This editorial reviews the most popular OE papers of 2018.

In an optical multiple input multiple output (MIMO) transmission system, mode coupling and mode dispersion have a significant impact on the signal recovery processes at the receiver. We describe a polarization-based MIMO transmission by using mode division multiplexing and polarization division multiplexing over few-mode multicore fiber. We used three optical modes with x-y polarizations for 6  ×  6 MIMO transmission over 75 km length of few-mode multicore fiber at 20 Gbps quadrature phase shift keying signals. Optical MIMO transmission is successfully recovered by using a recursive least squares constant modulus algorithm (RLSCMA). The simulation results show that RLSCMA enhanced the system performance of optical MIMO transmission system by employing sufficient adaptive equalization.

Photonic integrated circuits (PICs), the optical counterpart of traditional electronic integrated circuits, are paving the way toward truly portable and highly accurate biochemical sensors for Department of Defense (DoD)-relevant applications. We introduce the fundamentals of PIC-based biochemical sensing and describe common PIC sensor architectures developed to-date for single-identification and spectroscopic sensor classes. We discuss DoD investments in PIC research and summarize current challenges. We also provide future research directions likely required to realize widespread application of PIC-based biochemical sensors. These research directions include materials research to optimize sensor components for multiplexed sensing; engineering improvements to enhance the practicality of PIC-based devices for field use; and the use of synthetic biology techniques to design new selective receptors for chemical and biological agents.

Increasing demand for using the glass ceramic ZERODUR^® for optical elements with high mechanical loads called for strength data based on statistical samples larger than 20 specimens. The data now available for a variety of practical surface conditions (ground, lapped, and etched) allow stresses by factors 4 to 10 times higher than before. The larger samples revealed that breakage stresses of ground surfaces follow the three-parameter Weibull distribution. The threshold parameter of this distribution reflects the existence of an upper limit for the microcracks depth within such surfaces. It is equivalent to a minimum strength below which breakage probability is zero. Its use in the well-established crack growth theory allows calculating minimum lifetimes including fatigue using the stress corrosion constant for the prevailing environmental humidity. Long-term loading tests have confirmed the validity of the model. For fully etched surfaces, the Weibull statistics fails because in such cases failure mechanism is not unique anymore. Nevertheless, ZERODUR^® with fully etched surfaces that are free from other damages still exhibit minimum breakage stress above 100-MPa tensile stress. The successful satellite mission LISA Pathfinder has confirmed the possibility to apply ZERODUR^® for utmost precision experiments together with high mechanical loads.

We present a mode-optimized two-dimensional piezoelectric fiber actuator and a robust actuation scheme to establish three different scan patterns with the same device. Having multiple scan options, the operator of the actuator is free to choose between improved uniformity, high frame rate to image abrupt biological event, or circular field-of-view to better image cylindrical cavities (gastrointestinal tract, esophagus, etc.) that are cylindrical in nature. We demonstrate the capabilities of the device, through first presenting its mechanical frequency behavior, then offering an actuation scheme to achieve different scan patterns, and finally generating 256 pixel width or diameter raster, spiral, and Lissajous patterns at ≥20 frames per second. With further development, the presented actuator design and drive scheme could serve as a useful tool as part of a minimally invasive optomedical device in the clinic.

In recent years, the compressive line sensing (CLS) active imaging scheme has been proposed for imaging applications over strong scattering medium. This concept has been demonstrated to be effective in the particle-induced scattering mediums and in the turbulence environment through simulations and test tank experiments. Nevertheless, in many atmospheric and underwater surveillance applications, the degradation of the visual environment may come from both particle scattering (turbidity) and turbulence. We study the CLS imaging system in a hybrid environment consisting of simultaneous particle and turbulence-induced scattering for the first time. A CLS prototype is used to conduct a series of experiments at the Naval Research Lab Simulated Turbulence and Turbidity Environment. The imaging path is subjected to various turbulence intensities and turbidities, which maintained stably over experiment duration. The adaptation of the CLS sensing model to the hybrid scattering environment is discussed. The experimental results with different turbidities and turbulence intensities are presented.

Image denoising has become one of the fundamental problems in the field of image processing. We present an iterative image denoising method named as multitexton noise identification and local dissimilarity-based noise removal that incorporates a noise restoration phase followed by a noise identification phase for gray-scale images corrupted by random-valued impulse noise (RVIN). Multiple textons of distinct orientations arranged on the basis of radial symmetry are proposed that give sharp edges of restored images. The noise identification phase works on an adaptive threshold range by employing the local statistics of textons. Moreover, the proposed method can better be utilized to deal with the uncertainty observed in identification of noisy and edge pixels. The dissimilarity of the identified corrupted pixel with its four-connected neighboring pixels is computed by considering the similarity among these four-connected pixels, to restore the gray-level intensities of identified corrupted pixels at the second phase. Standard gray-scale benchmark test images are used to assess the quantitative and visual performance of the proposed method by comparing it with state-of-the-art denoising methods. The extensive experimental results reveal that the presented method outperforms in terms of peak-signal-to-noise ratio, structural similarity index measurement, miss detection, false detection, and visual results for both lower and higher intensities of RVIN as compared to other existing techniques.

Terahertz (THz) homodyne and direct spectroscopic images of low-absorbing materials packaged between up to six layers of a cotton fabric are recorded at 0.3 and 0.6 THz at room temperature. More than two orders of magnitude higher dynamic range is revealed due to the detection in a homodyne scheme, which is realized using paper sheets as a phase-shifting mechanism. It is demonstrated that the homodyne approach can serve as a convenient imaging tool to identify and resolve objects manifesting low absorbance of THz radiation, such as paper tissue, nitrile, and low-density polyethylene concealed in a textile environment.

With the development of artificial intelligence technology, intelligent fringe processing is a goal of relevant researchers in optical interferometry. We propose an intelligent method to achieve fully automated extraction of the fringe skeletons in electronic speckle pattern interferometry (ESPI) based on U-Net convolutional neural network. In the proposed method, the network is first trained by the samples that consist of the noisy ESPI fringe patterns and the corresponding skeleton images. After training, the other multiframe ESPI fringe patterns are fed to the trained network simultaneously; the corresponding skeleton images can be obtained in batches. Using our method, it is not necessary to process fringe patterns frame by frame. Our method is especially suitable for multiframe fringe patterns processing. We apply the intelligent method to one computer-simulated and one real-dynamic ESPI measurement, respectively. For the simulated measurement, it takes just 40 s to obtain the skeleton images of 20 noisy ESPI fringe patterns using our method. Even for low-quality experimental obtained ESPI fringe patterns, our method can also give desired results.

