This editorial recognizes reviewers who served Optical Engineering in 2019, as well as retired SPIE Director of Publications Eric Pepper.

This is a list of reviewers who served OE in 2019.

In the field of laser circumferential detection and laser imaging, there is demand for a simple and small system. A new kind of small linear laser circumferential laser radar using a single detector is proposed. The effects of surface characteristics of two typical targets (a plane and a cylinder) on echo waves of a linear laser are investigated via modeling, theoretical analysis, and an experiment. The results show that the echo waveform is affected by the detection range, target surface condition, and target size. The rising edge, broadening, and arrival time of the echo are affected by the shape of the target. Furthermore, a simple imaging method is proposed in the last part of our study using this circumferential system based on the echo characteristic studied.

The Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) instrument on NASA’s Ionospheric Connection Explorer’s mission will measure neutral winds in the Earth’s thermosphere. We investigate how thermal changes to the instrument’s optical bench affect the relative position of the image recorded by the camera. The thermal shift is measured by fitting the image of a series of reference notches and determining their current position on the camera with subpixel precision. Analyzing ground-based calibration data, we find that the image position is not affected within the uncertainty of the analysis for the applied thermal changes. We also address the question of the analysis uncertainty with signal-to-noise ratio.

To meet the increasing demand for practicality and rapidity of the optical components in aircraft folding optical systems, an aluminum mirror preparation method based on additive manufacturing (AM) technology is proposed. From the perspective of special processing technology for AM, a preparation process chain of additively manufactured mirrors is proposed to address limitations in the design process such as size, support, and connectivity constraints. First, the deposited mirror manufactured by AM technology is prepared. Second, the additively manufactured metal mirror is densified by the hot isostatic pressing process. Then, computer numerical control machining and single-point diamond turning is applied to realize the shape of the surface. Finally, the AM mirror is coated with the gold film. The experimental results show that the surface quality is 0.384λ (peak-to-valley method) and 0.093λ (root mean square method) (λ  =  632.8  nm), and the mirror surface roughness (Ra) is better than 8 nm. After the mirror is subjected to thermal cycles, the surface accuracy does not change significantly, which essentially meets the requirements of the aviation environment adaptability and shows good potential for application in the infrared band.

The flow-field-caused aero-optical effect is becoming a crucial issue in hypersonic flight. The degraded imaging results can seriously affect the aircraft’s ability to capture targets. Experiments were carried out in a Mach 6-gun tunnel, and the imaging quality of a hypersonic optical dome was investigated under six different values of pressure ratio of jet. Background oriented schlieren was used to obtain the point spread functions (PSFs), which were then used to evaluate the imaging quality of the optical dome. The experimental results showed that cooling film with higher pressure will reduce the support field of the PSF and will have a more severe influence on the imaging quality.

A machine vision method based on an eigenvalue realization algorithm was proposed to determine the FY-4 satellite structural modes of vibration. An experiment was designed to prove the feasibility of the proposed method. The experiment results show that the first mode 0.32 Hz and the second mode 1.57 Hz of the FY-4 satellite structure were both clearly distinguished from the background by the proposed method. The comparison of the results of the proposed method and that of simulation and gyroscope testing shows that the error rate of the proposed method is below 0.1% for the first mode and below 6% for the second mode, which is feasible for engineering applications.

Spread spectrum techniques have been implemented in laser remote sensing systems in order to solve the crosstalk problem. The performances of chaotic pulse position modulation (CPPM) as well as orthogonal codes modulation have been compared using correlation analysis and detection probability estimations. Characteristics, such as power efficiency and noise robustness, also have been emphasized. In addition, the experimental results of anticrosstalk and multiple-input multiple-output capabilities of the CPPM have been shown. The results show that the CPPM modulation exhibits excellent performance.

The quantification of remote sensing information requires spectral radiometric calibration technology support with high accuracy, and this technology can ensure the comparability, accuracy, and long-term stability of data acquisition for sensor. Now, the calibration technique traced to the absolute cryogenic radiometer (ACR) is the trend of development. To improve the accuracy of infrared absolute spectral responsivity, a gold-plated hemisphere reflector is added to a thermopile detector with electrical substitution pins. So an infrared electrical substitution radiometer (ESR) is developed as the standard transfer detector. The spectral response linearity, uniformity, and stability of ESR are tested by electric substitution technology. We also have established a radiometric calibration system based on ACR. The combined uncertainty in the spectral responsivity of this detector from 0.7 to 20  μm is about 0.99%. The application of infrared ESR to radiometric calibration can improve the calibration accuracy of detectors through its shorter calibration chain.

Three-dimensional (3-D) reconstruction of a shiny surface is a great challenge for existing structured light techniques because serious measurement errors will occur in image saturation regions in the captured image. To this end, an adaptive microphase measuring profilometry (AMPMP) is proposed. In this method, an optimal frequency selection strategy is introduced to select the frequencies of phase-shifted fringe patterns. The intensity relation between the projected pattern and the captured image is estimated by fitting a nonlinear function after projecting a series of uniform gray-level patterns onto the shiny surface, and the mapping regions of saturated areas in the captured image are computed with the correspondence between the projector image plane and the camera image plane, which is established through MPMP. Based on the estimated intensity relation and the mapping regions, the optimal projection intensities of saturated areas are adaptively adjusted. Then, the adjusted phase-shifted fringe patterns are projected to obtain the absolute phase distribution of the shiny surface. Therefore, accurate 3-D shape reconstruction is realized. The experimental results demonstrated that the proposed AMPMP can well tackle saturated areas of shiny surfaces and has greater performance than other commonly used adaptive fringe projection techniques.

