My first exposure to Optical Engineering occurred as a graduate student at Georgia Tech shortly after I attended an SPIE optical fibers conference in Atlanta and subsequently became a student member of SPIE. Perhaps for that reason, I have always maintained a sentimental connection to this prestigious journal. I recall receiving my first copy of Optical Engineering in the mail, attempting to read and understand each article, and realizing that I had much to learn about optics. Fortunately, that learning process has never ended.

A multimodal interferometer based on a new microstructured fiber tip is proposed for detection of the evaporation process of acetone. The new geometry consists of a capillary tube in which an offset Ge-doped core is fused and spliced at the end of a single-mode fiber. The fiber tip sensor structure was immersed in liquid acetone and the evaporation process of acetone was monitored in real time. Due to the refractive index variation of the external medium with increasing temperature, a short detection time of ∼1  s was achieved.

The past two decades have produced significant advancement in the state-of-the-art for many single-photon technologies. The use of single-photon generation and detection is being pursued over an enormous portion of the electromagnetic spectrum ranging from ultraviolet to millimeter wavelengths, and the breadth of applications that rely on these technologies—including fluorescence techniques, quantum information processing, and photon-starved imaging and communications—continues to grow rapidly. The papers in this special section of Optical Engineering provide a snapshot of some of the recent work that has focused on promising new single-photon component technologies and applications.

The scintillation index is a common measure of the effects of atmospheric turbulence. Using a photon-counting sensor, the integration time for each sample needs to be short enough to ensure that the intensity is constant during this time. Simultaneously, hardware limitations, including detector dead-time, restrict the count rate so that the number of counts in a single time segment is extremely low. The dead-time also introduces nonlinear effects. The variance calculation in the scintillation index formula is then dominated by quantization error, and the scintillation index is severely overestimated. We investigate two methods of correcting the scintillation index based on data from a time-correlated single-photon counting laser radar system. The first approach is based on the covariance calculation of the data and can be used for very low count rates and high temporal resolution. This method may also be useful in other cases where the variance of noisy, time-resolved data needs to be calculated. The second method is based on fitting the theoretical probability density function for the intensity fluctuation caused by propagation through turbulence to the experimental data. This method can take dead-time effects into account and be used for higher count rates.

A photon counting detector that has been optimized for optical time transfer experiments is presented. The newly developed detector’s operational scheme enables us to achieve very high useful data yields exceeding 50% while still maintaining pure single photon operation and wide dynamical range. The photon counting detector was designed and approved for operation also in a space environment. Its construction is extremely rugged and compact. The long-term detection delay stability in the sense of time deviation is excellent—typically 200 fs for averaging times of hours. The change of the detection delay with temperature is typically below 260  fs/K . Both the detector and its operational scheme might be employed in optical time transfer experiments on single photon signal level. Another attractive application is foreseen in future space missions, where a photon counting device providing subpicosecond timing stability and radiation tolerance is required.

Geiger-mode avalanche photodiodes (GM-APDs) use the avalanche mechanism of semiconductors to amplify signals in individual pixels. With proper thresholding, a pixel will be either “on” (avalanching) or “off.” This discrete detection scheme eliminates read noise, which makes these devices capable of counting single photons. Using these detectors for imaging applications requires a well-developed and comprehensive expression for the expected signal-to-noise ratio (SNR). This paper derives the expected SNR of a GM-APD detector in gated operation based on gate length, number of samples, signal flux, dark count rate, photon detection efficiency, and afterpulsing probability. To verify the theoretical results, carrier-level Monte Carlo simulation results are compared to the derived equations and found to be in good agreement.

A linear mode photon counting focal plane array using HgCdTe mid-wave infrared (MWIR) cutoff electron initiated avalanche photodiodes (e-APDs) has been designed, fabricated, and characterized. The broad spectral range (0.4 to 4.3 μm) is unique among photon counters, making this a “first of its kind” system spanning the visible to the MWIR. The low excess noise [F(M)≈1] of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500–1000. The readout integrated circuit (ROIC) design included a very high gain-bandwidth product resistive transimpedance amplifier (3×10 ¹³   Ω-Hz ) and a 4 ns output digital pulse width comparator. The ROIC had 16 high-bandwidth analogs and 16 low-voltage differential signaling digital outputs. The 2×8 array was integrated into an LN2 Dewar with a custom leadless chip carrier and daughter board design that preserved high-bandwidth analog and digital signal integrity. The 2×8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were approximately 1 pA at 13 V bias where the measured avalanche photodiode gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse signal-to-noise ratio of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a photon background limited false event rate of about 1 MHz. The measured jitter was in the range of 550–800 ps. The demonstrated minimum time between distinguishable events was less than 10 ns.

The behavior of the gain-voltage characteristic of the mid-wavelength infrared cutoff HgCdTe linear mode avalanche photodiode (e-APD) is discussed both experimentally and theoretically as a function of the width of the multiplication region. Data are shown that demonstrate a strong dependence of the gain at a given bias voltage on the width of then− gain region. Geometrical and fundamental theoretical models are examined to explain this behavior. The geometrical model takes into account the gain-dependent optical fill factor of the cylindrical APD. The theoretical model is based on the ballistic ionization model being developed for the HgCdTe APD. It is concluded that the fundamental theoretical explanation is the dominant effect. A model is developed that combines both the geometrical and fundamental effects. The model also takes into account the effect of the varying multiplication width in the low bias region of the gain-voltage curve. It is concluded that the lower than expected gain seen in the first 2×8 HgCdTe linear mode photon counting APD arrays, and higher excess noise factor, was very likely due to the larger than typical multiplication region length in the photon counting APD pixel design. The implications of these effects on device photon counting performance are discussed.

