It is an exciting time for me. I just left the U.S. federal government after 17 years of service with the Army and the Navy. I am off to start my own small business in the area of infrared systems. The name of the new company is St. John’s Optical Systems. My 17 years with the government was great. I had left a large aerospace company to go to work for the Army. In the large corporation, I was relegated to the measurement of modulation transfer function, or MTF, on a targeting pod of a fighter aircraft. No more, no less. When I wanted to do other things and help out in other areas of electro-optics, I was told to “get back to work,” as is the case with many huge programs. When I went to work for the Army, I was told to do whatever I wanted as long as I completed my required assignments. I flourished in such an open environment with a great deal of interesting projects underway. I worked hard, learned a lot, and aligned myself with those who were making a difference. I was lucky to have a good number of giants in the field as mentors.

With the development of new concepts and principles over the past century, helmet-mounted displays (HMDs) have been widely applied. This paper presents a review of avionic HMDs and shows some areas of active and intensive research. This review is focused on the optical design aspects and is divided into three sections to explore new optical design methods, which include an off-axis design, design with freeform optical surface, and design with holographic optical waveguide technology. Building on the fundamentals of optical design and engineering, the principles section primarily expounds on the five optical system parameters, which include weight, field of view, modulation transfer function, exit pupil size, and eye relief. We summarized the previous design works using new components to achieve compact and lightweight HMDs. Moreover, the paper presents a partial summary of the more notable experimental, prototype, fielded, and future HMD fixed-wing and rotary-wing programs.

In the past century, every component of an optical system has become lighter and smaller, except the lenses. Typical lenses have too few degrees of freedom—just the refractive index, and the front and back surface shapes—to meet the demands of the vast array of modern optical systems which collect, project, or otherwise manipulate light. (Even in imaging systems, where computational power has the potential to eliminate the tight coupling between lenses and performance, more capable lenses would increase the trade space that optical designers have available to them).

Radial and spherical polymer gradient-index (GRIN) eyepiece designs are presented. The chromatic behavior of GRIN profiles is constrained to real material properties of a polymethyl methacrylate polystyrene copolymer gradient-index system. Single-element, two-element, and multielement eyepiece design configurations each demonstrate significant spot diameter and modulation transfer function performance improvements with the use of a GRIN element. A high-performance spherical GRIN eyepiece design, with 48-deg full field-of-view and 3% distortion, is compared to a similar homogeneous glass solution.

Results that characterize a fast-diffusing titania silicate gradient-index (GRIN) glass in a sodium (Na ⁺ ) for lithium (Li ⁺ ) ion exchange are presented. Manufacturing challenges associated with diffusion in titania silicate glass are addressed in order to extend its manufacturability to large-diameter radial diffusions. Two 20-mm diameter radial GRIN lenses have been fabricated in less than 4-weeks diffusion time, demonstrating the potential for fabricating large-diameter radial GRIN glass elements.

The article explores the possibility of athermalizing a gradient-index (GRIN) lens so that the effective focal length (EFL) of the element remains constant over a change in temperature. This is accomplished by designing the lens so that the surface curvatures and index profile compensate for one another over a change in temperature to maintain constant optical power. The means to determine how the lens geometry and index profile change with temperature for both a homogeneous and radial GRIN are explained. An analytic model for the purpose of identifying athermalized GRIN singlets is described and validated against the previous work in this field. The model is used to identify an athermalized polymer radial GRIN element and compare it with four other polymer elements of the same focal length but different index profiles, including a homogeneous one. Comparison of these singlets in CODE V® optical design software shows that the athermalized GRIN element maintains its nominal EFL over a temperature change the best of the five in the group while the homogeneous element (having no GRIN profile to counteract the effect of temperature on the surface curvatures) has the poorest performance. A numerical model to analyze more complicated GRIN systems is discussed.

