This PDF file contains the editorial “Natural Synergy of Conferences and Journals” for OE Vol. 53 Issue 12

This PDF file contains the editorial “Special Section Guest Editorial: Advances of Precision Optical Measurements and Instrumentation for Geometrical and Mechanical Quantities” for OE Vol. 53 Issue 12

Frequency-modulated continuous wave (FMCW) laser ranging is one of the most interesting techniques for precision distance metrology. In order to ensure the theoretical measurement range and precision, a narrow linewidth external cavity tunable laser with large tuning range is chosen. In practical situations, the tuning nonlinearity of the laser reduces the measurement precision, hence an auxiliary interferometer is used to measure the laser tuning rate and linearize the frequency ramp. Then, fast Fourier transform algorithm is applied to the resampled signal of the main interferometer, and the full-width at half maximum of the frequency spectrum is narrowed. In the end, the experiments are carried out using the FMCW laser ranging system and demonstrate 50-μm range resolution at 8.7 m.

We revisit the method of synthetic wavelength interferometry (SWI) for absolute measurement of long distances using the radio-frequency harmonics of the pulse repetition rate of a mode-locked femtosecond laser. Our intention here is to extend the nonambiguity range (NAR) of the SWI method using a coarse virtual wavelength synthesized by shifting the pulse repetition rate. The proposed concept of NAR extension is experimentally verified by measuring a ∼13-m distance with repeatability of 9.5 μm (root-mean-square). The measurement precision is estimated to be 31.2 μm in comparison with an incremental He–Ne laser interferometer. This extended SWI method is found to be well suited for long-distance measurements demanded in the fields of large-scale precision engineering, geodetic survey, and future space missions.

The paper describes recent improvements of Physikalisch-Technische Bundesanstalt's (PTB) reference measuring instrument for length graduations, the so-called nanometer comparator, intended to achieve a measurement uncertainty in the domain of 1 nm for a length up to 300 mm. The improvements are based on the design and realization of a new sample carriage, integrated into the existing structure and the optimization of coupling this new device to the vacuum interferometer, by which the length measuring range of approximately 540 mm with sub-nm resolution is given. First, measuring results of the enhanced nanometer comparator are presented and discussed, which show the improvements of the measuring capabilities and verify the step toward the sub-nm accuracy level.

A multiprobe surface encoder for optical metrology of six-degree-of-freedom (six-DOF) planar motions is presented. The surface encoder is composed of an XY planar scale grating with identical microstructures in X - and Y -axes and an optical sensor head. In the optical sensor head, three paralleled laser beams were used as laser probes. After being divided by a beam splitter, the three laser probes were projected onto the scale grating and a reference grating with identical microstructures, respectively. For each probe, the first-order positive and negative diffraction beams along the X - and Y -directions from the scale grating and from the reference grating superimposed with each other and four pieces of interference signals were generated. Three-DOF translational motions of the scale grating Δx , Δy , and Δz can be obtained simultaneously from the interference signals of each probe. Three-DOF angular error motions θ _X , θ _Y , and θ _Z can also be calculated simultaneously from differences of displacement output variations and the geometric relationship among the three probes. A prototype optical sensor head was designed, constructed, and evaluated. Experimental results verified that this surface encoder could provide measurement resolutions of subnanometer and better than 0.1 arc sec for three-DOF translational motions and three-DOF angular error motions, respectively.

In order to obtain accurate position of the inner key components in the experimental advanced superconducting tokamak (EAST), a combined optical measurement method which is comprised of a laser tracker (LT) and articulated coordinate measuring machine (CMM) has been brought forward. LT, which is an optical measurement instrument and has a large measurement range and high accuracy, is employed for establishing the precision measurement network of EAST, and the articulated CMM is also employed for measuring the inner key components of EAST. The measurement uncertainty analyzed by the Unified Spatial Metrology Network (USMN) is 0.20 mm at a confidence probability of 95.44%. The proposed technology is appropriate for the inspection of the reconstruction of the EAST.

A coordinate measuring method with two operation modes, based on the adjustable articulated arms, is proposed to keep measurement capability in global space and improve the measurement precision in local space. The adjustable articulated arm coordinate measuring machine (AACMM) with an electromagnetic locking device can automatically switch between the all-free articulated arms operation mode and the partially bound articulated arms operation mode. In the former mode, three arms and six articulations can freely move and measure the coordinates of any point in global space. In the latter mode, the front two articulations are locked to improve the measurement precision by decreasing the importation of angle errors in the local space. A prototype of the adjustable AACMM has also been designed and developed. A mathematical model for the adjustable AACMM has been built. Theoretical analysis and numerical simulation show that the partially bound AACMM performed much better than the all-free AACMM in single point repeatability and length measurement precision in the local space. Therefore, the proposed coordinate measuring method based on the adjustable articulated arms is verified as being effective.

The measuring accuracy of laser optical sensors is gradually degraded over time as a result of geometrical fluctuations of the laser beam. In a previous study by the present group, a method was proposed for stabilizing the laser beam by means of a rotating optical diffuser. In the present study, the effects of the key diffuser parameters, namely the rotational speed (ω), the particle radius (r), and the particle concentration (c), on the performance of the proposed system are evaluated both numerically and experimentally. The results confirm that given an appropriate setting of the three parameters, the proposed system reduces the variation of the image centroid position and improves the measuring accuracy of the laser optical sensor as a result.

Spectrally resolved interferometry based on optical frequency comb is an effective way for absolute distance measurement. We introduce wavelet transform to spectrally resolve interferometry as an effective data analysis tool. A comparison has been presented between wavelet transform and conventional algorithm. The results demonstrate that the wavelet transform is a reliable technique and provides good performance over noise resistance for spectrally resolved interferometry measurement.

In this contribution, the extent to which the Nyquist criterion can be violated in optical imaging systems with a digital sensor, e.g., a digital microscope, is investigated. In detail, we analyze the subpixel uncertainty of the detected position of a step edge, the edge of a stripe with a varying width, and that of a periodic rectangular pattern for varying pixel pitches of the sensor, thus also in aliased conditions. The analysis includes the investigation of different algorithms of edge localization based on direct fitting or based on the derivative of the edge profile, such as the common centroid method. In addition to the systematic error of these algorithms, the influence of the photon noise (PN) is included in the investigation. A simplified closed form solution for the uncertainty of the edge position caused by the PN is derived. The presented results show that, in the vast majority of cases, the pixel pitch can exceed the Nyquist sampling distance by about 50% without an increase of the uncertainty of edge localization. This allows one to increase the field-of-view without increasing the resolution of the sensor and to decrease the size of the setup by reducing the magnification. Experimental results confirm the simulation results.

A full-scale three-dimensional profile measurement system with an innovative optical setup for measuring the geometric shape of large wind-turbine blades in high accuracy is developed. A normal full-scale wind blade geometry measurement system can be very expensive. The presented system is low cost, but it can yield a high accuracy for geometric dimensions by error compensation from its measured data. It consists of a low cost long linear stage driven by a direct current motor with linear scale feedback for position and velocity control, and two line-scan optical heads mounted on opposite sides. The line image of the sectional shape profile can be captured by two charge-coupled devices. By scanning the optical head throughout the full length of the blade, the image of the whole profile can be collected. The shape parameters of the wind-turbine blades can thus be determined. A special effort has been employed to improve the straightness and positioning accuracy of the linear stage by error compensation. With system calibration of the stage and the cameras, experimental results show high accuracy of the developed system. This low-cost optical system is expected to measure any full-scale wind blade profile up to several meters in length.

