This PDF file contains the editorial “Shifting Ground” for OE Vol. 46 Issue 01

We show through numerical simulation of 10.7-Gbits/s dense wavelength-division multiplexed (DWDM) duobinary transmission over 800 km of nonzero-dispersion-shifted fiber that uncompensated dispersion can introduce significant departures from Gaussian statistics in the receiver current.

We present a new configuration based on a fiber ring laser sensor with four-wave mixing effect generation for simultaneous measurement of strain and temperature. The fiber ring laser sensor, corresponding to the signal wavelength, is based on an erbium-doped fiber amplifier, a highly nonlinear bismuth-based optical fiber, and a Bragg grating filter. Four-wave mixing is generated when an external pump laser is combined with a ring laser sensor. Two new peaks appear and are named the converted and satellite wavelengths. With efficiency conversion, which corresponds to the optical power ratio between the converted signal and ring laser input signal, and a Bragg wavelength it is possible to discriminate strain and temperature. Maximum errors of ±1.8° C and ± 12 με are reported over 100°C and 1000 με measurement ranges, respectively.

A simple fabrication technique for a silica suspended-core holey fiber design is presented that features a higher air-filling fraction than most holey fibers, making it ideal for evanescent-field-sensing applications. The holes in the fiber are defined through mechanical drilling of the preform, which is a significantly quicker and more straightforward approach to the customary stacking method. During the draw, the shape of the holes are manipulated so that the final fiber design approximates that of an air-suspended rod with three fine struts supporting the core. Modeling reveals that the modal overlap is greater than 29% at 1550 nm for a core diameter of 0.8 µm, which is significantly higher than any previously reported index-guiding structure used for sensing. A basic gas sensor is demonstrated using acetylene as the sensing medium and the results are reported.

We describe a proof of concept for a snapshot dual-band visible hyperspectral imaging spectrometer. A commercially available digital camera was integrated into a computed tomographic imaging spectrometer to provide a means for this proof of concept. Two spatially coregistered data cubes covering the blue (400 to 500 nm) and red (600 to 700 nm) spectral regions were reconstructed and the results analyzed. We found that the system accurately reconstructs the spectral content of the scene and that the two data cubes are automatically spatially coregistered by virtue of the system design.

1×2 and 1×3 planar optical splitters based on Benzocyclobutene (BCB 4024-40) polymeric material are demonstrated for the first time. The devices are designed based on symmetric interference and fabricated on BK7 glass substrates with a thin layer of SiO2 as cover. A cost-effective chemical etching technique is used in the fabrication process to take advantage of the photosensitivity of the polymer. The waveguide loss was measured using the cutback method to be 3.5 dB/cm. The splitting uniformity is better than 0.02 and 1.5 dB at 1550-nm optical wavelength for the 1×2 and 1×3 splitters, respectively.

Wall movement in an underground mine must be continually monitored to provide real-time assessment of dynamic stress changes around mining excavation and to warn against rock failure. An in situ approach using optical spot projection and image processing can measure submillimeter movements. The system can be easily automated and poses no obstruction to mine traffic. The position of an off-normal laser spot will shift as the target wall moves. The centroid of this projected spot can be measured through image processing of a digital image. Multiple laser spots give the system simultaneous information over a larger sampled area. Software processing can measure subpixel shifts of the spot centroid. The resolution depends on the pixel count, the magnification of the camera lens (and hence the field of view), the optical-beam angle, and the spot characteristics. A laboratory table-top experiment demonstrated the system for distances up to 3 m and for a resolution of 0.1 mm. The results were linear in target movement, with standard errors between 0.83% and 5.38%. An automated system is described for a mine environment.

When testing an assembled image forming optical system to measure the on-axis wavefront shape, we have the practical impossibility of taking out the lens. However, there is a possibility of using the Hartmann test, as was previously done by some researchers, but some special precautions must be taken as described here. We will show that some extra spherical aberration is introduced and will make an estimation of its magnitude. Some proposals for reducing this aberration will be made.