A setup based on the differential interference contrast imaging technique is implemented by replacing the linearly polarized beams from the conventional technique with spatially inhomogeneous polarized beams. This setup works in a reflective mode for material discrimination and for surface measurement, exploiting the information delivered by an analysis of the polarization state. A genetic algorithm that considers Malus’s law adjusts the collected data from a flat reference, and the resulting model is applied to the testing objects. Our study shows that, under the same setup, spatially inhomogeneous polarized beams offer a better height and composite material discrimination, in comparison to the use of linearly polarized beams. We used, as testing objects, reflective composite materials and highly reflective surface structures that have lateral dimensions up to 8 mm and depth variations from 50  μm to 3 mm. These surfaces can be related to applications in the semiconductor and metallic materials industry.

We propose a direct two-dimensional Fourier domain fitting-free method to determine the period of a Ronchi ruling. A precise method to measure a spatial frequency target’s quality and fidelity is highly desired as the pattern period directly affects every aspect of a spatial frequency target-based metrology, including the accuracy and precision of the measurement or evaluations. A standard Talbot experimental apparatus and the Talbot effect are used to obtain and model our data. To determine the period of the ruling directly, only a common digital camera, with a protective glass and an air gap in front of the sensor array, and a Ronchi ruling of chrome deposited on a glass substrate are required. The Talbot effect-based crossing point modeling technique requires no calibration or a priori information but simply the pixel size of the digital camera and a precise means of measuring the spatial frequency from a Talbot image. For a Ronchi ruling with a period specification of 0.1 mm, the nanometric measurement was found to be 0.100010 mm with an error level of 5 nm.

We propose a fringe-projection profilometry technique for shape defects measurement, which can be employed for three-dimensional (3-D) quality inspection. The proposal consists of using a template surface to design phase-shifting algorithms with special-purpose phase response via the frequency transfer function. These algorithms can jointly and directly estimate the spatial phase deviations in a 3-D inspection. Phase deviations correspond to shape differences between the template shape and a testing one. The phase-unwrapping procedure is unnecessary when phase differences are small, as is usual in quality inspections. Experimental results show that our technique is so sensitive that the ripples in a fingerprint can be retrieved.

With regard to the three-dimensional (3D) shape measurement, based on speckle interferometry, of an object with a fine structure beyond the diffraction limit of the objective lens, phase distribution of each speckle region in a speckle pattern and a specklegram is discussed. The experimental results confirmed that the phase distribution with respect to the shape of an object exists spatially even in the speckle region of the speckle pattern. However, the measurement results of a superfine structure by speckle interferometry indicated that the spatial continuity of the phase distribution does not exist in each speckle but in the specklegram. This feature was used to confirm that a wide range of 3D shape measurements of an object with a fine structure can be realized by analyzing specklegrams calculated from speckle patterns before and after a lateral shift in speckle interferometry. Furthermore, it is confirmed that the shape measurement of a nonperiodical structure can be performed.

The characteristics and capability of a homemade all-fiber 1.54-μm pulsed coherent Doppler lidar (CDL) were validated in field experiments by comparing the detection results with a collocated lidar and sounding balloons. With the range gate of 30 m and temporal resolution of 16 s at velocity–azimuth display mode, the detection capability of the CDL ranged from 0.1 to 5 km, and the time sequence and height position of this CDL were calibrated by the collocated lidar. In the intercomparison experiments with sounding balloons, the discrepancy of 30-s averaged measurement results of horizontal wind speed and wind direction was nearly 0.7  m  /  s and 5.3 deg, respectively. The good agreement achieved in such a short averaged time period was a convincing case of intercomparison experiments between CDL and sounding balloon. The CDL system demonstrated good reliability and operational stability in field experiments.

Monitoring the static and dynamic displacements of large engineering structures, such as buildings and bridges, can provide quantitative information for evaluating structural safety and maintenance purposes. Camera calibration is a key process in vision-based sensor systems for remote displacement measurement. Due to the large field of view of engineering structures, conventional camera calibration methods using precise calibration boards are difficult to apply. A modified calibration method for a binocular stereo vision system based on the epipolar constraint relationship is proposed to simplify the calibration process. Due to the absence of reference points in outdoor applications, an unmanned aerial vehicle that carries a reference marker is adopted. During its flight in the field, sequential images are captured simultaneously with the left and right imaging stations. An alternative to determine the scale factor is also proposed, which provides adequate precision for camera calibration. Two important issues are discussed, including the number of reference points and their selection with regards to the depth of view. The experimental results show that the proposed method is convenient to apply in outdoor situations and can achieve high accuracy in displacement measurement.

