This PDF file contains the editorial “Top Reviewers of 2014” for OE Vol. 54 Issue 01

This PDF file contains the editorial “OE 2014 List of Reviewers” for OE Vol. 54 Issue 01

This PDF file contains the editorial “Special Section Guest Editorial: Fiber Lasers and Applications” for OE Vol. 54 Issue 01

In this paper, a figure-8 laser based on a semiconductor optical amplifier (SOA) for generating dark pulses is proposed. By adjusting the polarization controller, dark pulses with repetition frequency from 9.10 to 33.44 MHz are generated. The pulse widths are from 74.26 to 19.96 ns. The laser consists of an SOA asymmetrically placed in a short fiber loop. Its switching time is determined by the off-center position of the nonlinear element within the loop. Pulses transmitting in the clockwise and counterclockwise directions have the same widths. Then, the two pulses with the same widths overly together and form the dark pulse.

With microstrain resolution and the capability to sample at rates of 2000 Hz or higher, fiber Bragg grating (FBG) strain sensor offers exciting new possibilities for in situ deformation monitoring induced by blasting load in an open pit slope. Here, we are developing a new technology for measuring deformation in real time on the microstrain in an open pit slope during the blasting. A fiber optically instrumented rock mass strain sensor measured strain at 100-cm intervals along a two anchor rock bolt grouted in the slope intact rock mass. In field testing, a number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the slope stability and blasting cumulative effects. Therefore, FBG sensors are a useful tool for measuring in situ strain in intact rock masses.

Our laboratory is currently studying the experimental thulium fiber laser (TFL) as a potential alternative laser lithotripter to the gold standard, clinical Holmium:YAG laser. We have previously demonstrated the efficient coupling of TFL energy into fibers as small as 100-μm-core-diameter without damage to the proximal end. Although smaller fibers have a greater tendency to degrade at the distal tip during lithotripsy, fiber diameters (≤200  μm) have been shown to increase the saline irrigation rates through the working channel of a flexible ureteroscope, to maximize the ureteroscope deflection, and to reduce the stone retropulsion during laser lithotripsy. In this study, a 50-μm-core-diameter, 85-μm-outer-diameter, low-OH silica fiber is characterized for TFL ablation of human calcium oxalate monohydrate urinary stones, ex vivo. The 50-μm-core fiber consumes approximately 30 times less cross-sectional area inside the single working channel of a ureteroscope than the standard 270-μm-core fiber currently used in the clinic. The ureteroscope working channel flow rate, including the 50-μm fiber, decreased by only 10% with no impairment of ureteroscope deflection. The fiber delivered up to 15.4±5.9  W under extreme bending (5-mm-radius) conditions. The stone ablation rate measured 70±22  μg/s for 35-mJ-pulse-energy, 500-μs-pulse-duration, and 50-Hz-pulse-rate. Stone retropulsion and fiber burnback averaged 201±336 and 3000±2600  μm, respectively, after 2 min. With further development, thulium fiber laser lithotripsy using ultra-small, 50-μm-core fibers may introduce new integration and miniaturization possibilities and potentially provide an alternative to conventional Holmium:YAG laser lithotripsy using larger fibers.

We investigate the generation of a chirped pulse in a single-mode, ring-cavity, erbium-doped fiber laser employing carbon nanotubes (CNTs) as a saturable absorber (SA). The pulse propagation is simulated using analytical methods to understand and quantify the role of multiple SA properties, particularly in the propagation dynamics of the laser pulse. The soliton solution is obtained on the basis of nonlinear effects, such as gain dispersion, second anomalous group-velocity dispersion, self-phase modulation, and two-photon absorption for a generalized nonlinear Schrodinger equation. The influences of the SA parameter in the range from 0.1 to 0.4 on the chirp, power, and width of the soliton are calculated. A stable, passively mode-locked fiber laser using CNTs as an SA is modeled. In addition, the power, width, chirp, and phase of the soliton pulses can be tuned by choosing suitable SA parameters.