A polarization-insensitive dichroic filter was designed and fabricated at an angle of 45 deg with a slope of 13.0 and an average transmittance of 96.41% for a wavelength range about 300 nm for use in the automatic optical inspection (AOI) of a printed circuit board. The filter supports the inspection of PCBs with a high signal-to-noise ratio. The AOI had a resolution of 25.39 line pairs/mm in the tangential and sagittal directions.

We explore the possibility of using ternary Si_1-x-yGe_xC_y grown on Si as the material for detection of infrared light of wavelengths 1.3 µm and 1.55 μm. By using an empirical expression for the absorption coefficient α, it is ensured that the ternary alloy exhibits values of α as high as 5000 cm^-1 at these wavelengths. From the corresponding values of the band gap, the compositions of the alloy are then ascertained by using an existing theory of band gap and band offset of ternary SiGeC alloy grown on (001) oriented Si. The estimated compositions are displayed as plots of x versus y ( ≤0.03) with different values of α as parameters.

Tapered fiber phase-conjugating mirrors (PCMs) have been experimentally investigated in a master-oscillator power amplifier system. The dependence of PCM properties such as threshold and reflectivity on the geometry of the fiber is discussed. The beam quality factor M² of the output beam was 1.2, and the peak power reached 1.1 MW. In operation with 100-Hz repetition rates, the fluctuation of the output energy was 8%. A depolarization ratio of 3% and phase conjugation fidelity above 90% were also demonstrated.

We propose a simple semiconductor optical amplifier–based fiber laser configuration for achieving multiwavelength oscillation at room temperature, in which a few-mode fiber Bragg grating was used as the wavelength-selective component. An amplitude variation of 0.8 dB over a 180-sec period was obtained for triple-wavelength oscillation at room temperature, which is more stable than that of erbium doped fiber (EDF)–based fiber laser with similar configuration. This multiwavelength laser can be switched between dual- and triple-wavelength operations by properly adjusting the polarization controller in the cavity. The lasing wavelength could be effectively tuned using the thermal heating method, and the output power variation was very small compared with that of the EDF-based fiber laser. This multiwavelength laser has the advantage of simple configuration, good stability, and wavelength tuning with small output power variation.

An effective approach to enhance the performance of a room-temperature multiwavelength fiber ring laser is presented. By introducing self-lasing feedback in conventional phase-modulation-based multiwavelength fiber ring lasers, the number of lasing lines is increased and the differences among the lasing lines are diminished. Stable multiwavelength lasing with numbers of lasing lines up to 17 within the 3-dB spectral bandwidth and with wavelength spacing as small as 0.2 nm is achieved.

We report experimental results on an efficient singly resonant optical parametric oscillator (OPO) based on periodically poled MgO-doped LiNbO₃. The OPO was pumped by a diode-pumped passively Q-switched Nd:GdVO₄ laser, which could produce 3-ns laser pulses with a repetition rate of 5 kHz. By changing the crystal temperature and grating periods, the OPO generated signal and idler output in the ranges of 1.41 to 1.78 μm and 2.7 to 4.3 μm, respectively. The maximum output power at the signal wavelength of 1.55 μm was 35.4 mW with 120-mW pump power. The possibility of using the OPO for trace gas detection was demonstrated through measuring the absorption spectra of methane.

To the best of our knowledge, proposed is the first extreme-environment wireless pressure sensor design using a remoted single-crystal SiC chip within a pressurized capsule. A detailed theoretical analysis of the sensor system is performed, including the SiC chip's mechanical response within the pressure capsule and the pressure measurement technique's optical response based on image demagnification. The remote sensor was experimentally tested at room temperature for pressures up to 41 atm, and the sensor response is consistent with the theoretical analysis. The demonstrated sensor has a current experimental resolution of 1.17 atm with a designed maximum pressure range of 140 atm. Improved sensing resolution and range can be achieved via optimal selection of the SiC chip's dimensions and its seating in the pressure capsule. Applications for this sensor include extreme environments involving hot gases and corrosive fluids, as in power generation systems, oil field operations, and aerospace systems.