Conventional video extensometers, using a measurement mark on specimen to obtain material strain, have a problem with deformation of the measurement mark. Therefore, the accurate position of the measurement mark is difficult to evaluate, and measurement accuracy is limited. To solve this problem, a strain measurement method based on a laser mark automatically tracking a line mark on the specimen is proposed. This method is using an undeformed laser mark to replace the line mark to calculate the specimen strain and eliminates the measurement error induced by the deformation of specimen marks. The positions of the laser mark and the line mark are achieved by using digital image processing. Automatic tracking is realized by means of an intelligent motor control. Also, the strain of the specimen is obtained by analyzing the movement trace of the laser mark. A video extensometer experimental setup based on the proposed method is constructed. Two experiments were carried out. The first experiment verified the validity and the repeatability of the method via tensile testing of the specimens of low-carbon steel and cast iron. The second one demonstrated the high measurement accuracy of the method by comparing with a clip-on extensometer.

A unique absolute length measurement method is proposed and demonstrated for the first time. Since it takes advantage of both the high-accuracy measurement capability of a pulse train interference method and the ability of a two-color method to compensate for environmental changes, the present method is expected to be useful for high-precision length measurement for not only the purposes of laboratory science but also for satisfying the requirements of industry. A length measurement was performed to demonstrate the feasibility of the proposed method.

Based on the computer texture analysis method, a new noncontact surface roughness measurement technique is proposed. The method is inspired by the nonredundant directional selectivity and highly discriminative nature of the wavelet representation and the capability of the Markov random field (MRF) model to capture statistical regularities. Surface roughness information contained in the texture features may be extracted based on an MRF stochastic model of textures in the wavelet feature domain. The model captures significant intrascale and interscale statistical dependencies between wavelet coefficients. To investigate the relationship between the texture features and surface roughness R_a, a simple research setup, which consists of a charge-coupled diode camera without a lens and a diode laser, was established, and the laser speckle texture patterns are acquired from some standard grinding surfaces. The research results have illustrated that surface roughness R_a has a good monotonic relationship with the texture features of the laser speckle pattern. If this measuring system is calibrated with the surface standard samples roughness beforehand, the surface roughness actual value R_a can be deduced in the case of the same material surfaces ground at the same manufacture conditions.

This PDF file contains the editorial “Special Section Guest Editorial: Laser Damage II” for OE Vol. 53 Issue 12

Laser peening has recently emerged as a useful technique to overcome detrimental effects associated with other well-known surface modification processes such as shot peening or grit blasting used in the biomedical field. It is worthwhile to notice that besides the primary residual stress effect, thermally induced effects might also cause subtle surface and subsurface microstructural changes that might influence corrosion resistance and fatigue strength of structural components. In this work, plates of Ti-6Al-4V alloy of 7 mm in thickness were modified by laser peening without using a sacrificial outer layer. Irradiation by a Q-switched Nd-YAG laser (9.4-ns pulse length) working at the fundamental 1064-nm wavelength at 2.8  J/pulse and with water as a confining medium was used. Laser pulses with a 1.5-mm diameter at an equivalent overlapping density of 5000  cm⁻² were applied. Attempts to analyze the global-induced effects after laser peening were addressed by using the contacting and noncontacting thermoelectric power techniques.

Laser calorimetry is based on the measurement and evaluation of the temperature increase caused by absorption in the sample exposed to laser radiation. A temperature distribution develops in the irradiated sample as a result of dependence on the thermal diffusivity of the sample. Therefore, finding a correlation between the temperature increase and absorption becomes a complex task. This challenge was met by keeping the sample geometry at a standard size and simulating the thermal distribution for a number of optical materials. Using this method, Laser Zentrum Hannover e.V. (LZH) developed a calorimetric test setup that offers fully calibrated absorptance values for sample diameters of 25 mm (or 1 in.) with a total error of below 13% and a relative measurement error of below 5%. However, this technique is limited to the above-mentioned sample geometry. This work presents an approach to adjust the measurement configuration to numerous sample sizes for standard circular laser components. Finite element analysis and experimental verification are presented for exemplary values of the samples’ diameters. Based on the different sample mount concept, this procedure allows utilizing flexibility in test wavelength and angle of incidence, combined with the sensitivity level sufficient for current optical materials.

Hafnium oxide (HfO ₂ ) is the most frequently used high-index material in multilayer thin-film coatings for high-power laser applications ranging from near-infrared to near-ultraviolet (UV). Absorption in this high-index material is also known to be responsible for nanosecond-pulse laser-damage initiation in multilayers. In this work, modification of the near-UV absorption of HfO ₂ monolayer films subjected to irradiation by continuous-wave (cw), 355-nm or 351-nm laser light focused to produce power densities of the order of ∼100  kW/cm ² is studied. Up to a 70% reduction in absorption is found in the areas subjected to irradiation. Temporal behavior of absorption is characterized by a rapid initial drop on the few-tens-of-seconds time scale, followed by a longer-term decline to a steady-state level. Absorption maps generated by photothermal heterodyne imaging confirm the permanent character of the observed effect. Nanosecond-pulse, 351-nm and 600-fs, 1053-nm laser-damage tests performed on these cw laser–irradiated areas confirm a reduction of absorption by measuring up to 25% higher damage thresholds. We discuss possible mechanisms responsible for near-UV absorption annealing and damage-threshold improvement resulting from irradiation by near-UV cw laser light.

The cracks and scratches inevitably generated by previous grinding and polishing significantly lower the ability of laser resistance of optical substrates. In this study, the artificial indentations, scratches, and structural defects imbedded with metal nanoparticles are fabricated. The laser-induced damage characteristics of such defects in different types and sizes are investigated qualitatively and quantitatively under 1064-nm laser irradiation. Moreover, the etching effect on improving the laser-induced damage threshold (LIDT) of artificial defects under different etching conditions is analyzed. LIDT is then evaluated according to the etching depth and the morphologies of artificial defects.

The modeling of laser interaction with metals for various applications requires a knowledge of absorption coefficients for real, commercially available materials with engineering grade (unpolished, oxidized) surfaces. However, most currently available absorptivity data pertain to pure metals with polished surfaces or vacuum-deposited thin films in controlled atmospheres. A simple laboratory setup is developed for the direct calorimetric absorptivity measurements using a diode-array laser emitting at 780 nm. A scheme eliminating the effect of convective and radiative losses is implemented. The obtained absorptivity results differ considerably from existing data for polished pure metals and are essential for the development of predictive laser-material interaction models.

We present, to our knowledge, the first adaptation of the particle-in-cell (PIC) simulation method for use in the study of femtosecond pulse laser damage, including the first implementation of the Morse pair-potential for PIC codes. We compare the PIC method to a wide variety of currently used modeling schemes, ranging from purely ab initio molecular dynamics simulations to semi-empirical models with many fitting parameters and show how PIC simulations can provide a complementary approach by filling the gap in theoretical methodology between the two cases. We detail the necessity and implementation of an interatomic pair-potential in PIC studies of laser damage. Finally, we use our model to treat the full laser damage process of a copper target and show that our results compare well to simple scaling laws for crater size.

