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The wavelength spectra of ridge waveguide Fabry Perot lasers can be modified by perturbing the effective refractive index of the guided mode along very small sections of the laser cavity. One way of locally perturbing the effective index of the lasing mode is by etching features into the ridge waveguide such that each feature has a small overlap with the transverse field profile of the unperturbed mode, consequently most of the light in the laser cavity is unaffected by these perturbations. A proportion of the propagating light is however reflected at the boundaries between the perturbed and the unperturbed sections. Suitable positioning of these interfaces allows the mirror loss spectrum of a Fabry Perot laser to be manipulated. In order to achieve single longitudinal mode emission, the mirror loss of a specified mode must be reduced below that of the other cavity modes. Here we briefly review one procedure for calculating the mirror loss spectra of devices containing such features. We then go on to describe a method for synthesising onedimensional slot patterns. This technique allows the lasers emission wavelength to be specified to a high degree of accuracy.

In recent years quantum well infrared (IR) photodetector (QWIP) focal plane array (FPA) technology has developed to the point where it may be considered a candidate for insertion into 3rd generation FLIR systems. Large format 1024x1024 pixels FPAs have been produced using QWIP technology. We report on the application of a large format FPAs to the challenges facing today's military. These include the collection of signatures of military vehicles for long-range target detection/identification. The FPA used was a 1024x1024 pixel array which is available commercially. from QWIP Technologies, Inc. We show imagery of military targets at ranges from 500 m to 5 km acquired in the field. The results of the performance in the field are compared to that predicted by computer models and the performance of the large format QWIP FPA will be evaluated in terms of the capabilities of a notional 3rd generation FLIR system.

Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) 1024x1024 pixel quantum well infrared photodetector (QWIP) focal planes have been demonstrated with excellent imaging performance. The MWIR QWIP detector array has demonstrated a noise equivalent differential temperature (NE(Delta)T) of 17 mK at a 95K operating temperature with f/2.5 optics at 300K background and the LWIR detector array has demonstrated a NE(Delta)T of 13 mK at a 70K operating temperature with the same optical and background conditions as the MWIR detector array after the subtraction of system noise. Both MWIR and LWIR focal planes have shown background limited performance (BLIP) at 90K and 70K operating-temperatures respectively, with similar optical and background conditions. In addition, we are in the process of developing MWIR and LWIR pixel collocated simultaneously readable dualband QWIP focal plane arrays.

A novel uncooled long-wave infrared imaging technology with optical readout is proposed and developed targeted for low cost thermal imaging applications. This technology uses the thermo-optic effect in a semiconductor to detect infrared signals rather than the thermal-resistance effect used in traditional microbolometers. The key component of the imager, the focal plane array, is made up of thermally tunable thin film filter membrane pixels. Each thermal pixel acts as a wavelength translator, converting far infrared radiation signals into near infrared signals which are then detectable by off-the-shelf CCD or CMOS cameras. This approach utilizes optical filter and MEMS technologies, to build a low-cost passive long wavelength infrared focal plane array without electrical leads or active cooling. Within one year since the commencement, NETD values of 0.28K in a 160x120 array operating at 22Hz video frame rate have been achieved without temperature control.

Raytheon Vision Systems (RVS) is developing two-color, large-format infrared FPAs to support the US Army’s Third Generation FLIR systems**. RVS has produced 640 x 480 two-color FPAs with a 20 P m pixel pitch. Work is also underway to demonstrate a 1280 x 720 two-color FPA in 2005. The FPA architecture has been designed to achieve nearly simultaneous temporal detection of the spectral bands while being producible for pixel dimensions as small as 20 microns. Raytheon’s approach employs a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). This ROIC is coupled to bias-selectable two-color detector array with a single contact per pixel. The two-color detector arrays are fabricated from MBE-grown HgCdTe triple layer heterojunction (TLHJ) wafers. The single indium bump design is producible for 20 P m unit cells and exploits mature fabrication processes that are in production at RVS for Second Generation FPAs. This combination allows for the high temporal and spatial color registration while providing a low-cost, highly producible and robust manufacturing process. High-quality MWIR/LWIR (M/L) 640 x 480 TDMI FPAs with have been produced and imaged from multiple fabrication lots. These FPAs have LWIR cutoffs ranging to 11 P m at 78K. These 20 P m pixel FPAs have demonstrated excellent sensitivity and pixel operabilities exceeding 99%. NETDs less than 25 mK at f/5 have been demonstrated for both bands operating simultaneously. Keywords: HgCdTe, infrared detectors, focal plane arrays, multi-spectral, two-color, 3

We present a new technology for uncooled focal plane arrays, which is based on optical readout. An electrooptic crystal, operated in the paraelectric phase, is used as the temperature sensitive element. A lateral readout configuration is utilized, and an entire row of pixels is read simultaneously through electrical triggering. The optical readout diminishes the electronic noise mechanisms resulting in a predicted NETD of 5-7mk. This low NETD of the device is determined by the temperature fluctuations noise, thus approaching the theoretical limit for uncooled FPAs.