We describe a number of methods that have been pursued to develop superconducting nanowire single-photon detectors (SNSPDs) with attractive overall performance, including three systems that operate with <70% system detection efficiency and high maximum counting rates at wavelengths near 1550 nm. The advantages and tradeoffs of various approaches to efficient optical coupling, electrical readout, and SNSPD design are described and contrasted. Optical interfaces to the detectors have been based on fiber coupling, either directly to the detector or through the substrate, using both single-mode and multimode fibers with different approaches to alignment. Recent advances in electrical interfaces have focused on the challenges of scalability and ensuring stable detector operation at high count rates. Prospects for further advances in these and other methods are also described, which may enable larger arrays and higher-performance SNSPD systems in the future. Finally, the use of some of these techniques to develop fully packaged SNSPD systems will be described and the performance available from these recently developed systems will be reviewed.

In recent years, significant progress has been made in InP-based Geiger-mode single photon avalanche diodes (SPADs), and a variety of circuits for enabling Geiger-mode operation have been proposed and demonstrated. However, due to the inherent positive feedback of the impact ionization avalanche process, Geiger-mode SPADs are constrained by certain performance limitations, particularly with regard to counting rate and the inability to resolve photon number. To overcome some of the performance limitations of regular SPADs, we have developed negative feedback avalanche diodes (NFADs) that employ a negative feedback mechanism to regulate the avalanche process. The NFAD fabrication process is based on the design platform we use to achieve state-of-the-art performance SPADs and is very flexible. The operation of NFAD devices is also very simple, with only a direct current (DC) bias being required. Various discrete devices and matrices composed of different elements have been designed, fabricated, and characterized. For discrete devices, ∼10% photon detection efficiency has been realized consistent with acceptable afterpulsing probability. The negative feedback mechanism significantly improves the uniformity of the output pulse heights and avalanche charge per detection event, resulting in a low “charge excess noise” factor. When configured in a matrix format, the NFAD devices were demonstrated to have the ability to resolve photon number and work effectively as solid-state photomultipliers (SSPMs) in the shortwave infrared (SWIR) region. The InGaAs/InP NFAD SSPMs have the potential to replace photomultiplier tubes and provide a solid-state solution in applications where the requirement for single-photon sensitivity in the SWIR region beyond ∼0.9  μm  cannot be met by silicon photomultipliers. The NFAD devices have been used in various quantum optics and quantum key distribution applications and demonstrated an excellent performance.

This publication details CMOS foundry fabrication, reliability stress assessment, and packaged sensor test results obtained during qualification of the SensL B-Series silicon photomultiplier (SiPM). SiPM sensors with active-area dimensions of 1, 3, and 6 mm were fabricated and tested to provide a comprehensive review of SiPM performance highlighted by fast output rise times of 300 ps and photon detection efficiency of greater than 41%, combined with low afterpulsing and crosstalk. Measurements important for medical imaging positron emission tomography systems that rely on time-of-flight detectors were completed. Results with LSYO:Ce scintillation crystals of 3×3×20  mm³ demonstrated a 225±2-ps coincidence resolving time (CRT), and the fast output is shown to allow for simultaneous acquisition of CRT and energy resolution. The wafer level test results from ∼150  k 3-mm SiPM are shown to demonstrate a mean breakdown voltage value of 24.69 V with a standard deviation of 0.073 V. The SiPM output optical uniformity is shown to be ±10% at a single supply voltage of 29.5 V. Finally, reliability stress assessment to Joint Electron Device Engineering Council (JEDEC) industry standards is detailed and shown to have been completed with all SiPM passing. This is the first qualification and reliability stress assessment program run to industry standards that has been reported on SiPM.

The generation, measurement, and manipulation of light at the single- and few-photon levels underpin a rapidly expanding range of applications. These range from applications moving into the few-photon regime in order to achieve improved sensitivity and/or energy efficiency, as well as new applications that operate solely in this regime, such as quantum key distribution and physical quantum random number generation. There is intensive research to develop new quantum optical technologies, for example, quantum sensing, simulation, and computing. These applications rely on the performance of the single-photon sources and detectors they employ; this review article gives an overview of the methods, both conventional and recently developed, that are available for measuring the performance of these devices, with traceability to the SI system.

The image quality in off-axis digital holography (DH) is often degraded by inaccuracies in the reference wave used for reconstruction and the spatial filtering adopted to avoid twin images and zeroth order diffraction. To enhance the image quality in such cases, coherent diffraction imaging is combined with a DH technique to iteratively reconstruct the hologram. By using a small aperture on the sample plane as a spatial constraint and the recorded diffraction pattern as an intensity constraint, a higher spatial resolution than usual is obtained with the proposed method.

Coatings are commonly applied to products both for protection and to enhance the appearance. In the manufacturing process, it is essential for coatings to be applied uniformly and with appropriate thickness. However, the coating thickness measurement of nonmetallic thin film remains a challenging task. This paper reports the results of research to explore the potential of regression and artificial neural network (ANN) models for estimating the thickness of nonmetallic coatings. The developed ANN models yielded a lower-error rate than a corresponding regression model. Three parameters were used as inputs to the ANN model to train 1880 network models with six different training algorithms. The input parameters were: temperature increments collected from 90 samples at 1 Hz for 59 s; rising temperature rates and slope of scaled log-reconstructed Laplace-transformed temperature along the real axis; and dEx(T) values derived from the temperature increments. The relative interval radius (RR) goes below 1% in ANN models with 99.5% confidence when dEx(T) is used as an input parameter. In addition, the RR value varies directly as the standard deviation of the modeling sample size varies, but the RR values stay at the same order of magnitude. Further testing found that the ANN models will not guarantee an acceptable prediction under untrained conditions.