Output performances of fiber-based optical systems, in particular fiber lasers and amplifiers, can be controlled using tailored fiber designs, gain profiles, and pump light overlap with the gain medium. Here, the performances of 2-μm light, propagating in three large-mode area fibers, a step-index fiber, a photonic crystal fiber (PCF), and a leakage channel fiber (LCF), designed to deliver a single-mode (SM) beam at this wavelength, were compared. Using the S² imaging technique, the transverse mode content has been decomposed, and propagation losses, SM purity, and mode-field area (MFA) were measured for various input mode overlap and coiling diameters. It was experimentally demonstrated that coiling the PCF and LCF to 40 and 20 cm in diameter, respectively, resulted in efficient higher-order mode suppression, pure SM beam delivery, moderate (∼1  dB ) coil-induced losses in the fundamental mode, and nondistorted, large MFA (∼1600  μm² ) beam delivery.

A monolithic coherent combiner scheme for combining multiple fiber lasers based on a photonic crystal fiber is described. Beam propagation method (BPM) simulations show that the beam combiner efficiency can reach 96% for a 4×1 combiner, 94% for an 8×1 combiner, and 91% for a 16×1 combiner, provided the fiber lasers are phase matched. In addition, a 2×1 intensity polarization combiner is proposed and simulated through full vectorial BPM, yielding a combining efficiency of 95%. This concept can lead to a rugged and efficient combiner for multiple fiber lasers.

The process observation in selective laser melting (SLM) focuses on observing the interaction point where the powder is processed. To provide process relevant information, signals have to be acquired that are resolved in both time and space. Especially in high-power SLM, where more than 1 kW of laser power is used, processing speeds of several meters per second are required for a high-quality processing results. Therefore, an implementation of a suitable process observation system has to acquire a large amount of spatially resolved data at low sampling speeds or it has to restrict the acquisition to a predefined area at a high sampling speed. In any case, it is vitally important to synchronously record the laser beam position and the acquired signal. This is a prerequisite that allows the recorded data become information. Today, most SLM systems employ f-theta lenses to focus the processing laser beam onto the powder bed. This report describes the drawbacks that result for process observation and suggests a variable retro-focus system which solves these issues. The beam quality of fiber lasers delivers the processing laser beam to the powder bed at relevant focus diameters, which is a key prerequisite for this solution to be viable. The optical train we present here couples the processing laser beam and the process observation coaxially, ensuring consistent alignment of interaction zone and observed area. With respect to signal processing, we have developed a solution that synchronously acquires signals from a pyrometer and the position of the laser beam by sampling the data with a field programmable gate array. The relevance of the acquired signals has been validated by the scanning of a sample filament. Experiments with grooved samples show a correlation between different powder thicknesses and the acquired signals at relevant processing parameters. This basic work takes a first step toward self-optimization of the manufacturing process in SLM. It enables the addition of cognitive functions to the manufacturing system to the extent that the system could track its own process. The results are based on analyzing and redesigning the optical train, in combination with a real-time signal acquisition system which provides a solution to certain technological barriers.

We propose an approach to Brillouin optical frequency domain analysis for measuring temperature or strain along an optical fiber. In this approach, we modulate the pump by a cosine function over a range of frequencies. By measuring only the probe power at the edge of the fiber for each modulation frequency, we obtain the cosine transform of the Brillouin gain coefficient of the fiber. We describe the operation principle of the proposed method, introduce additional advantages of the frequency domain analysis, such as compressed representation of the spatial information and better field of view, and use simulations and experimental work to demonstrate the capabilities of the proposed technique.

The surfaces of laser-induced forward transfer (LIFT) printed metal structures show typical roughness characteristic of the metal droplet size (3 to 10  μm). Submicron voids are often observed in the bulk of such printed metal structures with consequences on the mechanical strength, chemical resistivity, and electrical conductivity. We present the results of our efforts to reduce surface roughness and bulk voids by controlled laser melting. We have used temporally shaped pulses from a fiber laser tunable in the range from 1 to 600 ns in order to improve the quality of LIFT printed copper and aluminum structures. For the best case shown, roughness was improved from R_RMS=0.8  μm to R_RMS=0.2  μm and the relative percentage of the voids was reduced from 7.3% to 0.9%.

Industrial production constraints often require technical tests and controls. Optical metrology methods allow a non destructive test of wide range of parameters, such as defects and displacements, with very good accuracy. The phase retrieval is an effective way that allows three-dimensional profile reconstruction from intensity shearograms. This research work focuses on the extraction of the phase from one uncarrier shearogram using the Hilbert–Huang transform. An algorithm for the phase calculation based on the bidimensional empirical mode decomposition, Hilbert transform (HT), and Fourier transform (FT) is presented. A spatial digital carrier has been superimposed before the application of the FT or HT which uses two π2 shifted shearograms, to get access to the phase map via a global analysis of intensity images. An evaluation was made through a numerical simulation to validate and confirm the performance of the proposed algorithm. The main advantage of this technique is its ability to provide a metrological solution for fast dynamic analysis.