An approach to characterizing the intrinsic phase response of an optical filter free of additional phase shift from the pigtail or the free space is proposed and experimentally demonstrated. The kernel of this approach is based on the fact that the intrinsic phase response of an optical filter with a minimum phase response has a unique relationship with its magnitude response. Thus, the intrinsic phase response can be obtained through the measurement of the magnitude response only. Most importantly, this method avoids the influence of the fiber pigtail or the free space on the phase response measurement, which cannot be easily calibrated in the traditional single-sideband modulation-based optical vector analyzer. A phase retrieval algorithm based on fast Fourier transform is presented, which can be used to accurately retrieve the intrinsic phase response of an optical filter with either a symmetric magnitude response or an asymmetric one. In the experiment, the intrinsic phase response of an active stimulated-Brillouin-scattering-based optical filter with an asymmetric magnitude response and a phase-shift fiber Bragg grating with pigtails is successfully retrieved from the measured magnitude response using the proposed method.

We report on the application of an interferometric system based on the low-coherence interferometry technique to the dimensional characterization of large opaque mechanical parts as well as microdeformations experienced by them. The implemented scheme is capable of simultaneously measuring very small deformations and relatively large dimensions or thicknesses (several centimeters) of the sample. By applying the chirp Fourier transform algorithm, it was possible to measure changes in thickness with an uncertainty of 0.35  μm when a 7-cm-thick sample was measured. The measurement scheme was implemented in optical fiber, which makes it highly adaptable to industrial conditions. It employs a tunable light source and a Sagnac–Michelson configuration of the interferometric system that allows the thickness of the opaque sample and the topography of both faces to be obtained simultaneously. The developed system can be used to perform profilometry of opaque samples and to analyze the dimensional behavior of mechanical pieces in production lines or under mechanical efforts capable of introducing some deformations on them. This feature enables the system to perform quality control in manufacturing processes.

Low-coherence interferometry (LCI) is a well-established optical method used for obtaining geometric measurements, suited for operating in nonideal environments as shown through its use in biomedical science, where it is referred to as optical coherence tomography. However, work on characterizing these technologies' ability to work in-situ within the area of manufacturing is yet to be demonstrated. This research is motivated by the need to develop robust sensors capable of operating in the harsh environment of manufacturing processes in near real-time, providing on-demand process control for the next generation of precise, and highly adaptive schemes of production. The evaluation of a common-path, lensless, spectral-domain, LCI-based sensor for measurements of step heights in air and in the nonideal operating environment of water is demonstrated. Calibration experiments have explored linearity of measurements over a 1-mm investigated axial range with deviations of the order of ±50  nm in air and ±100  nm in water. Step heights of 8, 7, 6, and 5  μm were measured in air and also with the sample and sensing probe submerged in water. Step heights in both media closely align with calibrated specifications given by the manufacturer demonstrating submicrometer accuracy and a precision of ±56  nm in air and ±76  nm in water.

In the measurement of the 3-D shape of a large curved object using traditional 3-D-digital image correlation (DIC), the left and right camera images are relatively rotated or deformed too much, which will cause the matching to fail. We propose a 3D-DIC matching method. Compared with traditional methods having fixed-size templates, this method is robust to relative rotation and deformation between dual cameras and is suitable for 3D-DIC measurement of nonplanar objects. In the proposed method, we use polar coordinates to define the matching template as a variable circle, which is convenient for interpolation calculation and changing the template size. The zero-mean normalized cross-correlation method is used to determine the position of an entire pixel on the distorted image. This step can provide an initial value close to the true value for subsequent iterations of the inverse compositional Gauss–Newton algorithm. The performance of the proposed method is verified by experiments. Compared with the traditional matching method, the proposed method is suitable for matching large curved surfaces between 3D-DIC dual cameras and extends the application of DIC technology.

Our study investigated an object-detection method based on the faster region-based convolutional neural network (faster R-CNN). The method was designed to determine the center of either a concentric circle or concentric ellipse. Specifically, the central spot of the image (as the object region) can be marked by the bounding box when the circular or elliptical image is used as input data for the faster R-CNN model. The center point of the bounding box can then be calculated according to the coordinates of the upper left and lower right corners, that is, the center position of the concentric circle or concentric ellipse. It is important to determine the center coordinates when taking optical measurements, as the curvature radius of optical components can thus be obtained. The effectiveness of this method is demonstrated through simulation images. Furthermore, we can obtain the center coordinates of the actual Newton’s rings image using the above method; according to the coordinate transformation method, the curvature radius can be estimated based on the center.

We aim to present a design for a submicron polarizer in the near-infrared (IR) wavelength range with a wide bandwidth that can be fabricated using conventional thin-film microfabrication technology to reduce costs. A near-IR transverse magnetic polarizer with 95% transmittance at 2400 nm, more than 1000-nm resonance peak bandwidth, and an extinction ratio (ER) of 300 was designed using a gold wire grid-based guided-mode-resonance filter with a 700-nm grating width and 1200-nm grating period. The ER can be increased by increasing the wire grid thickness, but in this case, its resonance peak moves to longer wavelengths. A cavity resonance was found to exist between two adjacent wire grids when the grid thickness is >800  nm.

A panoramic imaging system has been developed using two afocal lenses and a smart phone that consists of cameras on both sides. An individual afocal lens has been developed with a large field of view (FOV) and a large exit pupil, which can enlarge the FOV of a smart phone camera to above 180 deg. Some issues with the smart phone-based 360-deg panoramic system, such as relative illuminance and assembly tolerance, were analyzed in detail and taken into consideration during the design procedure. A prototype has been developed with a low fabrication cost, yet producing impressive panoramic image quality.