A type of distortion-free single-pixel ghost imaging without lenses is successfully realized by using a mobile phone. The proposed method is not sensitive to imaging distance. When the ratio of imaging distance to the size of the imaging object is close to 1  ∶  2, clear and distortion-free imaging can still be achieved. This imaging entails low requirements for system hardware. A clear image can be realized when the brightness of a mobile phone is 70 lumens. Benefiting from the small volume, high portability, and wide application of mobile phones, as well as the broad application prospects of ghost imaging, the ghost imaging based on mobile phones can provide a portable and distortion-free imaging and detection method with significant applications in biological recognition and health monitoring.

A technology to stabilize the process by inspecting fine cracks in advance before mass and fatal defects occur is important. We propose a vision inspection system for the edge cracks of cold-rolled steel strips. It is important to detect the edge cracks of cold-rolled steel sheets early because they can result in plate breakage. There are two major components to achieve a suitable defect inspection system. The first is an optical system design technique that generates an image that can easily distinguish a defect area from a nondefect area. The second is an automatic detection algorithm technique that can accurately detect the position and shape of a defect in an image generated from an optical system. The optical part is designed using a backlight technique, which places the camera on the top of the strip and irradiates light from the bottom to the top. The defect detection algorithm detects the defects based on morphological operations. It is important to design the structuring elements that determine the performance of the detection algorithm. We optimized the structuring elements using the exhaustive dynamic encoding algorithm for searches. Experiments were conducted on the obtained images by installing the proposed system in actual production lines. The experimental results show the effectiveness of the proposed system.

A three-dimensional (3-D) display with optimized view distribution is proposed. The proposed 3-D display consists of a two-dimensional (2-D) display panel, a pixel mask, and a lenticular lens. The 2-D display panel is used to provide parallax images. The pixel mask has a designed pinhole array and is located close to the 2-D display panel. This pinhole array can modulate the location and size of the pixels on the 2-D display panel to provide an optimized view distribution. Finally, the lenticular lens projects the pixels in different spatial directions to form 3-D images. Different from conventional 3-D displays, the proposed 3-D display can concentrate views into the center area and form a better parallax continuity. A prototype of the proposed 3-D display is developed. Experiment results agree well with the theory.

The image quality of the large-scale, long-focus, large field of view, off-axis three-mirror anastigmat space telescope will decrease dramatically due to the thermal effect. This paper studies the thermal deformation of the telescope, and based on its axial deformation, it proposes a passive thermal compensation design using truss rods made of carbon fiber-reinforced plastics, utilizing its thermal conducting design, which is determined by different spread quantity and spread angle of the lamination. Meanwhile, it satisfies the system stiffness requirement. First, the truss rods are divided into several independent triangles, according to the principle of equal axial distance after deformation, the function in terms of coefficient of thermal expansion (CTE) and length of truss rods is determined. Bonded by the optical tolerance requirement, the range of the CTE is obtained. Second, according to the CTE function, coupled with the stiffness requirement of the whole instrument, the CTE of each truss rod is determined. According to the CTE and stiffness requirement, the design of truss rods lamination is carried out. Finally, the group of truss rods with unequal CTE is obtained. The results have been validated by finite element analysis, vibration experiments, and image test, which indicate that the accuracy of the space telescope studied meets the requirements of optical tolerance; in addition, the rotation angle of the front panel around the X axis is competent, implying the stiffness requirement is solved. Moreover, the modulation transfer function at Nyquist frequency is better than 0.178 under the condition of 20  °  C  ±  5  °  C.

The spatial resolution of hyperspectral image (HSI) is severely limited as a cost for higher spectral resolution. We propose a deep learning-based HSI super resolution method named dual residual dense networks (DRDNs) to overcome the resolution limitation by fusing low-resolution (LR) HSI and high-resolution (HR) multispectral image (MSI). The proposed model uses two symmetric subnets based on residual dense block to fully exploit deep features from HR-MSI and LR-HSI, respectively. Then the features are fused through a feature fusion subnet to generate the final reconstructed HR-HSI. Our DRDNs have realized end-to-end mapping from the original input of LR-HSI and HR-MSI to the desired HR-HSI directly. All the parameters are trained through a unified framework. Some experiments are also carried out to evaluate the performance of our proposed method. The results show that our proposed method gains significant improvement in simultaneously enhancing spatial resolution and preserving spectral consistency when compared to other HSI/MSI fusion methods proposed recently.

Terahertz waves have unique propagation and penetration ability, and these reasons have been widely used in nondestructive testing. Compared with other terahertz imaging approaches, terahertz digital holography can retrieve both quantitative amplitude and phase information about an object wavefront in real time. A continuous-wave terahertz reflective digital holographic method is proposed for measuring the profile of covered objects. Subpixel image registration and image fusion algorithms are presented to expand the field-of-view (FOV) of the system. The validity of the proposed method is verified by the experiment using an optical pumped far-IR gas laser and a pyroelectric array detector. The profile of a metallic bookmark covered by a polytetrafluoroethylene plate is obtained. The FOV is expanded by a factor of 1.75 compared with that of a reconstruction performed employing single hologram.

Windowing the Fourier spectra is usually used to filter the zero-order and twin image in off-axis digital holography (DH). However, during the filtering process, manual operation is often needed. In our work, we present an automatic filtering method. The filtered zone is a circle in the Fourier domain. The point with the highest intensity in the +1st order is chosen as its center, the location of which is related to the angle between the reference and the objective beams. Considering that the size of the zero-order spectra is twice of that of the +1st order, the radius of the circle is set to be a third of the distance between the circle center and the very center of the Fourier spectra. With this filtering method, the desired spectra can be filtered out automatically and noniteratively. This method has been tested and validated on several samples. Compared with the iterative filtering method, it spends about 2‰ of the processing time and obtains the reconstructed images with high similarity. This approach can result in real-time use of the off-axis DH and holographic microscopy.

In past performance analyses and comparisons of midwave infrared (MWIR) and long-wave infrared (LWIR) systems, infrared systems scientists and engineers have not had the cumulative technologies that we will soon enjoy. Large format, small pitch, deep wells, and digital processing do not exist in a single focal plane, but they are a reality now individually and will exist collectively in the near future. How do we best use these technologies, and how do we compare sensors when we use these technologies? From a more fundamental aspect, how do you optimize a system given that practical limits are minimized and theoretical limits apply? Smaller pitch infrared detectors can provide longer range performance for a given aperture and higher photon collection duty cycles (deep wells and faster frame rates) can allow better modulation transfer function correction. Digital image processing allows for recovery of resolution by trading surplus signal-to-noise ratio. Nonuniformity correction becomes an important issue, but there are methods using higher duty cycles to address the problems. LWIR can compete with MWIR using the additional photons given an improved photon collection duty cycle. A holistic approach to system design can provide for an extremely high-performance system. It is also worth mentioning that infrared targeting sensor design in the future shall be quantified with more than just identification range. Since these technologies provide more than a human can consume, the sensors need to be designed to better utilize human consumption limits. An example is that small pitch high-density sensors (solid-state imaging) can provide faster target prosecution, which allows for faster target engagements. We show these possibilities using an LWIR targeting sensor to demonstrate the concept of optimizing pitch-well-processing.

Visible light positioning (VLP) is widely believed to be a cost-effective answer to the growing demand for indoor positioning. There are two critical elements in VLP: positioning accuracy and real-time ability. Nevertheless, because of the nonlinear and highly complicated relationship between three-dimensional world coordinate and two-dimensional image coordinate, developing an effective VLP location algorithm to locate the positioning facility using complementary metal–oxide–semiconductor image sensor is of great need. Additionally, due to the high computational cost of image processing, most existing VLP systems fail to deliver satisfactory performance regarding real-time ability and positioning accuracy, both of which are crucial for the performance of an indoor positioning algorithm. Therefore, we propose a positioning algorithm: the triple-light positioning algorithm based on a situation with three light-emitting diodes (LEDs). The proposed positioning algorithm takes positioning accuracy into consideration. As there are merely binary linear equations to solve, real-time ability is also reflected in the proposed algorithm, which is very worth mentioning because most current positioning algorithms fail to deliver the efficiency. In addition, we treated the LED-ID detection and recognition problem, which is an essential part of the algorithm, as a classification problem in machine learning field and machine learning algorithm into consideration. Experiment results show that the proposed algorithm provides an accuracy of 4.38 cm and the computational time is 65.50 ms, suggesting that the proposed algorithm has a good performance in accuracy as well as real-time ability.

We devote this article to the spectral analysis of volume holograms recorded in materials with diffusion-based formation mechanisms. Two media of different natures were examined: polymer material with dye molecules photoattachable to macromolecules (PQ-PMMA) and photochromic additively colored fluorite crystal (CaF₂). Differential spectra of holograms refractive index and absorbance were determined by two approaches: numerical approximation of both spectral and angular hologram response (selectivity hypercontour) by coupled-wave theory and processing hologram spectra by Kramers–Kronig dispersion relations. The principles of the two approaches, results of their application, and advantages and disadvantages are discussed. Kramers–Kronig relations can be usefully applied in cases where the nature of holograms formation is well studied, whereas the hypercontour approach is indispensable for obtaining the values of absorbance and refractive index modulation; therefore, a combination of both the techniques is a powerful tool for fundamental investigation of the hologram formation mechanisms.

There remains a wide proliferation of second-generation frequency-modulated conical-scan seekers in the hands of irregular forces while the understanding of what makes a jam signal effective remains unclear. It is generally known that the jam-to-signal (J/S) ratio, the jam signal frequency, and the duty cycle are the parameters that need consideration when developing an effective jam code, but the effect of using different jammer waveforms is not generally known. Our study investigates the effect of using different jammer waveforms namely: the fixed carrier, low frequency, amplitude modulation (AM), and frequency-modulation jam codes, for jam signal analysis. Of the tested jam signals, it was found that the AM jam code is most effective in countering the conical-scan seeker due to the amplitude variations created by the jam signal.

An illumination design method that incorporates the entire 3-D geometry between the optical surface and target plane into the ray mapping calculation has been developed. The resulting mapping relationship is integrable along the specific contours of the optical surface instead of a 2-D plane. The method is capable of creating extremely high performance, off-axis designs (97% uniformity at 54-deg off-axis), and since it explicitly accounts for system geometry, appears to be easily generalizable.

In light of the potential threat to aircraft from missiles using ultraviolet (UV) wavebands, it is important to understand the signature of an aircraft and how this can be predicted. This study compares empirical UV signature data to modeled data from camouflage electro-optical simulation (CAMEOSIM) to determine how well the contrast between the object and the background can be predicted using local knowledge of the atmosphere. CAMEOSIM uses the standard moderate resolution atmospheric transmission (MODTRAN) model to estimate the radiative transfer through the atmosphere. Both MODTRAN and CAMEOSIM are well validated in visible and infrared wavebands, and MODTRAN can accurately predict UV radiative transfer. Unfortunately, the work so far has concentrated on bulk transfer to describe the sky background in the UV where the aircraft scene is typically a negative contrast “hole” in a positive sky background. Importantly, path-to-path scattering is a key consideration in this scene since it is this that will tend to blur the edges of an object and reduce the contrast associated with it. A developed understanding of the limitations is required. It is determined that prediction is possible up to the ranges of 5 km. The local visibility (in km) is required for this prediction.

We propose that a magnetorheological (MR) polishing fluid should be considered as a polydispersed suspension system due to its composition and appearance after sedimentation and that fluid can be divided into flocculated MR polishing fluid and deflocculated MR polishing fluid. Two kinds of MR polishing fluid are prepared using different additives, and removal function experiments are carried out on BK7 glass and a silicon modification layer on reaction-bonded silicon carbide, respectively. The performance of flocculated and deflocculated MR polishing fluid is investigated by removal function experiments. The experimental results demonstrate that performance of flocculated MR polishing fluid is superior to deflocculated MR polishing fluid in material removal rate, surface roughness, and removal function stability in long-term polishing processes.

We present polymer-based holograms with varying structure angle for each individual micrometer-sized pixel. The holograms are numerically calculated utilizing an iterative optimization procedure, including a first-order Taylor series expansion to determine the slope of each pixel. The holograms are fabricated by three-dimensional laser lithography. Due to the small pixel sizes and the individual slope of each pixel, aliases can be avoided and the overall intensity of the desired projection is strongly increased compared to holograms consisting of larger and uniform pixels. Furthermore, the discrepancy between calculated and measured intensity distribution is strongly reduced.

Active feedback control is a feasible approach for maintaining the desired shape of a reflector. An architecture for an active reflector is presented in which both piezoelectric ceramic transducer (PZT) and macrofiber composite (MFC) actuators are introduced to reduce both the global and local (high-order) surface errors. An electromechanical finite element model with two types of actuators is developed using the Hamilton principle. An optimal shape controller is then developed to minimize the reflector surface error. Lower-order orthogonal Zernike polynomials are derived in a unit hexagon. The polynomials are considered to be the basic error modes of a reflector and are used to optimize the length of the PZT actuators. The arrangement of MFC actuators can be optimized in a single triangular component of the reflector because the deflections they induced are very local. An experiment is conducted to verify the performance of MFC actuators in a triangular component. The improvement in the control efficiency of the active reflector is demonstrated by two numerical examples.

To develop a white light-emitting diode (LED) phosphor with a better performance, we will make an Er^3  +  /  Tm^3  +  /  Yb^3  + tridoped ZrO₂  /  WO₃ composite nanophosphor through the sol–gel method. Using a 975-nm laser diode, we analyze the upconversion luminescence performance of the sample. The result shows that the Er^3  +  /  Tm^3  +  /  Yb^3  + tridoped sample can emit strong white light and its color temperature is adjustable. The study helps to explore efficient, complex nanophosphors for white LEDs.

A concept for a freeform primary lens system (FPLS) is proposed for designing freeform optics in the direct-backlight system of liquid crystal displays. This FPLS has a super small ratio of height of the optics system h to LED source with diameter D by avoiding the use of traditional secondary optics for LED illumination applications. The FPLS is first designed based on energy conservation principles and ray mapping techniques, and improved through feedback optimization algorithms. Both simulation data and experimental data are employed successively to improve the illumination uniformity on the target plane. Meanwhile, the divergence angle of the FPLS is further improved to achieve a more uniform irradiance distribution pattern of the LED luminaire array on the target plane with an ultrashort projection distance between source and target. We demonstrate the concept by designing and manufacturing an FPLS; the experimental data show that, with h  /  D  =  0.98 of the optical system, the relative standard deviation of the target area reaches 0.22 within the divergence angle of 120 deg.

Offner spectrometer is widely applied in hyperspectral imaging spectrometers, in which the design, fabrication, and testing of convex gratings are crucial to ensure the spectral and imaging performance of the whole system. A conical diffraction Offner spectrometer is proposed with the spectral range of 400 to 900 nm, spectral resolution of 5 nm, slit length of 1 mm, and spectral dispersive width of 10 mm. A finite-element analysis is adopted to optimize the groove parameters of the conical diffraction blazed convex grating that is used in the aforesaid spectrometer. Holographic scan ion beam etching method is employed to fabricate such convex grating. Experimental facilities for testing the diffraction efficiency are built in the lab, showing that the efficiency is higher than 50% in the whole waveband and the peak efficiency is over 75%, which is consistent with the design result. The result indicates that it is feasible to design and optimize the grating groove using the finite-element analysis method.

A self-adaptive probabilistically shaped (PS) star-carrier-less amplitude/phase (CAP)-24 modulated passive optical network (PON) is proposed to adaptively adjust the probability distribution of the signal points. The proposed scheme is capable of accommodating different channels and transmission distances to obtain the optimal probability distribution. The employment of the simulated annealing algorithm in the constellation can significantly reduce the average signal power, as well as facilitating the flexible deployment and enhancement of the PON system. The experimental results show that the self-adaptive optimal signal transmission can be acquired when the probability ratio of signal points located in the three well-designed rings of the CAP-24 constellation is set as 0.724:0.175:0.101. Compared with the general PS-star-CAP-32 signal, the self-adaptive PS-star-CAP-24 signal has a gain of 1.5 dB at the 7% hard decision forward error correction (HD-FEC) limit. What is more, the proposed PS self-adaptive PS-star-CAP-24 can outperform the general PS-star-CAP-24 constellation by 0.2 dB at the 7% HD-FEC limit.

A fine tracking system is crucial for maintaining the accuracy of the optical communication terminals that aim at each other. To ensure the reliability of the communication link, the fine tracking system requires high bandwidth to mitigate the effect of arrival angle fluctuation caused by atmospheric turbulence. Traditionally, the fine tracking system includes only a single feedback loop; a high bandwidth is obtained by increasing the high gain of the fine tracking system, which usually suffers considerably from the time delay engendered by sampling and data processing, the hysteresis nonlinearity of a fast steering mirror (FSM), and the limitations of dynamic response of FSM. To track the beacon in real time and with high precision, a pioneering control method is presented in our paper, namely, double closed-loop control (DCC), which performs better in a tracking system compared with a traditional single-feedback loop. In the inner feedback loop, the response of FSM is measured by a strain gauge sensor (SGS) and used as the inner feedback signal. Thus, by co-operating with the outer CCD-based feedback loop, a DCC scheme is proposed for the fine tracking system. With the SGS signal, the inner loop controller is designed to obtain a rapid response without overshooting; meanwhile, the hysteresis nonlinearity is diminished. Experimental results indicate that the static hysteresis nonlinearity of FSM is reduced from 15.6% to 1.4% by an inner feedback loop, and the dynamic response and stability of FSM is greatly improved, thereby simplifying the outer loop controller design. Then, with the SGS signal, the time delay of the outer loop can be compensated accurately with a predicted signal compensation method. The experimental results show that the −3  dB error rejection bandwidth is increased from 76 to 85 Hz, and the coupling efficiency in our optical communication system is improved by 16.87% after using the DCC fine tracking method. These results indicate that the DCC method can effectively achieve the goal of fast and accurate tracking for optical communications systems.

Multiple transition levels covering S and near-C bands (1460 to 1565 nm) of thulium-doped fiber amplifiers (TDFAs) allows them as potential alternative to erbium-doped fiber amplifiers (EDFAs) hence getting employed in dense wavelength division multiplexing (DWDM) in optical fiber communication. We present overmodulation gain dynamics (OGD) on TDFAs using the EDFA’s Bononi and Rusch (1998) model. The paper portrays overmodulation of digital optical communication signal, of which amplitude was modulated by low-frequency (∼100  kHz), for line-surveillance and management control of optical information in DWDM TDFA systems. Signal-to-signal and signal-to-amplified spontaneous emission (ASE) transfer functions were derived to predict the system’s performance under high input signal power conditions and its variation on input modulation indices. Difference of −10.3  dB (0.093 W) in output signal gain was observed at 1490 nm signal wavelength with ASE. Output modulation index of 15% having high pass signal characteristics as of a lead compensator was observed with the applied signal frequency sweep. Overmodulation sensitivity for various levels of input signal power demonstrates undesirable 30% increment in the output modulation index at 8 dBm and 1490 nm signal wavelength, respectively, at corner frequency.

The hole-coupling method, which is commonly used in the optically pumped THz gas laser, will reduce the beam quality due to the hard-edge diffraction. It is shown that by optimizing the parameters of the resonator a diffraction-limited output beam can be obtained. A method is proposed for the selection of a higher order waveguide mode, whose first zero point along the radial direction coincides with the edge of the output hole coupler and the effect of hard edge diffraction is depressed. Measurements of beam quality are taken on an experimental prototype whose optical setup is designed by this method. A quasi-Gaussian beam output is obtained, and the beam quality M² factor is about 1.15

In the environmental security monitoring application, an optical fiber prewarning system (OFPS) functions not only to locate the intrusion events but also recognize them. As a nonlinear network for recognition, the stochastic configuration network (SCN) is considered a promising method because it does not require setting the network scale beforehand. However, in the specific requirements of the application of OFPS, due to the small feature distance of different intrusion signals to be classified, it is necessary to set a smaller value of error tolerance. However, the side-effect is that meeting the constraint condition faces a challenge. To overcome this, we improve the configuration method of the hidden layer nodes in the SCN network. In the proceeding of the network process, the increment of the hidden layer nodes in each loop is gradually increased, and the space of the corresponding random parameters generated is enlarged. The SCN with variable increments of hidden nodes can adjust the number of hidden nodes added in each loop for continuous construction and obtaining higher classification accuracy. This study has a great significance for the application of SCN in the classification of intrusion signals in OFPS.

A single-mode fiber-optic Fabry–Perot interferometry (FPI) sensor based on optical cross-correlation demodulation is presented, with advantages of both low cost and small size. The sensor mainly consists of a light-emitting diode, a glass wafer, a linear charge-coupled device, a cylindrical lens, a fiber-optic coupler, a sensing head, and a circuit board. The cavity length of the FPI sensor is demodulated by the optical cross-correlation relationship between the glass wafer and the Fabry–Perot cavity. When the cavity length changes, the pixel number corresponding to the peak value of the cross-correlation image shifts accordingly. To optimize the demodulation parameters, a simulation model is established. The experimental results show that the resolution and variation range of cavity length measurement are achieved to be 0.6 nm and ±4  μm, respectively. The proposed fiber-optic sensor is promising for the temperature and pressure measurements in circumstances of intense electromagnetic interference and long distance.

An astigmatism compensated isosceles triangular ring resonator is designed and tested for a subjoule class Nd:YAG laser, considering the thermal lensing effects. The ABCD approach is used and the laser rod is considered as a lens like media whose optical power is induced by dissipated heat in the rod. The beam spot size, divergence angle, and some other parameters were studied for both sagittal and tangential planes and a minimal astigmatic configuration is designed applying dynamic stability condition. It is shown practically that the designed resonator emits a circular TEM₀₀ Gaussian beam (using an aperture). Comparative studies were also done considering a linear resonator especially for alignment sensitivity of the linear and ring resonator.

We propose a modeling methodology tailored to predicting the wavelength and power output from a distributed Bragg reflector laser for use in quantum measurements. The relationship between power, wavelength, current, and temperature is acquired with a genetic algorithm (GA). The function set and termination set for GA are determined from the physical mechanisms of laser current, temperature, and output performance. To verify the validity of the method, measured data are divided into a training group and a test group. The test results show that our models can accurately predict the value of power and wavelength at the given current and temperature, with the RMSE of 13.4  μW and 6.0  ×  10^−  5  nm, respectively. This method can help enhance the output performance of a laser.

A highly sensitive refractive index sensor based on a fiber in-line Mach–Zehnder interferometer sensor based on single-mode–tapered thin-core–single-mode fiber is proposed. We developed a theoretical model using wide-angle beam propagation method in cylindrical coordinates to simulate light propagation performance of such fiber devices. Tapered structures using different taper geometries (linear, exponential, and parabolic taper) with waist diameters as low as 30  μm are presented. The proposed sensor has an ultrahigh sensitivity of 996.4 nm/refractive index unit in glycerin solution with different concentrations in water using the linear taper geometry. The designed structure presents the merits of high sensitivity, which is 6 times higher than that of the thin-core fiber modal interferometer and makes it an excellent candidate for biochemical sensing applications.

Visible light radiated from light-emitting diodes is used to estimate the position of the receiver in a positioning system indoors. Wall reflection and noise are inevitable, which result in an error in the visible light indoor positioning system. Some indoor positioning algorithms based on fingerprint are compared through simulation. An improved indoor positioning algorithm of Bayesian theory is proposed. After using a Gaussian filter, several grid points that are close to each other are selected to be a cluster, and then the posterior probability of each cluster is calculated by the Bayesian algorithm. The cluster with the largest posterior probability is obtained to estimate the receiver’s position. The improved algorithm simplifies the Bayesian algorithm and improves the positioning accuracy, with an average error of 0.18 m.

We present an application of optonumerical methodology for improvement of functional parameters of polymer optical bridges that connect two optical fibers. To fully determine and improve the properties of this kind of microstructure, experimental methods are combined with numerical modeling. The parameters describing the functionality of the polymer optical bridges are optical losses (insertion and return), which determine the usability of those elements as optical fiber splices. These parameters are the function of such features as refractive index distribution, microstructure geometry, and propagation of the light wavelength. To analyze the relation of those features on the functional parameters of the studied microstructures, the experimental results are compared to the ones obtained with simulations. Numerical modeling of the aforementioned optical bridges is performed by means of the finite-difference time-domain method. Experimental methods consist of optical diffraction tomography, which is used to obtain a full three-dimensional refractive index distribution of optical bridge, and measurement of optical losses. Implementation of the proposed methodology in iterative procedure allows the optimization of the fabrication procedure to produce efficient and reliable optical splices with desired functional parameters—insertion loss at the level of 0.2 dB and return loss above 60 dB.