An overview of DRS' development of a very unique two-color infrared photodiode architecture for two-color infrared focal-plane arrays based on Hg 1-x Cd x Te is given. One unique feature of this architecture is that it requires only a mono-color based on Hg 1-x Cd x Te material growth technique for its implementation. Some data from this development are presented.

The authors describe a procedure they believe to be optimum for determining the path of a ray through an atmosphere described by a radially varying refractive index profile on the Earth. The result is exact for patched quadratic profiles inverse problem and is suitable for calculating ray paths in complex profiles. The method also facilitates greatly the inverse problem.

In silicon, direct electronic transitions leading to light emission have a low probability of occurrence due to the momentum mismatch between upper and lower electronic levels. Until recently, this had prevented the realization of the long waited silicon optical amplifier and laser. Raman scattering, which describes the interactions of light with vibrational levels, can be used as a way to bypass the indirect band structure of silicon and to obtain amplification and lasing. The Raman approach is very appealing because device can be made in pure silicon with a spectrum that is widely tuneable though the pump laser wavelength. While a new research topic, amplifiers with pulsed gain of 20dB and CW gain of 3 dB have already been demonstrated. Using parametric Raman coupling, wavelength conversion from 1550nm to 1300nm has been achieved. A distinguishing feature of silicon Raman devices, compared to fiber devices, is the electronic modulation capability. By integrating a p-n junction with the silicon gain medium, electrically switched lasers and amplifiers have already been demonstrated. These have many exciting applications. For example, the laser can be directly modulated to transmit data, and can be part of a silicon optoelectronic integrated circuit. At the same time, electrically switched amplifiers represent loss-less optical modulators.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present high-performance surface-normal modulators based on unique properties of stepped quantum wells (SQWs) around the eye-safe wavelength of 1550 nm. Fabricated devices show nearly two times better efficiency and 7 dB higher extinction ratio compared with the conventional devices with rectangular and coupled-quantum well active layers. Moreover, the optical bandwidth is about 70 nm at a 3dB modulation depth, which is more than five times wider than the optical bandwidth of the conventional devices. Such a wide optical bandwidth eliminates the need for a temperature controller. This is a critical advantage for many applications such as unmanned aerial vehicles (UAVs) and dynamic optical tags (DOTs), where limited volume, power, and weight can be allocated to the modulator system.

An approach that modifies an analog fiber optic link with a recirculating optical loop as a means to realize a high-speed, high-resolution Analog-to-Digital Converted (ADC) is presented. The loops stores a time-limited microwave signal so that it may be digitized by using a slower, conventional electronic ADC. Detailed analytical analysis of the dynamic range and noise figure shows that under appropriate conditions the microwave signal degradation is sufficiently small so as to allow the digitization of a multi-gigahertz signal with a resolution greater than 10 effective bits. Experimental data is presented which shows that a periodic extension of the input signal can be sustained for well over one hundred periods that in turn suggests an electronic ADC speed-up factor of over 100. The data also shows that polarization effects must be carefully managed to inhibit the loops tendency to lase even though the loop itself contains no frequency-selective elements.

We survey attempts to accurately diagnose skin cancer in vivo and in real time through the use of optics and automatic computation without the intervention of the clinician. Although no system has yet been shown to have such a capability, commercialization of a variety of diagnostic aids has taken place. We describe our own research into the diagnosis of malignant melanoma, the most lethal form of skin cancer, based on the spectroscopy of diffuse white light reflectance collected with a single multimode fiber after delivery with a multimode fiber bundle. Our clinical study, which collected 82 melanomas and 277 lesions in total, highlights the complexity of this challenging application.

We discuss the fundamental limits of fiber Bragg grating (FBG) wavelength metrology. High-accuracy wavelength measurements are critical for FBG strain sensors because a wavelength measurement uncertainty as small as 1 pm leads to an uncertainty of nearly 1 microstrain. We compare the measurement uncertainties for several common wavelength measurement systems, including tunable laser, optical spectrum analyzer (OSA), and interferometric. We show that when using an OSA it is difficult to achieve a measurement uncertainty better than 10 pm, and if the OSA is not accurately calibrated to a known wavelength reference, then the wavelength measurement uncertainty can be as large as 1 nm. We describe the uncertainties involved in determining peak and/or centroid wavelength from a measured data set. We also discuss calibration references for FBG sensor interrogation units. Wavelength references that are based on molecular absorption lines are often an excellent choice for FBG sensor calibration. However, some interrogation units require a wavelength reference unit based on narrow reflection lines rather than absorption lines. We investigated the application of athermally packaged FBGs as wavelength references, but we found that their wavelengths will drift with time and can undergo large jumps. We concluded that it is difficult to achieve stability better than 4 pm/year in athermally packaged FBGs.