We propose two energy-mapping methods to produce a uniformly illuminated area for a nontilted (straight) and a tilted light source toward a target plane. These energy-mapping methods define the positions of the desired points (the destination points of the refracted rays of a light source) on an illuminated area. The surface of the lenses can then be formed through the position of the desired points. Based on these design methods, two freeform lenses, (i) a symmetric lens and (ii) an asymmetric lens, were designed to provide uniformity within a rectangular illumination footprint for a nontilted and a tilted light source, respectively. This method can produce uniformity for a tilted light source within 0 deg to 45 deg toward the normal vector on the target plane. Two freeform lenses for 0 deg and 20 deg tilted toward a target plane were designed. The illumination footprint of the symmetric and asymmetric freeform lenses was evaluated through ray-tracing simulations and experiments. Both models produce over 90% uniformity within an illuminated area.

Determination of the removal function is an important step in magnetorheological finishing (MRF) of optical materials. However, the removal function is difficult to determine if a spot sample of the same material as that needing polish is unavailable, thus preventing establishment of an MRF process plan. A method to resolve this issue is proposed, wherein an MRF process plan for an item of a differing optical material is migrated. The theoretical basis for migration is investigated, and simulations reveal that narrow nonlinear regions of the source and target materials’ removal functions are a necessary condition for adequate convergence rate. The proposed method is experimentally verified by finishing a ZnS part with the process plan of a BK7 optical part of the same geometry, with a convergence rate of 45.7% after four iterations.

As efficient artificial light sources, nitride-based light-emitting diodes (LEDs) have been widely used. However, III nitride white light-emitting diodes (WLEDs) are mostly fabricated by combining a blue LED chip with yellow phosphors, which results in inevitable problems, such as low color-rendering index (CRI) and detrimental effect to human eyes. As a solution for healthy lighting, we report a strategy to produce a WLED with an emission spectrum perfectly matched with natural daylight. By utilizing near-ultraviolet LED chips and a mixture of blue/cyan/amber/red phosphors, a CRI (Ra) of 97.9 is achieved for the WLEDs at a correlated color temperature of around 5000 K. The resemblance ratio of these solar-spectrum WLEDs with the standard normalized daylight spectrum (5000 K) is found to as high as 93.4%. Residual UV light in the normalized spectrum is <5  %  . The method will benefit the development of high-efficiency healthy artificial light sources.

We describe the method for optical design of afocal achromatic systems that keep their characteristics stable in a wide thermal range without refocusing. Usually, thermal stabilization of the back focal length of an optical system is considered, but these methods are not directly applicable for afocal systems. The method proposed is based on the idea of using a proper combination of optical materials and powers of optical elements to keep the whole system as an afocal one in the desired temperature range. As an additional advantage, the method has shown that afocal systems have enough parameters to achieve achromatic correction and may take into account the F-number of optical elements, which is important for the aberration balance. The research presents both theoretical statements and the example of a laser beam expander for dual wavelengths. The method can also be used to design afocal achromatic compensators that help to achieve thermal stability.

The use of complementary wavelength bands in camera systems is a long-known principle. The camera system’s spectral range is split into several spectral channels, where each channel possesses its own imaging sensor. Such an optical setup is used, for example, in high-quality three-sensor color cameras. A three-sensor camera is less vulnerable to laser dazzle than a single-sensor camera. However, the separation of the individual channels is not high enough to suppress cross talk, and thus, all three channels will suffer from laser dazzling. To solve that problem, we suggest two different optical designs in which the spectral separation of the channels is significantly increased. The first optical design is a three-channel camera system, which was already presented earlier. The second design is a two-channel camera system based on optical multiband elements, which delivers undisturbed color images even under laser dazzle.

A multipoint fiber optic refractometer that uses an algorithm to compensate for the nonlinearity generated by an optical frequency sweeping laser is presented. This compensation allows for designing the multipoint sensing system. Otherwise, only single-point sensing is possible. Also, the nonlinearity compensation, applied to all sensing points simultaneously, permits an improvement in the resolution of each sensing point after the compensation is performed. The mathematical model and experimental results of the proposed multipoint refractive index sensor are presented.

An R-on-1 laser-induced damage-threshold (LIDT) testing method was applied to test coated and uncoated magnesium oxide-doped periodically poled lithium niobate samples pumped by femtosecond Yb:KGW laser pulses at kilohertz and megahertz pulse repetition rates. LIDT values decreased by ∼1.5 and ∼38 times when increasing repetition rate from 100 to 571 kHz and to 76 MHz, respectively. We also investigated nonlinear absorption changes in lithium niobate crystals at the pulse repetition range from 60 to 600 kHz with trains consisting of 100 identical femtosecond pulses. Laser beam transmission in the crystal experienced a drop of ∼18  %   from initial pulses of train to the next 40 pulses at intensities 40% to 15% lower than LIDT due to nonlinear absorption of 220 fs duration pulses at 1.03  μm.