An optical waveguide for measuring the change of refractive index (RI) by using a photonic crystal fiber is designed. The simulation results show that a variation of 3.5% refractive index unit (RIU) could shift the resonant wavelengths of 450 and 110 nm when the RIs of liquid are 1.5 and 1.7, respectively. The sensitivities of 8×10^-7 and 3.2×10^-6 RIU are thus able to be achieved for the RIs of liquid of 1.5 and 1.7, respectively.

The core diameter of a single-mode fiber is about 8 to 10µm. Any slight misalignment or deformation of the optical mechanism will cause significant optical losses at connections. Previous studies have concentrated on improving the manufacturing process to obtain high-precision components. Although the precision can be controlled, misalignment may still occur owing to the contact stress on the connecting interface. This study used the finite-element method to simulate the contact status, and used MT-series connectors as examples. The connectors use a guide-pin structure to align the two contact surfaces, and use a spring or clip to maintain tight contact. Because the MT ferrules are made of plastic materials, they are softer than ceramic ones and the deformation strains are more significant. For the finite-element analysis, a solid model of the MT ferrule with a convex end, oblique angle, and fiber protrusion can be constructed according to the JIS C5981 standard. Results of a simulation, integrating the optimization technique and ANSYS software, showed that the fiber center displacement for the commonly used oblique PC connectors would be 0.9 μm. This will significantly affect the eccentricity tolerances for single-mode applications, because a maximum of 1.5 µm is acceptable. Using the resulting new standard (eccentricity <0.6 μm) to select ferrules, and making them into connectors, it is found that the insertion losses in 98% of connectors are less than 0.3 dB. This result satisfies the requirement for single-mode applications.

This paper investigates the dynamic survivable routing problem and proposes an effective path-based protection algorithm, called the dynamic joint routing approach (DJRA), to completely protect against double-link failures in meshed wavelength division multiplexing optical networks. In DJRA, each connection is assigned a primary path and two link-disjoint backup paths to tolerate the simultaneous failures of two arbitrary fiber links. In order to improve the algorithm performance, we consider three key issues: (1) sharing spare backup resources, (2) failure recovery time, and (3) trap-path avoidance. Therefore, compared to previous algorithms, DJRA obtains better performance and is also able to yield flexible trade-off solutions.

The performances of optical and radio frequency communication systems are compared for long distance applications, such as deep space communications, where the signal-to-noise ratio (SNR) is crucial. We compare an optical communication system operating at 0.8 μm using intensity modulation and direct detection with an avalanche photodiode, an optical communication system operating at 1.5 μm using on-off keying and an optical preamplifier, and a radio frequency communication system operating in the X band. Assuming typical system parameters for the link budget analysis, we find that for distances between the transmitting and receiving antennas (R) of 10⁷ km, the SNR for the optical systems is proportional to R^-4, while for the radio frequency system it is always proportional to R^-2. The maximum data rate achievable with the radio frequency system is higher than that with the optical systems for distances beyond 10⁸ km. For near-Earth communication links, an optical system with optical preamplification is preferable when the data rate is higher than several gigabits per second. Clearly our results are based on specific system parameters. However, the equations involved and the method of comparison will be applicable for a wide range of system parameters.

We present a cone-beam image reconstruction algorithm for helical CT scanning with a tilted gantry and N-PI data acquisition. When the gantry is tilted, the effective source trajectory in the patient's reference frame lies on an elliptical cylinder, rather than on a circular cylinder as in the standard helical scanning mode. The aim of this work is to provide a means of reconstructing an image object directly from cone-beam projection data without transforming the image object into a virtual object and without rebinning projection data acquired for a real object into the projection data of the virtual object. This task has been accomplished by the application of an exact reconstruction algorithm, which utilizes an important geometrical property of the elliptical helical trajectory: the existence of generalized N-PI lines for a given image point. Based on this property, a mathematically exact image reconstruction scheme via filtering the backprojection image of differentiated projection data (FBPD) is applied to solve the reconstruction problem. Due to the gantry tilt, the required detector size is different from that of the standard helical trajectory (nontilted). A systematic analysis of the required detector size is presented. For an N-PI data acquisition scheme, an image may be reconstructed using data from an N-PI window, an (N-2)-PI window, and so on. Although the images reconstructed using an N-PI (N>1) window are noisier than the images reconstructed from a 1-PI window, a weighted-average scheme over reconstructed images is presented to generate a final image with significantly lower noise variance than that in the 1-PI data acquisition scheme. The image reconstruction algorithm was numerically validated using a mathematical phantom.

This study proposes an improved hybrid scheme of spectral polarization coding (SPC) and spectral amplitude coding (SAC) based on a specified orthogonal ternary sequence. In a previous SPC scheme, the entire wavelength was assigned either a vertical or a horizontal state of polarization. However, this study assigns positive, negative, or null chip values to individual wavelengths to create an orthogonal ternary sequence based on the bipolar Walsh-Hadamard matrix. This approach reduces the number of wavelength collisions at the photodetector and hence reduces the phase-induced intensity noise (PIIN). Neglecting the effects of shot noise and thermal noise, we consider only PIIN with a degree of polarization P=0. The proposed scheme improves the bit error rate by 40% and 100% over SPC and complementary unipolar SAC schemes. The number of simultaneous active users supported by the hybrid scheme for the worst case (P=1) is improved by 41.3% over the complementary unipolar SAC coding scheme for the ideal case (P=0).

A seven-chip, 280-Gchip/s OCDM system incorporating quaternary phase coding and decoding is experimentally demonstrated. The encoder and decoder, consisting of superstructured fiber Bragg gratings, are fabricated using the equivalent-phase-shift method; only ordinary phase masks and submicrometer precision in control are required. Interchannel interference noise is also considered and evaluated; 40-Gbit/s×2-channel multiplexing is demonstrated. In addition, a nonlinear amplifying loop mirror (NALM) within the receiver is introduced to improve the system performance under two-channel operation. Switching is obtained for peak signal power less than 10 mW. We show that the NALM can act as a nonlinear processing element capable of reducing both the pedestal associated with conventional matched filtering and the width of the associated code recognition pulse.

Polymer tapered multimode waveguides were modeled using the finite difference wide-angle beam propagation method to investigate whether tapered input waveguide couplers decreasing in width away from the waveguide entrance give improved tolerance to lateral misalignments of an optical source compared to straight waveguides and whether there is any effect on angular misalignment tolerance for use in optical backplane interconnections. Input tapered couplers having a larger entrance and tapering down in width do indeed improve the lateral misalignment tolerance compared to straight waveguides but do so at the expense of an increased loss. Tapers have no effect on angular tolerance for strongly driven vertical cavity surface-emitting laser (VCSEL) sources although they cause a loss of angular tolerance for single-mode fiber sources and VCSELs at low drive currents.

We have developed a simple and low-cost microlens for optical communication systems involving the coupling of laser diodes or integrated waveguides to single-mode fibers (SMFs), and fiber-to-fiber interconnection. More generally, the applications concern coupling active or passive components to SMFs. The novelty consists in using a graded-index section of 125-μm core diameter without cladding and taking into account the diffraction effects. This new microlens offers a wider range of mode field diameters (from around 5 to 90 μm) and working distances (up to 2000 μm) than previously demonstrated. This paper investigates the influence of the Fresnel diffraction of a Gaussian beam, during its propagation through the microlens, on the mode field intensity profile, the working distance, and the coupling efficiency of this kind of microlens in front of a SMF. It shows that while truncation causes multiple maxima of the coupling efficiency as a function of z displacement, it does not decrease the maximum coupling efficiency.

Integration technologies for board-to-board optical interconnect systems are presented. In the module architecture, optical transmitters and receivers are placed on the line cards and the signals are routed to the optically passive backplane through optical jumpers. The backplane contains a light guiding layer with embedded polymer waveguides (WGs) and 45-deg reflector micromirrors. The WGs are fabricated by direct lithographic patterning and have propagation losses as low as 0.05 dB/cm. The wedge dicing technology is developed for fabrication of the 45-deg micromirrors with 0.5-dB excess losses. The pluggable optical connectors with microlens adaptors couple the light from the optical jumpers into the backplane WGs. Evaluation of the connector alignment tolerances demonstrates a very weak dependence of the coupling efficiency on the axial displacement and a more significant effect of the radial shifts. The presented results show that the displacement tolerances can be substantially improved with auxiliary lenses formed on the substrate. Prototype optical interconnect modules with integrated channel WGs, mirrors, and assembled connectors are fabricated with insertion losses of 5 to 6 dB. The modules successfully pass the high-speed transmission tests at data rates up to 11 Gbits/s.

We have reconstructed a three-dimensional instantaneous temperature distribution inside the turbulent flame of a propane-air premix burner using multidirectional holographic interferograms by constructing a geometric model of the three-dimensional isothermal surfaces, and have read out the temperature values. To reconstruct this asymmetric temperature field, interference fringe data were acquired using an eight-directional Twyman-Green interferometer surrounding the object flame, and all data were simultaneously acquired. A ruby laser was used to obtain clear fringe patterns. The temperature distribution was reconstructed from the refractive index distribution, obtained from the fringe patterns, by a computed tomography technique using a convolution reconstruction algorithm. Isothermal profiles were calculated in horizontal cross sections from the temperature data. All of the multiple isotherms in each horizontal section were approximated with polygons and then stacked vertically to form polyhedra in a solid model format, having side facets with the corresponding isothermal values. Models, not only of the initial isothermal solids, but also of the solids resulting from multiple Boolean operations on them, were reconstructed to show the distribution in detail. This development will be of help in studying the local instantaneous structure of turbulent flames.

The holographic characteristics of two different photopolymer films containing methyl violet dyes in polyvinyl alcohol matrices were examined. One of the films contains only dye in the matrix. A holographic grating in the film is created by bleaching the dye. The other film contains acrylamide as a monomer and triethanolamine as a promoter as well as dye in the matrix, and a holographic grating is formed by the photopolymerization of acrylamide. In this study, the temporal evolution of diffraction efficiency was measured during holographic recording, by changing the dye concentration, exposure power, etc. For the film exposed to bleaching, the diffraction efficiency had a peak at a specific exposure time, and then decreased with overexposure. For the film exposed to photopolymerization, the diffraction efficiency was saturated at high exposure, and the maximum efficiency was approximately 40%, much higher than that of the other film.

A multiplexing scheme in volume holograms with a spherical reference wave implemented by rotating a mirror is presented. Diffraction angular selectivity of the mirror in the proposed holographic storage system is theoretically and experimentally investigated. The spherical reference waves in the presented multiplexing scheme can be replaced with plane reference waves; however, angular selectivity of the mirror with a plane reference wave is higher than that with a spherical reference wave in the system. Angular selectivity of the mirror using a spherical reference wave in the system can be increased by decreasing the distance between the mirror and the hologram. Unlike spherical reference waves, angular selectivity using a plane reference wave stays the same in this system. Although using spherical reference waves generates a lower angular selectivity, it also offers larger total accessible angular range for reference beams. Accordingly, using spherical reference waves in the proposed system will not sacrifice the storage capacity.

The signal-processing aspect of a 2-μm wavelength-coherent Doppler lidar system under development at NASA Langley Research Center in Virginia is investigated in this paper. The system is named VALIDAR (validation lidar), and its signal-processing program estimates and displays various wind parameters in real time as data acquisition occurs. The goal is to improve the quality of the current estimates of power, Doppler shift, wind speed, and wind direction, especially in the low signal-to-noise-ratio (SNR) regime. A novel nonlinear adaptive Doppler-shift estimation technique (NADSET) is developed for this purpose, and its performance is analyzed using the wind data acquired over a long period of time by VALIDAR. The quality of Doppler-shift and power estimations by conventional Fourier-transform-based spectrum estimation methods deteriorates rapidly as the SNR decreases. NADSET compensates this deterioration by adaptively utilizing the statistics of Doppler-shift estimates in a strong SNR range and identifying sporadic range bins where good Doppler-shift estimates are found. The authenticity of NADSET is established by comparing the trend of wind parameters with and without NADSET applied to the long-period lidar return data.

While tracking dim and small moving targets in the electro-optical (EO) tracking system, the numerous false alarms resulted from the low signal-to-noise ratio would seriously debase the performance of target recognition and tracking. The probabilistic data association filter in conjunction with a maximum likelihood approach (PDAF-ML) has been applied effectively to low observable or dim target motion analysis. Whereas, the PDAF-ML supposes that the amplitude of target is not correlative among different sampling instants, and that the greater the amplitude value is, the greater the probability of being the target of interest would be. In the EO imaging tracking system, the amplitude information and the motion of target are consistent and highly correlative in a short period. To resolve the problem that the PDAF-ML is inconsistent with the EO imaging tracking system, the two features, namely, the amplitude information and the motion as well as their consistency, are modeled as Markov stationary random signals and are fused by means of PDAF. Experiments are carried out, and the results show that, with the proposed approach, the uncertainty of trajectory association would be largely decreased, and the performance of target recognition and tracking could be significantly improved.

We present several different estimation methods to calculate the total luminous flux and luminous efficiency of a backlight module by using the measurement data from a luminance meter. We used the integrating sphere to measure a backlight module and found the luminous efficiency as 72.232%. With this result as a reference, we compared total luminous efficiencies obtained by five estimation methods, which are based on the luminance distributions along the x axis and y axis and measured by a luminance meter. In addition to these experimental results, we discussed the effect of dividing the luminous region into small areas on faster computations of total luminous flux of a backlight module.

With staring infrared sensors becoming more predominant, an increasing number of system Modulation Transfer Functions (MTFs) are being extracted from undersampled imagery. Contamination of the MTF measurement caused by aliasing must be avoided by using a superresolution technique. We compare differences in the utility of two of these techniques: the tilted edge technique, which is commonly used, and the tilted slit technique, which is not. While the tilted edge MTF measurement is taken for granted, the use of the tilted slit is both innovative and instructive. The differences between the two techniques are examined to determine their ability to successfully extract the MTF from an undersampled image without noise, with white noise, with fixed pattern noise, and with various angles of tilt. Ease of implementation is also considered. All things considered, the tilted edge technique is more easily accomplished and gives superior results. The tilted slit technique, however, gives valuable insight into the nature of aliasing in the real world. It is shown to be an excellent tool by which the difficult concept of aliasing may be easily illustrated.

We present a detailed analysis of the reconstruction of gray-level images using orthogonal moments with respect to the basis sets of Zernike, Fourier-Mellin, Chebyshev-Fourier, and pseudo-Jacobi-Fourier polynomials. As test images, we use Ronchigrams with different numbers of fringes as high-spatial-frequency components. The evaluation of image reconstruction between orthogonal moment sets is made in terms of different metrics. These measurements are the normalized image reconstruction error, the overall activity level in each image with respect to spatial frequency variations, the root-mean-square contrast, the total number of reconstructed fringes, the coordinate transformations of the input image, and the number of moment orders. Moreover, a method of denoising the input image based on the Daubechies wavelet transform is implemented to compute the signal-to-noise ratio. Numerical computations show that, for the Ronchigram reconstructions, the performance of Zernike moments is better than that of the other basis sets of orthogonal moments.

An efficient approach is proposed for moving object segmentation in the H.264 compressed domain. The normalized motion vector field is derived from the H.264 compressed video and is exploited to segment the frame into motion-homogenous regions. The following region merging process is efficiently represented using a binary partition tree (BPT). Selective nodes are extracted from the traversal process of BPT to represent different moving objects. Experimental results demonstrate the good segmentation performance of our approach. In fact, the work presents a general BPT-based compressed-domain moving object segmentation approach, which can be easily adapted to other compressed domains including the Moving Picture Expert Group (MPEG) domain.

In automated apple sorting and grading applications, one persistent problem is to identify apple stem-ends and calyxes from defects. To solve this problem, a Three-dimensional (3D) Shape Enhanced Transform (SET) approach is presented. The SET method enhances the apple stem-end/calyx area according to the 3D surface gradient difference between the stem-end/calyx and the apple surface region. In addition, the proposed SET approach does not depend on the location of the stem-end/calyx on the apple surface, making it more suitable for apples orientated randomly. SET is also an automated and robust method, which detects the apple stem-end/calyx without any human intervention, and performs well with noise and even incomplete image data. A total of 232 Golden Delicious apple images were tested, and an overall detection rate of 93.97% was achieved.

A conjugate gradient method based on an inverse algorithm is applied in this study to estimate the unknown time-dependent convection heat transfer coefficient under the varying ambient temperatures in multicoated optical fibers by reading the transient temperature measurement data at the fiber surface, when the optical fibers are subjected to transient thermal loading. No prior information is available on the functional form of the unknown convection heat transfer coefficient in the present study; thus, it is classified as the function estimation in inverse calculation. The accuracy of the inverse analysis is examined by using the simulated exact and inexact temperature measurements. Results show that the estimation of time-dependent convection heat transfer coefficient and transient temperature distributions can be obtained and are even effected by the varying ambient temperatures for all the test cases considered in this study.

An infrared imaging lens that produces beams with long focal depth while preserving the entire aperture for full light capturing and high transverse resolution is presented. Based on the conjugate-gradient algorithm, a diffractive optical lens with four times depth of focus improvement at the 10.6-μm wavelength has been designed and fabricated. ZEMAX simulation has confirmed the designed lens performance showing legible images over an extended image range. Through laser direct-write photoresist patterning and subsequent reactive ion etching on a germanium substrate, we have realized a diffractive lens with four times depth of focus improvement over a conventional infrared lens of the same numerical aperture. The extended depth of focus lens should yield a variety of applications in focus-free imaging without reducing lens aperture.

In this paper, both backward and forward-type lens axicons illuminated by a Gaussian-Schell model (GSM) beam are studied. A numerical calculation is performed to investigate the axial intensity distribution of the focal segment. It is shown that when a backward-type lens axicon is illuminated with an appropriate beam radius and spatial coherence of the incident GSM beam, one can achieve a focal segment with uniform axial intensity. The rapid oscillation of the axial intensity can be reduced by the decreasing of spatial coherence. This method is very simple and efficient. A forward-type lens axicon illuminated by a GSM beam is also investigated.

The issues of security and obstruction of line of sight are concerns of wireless optical communications (WOC). In this paper, an analysis of light beams carrying orbital angular momentum (OAM) used for WOC is undertaken. The basic description and conception of OAM of light are given. Encoding information both as the OAM states of a single light beam and as the spatial structure of the mixed light beams with OAM is discussed and compared. Using OAM to encode the data offers an inherent security enhancement. Mixed vortex fields with OAM possess a self-reconstruction property that is able to regenerate and to reconstruct their initial spatial distributions of the complex amplitude in the condition of obstruction of line of sight.