Michael T. Eismann summarizes journal progress and top downloads and citations for 2017.

A design for simultaneous bending-curvature and temperature measurement using a fiber Bragg grating (FBG) inserted between two peanut-shaped structures is presented. The peanut-shaped structure is fabricated in the single-mode fiber by a fusion splicer and then connected with another peanut-shaped structure to form a Mach–Zehnder interferometer (MZI). By measuring the wavelength variation of the MZI and FBG in the spectral response of this configuration, simultaneous bending-curvature and temperature measurement is obtained. The experiment results show that curvature sensitivity is −27.58  nm  /  m  ?  1 and FBG is 0.03869 and 0.01217  nm  /    °  C.

This editorial introduces the special section on Solid-State Lasers.

Q-peak has demonstrated a compact, pulsed eyesafe laser architecture operating with >10  mJ pulse energies at repetition rates as high as 160 Hz. The design leverages an end-pumped solid-state laser geometry to produce adequate eyesafe beam quality (M2∼4), while also providing a path toward higher-density laser architectures for pulsed eyesafe applications. The baseline discussed in this paper has shown a unique capability for high-pulse repetition rates in a compact package, and offers additional potential for power scaling based on birefringence compensation. The laser consists of an actively Q-switched oscillator cavity producing pulse widths <30  ns, and utilizing an end-pumped Nd:YAG gain medium with a rubidium titanyl phosphate electro-optical crystal. The oscillator provides an effective front-end-seed for an optical parametric oscillator (OPO), which utilizes potassium titanyl arsenate in a linear OPO geometry. This laser efficiently operates in the eyesafe band, and has been designed to fit within a volume of 3760  cm3. We will discuss details of the optical system design, modeled thermal effects and stress-induced birefringence, as well as experimental advantages of the end-pumped laser geometry, along with proposed paths to higher eyesafe pulse energies.

We detail the full characterization of the anisotropy of the absorption properties of two different Yb-doped monoclinic borate compounds under polarized light. The studied crystals are Li6(Gd)0.75Yb0.25(BO3)3 and Li6Y0.75Yb0.25(BO3)3, respectively, and were grown by the Czochralski method. We focused on the study of their absorption at the zero line transition as a function of the polarization direction of the incident light for two different crystal cuts of each compound. We discuss the different eigenframes that must be considered in these materials, due to their monoclinic character, as well as the optimum crystal orientation for the considered absorption and show the potential influences when used as laser materials.

We present an investigation into the characteristics of a normal dispersion mode-locked Yb-doped fiber laser using a birefringent plate as a spectral filter. Before implementing mode locking using a birefringent plate, the transmission characteristics of the birefringent plate were analyzed. It was observed that the center wavelength and the full width at half maximum (FWHM) of the transmitted light varied according to the azimuthal angle and optical axis of the birefringent plate. The total dispersion value of the laser cavity was 0.04673  ps2 at a center wavelength of 1.03  μm. The mode locking was therefore achieved in the normal-dispersion region. The total cavity length of the Yb-doped fiber laser was about 1.9 m including free space, and the repetition rate of the mode-locked pulse was ∼104.2  MHz. The FWHM of the optical spectrum was ∼18.09  nm, and the pulse width measured by an autocorrelator was ∼1.71  ps assuming a Gaussian pulse shape. To obtain the transform limited pulse, a pulse was compressed using a pair of diffraction gratings outside the cavity. We achieved a compressed pulse of ∼148  fs assuming a Gaussian pulse shape. The output characteristics of the mode-locked Yb-doped fiber laser were observed while rotating the optical axis of the birefringent plate in the laser cavity. It was found that the center wavelength and FWHM of the optical spectrum could be varied by rotating the optical axis of the birefringent plate. The center wavelength changed by about 18.7 nm from 1026.9 to 1045.6 nm during rotation of the optical axis of the birefringent plate.

In the field of atmospheric research, lidar is a powerful technology that can measure gas or aerosol concentrations, wind speed, or temperature profiles remotely. To conduct such measurements globally, spaceborne systems are advantageous. Pulse energies in the 100-mJ range are required to achieve highly accurate, longitudinal resolved measurements. Measuring concentrations of specific gases, such as CH4 or CO2, requires output wavelengths in the IR-B, which can be addressed by optical-parametric frequency conversion. An OPO/OPA frequency conversion setup was designed and built as a demonstration module to address the 1.6-μm range. The pump laser is an Nd:YAG-MOPA system, consisting of a stable oscillator and two subsequent Innoslab-based amplifier stages that deliver up to 500 mJ of output pulse energy at 100 Hz repetition frequency. The OPO is inherited from the OPO design for the CH4 lidar instrument on the French–German climate satellite methane remote-sensing lidar mission (MERLIN). To address the 100-mJ regime, the OPO output beam is amplified in a subsequent multistage OPA. With potassium titanyl phosphate as nonlinear medium, the OPO/OPA delivered more than 100 mJ of output energy at 1645 nm from 450 mJ of the pump energy and a pump pulse duration of 30 ns. This corresponds to a quantum conversion efficiency of about 25%. In addition to demonstrating optical performance for future lidar systems, this laser will be part of a laser-induced damage thresholds test facility, which will be used to qualify optical components especially for the MERLIN.

We report on a high-power, continuously tunable Tm:CaGdAlO4 (Tm:CALGO) laser with a volume Bragg grating (VBG) as a wavelength selector and a homemade ∼1.7-μm Raman fiber laser as a pump source. Taking the advantages of high diffraction efficiency of VBG, low quantum loss of in-band pumping, and smooth and flat gain spectrum of Tm:CALGO, >5-W output power was achieved in the range of 1945.6 to 1993.7 nm, within a total tuning range from 1919.7 to 1993.7 nm, i.e., 74 nm. The beam quality M2 factors at 5-W output power around 1959.5 nm were 1.31 and 1.32 in the x- and y-directions, respectively.

