This PDF file contains the editorial “For Some of Us, Science and Engineering is Hard Work” for OE Vol. 49 Issue 11

A compact bidirectional optical subassembly (BOSA) for a 1.25/10-Gbps passive optical network is developed. A vertically stacked 1.25-Gbps transmitter based on a silicon optical bench, and a 10-Gbps receiver based on a low temperature cofired ceramic are implemented to realize low-cost manufacturing and miniaturization for single package application. The proposed BOSA delivers an extinction ratio more than 10 dB at 1.25-Gbps modulation, optical output coupling efficiency is more than 60%, rise and fall time is under 300 ps, and the side mode suppression ratio is more than 35 dB for the transmitter part. For the receiver part, responsivity is more than 0.6 A/W, and sensitivity is lower than -17 dBm at a 10-Gbps bit error rate 10^-12 and -21 dBm at BER 10^-3 without forward error correction. The cross talk between receiver and transmitter is less than -53 dB up to 10 GHz, and optical isolation is 33 dB.

A novel all-optical wavelength converter for nonreturn-to-zero systems based on a nonlinear polarization switch (NPS) and a delayed interferometer (DI) is proposed and demonstrated experimentally. The performance of the proposed wavelength converter is compared with the performance of a conventional wavelength converter that only incorporates a NPS. Results show that the quality of the converted signal is significantly improved by inserting a DI after the NPS. The transmission performance of the wavelength-converted signal is also improved by the DI.

This PDF file contains the editorial “Special Section Guest Editorial: Quantum and Interband Cascade Lasers” for OE Vol. 49 Issue 11

The design and operating principles of quantum cascade lasers (QCLs) are reviewed along with recent developments in high-power cw and broadband devices. Cw power levels of several watts at room temperature have been achieved at 4.6-µm wavelength; broadband single-mode tuning (400 cm−^;1) has been achieved using an external-cavity QCL with a grating as a tuning element. An alternative approach, consisting of a monolithically integrated array of single-mode QCLs individually current-driven by a microcontroller, has led to broadband single-mode tuning over a range of 200 cm−^;1 without requiring the use of moving parts. This spectrometer on a chip holds promise for high-brightness compact trace-gas sensors capable of operating in harsh environments.

We present recent performance highlights of midinfrared quantum cascade lasers (QCLs) based on an InP material system. At a representative wavelength around 4.7 µm, a number of breakthroughs have been achieved with concentrated effort. These breakthroughs include watt-level continuous wave operation at room temperature, greater than 50% peak wall plug efficiency at low temperatures, 100-W-level pulsed mode operation at room temperature, and 10-W-level pulsed mode operation of photonic crystal distributed feedback quantum cascade lasers at room temperature. Since the QCL technology is wavelength adaptive in nature, these demonstrations promise significant room for improvement across a wide range of mid-IR wavelengths.

We report the life test results for 9 4.6-µm planar buried heterostructure quantum cascade lasers made of strain-balanced GaInAs/AlInAs/InP materials grown by metal organic vapor-phase epitaxy. No facet coating was deposited, and the devices were mounted on CuW submounts using AuSn solder. The aging condition is continuous wave operation at a heat-sink temperature of 22^ºC with a constant current of 0.85A, corresponding to current densities of 4.7 and 2.7 kA/cm², for lasers with widths of 4 and 7 µm, respectively. All lasers survived 5000 h aging, and most devices showed improved performance during the first 1000 h of aging.

Because of their compact size, reliability, tunability, and convenience of direct electrical pumping, quantum cascade lasers have found a number of important civilian and defense applications in the midwave infrared and long-wave-infrared spectral range. Most of these applications would benefit from higher laser optical power and higher wall-plug efficiency. We describe some of the most important features of high-efficiency quantum cascade laser design and realization of high-power quantum cascade laser systems. Specifically, optimization of the active region and waveguide, thermal management on the chip level, and impact of the laser facet coating on laser efficiency and scaling of optical power with cavity length are discussed. Also, we present experimental results demonstrating multiwatt operation with reliability of at least several thousands of hours on a system level.

We present an overview of our recent results on the growth, fabrication, and characterization of high-power long-wave infrared quantum cascade lasers with multimode and single-mode waveguides. Powers of up to 1.2 W at wavelengths of = 6.1 µm are obtained with InGaAs/InAlAs buried heterostructure lasers grown lattice matched on InP substrates. For longer wavelengths, up to = 9 µm, powers of P = 800 mW are delivered from room-temperature-operated devices. Distributed-feedback waveguides have been fabricated with buried grating geometry, leading to single-mode emission of more than P = 150 mW output at = 7.74 µm when the device is operated at room temperature in continuous mode.