CO₂ sensing using self-fabricated all-fiber gas cells based on a hollow-core photonic crystal fiber and chirped laser dispersion spectroscopy (CLaDS) is presented. I show that CLaDS enables significant reduction of optical fringes arising from the mutlimode nature of the air-core photonic bandgap (PBG) fiber, hence improving the gas sensing system’s stability and performance. By carefully selecting modulation frequency, it is possible to match the period of interference signal (resulting from coupling between fiber’s guided modes) and, therefore, suppress its contribution to the retrieved signal. CLaDS enables successful use of hollow-core PBG fibers in gas sensing applications.

We demonstrate a bidirectional radio-over-fiber system based on optical coupling sing-sideband modulation, which can effectively reduce the impact of nonlinear distortion in transmission. With sidebands heterodyne detection, the system achieves 60-GHz wireless coverage. By optical carrier reusing, the base station can omit the laser source to make the system colorless. Through simulation analysis, the system has the credible transmission property both in downlink and uplink transmissions.

An 808-nm laser diode-pumped narrow line-width Tm  :  Y₂O₃ ceramic laser at 2059 nm was realized using volume Bragg grating (VBG) to provide a spectrally controlled feedback. The reflecting-VBG was used as a wavelength selector and a pump input mirror simultaneously. A maximum output power of 1.81 W with a slope efficiency of about 15.7% with respect to absorbed power was obtained. The central wavelength of the Tm  :  Y₂O₃ ceramic laser was locked at 2059.7 nm with an FWHM linewidth of 0.1 nm. Our work provides a cost-effective method for developing a 2-μm solid-state laser with good wavelength stability.

The signal flow graph method based on Mason’s rule has been used to simulate a serially coupled triple microring resonator (SCTMRR) optical filter. One of the most important parameters of MRRs is the coupling coefficient. Therefore, calculating the optimum value of this parameter is one of the key challenges to design optical filters. The fuzzy logic system has been used to find mentioned coefficients. The fuzzy system inputs are the on–off ratio, shape factor (SF), and propagation loss parameters of the SCTMRR. The obtained results have been verified by comparison with authentic published works. It has been shown that to achieve minimum ripple and maximum SF of filter drop port response, the internal coupling coefficients should be smaller than the external ones. Just on the basis of fuzzy logic results, it has been proved that SF of the SCTMRR filter could not be >0.6.

We propose and demonstrate high-index elliptical epoxy-lensed fiber probes designed specifically for endoscopic common-path (CP) swept-source optical coherence tomography (OCT) imaging inside wet tissues. The high-index epoxy lens uses a beam expansion region of a few hundred microns, which is capped with a spherical surface mounted at the end of an optical fiber. The probe is designed to operate inside wet tissues and shows improved performance over a long imaging depth, compared to bare or simple half-ball-lensed fiber probes. It allows optimal CP-OCT imaging in wet tissues by providing a stable reference power level and optimal beam waist for a long imaging depth. We have fabricated an array of these probes and one with a beam expansion region length of 288  μm and with a lens surface radius of 61  μm exhibiting 25-dB signal-to-noise ratio improvement compared to a bare fiber probe at 1.5-mm distance in water. The proposed probe is tested using a phantom sample comprising several layers of tape in water. The OCT images show that the probe is capable of high-resolution imaging over a distance of 1.5 mm.

Micrometer-sized optical temperature sensors with graphene have been designed and analyzed in order to reach the highest temperature sensitivity. The main idea is to control the temperature of any system by surface graphene material in a micro-two-ring resonator path. By rising the temperature of the system, graphene with 5000  W  /  mK temperature conductivity senses the increase and applies it in the sensor resonance peaks. The prominent features of these sensors are small dimensions, high finesse and sensitivity, safe from electromagnetic interference, high bandwidth, low weight, and low cost. Electronic sensors suffer from electrical disturbances and electromagnetic interference, and sometimes do not function properly. Therefore, this micrometer optical temperature sensor with 44  ×  40  μm dimensions and high sensitivity and finesse in all industrial and medical fields is necessary. The materials used in this sensor are graphene, Si, and SiO₂, which are natural elements and have very high temperature stability. Free-spectral range reaches to 820 GHz and full width at half maximum to 21 GHz, and this quantity increases the finesse and sensitivity of the system, the time delay of sensor is 1.21 ps, which shows the low light dispersion and fast performance time.

A one-dimensional (1-D) photonic crystal (PhC) composed of multilayer thin films having the form (Si  /  Air)₉ is investigated. The selection of Si and air is because their use as a material platform provides the ability to fabricate sensors with other photonic devices on a single chip. We design a biosensor concept that uses an optical resonance in a 1-D silicon PhC with an air defect, which is theoretically studied for refractive index (RI) sensing in the infrared wavelength region (700 to 1900 nm). A similar crystal with a defect layer by changing the dielectric constant in different orders is also studied. A method is presented to maximize the sensitivity of the biosensor and to obtain a very narrow bandwidth cavity mode for good biosensor resolution. We optimized the thickness of the air defect layer in the PhCs on both sides of the nanocavity and show that a high sensitivity value of 1118 nm/refractive index unit (RIU) with a detection limit of 0.001 RIU, which proves the ability of the design to produce biosensor PhC. This optical biosensor has a very much higher sensitivity and simpler structure compared to other PhC biosensors reported previously.

Ceramic YAG  :  Ce^3  + powders and thin transparent coatings consisting of small (<70  nm) nanocrystals were prepared by polymer-salt method. The characterization of obtained materials was conducted by XRD, SEM, and luminescent analysis. The materials calcined at 900°C to 1250°C consist of small (30 to 70 nm) YAG  :  Ce^3  + nanocrystals. The highest quantum yield of 41% was obtained for powders containing cerium doping 0.2 wt.% and consisting of nanocrystals having size about 70 nm.