Optical coherence tomography benefits from the high brightness and bandwidth, as well as the spatial coherence of supercontinuum (SC) sources. The increase of spectral power density (SPD) over conventional light sources leads to shorter measuring times and higher resolutions. For some applications, only a portion of the broad spectral range can be used. Therefore, an increase of the SPD in specific limited spectral regions would provide a clear advantage over spectral filtering. This study describes a method to increase the SPD of SC sources by amplifying the excitation wavelength inside of a nonlinear photonic crystal fiber (PCF). An ytterbium-doped PCF was manufactured by a nanopowder process and used in a fiber amplifier setup as the nonlinear fiber medium. The performance of the fiber was compared with a conventional PCF that possesses comparable parameters. Finally, the system as a whole was characterized in reference to common solid-state laser-based photonic SC light sources. An order-of-magnitude improvement of the power density was observed between the wavelengths from 1100 to 1350 nm.

An integrated navigation system often uses the traditional gyro and star tracker for high precision navigation with the shortcomings of large volume, heavy weight and high-cost. With the development of autonomous navigation for deep space and small spacecraft, star tracker has been gradually used for attitude calculation and angular velocity measurement directly. At the same time, with the dynamic imaging requirements of remote sensing satellites and other imaging satellites, how to measure the angular velocity in the dynamic situation to improve the accuracy of the star tracker is the hotspot of future research. We propose the approach to measure angular rate with a nongyro and improve the dynamic performance of the star tracker. First, the star extraction algorithm based on morphology is used to extract the star region, and the stars in the two images are matched according to the method of angular distance voting. The calculation of the displacement of the star image is measured by the improved optical flow method. Finally, the triaxial angular velocity of the star tracker is calculated by the star vector using the least squares method. The method has the advantages of fast matching speed, strong antinoise ability, and good dynamic performance. The triaxial angular velocity of star tracker can be obtained accurately with these methods. So, the star tracker can achieve better tracking performance and dynamic attitude positioning accuracy to lay a good foundation for the wide application of various satellites and complex space missions.

The patient motion can damage the quality of computed tomography images, which are typically acquired in cone-beam geometry. The rigid patient motion is characterized by six geometric parameters and are more challenging to correct than in fan-beam geometry. We extend our previous rigid patient motion correction method based on the principle of locally linear embedding (LLE) from fan-beam to cone-beam geometry and accelerate the computational procedure with the graphics processing unit (GPU)-based all scale tomographic reconstruction Antwerp toolbox. The major merit of our method is that we need neither fiducial markers nor motion-tracking devices. The numerical and experimental studies show that the LLE-based patient motion correction is capable of calibrating the six parameters of the patient motion simultaneously, reducing patient motion artifacts significantly.

This paper presents a near-real-time stereo matching method using both cross-based support regions in stereo views. By applying the logical AND operator to the cross-based support region in the reference image and target image, we can obtain an intersection support region, which is used as an adaptive matching window. The proposed method aggregates absolute difference estimates in the intersection support region, which are combined with the census transform results. The census transform with a fixed window size and shape is applied, and only the resultant binary code of the pixel in the intersection support region is used. From Middlebury images and their ground truth disparity maps, we compute the area similarity ratio of support regions in stereo views. Then, a conditional probability of observing a correct disparity estimate with respect to the area similarity ratio is examined. By taking a natural logarithm of the probability, a relative reliability weight about the area similarity of support regions is obtained. The initial matching cost is then combined with the reliability weight to obtain the final cost, and the disparity with the minimum cost is chosen as the final disparity estimate. Experimental results demonstrate that the proposed method can estimate accurate disparity maps.

Polarization imaging methods are actively used to study anisotropic objects. A number of methods and systems, such as imaging polarimeters, were proposed to measure the state of polarization of light that passed through the object. Digital holographic and interferometric approaches can be used to quantitatively measure both amplitude and phase of a wavefront. Using polarization modulation optics, the measurement capabilities of such interference-based systems can be extended to measure polarization-dependent parameters, such as phase retardation. Different kinds of polarization rotators can be used to alternate the polarization of a reference beam. Liquid crystals are used in a rapidly increasing number of different optoelectronic devices. Twisted nematic liquid crystals are widely used as amplitude modulators in electronic displays and light valves or shutter glass. Such devices are of particular interest for polarization imaging, as they can be used as polarization rotators, and due to large-scale manufacturing have relatively low cost. A simple Mach–Zehnder polarized holographic setup that uses modified shutter glass as a polarization rotator is demonstrated. The suggested approach is experimentally validated by measuring retardation of quarter-wave film.

Terahertz (THz) radiation is able to penetrate many different types of nonpolar and nonmetallic materials without the damaging effects of x-rays. THz technology can be combined with computed tomography (CT) to form THz CT, which is an effective imaging method that is used to visualize the internal structure of a three-dimensional sample as cross-sectional images. Here, we reported an application of THz as the radiation source in CT imaging by replacing the x-rays. In this method, the sample cross section is scanned in all translation and rotation directions. Then, the projection data are reconstructed using a tomographic reconstruction algorithm. Two-dimensional (2-D) cross-sectional images of the chicken ulna were obtained through the continuous-wave (CW) THz CT system. Given by the difference of the THz absorption of different substances, the compact bone and spongy bone inside the chicken ulna are structurally distinguishable in the 2-D cross-sectional images. Using the filtered back projection algorithm, we reconstructed the projection data of the chicken ulna at different projection angle intervals and found that the artifacts and noise in the images are strikingly increased when the projection angle intervals become larger, reflected by the blurred boundary of the compact bone. The quality and fidelity of the 2-D cross-sectional images could be substantially improved by reducing the projection angle intervals. Our experimental data demonstrated a feasible application of the CW THz CT system in biological imaging.