The gain spectra of a continuous wave operating injectorless quantum cascade lasers are presented. These are extracted by the Hakki-Paoli method from electroluminescence measurements. The device operates around 7.4 µm and has threshold current densities in pulsed and continuous wave operation at room temperature that measure 1.1 and 1.76 kA/cm², respectively. The differential gain coefficients around 100 cm/kA at liquid nitrogen temperature and 44 cm/kA around room temperature are determined. The gain measurement values are compared with the 1/L threshold behavior versus temperature, indicating a nonlinear gain current dependence of injectorless devices.

The equations for threshold-current density J_th and external differential quantum efficiency _d of quantum cascade lasers (QCLs) are modified to include electron leakage and the electron-backfilling term corrected to take into account hot electrons in the injector. We show that by introducing both deep quantum wells and tall barriers in the active regions of 4.8-µm-emitting QCLs, and by tapering the conduction-band edge of both injector and extractor regions, one can significantly reduce electron leakage. The characteristic temperatures for J_th and _d, denoted by T₀ and T₁, respectively, are found to reach values as high as 278 and 285 K over the 20 to 90°C temperature range, which means that J_th and _d display 2.3 slower variation than conventional 4.5- to 5.0-µm-emitting, high-performance QCLs over the same temperature range. A model for the thermal excitation of hot injected electrons from the upper laser level to the upper active-region energy states, wherefrom some relax to the lower active-region states and some are scattered to the upper miniband, is used to estimate the leakage current. Estimated T₀ values are in good agreement with experiment for both conventional QCLs and deep-well QCLs. The T₁ values are justified by increases in both electron leakage and waveguide loss with temperature

We consider the use of strained quantum cascade (QC) laser designs in the 7.3- to 7.8-µm wavelength region to improve the cw operating characteristics of these lasers at room temperature. We compare the performance of a QC laser with a strain-balanced active region with that of two similarly designed QC lasers with lattice-matched active-region layers, and we show that the strain-balanced design significantly outperforms the unstrained-layer designs in terms of threshold-current density, power slope efficiency, and cw output power at room temperature. A epi-side-down-mounted, double-channel ridge-geometry laser that was fabricated from the strained design with a ridge width of 13 µm and a cavity length of 3 mm produced cw output power in excess of 500 mW at room temperature. The characteristic temperature T₀ for the strained-layer design as determined from pulsed measurements between 25 and 80^°C is 221.5 K, compared with a value of 194.0 K for one of the unstrained designs.

A simple rate-equation-based model for transport and gain in quantum cascade lasers is developed. According to the model, the IV characteristics of a quantum cascade laser can be well described by equivalent circuit containing a serial connection of a Schottky diode, a tunnel diode, and a field-effect transistor. As a consequence, a quantum cascade laser can be described in terms accessible to circuit designers.

Different approaches to power scaling of 4.5- to 5-µm emitting quantum cascade (QC) lasers by multiemitter beam combining are investigated. Spectral beam combining of linear arrays of QC lasers consisting of several individual emitters located side by side is demonstrated as a first variant, using an external cavity equipped with a diffraction grating and a partially transmitting output mirror providing wavelength-selective feedback to each emitter. In this way, spectral beam combining of up to eight individual QC lasers is achieved with an optical coupling efficiency of 60% for an array of six emitters. The resulting beam quality (M² < 2 for both fast and slow axes) is close to that observed for single emitters. As a second approach, a linear array of QC lasers is coupled to a custom-made array of silicon microlenses positioned in front of the output facets of the QC lasers. This technique produces a set of closely spaced parallel output beams, strongly overlapping in the far field, without introducing any coupling losses. The resulting beam divergence is given by the aperture size of the microlenses, which is limited by the center-to-center spacing of the QC lasers (500 µm in our case).

We describe a mechanism for plasmonic loss reduction in midinfrared metallic photonic crystals and apply it to surface-plasmon quantum cascade lasers. We obtain pulsed, room-temperature operation of surface-emitting photonic crystal quantum cascade lasers operating at 7.4 µm. The photonic crystal resonator is patterned in the device top metallization, and laser operation is obtained on a band-edge mode of the photonic band structure. The emission is spectrally single mode, with a side-mode suppression ratio of 20 dB, and on-chip tunability is obtained over a wavelength range of 0.52 µm. Simulations based on a finite elements approach and on the finite-difference time-domain method allow us to study the photonic-band structure, the electromagnetic field distributions, and especially, the influence of the device parameters on the losses. The comparison between the measured and simulated far-field emission patterns and polarization proves the lasers operate on a monopolar-symmetry mode

We present ring-cavity surface-emitting lasers (ring-CSELs) based on quantum cascade structures as an elementary building block for two-dimensional quantum cascade laser arrays. The devices operate at temperatures of 380 K and above. A reduction in threshold current density and enhanced radiation efficiency are observed as compared to Fabry-Pérot lasers. The devices exhibit single-mode emission at a wavelength around 8 µm with a side-mode suppression ratio of 30 dB at room temperature. Tuning of the resonance is achieved by variation of the grating period or change in temperature. The emitters exhibit a low-divergence ring-shaped beam pattern with a lobe separation of 1.5 deg. Based on a direct coupling scheme, phase locking of two ring-CSELs is demonstrated with a fringe visibility of 60%. Coherent operation of ring-type lasers results in a narrow spectral line and thus in an enhancement of the spectral brightness.