In wireless laser communication, the effects of atmospheric turbulence cause laser spots to appear as irregular speckles in the far-field plane, which may make automatic acquisition, pointing, and tracking systems unable to locate a spot position. We propose an innovative and precise anti-interference, spot-detection algorithm for far-field irregular speckles based on the intensity characteristics of the far-field spots. Before obtaining the spot center coordinates through two-dimensional vector space circle fitting, the proposed algorithm employs computer graphics combined with mathematical morphology to fill the connected domains and extracts light-spot edges using the Canny operator, thus resulting in an accurate detection of far-field spots. The algorithm validation, using an automated experimental platform (atmospheric turbulent refractive-index structure parameter Cn2 is 10^−  15  m^−  2/3), yields a detection threshold accuracy of 30 to 32 pixels (each pixel represents 3  μrad) for 10.2 km far-field spot images. The corresponding deviation satisfies the two-dimensional aiming platform for a minimum step distance of 17.45  μrad, verifying the feasibility and effectiveness of the algorithm.

A potassium dihydrogen phosphate (KDP) crystal is set to be a vital factor in inertial confinement fusion (ICF). This paper focuses on how to reduce the gravitational distortion and increase the second-harmonic generation (SHG) efficiency of the KDP crystal. Multipoint support mounting configurations are proposed. Their influence on distortion and SHG efficiency is analyzed. Mechanical and optical models are established, and the distortion of the KDP crystal is calculated using the finite-element methods. In addition, phase mismatch caused by distortion and SHG efficiency is calculated and analyzed. The numerical results reveal that multipoint support is a suitable mounting configuration as it has little distortion and increases SHG efficiency. Multipoint support mounting configuration is well applied in the ICF facility.

Photonic generation of frequency octupling microwave signal is proposed and demonstrated, which is based on a cascading polarization modulator and a dual-parallel Mach–Zehnder modulator. The method avoids using phase shifter and optical filter, which is suitable for generating a frequency octupling microwave signal with a wideband tuning range and fast tuning speed. In the experiment, a photonic generation frequency octupling microwave signal with 20 GHz is generated. The optical sideband suppression ratio, electrical sideband suppression ratio (ESSR), and phase noise of the photonic generation octupling microwave signal are measured. In addition, the tunability of the proposed scheme is investigated.

Nonline-of-sight (NLOS) ultraviolet (UV) communication is expected to play a prominent role in wireless communications with enhanced connectivity due to the aerosol and strong molecular scattering and availability of large unlicensed bandwidth. We investigate the performance of NLOS UV co-operative communication over a log-normal fading channel by considering the channel estimation error, as experienced in practical UV communication systems. An analytical framework for a dual-hop amplify-and-forward UV NLOS cooperative relay is developed. We derive the closed-form expressions of lower-bound outage probability and the average symbol error rate for the energy efficient modulation schemes, such as hexagonal quadrature amplitude modulation (QAM) and rectangular QAM schemes. The derived analytical results are validated through the Monte Carlo simulations.

The free space optical communication system has become remarkably important nowadays due to huge benefits associated with it. However, atmospheric induced laser beam scintillation remains a challenge that has been addressed using different techniques over the years. Multipulse position modulation has been analyzed over the generalized atmospheric turbulence model (M distribution) using bit error rate (BER), ergodic channel capacity, and outage probability as performance metric measures. Using Meijer’s G functions closed form expressions for the said performance metrics have been derived. Next, using the derived closed form expressions, a thorough investigation of how the BER, ergodic channel capacity, and the outage probability are affected by the atmospheric turbulence conditions and other parameters, such as operational wavelength, has been carried out. It has been observed that increasing the modulation level cannot significantly reduce the transmitted power required for a targeted BER in strong atmospheric turbulence channels. Also the outage probability increases with increasing threshold signal-to-noise ratio.

Here we report simultaneous measurement of strain and temperature, carried out by incorporating a long-period fiber grating (LPFG) written on high birefringence photonic crystal fiber (HBPCF), referred to as an HBPC-LPFG, and a Faraday rotator mirror (FRM) concatenated in series with the LPFG. The HBPC-LPFG used as a sensor head was fabricated by irradiating CO₂ laser pulses to unjacketed HBPCF with line-by-line technique. The HBPC-LPFG connected in series with the FRM exhibits a polarization-independent wavelength-dependent loss spectrum in its reflection spectrum, which has two resonance dips (RDs) with different cladding-mode orders, designated as RDs 1 and 2. As the two RDs in the fabricated HBPC-LPFG ended with the FRM have dissimilar cladding-mode orders, their strain sensitivities and their temperature sensitivities are different from each other. For the two RDs with resonance wavelengths of λ₁  =  ∼1470.5  nm and λ₂  =  ∼1495.1  nm, strain and temperature responses were investigated in an applied strain range from 0 to 1790  με (step: 179  με) and an ambient temperature range from 30°C to 95°C (step: 5°C), respectively. The strain sensitivities of the RDs 1 and 2 were measured to be approximately −0.77 and −1.07  pm  /  με at room temperature, and the temperature sensitivities of the RDs 1 and 2 were measured as ∼10.44 and ∼8.56  pm  /    °  C without applied strain, respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the HBPC-LPFG can be simultaneously estimated from the measured wavelength shifts of the RDs 1 and 2 using their predetermined strain and temperature sensitivities.

The mode of spatial optical in wireless laser communication systems directly affects the coupling efficiency of spatial optics to single-mode fibers. The mode conversion model of a free-spatial optical path based on a liquid crystal spatial light modulator is studied to improve the content of fundamental modes. According to the spatial spectrum filtering relationship between the conversion mode and the target mode, the corresponding conversion transfer function (CTF) is obtained. Combined with the simulated annealing algorithm, the CTF is accelerated locally, and its optimal value is obtained. Therefore, the conversion efficiency from higher-order modes to fundamental mode is generally improved. In addition, an experimental platform is established to verify the rationality of the simulation results. Further, it is indicated that the coupling efficiency can be improved by mode conversion.

