones by virtue of improved energy resolution (1.7±0.1)% at 662 keV and the ability to function in the presence of external acoustic noise (up to 100 dB) with virtually no degradation of spectrometric characteristics. These results have been achieved by developing a digital method of processing every electric signal coming from the XGD. For this method, digital electronics based on field-programmable gate array has also been developed.

The central parameter in the quantification of chemical vapor plumes via remote sensing is the mean concentration-path length (CL) product, which can be combined with scene geometry information to provide estimates of the absolute gas quantity present. We derive Cramer-Rao lower bounds on the variance of an unbiased estimator of CL in concert with other parameters of a nonlinear radiance model. These bounds offer a guide to feasibility of CL estimation that is not dependent on any given algorithm. In addition, the derivation of the bounds yields great insight into the physical and phenomenological mechanisms that control plume quantification, which we illustrate with examples representing a variety of experimental scenarios.

A number of optical techniques are available to perform active standoff trace explosive detection. Integrating a laser scanner provides the ability to detect explosives over a wide area as well as to assess the full extent of a threat. Risley prism laser-beam steering systems provide a robust alternative to conventional scanner solutions and are ideal for portable and mobile systems due to their compact size, low power, large field-of-view, and fast scan speed. The design of a long-wave infrared Risley prism-scanned diffuse reflectance spectroscopy system along with data obtained from a prototype system is presented for both simulant and live explosive materials.

We propose an algorithm for standoff quantification of chemical vapor plumes from hyperspectral imagery. The approach is based on the observation that the quantification problem can be easily solved in each pixel with the use of just a single spectral band if the radiance of the pixel in the absence of the plume is known. This plume-absent radiance may, in turn, be recovered from the radiance of the subset of spectral bands in which the gas species is transparent. This “selected-band” algorithm is most effective when applied to gases with narrow spectral features, and are therefore transparent over many bands. We also demonstrate an iterative version that expands the range of applicability. Simulations show that the new algorithm attains the accuracy of existing nonlinear algorithms, while its computational efficiency is comparable to that of linear algorithms.

Our team has pioneered an explosives detection technique based on hyperspectral imaging of surfaces. Briefly, differential reflectometry (DR) shines ultraviolet (UV) and blue light on two close-by areas on a surface (for example, a piece of luggage on a moving conveyer belt). Upon reflection, the light is collected with a spectrometer combined with a charge coupled device (CCD) camera. A computer processes the data and produces in turn differential reflection spectra taken from these two adjacent areas on the surface. This differential technique is highly sensitive and provides spectroscopic data of materials, particularly of explosives. As an example, 2,4,6-trinitrotoluene displays strong and distinct features in differential reflectograms near 420 and 250 nm, that is, in the near-UV region. Similar, but distinctly different features are observed for other explosives. Finally, a custom algorithm classifies the collected spectral data and outputs an acoustic signal if a threat is detected. This paper presents the complete DR hyperspectral imager which we have designed and built from the hardware to the software, complete with an analysis of the device specifications.

This paper presents a proof of concept sensor system based on a linear array of pyroelectric detectors for recognition of moving objects. The utility of this prototype sensor is demonstrated by its use in trail monitoring and perimeter protection applications for classifying humans against animals with object motion transverse to the field of view of the sensor array. Data acquisition using the system was performed under varied terrains and using a wide variety of animals and humans. With the objective of eventually porting the algorithms onto a low resource computational platform, simple signal processing, feature extraction, and classification techniques are used. The object recognition algorithm uses a combination of geometrical and texture features to provide limited insensitivity to range and speed. Analysis of system performance shows its effectiveness in discriminating humans and animals with high classification accuracy.