We study the multimode operation regimes of midinfrared quantum cascade lasers (QCLs), taking into account nonlinear phase-sensitive interactions between transverse modes. We show the possibility of the coherent coupling of several transverse modes, which results in a number of interesting effects including frequency and phase locking between transverse modes, bistability, and beam steering. We present an analytical model for the modal dynamics and its numerical analysis. Effects of amplitude and phase fluctuations on the modal stability are explored. The theoretical results are in agreement with our experimental measurements of buried heterostructure QCLs.

We review the basic microscopic phenomena controlling the extraction of energy from the active regions of terahertz quantum cascade lasers. Using time-resolved microprobe photoluminescence, we measure the in-plane (v_//) and cross-plane (v) heat transfer velocities and find a strong anisotropy, with v_// slightly reduced by 30% and v reduced by a factor of 6.5 with respect to the corresponding bulk values. Combining this information with measurements of the cross-plane thermal conductivity enables us to estimate a phonon mean free path value L ~ 70 nm at 80 K, reduced by about one order of magnitude with respect to the bulk value in GaAs, but much larger than the average spacing between the interfaces in the active region. The latter finding is consistent with a heat transport model considering both thermal boundary resistance and phonon dispersion modification.

We report an experimental study of how the light-current characteristics and lateral mode properties of interband cascade lasers depend on ridge width. Narrower ridges provide greater heat dissipation due to lateral flow, along with operation in a single lateral mode. However,sidewall imperfections increase the cw threshold current density somewhat, from J_th = 582 A/cm² at 300 K for an 11-µm-wide ridge to 713 A/cm² and 1.07 kA/cm² for 5- and 3-µm-wide ridges, respectively. The narrowest ridges similarly display a degradation of the slope efficiency. A 13-µm- wide ridge produced 45 mW per facet of cw output power and maximum wall-plug efficiency of 3.5% per facet at T = 20°C. A 5-µm-wide ridge with 3-mm cavity length and no facet coatings operated cw at = 3.5-µm to a new record temperature of 345 K for the 3 to 4-µm spectral range.

Interband cascade lasers (ICLs) are very attractive light sources to cover the important spectral range from 3.3 to 3.6 µm, which is of major interest for hydrocarbon detection. We present ICLs emitting in this wavelength range that have a notably shortened injector region. Compared to a well-established reference design, the total length of one cascade is reduced by about 16% to only 63 nm. The benefits are a reduction of the number of heterointerfaces, a higher intensity of the optical mode in the active region, and easier strain compensation. Threshold current densities of devices using the new injector design are cut in half throughout the temperature range of operation, compared to devices with the reference design. Deeply etched ridge waveguide devices processed from structures with 14 cascades yielded maximum operation temperatures of 73°C in pulsed operation. Peak output powers exceed 100 mW at a heat-sink temperature of 15°C.

We demonstrate a mid-IR semiconductor laser-based absorption spectrometer for field measurements of ambient CH₄. The field sensor uses a type II quantum cascade laser (or interband cascade laser, ICL) operating near 3.3 µm to monitor a well-isolated line in the 3 fundamental band of CH₄. The ICL operates in cw mode at cryogenic temperatures. The sensor uses a multipass cell that provides an optical path of ~7 m with a 0.25-m base path. Thermoelectrically cooled InAs detectors are used along with balanced ratiometric detection to achieve a precision of 15 ppbv for a 60-s integration time. Several successful field demonstrations are carried out at sites maintained by the University of New Hampshire.

The development of interband cascade lasers from concept to packaged devices is briefly reviewed. The application of a single-mode, mid-IR (3.27-µm) interband cascade laser packaged with a thermoelectric cooler for field measurements of methane and water is described.

The utilization of the broad gain bandwidth available from quantum cascade (QC) devices is considered. The performance of homogeneous and heterogeneous QC gain media is explored in an external-cavity configuration. Paradigms for realizing fixed wavelength or broad tuning performance of QC devices are considered.