Modeling the imaging chain has become a critical design tool to understand the relationship between design trades and image quality in camera systems. The mathematical models for the fundamental components of an imaging chain are well understood and have been validated using working camera systems. However, the complexity of camera designs continues to grow as the technology advances to drive higher performance using different approaches. The fundamental imaging chain models do not always meet the needs of the new imaging system designs, thus requiring the models to advance in complexity as well. Of particular interest to the optical designers is the development of mathematical models that enable more complex modeling of the wavefront errors for the optical transfer functions (OTF) in the image chain models. A tutorial on the imaging chain is given followed by an innovative approach using an η matrix for modeling the OTF in the imaging chain.

An active terahertz wave imaging method is investigated for improved system simplicity, inexpensive implementation, and distance approximation. The proposed technique is composed of a single-pixel setup that allows acquiring the two-dimensional (2-D) Fourier transform intensity map of the imaged object and additional depth data using time of flight method. A raster scan is performed to achieve the object’s 2-D Fourier transform intensity and depth information. Iterative phase retrieval methods are employed to accomplish good image reconstruction using only the measured object’s Fourier transform magnitude. The proposed method uses a glow discharge detector (GDD) as its single millimeter wave pixel and offers a simple noncalibrating scheme for 2-D imaging and distance approximation. This work shows experimental results of using the GDD as a distance approximation detector and specifies the advantages, disadvantages, and constraints when using such a sensor. Basic aperture imaging (transmission imaging) experimental results are also shown, and complex aperture imaging simulations and their corresponding reconstructions are presented. Finally, both 2-D imaging and acquired depth data are fused into a single three-dimensional reconstructed image to reveal the potential of the proposed method.

We develop a new double exposure Moire method for an optical registration metrology system in photolithography. Our method enables us to achieve at least a factor of 10 improvements in precise displacement metrology using a conventional optical sensor. We utilize a new registration mark printed to the photoresist on a bare silicon wafer using a double exposure of the gratings. The mark consists of two types of Moire with opposite phases. The two types of Moire are oriented in alternate directions. Displacement is measured from the distance between the positions of the two types of Moire in analogy with the conventional registration method. This concept is called alternating direction Moire. Performance is experimentally confirmed using an i-line wafer exposure apparatus. Precision is improved by up to 32 times as compared with the conventional method and can be applied to other Moire metrologies.

We investigate the time-invariant linear filter (TILF) approach to optimally parameterize the surface metrology of high-quality x-ray optics considered as a result of a stationary uniform random process. The approach is a generalization of autoregressive moving average (ARMA) modeling of one-dimensional slope measurements with x-ray mirrors considered. We show that the suggested TILF approximation has all the advantages of one-sided autoregressive and ARMA modeling, allowing a high degree of confidence when fitting the metrology data with a limited number of parameters. Compared to ARMA modeling, the TILF approximation gains in terms of better fitting accuracy and the absence of the causality limitation. Moreover, the TILF approach can be directly generalized to two-dimensional random fields. With the determined model parameters, the surface topography of prospective beamline optics can be reliably forecast before they are fabricated. These forecast metrology data, containing essential and reliable statistical information about the existing optics which are fabricated by the same vendor and technology, but generally, have different sizes, and slope and height root-mean-square variations, are vitally needed for numerical simulations of the performance of new x-ray beamlines and those under upgrade.

We show the design for a laser scanning microscopy defect detection system based upon the idea that the light can reflect off a photoresist-laden fused-silica sample containing defects, allowing height and depth information to be obtained through changes in light intensity. Image registration using predefined points is employed. Image processing techniques involving median and deconvolution filtering are used. Results show that the 2.1-μm resolution of these defects is obtainable, and receiver operating characteristic curves are used for quantifying results. Discriminabilities of 0.73 are achieved. Preliminary results for larger-array patterns through stitching processes are also shown.

A parallel guidance endoscopic optical coherence tomography (OCT) system is proposed for minimally invasive internal inspection of inner organs or complex structures for diagnosis. The system is maneuvered to access the target using an active cannulas’ steerable structures. The integration of a specially designed linkage device with the developed system allows the OCT endoscope to scan with enhanced signal-collective performance, while maintaining its tip at a constant distance from the target, as well as expanding the scanning range. The proposed system is integrated with flexible active cannulas, and this prototype is used for testing. The test results show that the device reliably performs for biological samples. Thus, it could be implemented for various types of noninvasive diagnoses in situations involving small entrances or crooked passage to a target.

For a high precision interferometric fiber optic gyroscope (IFOG) under temperature control, a short start-up time and small temperature drift are important for its applications. The start-up time and the temperature drift of IFOG with the same fiber length but with a different fiber coil layer number are investigated and compared. Simulation by finite difference time domain method is done to illustrate the existence of optimal layer number for the fiber coil wound by the quadrupolar method. Theoretical analysis is then provided and a closed-form formulation is given to calculate the optimal layer number of the fiber coil, which can effectively reduce both the start-up time and temperature drift of IFOG. Our study is meaningful in improving the thermal performance of the fiber coil.

The CO² laser mitigation method has been developed to mitigate the ultraviolet laser damage site on a fused silica surface. The mitigation process was monitored by an on-line white light scattering imaging system in order to ensure that the mitigation is successful. Additionally, a total internal reflection microscope was utilized to analyze the mitigation pit. By optimizing the laser mitigation parameters, the rough damage site can be replaced by a smooth Gaussian-shaped mitigation pit. The chemical composition of the damage sites and the CO² laser mitigation pits was also measured with energy dispersive x-ray spectroscopy. It reveals that the oxygen deficiency center defect of the ultraviolet laser damage site is removed after CO² laser mitigation, which helps us better understand the CO² laser mitigation process.