We report on the continuation of a comparative study of different fused silica materials for ArF laser applications. After selecting potentially suited fused silica materials from their laser-induced absorption and compaction obtained by a short-time testing procedure, accelerated lifetime tests have been undertaken by sample irradiating at liquid nitrogen temperature and subsequent direct absorption measurements were made using the laser-induced deflection technique. The obtained degradation acceleration strongly differs between fused silica materials, showing high and low oxygen hole (OH) contents, respectively. As a result, a difference in the absorption degradation mechanism between high and low OH-containing fused silica is proposed. Consequently, two different scenarios for an acceleration of the absorption degradation are derived.

Optical materials submitted to multiple subpicosecond irradiations are known to exhibit a decrease of the laser-induced damage threshold (LIDT) with the applied number of pulses, an effect referred to as “fatigue” or “incubation.” In this work, we experimentally investigate this effect for the case of optical thin films submitted to multiple exposures with 500 fs pulses at different wavelengths: 1030, 515, and 343 nm. Niobia, hafnia, and silica films made with dense coating techniques (magnetron sputtering, ion-assisted deposition, and reactive low voltage ion plating) are studied, as well as the surface of a fused silica substrate. These samples have been exposed to different pulse numbers (from 1 to 100,000) at a low-frequency repetition rate (less than 1 kHz) and the LIDT has been measured. The results reveal the differences between materials and for the various wavelengths such as the decrease rate of the LIDT or the stabilization level that is reached after multiple exposures. All the results evidence the role of native- and laser-induced defects that we discuss on the basis of published works on the subject.

Laser damage experiments were performed on painted and unpainted aluminum coupons using a 1.07-μm fiber laser at irradiances ranging from 0.2 to 1.4  kW/cm² in a wind tunnel operating at Mach 0.1 to 0.9. Coupon penetration times of ∼0.5 to 10 s were measured using a silicon photodiode viewing a Lambertian scatter plate placed behind the target. Despite the thin, 0.81 to 0.95 mm, samples and large laser spot diameters, 2 to 3 cm, the effects of radial heat conduction dominate for irradiances of <1  kW/cm². The fluence required to melt the back surface scales linearly with paint absorbance and the effects of paint aging have been observed. Penetration times for gray-painted aluminum at 287  W/cm² decrease by 45% as the airflow speed increases from M=0.1 to M=0.2, but remains constant for flow speeds up to M=0.7.

CdSiP ₂ (CSP) is a very promising nonlinear crystal for the mid-infrared spectral range with a nonlinear coefficient slightly larger than that of ZnGeP₂ (ZGP). In contrast to ZGP, CSP is phase-matchable and can be employed in 1.064-μm pumped optical parametric oscillators (OPOs) without two-photon absorption. Although low damage resistivity has been reported in such initial OPO tests of CSP, no reliable data on the damage threshold of uncoated CSP exists. In this work, we compare the damage resistivity of uncoated CSP with ZGP at two wavelengths, 2.09  μm (1 kHz, 21 ns) and 1.064  μm (100 Hz, 8 ns).

The degradation of 405-nm fiber-coupled diode laser systems with more than 50 mW power was investigated in detail with focus on the effects occurring at the input end. The coupling and transmission loss of the laser light were associated with the growth of a projection and a periodic structure on the input surface. To avoid this degradation, a short launch fiber with a good surface quality was used at the input end. In this way, the power transmission was stabilized for at least one month. However, structural degradation was noticed on the output surface of the single-mode fiber. To investigate this effect, the damaged samples were measured after different periods of time and examined with a scanning electron microscope and with an atomic force microscope. Reproducible spherical projections with a submicron periodic structure were found in the core region. Additionally, the spectral loss of the fiber was measured, showing the formation of color centers in the deep ultraviolet along the length of the fiber. These investigations were accompanied by simulations of the growth of the structure on the output surface. The influence of the structure was mainly on the divergence angle of the emitted laser beam, reducing the beam quality for applications.

The effective area of a laser spot is an important quantity used to characterize the laser-induced damage threshold of optical materials according to ISO 21254-1:2011 standard. A method for measuring the effective area/diameter of spots from pulsed laser beams using charge-coupled device camera-based beam profilers is presented. Factors affecting the measurement’s accuracy, as the background noise and the size of the summation area, were evaluated using MATLAB^®. To minimize the noise contribution, we use an iterative method similar to the one used to measure the second-moment-based spot sizes. We find that the two analyzed components of the background noise, its zero-mean noise and its offset, have an opposite effect on the measurements of the effective area/diameter as compared with the second-moment-based measurements. We prove that there is an upper limit of the relative error of such iterative measurements of effective area, the iteration limit parameter, and that it is a measurable quantity. We measure the effective area/diameter of laser spots with different sizes from a Nd:YAG laser at 1064 nm, 6 ns pulse duration, 10 Hz repetition frequency, and estimate the standard uncertainty of the measurements. Further, we generalize the effective area/diameter concept to include elongated (elliptical/rectangular) spots.

Micron and submicron scale artificial silica spheres were used to quantitatively detect the ultrasonic cleaning procedure for an ultrasmooth optical substrate. Silica spheres with sizes from 1 to 3  μm were explored to efficiently reduce the ultrasonic power for ultrasonic frequencies of 40, 80, and 120 kHz. On account of submicron silica spheres with sizes from 0.3 to 1.0  μm, the cleaning efficiencies for the ultrasonic frequencies of 170, 220, and 270 kHz were quantity evaluated, and the removal efficiency was effectively increased by employing a solution of NH₄OH, H₂O₂, and deionized water with an optimized configuration. Furthermore, for the range from 40 to 270 kHz, according to investigations of the ultrasonic time, the particles’ removal process was explored during the ultrasonic treatments, and the damages of ultrasound on the ultrasmooth substrate within the ultrasonic time were also detected. The quantitative results give us many clues for realizing the optimization of an ultrasonic cleaning procedure for ultrasmooth optical substrates.

Laser-induced ionization is a major process that initiates and drives the initial stages of laser-induced damage (LID) of high-quality transparent solids. The ionization and its contribution to LID are characterized in terms of the time-dependent ionization rate and conduction-band electron density. Considering femtosecond pulses of various durations (from 35 to 706 fs) and variable peak irradiances (from 0.01 to 60  TW/cm²), we use a single-rate equation to simulate time variations of conduction-band electron density and rates of the photoionization and impact ionization. The photoionization rate is evaluated with the Keldysh equation. At low irradiance, the electron density and total ionization rate demonstrate power scaling characteristic of multiphoton ionization. With the increase of irradiance, there is observed a saturation of the photoionization rate due to photoionization suppression by the Keldysh-type singularity during the increase in the number of simultaneously absorbed photons by 1. A striking result is that the saturation is followed by a stepwise transition from the ionization regime which is completely dominated by the photoionization to a regime totally dominated by the impact ionization. The transition results in the increase of the electron density by a few orders of magnitude induced by a variation of peak laser irradiance by about 15% to 20%. The physical effects that are involved are discussed.

We have examined how three different cleaning processes affect the laser-induced damage threshold (LIDT) of antireflection coatings for large dimension, Z-Backlighter laser optics at Sandia National Laboratories. Laser damage thresholds were measured after the coatings were created, and again 4 months later to determine which cleaning processes were most effective. Coatings that received cleaning exhibited the highest LIDTs compared to coatings that were not cleaned. In some cases, there is nearly a twofold increase in the LIDT between the cleaned and uncleaned coatings (19.4  J/cm² compared to 39.1  J/cm²). Higher LIDTs were realized after 4 months of aging. The most effective cleaning process involved washing the coated surface with mild detergent, and then soaking the optic in a mixture of ethyl alcohol and deionized water. Also, the laser damage results indicate that the presence of nonpropagating (NP) damage sites dominates the LIDTs of almost every optic, despite the cleaning process used. NP damage sites can be attributed to defects such as nodules in the coating or surface contamination, which suggests that pursuing further improvements to the deposition or cleaning processes are worthwhile to achieve even higher LIDTs.