We review the use of both pulsed and continuous wave quantum cascade lasers in high-resolution spectroscopic studies of gas phase species. In particular, the application of pulsed systems for probing kinetic processes and the inherent rapid passage structure that accompanies observations of low-pressure samples using these rapidly chirped devices are highlighted. Broadband absorber spectroscopy and time-resolved concentration measurements of short-lived species, respectively exploiting the wide intrapulse tuning range and the pulse temporal resolution, are also mentioned. For comparison, we also present recent sub-Doppler Lamb-dip measurements on a low-pressure sample of NO, using a continuous wave external cavity quantum cascade laser system. Using this methodology the stability and resolution of this source is quantified. We find that the laser linewidth as measured via the Lamb-dip is ca. 2.7 MHz as the laser is tuned at comparably slow rates, but decreases to 1.3 MHz as the laser scan rate is increased such that the transition is observed at 30 kHz. Using this source, wavelength modulation spectroscopy of NO is presented.

Despite the growing interest that quantum cascade lasers (QCLs) are gaining, they still present a few unclear aspects of their fundamental properties, such as spectral purity, that need to be deeply investigated when aiming to make these innovative laser sources suitable for high-resolution spectroscopy and metrology. This paper is a review of our efforts towards QCL-based high-resolution spectroscopy and of our experimental investigation of QCLs' frequency noise, aimed to discover the ultimate performances attainable by QCLs and to develop the experimental techniques required to achieve them. Our results, confirmed by several independent measurements, show that QCLs have a very small intrinsic linewidth buried under a large frequency-noise background. The development of appropriate frequency stabilization techniques will make QCLs well suited for high-resolution spectroscopy and metrology in the mid and far IR.

State-of-the-art quantum- and interband-cascade-based chemical sensors may be effective new tools for the identification and quantification of trace gases in human breath for clinical uses. Increased or decreased concentrations of these molecules are associated with the pathogenesis of a large number of diseases. Current technologies enable breath analyses to be performed on a single breath and the results are available in real time. Critical parameters including sensor sensitivity, selectivity, real-time monitoring capability, robustness, cost, size, and weight determine the progress made toward the development and availability of commercial diagnostic material.

We review our recent results in development of high-precision laser spectroscopic instrumentation using midinfrared quantum cascade lasers (QCLs). Some of these instruments have been directed at measurements of atmospheric trace gases where a fractional precision of 10−^;3 or better of ambient concentration may be required. Such high precision is needed in measurements of fluxes of stable atmospheric gases and measurements of isotopic ratios. Instruments that are based on thermoelectrically cooled midinfrared QCLs and thermoelectrically cooled detectors have been demonstrated that meet the requirements of high-precision atmospheric measurements, without the need for cryogens. We also describe the design of and results from a new dual QCL instrument with a 200-m path-length absorption cell. This instrument has demonstrated 1-s noise of 32 ppt for formaldehyde (HCHO) and 9 ppt for carbonyl sulfide (OCS).

Exploiting several key characteristics of quantum cascade (QC) lasers, including wide tunability and room-temperature operation, the Quantum Cascade Laser Open-Path System (QCLOPS) was designed for the detection of a range of trace gases and for field deployment in urban environments. Tunability over a wavelength range from 9.3 to 9.8 µm potentially provides the capability for monitoring ozone, ammonia, and carbon dioxide, a suite of trace gases important for air quality and regional climate applications in urban environments. The 2008 Olympic Games in Beijing, China drew attention to air quality problems in urban environments. Prior to and during the Olympic games, regional air quality modifications through factory shutdowns, car restrictions, and construction halts in Beijing and its surrounding areas created a unique test bed for new sensor technologies such as the QCLOPS sensor. We report the design of this novel, open-path air quality sensor and the results of both laboratory tests and field trials during the 2008 Olympic Games in Beijing, China.

Detection of explosives is an emerging task for maintaining civil security. Optical methods and especially tunable diode laser spectroscopy are discussed as means for providing fast and reliable data. Selective and sensitive detection is possible in the midinfrared spectral region; however, until recently, small and easy to operate laser sources were not readily available for applications outside the laboratory. The situation changes with the maturation of quantum cascade lasers (QCLs). We present detection methods based on photofragmentation and subsequent midinfrared detection of the fragments for the detection of nitrogen-based explosives. For this type of explosive, the very low vapor pressure makes the use of direct spectroscopic techniques extremely difficult, since the equilibrium concentrations are in the ppb to ppt range. Peroxide-based explosives like triacetone triperoxide possess a much higher vapor pressure, making direct absorption spectroscopy and also a quartz-enhanced photoacoustic spectroscopy sensor possible. The progress and challenges of the application of QCLs, also with respect to interferences with other molecules present, are discussed.