The workspace measurement and positioning system is a three-dimensional (3-D) coordinate system based on laser scanning, which is widely applied in large-scale metrology. As a core part ensuring measurement accuracy, the transmitter parameters’ calibration is the critical technique of the system. The present transmitter parameter calibration method relies on auxiliary measurement equipment, which is more error prone and less efficient. This paper will focus on the improvement of the transmitter parameters’ calibration by using a highly precise 3-D coordinate control network. Several calibration points with known coordinates are set in the workspace to establish the precise 3-D coordinate control network. After the new model of transmitter parameter calibration has been explained, both the calculation method for optimization and the production of the initial iteration value are given. As indicated by the results of the verifying experiment, the accuracy and efficiency of the transmitter calibration can be distinctly improved by using the proposed method. The experimental data show that the 3-D coordinate measurement error has obviously decreased from 0.3 to 0.15 mm as a merit of the proposed method.

A camera-independent method of avoiding image saturation using modified sinusoidal fringe-pattern projection to reduce surface measurement error and thus accommodate variable illuminance in phase-shifting surface-shape measurement is presented. The maximum input gray level (MIGL) in the projected patterns is reduced to an optimal tradeoff point, below which the intensity modulation, contrast, and signal-to-noise ratio would diminish the advantage of further MIGL reduction. Measurement simulations using 31 MIGL values, from 105 to 255 in increments of 5, demonstrated reductions in root-mean-square errors for ambient illuminance of 400, 500, 600, 700, 800, and 900 lx, from 0.38, 0.56, 0.86, 1.21, 85, and 373 mm, respectively, at 255 MIGL, to 0.31 to 0.32 mm at the optimum MIGL. The advantage of the method was confirmed in real measurements of a flat plate and human masks. The ability to perform camera-independent measurements under variable lighting conditions and surface reflectivity may lead to more practical measurements in uncontrolled environments.

A sinusoidal wavelength scanning interferometer with the four-step phase-shift method is demonstrated for a three-dimensional profile measurement. The interference phase-shift signal generated by the sinusoidal wavelength scanning contains information about optical path difference covering a nm-μm scale structure. The interference phase-shift signal was obtained by the four-step phase-shifting method. The sinusoidal wavelength shifting with a frequency of approximately 180 Hz and tunable range of 5.7 nm was performed by the Littman–Metcalf external resonator-type tunable laser with a center of 772.1 nm. The full-field surface profile measurement was conducted by a charge coupled device image sensor with an accuracy on a nm scale. The surface profiles of gauge blocks with a step height up to about 10  μm were successfully measured with a surface profile irregularity of 3.8×10⁻² μm . The deviations of two measured surfaces of upper and lower steps were estimated to be 1.6×10⁻²  μm and 3.1×10⁻²   μm , respectively.

The installation position errors of a measured gear are unavoidable when measuring gear tooth flank form deviation with laser interferometry, causing mistakes when using the simulated phase difference as a measurement reference to calculate the form deviation. In order to avoid manufacturing many standard gears as measurement references, a compensation method for the installation errors of the measured gear has been developed. In this process, the actual installation position of the measured gear can be traced by modifying the theoretical position parameters using the ray-tracing method. Experimental results show that this method is feasible. Thus, the simulated phase difference that compensate for installation errors can be utilized as a measurement reference in calculating the form deviation of the measured gear.

Free-space quantum key distribution links in an urban environment have demanding operating needs such as functioning in daylight and under atmospheric turbulence, which can dramatically impact their performance. Both effects are usually mitigated with a careful design of the field of view of the receiver. However, a trade-off is often required, since a narrow field of view improves background noise rejection but is linked to an increase in turbulence-related losses. We present a high-speed automatic tracking system to overcome these limitations. Both a reduction in the field of view to decrease the background noise and a mitigation of the losses caused by atmospheric turbulence are addressed. Two different designs are presented and discussed, along with technical considerations for the experimental implementation. Finally, preliminary experimental results of beam wander correction are used to estimate the potential improvement of both the quantum bit error rate and secret key rate of a free-space quantum key distribution system.

For some critical applications, the location of fiber Bragg gratings (FBGs) in draw tower grating (DTG) arrays needs to be determined to sub-mm accuracy; for example, successful packaging of the FBGs in mm scale packages. The DTG manufacturing process leaves no external visible identification marks on the fiber, hence location needs to be determined prior to packaging. This work presents an automated fiber marking system that can accurately locate the positions of the FBGs within ±0.1  mm . The simple, low cost, and automated system avoids manual fiber handling and allows accurate packaging of the FBGs for sensing applications.

Hot slumping technology is under development by several research groups in the world for the realization of grazing-incidence segmented mirrors for x-ray astronomy, based on thin glass plates shaped over a mold at temperatures above the transformation point. The performed thermal cycle and related operations might have effects on the strength of the glass, with consequences for the structural design of the elemental optical modules and, consequently, on the entire x-ray optic for large astronomical missions such as IXO and ATHENA. The mechanical strength of glass plates after they underwent the slumping process was tested through destructive double-ring tests in the context of a study performed by the Astronomical Observatory of Brera with the collaboration of Stazione Sperimentale del Vetro and BCV Progetti. The entire study was done on more than 200 D263 Schott borosilicate glass specimens of dimensions 100  mm×100  mm and a thickness 0.4 mm, either flat or bent at a radius of curvature of 1000 mm through the pressure-assisted hot slumping process developed by INAF-OAB. The collected experimental data have been compared with nonlinear finite element model analyses and treated with the Weibull statistic to assess the current IXO glass x-ray telescope design, in terms of survival probability, when subjected to static and acoustic loads characteristic of the launch phase. The paper describes the activities performed and presents the obtained results.