This paper introduces a sensitivity test for characterizing the laser damage behavior of a sample. A sensitivity test analyzes unbinned laser damage test data to estimate the damage probability curve. The means of estimation is by employing a parametric model of the probability of damage and identifying the parameters most likely to produce the observed results using the maximum-likelihood (ML) method. The ML method applied to laser damage measurements is reviewed. The sensitivity test is analyzed for its performance using Monte Carlo methods. A series of laser damage tests are simulated on a test of a hypothetical test optic. A Weibull distribution is selected for the damage probability model, while the virtual test optic was chosen to have a non-Weibull shaped damage probability curve. Damage measurements for varying the number of sites exposed are modeled to show the convergence of the Weibull parameters. This paper concludes by showing how the underlying defect distribution is calculated from results of the sensitivity test.

This PDF file contains the editorial “Special Section Guest Editorial: Optical Frequency Combs” for OE Vol. 53 Issue 12

We theoretically and experimentally investigate some effects related to the Kerr optical frequency comb generation, using a millimeter-size magnesium fluoride ultrahigh quality disk resonator. We show that the Kerr comb tunability can be extremely wide in the Turing pattern (or primary comb) regime, with an intermodal frequency that can be tuned from 4 to 229 multiple free spectral ranges (corresponding to a frequency spacing ranging from 24 GHz to 1.35 THz). We also discuss the role played by thermal locking while pumping the resonator, as well as the effect of modal crossing when broadband combs are generated.

We experimentally study two Ti:sapphire optical frequency comb femtosecond regimes, respectively, with a linear and a nonlinear dependence of the carrier-envelope offset frequency (f_CEO) on pump intensity. For both regimes, we study the effect of single- and multimode pump lasers on the f_CEO phase noise. We demonstrate that the femtosecond regime is playing a more important role on the f_CEO phase noise and stability than the pump laser type.

The second harmonic generation (SHG) of a femtosecond optical frequency comb (FOFC) has been studied. This work focuses on the SHG frequencies that are generated by the mixing of even-numbered frequency components from the original comb with odd-numbered components. It is observed that the generation of those frequencies is the reason the original FOFC and FOFC-based SHG signal have the same repetition frequency. The theoretical derivation agrees with the result of an optical experiment. Our results may be of use with the high-harmonic-generation process and FOFC-based SHG applications, including high-resolution spectroscopy, attosecond pulse generation, and precision length measurement.

We discuss recent advances in the stabilization and application of femtosecond frequency combs based on optical parametric oscillators (OPOs) pumped by femtosecond lasers at 800 and 1060 nm. A method for locking to zero the carrier-envelope-offset of a Ti:sapphire-pumped OPO comb is described. The application of Yb:KYW-laser-pumped dual-combs for mid-infrared spectroscopy is detailed, specifically methane spectroscopy at approximately a 0.7% concentration at 1 atm.

The property of a three-branch waveguide interferometer (TBWI) used to achieve the desired single-side-band (SSB) frequency shift has been theoretically analyzed in detail. Optical frequency comb generation (OFCG) based on TBWI with a recirculating loop has been also proposed and analyzed. The simulation has been implemented to show the properties of TSSBM and TSSBM-based OFCG (TOFCG). Simulation results show that the TOFCG has the ability to generate high-quality multiple optical frequency combs by applying optimized parameters.

We study theoretically and demonstrate experimentally generation of coherent optical frequency combs in a sapphire whispering gallery mode resonator. We verify phase locking of the comb harmonics by demodulating the comb on a fast photodiode and by observing the production of a spectrally pure radio frequency signal. Because of excellent optical and superior mechanical properties of the material, the sapphire resonator is an excellent candidate for the on-chip device integration of the comb oscillator.

A comprehensive investigation is done on a high-quality frequency comb or multicarrier generation using a single-sideband modulation based recirculating frequency shifter (SSB-RFS), which has shown a good capability to provide a large number of frequency-locked carriers for terabits transmission. The dominant factors, such as I/Q modulator, direct current-bias, radio-frequency signal, and band pass filter, are analyzed theoretically and experimentally in terms of the metrics such as flatness, signal-to-noise ratio, and stability. The achieved 69 flat tones by noise suppressed SSB-RFS are presented. An example of SSB-RFS based 2.56  Tb/s PDM-RZ-16QAM 792 km transmission is illustrated, which is enabled by the 20 high-quality carrier tones generated with the optimally operated SSB-RFS source.

We experimentally study the factors that influence the span in frequency combs derived from the crystalline whispering gallery mode resonators. We observe that cavity dispersion plays an important role in generation of combs by a cascaded four-wave mixing process. We observed combs from the resonators with anomalous dispersion and nearly zero dispersion at the pump wavelength. In addition, the comb generation efficiency is found to be affected by the crossing of modes of different families. The influence of Raman gain is discussed as well as the roles of the cavity diameter and pump power.

Yarn hairiness has been an important indication of yarn quality that affects weaving production and fabric appearance. In addition to many dedicated instruments, various image analysis systems have been adopted to measure yarn hairiness for potential values of high accuracy and low cost. However, there is a common problem in acquiring yarn images; that is, hairy fibers protruding beyond the depth of field of the imaging system cannot be fully focused. Fuzzy fibers in the image inevitably introduce errors to the hairiness data. This paper presents a project that attempts to solve the off-focus problem of hairy fibers by applying a new imaging scheme—multifocus image fusion. This new scheme uses compensatory information in sequential images taken at the same position but different depths to construct a new image whose pixels have the highest sharpness among the sequential images. The fused image possesses clearer fiber edges, permitting more complete fiber segmentation and tracing. In the experiments, we used six yarns of different fiber contents and spinning methods to compare the hairiness measurements from the fused images with those from unfused images and from the Uster tester.

With the advent of modern computing and imaging technologies, digital holography is becoming widespread in various scientific disciplines such as microscopy, interferometry, surface shape measurements, vibration analysis, data encoding, and certification. Therefore, designing an efficient data representation technology is of particular importance. Off-axis holograms have very different signal properties with respect to regular imagery, because they represent a recorded interference pattern with its energy biased toward the high-frequency bands. This causes traditional images’ coders, which assume an underlying 1/f² power spectral density distribution, to perform suboptimally for this type of imagery. We propose a JPEG 2000-based codec framework that provides a generic architecture suitable for the compression of many types of off-axis holograms. This framework has a JPEG 2000 codec at its core, extended with (1) fully arbitrary wavelet decomposition styles and (2) directional wavelet transforms. Using this codec, we report significant improvements in coding performance for off-axis holography relative to the conventional JPEG 2000 standard, with Bjøntegaard delta-peak signal-to-noise ratio improvements ranging from 1.3 to 11.6 dB for lossy compression in the 0.125 to 2.00 bpp range and bit-rate reductions of up to 1.6 bpp for lossless compression.