The use of a tunable midinfrared external cavity quantum cascade laser for the standoff detection of explosives at medium distances between 2 and 5 m is presented. For the collection of the diffusely backscattered light, a high-performance infrared imager was used. Illumination and wavelength tuning of the laser source was synchronized with the image acquisition, establishing a hyperspectral data cube. Sampling of the backscattered radiation from the test samples was performed in a noncooperative geometry at angles of incidence far away from specular reflection. We show sensitive detection of traces of trinitrotoluene and pentaerythritol tetranitrate on real-world materials, such as standard car paint, polyacrylics from backpacks, and jeans fabric. Concentrations corresponding to fingerprints were detected, while concepts for false alarm suppression due to cross-contaminations were presented.

A high-resolution midwave infrared panoramic periscope sensor system has been developed. The sensor includes an f/2.5 catadioptric optical system that provides a field of view with 360-deg horizontal azimuth and -10- to +30-deg elevation without requiring moving components (e.g., rotating mirrors). The focal plane is a 2048×2048, 15-µm-pitch InSb detector operating at 80 K. An onboard thermoelectric reference source allows for real-time nonuniformity correction using the two-point correction method. The entire system (detector-Dewar assembly, cooler, electronics, and optics) is packaged to fit in an 8-in.-high, 6.5-in.-diameter volume. This work describes both the system optics and the electronics and presents sample imagery. We model both the sensor's radiometric performance, quantified by the noise-equivalent temperature difference, and its resolution performance. Model predictions are then compared with estimates obtained from experimental data. The ability of the system to resolve targets as a function of imaged spatial frequency is also presented.

Acousto-optic tunable filters (AOTFs) can be used as spectral filters in multispectral imaging applications. Acousto-optic crystals diffract a single wavelength from a broadband light beam, depending on the applied radio frequency signal. However, experimental measurements show that the actual performance is far from the expected behavior. We present an experimental characterization of several commercial off-the-shelf AOTFs for the implementation of multispectral imaging instruments. The diffraction performance of three bare crystals is compared, while a fourth AOTF crystal is mounted on the optical path of a multispectral imager to evaluate its performance. The experiments show that the behavior of all the analyzed AOTFs differs from the theoretical expectations and presents uneven diffraction efficiency, a Gaussian dependency with the applied power, and a strong nonlinear relationship with the driving signal frequency. The different behavior of each AOTF in terms of all the analyzed parameters shows the necessity for an in-depth characterization of the AOTF performance once mounted on a multispectral imaging device if quantitative measurements are required. Finally, several recommendations for use are derived from these experimental results.

This paper describes the use of microwaves to accurately image objects behind dielectric walls. The data are first simulated by using a finite-difference time-domain code. A large model of a room with walls and objects inside is used as a test case. Since the model and associated volume are big compared to wavelengths, the code is run on a parallel supercomputer. A fixed 2-D receiver array captures all the return data simultaneously. A time-domain backprojection algorithm with a correction for the time delay and refraction caused by the front wall then reconstructs high-fidelity 3-D images. A rigorous refraction correction using Snell's law and a simpler but faster linear correction are compared in both 2-D and 3-D. It is shown that imaging in 3-D and viewing an image in the plane parallel to the receiver array is necessary to identify objects by shape. It is also shown that a simple linear correction for the wall is sufficient.

Digital web cameras (popularly known as webcams) have recently gained a significant increase of relevance in the field of optical microscopy, in particular to allow for quick and do-it-yourself methods in developing low-cost and portable microscopes suitable for life sciences and engineering applications in low-resource areas. Unfortunately, these methods were published without any systematic explanation and quantitative assessment of the imaging performances. We reproduce these do-it-yourself methods, discuss the optical considerations that are relevant for them, and quantitatively compare their imaging performances to a commercial digital microscope in order to clarify both the advantages and disadvantages of the webcam-based microscopes.

Many methods have previously been devised to estimate the relative amounts of chemicals present in a measured Raman spectrum. However, relatively little work has been done on developing physics-based probabilistic models for the measurement system. Drawing from previous work in astronomical image restoration, we model the acquired data based on the physics of two key components in our Raman instrumentation: the spectrometer and the charge-coupled device detector. Under this model, we derive Cramér-Rao lower bounds for the mixing coefficients of the target spectra. This bound is compared against the performance of several classification algorithms. The non-negative iteratively reweighted least-squares algorithm is seen to give performance that is nearly identical to the more computationally demanding expectation-maximization approach; this is true even at weak signal levels.

The performance of a high-power fast-axial-flow CO₂ laser is directly determined by the characteristics of the turbulent flow of the active medium in the discharge region. To research the influence of the discharge tube structure on the internal gas flow field and determine the optimum design of the discharge tube, we use the computational fluid dynamics method and a set of governing equations to predict and compare the internal gas flow field of various sizes of discharge tubes. The influence of the tube diameter and gas inflow opening size on the gas flow velocity and turbulence intensity is discussed. There is good agreement between the theoretical prediction and the experimental results. The results obtained provide a basis for the optimum design of a high-power industrial fast-axial-flow CO₂ laser.