Optical design of an image magnifier based on a lens array is presented. The proposed image magnifier does not have a limit for object size due to off-axis aberration because the object is divided into segments and is kept apart from the nearest neighbor segment with the period of the lens array. Several images can be sequentially generated by magnifying sequentially specific objects among mosaic objects, which are composed of multi-images. The designed lens for the image magnifier corresponds to a lens array with an aperture of 2 mm and a magnification ratio of 3. Furthermore, an application for a display system of the designed image magnifier is also presented.

A refractive laser beam shaper typically consists of either two plano-aspheric lenses or one thick lens with two aspherical surfaces. Ray mapping is a general optical design technique for irradiance reshaping based on geometric optics. Although ray mapping, in principle, allows generating any rotational-symmetric irradiance profile, in the literature this technique is mainly used to transform a Gaussian irradiance profile to a uniform rotational-symmetric profile. For more complex profiles, especially with low intensity in the inner region (such as annular profiles), a high sampling rate is required to ensure an accurate calculation of the surfaces. In practice, the high sampling rate increases the numerical effort to calculate the aspherical surfaces and the simulation time to verify the design considerably. In this work, we evaluate different sampling approaches and surface construction methods. This allows us to propose and demonstrate a comprehensive numerical approach to efficiently design refractive laser beam shapers to generate rotational-symmetric collimated beams with annular irradiance profiles. Ray tracing analysis for several annular irradiance profiles demonstrates the excellent performance of the designed lenses and the versatility of our design procedure.

Currently, maintaining image quality during conjugate change is most frequently achieved through careful lens design with the multiconfiguration optimizing method and physical shifts of the lenses within the system such that the system remains in focus. However, in applications with operational limitations, such as endoscopy where the space available cannot allow for moving parts, the lens system needs to be designed such that the system is in focus over a wide range of conjugates. A lens design method that is integrable into existing commercial lens design software is presented. This method derives and maintains an optimal condition for astigmatism and distortion to control and reduce the overall aberration variation during the conjugate change, and extends the depth of field of the system. A side-by-side lens design comparison between the method illustrated in this study and the conventional lens design method commonly employed by designers of zoom lenses is also presented and is demonstrated to produce better results in designing conjugate change optical systems.

In this study, a theoretical model of embedded fiber Bragg grating sensors was developed to provide predictions of the strain transfer rate and average strain transfer rate without the assumption that the host material is subjected to uniform axial stress. Further, a finite element (FE) analysis was performed to validate the present model. It was shown that the theoretical results with the present model are in good agreement with those by FE analysis. Finally, the parametric analysis was used to quantitatively investigate the effect of the parameters of the adhesive layer and host material on the strain transfer rate and average strain transfer rate.

The software configurable optical test system (SCOTS) is an efficient metrology technology based on reflection deflectometry that uses only a liquid-crystal display and a camera to measure surface slope. The surface slope is determined by triangulation using the co-ordinates of the display screen, camera, and test mirror. We present our SCOTS test results concentrated on high dynamic range measurements of low order aberrations. The varying astigmatism in the 910-mm diameter aspheric deformable secondary mirror for the large binocular telescope was measured with SCOTS, requiring no null corrector. The SCOTS system was designed on-axis with camera and screen aligned on the optical axis of the test mirror with the help of a 6-inch pellicle beam splitter. The on-axis design provides better control of the astigmatism in the test. The high dynamic range of the slope provided a measurement of astigmatism within 0.2-μm root-mean-square accuracy in the presence of 231-μm peak-to-valley aspheric departure. The simplicity of the test allowed the measurements to be performed at multiple gravity angles.

Employing optical fiber to deliver the trapping laser to the sample chamber significantly reduces the size and costs of optical tweezers (OT). The utilization of fiber decouples the OT from the microscope, providing scope for system portability, and the potential for uncomplicated integration with other advanced microscopy systems. For use with an atomic force microscope, the fiber must be inserted at an angle of 10 deg to the plane of the sample chamber floor. However, the literature states that optical trapping with a single fiber inserted at an angle ≤20  deg is not possible. This paper investigates this limitation and proposes a hypothesis that explains it. Based on this explanation, a tapered-fiber optical tweezer system is developed. This system demonstrates that such traps can indeed be made to function in three-dimensions (3-D) at insertion angles of ≤10  deg using relatively low optical powers, provided the fiber taper is optimized. Three such optimized tapered fiber tips are presented, and their ability to optically trap both organic and inanimate material in 3-D is demonstrated. The near-horizontal insertion angle introduced a maximum trapping range (MTR). The MTR of the three tips is determined empirically, evaluated against simulated data, and found to be tunable through taper optimization.

We propose an integrated structure that combines chip and fiber array blocks for optical interconnection with a polymeric planar lightwave circuit (PLC) device using the roll-to-roll imprint process. The fiber array blocks and PLC chip of the integrated structure are fabricated on the same substrate, and the alignments in the three spatial directions were established with the insertion of an optical fiber. The characteristics of the integrated structure were evaluated by fabricating a 1×2 optical splitter device. The structure had an insertion loss of 3.9 dB, and the optical uniformity of the channel was 0.1 dB, indicating that the same performance for an active alignment can be expected.