We propose an approach for multiple change detection in multispectral remote sensing images based on joint sparse representation. The principal idea is that each change class lies in a low-dimensional space, in which the change vectors can be represented by a linear combination of a few representation atoms. Our method includes two stages: (1) in the learning stage, we learn a subdictionary for each change class from the training samples; and (2) in the reference stage, each change pixel vector is represented with respect to all subdictionaries and assigned to the class with minimum representation errors. Furthermore, taking into account the spatial contextual information, we propose a joint sparsity model to represent each pixel and its similar neighbors simultaneously, thereby enhancing the robustness of the representation to noise. To validate the effectiveness of the proposed method, we choose Shenzhen, China, as the study area in the context of fast urban growth. During the experiments, the proposed method achieves better results on two Landsat Enhanced Thematic Mapper Plus images than does another state-of-the-art supervised change-detection method.

We present a digital video stabilization approach that provides both robustness and efficiency for practical applications. In this approach, we adopt a stabilization model that maintains spatio-temporal information of past input frames efficiently and can track original stabilization position. Because of the stabilization model, the proposed method does not need accumulative global motion estimation and can recover the original position even if there is a failure in interframe motion estimation. It can also intelligently overcome the situation of damaged or interrupted video sequences. Moreover, because it is simple and suitable to parallel scheme, we implement it on a commercial field programmable gate array and a graphics processing unit board with compute unified device architecture in a breeze. Experimental results show that the proposed approach is both fast and robust.

To avoid the vergence-accommodation mismatch and provide a strong sense of presence to users, we applied a prism array-based display when presenting three-dimensional (3-D) objects. Emotional pictures were used as visual stimuli to increase the signal-to-noise ratios of steady-state visually evoked potentials (SSVEPs) because involuntarily motivated selective attention by affective mechanisms can enhance SSVEP amplitudes, thus producing increased interaction efficiency. Ten male and nine female participants voluntarily participated in our experiments. Participants were asked to control objects under three viewing conditions: two-dimension (2-D), stereoscopic 3-D, and prism. The participants performed each condition in a counter-balanced order. One-way repeated measures analysis of variance showed significant increases in the positive predictive values in the prism condition compared to the 2-D and 3-D conditions. Participants’ subjective ratings of realness and engagement were also significantly greater in the prism condition than in the 2-D and 3-D conditions, while the ratings for visual fatigue were significantly reduced in the prism condition than in the 3-D condition. The proposed methods are expected to enhance the sense of reality in 3-D space without causing critical visual fatigue. In addition, people who are especially susceptible to stereoscopic 3-D may be able to use the affective brain–computer interface.

Advances in microbolometer long-wave infrared (LWIR) detectors have led to the common use of infrared cameras that operate without active temperature stabilization, but the response of these cameras varies with their own temperature. Therefore, obtaining quantitative data requires a calibration that compensates for these errors. This paper describes a method for stabilizing the camera’s response through software processing of consecutive images of the scene and images of the camera’s internal shutter. An image of the shutter is processed so that it appears as if it were viewed through the lens. The differences between the scene and the image of the shutter treated as an external blackbody are then related to the radiance or temperature of the objects in the scene. This method has been applied to two commercial LWIR cameras over a focal plane array temperature range of ±7.2°C, changing at a rate of up to ±0.5°C/min. During these tests, the rms variability of the camera output was reduced from ±4.0°C to ±0.26°C.

Flat field calibration of broadband imaging systems is widely used, and it has been said that users should try to make the spectrum of the flatfield calibration light source as close as possible to that of the measurement object. However, a quantitative analysis of the error induced by a mismatch of calibration and object spectra has been lacking. In order to develop this quantitative analysis, we provide a theoretical radiometric model for flatfield calibration and show how this spectral mismatching error arises. Simulations covering a variety of measurement scenarios indicate that spectral mismatching can create quantitative errors of up to a factor of 5 in situations that are regularly encountered by researchers performing quantitative work.

Effects of different structural parameters on the optical characteristics of multistage microring resonators (MRRs) have been investigated while a soliton pulse is propagated. Effects of MRR radius, width, and gap between the MRR and the waveguide on the free spectral range (FSR) and the coupling coefficient have been studied. It has been shown that in the structure consisting of multistage MRRs, increasing the MRR radius and width would decrease the FSR. It is also indicated that by increasing the gap the width coupling coefficient would be enhanced at first, and then it would be diminished. The goal of this research is to improve the functionality of multistage MRRs by decreasing the FSR and increasing the coupling coefficient for soliton-based communication systems.

Laser radar (LADAR) is a three-dimensional (3-D) imaging system which provides very high-resolution 3-D images and is used for automatic recognition of 3-D targets. In order to achieve a high recognition ability for this system, a new LADAR image descriptor is proposed. This descriptor extracts features from the LADAR images by using special chromatic processors called invariant half-height overlapping triangular processors. The descriptor developed has a high discrimination capability and is robust to the effects that disturb LADAR images. The performance of the proposed LADAR descriptor is evaluated using simulated and real LADAR images. The results show the robustness of the proposed descriptor in the presence of noise, low resolution, view change, rotation, translation, and scaling effects. The results also show the high discrimination capability for the new descriptor over the traditional techniques such as invariant moments descriptor, which is used to benchmark the simulation and the experimental results.

A noncontacting combined measuring system comprising a nondiffracting beam attitude angle sensor, laser rangefinder, and total station is proposed for the coordinate measurement of hidden parts in large-scale spaces. The laser from the total station is aimed at the retro-reflector of the attitude angle sensor to obtain the spatial coordinates. Moreover, parts of the laser pass through the retro-reflector and generate a nondiffracting beam spot on the charge-coupled diode through an axicon lens. The horizontal angle of the attitude angle sensor can be obtained by using center fitting of the nondiffracting beam spot. An inclinometer can be used to obtain the rolling angle and the pitching angle. The laser from the rangefinder is directed at the measured point to measure the distance from the attitude angle sensor to the measured point. Finally, the spatial coordinates of the measured point can be calculated by using the measured spatial coordinates, the three attitude angles, and the distance. The experimental results of the combined measuring system are compared with those of the total station. The two results show good agreement.

Direct detection Doppler wind lidar (DWL) has been demonstrated for its capability of atmospheric wind detection ranging from the troposphere to stratosphere with high temporal and spatial resolution. We design and describe a fiber-based optical receiver for direct detection DWL. Then the locking error of the relative laser frequency is analyzed and the dependent variables turn out to be the relative error of the calibrated constant and the slope of the transmission function. For high accuracy measurement of the calibrated constant for a fiber-based system, an integrating sphere is employed for its uniform scattering. What is more, the feature of temporally widening the pulse laser allows more samples be acquired for the analog-to-digital card of the same sampling rate. The result shows a relative error of 0.7% for a calibrated constant. For the latter, a new improved locking filter for a Fabry–Perot Interferometer was considered and designed with a larger slope. With these two strategies, the locking error for the relative laser frequency is calculated to be about 3 MHz, which is equivalent to a radial velocity of about 0.53  m/s and demonstrates the effective improvements of frequency locking for a robust DWL.

A method for simple and reliable displacement measurement with nanoscale resolution is proposed. The measurement is realized by combining a common optical microscopy imaging of a specially coded nonperiodic microstructure, namely two-dimensional zero-reference mark (2-D ZRM), and subsequent correlation analysis of the obtained image sequence. The autocorrelation peak contrast of the ZRM code is maximized with well-developed artificial intelligence algorithms, which enables robust and accurate displacement determination. To improve the resolution, subpixel image correlation analysis is employed. Finally, we experimentally demonstrate the quasi-static and dynamic displacement characterization ability of a micro 2-D ZRM.

Hilbert transform (HT) is widely used in temporal speckle pattern interferometry, but errors from low modulations might propagate and corrupt the calculated phase. A spatio-temporal method for phase retrieval using temporal HT and spatial phase unwrapping is presented. In time domain, the wrapped phase difference between the initial and current states is directly determined by using HT. To avoid the influence of the low modulation intensity, the phase information between the two states is ignored. As a result, the phase unwrapping is shifted from time domain to space domain. A phase unwrapping algorithm based on discrete cosine transform is adopted by taking advantage of the information in adjacent pixels. An experiment is carried out with a Michelson-type interferometer to study the out-of-plane deformation field. High quality whole-field phase distribution maps with different fringe densities are obtained. Under the experimental conditions, the maximum number of fringes resolvable in a 416×416 frame is 30, which indicates a 15λ deformation along the direction of loading.

A method for tuning the chirp ratio of a fiber grating without the central wavelength shift is proposed by using a two-fixed-end compressive beam, which induces a linear strain distribution along the grating. This technique allows dynamic control of the grating’s chirp ratio by changing the displacement of the translation stage. The 3-dB bandwidth tuning range is from 6.52 to 13.5 nm when the grating is tuned in the broadening way, while the bandwidth tuning range is from 6.52 to 0.95 nm in the compressing way. The proposed method has potential applications for the dynamic dispersion compensation and the sensors of pressure and displacement, etc., by detecting the change of bandwidth information.

Cooled infrared detectors are typically characterized by well-known electro-optical parameters: responsivity, noise equivalent temperature difference, shot noise, 1/f noise, and so on. Particularly important for staring arrays is also the residual fixed pattern noise (FPN) that can be obtained after the application of the nonuniformity correction (NUC) algorithm. A direct measure of this parameter is usually hard to define because the residual FPN strongly depends, other than on the detector, on the choice of the NUC algorithm and the operative scenario. We introduce three measurable parameters: instability, nonlinearity, and a residual after a polynomial fitting of the detector response curve, and we demonstrate how they are related to the residual FPN after the application of an NUC (the relationship with three common correction algorithms is discussed). A comparison with experimental data is also presented and discussed.

A simple method employing an optical probe is presented to measure density variations in a hypersonic flow obstructed by a test model in a typical shock tunnel. The probe has a plane light wave trans-illuminating the flow and casting a shadow of a random dot pattern. Local slopes of the distorted wavefront are obtained from shifts of the dots in the pattern. Local shifts in the dots are accurately measured by cross-correlating local shifted shadows with the corresponding unshifted originals. The measured slopes are suitably unwrapped by using a discrete cosine transform based phase unwrapping procedure and also through iterative procedures. The unwrapped phase information is used in an iterative scheme for a full quantitative recovery of density distribution in the shock around the model through refraction tomographic inversion. Hypersonic flow field parameters around a missile shaped body at a free-stream Mach number of 5.8 measured using this technique are compared with the numerically estimated values.

Impulsive noise is a major problem that seriously degrades the performance of self-mixing interferometry (SMI). A new method to rectify this issue is proposed. First, an outlier detection approach is employed to detect the data samples corrupted by the impulsive noise, and then the SMI signal waveform is rectified by means of least square (LS) curve fitting. The results show that the proposed method can effectively remove the impulsive noise without introducing distortion to the original waveform and thus lead to improvement in the performance of an SMI system.

A beat-note frequency stabilization system using a distributed-feedback laser and external cavity laser diode has become a very important technique for laser spectroscopy, where highly stabilized high-frequency beat notes are required. We have developed a simple and versatile system capable of stabilizing the high-frequency beat notes (3 to 11 GHz) of two lasers using a delayed radio frequency self-heterodyne interferometer and have confirmed its basic operation. The frequency stability of the obtained beat notes is higher than 1 MHz in the 3- to 11-GHz frequency range with an average time of 20 s.

This work demonstrates the use of adaptive polymer lenses (APLs) for short-wavelength infrared (SWIR) applications. First, we present a push-button adaptive optical zoom system for variable magnification with a SWIR focal plane array. We then present a push-button, variable divergence, SWIR laser system for pointing and illumination. Last, we outline a system that combines the two: an SWIR adaptive zoom coupled with an APL-enhanced designator/illuminator. The result would allow a user to toggle between different fields of view (magnification), while optimizing illumination (beam divergence) for each field of view. This could be critical for situational awareness and target identification/designation in tactical applications.

We propose a laser fabrication approach for single monolith structure by varying beam focusing lenses (NA₁=1.4 and NA₂=0.45). With this, we show great improvement of the manufacturing throughput while keeping the desired spatial resolution and feature definition. To quantitatively evaluate its efficiency, we theoretically calculate and experimentally measure the polymerized volume using different focusing conditions and show that the augmentation can reach more than sevenfold. Based on this, sample structures of hybrid organic-inorganic SZ2080 and hydrogel polyethylene (glycol) diacrylate photopolymers were successfully manufactured. These three-dimensional structures included prototype scaffolds for cell growth and microfluidic components. Their quality was assessed and possible difficulties as well as limitations that arose were discussed. The new structures were compared to alternative direct laser writing throughput increasing techniques. Finally, we discussed potential applications in manufacturing various elements for regenerative medicine and microfluidics.

A half cycle 64-quadrature amplitude modulation (QAM) Nyquist subcarrier modulation (SCM) polarization division multiplexing (PDM) intensity modulation direct detection optical communication system is experimentally demonstrated. At the receiver, training sequences-based channel estimation and an overlap frequency domain equalization method are proposed to enhance the system performance. The experimental results show that the half cycle 64-QAM Nyquist-SCM PDM signal can be transmitted over 43-km standard single-mode fibers with a bit error rate below the forward error coding threshold of 2.4×10⁻².

Using intensity feedback, the closed-loop behavior of an acousto-optic hybrid device under profiled beam propagation has been recently shown to exhibit wider chaotic bands potentially leading to an increase in both the dynamic range and sensitivity to key parameters that characterize the encryption. In this work, a detailed examination is carried out vis-à-vis the robustness of the encryption/decryption process relative to parameter mismatch for both analog and pulse code modulation signals, and bit error rate (BER) curves are used to examine the impact of additive white noise. The simulations with profiled input beams are shown to produce a stronger encryption key (i.e., much lower parametric tolerance thresholds) relative to simulations with uniform plane wave input beams. In each case, it is shown that the tolerance for key parameters drops by factors ranging from 10 to 20 times below those for uniform plane wave propagation. Results are shown to be at consistently lower tolerances for secure transmission of analog and digital signals using parameter tolerance measures, as well as BER performance measures for digital signals. These results hold out the promise for considerably greater information transmission security for such a system.

A reflective graphene saturable absorber mirror (SAM) was successfully fabricated by chemical vapor deposition technology. A stable diode-pumped passively mode-locked Yb³+:Sc₂SiO₅ laser using a graphene SAM as a saturable absorber was accomplished for the first time. The measured average output power amounts to 351 mW under the absorbed pump power of 12.5 W. Without prisms compensating for dispersion, the minimum pulse duration of 7 ps with a repetition rate of 97 MHz has been obtained at the central wavelength of 1063 nm. The corresponding peak power and the maximum pulse energy were 516 W and 3.6 nJ, respectively.

High spatial resolution information of the rare-earth dopant distribution in optical fibers enriches our understanding of the fiber manufacture processes and enables improvement in the design of active photonic devices including optical fiber lasers and amplifiers. Here, data from an investigation of the backscattered fluorescence signal off the end-face of an erbium (Er³+) doped silica optical fiber obtained with a near-field scanning optical microscope (NSOM) are presented. It has been recently confirmed via a theoretical model that information about the relative Er³+ ion distribution in fibers can be inferred by simply monitoring the fluorescence signal originating from the de-excitation of specific energy levels in the investigated samples. A comparison of the Er³+ ion distribution profiles extracted from the fluorescence measurements acquired through the NSOM system with those obtained from the application of a powerful analytical ion probe is also presented.