The image spectrum signal-to-noise ratio (SNR) provides a means of estimating the noise effective spatial resolution of an imaging system and a means of estimating the highest spatial frequency which can be reconstructed with a postdetection image reconstruction algorithm. Previous work has addressed the effects of aerosol scattering on the overall point spread function (PSF). Here, we seek to extend these results to also account for the effects of measurement noise and to then estimate the noise effective resolution of the system, which accounts for scattering effects on the PSF and measurement noise in the detector. We use a previously published approach to estimating the effective PSF and radiometric calculations to estimate the mean numbers of direct and scattered photons detected by an imaging system due to reflected radiation in the visible and near-infrared, and emitted radiation in mid-infrared (MIR) band, for a horizontal near-ground imaging scenario. The analysis of the image spectrum SNR presented here shows a reduction in the value of noise effective cutoff spatial frequency for images taken through fog aerosol media, and hence emphasizes the degrading effect of fog aerosol models on the spatial resolution of imaging systems.

A method has been proposed to realize a transparent volumetric display using multiple mini-projectors and a rotating screen. Correct two-dimensional cross-sectional images are projected on a bidirectional scattering projection screen, which rotates to form a three-dimensional (3-D) image due to human vision persistence. An illumination subsystem is designed to ensure the accurate synchronization between the projectors and the rotating screen. Therefore, low-speed and low-cost miniature display devices can be used in the mini-projectors to realize dynamic volumetric imaging, which can satisfy all criteria of real 3-D vision with full color and high resolution. Experimental results of volumetric imaging realized by this method are also presented.

For the purpose of image distortion caused by the oblique photography of a zoom lens aerial camera, a fast and accurate image autorectification and mosaicking method in a ground control points (GCPs)-free environment was proposed. With the availability of integrated global positioning system (GPS) and inertial measurement units, the camera’s exterior orientation parameters (EOPs) were solved through direct georeferencing. The one-parameter division model was adopted to estimate the distortion coefficient and the distortion center coordinates for the zoom lens to correct the lens distortion. Using the camera’s EOPs and the lens distortion parameters, the oblique aerial images specified in the camera frame were geo-orthorectified into the mapping frame and then were mosaicked together based on the mapping coordinates to produce a larger field and high-resolution georeferenced image. Experimental results showed that the orthorectification error was less than 1.80 m at an 1100 m flight height above ground level, when compared with 14 presurveyed ground checkpoints which were measured by differential GPS. The mosaic error was about 1.57 m compared with 18 checkpoints. The accuracy was considered sufficient for urgent response such as military reconnaissance and disaster monitoring where GCPs were not available.

Laser scanning systems that simultaneously measure multiple wavelength reflectances integrate the strengths of active spectral imaging and accurate range measuring. The Finnish Geodetic Institute hyperspectral lidar system is one of these. The system was tested in an outdoor experiment for detecting man-made targets from natural ones based on their spectral response. The targets were three camouflage nets with different structures and coloring. Their spectral responses were compared against those of a Silver birch (Betula pendula), Scots pine shoots (Pinus sylvestris L.), and a goat willow (Salix caprea). Responses from an aggregate clay block and a plastic chair were used as man-made comparison targets. The novelty component of the experiment was the 26-h-long measurement that covered both day and night times. The targets were classified with 80.9% overall accuracy in a dataset collected during dark. Reflectances of four wavelengths located around the 700 nm, the so-called red edge, were used as classification features. The addition of spatial aggregation within a 5-cm neighborhood improved the accuracy to 92.3%. Similar results were obtained using a set of four vegetation indices (78.9% and 91.0%, respectively). The temporal variation of vegetation classes was detected to differ from those in man-made classes.