We have compared the bit error rate (BER) performance of precoding-based asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT) optical wireless (OW) systems in additive white Gaussian noise (AWGN) and indoor multipath frequency selective channel. Simulation and analytical results show that precoding schemes such as discrete Fourier transform, discrete cosine transform, and Zadoff-Chu sequences do not affect the performance of the OW systems in the AWGN channel while they do reduce the peak-to-average power ratio (PAPR) of the OFDM output signal. However, in a multipath indoor channel, using zero forcing frequency domain equalization precoding-based systems give better BER performance than their conventional counterparts. With additional clipping to further reduce the PAPR, precoding-based systems also show better BER performance compared to nonprecoded systems when clipped relative to the peak of nonprecoded systems. Therefore, precoding-based ACO-OFDM and PAM-DMT systems offer better BER performance, zero signaling overhead, and low PAPR compared to conventional systems.

Quantum communication (QC) systems harness modern physics through state-of-the-art optical engineering to provide revolutionary capabilities. An important concern for QC engineering is designing and prototyping these systems to evaluate the proposed capabilities. We apply the paradigm of software-defined communication for engineering QC systems to facilitate rapid prototyping and prototype comparisons. We detail how to decompose QC terminals into functional layers defining hardware, software, and middleware concerns, and we describe how each layer behaves. Using the superdense coding protocol as an example, we describe implementations of both the transmitter and receiver, and we present results from numerical simulations of the behavior. We conclude that the software-defined QC provides a robust framework in which to explore the large design space offered by this new regime of communication.

In direct-detection optical orthogonal frequency division multiplexing (OFDM) systems, the high peak-to-average power ratio (PAPR) will cause nonlinear effects in both electrical and optical devices and optical fiber transmission when the nonlinear amplifiers are employed. A new hybrid technique based on carrier interferometry codes and companding transform has been proposed and experimentally demonstrated to reduce the high PAPR in an optical direct-detection optical OFDM system. The proposed technique is then experimentally demonstrated and the results show the effectiveness of the new method. The PAPR of the hybrid signal has been reduced by about 5.7 dB when compared to the regular system at a complementary cumulative distribution function of 10⁻⁴. At a bit error rate of 10⁻⁴, after transmission over 100-km single-mode fiber with a μ of 2, the receiver sensitivity is improved by 3.7, 4.2, and 5 dB with launch powers of 3, 6, and 9 dBm, respectively.

We present a straightforward method for the calculation of phase-matching conditions based on the angle-dependent refractive index in biaxial crystals. Considering the refractive indices relying on angular orientation of wave vector and optical axis angle, we acquire the numerical curves of phase-matching angles. We then obtain the effective nonlinear coefficient for a spontaneous parametric down conversion (SPDC) process in BiB₃O₆ (BIBO) crystal, and ascertain the optimal phase-matching directions for types I and II. Moreover, we focus on the angular gradient of the pump and emission wave refractive indices near the exact phase-matching direction, and compare the SPDC with the double frequency process in the geometrical relations of the refractive index ellipsoids. This method is superior in ascertaining the space distribution of pump and emission waves, and it is convenient for deciding the parameters of phase matching without solving the quadratic Fresnel equations.

We investigate the effects of cavity detuning on squeezing in the generation of the squeezed light at 1064 nm with a degenerate optical parametric amplifier (OPA) based on a periodically poled KTiOPO₄ crystal. We theoretically analyze several effects that lead to cavity detuning, including the displacement and tilt of the incident laser light and the cavity length fluctuations of the OPA. To reduce the influence of the cavity detuning and increase the degree of generated quantum noise reduction, a side-of-fringe locking technique is applied to achieve the resonance between the OPA cavity and the injected laser beam. The experimental results show that the transmitted power through the locked cavity is increased and the cavity detuning is greatly suppressed. After locking of the cavity, we could expect to get a value for the control loop with an accuracy of 10⁻⁹ and the locking time is not less than 2 h, which paves the way for the preparation of the squeezed light.

A time-varying gain amplification method is proposed and demonstrated in an optical fiber distributed vibration sensing system based on a phase-sensitive optical time-domain reflectometer. An acoustic optical modulator is adopted to generate narrow pulses, which are injected to the sensing fiber to generate Rayleigh backscattering light. The experimental results show that a 5-m spatial resolution and up to 44-km sensing range with 50-ns pulse width are achieved. The amplitude of vibrations is equal in three positions including 804, 20,054, and 43,455 m.

Optical beam spread and beam quality factor in the presence of both an initial quartic phase aberration and atmospheric turbulence are studied. We obtain the analytical expressions for both beam radius-squared and the beam quality factor using the moment method, and we compare these expressions with the results from Monte Carlo simulations, which allow us to mutually validate the theory and the Monte Carlo simulation codes. We then analyze the first- and second-order statistical moments of the fluctuating intensity of a propagating laser beam and the probability density function versus intensity as the beam propagates through a turbulent atmosphere with constant C²_n . At the end, we compare our analytical expression and our simulations with field test experimental results, and we find a good agreement.

We demonstrate an all-normal dispersion passively mode-locked Yb-doped fiber laser for the supercontinuum (SC) generation based on the nonlinear polarization rotation technique. A piece of polarization-maintaining fiber is utilized in the cavity as a spectral filter. By changing the pump power as well as the polarization states, pulses at repetition rates of 22.85, 45.7, 91.40 MHz can be achieved, corresponding to the fundamental and the second- and the fourth-ordered repetition rates, respectively. The output spectrum has a central wavelength of 1040 nm with steep edges, which indicates that it is a dissipative soliton (DS) mode-locking state. By launching the amplified DS pulses into a piece of photonic crystal fiber at a fundamental repetition rate, SC exhibiting a spectral range from 700 to 1700 nm is realized.