An equalization scheme consisting of two square-root-raised-cosine filters and a finite impulse response (FIR) filter is proposed to compensate the channel fading at high frequency induced by light-emitting diodes (LEDs) in visible light communication (VLC). Using the digital equalizer based on this scheme, the performance of the VLC system with different multilevel baseband signals is analyzed experimentally. It is demonstrated that, when the proposed digital equalizer is introduced in the VLC system, the transmission rate is shown to double under forward error correction threshold. As a result, a transmission rate of 120 Mb/s is realized when the LED’s 3-dB bandwidth is only 13 MHz. The number of the FIR tap coefficients was also optimized by considering the transmission performances and computational complexity, which is set to 32 in the adopted digital equalizer. The relation between the system performance improvement and signal Euclidean distance with the same bit per symbol is further investigated. The results reveal that the longer Euclidean distance is beneficial to the improvement of transmission performance for the VLC system with the proposed equalizer.

We investigate the influences of the phase noise induced by the demodulation module on the performance of subcarrier M-ary phase-shift keying (MPSK) optical communication system over turbulence channel. Novel asymptotic expression of average symbol error probability (SEP) is derived based on the Meijer’s G function and Fourier series over Malaga turbulence. The Malaga channel model used in our study can be potentially applied to the performance analysis problems involving other turbulence channels. The influences of the phase noise and its truncation error on the SEP performance are analyzed under different turbulence conditions, phase noise deviation, and modulation order. It is illustrated that the phase noise significantly degrades the performance of the system over turbulence channel. The more truncated number of Fourier series is required to get the minimum truncation error when the phase noise is lower. Under turbulence conditions, the appearance of symbol error rate floor depends predominantly on the phase noise.

We experimentally demonstrated a novel structure for generating an optical frequency comb source for multicarrier modulation in an optical transmission system. In the proposed scheme, the integration of an electroabsorption-modulated laser cascaded with a phase modulator is employed, both of which are driven and synchronized via a common sinusoidal radio frequency signal. The optimal operating range defined as a spectral flatness with less than 3-dB power fluctuation can be obtained through numerical simulation. Using the proposed scheme, we can achieve 10 flat-topped and frequency-locked optical carriers with a 12.5-GHz frequency spacing. On–off-keying intensity-modulated signals with 3.125 and 12.5  Gb  /  s are transmitted error-free over 20 km standard single-mode fiber utilizing the proposed optical frequency comb source for an optical wavelength-division multiplexing transmission system.

A novel Zeonex-based photonic crystal fiber (PCF) with a honeycomb-like cladding structure and a hexagonal slotted core is proposed. The finite-element method is used to numerically analyze the guiding characteristics of the proposed fiber. The investigations are carried out by optimizing various geometrical parameters of the fiber and varying the frequency, core diameter, and core porosity. The results indicate that the designed PCF demonstrates an ultrahigh birefringence of 0.083, extremely low confinement and effective material loss of 10^−  8 and 0.095  cm^−  1, respectively, and a very high core power fraction of 52.2% at an operating frequency of 1.5 THz. In addition, other guiding properties such as numerical aperture, dispersion, and effective area are also analyzed and discussed in detail. The obtained results show that the proposed PCF is extremely suitable for use in numerous low-loss, polarization-maintaining applications in the terahertz frequency range.

The impact of temperature gradients on the average bit error rate (ABER) performance of low-density parity-check (LDPC)-coded underwater wireless optical communication (UWOC) systems is investigated over the generalized gamma fading channels. The effects of absorption and scattering are also taken into account in our computation on the basis of the Monte Carlo (MC) ray-tracing method. The decoded-and-forward multihop communication is applied to extend the viable communication range of UWOC systems. In particular, using the generalized Gauss–Laguerre quadrature rule, the ABER analytical expressions of the multihop UWOC system with binary phase shift keying and multiple phase shift keying modulation schemes are mathematically derived under weak temperature-induced oceanic turbulence condition, respectively, which are also confirmed by MC simulation. In addition, LDPC codes are adopted to improve the UWOC system performance. The results reveal that the ABER performance of this multihop UWOC system degrades with increasing temperature gradients, hop numbers, and modulation orders, while it can be significantly enhanced by LDPC codes and the coding gains increase with the increase of temperature gradients and hop numbers. Our work will benefit the design and development of UWOC system.

The increasing trend of global IP traffic is mainly driven by high-definition video services and cloud computing and storage. Moreover, to maintain a high quality of service in content delivery networking, data are geographically replicated in data centers distributed within network topologies. Thus, data centers are an emerging scenario for research and development aimed at energy-efficient transmission and networking solutions. Previous research work has focused on intradata center energy efficiency while interdata center energy issues have not been extensively analyzed yet. We propose heuristics and meta-heuristics for optimal placement of data centers with minimum power consumption over a network topology relying on flex-grid spectral use. In order to minimize the network’s power consumption, we have performed a detailed comparison of heuristic and meta-heuristic designs for different network scenarios based on real topologies. Moreover, our results show that meta-heuristic provides an optimum data center placement in a reasonable amount of time for a small- to medium-sized network.