The modulus of the gradient of the color planes (MGC) is implemented to transform multichannel information to a grayscale image. This digital technique is used in two applications: (a) focus measurements during autofocusing (AF) process and (b) extending the depth of field (EDoF) by means of multifocus image fusion. In the first case, the MGC procedure is based on an edge detection technique and is implemented in over 15 focus metrics that are typically handled in digital microscopy. The MGC approach is tested on color images of histological sections for the selection of in-focus images. An appealing attribute of all the AF metrics working in the MGC space is their monotonic behavior even up to a magnification of 100×. An advantage of the MGC method is its computational simplicity and inherent parallelism. In the second application, a multifocus image fusion algorithm based on the MGC approach has been implemented on graphics processing units (GPUs). The resulting fused images are evaluated using a nonreference image quality metric. The proposed fusion method reveals a high-quality image independently of faulty illumination during the image acquisition. Finally, the three-dimensional visualization of the in-focus image is shown.

To ensure flight safety of an aerial aircraft and avoid recurrence of aircraft collisions, a method of multi-information fusion is proposed to design the key parameter to realize aircraft target detection on a space-based platform. The key parameters of a detection wave band and spatial resolution using the target-background absolute contrast, target-background relative contrast, and signal-to-clutter ratio were determined. This study also presented the signal-to-interference ratio for analyzing system performance. Key parameters are obtained through the simulation of a specific aircraft. And the simulation results show that the boundary ground sampling distance is 30 and 35 m in the mid- wavelength infrared (MWIR) and long-wavelength infrared (LWIR) bands for most aircraft detection, and the most reasonable detection wavebands is 3.4 to 4.2  μm and 4.35 to 4.5  μm in the MWIR bands, and 9.2 to 9.8  μm in the LWIR bands. We also found that the direction of detection has a great impact on the detection efficiency, especially in MWIR bands.

Inspired by the extensive application of terahertz (THz) imaging technologies in the field of aerospace, we exploit a THz frequency modulated continuous-wave imaging method with continuous wavelet transform (CWT) algorithm to detect a multilayer heat shield made of special materials. This method uses the frequency modulation continuous-wave system to catch the reflected THz signal and then process the image data by the CWT with different basis functions. By calculating the sizes of the defects area in the final images and then comparing the results with real samples, a practical high-precision THz imaging method is demonstrated. Our method can be an effective tool for the THz nondestructive testing of composites, drugs, and some cultural heritages.

Linear system theory is employed to make target acquisition performance predictions for electro-optical/infrared imaging systems where the modulation transfer function (MTF) may be imposed from a nonlinear degradation process. Previous research relying on image quality metrics (IQM) methods, which heuristically estimate perceived MTF has supported that an average perceived MTF can be used to model some types of degradation such as image compression. Here, we discuss the validity of the IQM approach by mathematically analyzing the associated heuristics from the perspective of reliability, robustness, and tractability. Experiments with standard images compressed by x.264 encoding suggest that the compression degradation can be estimated by a perceived MTF within boundaries defined by well-behaved curves with marginal error. Our results confirm that the IQM linearizer methodology provides a credible tool for sensor performance modeling.

The convergence of recent advances in optical fabrication and digital processing yields a generation of imaging technology—light-field (LF) cameras which bridge the realms of applied mathematics, optics, and high-performance computing. Herein for the first time, we introduce the paradigm of LF imaging into laryngoscopy. The resultant probe can image the three-dimensional shape of vocal folds within a single camera exposure. Furthermore, to improve the spatial resolution, we developed an image fusion algorithm, providing a simple solution to a long-standing problem in LF imaging.

Atmospheric turbulence is a fundamental problem in imaging through long slant ranges, horizontal-range paths, or uplooking astronomical cases through the atmosphere. An essential characterization of atmospheric turbulence is the point spread function (PSF). Turbulence images can be simulated to study basic questions, such as image quality and image restoration, by synthesizing PSFs of desired properties. In this paper, we report on a method to synthesize PSFs of atmospheric turbulence. The method uses recent developments in sparse and redundant representations. From a training set of measured atmospheric PSFs, we construct a dictionary of “basis functions” that characterize the atmospheric turbulence PSFs. A PSF can be synthesized from this dictionary by a properly weighted combination of dictionary elements. We disclose an algorithm to synthesize PSFs from the dictionary. The algorithm can synthesize PSFs in three orders of magnitude less computing time than conventional wave optics propagation methods. The resulting PSFs are also shown to be statistically representative of the turbulence conditions that were used to construct the dictionary.

A simple dichroic splitter for dual-band line scanning is described. It comprises prisms elements that enable cheapness of the whole prototype by using only one linear detector. Validity of the design is demonstrated via in-line moisture measurement.

Direction finding (DF) systems are fundamental electronic support measures for electronic warfare. A number of DF techniques have been developed over the years; however, these systems are limited in bandwidth and resolution and suffer from a complex design for frequency downconversion. The design of a photonic DF technique for the detection and DF of low probability of intercept (LPI) signals is investigated. Key advantages of this design include a small baseline, wide bandwidth, high resolution, minimal space, weight, and power requirement. A robust postprocessing algorithm that utilizes the minimum Euclidean distance detector provides consistence and accurate estimation of angle of arrival (AoA) for a wide range of LPI waveforms. Experimental tests using frequency modulation continuous wave (FMCW) and P4 modulation signals were conducted in an anechoic chamber to verify the system design. Test results showed that the photonic DF system is capable of measuring the AoA of the LPI signals with 1-deg resolution over a 180 deg field-of-view. For an FMCW signal, the AoA was determined with a RMS error of 0.29 deg at 1-deg resolution. For a P4 coded signal, the RMS error in estimating the AoA is 0.32 deg at 1-deg resolution.