Laser gated viewing is a prominent sensing technology for optical imaging in harsh environments and can be applied for vision through fog, smoke, and other degraded environmental conditions as well as for the vision through sea water in submarine operation. A direct imaging of nonscattered photons (or ballistic photons) is limited in range and performance by the free optical path length, i.e., the length in which a photon can propagate without interaction with scattering particles or object surfaces. The imaging and analysis of scattered photons can overcome these classical limitations and it is possible to realize a nonline-of-sight imaging. The spatial and temporal distributions of scattered photons can be analyzed by means of computational optics and their information of the scenario can be restored. In particular, the information outside the line of sight or outside the visibility range is of high interest. We demonstrate nonline-of-sight imaging with a laser gated viewing system and different illumination concepts (point and surface scattering sources).

In order to accurately characterize the radio frequency (RF) responsivity of the antenna-coupled detector in the terahertz (THz) band, we introduce an iterative deconvolution algorithm to extract the effective receiving area. We use this method to characterize an antenna-coupled Nb ₅ N ₆ microbolometer as an example. The effective receiving area is approximately 0.7  mm ² , which is 7.4 times larger than the physical area of the detector. The RF responsivity of the Nb ₅ N ₆ microbolometer is 480  V/W at 0.28 THz, including the effect of the antenna coupling and the substrate interference. This work offers an effective way to characterize antenna-coupled THz detectors and to analyze the element-to-element spacing along two orthogonal directions in THz focal plane array chips.

This article proposes a surface reconstruction method from multiview projectional data acquired by means of a rotationally mounted microlens array-based light detector (MLA-D). The technique is adapted for in vivo small animal imaging, specifically for imaging of nude mice, and does not require an additional imaging step (e.g., by means of a secondary structural modality) or additional hardware (e.g., laser-scanning approaches). Any potential point within the field of view (FOV) is evaluated by a proposed photo-consistency measure, utilizing sensor image light information as provided by elemental images (EIs). As the superposition of adjacent EIs yields depth information for any point within the FOV, the three-dimensional surface of the imaged object is estimated by a graph cuts-based method through global energy minimization. The proposed surface reconstruction is evaluated on simulated MLA-D data, incorporating a reconstructed mouse data volume as acquired by x-ray computed tomography. Compared with a previously presented back projection-based surface reconstruction method, the proposed technique yields a significantly lower error rate. Moreover, while the back projection-based method may not be able to resolve concave surfaces, this approach does. Our results further indicate that the proposed method achieves high accuracy at a low number of projections.

Tracking pedestrian targets in forward-looking infrared video sequences is a crucial component of a growing number of applications. At the same time, it is particularly challenging, since image resolution and signal-to-noise ratio are generally very low, while the nonrigidity of the human body produces highly variable target shapes. Moreover, motion can be quite chaotic with frequent target-to-target and target-to-scene occlusions. Hence, the trend is to design ever more sophisticated techniques, able to ensure rather accurate tracking results at the cost of a generally higher complexity. However, many of such techniques might not be suitable for real-time tracking in limited-resource environments. This work presents a technique that extends an extremely computationally efficient tracking method based on target intensity variation and template matching originally designed for targets with a marked and stable hot spot by adapting it to deal with much more complex thermal signatures and by removing the native dependency on configuration choices. Experimental tests demonstrated that, by working on multiple hot spots, the designed technique is able to achieve the robustness of other common approaches by limiting drifts and preserving the low-computational footprint of the reference method.

A possible variant of the direct observation of Earth-like planets with the help of a hypertelescope containing circular and/or annular collecting mirrors is considered. Analytical expressions describing the imaging in the focal plane are presented. They give the opportunity to estimate the influence of the photodetector pixel exposure time and accuracy parameters of the mirror control/stabilization system on the quality of the image in the plane of the photodetector and to evaluate a signal-to-noise ratio (SNR) at its output. Numerical results obtained for the hypertelescope containing 126 (for detection) and 1164 (for recognition) identical annular mirrors of diameter 1 m illustrate an opportunity of detecting an Earth-like exo-planet moving around a Sun-like star at a distance of 1 au and recognizing its surface area at a distance of 1 pc on conditions that some technique blocking out the radiation from the exo-planet’s star is used. Some dependencies of the corresponding SNR values on the pixel exposure time and the accuracy parameters of the mirror stabilization system are shown. Several typical aperture configurations are investigated for the case of recognition and the most appropriate configuration is defined.

Carbon foils have been used reliably for many decades in space plasma instrumentation to detect ions and energetic neutral atoms (ENA). When these particles pass through a foil, secondary electrons are emitted on both sides. Those electrons are used for coincidence detection and/or timing signals. Ultrathin carbon foils are also used to convert an ENA into an ion for further analysis. The interaction of particles with carbon foils also includes unwanted effects such as energy straggling and angular scattering, both of which scale with foil thickness. Therefore, it has always been a goal to use foils as thin as practically possible. The foils used in space are usually made of amorphous carbon of roughly a hundred atomic layers. Graphene can be made much thinner, even down to a single atomic layer, and is therefore a natural candidate for this kind of application. We evaluate one aspect of the interaction of particles with foils: charge exchange. We show the first measurements of exit charge state distributions of ∼1 to 50 keV ions passing through self-supported graphene foils. We compare the charge state fraction of exiting particles with state-of-the-art amorphous carbon foils.

A simple and efficient phase-unwrapping algorithm based on a rounding procedure and a global least-squares minimization is proposed. Instead of processing the gradient of the wrapped phase, this algorithm operates over the gradient of the phase jumps by a robust and noniterative scheme. Thus, the residue-spreading and over-smoothing effects are reduced. The algorithm’s performance is compared with four well-known phase-unwrapping methods: minimum cost network flow (MCNF), fast Fourier transform (FFT), quality-guided, and branch-cut. A computer simulation and experimental results show that the proposed algorithm reaches a high-accuracy level than the MCNF method by a low-computing time similar to the FFT phase-unwrapping method. Moreover, since the proposed algorithm is simple, fast, and user-free, it could be used in metrological interferometric and fringe-projection automatic real-time applications.

A method for the measurement of the concentration of methane is experimentally demonstrated. The broad light source is filtered by using a Fabry–Perot filter, whose wavelength peaks are matched with the methane absorption lines. The matched guided light is absorpted by the methane in the gas cell, which is coated with a film with 100% reflectivity. Therefore, the guided light is reflected a number of times and the sensitivity of the sensor is increased. The experimental results show that a resolution of 0.091% is achieved. In addition, due to the matched wavelength peaks, the influence of the interfering gas can be removed by using the proposed method. Compared with the conventional method, the proposed method possesses low cost, high sensitivity, and high stability.

We report on the physical observation of mixtures of volume contractional monohydric alcohol solutions interrogated by a planar Bragg grating sensor. Via evanescent field interaction, the sensor detects any changes of the composition of a liquid compound on the sensor surface that are associated with a change of the refractive index. Studying various aqueous solutions of monohydric alcohols, we found a nonlinear relationship of the analytes refractive index on its mixing ratio, indicating the different nature of the mixed phases in the solution and the effect of volume contraction. Analyzing aqueous solutions of methanol, ethanol, and 2-propanol reveals a local maximum of the refractive index for the application of smaller monohydric alcohol molecules, which is displaced in the direction of a higher alcohol content with an increasing number of carbon atoms in the chain of the alcohols. In turn, this sensor can be used for the online monitoring and observation of the water content in aqueous alcohol solutions, including volume contractional solutions.

A three-dimensional (3-D) shape measurement system that can simultaneously achieve 3-D shape acquisition, reconstruction, and display at 30 frames per second (fps) with 480,000 measurement points per frame is presented. The entire processing pipeline was realized on a graphics processing unit (GPU) without the need of substantial central processing unit (CPU) power, making it achievable on a portable device, namely a laptop computer. Furthermore, the system is extremely inexpensive compared with similar state-of-art systems, making it possible to be accessed by the general public. Specifically, advanced GPU techniques such as multipass rendering and offscreen rendering were used in conjunction with direct memory access to achieve the aforementioned performance. The developed system, implementation details, and experimental results to verify the performance of the proposed technique are presented.