A stereo microscopic system as a high-precision visual feedback is widely used in the fields of micro-three-dimensional (3-D) measurement and micromanipulation tasks. A new stereo binocular visual servoing model based on a Greenough-type stereoscopic light microscope to solve the 3-D micropositioning problem is proposed. The new model contains no depth information, but the information at the left and right images is used to obtain the image Jacobian matrix. Visual information can be directly obtained from the 3-D space without measuring or estimating the depth information of the unknown points of the object via this new model. The new model can not only accurately and rapidly realize automatic control for a micromanipulation system, but also improve the system control performance. We design an image-based controller with consideration of the kinematics characteristics of a microrobot. Experimental results verify the validity of the model.

The main aim of this paper is to study image enhancement by using sparse and redundant representations of the reflectance component in the Retinex model over a learned dictionary. This approach is different from existing variational methods, and the advantage of this approach is that the reflectance component in the Retinex model can be represented with more details by the dictionary. A variational method based on the dynamic dictionaries is adopted here, where it changes with respect to iterations of the enhancement algorithm. Numerical examples are also reported to demonstrate that the proposed methods can provide better visual quality of the enhanced high-contrast images than the other variational methods, i.e., revealing more details in the low-light part.

We present an automatic mutual information (MI) registration method for mobile LiDAR and panoramas collected from a driving vehicle. The suitability of MI for registration of aerial LiDAR and aerial oblique images has been demonstrated under an assumption that minimization of joint entropy (JE) is a sufficient approximation of maximization of MI. We show that this assumption is invalid for the ground-level data. The entropy of a LiDAR image cannot be regarded as approximately constant for small perturbations. Instead of minimizing the JE, we directly maximize MI to estimate corrections of camera poses. Our method automatically registers mobile LiDAR with spherical panoramas over an approximate 4-km drive, and is the first example we are aware of that tests MI registration in a large-scale context.

The direct ellipse-specific algebraic fitting (DESAF) method proposed by Fitzgibbon is a classical method to fit an ellipse from discrete points. Generally, for a complete spot, an ellipse, which coincides well with the complete contour of the spot, can be fitted by DESAF. However, for an incomplete spot damaged by some noises such as nonuniform optical surfaces, depth steps, and occlusion, DESAF would fit an incorrect ellipse which could not accurately match the complete contour of the spot. We analyze this problem encountered in the onsite three-dimensional measurement of hull plates and propose a method to remove these outlier points from the contours of incomplete spots. The experiments of computer simulated data and real data demonstrate that the proposed method can dramatically remove the outlier points from the contour and improve the detection accuracy of the center of the incomplete spot.

The principal idea of a method using two identical laser vibrometers to eliminate pseudovibrations occurring as structured noise in laser-vibrometer measurements of angular velocity of a rotating object is investigated. The two vibrometers monitor the same surface path on the rotating object, but are separated by a known angle. In addition, they are aligned in such a way that they observe the same speckle patterns, but with a relative time lag given by the angular separation of the vibrometers and the angular velocity of the object. However, any physical variations in angular velocity of the object occur simultaneously at the two vibrometers. Knowing the angular separation between the vibrometers, simple trigonometry can be used to suppress the pseudovibrations. The experiments demonstrate the principle of the method only and no real-time measurements are presented.

Point diffraction interferometry (PDI) combined with annular subaperture stitching is proposed for ultrahigh-accuracy measurements of aspheric surfaces. By adding an axial movement to the test optics in the PDI system, aspheric surfaces with large departures can be measured with high accuracy by stitching the annular measurement data of different axial positions. We examine the principle of PDI-based annular subaperture stitching and the stitching algorithm. Simulations and experiments demonstrate the feasibility and effectiveness of our proposed method. Our method retains the ultra-high accuracy of PDI while extending the vertical dynamic range of the interferometer, enabling nanometer or even subnanometer accuracy measurements of large-departure rotationally symmetric aspheric surfaces.

An off-axis annular subaperture stitching interferometry (OASSI) is presented to test off-axis aspheric surfaces. In view of this, the relationship between misalignment and wavefront aberration is deduced with a strict theoretical analysis. The analytic result shows that the relative misalignment errors between the interferometer and the off-axis mirror tested will lead to complex wavefront aberrations in the measurement result other than the ordinary terms of piston, tilts, and power. Based on the analytic result, a suitable off-axis stitching algorithm is developed for stitching the off-axis subaperture. Both the numerical simulations and preliminary experimental results prove the potential of the proposed approach for the measurement of off-axis aspheric surfaces. As far as we know, this is the first time that OASSI has been used to test the off-axis aspheric surface.