A nanolayered polymer films approach to designing and fabricating gradient refractive index (GRIN) lenses with designer refractive index distribution profiles and an independently prescribed lens surface geometry have been demonstrated to produce a new class of optics. This approach utilized nanolayered polymer materials, constructed with polymethylmethacrylate and a styrene-co-acrylonitrile copolymer with a tailorable refractive index intermediate to bulk materials, to fabricate discrete GRIN profile materials. A process to fabricate nanolayered polymer GRIN optics from these materials through thermoforming and finishing steps is reviewed. A collection of technology-demonstrating previously reported nanolayered GRIN case studies is presented that include: (1) the optical performance of a f/# 2.25 spherical GRIN plano-convex singlet with one quarter (2) the weight of a similar BK7 lens and a bio-inspired aspheric human eye GRIN lens. Original research on the fabrication and characterization of a Luneburg inspired GRIN ball lens is presented as a developing application of the nanolayered polymer technology.

Over 100 doublets were designed using a polychromatic gradient-index (GRIN) design model to analyze the benefits of radial GRIN profiles in broadband visible to short-wave infrared (vis-SWIR) imaging system applications. The polychromatic GRIN design model can be applied to any GRIN material, but for this work, titania silicate glass was investigated. A multielement design study with Petzval lenses was performed to show improved color correction when using GRIN elements. Results from the doublet and Petzval designs illustrate that in broadband vis-SWIR imaging applications, GRIN can either improve system performance or reduce a cemented homogeneous doublet to a GRIN singlet.

A 40-deg full field-of-view high-performance eyepiece design utilizing a polymer spherical gradient-index (GRIN) optical element is presented. In the design process, the GRIN lens material is constrained to current manufacturing capabilities. Several spherical GRIN lens blanks are fabricated from a thermoformable axial GRIN polymethyl methacrylate polystyrene copolymer material. One is diamond turned into a lens for the eyepiece, and the additional blanks are used to characterize the fabrication process. The spherical GRIN profile is evaluated in the original design, and a tolerance analysis is provided.

The refractive index profile of one-dimensional gradient-index (GRIN) samples can be measured using the incident and exit beam angles of multiple beams passing through the sample at different positions along the index gradient. Beginning from a region of known refractive index, the collective angular deflection measurement of multiple beams is bootstrapped to compute the index profile of the entire sample. An alternative method using an approximate beam displacement model and a corrective algorithm is also presented. The two techniques are used to measure the index profile of a thick GRIN sample, and experimental results show good agreement with a maximum discrepancy of 1.5×10 [sup]−3 in the calculated index. An index accuracy of 5×10 ⁻⁴ is predicted for the bootstrap method employing typical micron-level spatial measurements.

The ability of gradient index (GRIN) materials to correct color in the infrared is explored. An overview of GRIN materials is provided, including a review of the most common index profiles (axial, radial, and spherical). Due to the high potential for color correction, a design study is carried out to compare a single ZnS/ZnSe GRIN element’s imaging performance to that of homogeneous singlets and doublets. A radial GRIN element is able to correct axial color and provide superior performance over an aspheric homogeneous singlet and offer a reduction in weight when compared to a homogeneous doublet. The ZnS/ZnSe GRIN is also shown to be superior to other infrared GRIN materials between 3 and 5 μm.

A laser collimator is a narrow-field imaging system that is designed for near-diffraction-limited performance when operating at a single wavelength. Although a traditional design might consist of a homogeneous doublet, the design of an axial gradient-index (GRIN) singlet for such an application is described. The analysis begins with third-order aberration thin lens optical design principles in order to examine the effects of spherochromatism from the GRIN. The final design is optimized in CODE V® using GRIN aluminum oxynitride (ALON®), which allows for diffraction-limited operation from the visible through 5 μm. A tolerance analysis is performed on the slope, nonlinearity, and tilt of the GRIN profile in order to assess sensitivity to fabrication errors.

In recent years, there has been an ever-growing interest in exploring different optical materials and components to develop compact and effective optical systems. The design and fabrication of high-performance optics require nondestructive metrology techniques to inspect the samples. We have investigated the capability of optical coherence tomography (OCT) to nondestructively characterize layered polymeric materials. Using a custom developed Gabor-domain optical coherence microscopy system centered at 800 nm with 120 nm full width at half maximum enabling unprecedented 2 μm resolution both laterally and axially in an 8  mm ³ volume, we investigated the internal structure of 50 μm thick films and layered sheets, which prompted the manufacturing process to adopt a compatibilization technique. Based on a custom swept-source OCT system centered at 1320 nm with expanded imaging field-of-view and latest depth of imaging extended to ∼5  mm , we performed nondestructive metrology of the layer thickness profiles over the depth of a monolithic layered sheet and diagnosed a film compression issue within the sheet. With the OCT metrology, the manufacturing process has been advanced and the layer thickness profile of a recent layered gradient refractive index sheet shows improved uniformity through depth.