The manufacturing cost of coaxial optical communication lasers mainly occurs during the packaging process, and the key technology of the packaging process is the coupling alignment of photonic devices. Therefore, a high-precision and high-efficiency alignment algorithm is of great importance to reduce the cost and improve the quality of coaxial optical communication lasers. The traditional alignment algorithm that is currently used has the following shortcomings: (1) it cannot achieve multidegree-of-freedom alignment at the same time; (2) it easily falls into a local extreme point. Particle swarm optimization (PSO) can achieve simultaneous alignment of multiple degrees of freedom, and it does not easily fall into a local optimum. According to the optical principle, the coupling efficiency function of a coaxial optical communication laser can be obtained. Then, PSO can be used to find the maximum value of the function. The simulation results showed that the PSO algorithm could locate 99% of the maximum optical power. Coupling search can be used for coaxial optical communication laser devices using PSO on packaged devices. The experimental results also proved that the PSO algorithm has high reliability and can realize multidegree of freedom simultaneous search.

A highly linearized microwave photonic link (MPL) based on a dual-drive Mach–Zehnder modulator (DDMZM) is proposed. Eliminating or suppressing these nonlinear distortions is achieved by adjusting the polarization of the light and the frequency of the radio frequency (rf) signal as well as controlling the dc bias voltage of DDMZMs. The proposed technique is verified by the simulation test of two-tone and three-tone. Theoretical analysis and simulation experiments show that under the two-tone condition, when the modulation depth is <0.8, intermodulation distortion (IMD) and even-order harmonic distortion (HD) can be completely eliminated, and third-order HD can be suppressed under the background noise level. Compared with traditional MPL, the spurious free dynamic range of MPL is improved by 11 dB. In addition, in the three-tone test, when the modulation depth is <1, the second-order distortion is completely eliminated and the IMD3 third-order HD are suppressed. The scheme can be used in single octave or multiple octave systems.

The mode, loss, and dispersion characteristics of a buried strip waveguide have been calculated for coarse wavelength division multiplexing wavelengths. Process simulation has been used to form the graded-index core by germanium implantation in silicon. Quasivectorial finite-difference method has been used to calculate the number of propagating modes, effective indices, material absorption, mode confinement, and the dispersion parameters. The scattering loss has been determined from the Payne–Lacey model. The zero dispersion for higher-order modes occurs in the O-band, which is suitable for short-reach multimodal applications. The usable waveguide length and number of modes have been characterized and are limited by the mode with highest propagation loss/dispersion and lowest mode confinement, respectively.

We use the iterative split-step Fourier method to simulate the eight quadrature amplitude modulation all-optical orthogonal frequency division multiplexing (OFDM) system employing different constellation design schemes. It is found that properly narrowing down the modulus difference between constellation points has an impact on the whole system’s ability to resist nonlinear effects. The results show that by means of changing the modulus values of the constellation points, the system bit error rate will be significantly reduced when the optical signal-to-noise ratio is high. Furthermore, by comparing four different constellation diagrams, we propose an alternative solution to reduce the distortion brought by the nonlinear effects on all-optical OFDM systems, that is, the constellation points can be appropriately redesigned to improve the performance of high nonlinear effect systems.

A compact silicon-based polarization beam splitter (PBS) using directional couplers assisted with subwavelength grating (SWG) is proposed and analyzed in detail, where a conventional directional coupler at the input/output end and an SWG coupler in the middle together with SWG-based transitions between them are used. The introduced SWG structures can significantly improve the coupling strength for TE mode, whereas that for TM mode is hardly affected. Subsequently, by carefully choosing structural dimensions, the total coupling length for TM mode can be exactly twice that of TE mode, thus the two polarization states can be effectively separated so that a PBS is realized. Results show that a PBS with a coupling length of 10  μm is achieved, and the extinction ratio, insertion loss, and reflection loss are 23.38/31.78, 0.136/0.104, and −22.03  /    −  26.91  dB, respectively, for TE/TM mode. In addition, fabrication tolerance to the key structural parameters and field evolution through the present device are both presented.

For an image-based tracking loop system of tip-tilt mirror, the traditional control methodologies mainly include a single-position loop or two-position loop. The most effective method for enhancing tracking performance is to increase control gain for a high bandwidth. However, the image sensor sampling rate and time delay engendered by data processing restricts the bandwidth. Therefore, a position-rate control method is proposed to improve the performance of a tip-tilt mirror control system. The angular rate of tip-tilt mirror is calculated from the angular position measured by the linear encoder. The open-loop rate transfer function of tip-tilt mirror features differential in the low-frequency domain because the original tip-tilt control system is zero-type. When the inner rate feedback loop is implemented, an integrator is introduced into the original position loop. A PI (proportional-integral) controller can stabilize the position loop such that two integrators are in the tracking loop, so the low-frequency performance can be improved compared to the original control method. The experimental results coincide with the theoretical analysis and then verify the correctness of the presented theories.