We compared the photometric and radiometric quantities in the visible, ultraviolet, and infrared spectra of white light-emitting diodes (LEDs), incandescent light bulbs and a compact fluorescent lamp used for home illumination. The color-rendering index and efficiency-related quantities were also used as auxiliary tools in this comparison. LEDs have a better performance in all aspects except for the color-rendering index, which is better with an incandescent light bulb. Compact fluorescent lamps presented results that, to our knowledge, do not justify their substitution for the incandescent light bulb. The main contribution of this work is an approach based on fundamental quantities to evaluate LEDs and other light sources.

We aim at the intrinsic parameterization of a computational optical system applied in long-distance displacement measurement of large-scale structures. In this structural-monitoring scenario, the observation distance established between the digital camera and reference targets, which is composed of the computational optical system, can range from 100 up to 1000 m, requiring the use of long-focal length lenses in order to obtain a suitable sensitivity for the three-dimensional displacement measurement of the observed structure which can be of reduced magnitude. Intrinsic parameterization of long-focal length cameras is an emergent issue since conventional approaches applied for reduced focal length cameras are not suitable mainly due to ill-conditioned matrices in least squares estimation procedures. We describe the intrinsic parameterization of a long-focal length camera (600 mm) by the diffractive optical element method and present the obtained estimates and measurement uncertainties, discussing their contribution for the system’s validation by calibration field test and displacement measurement campaigns in a long-span suspension bridge.

Compressed sensing (CS) is a new jointly sampling and compression technology for remote sensing. In hyperspectral imaging, a typical CS method encodes the two-dimensional (2-D) spatial information of each spectral band or encodes the third spectral information simultaneously. However, encoding the spatial information is much easier than encoding the spectral information. Therefore, it is crucial to make use of the spectral information to improve the compression rate on 2-D CS data. We propose to encode the third spectral information with an adaptive Karhunen–Loève transform. With a mathematical proof, we show that interspectral correlations are preserved among 2-D randomly encoded spatial information. This property means that one can compress 2-D CS data effectively with a Karhunen–Loève transform. Experiments demonstrate that the proposed method can better reconstruct both spectral curves and spatial images than traditional compression methods at the bit rates 0 to 1.

Saturation correction for photon counting near the Earth’s surface can improve the accuracy of temperature measurement in the troposphere by pure rotational Raman lidar (PRRL). The PRRL system uses a 532-nm band as a light source, and the echo signals of high-J and low-J channels separated by a double grating monochromator are used for temperature profile inversion. The pulse-height distribution and the discriminator level selection in the system, which comprise a photomultiplier and a photon counter, saturate photon counting near the ground and affect the echo signals from the higher layer for calibration. Given its nonlinear effect on the photomultiplier tubes, the correction method is applied to the echo signals of the calibration interval in troposphere temperature measurements. The method not only reduces the error in temperature measurement (the root-mean-square error of temperature reaches 1.5 K), but also accurately indicates the distribution of the inversion layer within the boundary layer. It can be applied to deal with the saturation phenomenon of a photomultiplier tube.

The design and realization of high-quality bandpass optical filters are often very difficult tasks due to the strong correlation of the optical index of dielectric thin films to their final thickness, as observed in many industrial deposition processes. We report on the optimization of complex optical filters in the visible and NIR spectral ranges as realized by ion beam-assisted electron beam deposition of silica and titanium oxide multilayers. We show that this process always leads to amorphous films prior to thermal annealing. On the contrary, the optical dispersion of TiO₂ nanolayers is highly dependent on their thickness, while this dependence vanishes for layers thicker than 100 nm. We demonstrate that accounting for this nonlinear dependence of the optical index is both very important and necessary in order to obtain high-quality optical filters.

In this work, a multifields optical design method aiming to calculate two high-order aspheric lens profiles with an embedded entrance pupil is proposed. This direct design algorithm is capable of partially coupling more than three ray bundles that enter the same pupil with only two surfaces. Both infinite and finite conjugate objectives can be designed with this approach. Additional constraints such as surface continuity and smoothness are taken into account to calculate smooth and accurate surface contours described by point clouds. The calculated points are then fitted with rotationally symmetric functions commonly used in optical design tools. A presented subaperture sampling strategy that introduces a weighting function for different fields allows for a very well-balanced imaging performance over a wide field of view (FOV). As an example, a ±45  degf/7.5 wide-angle objective is designed and analyzed to demonstrate the potential of this design method. It provides an excellent starting point for further optimization of the surfaces’ coefficients and initial design parameters, resulting in a very good and well-balanced imaging performance over the entire FOV.