Interactions between acoustic waves and optical modes both propagating along an optical fiber are reviewed for applications in tunable photonic sensory devices. Fiber design parameters in Brillouin scattering sensors, and acoustooptic tunable filters are discussed.

An efficient multiplexing method for fiber Bragg grating sensors based on a broadband mode-locked laser source and interferometric interrogation is described. The system has the capability to interrogate several hundred fiber Bragg grating sensors on a single fiber, whilst achieving sub-microstrain resolution over bandwidths greater than 100 kHz. A demonstration system with 8 sensors is presented, which achieves strain resolutions less than 213 n epsilon/Hz(1/2) up to 100 Hz.

The propagation properties of microstructured optical fibers useful for sensing applications are reviewed. The interaction between light and sample can reach 95 % in singlemode hollow core fibers and examples of structures exhibiting such large overlap ratios are described. The generation of 300 nm and 700 nm flat continua of visible and IR light in a single highly nonlinear holey fiber, well suited for the detection of biological species by spectroscopy, is reported. The low temperature sensitivity of long period gratings and of the birefringence in holey fibers is attractive for sensors operating in varying environmental conditions.

POF has found wide use in multimode intensity-based sensor applications. The characteristics of POF, limit the link (sensor)length to a few tens of meters and the sensor operational temperature cannot exceed 80°C. However, when these limitations are not important, POF sensors can offer their most significant advantage over corresponding glass fibre sensors, and this is a significantly lower system cost. Following recent new developments in POF technology, such as fibre gratings and microstructured POF, one looks forward to new and more exciting POF sensor applications. The future of POF sensors is concluded as optimistic.

We have been developing "fiber optic nerve systems" for "smart structures and smart materials", in which an optical fiber acts as sensor to measure distribution of strain along it. The original technology, "Brillouin Optical Correlation Domain Analysis: BOCDA" has been proposed and developed to analyze the distributed strain along the fiber by use of synthesis of correlation characteristics of continuous lightwave. Adopting this technology, "fiber optic nerve systems" with quite a high spatial resolution and measurement speed, have been established.

Light emitting diodes (LEDs) are excellent candidates for the applications requiring low noise light sources with wavelengths ranging from 200 nm to 900 nm. These applications include the detection of fluorescence from protein molecules excited with the ultraviolet (UV) light (200-300nm) for identifying miniscule amounts of hazardous biological pathogens. The detection system including the light source must exhibit low noise and high stability over tens of minutes. In comparison with xenon, tungsten halogen lamps, lasers, and other conventional UV sources, UV LEDs are more stable, have lower noise, are smaller, cheaper, and easier to use. We report on the low frequency fluctuations of the current and light intensity of LEDs (fabricated by SET, Inc.) with wavelengths ranging from 265nm to 340nm. The results are compared with the noise properties of the halogen lamps and other commercially available LEDs with the wavelengths of 375nm, 505nm and 740nm. We show that the LEDs fabricated by Sensor Electronic technology, Inc. are suitable for studying steady state and time-varying UV fluorescence of biological materials. The correlation coefficient between the current and light intensity fluctuations varies with the LED current and load resistance. This dependence is explained in terms of the contributions to the 1/f noise from the active region and from the LED series resistance. The noise level could be reduced by operating the LEDs at a certain optimum current level and by using a large external series resistance (in the current source driving mode).

A detailed understanding of noise characteristics is essential for the design of a high signal-to-noise ratio (SNR) reader sensor. We intend to correlate the microstructure to the source of magnetic noises for improving the magnetic stability of the recording heads. A dynamic magnetic sensitivity mapping (MSM) system is designed to image the magnetic noise sources in sub-micrometer sized recording heads. A nanometer sized magnetic force microscopy (MFM) tip was used to apply a well-defined, localized magnetic field on the air bearing surface (ABS) of the head. For a certain area position of the free layer with incoherent rotation of the magnetic moment, this localized magnetic field will cause magnetic instability in the head, and this instability will show up as electrical noise on the output signal. Because most of the noise related to magnetic domain fluctuation is dominated at the low frequency region, our study concentrates on the spatial characterization of the noise source in a frequency range of 20 kHz to 60 kHz. Recording the average amplitude of the noise spectrum due to the excitation in the measured frequency range as a function of the tip's position, the location of the magnetic noise source can be identified. Magnetic noise images have been obtained by our system for some recording giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) recording heads. Noise MSM images of some unstable recording heads clearly show the spatially uneven noise.

We report our direct observation of the Barkhausen avalanche in ferromagnetic thin film systems, where a collective spin behavior produces nontrivial fluctuations in magnetization change under an external magnetic field. For this study, we develop and use two direct full-field magnetic imaging techniques: magneto-optical microscope magnetometer (MOMM) and magnetic transmission X-ray microscopy (MTXM). From a direct visualization and a statistical analysis of the fluctuating domain images for Co thin films, we investigate the scaling behavior of the Barkhausen avalanche both on spatial and temporal scales using MOMM. We also investigate the reproducibility of the Barkhausen avalanche process. Interestingly, the partially stochastic nucleation behavior is observed for CoCrPt alloy films by means of MTXM on a nanometer scale comparable to the fundamental length scales such as the Barkhausen volume and the grain size of the polycrystalline films. Via these direct full-field observation techniques, dynamic details of Barkhausen avalanche are revealed.

Detecting quantum fluctuations of a mechanical resonator is a long-standing goal of experimental physics. Recent progress has been focussed on high frequency (MHz to GHz) resonators inserted in a milli-Kelvin environment, with motion detection performed by single electron transistor means. Here we propose a novel experimental approach based on high-sensitivity optical monitoring of the displacement of the resonator and feedback cooling. The experimental setup is based on a micro-mechanical resonator inserted in a high-finesse optical cavity and monitored by a highly-stabilized laser system. Available optical technologies provide an unequalled sensitivity, in the 10-20m/√Hz range. The displacement signal is used in real-time to perform a feedback cooling in order to set the resonator's fundamental mode of vibration in its quantum ground state. With the resonator at cryogenic temperature, the feedback cooling mechanism should allow to reach an effective temperature in the micro-Kelvin range.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe proof-of principle experiments demonstrating a novel approach for generating pulses of light with controllable photon numbers, propagation direction, timing, and pulse shapes. The approach is based on preparation of an atomic ensemble in a state with a desired number of atomic spin excitations, which is later converted into a photon pulse by exploiting long-lived coherent memory for photon states and electromagnetically induced transparency (EIT). We discuss our progress toward applying these techniques to transmit quantum states between atomic memory nodes connected by a photonic channel.

Data from a recent “first-look” at using Long Wave InfraRed Imaging Polarimetry (LWIR-IP) to detect surface swimmers is presented and discussed. A significant increase in detection SNR over conventional IR imaging techniques was discovered. The physical phenomena that produces the increased SNR is discussed along with data that shows range effects and their degradation on the SNR. Most significantly, a method to classify the detected object using the same dataset is discussed. Augmenting current swimmer detection systems using this technique will likely significantly decrease the false alarm rates of the system, thus saving manpower resources and preserving force readiness.

Some of our experimental and theoretical results on optical properties of atmospheric turbulence and light propagation through atmosphere are introduced as follows. 1) A mixed model for the scaling property of high-order structure functions of temperature is proposed to study the turbulence intermittency effect in numerical simulation of light propagation through turbulence. 2) The difference of refractive index structure constant C2n of atmosphere and light beam wander measured in horizontal and vertical directions suggest that the atmospheric turbulence is not isotopc in many cases. 3) The fluctuation of the scaling exponent of optical turbulence spectrum and light intensity fluctuation spectrum indicate taht the atmospheric turbulence is not a Kolmogorov one in many cases. 4) A general description of turbulence strength is proposed using the variance of the refractive index σ2n, the outer scale L0, the scaling power γ and the inner scale l0. 5) Characteristics of the power spectrum of laser scintillation in atmosphere depend on the laesr beam property, diameter of the receiving aperture, turbulence spectrum in the dissipation range. 6) The intermittency of ligh scintillation is rather stable at different scintillation index for light propagation through turbulence over both land and sea surfaces. 7) The number density of phaser branch points of light wave field in turbulence generally increases with turbulence strength but present large diversity.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Some time ago a near-field optical imaging technique had been introduced (Appl. Phys. Lett. 73, 1669 (1998)), which achieves high spatial resolution and excellent sensitivity by exploiting the highly localized and mutual near-field interactions between a Au-nanosphere and a sharp Si-probe under evanescent field illumination. Specifically, the scattering of Au-nanoparticles is significantly enhanced by the presence of a sharp nanoscopic probe demonstrating that the probe acts as an efficient antenna. The present study focuses on the underlying physics of the original results by investigating more systematically nanoparticle-probe interactions: (1) The polarization pattern of the scattered field of an evanescent wave excited Si-probe is studied, which demonstrates that the probe scatters as a single dipole. (2) The enhanced scattering signal is measured as a function of sample size, which allows us to predict the signal strength for different size samples. (3) The wavelength dependence of the probe-sample scattering is investigated by exciting Au-nanospheres on (@543 nm) and off plasmon resonance (@633nm). The data shows a pronounced wavelength dependence reflecting the near-field spectrum of the Au-nanocrystals. (4) Finally, a simple, but intuitive model describing these mutual near-field interactions is presented, which explains qualitatively both the size and wavelength dependence of the enhanced scattering signals.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present a low vacuum tool for ultra high resolution scanning electron microscopy of insulators and floating conductors. Charging is stabilized by ionized gas molecules generated using an environmental secondary electron detector designed to operate within the magnetic field on an immersion objective lens. The charge stabilization mechanism yields consistent charge control that is transparent to the operator, and independent of the tasks performed during imaging. This is illustrated by series of artifact-free, high resolution images of an insulating test sample acquired as a function of magnification and scan speed, at a number of accelerating voltages. The low vacuum method is compared to the high vacuum technique of adjusting the electron beam landing energy so as to minimize charging artifacts (i.e., the "total yield" method). The low vacuum approach is less sensitive to changes in beam current density (determined by the beam current, magnification, scan speed and beam diameter) and yields higher ultimate image resolution. The resolution improvement results from effective suppression of both charge-induced defocusing of the electron beam and distortion of the scan pattern.

X-Ray Refraction Topography techniques are based on Ultra Small Angle Scattering by micro structural elements causing phase related effects like refraction and total reflection at a few minutes of arc as the refractive index of X-rays is nearly unity (1x10-5). The extraordinary contrast of inner surfaces is far beyond absorption effects. Scanning of specimens results in 2D-imaging of closed and open pore surfaces and crack surface density of ceramics and foams. Crack orientation and fiber/matrix debonding in plastics, polymers and ceramic composites after cyclic loading and hydro thermal aging can be visualized. In most cases the investigated inner surface and interface structures correlate to mechanical properties. For the exploration of Metal Matrix Composites (MMC) and other micro structured materials the refraction technique has been improved to a 3D Synchrotron Refraction Computed Tomography (SR-CT) test station. The specimen is situated in an X-ray beam between two single crystals. Therefore all sample scattering is strongly suppressed and interpreted as additional attenuation. Asymmetric cut second crystals magnify the image up to 50 times revealing nanometer resolution. The refraction contrast is several times higher than "true absorption" and results in images of cracks, pores and fiber debonding separations below the spatial resolution of the detector. The technique is an alternative to other attempts on raising the spatial resolution of CT machines. The given results yield a much better understanding of fatigue failure mechanisms under cyclic loading conditions.

High-Speed silicon modulators, based on carrier dependent absorption effects, have recently been reported in the literature. For improved performance, these modulators rely on a MOS configuration to control carrier accumulation, rather than on carrier injection from the contacts, to induce an index perturbation for controlling the phase of a propagating signal. Accurate simulation of the carrier distribution is required for the analysis of such a device. This entails the self-consistent solution of the coupled electro-thermal transport equations. An appropriate absorption model is also required in order to couple the carrier distribution to the propagating optical field, via a complex index perturbation. Finally, in order to determine performance, the full optical problem must be solved throughout the device domain. The present work integrates the Box Integral Method of solving the active device transport equations with the Vector Beam Propagation Method (BPM) typically used to analyze passive waveguide structures. A modified Drude Model and Kramers-Kronig relations are used to determine the carrier density dependent absorption and refractive index perturbations. This complex index perturbation is determined as a function of the applied voltage, and used by a simulator based on the BPM to determine the optical performance of an example silicon modulator. Both steady-state and frequency responses are considered. This comprises a general methodology for analyzing realistic semiconductor photonic devices in which the optical propagation is affected by the electro-thermal transport within the device.

We consider the paraxial model for a nonlinear resonator with a saturable absorber beyond the mean-field limit. We introduce a general stability analysis to evidence modulational-instabilities leading to the destabilization of a homogeneous field profile, eventually causing the formation of 3D structures. Further on, for accessible parametric domains, we show in simulations the phenomenon of total radiation confinement leading to the formation of 3D localized bright structures. Such structures are a direct generalization of 2D Cavity Solitons, r ecently observed in broad-area VCSELs, but they are confined also in the propagation dimension. At difference from freely propagating light bullets, here the self-organization proceeds from the resonator feedback/dissipation, combined with diffraction and nonlinearity. We show that such cavity light bullets can be independently excited and erased by appropriate pulses. They can be addressed to form arrays in the transverse field profile as well as serial trains in the longitudinal direction of the resonator thus combining serial and parallel encoding in the same device. Once created, they endlessly travel the cavity roundtrip. Keywords: 3D structures, cavity solitons, localized structures, pattern formation, nonlinear resonators

Single-photon sources rarely emit two or more photons in the same pulse, compared to a Poisson-distributed source of the same intensity, and have numerous applications in quantum information science. The quality of such a source is evaluated based on three criteria: high efficiency, small multi-photon probability, and quantum indistinguishability. We have demonstrated a single-photon source based on a quantum dot in a micropost microcavity that exhibits a large Purcell factor together with a small multi-photon probability. For a quantum dot on resonance with the cavity, the spontaneous emission rate has been increased by a factor of five, while the probability to emit two or more photons in the same pulse has been reduced to 2% compared to a Poisson-distributed source of the same intensity. The indistinguishability of emitted single photons from one of our devices has been tested through a Hong-Ou-Mandel-type two-photon interference experiment; consecutive photons emitted from such a source have been largely indistinguishable, with a mean wave-packet overlap as large as 0.81. We have also designed and demonstrated two-dimensional photonic crystal GaAs cavities containing InAs quantum dots that exhibit much higher quality factors together with much smaller mode volumes than microposts, and therefore present an ideal platform for construction of single photon sources of even higher quality.