The traditional measurement method for a hand-held probe with two sensors in large-scale measurements magnifies the uncertainty significantly when the distance between the tip and the nearest sensor is increased in order to touch the measurement points in the shadowed zone. To reduce the precision loss in that case, we propose a new method that only utilizes the colinear characteristic of the sensors on our newly designed probe. Hence, the principle of intersection can be used to determine the coordinate of the measurement point by holding the probe to touch the point in two or more different directions. This method is helpful to keep a low uncertainty in the case where increasing the distance between the tip and the nearest sensor is necessary. Correspondingly, the algorithms used to make the probe work its best, as well as the detailed derivations, are given. An experiment is then shown to verify the efficacy of the new design and the proposed method. The results indicate that the new design and the new method can reduce the precision loss remarkably in the case where the distance between the tip and the nearest sensor has to be increased.

An adaptive method is proposed in this paper for design of an ideal low-pass filter based on the Fourier-transform spectrum distribution of digital speckle interference fringe patterns. The size and shape of the ideal low-pass filter can be determined automatically by using an adaptive method based on the edge detection and morphological operation. The cutoff frequency of the ideal low-pass filter along arbitrary direction can also be determined automatically by using an adaptive method based on the Radon transform. The ideal low-pass filter designed in the paper can be used effectively for denoising of digital speckle interference fringe patterns in phase-shifting digital speckle pattern interferometry.

A calculation reduction method for color digital holography (DH) and computer-generated holograms (CGHs) using color space conversion is reported. Color DH and color CGHs are generally calculated on RGB space. We calculate color DH and CGHs in other color spaces for accelerating the calculation (e.g., YCbCr color space). In YCbCr color space, a RGB image or RGB hologram is converted to the luminance component (Y), blue-difference chroma (Cb), and red-difference chroma (Cr) components. In terms of the human eye, although the negligible difference of the luminance component is well recognized, the difference of the other components is not. In this method, the luminance component is normal sampled and the chroma components are down-sampled. The down-sampling allows us to accelerate the calculation of the color DH and CGHs. We compute diffraction calculations from the components, and then we convert the diffracted results in YCbCr color space to RGB color space. The proposed method, which is possible to accelerate the calculations up to a factor of 3 in theory, accelerates the calculation over two times faster than the ones in RGB color space.

This paper proposes a parallax barrier with an elliptical pattern that reduces the cross talk caused by light leakage from adjacent subpixels in autostereoscopic three-dimensional (3-D) displays. To find the optimum size of the elliptical barrier pattern, the relationship between the reduction of the light leakage and that of the luminance is analyzed. In addition, we analyze the relationship between the cross talk and the luminance. By using these relationships, we propose an optimum size of the ellipse. An autostereoscopic 3-D display with the elliptical barrier is compared with 3-D displays with the slanted barrier and the rectangular one. The measured cross talk of the slanted-type 3-D display whose pixel size is 98×294  μm was 57%. However, the cross talk of the ellipse-type 3-D display was 32% at the similar luminance condition when the minor and major axes are 92 and 278 μm, respectively. For generalization, we investigate autostereoscopic 3-D displays with different pixel sizes and different viewing distances. We find the optimum area of the ellipse is 70% of the subpixel area to reduce the cross talk.

A new optical architecture for holographic data storage system which is compatible with a Blu-ray Disc™ (BD) system is proposed. In the architecture, both signal and reference beams pass through a single objective lens with numerical aperture (NA) 0.85 for realizing angularly multiplexed recording. The geometry of the architecture brings a high affinity with an optical architecture in the BD system because the objective lens can be placed parallel to a holographic medium. Through the comparison of experimental results with theory, the validity of the optical architecture was verified and demonstrated that the conventional objective lens motion technique in the BD system is available for angularly multiplexed recording. The test-bed composed of a blue laser system and an objective lens of the NA 0.85 was designed. The feasibility of its compatibility with BD is examined through the designed test-bed.