To improve the quality of diamond turned optics, it is necessary to employ a laborious postprocessing approach. However, the approach is often very hard to implement on freeform surfaces, and it greatly decreases the processing efficiency even for simple optics. An evolutionary diamond turning method is first proposed to comprehensively improve the quality of the turned optics. The evolutionary goal is to attenuate the peak value of the power spectral density of residual tool marks, and the special mutation point is observed during the evolution. Taking advantage of the modulation operation of a diamond tool in the spatial frequency domain, form accuracy, surface roughness, and surface reflectivity are all significantly improved, demonstrating the superiority of the EDT method for optics fabrication.

An algorithm to solve the inverse problem of synchrotron radiation adaptive mirrors’ tuning is presented. The influence functions are modeled and calculated for a generic bimorph mirror. An error function minimization method is used to simulate the correction of the surface figure of the mirror in some particular conditions. Possible applications to free-electron-laser mirror simulations are pointed out.

We report a simple budget heuristic for a fast optimization of multipump Raman amplifiers based on the reallocation of the pump wavelengths and the optical powers. A set of different optical fibers are analyzed as the Raman gain medium, and a four-pump amplifier setup is optimized for each of them in order to achieve ripples close to 1 dB and gains up to 20 dB in the C band. Later, a comparison between our proposed heuristic and a multiobjective optimization based on a nondominated sorting genetic algorithm is made, highlighting the fact that our new approach can give similar solutions after at least an order of magnitude fewer iterations. The results shown in this paper can potentially pave the way for real-time optimization of multipump Raman amplifier systems.

A tunable optoelectronic oscillator (OEO), which employs an all-optical microwave photonic filter (MPF) consisting of two laser sources (LD1 and LD2), an optical coupler (OC, 50:50), a Mach-Zehnder modulator (MZM), and a chirped fiber Bragg grating, is proposed. Because the central frequency of the all-optical MPF can be shifted by changing the wavelength spacing between the two laser sources, the frequency tunability of the OEO can be realized by incorporating such an all-optical MPF into an optical domain dual-loop OEO without any electronic microwave filters. A detailed theoretical analysis is presented and the results are confirmed by an experiment. A microwave signal with a frequency-tuning range from 4.057 to 8.595 GHz is generated. The phase noise, the long-term stability, and the side-mode suppression performance of the generated microwave signal are also investigated.

A full-duplex multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband over fiber (UWBoF) system is proposed, and bidirectional transmission of a 1.28-Gbps MB-OFDM UWB signal over 50-km standard single-mode fiber (SSMF) is demonstrated. An optical remote heterodyning mixing scheme is employed to generate a 60-GHz optical millimeter wave. Meanwhile, an optical carrier without modulation data is extracted by using a fiber Bragg grating for the uplink MB-OFDM UWB signal transmission. After 50-km SSMF transmission at a bit error rate of 1×10−⁴, the power penalties are 0.7 dB for a 4 quadrature amplitude modulation (QAM)-uplink and 1.0 dB for a 16QAM-uplink, respectively. The proposed scheme would greatly reduce the cost and significantly improve the spectrum utilization efficiency in the full-duplex MB-OFDM UWBoF systems.

Supercontinuum (SC) generation contained in the normal dispersion range of an optical fiber has been shown to be limited primarily by the available peak power and length of the pump pulse. In this work, we numerically investigate the SC spectral width and flatness for various pump pulse conditions in a nonlinear, all-solid, soft-glass, photonic crystal fiber (PCF) with a flattened dispersion profile. We assume a range of pump pulse parameters with pulse lengths between 250 and 100 fs (60 to 150 kW of peak power), and input pulse energies between 10 and 30 nJ, numerically reaching a maximum SC width of 800 to 2600 nm. The presented theoretical study provides a guideline for the selection of a fiber laser pump source, or in other words, it enables one to expect the extent of spectral broadening in the developed, all-normal dispersion PCF, when presently available fiber laser pump pulse parameters are assumed.