Spectrum properties of apodized fiber Bragg gratings (AFBGs) are well studied, in particular, when there is a need to reduce transmission side lobes. Otherwise, the polarization properties of these gratings are rarely reported when evaluating system performance. We analyze the reflected spectrum, the polarization-dependent loss (PDL), and the differential group delay (DGD) of an AFBG written in a single-mode fiber (SMF28). The evolution of these properties versus the grating parameters is studied based on the coupled mode theory. We mainly focus on the PDL and DGD maximum amplitudes and their wavelength separation. The simulation analysis is developed by means of the transfer matrix method. We demonstrate that the apodization function induces some asymmetry between the left and right parts of the PDL and DGD curves. The maximum amplitudes of the PDL and DGD converge to constant values versus birefringence but increase with grating length.

An innovative all-fiber low-pedestal spectral compression scheme based on a two-stage structure employing a high nonlinear fiber (HNLF) connected with a nonlinear optical loop mirror (NOLM) is proposed and demonstrated. Both numerical and experimental results showed that the spectral pedestal and side-lobe component after spectral compression in the HNLF can be efficiently suppressed by the NOLM, simultaneously improving the spectral compression ratio. The measured spectral compression ratio increases by a factor of 2 from 3.39 to 6.91 and the side-lobe level is reduced from –7.47 to –9.36 dB. The spectral pedestal ratio is 15.7% using the proposed scheme, which is nearly one-third of that using the conventional feedthrough HNLF alternative.

The analytical expression for hollow sinh-Gaussian (HsG) beams propagating through a paraxial ABCD optical system is derived and used to investigate its propagation properties in a fractional Fourier transform (FrFT) optical system. Several influence parameters of both the HsG beams and the FrFT optical system are discussed in detail. Results show that the FrFT optical system provides a convenient way for modulating HsG beams: HsG beams maintain their dark-centered distribution when the fractional order p is low, and low-ordered HsG beams lose their original dark-centered distribution more quickly than high-ordered ones when the value of p increases. Eventually all HsG beams’ intensities evolve into peak-centered distributions with some side lobes located sideways. Furthermore, our results also show that HsG beam intensity distribution versus the fractional order is periodical and the period is 2. The results obtained in this work are valuable for HsG beam shaping.

The shadowing effect caused by the human body can have a considerable influence on indoor visible light communications (VLC) channel characteristics. A shadowing ray tracing (SRT) algorithm to investigate the human shadowing channel characteristics is proposed. To validate the effectiveness of the SRT model, an extensive indoor human shadowing channel measurement has been presented. Both scenarios, the line-of-sight (LOS) and the non-line-of-sight (NLOS), have been measured. Theoretical results from the SRT model are validated against the measurement results for channel path loss. The results from measurements and simulations show that receiver rotations can mitigate human shadowing effect under the LOS scenario. Under the NLOS scenario, the VLC system becomes more sensitive to human shadowing when the distance between the LED and the receiver is large. Finally, more simulation results on the channel root mean square delay spread are also presented to analyze its relation with human shadowing distance to the receiver.

A long-range air-gaps assisted subwavelength waveguide is proposed and demonstrated. The configuration can be interpreted as a combination of a silicon waveguide and a metal–dielectric–metal (MDM) waveguide. Unlike the conventional MDM waveguide, the proposed waveguide is polarization insensitive because long propagation lengths can be achieved by both the transverse electric (TE) and transverse magnetic (TM) polarizations. The propagation length of this waveguide can even be up to decimeters. Moreover, this design is very compact with only a 1.05  μm center-to-center separation between two adjacent waveguides, while the waveguide isolation is <58  dB. Also, it is capable of guiding light effectively (<97% transmittance) through a 90 deg bend with a small bending radius (1  μm). These unique features make the proposed waveguide a promising candidate for the future large-scale photonic integrated circuits.

To overcome the downhole space limitation, eccentric tubing was designed. Two fiber Bragg grating (FBG) sensors written in one optical fiber were integrated into the specially designed tubing to form an on-line monitoring system, and a protective casing was designed to isolate the FBG sensors from the harsh environment. The wavelengths of the FBGs were recorded at certain temperatures and pressures in order to calibrate the temperature and pressure sensitivity, respectively. Both the temperature and the pressure exhibit good linear relationships with the wavelengths of the FBGs, which suggest that they can be represented as a function of the wavelength and generated via a data analysis system. The good linear relationships also proved that the sensing area was well sealed. The temperature was measured by the FBG sensor and a thermocouple at the same time, and good agreement was found between these results, validating the credibility of this system.

A vertical pin photodiode with a thick intrinsic layer is integrated in a 0.5-μm BiCMOS process. The reverse bias of the photodiode can be increased far above the circuit supply voltage, enabling a high-drift velocity. Therefore, a highly efficient and very fast photodiode is achieved. Rise/fall times down to 94  ps/141  ps at a bias of 17 V were measured for a wavelength of 660 nm. The bandwidth was increased from 1.1 GHz at 3 V to 2.9 GHz at 17 V due to the drift enhancement. A quantum efficiency of 85% with a 660-nm light was verified. The technological measures to avoid negative effects on an NPN transistor due to the Kirk effect caused by the low-doped I-layer epitaxy are described. With a high-energy collector implant, the NPN transit frequency is held above 20 GHz. CMOS devices are unaffected. This photodiode is suitable for a wide variety of high-sensitivity optical sensor applications, for optical communications, for fiber-in-the-home applications, and for optical interconnects.

Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers a unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here the realization of BH-QCLs with a single-step regrowth of highly resistive (<1×10⁸   ohm⋅cm ) semi-insulating InP:Fe in <45  min for the first time in a flexible hydride vapor phase epitaxy process for burying ridges etched down to 10 to 15  μm depth, both with and without mask overhang. The fabricated BH-QCLs emitting at ∼4.7 and ∼5.5  μm were characterized. 2-mm-long 5.5-μm lasers with a ridge width of 17 to 22  μm , regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for continuous wave (CW) operation. 5-mm-long 4.7-μm BH-QCLs of ridge widths varying from 6 to 14  μm regrown without mask overhang, besides being spatially monomode, TM₀₀ , exhibited wall plug efficiency (WPE) of ∼8 to 9% with an output power of 1.5 to 2.5 W at room temperature and under CW operation. Thus, we demonstrate a quick, flexible, and single-step regrowth process with good planarization for realizing buried QCLs leading to monomode, high power, and high WPE.