Asteroids and comets that cross Earth’s orbit pose a credible risk of impact, with potentially severe disturbances to Earth and society. We propose an orbital planetary defense system capable of heating the surface of potentially hazardous objects to the vaporization point as a feasible approach to impact risk mitigation. We call the system DE-STAR, for Directed Energy System for Targeting of Asteroids and exploRation. The DE-STAR is a modular-phased array of kilowatt class lasers powered by photovoltaic’s. Modular design allows for incremental development, minimizing risk, and allowing for technological codevelopment. An orbiting structure would be developed in stages. The main objective of the DE-STAR is to use focused directed energy to raise the surface spot temperature to ∼3000  K, sufficient to vaporize all known substances. Ejection of evaporated material creates a large reaction force that would alter an asteroid’s orbit. The baseline system is a DE-STAR 3 or 4 (1- to 10-km array) depending on the degree of protection desired. A DE-STAR 4 allows initial engagement beyond 1 AU with a spot temperature sufficient to completely evaporate up to 500-m diameter asteroids in 1 year. Small objects can be diverted with a DE-STAR 2 (100 m) while space debris is vaporized with a DE-STAR 1 (10 m).

Deformable optical elements may be physically altered through fluid displacement. We imprint a grating surface onto an elastomeric membrane by soft lithography. The membrane is installed as a flexible wall of a sealed fluidic chamber. Injection of excess fluid into the chamber induces expansion of the membrane, effectively varying the groove spacing of the imprinted grating. In transmission experiments using a broadband light source, we are able to achieve a 127.2-nm shift in peak wavelength with an injected fluid volume of 0.56 ml. A paraboloid model of the change in curvature of the grating during expansion accurately describes our experimental results.

A thermal model coupled to statistics is proposed based on damage initiation by heating of size-distributed inclusions to a critical temperature. The data points of damage probability on the surface of fused silica containing different levels of impurities are measured. By linking the contents of various impurities measured to the calculation for damage probability, the influence of various impurities on damage probability is obtained. The purpose of the work is to present a more thorough analysis of the correlation of subsurface impurities to laser damage probability.

The output window of a high-power laser system is vulnerable to damage, and this is the main limiting factor on the power scaling and structure integrity of the laser system. In endeavoring to obtain higher output powers from the laser system, the impact of the thermal and mechanical effects and the damage mechanism of the output window must be considered. In order to study these issues, a thermal model of the laser window is established based on the heat transfer and thermoelastic theories, and the expressions for the transient thermal and mechanical stress distributions of the output window are deduced in terms of the integral-transform method. Taking the infrared quartz window material as an example, the temperature and mechanical field distributions of a high-power all-solid-state 2-μm laser system window are simulated, and the laser-induced damage mechanism is deeply analyzed. The calculation results show that the laser window-induced damage is mainly caused by melting damage when the temperature exceeds the melting point of the material. The presented theoretical analysis and numerical simulation results are significant for the design and optimization of high-power laser windows.

We present a scheme based on precoding carrier allocation and independent component analysis (ICA) for indoor multiple-input-multiple-output (MIMO) visible light communication (VLC). In order to improve the reliability of the ICA algorithm for the mixed signal separation, at the sending end, frequency doubled-carriers are employed to module the parallel data which ensure that the modulated signals are independent of each other. The ICA algorithm is applied to separate the mixed signal at the receiving end directly without the requirement of the channel information. Simulation results show that this indoor MIMO VLC scheme can achieve excellent performance. When the signal-to-noise ratio is equal to 14 dB, the bit error ratio (BER) reaches the level of 10 ⁻⁵ . So, the communication performance is superior to that of the commonly used schemes based on the channel estimation.

This contribution investigates the effectiveness of optical communication links in enabling high-speed data transfer from deep-space (DS) probes directly to Earth ground stations. In particular, the propagation impairments induced by clouds are estimated by exploiting long-term radiosonde observation data collected in some European sites. The impact of different cloud types on optical links operating at 1.55 μm is first quantified in terms of total path attenuation, and afterward, the implementation of multisite diversity schemes is discussed to counteract the extremely high attenuation levels caused by clouds. Results show that a three-site diversity system with target availability of 90% allows reduction of the link margin to counteract cloud attenuation from at least 40 dB to ∼6 dB, which makes optical communications a viable option also for DS missions.

It has been demonstrated experimentally that pulsed pumping can significantly improve thermal management, thus upgrade the output power of an optically pumped semiconductor disk laser (SDL). The transient heat conduction equation is solved by the use of the finite element method, and the maximum temperature rise of the active region in an InGaAs quantum well SDL under pulsed pumping is focused. Based on the numerical results, the influences of width, repetition rate, and shape of pump pulses on the maximum temperature rise are discussed, the optimized design of width, repetition rate, and shape of pump pulses are concluded, and the theoretical results are in good agreement with the reported experiments.

A location technique based on power spectrum analysis for the phase-sensitive optical time-domain reflectometry is proposed. The frequency characteristics of the backscattered signal at a time interval over the sensing fiber are provided to discriminate the disturbance region from other regions. Compared with conventional location techniques, the proposed method significantly enhances the signal-to-noise ratio (SNR). Therefore, it can provide a high-performance and cost-effective solution avoiding the use of the laser with super low-frequency drift. Although the frequency drift of the laser utilized in the experiment is 230  MHz/min , the average SNR is improved to be 20.3 dB and the maximum location error is 100 m over the monitored length of 9 km during 20 experiments.

Linewidth optimization of a fiber grating Fabry–Perot (FGFP) laser is performed numerically. In addition to the external optical feedback (OFB), the effect of temperature, injection current, cavity volume, gain compression factor, and external cavity parameters [i.e., coupling coefficient (C _o ) and external cavity length (L _ext )] on linewidth characteristics are investigated. The effects of external OFB and temperature on linewidth characteristics are calculated according to their effect on threshold carrier density (N _th ). The temperature dependence (TD) of linewidth characteristics is calculated according to the TD of laser parameters instead of the well-known Pankove relationship. Results show that the optimum external cavity length (L _ext ) is 3.1 cm and the optimum range of operating temperature is within ±2°C from the fiber Bragg grating (FBG) reference temperature (T _o ). In addition, the antireflection (AR) coating reflectivity value of 1×10 ⁻² is sufficient for the laser to operate at narrow linewidth and low fabrication complexity. The linewidth can be reduced either by increasing the laser injection current or the strength of external OFB level.

A double-image encryption scheme is proposed based on an asymmetric technique, in which the encryption and decryption processes are different and the encryption keys are not identical to the decryption ones. First, a phase-only function (POF) of each plain image is retrieved by using an iterative process and then encoded into an interim matrix. Two interim matrices are directly modulated into a complex image by using the convolution operation in the fractional Fourier transform (FrFT) domain. Second, the complex image is encrypted into the gray scale ciphertext with stationary white-noise distribution by using the FrFT. In the encryption process, three random phase functions are used as encryption keys to retrieve the POFs of plain images. Simultaneously, two decryption keys are generated in the encryption process, which make the optical implementation of the decryption process convenient and efficient. The proposed encryption scheme has high robustness to various attacks, such as brute-force attack, known plaintext attack, cipher-only attack, and specific attack. Numerical simulations demonstrate the validity and security of the proposed method.