With the widespread availability of electromagnetic (vector) analysis codes for describing the diffraction of electromagnetic waves by periodic grating structures, the insight and understanding of nonparaxial parametric diffraction grating behavior afforded by approximate methods (i.e., scalar diffraction theory) is being ignored in the education of most optical engineers today. We show that the linear systems formulation of nonparaxial scalar diffraction theory enables the development of a scalar parametric diffraction grating model [for transverse electric (TE) polarization] for sinusoidal reflection gratings with arbitrary groove depths and arbitrary nonparaxial incident and diffracted angles. This scalar parametric analysis is remarkably accurate as it includes the ability to redistribute the energy from evanescent orders into the propagating ones, thus allowing the calculation of nonparaxial diffraction efficiencies to be predicted with an accuracy usually thought to require rigorous electromagnetic theory. These scalar parametric predictions of diffraction efficiency are compared to paraxial scalar and rigorous electromagnetic (vector) predictions for a variety of nonparaxial diffraction grating configurations, thus providing quantitative limits of applicability of nonparaxial scalar diffraction theory to sinusoidal reflection gratings as a function of the grating period-to-wavelength ratio (λ  /  d).

We theoretically investigate the characteristics of the multilayered hyperbolic metamaterial (HMM) composed of graphene and discuss the transmission properties from another angle of Fabry–Perot (F–P) resonance analysis. Dispersion characteristics of graphene-dielectric multilayered hyperbolic metamaterials (GDM HMMs) can be adjusted by changing the chemical potential of graphene. Transfer matrix method is improved to adapt the condition of large tangential vectors, and transmission properties are analyzed numerically. Calculated results indicate that dielectric material and graphene codetermine the dispersion properties of the HMMs, and the optical properties can be dynamically adjusted due to the introduction of graphene. Transmission spectra exhibit F–P resonance properties and discussions prove the validity of the F–P cavity theory. However, the transmission characteristics of GDM HMMs are different from the phenomena and laws of the traditional F–P cavity. Further analysis reveals that the mechanism originates from the contribution of graphene and high-k waves in HMMs. We present an innovative perspective for investigating and understanding transmission properties of GDM HMMs and provide references for design of HMMs and other related photonic devices.

Based on the beam propagation method, a compact 1  ×  16 polymeric optical splitter with center wavelength ∼1550  nm is designed for electro-optical printed circuit board (EOPCB). The length of the optical splitter is 20,000  μm. Spacing between each output port is 127  μm. The cross section of the optical splitter is 10  μm  ×  10  μm. The silicon glass is chosen as a substrate material of the optical splitter, the cladding material of the optical splitter is air, and the core material of the optical splitter is SU8 polymer. It is fabricated for a 1  ×  16 chip-to-chips optical interconnect by femtosecond laser. The testing results show that average insertion loss per channel for the optical splitter is I_L  ≤  20  dB and the uniformity for the optical splitter is C_u  ≈  1.42  dB. It is very suitable for the optical interconnection of EOPCB.

A double series configuration of a microresonator is proposed to measure the amount of corrosion on iron metal. A numerical computation has been performed for analyzing the sensing operation in which the metal is attached to the waveguide as a top cladding material. The transparency peak profile and transfer function of the output transmission spectrum is obtained using a signal flow graph method and Mason’s rule. The output spectrum of the microresonator shows that the linear free spectral range (FSR) changes as the iron begins to oxidize, which affects the cladding index of the sensing system. The FSR changes with respect to the amount of corrosion present in iron metal. In addition, the microresonator is able to distinguish two different types of corrosion, which are hematite and magnetite. The sensitivities of the practical application design are obtained as 22.62 nm/RIU for magnetite and 7.17 nm/RIU for hematite detection. The FSR analysis is an alternative approach in all-optical sensing to wavelength shift. It is promising for applications in additional high-sensitivity all-optical corrosion sensors.

For applications in portable electronic devices and machine vision, a small sized focus-tunable lens or lens system is highly desired. Materials with electrically tunable optical refractive indices are required in these fields. A functional material consisting of a WS₂ thin film on a SiC substrate was synthesized and reported in our investigation. A 15-nm-thick WS₂ film was deposited on an n-doped SiC substrate (7.37  ×  10¹⁹  cm^−  3) by pulsed laser deposition. The optical refractive index of the WS₂  /  SiC material was found to be electrically tunable. The value of the effective optical refractive index ranged from 4.03 to 5.84, which implies a 44.9% variation in the refractive index change, at a wavelength of 460 nm under an applied external DC voltage modulated in the range of 0 to 7 V. We also found experimentally that the focal length of the plane lens was tunable from 184 to 156 mm at a wavelength of 650 nm with the applied voltage ranging from 0 to 7 V. Our investigation presents a new way to modulate the refractive index and make the compact focus-tunable lens design convenient.

A compact, dielectric–metal–dielectric surface plasmon polariton (SPP) waveguide is designed that can have propagation lengths as high as several thousand microns. The dielectric layers surrounding the metal waveguide are actually periodic layers of different nanometer-sized dielectric pairs. By proper selection of the surrounding layer widths, a device with a controllable effective index and increased compactness can be designed. It can have a propagation loss <1  dB  /  cm, which is much less than any conventional SPP waveguide. Introduction of a metallic layer outside the dielectric layer of this structure results in a hybrid Tamm plasmon polariton (TPP), which can be used for sensing applications. Its sensitivity and the figure of merit are reported at different operating wavelengths. It is seen that the proposed hybrid TPP structure can be used to detect a larger refractive index range with comparable or better sensitivity than other plasmonic sensors. Determination of different optical parameters required for the design is done using a semianalytical approach. All the necessary steps of the present analysis are discussed and the calculated results are matched and compared with similar results of the other researchers whenever required.