There is an increasing need for single-spatial-mode, edge-emitting semiconductor lasers with reliable cw output power of around 1 W for applications such as pumps for rare-earth-doped fiber amplifiers and free-space communications. The design of respective devices is still a challenging task for experimenters, and software can assist very much in doing analyses of potentially perspective designs. We have developed a 3D numerical code supplied with a user-friendly interface for active diode-laser structures, taking into account light diffraction and carrier diffusion. The Watt-Ampere characteristics are calculated by changing the drive current density in the equation for the carrier-number density. To evaluate a single-mode stability limit, a procedure is developed to calculate 3-5 higher order optical modes on a 'frozen background': gain, carrier-induced index variation, as produced by the operated mode at a fixed drive level. Modal gains of these modes are compared to the calculated threshold gains for each mode. Because of non-uniform gain saturation by the operated mode, modal gains for higher-order modes increase with drive current due to beneficial overlap of their fields with the gain. When one of the higher-order modes approaches its threshold, this puts an upper limit for stable single-mode operation. A graphical interface allows for viewing near- and far-field patterns of any mode in the form of 3D surfaces or contour-plots. Scanning of profiles of mode intensity in an arbitrary cross section in the output plane and in far-field zone is available, too. Results of analyses of a number of published designs are reported.

The Tilted Cavity (TC) concept has been proposed to combine advantages of edge- and surface-emitting lasers (detectors, amplifiers, switches, etc.). Tilted Cavity Lasers (TCL) enable wavelength-stabilized high-power edge and surface emitters (TCSEL) in low-cost single-epitaxial step design. The concept covers numerous applications including mode-locked TCL for light speed control, dispersion and linewidth engineering, GaN-based light-emitters, electrooptic wavelength tunable devices, and other applications. Presently, wavelength stabilized TC operation is realized between -200°C and 70°C in broad TCL diodes with cleaved facets based on quantum dots (QDs). The spectral width is below 0.6 nm in broad area 100 μm-wide-stipe devices. The far fields are: 4° (lateral) and 42° (vertical). Wavelength-stabilized 1.16 μm and 1.27 μm edge-emitting QD TCL lasers are demonstrated. Quantum well TCL demonstrate high-temperature operation up to 240°C with a low threshold, high temperature stability and improved wavelength stability. The tilted cavity approach can also be applied in wavelength-optimized photodetectors, switches, semiconductor optical amplifiers, including multi-channel devices, in optical fibers, in photodetectors, in light-emitting diodes and in many other applications. Moreover, microelectronic devices based on similar tilted angle resonance phenomena in quantum wells and superlattices can be realized in electron- or hole-wavefunction-engineered structures, thus, merging the fields of nanophotonics and nanoelectronics. The tilted cavity concept can be further complimented by lateral patterning and (or) processing of three-dimensional photonic crystal structures further extending horizons of modern optoelectronics.

We review progress in realizing a semiconductor source of single photons and photon pairs based on the emission of individual self-assembled quantum dots. Integration of the quantum dot into a pillar microcavity produces a strong Purcell enhancement of the radiative recombination rate, resulting in photon collection efficiencies into a lens of ~10%. The residual multi-photon emission is found to derive from the emission of other layers within the structure, such that under resonant laser excitation of the dot a greater than 50-fold reduction in the 2-photon rate can be achieved compared to a laser of the same average intensity. The polarization of the emitted photons can be controlled and selected in appropriately designed cavities. Through careful control of the dot growth conditions, we realize a single photon source at the fiber compatible wavelength of 1300nm. This is achieved by utilizing a second critical InAs coverage to produce a low density of large, long wavelength InAs quantum dots. We demonstrate also an electrically driven planar cavity structure with photon collection efficiencies into a lens of ~5%, corresponding to an order of magnitude enhancement in the photon collection compared to dots in a bulk semiconductor LED. Single photon emission is demonstrated for both the biexciton and exciton state of the quantum dot.

By introducing triangular holes into oxide confined 850nm monolithic vertical-cavity surface-emitting lasers (VCSELs), single-transverse-mode operation has been obtained for large oxide apertures of 14-15 microns. The two-dimensional triangular holes etched on the device surface were aligned circumferentially along the aperture perimeter, with their tips surrounding the device center. When the holes had a relatively large lateral penetration into the oxide aperture, the holey VCSEL lased with a single spot near field pattern with a high side-mode suppression ratio (SMSR) of 45-50dB, and an output power of 2mW. In this case, it is assumed that the triangular holes are acting as a highly mode selective loss mechanism. On the other hand, when the penetration of the holes was relatively small, an SMSR of 40dB was obtained from a large area "floral" near field pattern, with a record high single-mode output power of 7mW. The lasing spectrum and far field intensity profile of this "floral" type emission indicates that it is a somewhat deformed fundamental mode that is extending over the whole device, and oscillating in-phase. The ability of triangular holes to suppress high order modes in large area oxide confined VCSELs should be effective for systems with wavelengths other than 850nm as well.