Waveguide bends are essential structures for designing an integrated optical waveguide modulator (IOWM). However, reducing bending loss for strip and slot waveguide bends and obtaining a small increment of footprint size by bending the radius are challenging issues. We aim to evaluate whether using strip waveguides with an adiabatic bend reduces the bending loss and to obtain a specific length suitable for a V-shaped mode converter with a small increment of footprint size in the IOWM. Furthermore, a new 180-deg-bend structure for slot waveguides, to reduce bending loss, is investigated. The results of reducing bending radius (R) with a large coverage angle (α_c) for strip waveguide bends revealed a low bending loss and simultaneously a reduction of the footprint size in the IOWM. For the 180-deg slot waveguide bend, both the bending and coupling losses were reduced by optimizing the taper and distance offset, which enhanced transmission power efficiency. We validated the feasibility and efficiency of the proposed bending strip and slot structures in IOWM, which is important for integrated optical applications.

The measurement of plant growth’s light environment is the basis of urban agriculture, such as light-emitting diode plant factories. The photosynthetic photon flux density (PPFD) sensors, which measure the photon flux density of radiation in the spectral interval from 400 to 700 nm, cannot measure the spectral components, including the photon flux density of red light band and the photon flux density of blue light band, the light in these two bands are widely used in plant lighting. Facility agriculture, especially plant factories with artificial lighting, needs these data and their ratios by far, so a type of multichannel PPFD sensor is designed to solve these problems. First, we expound the design principle, circuit, and structure of the sensor, and calibrate the unregulated sensors by a standard PPFD sensor (PQS-1, Holland) under the calibration device constructed by controller based on DMX512 protocol. The test results show that the sensor’s average variance is 0.9956, which approximates the ideal linear response value 1. Meanwhile, we make a black opaque soft plastic anticrosslight sleeve using three-dimensional printing to solve the problem of crosslight on the sidewall of the filter, which let the crosslight error coefficient of the red and blue channels reduced by an order of magnitude and approached 0. Finally, we designed a set of light environment control systems, which can autocontrol the PPFD and light quality ratios, and a set of supplemental lighting systems, which can control the daily light integral, based on the sensors.

The optical decoding and forward (ODF) performance with long systematic Hamming distance-4 (LSD_4) codes is suggested. Hereby, the LSD_4 codes are produced via generating 224 codewords of 15 bits length. The LSD_4 codes length is extremely long (equal to 4096 bits); accordingly, the encoding/decoding by the LSD_4 codes becomes more compatible with optical speed and consistency. Apparently, increasing the length of coding is concurrent with the ODF speed. Our model is grounded on hiring the LSD_4 codes in the encoding/decoding process of the onboard ODF for optical routing serial. The net gain of plugging the planned LSD_4 codes in optical routing is obtained within 5- to 15-dB range for diverse optical routing forms at code length 4096 bits with Q-ary pulse-position modulation. The numeric and simulated results confirmed the significant improvements of the planned LSD_4 codes performance with ODF over the non-LSD_4 codes ODF counterparts.

Plastic optical fibers (POFs) have many advantages compared with conventional optical fibers for sensing and monitoring applications in civil engineering. The monitoring capability of POF sensors depends on the strain transfer characteristics between POFs and host materials to a great extent. Theoretical analysis is performed to develop the strain transfer relationship between surface-bonded POFs and concrete. Besides, finite-element method is conducted for verification, and the main influencing parameters are studied respectively. The calculated results demonstrate that the effect of strain transfer is more significant when the POF is bonded close to the surface of concrete and the length of the adhesive layer ought to exceed 300 mm simultaneously, and the width, thickness, and shape of the adhesive layer have slight impact on the strain transfer.

Several commercially available color standards exist, generated by a variety of manufacturers including LabSphere, Lucideon, and Avian Technologies. Previous work has characterized the photometric properties of LabSphere Spectralon targets. Here, we measure the visible and shortwave infrared (VSWIR; 0.4 to 2.5  μm) reflectance at multiple angles and determine the photometric properties of materials manufactured by Lucideon and Avian Technologies for potential use as calibration target materials for the Mars 2020 Mastcam-Z instrument. The Lucideon black, gray 33, green, and cyan samples are found to be significantly forward scattering. The yellow, red, and gray 70 samples are found to be weakly forward scattering. The Avian Technologies AluWhite98 sample was found to be weakly backward scattering. We characterize the absorptions observable and note the occurrence of wavelength-dependent photometric properties. The reflectance and photometric data collected and released here enable the use of these color standards for calibration of data from Mastcam-Z and other Mars-2020 rover instruments as well as provide key information for many other imaging and spectroscopy applications that require the calibration of data from multiple lighting or viewing geometries.

Present research on strain transfer in optical fiber sensors focuses on high-precision, point-bonded fiber Bragg grating sensors. The spatial resolution of a traditional distributed fiber optic sensor (DOFS) reaches submeter or meter, and strain transfer in these sensors is rarely noticed. The accuracy of the DOFS based on optical frequency domain reflectometry used in our paper is improved significantly compared to previous sensors. It has an accuracy of 0.1 and a spatial resolution of 1 mm. Therefore, the strain transfer performance of such a high-precision DOFS should be investigated further. We fixed a DOFS on a host structure using an adhesive, and the factors influencing strain transfer were studied, including the “endpoint effect,” bond length, shear modulus of the coating and adhesive layers, and thickness of the adhesive layer. A theoretical strain transfer model for a DOFS was designed and the theoretical strain transfer coefficient was calculated using numerical simulations. Furthermore, an evaluation of the theoretical model was implemented using a uniform strength beam. The experimental results are consistent with the simplified two-layer strain transfer model presented in our paper. The effect of the bond length on strain transfer is summarized from experimental and numerical simulation results, i.e., a larger adhesive length leads to a higher strain transfer coefficient and smaller “endpoint effect.” If the shear modulus of the fiber coating layer and the glue is high, we obtain a high train transfer coefficient and small “endpoint effect.” These conclusions provide a useful reference for the application of DOFS in practice.