For media with heterogeneous index of refraction, rays follow curved paths. Methods exploiting knowledge of the relationship between refractive index distribution and ray paths to estimate the refractive index distribution based on laser beam deflection, displacement, and mode conversion on passage through the media are described. The work covers axial, radial (cylindrical), and spherical refractive index distributions.

The key contribution is the optical design of a 2.75 g 2.5× magnification visual loupe developed within the Defense Advanced Research Projects Agency Manufacturable Gradient Index (M-GRIN) phase 2 program. We present a visual loupe (i.e., a Galilean telescope) that is constructed by a positive optical power objective lens that makes use of a spherical gradient index profile and a negative optical power eye lens that collimates the light for visual use. The optical materials and the preform thickness in the design are judiciously designed to be manufacturable within the spherical gradient index design rules available today. A comparison of the M-GRIN design to an all-plastic homogeneous baseline design shows that the M-GRIN design reduces the weight from 4.15 to 2.75 g while maintaining equivalent optical performance of the baseline.

An approach is proposed to reduce the tracking jitter of the extended target in boost phase for plume tracker in a photoelectric acquisition, tracking, and pointing system. The characteristics of the vehicle imaging are analyzed and the causes of jitters are identified. The target moving direction and its principal axis are combined to calculate the optimal frontal direction. A contour smoothing method based on the chord-arc ratio filtering is introduced to obtain a preliminary extraction point with lower jitters. Then a fine tracking point extraction method based on the minimal inscribed circle of contour after filtering is presented. Experimental results confirm that the proposed method significantly improves the tracking precision and stability.

Face liveness detection aims to distinguish genuine faces from disguised faces. Most previous works under visible light focus on classification of genuine faces and planar photos or videos. To handle the three-dimensional (3-D) disguised faces, liveness detection based on multispectral images has been shown to be an effective choice. In this paper, a gradient-based multispectral method has been proposed for face liveness detection. Three feature vectors are developed to reduce the influence of varying illuminations. The reflectance-based feature achieves the best performance, which has a true positive rate of 98.3% and a true negative rate of 98.7%. The developed methods are also tested on individual bands to provide a clue for band selection in the imaging system. Preliminary results on different face orientations are also shown. The contributions of this paper are threefold. First, a gradient-based multispectral method has been proposed for liveness detection, which considers the reflectance properties of all the distinctive regions in a face. Second, three illumination-robust features are studied based on a dataset with two-dimensional planar photos, 3-D mannequins, and masks. Finally, the performance of the method on different spectral bands and face orientations is also shown in the evaluations.

Signal processing enables the detection of more returns in a digital ladar waveform by computing the surface response. Prior work has shown that obtaining the surface response can improve the range resolution by a factor of 2. However, this advantage presents a problem when forming a range image—each ladar shot crossing an edge contains multiple values. To exploit this information, the location of each return inside the spatial beam footprint is estimated by dividing the footprint into sections that correspond to each return and assigning the coordinates of the return to the centroid of the region. Increased resolution results on the edges of targets where multiple returns occur. Experiments focus on angled and slotted surfaces for both simulated and real data. Results show that the angle of incidence on a 75-deg surface is computed only using a single waveform with an error of 1.4 deg and that the width of a 19-cm-wide by 16-cm-deep slot is estimated with an error of 3.4 cm using real data. Point clouds show that the edges of the slotted surface are sharpened. These results can be used to improve features extracted from objects for applications such as automatic target recognition.

We present a simulation method for studying turbid media in the optical field based on circular Gaussian distribution (CGD) model, by which the transmission matrix, representing the modulation of a turbid medium on the amplitude and the phase of incident light, can be generated directly and efficiently. As an application example, light refocusing through a turbid medium is realized employing the CGD model approach, combining with wavefront–phase modulation technique and Fresnel diffraction theory, which is applied to describe the light propagation between optical elements of the entire system. Simulation results based on this approach agree well with theoretical analysis for light refocusing, which can validate the feasibility of CGD model. This work can be used for exploring the potential applications of turbid media in the optical field further, especially for developing new microscopic imaging technologies beyond the diffraction limit.

This paper presents a fast and robust human tracking method to use in a moving long-wave infrared thermal camera under poor illumination with the existence of shadows and cluttered backgrounds. To improve the human tracking performance while minimizing the computation time, this study proposes an online learning of classifiers based on particle filters and combination of a local intensity distribution (LID) with oriented center-symmetric local binary patterns (OCS-LBP). Specifically, we design a real-time random forest (RF), which is the ensemble of decision trees for confidence estimation, and confidences of the RF are converted into a likelihood function of the target state. First, the target model is selected by the user and particles are sampled. Then, RFs are generated using the positive and negative examples with LID and OCS-LBP features by online learning. The learned RF classifiers are used to detect the most likely target position in the subsequent frame in the next stage. Then, the RFs are learned again by means of fast retraining with the tracked object and background appearance in the new frame. The proposed algorithm is successfully applied to various thermal videos as tests and its tracking performance is better than those of other methods.

We propose a new system that can generate digital holograms using natural color information. The system consists of a camera system for capturing images (object points) and software (S/W) for various image processing. The camera system uses a vertical rig, which is equipped with two depth and RGB cameras and a cold mirror, which has different reflectances according to wavelength for obtaining images with the same viewpoint. The S/W is composed of the engines for processing the captured images and executing computer-generated hologram for generating digital holograms using general-purpose graphics processing units. Each algorithm was implemented using C/C++ and CUDA languages, and all engines in the form of library were integrated in LabView environment. The proposed system can generate about 10 digital holographic frames per second using about 6 K object points.

An automatic technique to improve the surface model of the shoe-last bottom via Bezier networks is presented. In this technique, the surface model is generated via Bezier networks based on surface points, which are measured via line projection. The measurement procedure is performed by a Bezier network based on the line position. Thus, the model provides a continuous surface pattern of shoe-last bottom with high accuracy because the model passes through all control points of the physical surface. Furthermore, the network reduces the operations and memory size to calculate the surface. It is because the computational model is implemented with less mathematical terms than the traditional models. Additionally, the network and the laser line compute the vision parameters to avoid external calibration, which increases the inaccuracy of the surface model. Also, the model adapts the surface of the shoe-last bottom to the plantar surface. Thus, the proposed technique improves the accuracy, speed, and memory size of the traditional surface models. This improvement is proven by an evaluation based on the traditional surface models, which perform the model of the shoe-last bottom. The evaluation shows experimental results, which provide evidences of the contribution of the proposed technique.

Range performance is often the key requirement around which electro-optical and infrared camera systems are designed. This work presents an objective framework for evaluating competing range performance models. Model selection based on the Akaike’s Information Criterion (AIC) is presented for the type of data collected during a typical human observer and target identification experiment. These methods are then demonstrated on observer responses to both visible and infrared imagery in which one of three maritime targets was placed at various ranges. We compare the performance of a number of different models, including those appearing previously in the literature. We conclude that our model-based approach offers substantial improvements over the traditional approach to inference, including increased precision and the ability to make predictions for some distances other than the specific set for which experimental trials were conducted.

Occlusion is a major problem for real-time position and orientation measurement with distributed optical large-scale metrology systems. This paper presents two novel methods with occlusion handling to address this issue, which should be used in combination for practical applications. These two approaches are based on the constraints established by three control points and six control points, respectively, and then the position and orientation can be calculated through iterative optimization algorithms. In this paper, all the work is carried out by using the workspace measuring and positioning system as a verification platform. The experimental results show that the orientation accuracies of the three-point method and the six-point method are kept within 0.1 and 0.04 deg, respectively, and the position accuracy exceeds 0.15 and 0.08 mm.

Microscopes are widely applied in characterizing feature sizes at the micro-/nanoscale, and magnification calibration plays a key role in achieving precise measurements. However, it is difficult to obtain accurate results by using the general magnification calibration method if comparing the displayed size of a test-piece under microscope and its original one. In this study, a high-accuracy and automatic magnification calibration method that could be applied to different types of microscopes is proposed. A standard grating is employed as the reference, and a high-resolution discrete Fourier transform is used to analyze the images captured under various magnifications in this method. With utilization of the high-order harmonic component in the Fourier spectrum, the proposed method is capable of performing the calibration over a wide range of magnifications while maintaining identical precision. The relative error of the proposed method can be theoretically limited to 0.01%; moreover, the image noise can be tolerated. Furthermore, the validation and extensive adaptability of this method are demonstrated by calibrating the magnification of a scanning electron microscope and an optical microscope.