The small-signal parameters of quantum dash (QDash) lasers with multiple coupled energy states have been derived using a rate equation model. Analytical expressions for the small-signal differential gain, resonance frequency, and recombination lifetime have been derived. The linewidth enhancement factor of QDash lasers with multiple coupled energy states is studied using our derived model. With the help of our model, we find that introducing p-type doping in the active region of the QDash layer does not enhance the small-signal resonant frequency, but reduces the small-signal recombination lifetime and the linewidth enhancement factor. Also, we find that increasing the n-type doping concentration decreases slightly the small-signal recombination lifetime and the resonant frequency, and increases slightly the linewidth enhancement factor. Our analysis reveals that an undoped QDash laser can be designed to operate at the second excited state energy and to yield high modulation bandwidth and low linewidth enhancement factor.

A model of deicing with Nd:YAG and CO₂ lasers for simulation using ANSYS software is presented. Experiments with a 300-W, 1-ms, 60-Hz Nd:YAG laser and a 500- to 2000-W cw CO₂ laser are reported. The Nd:YAG laser is considered as a volume thermal source, and the CO₂ laser as a plane thermal source. The model and the simulation results can describe both Nd:YAG and CO₂ laser deicing well. The results of the simulation and experiments suggest that the melting rates for the two lasers are almost equal at the same laser power density. So are the melting efficiencies. The hard and transparent ice irradiated by the Nd:YAG laser becomes opaque and loose, because the thermal stress is distributed in the body of the ice, while the ice irradiated by the CO₂ laser is still transparent and hard, because thermal stress hardly occurs. So the laser with characteristics of high output power and large ice absorbing length can be selected for the power line laser deicing system, and Nd:YAG laser is more appropriate for power-line deicing than CO₂ laser.

Modulation phase drift is observed in the interferometric fiber optic gyroscope (IFOG) and is attributed to the use of a multifunction integrated optical device. The influence of modulation phase drift on the IFOG's properties is analyzed in theory. The analysis results indicate that the drift causes a nonnegligible dead zone in the IFOG's response. To suppress the drift, three methods are proposed and validated experimentally. It is confirmed that a multifunction integrated optical device without a buffer layer can suppress the modulation phase drift effectively.

Crosstalk and power penalty analysis for optical code-switched static wavelength routing wavelength-division multiplexing nodes of a generalized multiprotocol label switched (GMPLS) network is presented. The performance of the system is analyzed for coherent crosstalk, end-to-end bit error rate, and the code length. We introduce a new approach to estimate the blocking probability in crosstalk-impaired GMPLS networks. Packet error rate is estimated for an increasing traffic condition. A well-known Erlang's traffic model is used to examine the performance of the system.

Cone tracking is a well-known method to optimize the pointing of a beam, and has been reported previously in the case of direct-mode free-space optical communication links, using a beacon or a dedicated wavelength to carry the feedback. In a retro link, because the beam is reflected back to the transmitter, feedback data are locally available. We present here the results of an evaluation (both simulation and experiments) of the cone-tracking technique applied to a retro link and how the implementation can be optimized for this specific case. We show that, using only a small number of samples (as few as 16 with 625-Hz modulation frequency), we can retrieve the pointing error. We finally demonstrate cone tracking (a closed loop maintaining the beam center on the retroreflector) with modulation amplitude as low as 1% of the beam divergence.

A simple distortion-invariant optical identification (ID) tag is presented for real-time vehicle identification and verification. The proposed scheme is composed of image encryption, ID tag creation, image decryption, and optical correlation for verification. To create the ID tag, a binary-phase computer-generated hologram (BPCGH) of a symbol image representing a vehicle is created using a simulated annealing algorithm. The BPCGH is then encrypted using an XOR operation and enlargement transformed into polar coordinates. The resulting ID tag is then attached to the vehicle. As the BPCGH consists of only binary phase values, it is robust to external distortions. To identify and verify the vehicle, several reverse processes are required, such as capturing the ID tag with a camera, extracting the ID tag from the captured image, transformation of the ID tag into rectangular coordinates, decryption, an inverse Fourier transform, and correlation. Computer simulation and experimental results confirm that the proposed optical ID tag is secure and robust to such distortions as scaling, rotation, cropping (scratches), and random noise. The ID tag can also be easily implemented, as it consists of only binary phase components.

Standoff discrimination of bioaerosols based on lidar measurements of depolarized backscattered light is herein studied. Measurements were performed at four wavelengths (355, 532, 1064, and 1570 nm) over 25 pollens and 2 dusts under controlled environment at a distance of 100 m. Linear polarization measurements were performed. It is shown that discrimination between pollens can be achieved using the linear polarization of at most three of the four wavelengths, and statistical discrimination based on Mahalanobis distance is obtained for most of the 27 cases studied.