A simple fiber-optic sensor based on a fiber Bragg grating (FBG) embedded in a fiber modal interferometer (MI) for simultaneous measurements of temperature and strain is proposed and experimentally demonstrated. The fiber MI is constructed by splicing two sections of the no-core fiber (NCF) between the single-mode fibers. Due to the different responses of the NCF-based MI and the FBG to the same temperature and strain, the discrimination between temperature and strain can be easily achieved. For a 0.01-nm wavelength resolution, the resolution of the sensor is 0.216°C and 6.75  με in temperature and strain, respectively.

Polymers with individually adjusted optical and rheological properties are gaining more and more importance in industrial applications, like in information technology. To modify the refractive index n, an electron-rich organic dopant is added to a commercially available polymer-based resin. Changes in viscosity for applications like inkjet printing can be achieved by using a comonomer with suitable properties. Therefore, we used a commercially available epoxy acrylate based UV-curable polymer matrix to investigate the influence of ethylene glycol dimethacrylate on viscosity and phenanthrene on refractive index. Refractive index was measured at a wavelength of 589 nm and 20°C using an Abbe refractometer. As a result, the change in viscosity decreased linearly from 47 Pa·s to 4 mPa·s, which is a more suitable region for inkjet printing. However, the refractive index decreased, at the same time, from 1.548 to 1.514. On adding phenanthrene, the refractive index increased linearly from 1.548 up to 1.561. It was shown that both viscosity and refractive index can be successfully adjusted in a wide range depending on desired properties.

An appearance of the photoluminescence anisotropy induced by photochemical reaction under continuous irradiation with polarized light is demonstrated for an ensemble of CdSe/ZnS quantum rods chaotically embedded in the pores of filter paper. A specific relaxation of the photoinduced anisotropy is revealed in darkness after an extended period of irradiation. This process is attributed to rotational diffusion of the nanocrystals in the pores of filter paper. The suggested theoretical model is used to estimate the rotational diffusion constant D , which is calculated to be 9.7×10 ⁻³  s−1 for the sample after 5 h of irradiation.

Ga-doped ZnO films of thicknesses 3 to 500 nm were grown on Si at 200°C by pulsed-laser deposition in 10 mTorr of Ar. Sheet carrier concentration ns and mobility μ were measured at room temperature by the Hall effect and were fitted, respectively, to the equations n s (d)=n(∞)(d−δd) and μ(d)=μ(∞)/[1+d ∗ /(d−δd)] , where n(∞) is the predicted volume carrier concentration at d=∞ (the bulk value), δd is the thickness of the dead layer (if any), μ(∞) is the predicted mobility at d=∞ , and d ∗ is a figure of merit for the electrical properties of the interface. The fitted values of d ^∗[/sup] and δd were ∼23 and ∼11  nm , respectively. X-ray reflectance results were consistent with an ∼3-nm -thick interfacial layer of significantly lower density than that of bulk ZnO. X-ray photoelectron spectroscopy measurements showed an ∼10-nm -thick interfacial region in which the Ga peaks at ∼3.8% , well above the average value of 2.4% found in the bulk of the film.

High-quality epitaxial ZnO films on c-plane sapphire substrates have been obtained by utilizing an off-axis sputtering configuration together with buffer layers prepared via nitrogen-mediated crystallization (NMC). The role of NMC buffer layers is to provide high density of nucleation site and, thus, to reduce the strain energy caused by the large lattice mismatch (18%) between ZnO and sapphire. The NMC buffer layers allow two-dimensional growth of subsequently grown ZnO films, being particularly enhanced by employing an off-axis sputtering configuration in which the substrate is positioned out of the high-energy particles, such as negative oxygen ions originating from the targets. As a result, ZnO films with smooth surfaces (root-mean-square roughness: 0.76 nm) and a high electron mobility of 88  cm² /V⋅s are fabricated. Photoluminescence spectra of the ZnO films show strong near-band-edge emission, and the intensity of the orange-red defect emission significantly decreases with increasing horizontal distance between the target and the substrate. From these results, we conclude that off-axis sputtering together with NMC buffer layers is a promising method for obtaining high-quality epitaxial ZnO films.

A linear tapered double S-shaped arrayed waveguide grating (AWG) was designed as an alternative to a U-shaped AWG, and a complete transmission spectrum for 18 channels of coarse wavelength-division multiplexing (CWDM) was demonstrated. The silicon-on-insulator based AWG with a rib waveguide structure with a broad channel spacing of 20 nm was designed to serve as a multiplexer/demultiplexer. A beam propagation method modeling simulation under transverse electric mode polarization over a free spectrum range of 700 nm was used for the design process. The geometrical dimensions of the AWG rib structure were optimized to achieve the lowest reported insertion loss of 1.07 dB and adjacent crosstalk of −38.83  dB . The influence of different etching depths on the top Si layer of the AWG for a constant core width of 0.6  μm as well as birefringence effects were also investigated. A transmission spectrum response at the output port close to the standard CWDM wavelength grid range of 1271 to 1611 nm with an average channel spacing of 2485 GHz was obtained.

This article [Opt. Eng.. 53, (8 ), 081910 (2014)] was originally published on 10 July 2014 with an error in the denominator of Eq. (6). The denominator should be the letter eta, as shown in the corrected equation below: