This PDF file contains the editorial “A New Editor” for OE Vol. 48 Issue 08

We propose a spectral flat-top, single resonant, and ultrabroadband-more than 180 nm in a -20-dB bandwidth-long-period fiber grating (LPG) filter. The ultrabroadband LPG is based on a thin cladding layer LPG synthesized by the Lagrange multiplier optimization (LMO) algorithm. As the bandwidth and resonant spectra cover a very wide band, both material dispersion and waveguide dispersion were included in the calculations of the LMO method. To the best of our knowledge, the bandwidth of the designed flat-top LPG filter in the -20-dB coupling is the broadest currently existing in the literature. Such designed LPG devices can be very useful for a variety of applications in broadband optical communication systems.

We propose a novel method to realize silicon-on-insulator (SOI)-based air slot waveguides for sensing applications. The method, based on feature size reduction using conformal thin films grown by atomic layer deposition (ALD), enables a guided slot mode in a silicon slot waveguide with a patterned slot width of more than 200 nm. Feature size reduction of slot structures with ALD grown amorphous TiO₂ is demonstrated.

A compact slot waveguide polarization splitter based on the silicon (Si) material system is proposed and analyzed. The slot waveguide structure introduces significant beat-length differences for transverse electric and transverse magnetic polarizations at operation wavelength of 1.55 μm. The special self-imaging design with limited modes and weak second-mode excitation shortens the device length to <50 μm, while still delivering good performance. The polarization extinction ratio is 13 dB, and the excess loss is only 0.3 dB. The splitter also has a relaxed length tolerance of 550 nm. These attributes make it an excellent candidate for polarization diversity circuits and compact electronic-photonic integrated circuits.

We describe a technique to optimally tune and calibrate bendable X-ray optics for submicron focusing. The focusing is divided between two elliptically cylindrical reflecting elements, a Kirkpatrick-Baez pair. Each optic is shaped by applying unequal bending couples to each end of a flat mirror. The developed technique allows optimal tuning of these systems using surface slope data obtained with a slope-measuring instrument, the long trace profiler. Because of the near linearity of the problem, the minimal set of data necessary for the tuning of each bender consists of only three slope traces measured before and after a single adjustment of each bending couple. The data are analyzed with software realizing a method of regression analysis with experimentally found characteristic functions of the benders. The resulting approximation to the functional dependence of the desired shape provides nearly final settings. Moreover, the characteristic functions of the benders found in the course of tuning can be used for retuning to a new desired shape without removal from the beamline and remeasuring. We perform a ray trace using profiler data for the finally tuned optics, predicting the performance to be expected during use of the optics on the beamline.

We measure the internal attenuation of bulk crystals of chemical vapor deposition zinc selenide (CVD-ZnS), chemical vapor deposition zinc sulfide (CVD-ZnSe), Si, and GaAs in the short near-infrared (sNIR) region to evaluate the possibility of astronomical immersion gratings with those high refractive index materials. We confirm that multispectral grade CVD-ZnS and CVD-ZnSe are best suited for the immersion gratings, with the smallest internal attenuation of αatt=0.01 to 0.03 cm^-1 among the major candidates. The measured attenuation is roughly in proportion to λ^-2, suggesting it is dominated by bulk scattering due to the polycrystalline grains rather than by absorption. The total transmittance in the immersion grating is estimated to be at least >80%, even for the spectral resolution of R=300,000. Two potential problems, the scattered light by the bulk material and the degradation of the spectral resolution due to the gradient illumination in the diffracted beam, are investigated and found to be negligible for usual astronomical applications. Since the remaining problem, the difficulty of cutting grooves on CVD-ZnS and CVD-ZnSe, has recently been overcome by the nanoprecision fly-cutting technique, ZnS and ZnSe immersion gratings for astronomy can be technically realized.

Passive harmonically mode-locked fiber ring laser with long single-mode fiber (SMF) has been demonstrated. With the increase of nonlinearity due to the incorporation of a long SMF, it is found that the laser can operate in harmonic mode-locking states with various repetition rates. Furthermore, based on the passively mode-locked fiber ring laser, a novel and simple technique for fiber length measurement with resolution on the order of centimeters has been proposed. The fiber length can be easily obtained from the fundamental cavity frequency of the mode-locked fiber laser because the transit time in a laser cavity is exactly proportional to the cavity length.

In this study, we propose a pulsed Nd:YAG laser system that uses a sequential charge and discharge circuit to adhere polymethylmethacryl (PMMA) plastics. Two PMMA plastics adhere to each other when the red one transmits the laser beam and the black one absorbs it. The optimal adhesion depends on several process parameters, such as the charging voltage of the capacitor, the pulse rate [in pulses per second (pps)], the velocity of the target, and the laser beam diameter. We try to optimize the adhesion process parameters from trial-and-error experiments. It has proposed that the optimal adhesion process parameters are a charging voltage of 650 V, a pulse rate of 11 pps, a target velocity of 4.20 mm/s, and a laser beam diameter of 4.00 mm in this pulsed Nd:YAG laser system. In addition, we generalize the optimal conditions for plastic adhesion, such as energy per pulse, peak power, and the velocity of the target.

Measurements and simulations of a dual gas-cell system, which uses Raman scattering to convert infrared laser radiation from 1064 to 1560 nm, are presented. The cells contain deuterium at a pressure of ~25 atm. A beamsplitter is used to distribute the energy from the YAG laser between the seed and pump cells. The laser light is focused into the seed cell, which generates a lower-power, backward-propagating, pulse. This seed pulse and a laser pulse counterpropagate in the pump cell to generate a stronger 1560-nm pulse. The goal is to create a single pulse without any shorter pulses or oscillations. Simulation results are presented, which indicate that a beamsplitter directing 25% of the laser energy into the seed cell and the rest into the pump cell is optimal for 1560-nm generation. Laboratory testing of a dual-cell system using a 32% beamsplitter shows the generation of a 1560-nm pulse with 250 mJ/pulse with an efficiency of 35%.

We present a formula connecting the change in the output of a multiple-channel waveguide to a micron-scale alteration in the physical properties of a segment of the waveguide. The analysis is motivated by the need to "tune" the properties of photonic devices after fabrication, necessitated by the present impossibility of fabrication to within the severe tolerances required to produce desired output characteristics. The results point to a method by which tuning can be carried out in an optimal manner. Example device application is illustrated by use of a simulation program.

We propose an optical parametric amplifier model based on highly nonlinear fiber with a single pump, and the impacting factors on the bandwidth and maximum value of the optical parametric gain are investigated. The analytic solutions indicate that by properly adjusting the structure of the optical parametric gain, ~400-nm bandwidth of the signal gain can be obtained, with its maximum value 55 dB. The optimal parameters designed for better performance of signal gain in the presence of dispersion fluctuations are also demonstrated in single-pump fiber-optic parametric amplifier (FOPA) architecture.

A design of a fast and accurate optical Gaussian noise generator is proposed and demonstrated. The noise sample generation is based on the Box-Muller algorithm. The functions implementation was performed on a high-speed Altera Stratix EP1S25 field-programmable gate array (FPGA) development kit. It enabled the generation of 150 million 16-bit noise samples per second. The Gaussian noise generator required only 7.4% of the FPGA logic elements, 1.2% of the RAM memory, 0.04% of the ROM memory, and a laser source. The optical pulses were generated by a laser source externally modulated by the data bit samples using the frequency-shift keying technique. The accuracy of the noise samples was evaluated for different sequences size and confidence intervals. The noise sample pattern was validated by the Bhattacharyya distance (Bd) and the autocorrelation function. The results showed that the proposed design of the optical Gaussian noise generator is very promising to evaluate the performance of optical communications channels with very low bit-error-rate values.

An enhanced mathematical model is introduced to study and evaluate the performance of a core node in an optical burst switched network. In the proposed model, the exact Poisson traffic arrivals to the optical burst switching (OBS) node is approximated by assuming that the maximum allowed number of arrivals to the OBS node, in a given time slot, is 2 (instead of ∞). A detailed state diagram is outlined to illustrate the problem, and then a mathematical model based on the equilibrium point analysis technique is presented. Two performance measures, namely, the steady-state system throughput and the average blocking probability, are derived from the model, which is built in the absence of wavelength conversion capability. Our proposed model is aided by a simulation work that studies the performance of an OBS core node under the assumption of Poisson traffic arrivals (the exact case) and calculates the steady-state system throughput. The results obtained from the proposed mathematical model are consistent with that of simulation when assuming Poisson traffic arrivals, and this consistency holds for a certain range of traffic load. The effect of varying different network parameters on the average blocking probability is discussed.

The performance of a single semiconductor optical amplifier (SOA)-based ultrafast nonlinear interferometer that is simultaneously driven by two ultrafast data streams with respect to the timing deviation between these signals and the standard clock input is theoretically studied and investigated. For this purpose, a numerical model is applied to simulate the operation of the specific module in pattern-operated dual rail-switching mode and under the presence of such imperfect synchronization. The thorough analysis and interpretation of the obtained results allows one to evaluate the impact of this temporal offset on the achievement of both bitwise logical correctness and high quality at the output. In this manner, the conditions that it must necessarily fulfill are derived and the dependence of its permissible margin and accordingly the way the latter can be extended is revealed, while its optimal amount for maximizing the defined metric is quantified by the difference between the orthogonal polarization clock components' relative walk-off and the control pulse width. These findings can help compensate for the existence of this effect as well as strengthen the tolerance against it so that it can be properly handled in the context of the considered type of SOA-based interferometric switch.

As a very effective scheme to realize channel acquisition, clustering-based channel estimation algorithms have been proposed for optical fiber communication systems with a maximum-likelihood sequence estimation receiver. These algorithms can estimate the key channel parameters needed by the Viterbi processor accurately without assuming that the channel memory length is known a priori to the receiver. In this work, a corresponding adaptive channel tracking algorithm is proposed, which is proved to be very powerful in tracking the variation of the communication channel. Cooperation of the clustering-based channel acquisition and channel tracking is realized.

Thermal actuated optoelectronic ring switch provides the advantages of high accuracy, easy actuation, and reasonable switching speed. However, when scaled up, the thermal ring switch may encounter issues related to fabrication error, nonaccurate wavelength response, and large terminal numbers in the control circuit. We propose the employment of an integrated control circuit to compensate for the fabrication error and tune as well as lock the wavelength in a thermal-actuated ring-type optical switch through an amplitude modulation scheme. Additional functionalities can also be added in this circuit by externally tailoring the round-trip loss or coupling constants of the ring. The design concept can be easily scaled up for large array optical switch system without much change in the terminal numbers thanks to the three-dimensional hierarchy of high gray-scale control circuit design, which effectively reduces the terminal numbers into the cubic root of the total control unit numbers. The integrated circuit has been designed, simulated, as well as fabricated, and demonstrated a decent performance with free spectral range equal to 1.5 nm at 1534 nm and very accurate wavelength modulation to 0.3 nm within 0.01 nm fluctuation for the thermal actuated ring-type optical switch.

We present a modification to the classic Michelson interferometer that allows the interference of multiple beams with equal amplitude. The proposed architecture presents the same advantages and simplicity as those of a classic Michelson interferometer. The basic unit of the device consists of a beamsplitter and two mirrors arranged as in a Michelson interferometer. To increase the number of interfering beams, the mirrors are replaced by a basic unit. In order to demonstrate the type of interference patterns that can be obtained, we present interferograms corresponding to three to eight interfering beams. The system can be used to optically induce photonic lattices.

A new algorithm for fringe phase analysis is proposed for shape and deformation measurement of diffusely reflecting surfaces by digital holography. It derives the phase of the averaged conjugate product of complex amplitudes before and after wavelength change or object deformation, corresponding to the complex coherence factor, and is consistent with a theory on fringe formation in holographic interferometry. By experiments and computer simulations, it is demonstrated that the algorithm is much more immune to speckle noise in phase analysis and unwrapping than conventional methods.

We have constructed a new type of modal wavefront sensor that uses a multiplexed hologram and position-sensing detectors to measure the amplitudes of a preselected set of eight Zernike modes in an input beam. The measurement is all optical, with the calculations made in encoding the holograms themselves. The result is a sensor with no computational overhead or postprocessing that has the potential to operate at megahertz speeds without the need for computer calculations. We have built and tested a prototype device, demonstrating its operation both as a stand-alone wavefront sensor and in a closed-loop adaptive optics control system.

Numerical models of holograms are available but their evaluation is a very computationally intensive task. We present an acceleration algorithm for optical field synthesis suitable for the reduced occlusion method of hologram synthesis. The acceleration uses an approximation that is designed for a field programmable gate array (FPGA) and therefore it mostly uses fixed point numbers. The work describes the approximation, its fixed-point modification, and the resulting FPGA structure. The results presented show that the solution produces high-quality holograms in a significantly reduced time due to efficient FPGA implementation.

A new approach to linear discriminant analysis (LDA), called orthogonal rotational LDA (ORLDA) is presented. Using ORLDA and properly accounting for target size allowed development of a new clutter metric that is based on the Laplacian pyramid (LP) decomposition of clutter images. The new metric achieves correlation exceeding 98% with expert human labeling of clutter levels in a set of 244 infrared images. Our clutter metric is based on the set of weights for the LP levels that best classify images into clutter levels as manually classified by an expert human observer. LDA is applied as a preprocessing step to classification. LDA suffers from a few limitations in this application. Therefore, we propose a new approach to LDA, called ORLDA, using orthonormal geometric rotations. Each rotation brings the LP feature space closer to the LDA solution while retaining orthogonality in the feature space. To understand the effects of target size on clutter, we applied ORLDA at different target sizes. The outputs are easily related because they are functions of orthogonal rotation angles. Finally, we used Bayesian decision theory to learn class boundaries for clutter levels at different target sizes.

We present the design and simulations of the expected performance of a novel 2-D x-ray shearing interferometer. This interferometer uses crossed phase gratings in a single plane, and is capable of operation over a wide range of energies extending from several hundred electron volts to tens of kiloelectron volts by varying the grating material and thickness. This interferometer is insensitive to vibrations and, unlike Moiré deflectometers implemented in the hard x-ray regime, recovers the full 2-D phase profile of the x-ray beam rather than the gradient in only one dimension.

We propose a CIECAM02-based tone mapping technique for color image contrast enhancement that works especially well in bright surrounding conditions. The CIECAM02 is inherently strong when considering human visual systems (HVS) and surrounding conditions. In general, dark regions look darker in bright surrounding conditions, so some details in these dark regions tend to disappear. To overcome this problem, we propose a global tone mapping technique for color image contrast enhancement that uses a gamma encoding and the I/O relationship of the CIECAM02 in bright surrounding conditions. The proposed method makes the contrast of the source image stronger in bright surrounding conditions. We confirmed the superior performance of the proposed method in bright surrounding conditions by psychophysical experiment.

The problem of spreading secret data to embed into multiple cover images is called batch steganography and has been theoretically considered recently. Few works have been done in batch steganography, and in all of them, the payload is spread between cover images unwisely. We present the Adaptive batch steganography (ABS) approach and consider embedding capacity as a property of images. ABS is an approach to adaptively spread secret data among multiple cover images based on their embedding capacity. By splitting the payload based on image embedding capacity constraint, embedding can be done more secure than the state when the embedder does not know how much data can be hidden securely in an image. Furthermore, the number of required cover images to embed a piece of secret data in ABS is smaller than the number of required cover images in usual batch steganography. We apply an ensemble system that uses different steganalyzer units to determine the embedding capacity of a cover image. Each steganalyzer unit is formed by a combination of multiple steganalyzers from a same type, but each one trained to detect a certain payload. Experimental results showed the effectiveness of embedding in ABS and security enhancement of produced stego images.

A method for solving the problem of unwarping a distorted omni-image taken by a lateral-direction misaligned omni-camera with its optical axis incoincident with its mirror axis is proposed. The method does not conduct camera calibration and is based on a new concept of two-stage image mapping from the real-world space to the distorted image space. The first stage is conducted in the camera manufacturing process and includes the generation of a pano-mapping function for mapping the real-world space to an undistorted image taken by an omni-camera with its optical and mirror axes being coincident. The second stage is conducted in an in-field environment when the omni-camera becomes lateral-direction misaligned and includes the generation of a distortion-mapping function that maps undistorted image pixels to distorted ones and the generation of a misalignment adjustment table that combines the pano-mapping and distortion-mapping functions to map the real-world space to the distorted image space. The distortion mapping function is generated by a new technique of curved quadrilateral morphing using quadratic pattern classifiers. The misalignment adjustment table is last used to unwarp distorted images conveniently by table lookup. Experimental results using simulated and real-image data show the feasibility of the proposed method.

We propose a rate-distortion optimized transform coding method that adaptively employs either integer cosine transform that is an integer-approximated version of discrete cosine transform (DCT) or integer sine transform (IST) in a rate-distortion sense. The DCT that has been adopted in most video-coding standards is known as a suboptimal substitute for the Karhunen-Loève transform. However, according to the correlation of a signal, an alternative transform can achieve higher coding efficiency. We introduce a discrete sine transform (DST) that achieves the high-energy compactness in a correlation coefficient range of -0.5 to 0.5 and is applied to the current design of H.264/AVC (advanced video coding). Moreover, to avoid the encoder and decoder mismatch and make the implementation simple, an IST that is an integer-approximated version of the DST is developed. The experimental results show that the proposed method achieves a Bjøntegaard Delta-RATE gain up to 5.49% compared to Joint model 11.0.

We propose an image encryption algorithm by using random position scrambling of the amplitude and phase functions in the frequency domain of optical transform. The positions of amplitude and phase data are scrambled randomly in the horizontal or vertical direction. The random position orders can be regarded as the key of the algorithm. Moreover, random phase encoding is not used in the proposed algorithm. A feasible optical implementation of the encryption algorithm is given. Some numerical simulations have demonstrated the capability of the algorithm.

We present a compressive sensing video acquisition scheme that relies on the sparsity properties of video in the spatial domain. In this scheme, the video sequence is represented by a reference frame, followed by the difference of measurement results between each pair of neighboring frames. The video signal is reconstructed by first reconstructing the frame differences using [script l]₁ minimization algorithm, then adding them sequentially to the reference frame. Simulation results on both simulated and real video sequences show that when the spatial changes between neighboring frames are small, this scheme provides better reconstruction results than existing compressive sensing video acquisition schemes, such as 2-D or 3-D wavelet methods and the minimum total-variance (TV) method. This scheme is suitable for compressive sensing acquisition of video sequences with relatively small spatial changes. A method that estimates the amount of spatial change based on the statistical properties of measurement results is also presented.