Infrared search and track pursues the detection of sea-skimming infrared targets incoming from long distance. This paper presents a realistic synthetic target simulator for the development of infrared target detection and tracking algorithms. The proposed simulator consists of a 2-D background modeling part and a 3-D infrared target modeling part. Real infrared background images are used for the realistic modeling of background. Synthetic infrared target images are obtained by the consecutive processing of 3-D geometric modeling and radiometric modeling of targets according to target types, target distances, and atmospheric transmissivity. The experimental results validate the realistic modeling of the proposed method by comparing real observation sequence data.

Breast cancer is one of the leading cancers of women. Ultrasound is often used for breast cancer diagnosis because it is harmless, portable, and low-cost. However, the segmentation of breast ultrasound (BUS) images is a difficult task due to their low contrast and speckle noise. Neutrosophy studies the origin, nature, and scope of neutralities and their interactions with different ideational spectra. It is a new philosophy to extend fuzzy logic and is the basis of neutrosophic logic, neutrosophic probability theory, neutrosophic set theory, and neutrosophic statistics. In this paper, we employ neutrosophy and develop a fully automatic algorithm for BUS image segmentation. By using neutrosophy, we integrate two conflicting opinions about speckle in ultrasound image: speckle is noise and speckle includes pattern information. The experiments demonstrate that the proposed approach is accurate, effective, and robust.

This paper proposes a refinement algorithm for equivalent numbers of nonmatching points on surfaces for binary-block motion estimation. The conventional binary-block motion estimation algorithms use the number of nonmatching points as a distortion criterion, and that limits the dynamic range of the distortion. The proposed algorithm performs refinement using the sum of absolute differences on surfaces that have equal numbers of nonmatching points. The refinement step improves the visual quality of binary-block motion estimation at the cost of higher computational complexity.

This paper proposes a new method for natural-image deblur based on a single blurred image. The natural image prior, a sparse gradient distribution, is enforced using a gradient histogram remapping method in the proposed deblur algorithm. The proposed objective function for blind deconvolution is solved by an alternating minimization method. The point spread function and the unblurred image are updated alternately. The proposed method is able to produce high-quality deblurred results with low computational costs. Both synthetic and real blurred images are tested in the experiments. Encouraging experimental results show that the newly proposed method could effectively restore images blurred by complex motion.

The paper proposes a nonaggressive median filtering scheme, with two settings, that can be used to restore images moderately corrupted with either of the two distinct types of impulse noise-fixed-valued and random-valued. The scheme benefits from the fact that its design requires neither the difficult selection of optimal image-data dependent threshold(s), nor any kind of prior training. The filter has been tested on various standard images contaminated with both varieties of impulse noise (not added simultaneously); the results obtained strongly validate the good performance of the process. The proposed approach fulfils all the requirements needed for use as a real-time video-processing component.

Overdrive is commonly used to reduce the liquid-crystal response time and motion blur in liquid-crystal displays (LCDs). However, overdrive requires a large frame memory in order to store the previous frame for reference. In this paper, a high-compression-ratio codec is presented to compress the image data stored in the on-chip frame memory so that only 1 Mbit of on-chip memory is required in the LCD overdrives of mobile devices. The proposed algorithm further compresses the color bitmaps and representative values (RVs) resulting from the block truncation coding (BTC). The color bitmaps are represented by a luminance bitmap, which is further reduced and reconstructed using median filter interpolation in the decoder, while the RVs are compressed using adaptive quantization coding (AQC). Interpolation and AQC can provide three-level compression, which leads to 16 combinations. Using a rate-distortion analysis, we select the three optimal schemes to compress the image data for video graphics array (VGA), wide-VGA LCD, and standard-definitionTV applications. Our simulation results demonstrate that the proposed schemes outperform interpolation BTC both in PSNR (by 1.479 to 2.205 dB) and in subjective visual quality.

We propose a new approach for detecting interest points using the support value of Gaussian function, which uses the support value to represent the salient features of the image. The support values image is computed by convolving the image with the support value filter deduced from the mapped least-squares support vector machines. The multiscale representation is built by successive smoothing of the support values image with a Gaussian kernel. Then, the normalized support value of Gaussian function is used to find the location of interest points and to select the points at which maxima are over scale. We compared our approach to the state of art of approaches using a standard data set. The experimental results show that the proposed approach performs better than other detectors for scenes under scale and blur changes, in terms of the repeatability score. The performance of the proposed approach is also confirmed by the image registrational results. Moreover, an extension of our method to airport recognition is presented.