A pulsed TOF laser radar utilizing the single-photon detection mode has been implemented, and its performance is characterized. The transmitter employs a QW double-heterostructure laser diode producing 0.6  nJ/100  ps laser pulses at a central wavelength of ∼810  nm. The detector is a single-chip IC manufactured in the standard 0.35-μm HV CMOS process, including a 9×9 single-photon avalanche diode (SPAD) array and a 10-channel time-to-digital converter (TDC) circuit. Both the SPAD array and the TDC circuit support a time gating feature allowing photon detection to occur only within a predefined time window. The SPAD array also supports a 3×3 SPADs subarray selection feature to respond to the laser spot wandering effect due to the paraxial optics and to reduce background radiation-induced detections. The characterization results demonstrate a distance measurement accuracy of +/−0.5  mm to a target at 34 m having 11% reflectivity. The signal detection rate is 28% at a laser pulsing rate of 100 kHz. The single-shot precision of the laser radar is ∼20  mm (FWHM). The deteriorating impact of high-level background radiation conditions on the SNR is demonstrated, as also is a scheme to improve this by means of detector time gating.

By exploiting the non-Kolmogorov model and Rytov approximation theory, a propagation model of Bessel–Gaussian vortex beams (BGVB) propagating in a subway tunnel is derived. Based on the propagation model, a model of orbital angular momentum (OAM) mode probability distribution is established to evaluate the propagation performance when the beam propagates along both longitudinal and transverse directions in the subway tunnel. By numerical simulations and experimental verifications, the influences of the various parameters of BGVB and turbulence on the OAM mode probability distribution are evaluated, and the results of simulations are consistent with the experimental statistics. The results verify that the middle area of turbulence is more beneficial for the vortex beam propagation than the edge; when the BGVB propagates along the longitudinal direction in the subway tunnel, the effects of turbulence on the OAM mode probability distribution can be decreased by selecting a larger anisotropy parameter, smaller coherence length, larger non-Kolmogorov power spectrum coefficient, smaller topological charge number, deeper subway tunnel, lower train speed, and longer wavelength. When the BGVB propagates along the transverse direction, the influences can be also mitigated by adopting a larger topological charge number, less non-Kolmogorov power spectrum coefficient, smaller refractive structure index, shorter wavelength, and shorter propagation distance.

This paper reports display luminance levels for good visibility under nine ambient illuminance conditions (50, 100, 200, 500, 1000, 2000, 5000, 10,000, and 20,000 lx) for a given white luminance level, chosen from five candidates (100, 200, 500, 1000, and 2000  cd  /  m2), through a psychophysical experiment. This work reveals that the luminance levels for good visibility increase as the maximum white luminance of the display increases. The white luminance dependency of display luminance is caused by the fact that the human visual system adapts to the maximum white luminance and evaluates the brightness of the display based on it. Based on the experimental results, an appropriate luminance zone under various illuminance conditions is proposed. The appropriate luminance zone varies with the maximum white luminance of the displays. This may be understood to mean that there is no absolute luminance level under a given lighting condition. To solve this issue, a new method is proposed to determine optimum luminance levels by considering both visibility and power consumption. By the proposed method, it is reported that the optimum maximum luminance lies between 200 and 500  cd  /  m2 for indoor use (below 500 lx). These results were verified by young adults with normal vision.

A two-dimensional (2-D) shearing interferometer based on an amplitude chessboard grating was designed to measure the wavefront aberration of a high numerical-aperture (NA) objective. Chessboard gratings offer better diffraction efficiencies and fewer disturbing diffraction orders than traditional cross gratings. The wavefront aberration of the tested objective was retrieved from the shearing interferogram using the Fourier transform and differential Zernike polynomial-fitting methods. Grating manufacturing errors, including the duty-cycle and pattern-deviation errors, were analyzed with the Fourier transform method. Then, according to the relation between the spherical pupil and planar detector coordinates, the influence of the distortion of the pupil coordinates was simulated. Finally, the systematic error attributable to grating alignment errors was deduced through the geometrical ray-tracing method. Experimental results indicate that the measuring repeatability (3σ) of the wavefront aberration of an objective with NA 0.4 was 3.4  mλ. The systematic-error results were consistent with previous analyses. Thus, the correct wavefront aberration can be obtained after calibration.

We present a quasicommon-path digital holographic microscopy with phase aberration compensation, which is based on a long-working distance objective and can be used for the quantitative characterization of microstructure specimens. The quasicommon-path arrangement makes the holographic system very compact and stable. Meanwhile, the object and reference beams all travel along the same path, which can effectively eliminate the system aberration, and the mirror in the reference arm can be adjusted precisely for the phase tilt compensation. In the experiment, a wafer with orderly patterns and unified height of 180 nm is measured, and its three-dimensional surface topography is obtained. A long-term system stability of 1.39 nm is achieved in measurement with the proposed method.

We developed a pulsed digital shearography system that utilizes the spatial phase-shifting technique. The system employs a commercial micropolarizer camera and a double pulse laser, which allows for instantaneous phase measurements. The system can measure dynamic deformation of objects as large as 1 m at a 2-m distance during the time between two laser pulses that range from 30  μs to 30 ms. The ability of the system to measure dynamic deformation was demonstrated by obtaining phase wrapped and unwrapped shearograms of a vibrating object.

A deflection angle detecting system (DADS) using a quadrant detector (QD) is developed to achieve the large deflection angle and high linearity for the fast steering mirror (FSM). The mathematical model of the DADS is established by analyzing the principle of position detecting and error characteristics of the QD. Based on this mathematical model, the method of optimizing deflection angle and linearity of FSM is demonstrated, which is proved feasible by simulation and experimental results. Finally, a QD-based FSM is designed and tested. The results show that it achieves 0.72% nonlinearity, ±2.0  deg deflection angle, and 1.11-μrad resolution. Therefore, the application of this method will be beneficial to design the FSM.

Two kinds of polarizer coatings were prepared by electron beam evaporation, using HfO2–SiO2 mixture and HfO2 as the high-refractive-index materials, respectively. The HfO2–SiO2 mixture layer was implemented by coevaporating SiO2 and metal Hf, the materials were deposited at an oxygen atmosphere to achieve stoichiometric coatings. The certain HfO2 and SiO2 content ratio is controlled by adjusting the deposition rate of HfO2 and SiO2 using individual quartz crystal monitor. The spectral performance, surface and interfacial properties, as well as the laser-induced damage performance were studied and compared. Comparing with polarizer coating using HfO2 as high-refractive-index material, the polarizer coating using HfO2–SiO2 mixture as high-refractive-index material shows better performance with broader polarizing bandwidth, lower surface roughness, better interfacial property while maintaining high laser-induced damage threshold.

Light-emitting diode (LED)-based automotive lighting systems pose unique challenges, such as dual-side packaging (front side for LEDs and back side for driver electronics circuit), size, harsh ambient, and cooling. Packaging for automotive lighting applications combining the advanced printed circuit board (PCB) technology with a multifunctional LED-based board is investigated with a focus on the effect of thermal conduction-based cooling for hot spot abatement. A baseline study with a flame retardant 4 technology, commonly known as FR4 PCB, is first compared with a metal-core PCB technology, both experimentally and computationally. The double-sided advanced PCB that houses both electronics and LEDs is then investigated computationally and experimentally compared with the baseline FR4 PCB. Computational models are first developed with a commercial computational fluid dynamics software and are followed by an advanced PCB technology based on embedded heat pipes, which is computationally and experimentally studied. Then, attention is turned to studying different heat pipe orientations and heat pipe placements on the board. Results show that conventional FR4-based light engines experience local hot spots (ΔT>50°C) while advanced PCB technology based on heat pipes and thermal spreaders eliminates these local hot spots (ΔT<10°C), leading to a higher lumen extraction with improved reliability. Finally, possible design options are presented with embedded heat pipe structures that further improve the PCB performance.

Yarn hairiness is one of the essential parameters for assessing yarn quality. Most of the currently used yarn measurement systems are based on two-dimensional (2-D) photoelectric measurements, which are likely to underestimate levels of yarn hairiness because hairy fibers on a yarn surface are often projected or occluded in these 2-D systems. A three-dimensional (3-D) test method for hairiness measurement using a multiperspective imaging system is presented. The system was developed to reconstruct a 3-D yarn model for tracing the actual length of hairy fibers on a yarn surface. Five views of a yarn from different perspectives were created by two angled mirrors and simultaneously captured in one panoramic picture by a camera. A 3-D model was built by extracting the yarn silhouettes in the five views and transferring the silhouettes into a common coordinate system. From the 3-D model, curved hair fibers were traced spatially so that projection and occlusion occurring in the current systems could be avoided. In the experiment, the proposed method was compared with two commercial instruments, i.e., the Uster Tester and Zweigle Tester. It is demonstrated that the length distribution of hairy fibers measured from the 3-D model showed an exponential growth when the fiber length is sorted from shortest to longest. The hairiness measurements, such as H-value, measured by the multiperspective method were highly consistent with those of Uster Tester (r=0.992) but had larger values than those obtained from Uster Tester and Zweigle Tester, proving that the proposed method corrected underestimated hairiness measurements in the commercial systems.

The continuous zoom system with a large zoom ratio can search for a target in the wider field-of-view and distinguish a target at a further distance. As the range of focal length is limited for a traditional continuous zoom system, we present a compound zoom method to greatly increase the range of focal length for a traditional continuous zoom system. The compound zoom method combines the traditional zoom system with a dual field-of-view zoom system to achieve a large zoom ratio. The differential equation model of the compound zoom method is established. Based on this model, a long-wave infrared zoom system with a zoom ratio of 36 and F/1.6 is designed, and it has such advantages as large zoom ratio, simple mechanical structure, good image quality, and smooth cam curves.

Diffractive telescope technology is an innovation solution in construction of large light-weight space telescope. However, the nondesign orders of diffractive optical elements (DOEs) may affect the imaging performance as stray light. To study the stray light characteristics of a diffractive telescope, a prototype was developed and its stray light analysis model was established. The stray light characteristics including ghost, point source transmittance, and veiling glare index (VGI) were analyzed. During the star imaging test of the prototype, the ghost images appeared around the star image as the exposure time of the charge-coupled device improving, consistent with the simulation results. The test result of VGI was 67.11%, slightly higher than the calculated value 57.88%. The study shows that the same order diffraction of the diffractive primary lens and correcting DOE is the main factor that causes ghost images. The stray light sources outside the field of view can illuminate the image plane through nondesign orders diffraction of the primary lens and contributes to more than 90% of the stray light flux on the image plane. In summary, it is expected that these works will provide some guidance for optimizing the imaging performance of diffractive telescopes.

This article presents a method of adjusting steering mirrors to achieve alignment in just two adjustments. The steering mirrors are adjusted in a way that creates a point that the beam always passes through. The beam pivots about this point as the pointing angle is changed. If the fixed point is located at the center of an aperture, the beam can then be pointed straight to a second aperture, thus completing the alignment. The pivot effect happens naturally when the second mirror is adjusted proportionally more than the first. If the extra amount was 10% and the adjustment to mirror 1 was 3 turns then mirror 2 gets 3.3 turns. The first three turns on both mirrors translates the beam sideways and parallel. The extra 0.3 turns the beam inward slightly in the direction opposite translation, so it crosses over the original beam line at a point that remains fixed. Further adjustments at this same ratio will change the angle through that point, and it appears like it pivots. The extra portion is the ratio A  /  B, which is the distance between steering mirrors divided by the distance from mirror 2 to the pivot.

The surface distortion due to gravity can be minimized when the flexure is located at the mirror’s neutral surface. When the mirror has relatively small surface curvature and simple lightweight structure, the location of neutral surface can be calculated by empirical formula or can approximately coincide with the mirror’s center of gravity. Otherwise, it is insufficient to determine the exact location of neutral surface. In addition, when the torque stiffness of flexure cannot be neglected, the optimum mount location may deviate from neutral surface and should be determined by more sophisticated analysis. The RMS surface distortions will degenerate significantly when the flexure has deviations from the optimum axial mount location due to fabrication and assembly errors. The distortion sensitivity to support axial location is critical to achieving low surface distortions in practice. We demonstrate a method of calculating the exact location of neutral surface and optimum mount location. The sensitivity of surface distortion to support axial location is theoretically calculated and compared with the results obtained from finite element analysis.

Three soft-input-soft-output (SISO) detection methods for dual-polarized quadrature duobinary (DP-QDB), including maximum-logarithmic-maximum-a-posteriori-probability-algorithm (Max-log-MAP)-based detection, soft-output-Viterbi-algorithm (SOVA)-based detection, and a proposed SISO detection, which can all be combined with SISO decoding, are presented. The three detection methods are investigated at 128  Gb/s in five-channel wavelength-division-multiplexing uncoded and low-density-parity-check (LDPC) coded DP-QDB systems by simulations. Max-log-MAP-based detection needs the returning-to-initial-states (RTIS) process despite having the best performance. When the LDPC code with a code rate of 0.83 is used, the detecting-and-decoding scheme with the SISO detection does not need RTIS and has better bit error rate (BER) performance than the scheme with SOVA-based detection. The former can reduce the optical signal-to-noise ratio (OSNR) requirement (at BER=10−5) by 2.56 dB relative to the latter. The application of the SISO iterative detection in LDPC-coded DP-QDB systems makes a good trade-off between requirements on transmission efficiency, OSNR requirement, and transmission distance, compared with the other two SISO methods.

Wireless sensor networks have tremendous applications, such as civil, military, and environmental monitoring. In most of the applications, sensor data are required to be propagated over the internet/core networks, which result in backhaul setback. Subsequently, there is a necessity to backhaul the sensed information of such networks together with prolonging of the transmission link. Passive optical network (PON) is next-generation access technology emerging as a potential candidate for convergence of the sensed data to the core system. Earlier, the work with single-optical line terminal-PON was demonstrated and investigated merely analytically. This work is an attempt to demonstrate a practical model of a bidirectional single-sink wireless sensor network-PON converged network in which the collected data from cluster heads are transmitted over PON networks. Further, modeled converged structure has been investigated under the influence of double, single, and tandem sideband modulation schemes incorporating a corresponding phase-delay to the sensor data entities that have been overlooked in the past. The outcome illustrates the successful fusion of the sensor data entities over PON with acceptable bit error rate and signal to noise ratio serving as a potential development in the sphere of such converged networks. It has also been revealed that the data entities treated with tandem side band modulation scheme help in improving the performance of the converged structure. Additionally, analysis for uplink transmission reported with queue theory in terms of time cycle, average time delay, data packet generation, and bandwidth utilization. An analytical analysis of proposed converged network shows that average time delay for data packet transmission is less as compared with time cycle delay.

A joint compensation scheme based on cascaded Kalman filter is proposed, which can implement polarization tracking, channel equalization, frequency offset, and phase noise compensation simultaneously. The experimental results show that the proposed algorithm can not only compensate multiple channel impairments simultaneously but also improve the polarization tracking capacity and accelerate the convergence speed. The scheme has up to eight times faster convergence speed compared with radius-directed equalizer (RDE) + Max-FFT (maximum fast Fourier transform) + BPS (blind phase search) and can track up polarization rotation 60 times and 15 times faster than that of RDE + Max-FFT + BPS and CMMA (cascaded multimodulus algorithm) + Max-FFT + BPS, respectively.

A photonic approach to generating square waveforms using a single dual-parallel Mach–Zehnder modulator driven by a single-tone radio frequency signal is proposed and demonstrated. The two sub MZM are set to be biased at the minimum transmission point and the maximum transmission point, respectively. By simply adjusting the modulation index to have a proper value, optical intensity with expression corresponding to the Fourier expansion of a square waveform is obtained, and square waveforms will be generated. Neither an optical filter nor an electrical component of frequency multiplier or hybrid is needed to guarantee the simplicity and the wide working frequency range. A proof-of-concept experiment is taken. Both 1- and 4-GHz square waveforms fitting well with the ideal ones are successfully generated.

We present a reconfigurable microwave photonic filter (MPF) based on polarization modulation and a tunable optical bandpass filter (TOBPF). The MPF is possible to switch between a low-pass and bandpass responses by simply setting a polarizer. Furthermore, the cutoff frequency of the low-pass MPF, the central frequency, and the bandwidth of the bandpass MPF are widely tunable by adjusting the TOBPF. The proposed MPF is theoretically analyzed and experimentally verified.

Nonintrusive temperature measurements for a real ammonium dinitramide (ADN)-based thruster by using tunable diode laser absorption spectroscopy and monochromatic radiation thermometry are proposed. The ADN-based thruster represents a promising future space propulsion employing green, nontoxic propellant. Temperature measurements in the chamber enable quantitative thermal analysis for the thruster, providing access to evaluate thermal properties of the thruster and optimize thruster design. A laser-based sensor measures temperature of combustion gas in the chamber, while a monochromatic thermometry system based on thermal radiation is utilized to monitor inner wall temperature in the chamber. Additional temperature measurements of the outer wall temperature are conducted on the injector, catalyst bed, and combustion chamber of the thruster by using thermocouple, respectively. An experimental ADN thruster is redesigned with optimizing catalyst bed length of 14 mm and steady-state firing tests are conducted under various feed pressures over the range from 5 to 12 bar at a typical ignition temperature of 200°C. A threshold of feed pressure higher than 8 bar is required for the thruster’s normal operation and upstream movement of the heat release zone is revealed in the combustion chamber out of temperature evolution in the chamber.

A rate-adaptive multilevel coded modulation (RA-MLC) scheme based on fixed code length and a corresponding decoding scheme is proposed. RA-MLC scheme combines the multilevel coded and modulation technology with the binary linear block code at the transmitter. Bits division, coding, optional interleaving, and modulation are carried out by the preset rule, then transmitted through standard single mode fiber span equal to 100 km. The receiver improves the accuracy of decoding by means of soft information passing through different layers, which enhances the performance. Simulations are carried out in an intensity modulation-direct detection optical communication system using MATLAB®. Results show that the RA-MLC scheme can achieve bit error rate of 1E-5 when optical signal-to-noise ratio is 20.7 dB. It also reduced the number of decoders by 72% and realized 22 rate adaptation without significantly increasing the computing time. The coding gain is increased by 7.3 dB at BER=1E-3.

With the rapid development of the smart grid, the data aggregation point (AP) in the neighborhood area network (NAN) is becoming increasingly important for forwarding the information between the home area network and wide area network. Due to limited budget, it is unable to use one-single access technology to meet the ongoing requirements on AP coverage. This paper first introduces the wired and wireless hybrid access network with the integration of long-term evolution (LTE) and passive optical network (PON) system for NAN, which allows a good trade-off among cost, flexibility, and reliability. Then, based on the already existing wireless LTE network, an AP association optimization model is proposed to make the PON serve as many APs as possible, considering both the economic efficiency and network reliability. Moreover, since the features of the constraints and variables of this NP-hard problem, a hybrid intelligent optimization algorithm is proposed, which is achieved by the mixture of the genetic, ant colony and dynamic greedy algorithm. By comparing with other published methods, simulation results verify the performance of the proposed method in improving the AP coverage and the performance of the proposed algorithm in terms of convergence.

A passively Q-switched 2.05-μmTm:Y2O3 ceramic laser, employing Ho:LuAG ceramic as a saturable absorber, was demonstrated for the first time. Under the absorbed pump power of 20.5 W, a maximum output power of 497 mW was obtained. Pulses with a minimum pulse width of 642 ns under the repetition rate of 33 kHz were achieved. Our works validate that Ho-doped materials have good potential for passive Q-switching of Tm-doped lasers at 2-μm wavelength region.

We report the experimental results of ultraviolet-extended broadband supercontinuum (SC) generation in a carefully designed uniform seven-core photonic crystal fiber (PCF) pumped by Ti:sapphire femtosecond laser at 800 nm. Three different PCFs of various core diameters are fabricated to achieve group-velocity matching for ultraviolet components. A wide optical spectrum spanning down to 350 nm is obtained, which is the shortest wavelength SC generation in multicore PCF to date. High spectral flatness (10 dB) has been achieved in the entire visible window.

In a high-power laser system, thermally induced aberrations, which can influence the laser beam quality, are caused by thermal deformations of the optical elements when they are irradiated by the laser. We evaluate the mechanism responsible for the influence of thermally induced aberrations on the laser beam quality factor (M2) using the finite element method and angular spectrum theory. The thermally induced aberrations are decomposed into their constituent Zernike terms, showing that the influence of the thermally induced aberrations on M2 comprises a mixed contribution of different aberrations. The M2 of a fiber laser is simulated and measured when the laser transmits through organic glass. The simulation results have a good consistency with the experimental results, indicating that the method used in this study is effective when analyzing the influence of thermally induced aberrations on M2.

A simple and low-cost continuous liquid-level sensor based on two parallel plastic optical fibers (POFs) in a helical structure is presented. The change in the liquid level is determined by measuring the side-coupling power in the passive fiber. The side-coupling ratio is increased by just filling the gap between the two POFs with ultraviolet-curable optical cement, making the proposed sensor competitive. The experimental results show that the side-coupling power declines as the liquid level rises. The sensitivity and the measurement range are flexible and affected by the geometric parameters of the helical structure. A higher sensitivity of 0.0208  μW/mm is acquired for a smaller curvature radius of 5 mm, and the measurement range can be expanded to 120 mm by enlarging the screw pitch to 40 mm. In addition, the reversibility and temperature dependence are studied. The proposed sensor is a cost-effective solution offering the advantages of a simple fabrication process, good reversibility, and compensable temperature dependence.

The ultraviolet to visible continuum generations are experimentally demonstrated by coupling femtosecond pump pulses around 800 nm into the normal dispersion region close to the zero-dispersion wavelength in the fundamental mode of a silica photonic crystal fiber (PCF). The ultraviolet to visible continuum is generated through a combination of nonlinear effects including degenerate four-wave mixing, cross-phase modulation, and stimulated Raman scattering. When pump pulses with the wavelength of 800 nm and the input average power of 500 mW are used, more than 40% of the incident pump power is converted to the ultraviolet to visible spectral region. The evolution of the output spectra for different PCF lengths is studied to understand the nonlinear propagation dynamics during the process of the continuum generation. The ultraviolet to visible continuum generation will find important applications in the ultraviolet and visible-based biophotonics and spectroscopy.

This paper presents the construction of unipolar transposed modified Walsh code (TMWC) and analysis of its performance in optical code-division multiple-access (OCDMA) systems. Specifically, the signal-to-noise ratio, bit error rate (BER), cardinality, and spectral efficiency were investigated. The theoretical analysis demonstrated that the wavelength-hopping time-spreading system using TMWC was robust against multiple-access interference and more spectrally efficient than systems using other existing OCDMA codes. In particular, the spectral efficiency was calculated to be 1.0370 when TMWC of weight 3 was employed. The BER and eye pattern for the designed TMWC were also successfully obtained using OptiSystem simulation software. The results indicate that the proposed code design is promising for enhancing network capacity.

A scheme to realize low-phase-noise frequency-quadrupled microwave generation without any filter is demonstrated. In this scheme, a multimode optoelectronic oscillator is mainly contributed by dual-parallel Mach–Zehnder modulators, fiber, photodetector, and microwave amplifier. The local source signal is modulated by a child MZM (MZMa), which is worked at maximum transmission point. Through properly adjusting the bias voltages of the other child MZM (MZMb) and the parent MZM (MZMc), optical carrier is effectively suppressed and second sidebands are retained, then the survived optical signal is fed back to the photodetector and MZMb to form an optoelectronic hybrid resonator and realize frequency-quadrupled signal generation. Due to the high Q-factor and mode selection effect of the optoelectronic hybrid resonator, compared with the source signal, the generated frequency-quadrupled signal has a lower phase noise. The approach has verified by experiments, and 18, 22, and 26 GHz frequency-quadrupled signal are generated by 4.5, 5.5, and 6.5 GHz local source signals. Compared with 4.5 GHz source signal, the phase noise of generated 18 GHz signal at 10 kHz frequency offset has 26.5 dB reduction.