Chalcogenide glasses are a matchless material as far as mid-infrared (IR) applications are concerned. They transmit light typically from 2 to 12 μm and even as far as 20 μm depending on their composition, and numerous glass compositions can be designed for optical fibers. One of the most promising applications of these fibers consists in implementing fiber evanescent wave spectroscopy, which enables detection of the mid-IR signature of most biomolecules. The principles of fiber evanescent wave spectroscopy are recalled together with the benefit of using selenide glass to carry out this spectroscopy. Then, two large-scale studies in recent years in medicine and food safety are exposed. To conclude, the future strategy is presented. It focuses on the development of rare earth-doped fibers used as mid-IR sources on one hand and tellurium-based glasses to shift the limit of detection toward longer wavelength on the other hand.

A crystalline Si thin film solar cell with elliptical silicon nanowire (SiNW)-decorated surface is presented. The elliptical SiNW solar cell performs broadband light absorption in the low energy region while maintaining strong absorption in the high energy region. For the elliptical SiNW solar cells with 1-μm underlying Si substrate, J_sc can be improved to 24.2  mA/cm ² , which is 16% higher than that of the circular SiNW-decorated solar cells. After integration with an Ag reflector, the elliptical SiNW solar cells can achieve higher J_sc of 26.3  mA/cm² , demonstrating a photocurrent enhancement of 12.6% as compared with circular SiNW control device.

Ultracompact wavelength splitters in LiNbO ₃ photonic wires with three different structures for 1.31 and 1.55 μm wavelengths with transverse magnetic (TM) polarization have been presented. The devices are based on the operating principle of a directional coupler in two parallel dielectric waveguides and can be designed by using the finite element method. Calculated steady-state field distributions indicate that 1.31 and 1.55 μm wavelengths are spatially separated. In a particular structure, the transmittances at 1.31 and 1.55 μm wavelength are 99.4% and 95%, respectively. The total lengths of the three different structures are 39.8, 40, and 42.6 μm, respectively. Moreover, the coupling length dependences of the transmissions, the optical transmission bandwidths, and fabrication sensitivities are evaluated, respectively.

Efficient coupling into inner photonic crystal waveguides (IPCW) is critical to the applications of PC in photonic integrated circuits. This paper discusses the highly efficient coupling from surface PCWs into IPCW based on all-PC structures by a modified taper at the interface. The introduction of the modified taper is aimed to compress gradually the extended field propagating at the surface as well as excite the surface mode at the modified oblique surface. Good matching of modal field profiles on the both sides of the interface is achieved by keeping the waveguide structures changeless. The numerical results based on finite-difference time-domain simulations show that the bandwidth for coupling efficiency <90% can be as broad as about 80 nm.

Linear, nonlinear, and total absorption coefficient and refractive index changes of double-quantum well (DQW) systems are studied theoretically in the presence of external static electric field applied along the growth direction. The analytical expression for the linear and nonlinear optical properties is obtained using density matrix method. Emphasis is laid on the effect of asymmetry in the shapes of DQW system on optical properties. Some interesting results are obtained and explained.

High-precision optical measurements depend on the optical sensor’s stability. Light source’s intensity fluctuations are the dominant cause for output instability of the sensor. In order to address this issue, we have developed an optical sensor for reducing influence of the intensity fluctuations on the output stability. The sensor comprises a light source generating smoothly modulated intensity, a measuring channel, a reference channel, and a beam splitter dividing the light source intensity between the channels. The reference channel opens the measuring channel when the light source intensity equals a specific value and the measuring channel converts the input intensity into the output of the sensor. The sensor was designed and studied experimentally at a wavelength of 635 nm and a modulation frequency of 285 Hz. With the sensor, the influence of the intensity fluctuations on the output stability was reduced 110 times and the relative rms instability of 4.5×10 ⁻⁵ was obtained. Techniques to achieve the instability of 1.0×10 ⁻⁵ are also shown.

This article [Opt. Eng.. 53, (1 ), 013109 (2014)] was originally published on 3 February 2014 with Fig. 18 duplicated as Fig. 19. The missing Fig. 19 is reprinted below.