A novel region-based adaptive anisotropic diffusion (RAAD) is presented for image enhancement and denoising. The main idea of this algorithm is to perform the region-based adaptive segmentation. To this end, we use the eigenvalue difference of the structure tensor of each pixel to classify an image into homogeneous detail, and edge regions. According to the different types of regions, a variable weight is incorporated into the anisotropic diffusion partial differential equation for compromising the forward and backward diffusion, so that our algorithm can adaptively encourage strong smoothing in homogeneous regions and suitable sharpening in detail and edge regions. Furthermore, we present an adaptive gradient threshold selection strategy. We suggest that the optimal gradient threshold should be estimated as the mean of local intensity differences on the homogeneous regions. In addition, we modify the anisotropic diffusion discrete scheme by taking into account edge orientations. We believe our algorithm to be a novel mechanism for image enhancement and denoising. Qualitative experiments, based on various general digital images and several T1- and T2-weighted magnetic resonance simulated images, show significant improvements when the RAAD algorithm is used versus the existing anisotropic diffusion and the previous forward and backward diffusion algorithms for enhancing edge features and improving image contrast. Quantitative analyses, based on peak signal-to-noise ratio, the universal image quality index, and the structural similarity confirm the superiority of the proposed algorithm.

Graph-based dimensionality reduction methods are popular in pattern recognition and machine learning. In contrast to the manifold learning approaches, the dot product representation of graphs (DPRG) seeks a solution to dimensionality reduction by assigning vectors to each node of a graph such that the dot product of every pair of nodes approximates the similarity between them. The DPRG has many potential applications, for the reason that there is no prior assumption of the data distribution. It has been found, however, that the DPRG tends to reduce the distances of the graph nodes represented in a low-dimensional space, which in turn degrades the performance of data clustering. Motivated by this observation, we propose an extended DPRG (EDPRG) model by simply employing negative similarity values. The theoretical analysis and experiments on synthetic data show that the modification is effective in increasing between-class distances. We demonstrate the effectiveness of the EDPRG model by experiments on synthetic aperture radar (SAR) image segmentation. The proposed image segmentation method has two steps. The first one presegments the image by the mean shift algorithm. The second merges the resulting regions by means of the EDPRG model.

Facial expression recognition can be widely used for various applications, such as emotion-based human-machine interaction, intelligent robot interfaces, face recognition robust to expression variation, etc. Previous studies have been classified as either shape- or appearance-based recognition. The shape-based method has the disadvantage that the individual variance of facial feature points exists irrespective of similar expressions, which can cause a reduction of the recognition accuracy. The appearance-based method has a limitation in that the textural information of the face is very sensitive to variations in illumination. To overcome these problems, a new facial-expression recognition method is proposed, which combines both shape and appearance information, based on the support vector machine (SVM). This research is novel in the following three ways as compared to previous works. First, the facial feature points are automatically detected by using an active appearance model. From these, the shape-based recognition is performed by using the ratios between the facial feature points based on the facial-action coding system. Second, the SVM, which is trained to recognize the same and different expression classes, is proposed to combine two matching scores obtained from the shape- and appearance-based recognitions. Finally, a single SVM is trained to discriminate four different expressions, such as neutral, a smile, anger, and a scream. By determining the expression of the input facial image whose SVM output is at a minimum, the accuracy of the expression recognition is much enhanced. The experimental results showed that the recognition accuracy of the proposed method was better than previous researches and other fusion methods.

The huge amount of video data generated by surveillance systems necessitates the use of automatic tools for their efficient analysis, indexing, and retrieval. Automated access to the semantic content of surveillance videos to detect anomalous events is among the basic tasks; however, due to the high variability of the audio-visual features and large size of the video input, it still remains a challenging task, though a considerable amount of research dealing with automated access to video surveillance has appeared in the literature. We propose a keyframe labeling technique, especially for indoor environments, which assigns labels to keyframes extracted by a keyframe detection algorithm, and hence transforms the input video to an event-sequence representation. This representation is used to detect unusual behaviors, such as crossover, deposit, and pickup, with the help of three separate mechanisms based on finite state automata. The keyframes are detected based on a grid-based motion representation of the moving regions, called the motion appearance mask. It has been shown through performance experiments that the keyframe labeling algorithm significantly reduces the storage requirements and yields reasonable event detection and classification performance.

Inter-view prediction is introduced in multiview video coding (MVC) to exploit the inter-view correlation. Statistical analyses show that the coding gain benefited from inter-view prediction is unequal among pictures. On the basis of this observation, a picturewise interview prediction selection scheme is proposed. This scheme employs a novel inter-view prediction selection criterion to determine whether it is necessary to apply inter-view prediction to the current coding picture. This criterion is derived from the available coding information of the temporal reference pictures. Experimental results show that the proposed scheme can improve the performance of MVC with a comprehensive consideration of compression efficiency, computational complexity, and random access ability.