An infrared (IR, 3–5 and 8–12 μm)/microwave (MW, 2.7 GHz) micromirror array type of beam combiner was fabricated for test and analysis. A model based on transmission line theory, Snell’s law, and phase monopulse radar theory was built to analyze the boresight angle errors. Three types of boresight angle errors—inherent error, refraction-induced error and combiner inserting error—were calculated. The results indicated that combiner inserting error was the most important error. Using this theoretical model, a beam combiner was optimized, fabricated, and investigated experimentally. Measured by a phase monopulse radar, the boresight angle error introduced by the beam combiner was <0.1  deg , which coincided with the theoretical calculations. Infrared images were reflected by the beam combiner, the angular resolution of the reflected images in azimuth, and elevation directions were 40 and 80 in., respectively.

We propose a prototype for single-sideband (SSB) transmission based radio-over-fiber system by employing a phase-shifted fiber Bragg grating. The grating has different transmission spectrum characteristics with different phase-shift magnitudes. Then, it is used in a millimeter-wave (mm-wave) SSB modulation scheme. In the scheme, only one PS-FBG is used to realize the following functions: first, the conversion from optical dual-sideband to optical single-sideband with carrier reduction modulation can be easily achieved and second, the optical carrier-to-sideband ratio (OCSR) can be eliminated completely. In our work, the OCSR can be reduced from 33.95 dB to the optimum 0 dB and a 60 GHz mm-wave can be detected after photodiode.

Next generation elastic optical networks will very likely require dual polarization (DP) optical transmitter with inherent flexibility to dynamically change its baud rate and/or modulation format. We develop a prototype of compact flexible DP M-ary quadrature amplitude modulation (MQAM) optical transmitter and demonstrate its reconfigurability to accommodate baud rates ranging from 8 to 32  Gbaud/s using the same hardware. The prototype has another advantage in that the modulation format can also be dynamically changed from binary phase shift keying up to 64 quadrature amplitude modulation (QAM) for single and DPs, all over a single optical carrier. This allows the generation of variable data rate up to 384-Gb/s over a single wavelength. Experimental results show that for the most challenging setting of DP-64 QAM/32 Gbaud, the worst case values of error vector magnitude and bit error rate are 6.7 and 3×10⁻² , respectively. For less stringent settings, i.e., lower baud rate and/or lower modulation format, forward error correction limit error free transmission is easily obtained. Further results are also reported to demonstrate transmitter flexibility and software definability, by measuring symbol streams showing instantaneous swapping between modulation schemes with a swapping time less than 10 symbols duration, i.e., 0.3 ns.

The generalized Harvey–Shack (GHS) surface scatter theory is numerically compared to the classical small perturbation method, the Kirchhoff approximation method, and the rigorous method of moments for one-dimensional ideally conducting surfaces whose surface power spectral density function is Gaussian or exhibits an inverse power law (fractal) behavior. In spite of its simple analytic form, our numerical comparison shows that the new GHS theory is valid (with reasonable accuracy) over a broader range of surface parameter space than either of the two classical surface scatter theories.

Band-stop optical fiber filters based on linear chirped long-period fiber gratings (LPFGs) were investigated. The transmission characteristics of the linear chirped LPFGs were analyzed in detail with transfer matrix method. Linear chirp coefficient had a remarkable influence on the bandwidth of the transmission spectra. When the linear chirp coefficient was 10 ⁻⁹ and the transmission spectra corresponding to multiple cladding modes were superposed, the bandwidth of the transmission spectra could be expanded to <1000  nm . So the linear chirped LPFGs can be used for single-channel broadband band-stop filters, and the relationships between the grating parameters and the bandwidth of the transmission spectra were given. When the linear chirp coefficient is 10 ⁻⁸ , several narrow and separated loss peaks appeared in the transmission spectra, which indicated that the linear chirped LPFGs can be used for multichannel narrowband filters. The variations of the grating parameters caused the shifts of the transmission spectra and the changes of the intervals between separated loss peaks. The conclusions that are significant in optimizing the fiber grating filters’ design were obtained.

We present a high-data rate optical wireless system. The implemented system exploits polarization (PM) and wavelength multiplexing, achieving the transmission of a total capacity of 1.6  Tbit/s over hybrid fiber free-space optics (FSO) system with no optical-electronic-optical conversion at the interfaces with air. Quadrature phase shift keying modulation in each channel and coherent detection were used. The system allows enough power budget to support the record transmission of 16 channels, operating each at 100  Gbit/s over 40 km of fiber and 80 m of FSO between two buildings. Performance of the fully transparent connection is presented in terms of bit-error rate.

Based on recirculating frequency shifter, we generate 20 high-quality multicarrier optical combs with a tone-to-noise ratio <25  dB . We also experimentally demonstrate 2.56-Tb/s , polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, coherent optical wavelength division multiplexing transmission over 800 km standard single-mode fiber with 25-GHz channel spacing, and the spectrum efficiency is 5.1  b/s/Hz .

An all-optical multibit correlator using the multistage cascaded quantum dots semiconductor optical amplifiers Mach-Zehnder interferometer (QD-SOA MZI) is presented with the example of an 8-bits correlator. The simulations demonstrate the correlator pulse with ultrahigh quality at the bit rate of 500 Gbps. For the ultrahigh bit-rate applications, the ultrafast dynamics of the QD-SOA are investigated with the pulse/spectra distortion, gain dynamics, phase dynamics, and the frequency chirp of the optical pulse. All-optical logical gates XOR and AND based on the QD-SOA MZI are simulated with the consideration of many nonlinear dynamics, such as the carrier injection, carrier depletion, carrier density fluctuation, carrier heating, and the spectra hole-burning in the rate equations scheme.

We propose some modifications in the conventional index-guided photonic crystal fiber (PCF) structure having circular holes of constant radii. In one of the proposed structures, the hole dimensions of the conventional PCF are modified such that the ratio of the radii of the holes in a particular layer with the nearest layer is constant while maintaining the same refractive index in all the holes. In the other structure, we propose to use different dielectrics in different layers of holes in the conventional PCF structure such that the ratio of the refractive index of the dielectric material in the holes in a particular layer with the nearest layer is constant. The simulations of the proposed structures are carried out using OptiFDTD simulator with full-vector mode solver using finite difference time domain method, and the results are compared with the conventional PCF structure having four layers of air holes. It is observed that the proposed structures exhibit lower waveguide dispersion and confinement loss than the conventional PCF structure over a wide range of wavelengths, making them suitable candidates for applications such as long-distance optical communications or high-data rate data transfer applications. One of the proposed structures exhibits large negative dispersion, and it can be used as a dispersion compensating fiber.

The laser-damage growth characteristics of initial damage sites play an important role in investigating the laser-induced damage mechanisms and determining the lifetime of the optics in application. We investigate the damage growth behaviors of fused silica under 1064, and 532-nm nanosecond-pulsed laser irradiation. The relationships between the sizes of initial damage sites and the laser fluence, as well as the initial damage morphology evolutions and the laser shot numbers, are discussed. Moreover, the growth characteristics of the initial damage sites generated by the same fluence but under different subsequent fluence irradiation are compared with those of initial damage sites generated by different fluence but under the same subsequent fluence irradiation. Finally, the differences between the damage morphologies and the growth features at the same conditions under different wavelengths are also provided and discussed.

Robust optical phase modulation (PM) schemes have been widely employed in the development of radio-over-fiber (RoF) transport systems. However, the signals produced by these PM schemes require a delay line interferometer (DI), a fiber Bragg grating (FBG), an optical band-pass filter (OBPF), or dedicated dispersive devices to convert them back into the intensity modulation (IM) signal format prior to detection by a photodetector (PD). Inserting a costly DI into a transport link can significantly increase overall cost, and the use of a fixed working window OBPF, FBG, or dedicated dispersive devices to achieve the conversion is inflexible. To overcome these problems in PM-based RoF transport systems, a long-reach RoF link with an innovative PM-to-IM converter is proposed and experimentally demonstrated. Compared with the published PM schemes, the proposed architecture can utilize a vertical-cavity surface-emitting laser (VCSEL) to achieve the same performance of the published PM-to-IM converter at a lower cost. In addition, by adjusting the VCSEL driving current, a tunable range exceeding 350 GHz is experimentally obtained for the converter working window. The proposed long-reach RoF transport system is the first to achieve such PM-to-IM conversion by using a VCSEL and can contribute to the development of RoF transport systems.

We describe a semi-analytic model used to deterministically and exactly calculate the variance of degenerate and nondegenerate four-wave mixing (FWM) noises for 128×10  Gb/s return-to-zero frequency-shift-keying dispersion-managed dense-wavelength-division-multiplexing system. The analytic model includes various important light-propagating effects such as walk-off between channels, oscillation of pulse width with transmission distance, and stochastic variation of birefringence strength and orientation along fiber. The achievable maximum Q-factor and allowed maximum input powers for different dispersion management schemes and transmission distances are shown including overall FWM noises from different channel combinations, signal-amplified spontaneous emission (ASE) beat noise, and others. Achievable maximum transmission distances for such systems are studied. The upper limit of local dispersion of the dispersion map is discussed. In the case of allowed maximum input powers for the system, the variances of overall FWM noise are observed to be about half of those of signal-ASE beat noise when averaged for different transmission distances. It is found that for high-local dispersion, when transmission distance is long enough, effect of nondegenerate FWM noise, compared with degenerate FWM noise, can be ignored.

A dispersion-tolerant full-duplex radio-over-fiber (RoF) system based on modified quadrupling-frequency optical millimeter (mm)-wave generation using an integrated nested Mach–Zehnder modulator (MZM), an electrical phase modulator, and an electrical gain is proposed. Not only does the scheme reduce the cost and complexity of base station by reusing the downlink optical carrier, but also the generated optical mm-wave signal with base-band data carried only by 1-s order sideband can overcome both the fading effect and bit walk-off effect caused by the fiber dispersion. Simulation results show that the eye diagram keeps open and clear even when the quadrupling-frequency optical mm-wave is transmitted over 120-km single-mode fiber, and the bidirectional 2.5  Gbit/s data are successfully transmitted over 40 km for both upstream and downstream channels with <1-dB power penalty.

Empirical models are proposed for recalculating non-Lambertian reflectance based on the transmitted pulses and returned pulses that are recorded by a lab-built full-waveform laser detection system. The experiments were implemented on three objects, which were gray-cement concrete, red dull paper, and glazed indoor tile. The pulse energy was calculated based on the pulse waveforms in two ways, integral of the waveform (IW) method and multiplying peak by the full width at half maximum of the waveform (PF) method. The newly introduced empirical parameters semi-ellipsoid distribution ratio (SEDR) of the semi-ellipsoid model and ellipsoid distribution ratio (EDR) of the ellipsoid model were put forward to evaluate the degree of the non-Lambertian reflectance of material surface, instead of Lambertian-based factor that is the cosine of the incidence angle. We conclude that the bigger values of SEDR and EDR indicate more significant deviation from Lambertian for material surface. The modified reflectance results estimated by using the semi-ellipsoid model display better approximations than those obtained from Phong cosine. Moreover, it is obvious that the modified reflectance using a combined method of PF and modified semi-ellipsoid model outweighs the results estimated by other manners.

We present a residual dispersion compensating highly birefringent photonic crystal fiber (PCF) based on an octagonal structure for broadband dispersion compensation in the wavelength range 1460–1625 nm. The finite element method with perfectly matched boundary condition is used as the numerical design tool. It has been shown theoretically that it is possible to obtain a negative dispersion coefficient of about −418 to −775  ps/nm/km over the S-, C-, and L-bands, relative dispersion slope (RDS) close to that of single mode fiber (SMF) of about 0.0036  nm⁻¹ at 1550 nm. According to the simulation, birefringence of 2×10⁻² is obtained at 1550-nm wavelength. The variation of structural parameters is also studied to evaluate the tolerance of the fabrication. The proposed octagonal PCF can be a potential candidate for residual dispersion compensation as well as maintaining single polarization in optical fiber transmission system.

A photonic crystal fiber design is presented, which has simultaneously ultra-high birefringence, high nonlinearity, and high negative dispersion. The relative dispersion slope matches with that of standard single-mode fiber of about 0.0036  nm ⁻¹ . The finite element method with circular perfectly matched boundary layer is used to investigate the guiding properties. The proposed fiber ensures a large negative dispersion coefficient of about −639.16  ps/(nm km) , birefringence of order 3.55×10⁻² , and nonlinear coefficient of 41  W⁻¹  km⁻¹ at 1550-nm wavelength.

The scale factor of the in-line Sagnac interferometer current sensor associated with the polarization-dependent crosstalk introduced by optical devices is theoretically investigated. The variations of output pigtail polarization crosstalk of the Ti-indiffused LiNbO ₃ phase modulator and polarization crosstalk of the polarization-maintaining (PM) delay optical fiber with the temperature can lead to the scale-factor error. The white-light interferometry is utilized to measure the pigtail polarization crosstalk of the phase modulator. The variation of the scale factor with the temperature has been tested over a range from −40°C to 60°C experimentally. The results confirm the influence of output pigtail polarization crosstalk of phase modulator and polarization crosstalk of the PM delay optical fiber on the scale factor. To ensure the scale factor error to be within ±0.2% , the output pigtail polarization crosstalk of phase modulator and polarization crosstalk of the PM delay optical fiber should not exceed −30  dB for the current sensor operating in the outdoor environment.

We investigated optical damage (surface and bulk) in one of the most promising wide bandgap nonoxide nonlinear crystals, HgGa₂ S₄ , that can be used in ∼1 -μm pumped optical parametric oscillators (OPOs) and synchronously pumped OPOs (SPOPOs) for generation of idler pulses above 4 μm without two-photon absorption losses at the pump wavelength. The optical damage has been characterized at the pump wavelength for different repetition rates using uncoated and antireflection-coated (mainly with a single layer for pump and signal wavelengths) samples. HgGa₂ S₄ is the most successful nonlinear crystal (both in terms of output energy and average power) for such OPOs, but optical damage inside the OPO has a lower threshold and represents at present the principal limitation for the achievable output. It is related to peak pulse and not to average intensity, and bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rates. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. The safe pump fluence in extracavity measurements is <1  J/cm² , which corresponds to ∼100  MW/cm² for the 8-ns pulse duration (both values peak on-axis). In the OPO, however, peak on-axis fluence should not exceed 0.3  J/cm² limited by the formation of bulk scattering centers in orange-phase HgGa₂ S₄. In the nanosecond OPO regime, the damage resistivity of Cd-doped HgGa₂ S₄ is higher and that of the almost colorless CdGa₂ S₄ is roughly two times higher, but the latter has no sufficient birefringence for phase-matching. In SPOPOs operating in the ∼100  MHz regime, the damage limitations are related both to the peak pulse and the average intensities, but here HgGa₂ S₄ seems the best nonoxide candidate to obtain first steady-state operation with Yb-based mode-locked laser pump sources.

Hollow, metal-lined capillary waveguides have recently been utilized in spontaneous gas-Raman spectroscopy to improve signal strength and response time. The hollow waveguide is used to contain the sample gases, efficiently propagate a pump beam, and efficiently collect Raman scattering from those gases. Transmission losses in the waveguide may be reduced by using an azimuthally polarized pump beam instead of a linearly or radially polarized pump. This will lead to improved Raman signal strength, accuracy, and response time in waveguide-based Raman gas-composition sensors. A linearly polarized laser beam is azimuthally polarized using passive components including a spiral phase plate and an azimuthal-type linear analyzer element. Half-wave plates are then used to switch between the azimuthally polarized beam and the radially polarized beam with no change in input pump power. The collected Raman signal strength and laser throughput are improved when the azimuthally polarized pump is used. Optimization of the hollow waveguide Raman gas sensor is discussed with respect to incident pump polarization.

This article [Opt. Eng.. 52, (4 ), 043401 (2013)] was originally published on 15 April 2013 with an error on page 6, Sec. 4, paragraph 2. The first sentence “Figure 8 shows the AFM image of surface microstructure images of 10×10  μm on the silica glass surface…” has been corrected to read “Figure 8 shows the AFM image of surface microstructure images of 5×5  μm on the silica glass surface…”