Recording of an ultrasonic vibration potential when a burst of ultrasound traverses a body containing a colloidal object can be used as the basis for an imaging method. The fundamentals of the theory of signal production and experimental demonstration of the imaging method are given. In a second imaging method, the use of ultrasound to modify x-ray phase contrast images where the ultrasound acts as a kind of "phase contrast" agent used to translate objects in space is demonstrated.

We discuss the dramatic development of high-power fiber laser technology in recent years and the prospects of kilowatt-class-single-frequency fiber sources. We describe experimental results from an ytterbium-doped fiber-based multi-hundred-watt single-frequency, single-mode, plane-polarized master-oscillator power amplifier (MOPA) operating at 1060 nm and a similar source with 0.5 kW of output power, albeit with a degraded beam quality (M2 = 1.6) and not linearly polarized. Experiments and simulations aimed at predicting the Brillouin limit of single-frequency system with athermally broadened Brillouin gain are presented. These suggest that single-frequency MOPAs with over 1 kW of output power are possible. In addition, the power scalability of a simple single-strand fiber laser to 10 kW is discussed

Advances in high power fibre lasers and amplifiers and in novel non-linear fibres that can be readily integrated with such pumps have led to a family of high power super-continuum sources that extend throughout the complete window of transparency of silica based fibres. The systems have been operated femtosecond, picosecond and nanosecond as well as cw. Average powers of 10’s Watts can be easily achieved, giving flat spectral power densities in excess of 10’s mW per nm from 400 nm to beyond 2.2 μm.

We demonstrated pulse compression at 1 μm based on the use of microstructured optical fibers. Two different approaches have been investigated. On the first hand, we used fibers with negative group velocity dispersion at the laser emission to obtain self-compression based on solitonic effect. On the second hand, we used fibers with zero-dispersion at the laser emission to generate very broad spectra (by self-phase modulation) recompressed afterward using prism-based compressor. We studied experimentally and theoretically these two techniques of compression with a special care regarding their limitations. For the experiments, the compressions have been obtained using a home-made diode-pumped Yb:SYS laser producing 110-fs pulses directly injected in the microstructured optical fibers. We demonstrated compressed pulses as short as 20-fs centered at 1070 nm which is to our best knowledge the shortest pulses obtained with a diode-pumped system in this wavelength range.

We report on the high power fiber based amplification of parabolic pulses. The output is compressed using transmission gratings to 300 fs and an average power of 38 W at 75 MHz repetition rate.

In the last years a dramatic increase of the output power of rare-earth-doped fiber lasers and amplifiers with diffraction limited beam quality has been observed. These demonstrates impressively that fiber lasers and amplifiers are an attractive and power scalable solid-state laser concept. The main limiting factors for the laser output power are the damage of the fiber ends, heating of the fiber due to the quantum defect and nonlinear effects. To overcome these problems, an increasing of the core diameter and keeping the fiber single mode, by using solid core step-index large-mode-area fibers, allow the power scaling beyond 1 kW at diffraction limited beam quality. A further scaling is possible by using novel highly doped air-clad photonic crystal fibers with increased mode field diameters of the active core. This type of fibers has several new preferable features. In our contribution we will discuss the advantages of microstructured fibers to reduce nonlinear effects inside the fiber and the possibility to scale the output power of fiber lasers and amplifiers with excellent beam quality. We also show experiments with pulsed fiber amplifier systems using these microstructured large mode area fibers.

Significant progress has been recently reported on beam combining of arrays of fiber and semiconductor lasers using both coherent (phased array) and wavelength (spectral) techniques. The choice between these two classes of beam-combining techniques has implications for the degree of control required on the array elements and on the optical system that is using the beam-combined array. Coherent beam combining imposes more stringent requirements on array-element control (spectrum, phase, amplitude, and polarization) than wavelength combining. Also, wavelength-combined systems should degrade more gracefully than coherent systems and require fewer changes to optical systems as the number of elements is varied.

We review recent advances in Yb fiber lasers and amplifiers for high power short pulse systems. We go on to describe associated recent developments in fiber components for use in such systems. Examples include microstructured optical fibers for pulse compression and supercontinuum generation, and advanced fiber grating technology for chirped-pulse amplifier systems.

A tapered fiber bundle is one of the leading approaches to coupling pump light into cladding-pumped fibers. This article compares it to other pump coupling schemes, and describes the tapered fiber bundle in detail. In addition tapered fiber bundles which maintain the polarization state of the signal and transform the size of the mode are described.

Ultrahigh resolution OCT using broadband light sources achieves improved axial image resolutions of ~2-3 um compared to standard 10 um resolution OCT used in current commercial instruments. High-speed OCT using Fourier/spectral domain detection enables dramatic increases in imaging speeds. 3D OCT retinal imaging is performed in human subjects using high-speed, ultrahigh resolution OCT, and the concept of an OCT fundus image is introduced. Three-dimensional data and high quality cross-sectional images of retinal pathologies are presented. These results show that 3D OCT may be used to improve coverage of the retina, precision of cross-sectional image registration, quality of cross-sectional images, and visualization of subtle changes in retinal topography. 3D OCT imaging and mapping promise to help elucidate the structural changes associated with retinal disease as well as to improve early diagnosis and monitoring of disease progression and response to treatment.

Third-order optical nonlinearity in optical fibers has many attractive applications to all-optical signal processing that will be employed in future large-capacity photonic networks. After reviewing the third-order nonlinear optical property of optical fibers, we describe our recent experimental results on all-optical signal processing functions such as wideband wavelength conversion, ultrafast gate switching, and ultrafast pulse reshaping. These functions are based on self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM) in nonlinear fibers.

Spin dynamics in the III-V dilute magnetic semiconductor GaMnAs is investigated by photo-induced demagnetization. Experimental results obtained from two different time-dependent characterization techniques - "two color-probe" magneto-optical Kerr effect (TR-MOKE) and mid-infrared differential transmittance -- are compared. Upon photo-excitation with a 100 fs, 3.1 eV light pulse, a long demagnetization time in the hundreds of picoseconds timescale is found by TR-MOKE, indicating a spin-dependent band structure in this material. In mid-infrared measurements, a positive increase of the differential transmittance is observed in the same time interval when the sample is cooled below its Currie temperature. It is shown that this mid-infrared absorption feature is directly related to ferromagnetism in this material. The magnetism-related component of the broad DC mid-infrared absorption peak characteristic of this p-type material could be observed with this time-resolved measurement. Experimental results were simulated with a model describing the interaction between three thermal reservoirs (hole, spin and lattice) and taking thermal diffusion into account.

Recent progress on ultrashort pulse lasers has made it possible to control the pulse timing and optical phases from different cavities very precisely. By applying detection and control techniques of the carrier-envelope phase to multicolor pulse sources with synchronized pulse timing, we obtained phase-coherent pulse-trains in different wavelengths. The pulse-trains have sufficient stability to achieve Fourier synthesis among optical fields. One of our test sources was a femtosecond optical parametric oscillator (OPO). Using a specially designed OPO whose signal and idler are sub-harmonics of the pump frequency, we obtained long-term stabilization of the optical phase among those three waves. Since the OPO also generates sum-frequencies of the three waves, six-color coherent pulse-trains from 425nm to 2550nm are available. Another test source was a passively synchronized mode-locked laser with two kinds of gain media, Ti:sapphire and Cr:forsterite. Although it is harder to reduce phase-noise between the different color pulses than in the OPO example, we can expect shorter pulse duration and higher average power from this type of coupled laser. These coherent multicolor pulse sources will be applied not only to shorter-pulse generation by field summation, but also to some applications that include competing non-linear processes among multicolor pulses.

The operation of the Hot Electron Light Emitting and Lasing in Semiconductor Heterostructure -- Vertical Cavity Surface Emitting Laser (HELLISH-VCSEL) devices is based on hot carrier transport parallel to the layers of Ga1-xAlxAs p-n junction. It is therefore a field - effect device and the light emission from the device is independent of the polarity of the applied voltage. In this study, we present the temperature dependence of the operational characteristics of the device. Experimental studies comprising of the measurements of the I-V characteristics, electroluminescence, reflectivity, and temperature dependent light-applied electric field (L-F) characteristics are conducted to find the optimum operating temperature of the device.

Terahertz scanning near-field infrared microscopy (SNIM) below 1 THz is demonstrated. The near-field technique benefits from the broadband and highly brilliant coherent synchrotron radiation (CSR) from an electron storage ring and from a detection method based on locking on to the intrinsic time structure of the synchrotron radiation. The scanning microscope utilizes conical waveguides as near-field probes with apertures smaller than the wavelength. Different cone approaches have been investigated to obtain maximum transmittance. Together with a Martin-Puplett spectrometer the set-up enables spectroscopic mapping of the transmittance of samples well below the diffraction limit. Spatial resolution down to about λ/40 at 2 wavenumbers (0.06 THz) is derived from the transmittance spectra of the near-field probes. The potential of the technique is exemplified by imaging biological samples. Strongly absorbing living leaves have been imaged in transmittance with a spatial resolution of 130 μm at about 12 wavenumbers (0.36 THz). The THz near-field images reveal distinct structural differences of leaves from different plants investigated. The technique presented also allows spectral imaging of bulky organic tissues. Human teeth samples of various thicknesses have been imaged between 2 and 20 wavenumbers (between 0.06 and 0.6 THz). Regions of enamel and dentin within tooth samples are spatially and spectrally resolved, and buried caries lesions are imaged through both the outer enamel and into the underlying dentin.