We present the performance analysis of a spectral amplitude code labeled system with 100  Gb/s polarization division multiplexed (PDM) differential quadrature phase shift keying payload in simulation. Direct detection is chosen to demodulate the PDM payload by applying a polarization tracker, while 4-bits of the 156 Mb/s spectral amplitude code label is coherently detected with a scheme of frequency-swept coherent detection. We optimize the payload laser linewidth as well as the frequency spacing between the payload and label. For back-to-back system and 96 km transmission, label eye opening factors are 0.95 and 0.94, respectively, while payload optical signal-to-noise ratios are 20.6 dB and 22.0 dB, and the payload received optical powers are −15.0  dBm and −14.5  dBm for a bit error rate value of 10⁻⁹. The results show that both the payload and label have good transmission performances after long-haul transmission in a standard single mode fiber and dispersion compensating fiber, and the payload could be well demodulated after 288 km transmission.

A module of an all-optical 2-bit comparator is analyzed and implemented using semiconductor optical amplifiers (SOAs). By employing SOA-based cross phase modulation, the optical XNOR logic is used to get an A=B output signal, where as AB¯ and A¯B; logics operations are used to realize A>B and A<B output signals, respectively. All input output bit operations results along with the wide open eye diagrams are obtained. It is suggested that the proposed system would be promising in all-optical high speed networks and computing systems.

An accurate fiber Bragg grating (FBG) phase-shift tuning system is proposed and demonstrated. It depends on a localized pressure applied to a specially packaged FBG filter using an accurate micrometer single-axis stage. Continuous range of phase shift from 0 to π is achieved with linear relation to the stage position. An acceptable degree of repeatability of the process is noticed in the experiment.

A 64×10  Gb/s nonreturn-to-zero wavelength division multiplexing system (WDM) for 100 GHz spaced with an efficient high gain optical amplifier is demonstrated using a fiber Raman amplifier (FRA). The WDM signals are propagated through a span comprising a single-mode fiber (96 km), a dispersion compensated fiber (16 km), and a Raman fiber (10 km). The FRA is pumped by only two pump wavelengths in the counterpropagating mode. The performance of this system is analyzed for different placements of Raman amplifier in the span, i.e., as pre, post, and both (pre and post), and it is concluded theoretically that the post configuration of the Raman amplifier is the best choice for the WDM system. With an input signal power of 0 dBm, a gain flatness of 4.4 dB with quality >11  dB is obtained across the frequency range of 188.35 to 194.65 THz without using any hybrid configuration or gain equalization technique for a transmission distance of 424 km. We present a comprehensive comparison of the performance of the WDM system by analyzing the results obtained from different configurations for different spans of fiber length.

The electro-optical efficiency of vertical-cavity surface-emitting lasers (VCSELs) strongly depends on the efficient carrier injection into the quantum wells (QWs) in the laser active region. Carrier injection degrades with increasing temperature, which limits VCSEL performance in high-power applications where self-heating imposes high-operating temperatures. In a numerical model, we investigate the transport of charge carriers in an 808-nm AlGaAs multi-quantum-well structure with special attention to the temperature dependence of carrier injection into the QWs. Experimental reference data were extracted from oxide-confined, top-emitting VCSELs. The transport simulations follow a drift-diffusion-model complemented by an energy-resolved carrier-capture model. The QW gain was calculated in the screened Hartree–Fock approximation. With the combination of the gain and transport model, we explain experimental reference data for the injection efficiency and threshold current. The degradation of the injection efficiency with increasing temperature is not only due to increased thermionic escape of carriers from the QWs, but also to state filling in the QWs initiated from higher threshold carrier densities. With a full opto-electro-thermal VCSEL model, we demonstrate how changes in VCSEL properties affecting the threshold carrier density, like mirror design or optical confinement, have consequences on the thermal behavior of the injection and the VCSEL performance.

Nonlinear pulse dynamics in two stages of different active or passive fibers are investigated in this work. Numerical approach of the symmetrized split step Fourier method is used to solve the nonlinear Schrödinger equation in the presence of fiber gain, nonlinearity, and dispersion. An input Gaussian pulse evolves into a linearly chirped perfect parabolic pulse (PP) when it propagates through a standard normal dispersion decreasing fiber amplifier. At the same time, for an erbium-doped dispersion decreasing fiber amplifier with a similar dispersion variation with length, the semiparabolic pulse (SPP) is produced at the output end of the fiber. To our knowledge, this is shown for the first time. In second stage, the so-obtained perfect PP, SPP, and also a chirp-free perfect PP are fed into the input of several normal dispersion fibers and the comparative pulse evolution is studied in detail with the variations of dispersion coefficient, gain, and nonlinearity. While using these pulses as the input of an anomalous dispersion fiber, our result shows that the linearly chirped PP is most efficient for compressing the pulses with a good quality factor without dropping significant pedestal energy.