Cerium-doped yttrium aluminum garnet (YAG  :  Ce^3  +) powder phosphor have been extensively studied as phosphors for blue to white light conversion. Y_{(3  −  x)}Al₅O₁₂  :  x Ce^3  + (x  =  2.0 at. %) were successfully synthesized by microwave-assisted combustion (MW) using the organic fuel urea. A direct conversion from the amorphous phase to the cubic one was obtained at 1050°C for 5 h, together with an increase in the particle size into the range of 50 to 60 nm and decrease in the specific surface area. The as-prepared precursors and powder sintered at 1050°C were characterized for their structure, particle size, morphology, electroluminescence properties, and chromaticity by x-ray diffraction, Brunauer–Emmett–Teller method, Fourier transform infrared spectroscopy, field emission-scanning electron microscopy, transmission electron microscopy, electroluminescence, and standard CIE 1931 chromaticity analysis (CIE) chromaticity diagram, respectively. The results show that the obtained sintered YAG  :  Ce^3  + phosphor powder has spherical-shaped particles and strong yellow emission compared to as-prepared phosphor powder. White light emitting diodes with proper color rendering index, and tunable correlated color temperature properties can be produced by controlling the injection currents and coating thickness of the sample, offering daylight white and neutral white LEDs.

We propose an intensity-modulated surface plasmon resonance (SPR) sensor based on a four-layer Kretschmann structural model. To achieve high-sensitivity intensity detection for the measured sample, a silver/gold (Ag/Au) composite film structure is proposed and optimized by a numerical simulation method, and then the chitosan thin film modified by glutaraldehyde cross-linked as an active layer for adsorbing measured sample is applied and immobilized onto the surface of the Ag/Au composite film by spin-coating technique. In addition, a reference beam is introduced into the system to raise its resolution and stability. Measurements are taken while varying the trace copper(II) ion (Cu^2  +) concentration from 0 to 5 ppm, and the SPR sensor is found to possess sensitivities of 0.9147 and 0.4466  ppm^−  1 at Cu^2  + concentrations of 0 to 3 and 3 to 5 ppm, respectively. The concentration resolution is 0.015 and 0.030 ppm for the trace Cu^2  + concentration of 0 to 3 and 3 to 5 ppm, respectively.

A photoconductive antenna (PCA) is modeled using a higher order solution derived from the Boltzmann’s equation to accurately predict anomalous behaviors in this system. In the case concerning the application of high bias fields, photoexcited carriers gain sufficient energies, so that they become hot carriers. Moreover, screening fields in PCAs are numerically investigated in the first picoseconds subsequent to the excitation using hot-carriers effects. To explore another exotic phenomenon in a PCA, its driving photocurrent is decomposed into DC and THz components. Then, the saturation behavior of the THz component peak and the sublinear dependence of the DC component with respect to an externally applied bias are numerically investigated. Furthermore, depending on the accumulation of photoexcited carriers, velocity overshoot phenomenon may occur in the driving photocurrent against the applied bias. These phenomena cannot be predicted by the conventional drift-diffusion model. To simplify the analysis of PCAs, a time-dependent equivalent circuit model is proposed for describing the associated operational principles. Finally, using a coherent detection scheme and the circuit model, it will be shown that owing to the screening fields, the THz power saturates with respect to the applied laser power.

Photopolymer luminescent compositions containing chelate complexes of Eu^3  + with 2-naphthoyltrifluoroacetone and trioctylphosphine oxide as a luminescent dopant have been developed for the photopolymer-based 3-D printing techniques. Photocured samples of photopolymer compositions are transparent in the visible range and have bright red luminescence under the UV excitation. Photophysical studies have revealed that photocured samples possess high quantum yields of luminescence up to 0.59 and monoexponential luminescence decay with 640  μs lifetime at 293 K. It was found that the luminescence of photocured samples is temperature-dependent; the luminescence lifetime of Eu^3  + ions decreases threefold during the heating from 293 to 373 K. The combination of monoexponential luminescence decay and high quantum yield is very promising for printing of temperature sensors.

We demonstrate the design and simulation of intense light beam (ILB) achieved by concave microlens engraved lenticular fibers. Both single and multiple microlenses of spherical and elliptical shapes have been incorporated just inside the fiber core. The ILBs have been designed using commercial OptiFDTD simulator. We investigate the average light intensity and beam profile at several locations including near field, far field-1, and far field-2 after the end face of the optical fibers. For single microlens, elliptical-shaped microlens engraved optical fibers show higher light intensity compared to spherical-shaped microlens engraved fibers. Light intensity at the output of the single-microlens incorporated fibers shows increasing trend with the increase of the radius of curvature (R) of the microlenses, where the highest light intensity has been achieved for R  =    −  17.65  μm. For microlens array engraved lenticular fibers, 9-lens model having R  =    −  8  μm has shown the highest light intensity. Due to the interference of light coming out from neighbor microlenses, the diffraction pattern, i.e., beam profile at any particular plane, is different for different arrangements of the microlenses. The proposed lenticular fibers are expected to have a wide range of applications in micro-optical systems, especially in biomedical engineering.

Absorption enhancement is very important to utilize solar energy in a sustainable way to reduce the excessive consumption of traditional energy resources. We propose a polarization-insensitive broadband one-dimensional grating absorber to investigate the absorption characteristics by using finite-difference time-domain simulations. Results show that the total weighted absorptions are above 98.91% for both TM and TE polarizations at normal incidence covering the whole visible regime. The absorption enhancements are attributed to the Brewster angle and the gradient refractive index for TM polarization and to the gradient refractive index for TE polarization.

We present a high-quality Cr,Nd:YAG crystal with 1.0 at. % Cr^3  + and 1.1 at. % Nd^3  + grown successfully by the Czochralski method. Spectroscopy results indicate that codoped Cr^3  + ions are advantageous for improving pumping efficiency. The maximum average power of 20.24 W operated at 40 Hz and 1.06  μm is obtained on the Cr,Nd:YAG crystal, with electrical-to-optical and slope efficiencies that correspond to 3.00% and 3.77%. Meanwhile, the maximum average power of only 13.32 W is obtained on the Nd:YAG crystal, with the electrical-to-optical and slope efficiencies of 1.97% and 2.47%, respectively. In addition, the laser beam quality of the Cr,Nd:YAG crystal is comparative with that of the Nd:YAG crystal. Therefore, the Cr,Nd:YAG is a promising laser crystal with higher laser performance and stronger radiation-resistant ability than the Nd:YAG crystal. Thus, the Cr,Nd:YAG crystal can be applied to high-power systems and radiant environments.