A flexible flat optical frequency comb (OFC) generator based on a single integrated polarization-multiplexing dual-drive Mach–Zehnder modulator (PM-DMZM) and a single RF source is proposed and demonstrated. Only one PM-DMZM is required, which guarantees the simplicity. Due to the additional controllable parameter of the polarization as compared with conventional schemes, flat coherent OFCs can be obtained with relatively small modulation indices. A theoretical model is established and an experiment is carried out. Five-, 7-, 9-, 11- and 13-line OFCs with flatness of 0.5, 0.7, 1.2, 1.65, and 2.7 dB are experimentally demonstrated, respectively.

In general, the sound waves can cause the vibration of the objects that are encountered in the traveling path. If we make a laser beam illuminate the rough surface of an object, it will be scattered into a speckle pattern that vibrates with these sound waves. Here, an efficient variance-based method is proposed to recover the sound information from speckle patterns captured by a high-speed camera. This method allows us to select the proper pixels that have large variances of the gray-value variations over time, from a small region of the speckle patterns. The gray-value variations of these pixels are summed together according to a simple model to recover the sound with a high signal-to-noise ratio. Meanwhile, our method will significantly simplify the computation compared with the traditional digital-image-correlation technique. The effectiveness of the proposed method has been verified by applying a variety of objects. The experimental results illustrate that the proposed method is robust to the quality of the speckle patterns and costs more than one-order less time to perform the same number of the speckle patterns. In our experiment, a sound signal of time duration 1.876 s is recovered from various objects with time consumption of 5.38 s only.

We experimentally studied a passive mode-locked square-wave fiber laser with net-normal dispersion by using a nonlinear amplifying loop mirror. The output pulse width of the square-wave pulse (SWP) can be extended with the increasing of the pump power as well as single pulse energy. The SWP can be amplified without shape distortion when the mode-locked laser is injected into an erbium-ytterbium codoped fiber amplifier. Then, by inputting the amplified SWPs into a single mode fiber (SMF), a supercontinuum (SC) can be observed with the spectral range of about 1550 to 1850 nm. Broader and flatter SC with the 20-dB spectral range of about 1550 to 1950 nm can also be generated using a segment of dispersion-shifted fiber instead of SMF.

We investigate the Fresnel zone plate (FZP) inscribed on multimode fiber endface using femtosecond laser ablation and its application in sensing. The mode transmission through fiber tips with FZP is investigated both by the beam propagation method theoretically and by measuring the beam images with a charge-coupled device camera experimentally, which show a good agreement. Such devices are tested for surface-enhanced Raman scattering (SERS) using the aqueous solution of rhodamine 6G under a Raman spectroscopy. The experimental results demonstrate that the SERS signal is enhanced benefiting from focal ability of FZP, which is a promising method for the particular biochemical spectra sensing applications.

A 1645-nm injection-seeded Q-switched Er:YAG master oscillator and power amplifier system is reported. The master oscillator generates single-frequency pulse energy of 11.10 mJ with a pulse width of 188.8 ns at 200 Hz. An Er:YAG monolithic nonplanar ring oscillator is employed as a seed laser. The output pulse from the master oscillator is amplified to 14.33-mJ pulse energy through an Er:YAG amplifier, with a pulse width of 183.3 ns. The M2-factors behind the amplifier are 1.14 and 1.23 in x- and y-directions, respectively. The full width at half maximum of the fast Fourier transformation spectrum of the heterodyne beating signal is 2.84 MHz.

The high peak power hybrid polycrystalline ceramic Ho:YAG laser, resonantly pumped by a Tm:fiber laser at the wavelength of 1907 nm, was reported. With an output coupler of 20% transmission and a 1.5 at.% Ho-doped YAG ceramic sample, up to 1.90 mJ energy/pulse at pulse repetition frequency of 500 Hz and 280-kW peak power with the shortest pulse duration of 6.8 ns, were demonstrated under 5.4 W of incident pump power.

Inspired by the soft, deformable human eye lens, a synthetic polymer gradient refractive index distribution (GRIN) lens with an adaptive geometry and focal power has been demonstrated via multilayer coextrusion and thermoforming of nanolayered elastomeric polymer films. A set of 30 polymer nanolayered films comprised of two thermoplastic polyurethanes having a refractive index difference of 0.05 were coextruded via forced-assembly technique. The set of 30 nanolayered polymer films exhibited transmission near 90% with each film varying in refractive index by 0.0017. An adaptive GRIN lens was fabricated from a laminated stack of the variable refractive index films with a 0.05 spherical GRIN. This lens was subsequently deformed by mechanical ring compression of the lens. Variation in the optical properties of the deformable GRIN lens was determined, including 20% variation in focal length and reduced spherical aberration. These properties were measured and compared to simulated results by placido-cone topography and ANSYS methods. The demonstration of a solid-state, dynamic focal length, GRIN lens with improved aberration correction was discussed relative to the potential future use in implantable devices.

In order to achieve tunable parity-time (PT) symmetry system, we design a PT-symmetry structure including magneto-optical material. We use the transfer matrix method to study the optical properties of the designed structure under the modulation of magnetic field. The structure system takes on twofold unidirectional properties, i.e., the reflectance and the transmittance are dependent on both the incidence direction and the incidence angle. The unique band-edge modes with the system have both enhanced reflectance and transmittance in one incidence direction or angle but lead to an absorption effect in the opposite incidence direction or angle. The reversal of magnetic field direction will all lead to the exchanging of the left band-edge modes and the right band-edge modes.