A method for holographic femtosecond laser parallel processing is proposed, which can suppress the interference of zero-order light effectively and improve the energy utilization rate. In order to blaze the target pattern to the peak position of zero-order interference, a phase-only hologram containing a digital blazed grating is designed and generated, and the energy of the target pattern can be increased by 3.793 times in theory. In addition, by subsequently increasing the phase of the divergent spherical wave, the focal plane of the target pattern and the plane of the multiorder diffraction beam resulting from the pixelated structure of the spatial light modulator (SLM) can be separated. Both a high-pass filter and aperture are used to simultaneously eliminate the influences of zero-order light and multiorder interferential patterns. A system based on the phase-only SLM (with resolution of 1920×1080) is set up to validate the proposed method. The experimental results indicate that the proposed method can achieve high-quality holographic femtosecond laser parallel processing with a significantly improved energy utilization rate.

Terahertz (THz) detector based on a GaAs/AlGaAs heterostructure was investigated at low temperatures and high magnetic fields. The response of the detector showed a feature caused by a cyclotron resonance that was accompanied by several peaks originated from excitations of magnetoplasmons. Illumination with a visible light (VIS) caused an increase of a plasma concentration and resulted in a change of a magnetoplasmon spectrum. An analysis of spectra allowed to determine changes in the plasmon dispersion with a VIS, which gives a tool to tune the THz response of a plasmonic detector.

We propose a nonmechanical cross-sectional scanning laser Doppler velocimeter (LDV) with directional discrimination of the transverse velocity component. In the proposed LDV, a combination of changes in wavelength and the port of the fiber array between the main body and probe are used to scan the measurement position on a two-dimensional (2-D) cross-sectional plane that is perpendicular to the direction of flow. Optical frequency shifting using acousto-optic modulators (AOMs) is employed to discriminate the direction of the transverse velocity component. Polarization multiplexing is used to transmit frequency-shifted beams from the main body to the probe. The experimental results reveal that the 2-D scanning function and introduction of directional sensitivity are successfully achieved.

We numerically investigated the pulse trapping in high nonlinear silicon waveguides. The two orthogonally polarized components of the pulse can trap and copropagate as a unit in a silicon waveguide. Our numerical results show that the trapping pulse can stably propagate when the polarization mode dispersion is compensated by shifting the frequencies of two orthogonally polarized components. We also analyze the effects of the free-carrier absorption and initial polarization angles on the pulse propagation in a silicon waveguide. The proposed on-chip trapping pulse in the silicon waveguide exhibits compact configuration and can potentially have important applications in integrated optics.

Terbium acetylacetonate hydrate (TAH)-doped poly methyl methacrylate (PMMA) with varying proportions was synthesized using the solvent evaporation method. Absolute spectral parameters of the samples under the excitation of a 308-nm light-emitting diode were derived adopting the integrating sphere method. The total radiant fluxes of typical visible emissions in 1, 2, and 3 wt% TAH doping PMMA samples are calculated to be 67.84, 80.16, and 93.90  μW, respectively, and the total luminous fluxes are determined to be 36.95, 43.16, and 50.18 millilumens. The results indicated that PMMA including TAH generates new applications as an efficient ultraviolet→visible conversion layer in enhancing the photovoltaic efficiency of solar cells.

The proposed four-quadrant Moiré alignment scheme to detect the misalignment between mask and wafer for proximity lithography can achieve the alignment accuracy with nanometer level. When implementing the scheme, however, the distribution of Moiré fringes associated with the mask–wafer gap indeed goes against the alignment, making the gap optimization highly urgent. The optimization model is established, and numerical simulation as well as experimental verification is also provided. Furthermore, an alignment accuracy of ∼3  nm with the illumination wavelength of 632.8 nm is experimentally attained. Simultaneously, the design mechanism of alignment marks for improving the availability of the alignment scheme is discussed.