Laser microforming is extensively used to align components with submicrometer accuracy, often after assembly. While laser-bending sheet metal is the most common laser-forming mechanism, the in-plane upsetting mechanism is preferred when a high actuator stiffness is required. A three-bridge planar actuator made out of Invar 36 sheet was used to characterize this mechanism by experiments, finite element method modeling, and a fast-reduced model. The predictions of the thermal models agree well with the temperature measurements, while the final simulated displacement after 15 pulses deviates up to a factor of 2 from the measurement, using standard isotropic hardening models. Furthermore, it was found from the experiments and models that a small bridge width and a large bridge thickness are beneficial to decrease the sensitivity to disturbances in the process. The experiments have shown a step size as small as 0.1  μm, but with a spread of 20%. This spread is attributed to scattering in surface morphology, which affects the absorbed laser power. To decrease the spread and increase the positioning accuracy, an adapted closed-loop learning algorithm is proposed. Simulations using the reduced model showed that 78% of the alignment trials were within the required accuracy of ±0.1  μm.

Subpicosecond pulse spectral compression without the limitation of the adiabatic soliton transform status in a comb-like distributed fiber (CDF) is proposed and numerically demonstrated. The principle of spectral compression in a single concatenation of single-mode fiber and high nonlinear fiber is theoretically analyzed and numerically proved. A three-stage CDF is designed and simulated following the principle. The simulation results show that an overall spectral compression ratio of 56.23 is obtained. The designed three-stage CDF is used in an all-optical analog-to-digital system to improve the quantization resolution to 7.1 bit.

The impact of atmospheric phase turbulence on Gaussian beam propagation along propagation paths of varying lengths is examined using multiple random phase screens. The work is motivated by research involving generation and encryption of acousto-optic chaos, and the interest in examining propagation of such chaotic waves through atmospheric turbulence. A phase screen technique is used to simulate perturbations to the refractive index of the medium through the propagation path. A power spectral density based on the modified von Karman spectrum model for turbulence is used to describe the random phase behavior of the medium. In recent work, results for the numerical simulation of phase turbulence over a narrow region of space implemented by placing a planar aperture representing a (narrow) random phase screen were presented. Results are presented pertinent to extended phase screens (via multiple random-phase apertures) through which an incident Gaussian beam propagates incrementally via alternate phase transmission and diffraction along the propagation path. Additionally, for profiled electromagnetic waves (such as Gaussian), the scintillation index is evaluated for extended phase turbulence, and finally, fringe visibility due to the interference of double-Gaussian beams passing through extended turbulence is examined.

Free-space optical communication (FSO) is emerging as a captivating alternative to work out the hindrances in the connectivity problems. It can be used for transmitting signals over common lands and properties that the sender or receiver may not own. The performance of an FSO system depends on the random environmental conditions. The bit error rate (BER) performance of differential phase shift keying FSO system is investigated. A distributed strong atmospheric turbulence channel with pointing error is considered for the BER analysis. Here, the system models are developed for single-input, single-output–FSO (SISO-FSO) and single-input, multiple-output–FSO (SIMO-FSO) systems. The closed-form mathematical expressions are derived for the average BER with various combining schemes in terms of the Meijer’s G function.

Dense wavelength division multiplexing has been proposed as a means of implementing communications on aircraft. In such applications, power consumption is a critical consideration. The impact of reducing the transmitter power and recovering losses using a shared amplifier has been investigated. By recovering the power loss using a shared amplifier transmitter, power savings can be made. This network has been modeled and savings of 20% are predicted in a realistic aircraft environment.

To relax the limited sampling rate of an analog-to-digital converter (ADC) and to reduce the complexity of conventional fiber-optical superchannel coherent detection, we propose and demonstrate a joint digital signal processing (DSP) technique of Nyquist-wavelength division multiplexing superchannel with group detection. At the receiver side, three Nyquist-spaced channels with 12.5 Gbaud polarization multiplexing-quadrature phase shift keying signals are group detected with a sampling rate per channel of 1.33 times over the normal sampling rate. A modified carrier separation technique is then put forward to retrieve the high-frequency interference component of both the designated channel and its adjacent channels, which can subsequently be used to recover the designated channel with new constant modulus algorithm-based joint multiinput-multioutput equalizers. The results show that the proposed group detection and joint DSP algorithm can simultaneously improve the transmission performance and reduce the complexity of both the transmitter and receiver, regardless of bandwidth restrictions from the waveshaper, ADC module, and coherent receiver.

We propose a prototype for frequency-reconfigurable terahertz (THz) wireless transmission using an optical comb based on radio-over-fiber technology. In the proposal, an electro-absorption modulator (EAM) followed by a phase modulator and an intensity modulator are used to generate a flat optical comb with a tunable frequency spacing. Then, a different THz signal can be generated by photo-mixing of optical two-tone signals from the comb lines. In the scheme, we obtain 46 comb lines within a 1-dB power deviation with an interval of about 10 GHz, and a THz up to 440 GHz is obtained. Moreover, a much higher bandwidth can be easily reached by adjusting the driving signal of the EAM. The feasibility and tunability of the proposed scheme are confirmed by the simulations. This method shows a simple cost-efficient configuration and good performance over long-distance delivery.

We report a continuous wave (CW) Tm:YLF laser-resonantly pumped Ho:LuAG laser in CW mode and passive Q-switched (PQS) mode with multilayer graphene as a saturable absorber. The lasing performance with different output couplers is analyzed. The best result was obtained with −150-mm radius curvature and 10% transmission output coupler. Under the CW mode, Ho:LuAG laser generated a maximum 1.3-W output power at the center wavelength of 2100.65 nm, corresponding to a slope efficiency of 34.6%. In PQS mode, the minimum pulse width observed is 784.7 ns with a center wavelength of 2100.65 nm at the pump power of 5.4 W. Meanwhile, the maximum output power is 0.37 W with a pulse repetition frequency of 47.3 kHz. The results show that the multilayer graphene can be used as a saturable absorber in a Ho³⁺-doped laser around a 2-μm wavelength.

We demonstrate a channel-reuse bidirectional 10-Gb/s/λ long-reach DWDM-PON and an optical beat noise-based automatic wavelength control method for a tunable laser used in a colorless optical network unit. 100-GHz channel spacing 55- and 100-km full-duplex bidirectional 320  Gb/s (32×10  Gb/s) capacity transmissions are achieved without and with optical amplification. Transmission performance is also measured with different optical signal to Rayleigh backscattering noise ratios and different central wavelength shifts between upstream and downstream in the channel-reuse system.

An ultrawideband single-polarization single-mode (SPSM) and nearly zero flattened dispersion photonic crystal fiber (PCF) based on a rectangular lattice with square air holes is proposed. The performance of the PCF is investigated by using the full-vector finite element method, and the impact of variation of geometric parameters is also numerically analyzed. The simulation results reveal that the single-polarization bandwidth of the proposed PCF can achieve 1.4  μm over 0.68 to 2.08  μm for the optimized designed parameters and can be freely adjusted by tuning the air hole dimensions. The nearly zero flattened chromatic dispersion about 0±0.8  ps/(km.nm) can be obtained by controlling the size of the central hole, but its existence makes the power distribution extraordinary. Moreover, the confinement loss is less than 10⁻⁶  dB/m over 0.68 to 1.86  μm for the PCF with 10 rings of air holes. The proposed PCF may be useful for applications with a wide SPSM operating bandwidth or flattened dispersion requirement.

A pump-probe photothermal mirror (PTM) method has been developed to determine the thermal diffusivity of opaque solid samples. The method involves the detection of the distortion of a probe beam whose reflection profile is affected by the photoelastic deformation of a polished material surface induced by the absorption of a focused pump field. We have measured the time dependence of the PTM signal of Ti, Al, Cu, Sn, Ag, and Ni samples. We show theoretically and experimentally that the time derivative of the signal in the first microseconds is proportional to the square root of the thermal diffusivity coefficient. The method affords a simple calibration and efficient interpretation of experimental data for a sensitive determination of the thermal diffusivity coefficient for materials. We demonstrate the applicability of the technique by measuring the thermal diffusivities of wadsleyite (β-Mg₂SiO₄) and diopside (MgCaSi₂O₆), two important minerals relevant to geophysical studies.

We propose and experimentally demonstrate an industrial wavelength selective switch (WSS) architecture based on a microelectromechanical system (MEMS) micromirror array. An anisotropic uniaxial crystal and a half-wave plate are employed as the polarization-diversity solution to reduce the polarization-dependent loss (PDL) of the WSS, and an achromatic lens composed of a positive lens and a negative lens is adopted to eliminate chromatic aberration. Experimental results show that it is possible to select and switch any wavelength channel from an input port to any output port through a two-stage beam steering process. The measured insertion loss and PDL are 4.78 and 0.5 dB, respectively. Benefiting from the customized MEMS micromirror array with a high fill factor of 96%, the 3 dB bandwidth is about 90 GHz. The results also show that the proposed WSS possesses uniform and superflat passbands for different wavelength channels.

By developing a pyramid-shaped reflector, we were able to remove the optical filter that causes the decline in light efficiency in existing systems. A pyramid-type glass optical multiplexer (MUX) can be designed for beam coupling efficiency <60% in a combined module. The module was designed and optimized using CODE V, which utilizes nonlinear curve fitting numerical analysis. Based on optical design data, aspheric grinding paths were developed using ULG APS software. Tungsten carbide optical MUX mold cores were fabricated with an ultraprecision grinding machining device [ULG-100C(H³)] and optimum grinding machining of optical surface roughness (<60  nm, angle tolerance <±0.05  deg). Pyramid-type optical MUX was fabricated using the hot press molding technique, and it was measured by using a contact geometry measuring device for reflection angle and angle uniformity. The measurement data were suitable for 49±0.05  deg, which was the design criterion. In addition, angle uniformity was <99.985%. A pyramid-type glass optical MUX molding technique was developed using an optical mold design, ultraprecision grinding machining technology, and a hot press molding system.

The experimental results of the system test of an optical resonant passive gyroscope based on a high Q-factor ring resonator in InP technology are reported. The open loop configuration based on the phase modulation was preferred among the analyzed configuration options, especially because it is potentially suitable for the monolithic integration of the entire sensor on a single chip. The setup components are described with a special emphasis on a custom digital readout board based on a field-programmable gate array. The board processes the input signals according to the proportional-integral algorithm which has been implemented through an optimized firmware. For the system test, the sensor rotation has been simulated using two properly driven acousto-optic modulators. The results reported here prove the gyro functionality and are a good starting point for the full development of the sensor.

Surface-plasmon-polariton (SPP) wave excitation in a composite structure obtained by depositing a columnar thin film (CTF) on a metal (silver) substrate decorated periodically by an array of rectangular grooves was analyzed using the rigorous coupled-wave approach. The SPP wave is excited as a Floquet harmonic of different orders by light incident from different directions, the incidence plane being the same as the grating plane. But, in some instances, the SPP wave appears to be excited as a doublet, i.e., a Floquet harmonic of the same order at two closely spaced angles of incidence, the excitation being less efficient for one angle of incidence than for the other. The characteristics of the SPP wave are affected by the vapor incidence angle chosen to deposit the CTF.

The theory of silicon optical modulators of ring resonators based on PN diode in reverse bias is primarily discussed. It secondarily provides a full-featured simulator to investigate the behavior of such modulators. Wave equation for ring structure will be solved by using the conformal transformation method and the matrix method as it was used to analyze bent planar optical waveguides. Power coupling between ring and straight waveguides will be calculated by coupled theory of nonparallel waveguides based on experimental results. The time response demonstrates the capability of this device to operate correctly at up to 10  Gbs⁻¹ bitrate, and the frequency spectrum analysis of device shows a <17  dB drop in transmission at the resonant wavelength of ∼1574.9  nm.

Planar photonic crystal nanocavities, made in a thin photonic-crystal membrane surrounded by symmetric cladding layers, are designed for integrated optics. Following the design, the quality factor (Q) and the resonant wavelength of the cavity are numerically analyzed by the three-dimensional finite-difference time-domain method and filter diagonalization approach. Optimization of the cavity design by modulating the structure parameter yields a high Q-factor. A nanocavity with symmetric cladding layers is studied and it is found that when the cavity has a low-refractive index (RI) symmetric cladding, the reflection losses of increase due to the smaller RI difference, and the Q-values drop considerably. Furthermore, we demonstrate that the Q-factor of the cavity depends on the geometric structure of the cladding and the calculated Q-factor for the designed cavity with periodic cladding increases by a factor of ∼1.4 relative to that for a designed cavity with a solid cladding. For operation at a telecommunication wavelength of 1550 nm, the Q-factor of the designed cavities is higher than 10⁴. This cavity design can help to enhance the optical nonlinearities and mechanical stability of a photonic crystal membrane structure.

Scale factor nonlinearity (SFN) and dynamic range (DR) are the two significant parameters used to evaluate the performance of a resonator fiber optic gyro (RFOG). The inherent SFN of an open-loop RFOG with triangular phase modulation is first analyzed theoretically, and its relationship with the DR is simulated, showing that the DR is significantly constrained by the SFN. For our system, when the SFN is 1%, the DR is less than ±82  deg/s. To decrease the SFN in a certain DR, a real-time compensation method based on a field-programmable gate array is proposed. The compensation model is set up and the compensation scheme is illustrated. With the proposed method, the SFN of the RFOG is decreased from 1.53% to 0.057% with a DR of ±100  deg/s.

Graphene is used as an ideal platform for plasmonic optical devices due to its unique electrical and optical characteristics. We proposed a unique effective index method to evaluate a graphene-based half Maxwell fish-eye (HMFE) plasmonic lens on a single flake of graphene, which can focus a surface plasmon polariton plane wave and transform into a spherical wave. Similar to traditional optics, the proposed method can be used to investigate the focusing performance of an HMFE lens versus different parameters, including the incident frequencies, the discrete semiring numbers, the chemical potentials, and the size of the proposed lens. In addition, we use two proposed lenses to build an optical-coupling element, which can transform the beam of a plane wave into a different beam width. The proposed effective index method can be used as a reference in designing plasmonic optical devices with a variable effective index profile on a flake of graphene. The finite element method is employed to realize the numerical simulation, which demonstrates results almost consistent with our design methodology.

This PDF file contains the errata for “OE Vol. 53 Issue 12 Paper OE-2014-1201-ERR” for OE Vol. 53 Issue 12