An Optical Imaging Technique Using Deep Illumination in the Angular Domain

This paper describes a novel optical imaging method, deep illumination angular domain imaging (ADI), for detecting micron-scale objects within highly scattering media. The new optical imaging is a much simpler and less expensive solution as compared to other available optical imaging techniques. In principle, deep illumination ADI uses collimation detection capabilities of small acceptance angle devices to extract photons emitted from the scattered light created by a laser source, aimed deep beneath the turbid medium surface. The laser source forms an illumination ball within the medium that emits scattered light in all directions and illuminates objects near the surface from behind. Consequently, when photons from this illumination ball pass an object and reach the angular filter, light that is not subsequently scattered passes through to a camera detector, whereas scattered photons are rejected by the filter. Image results obtained are recorded for different phantom locations, phantom sizes, and medium scattering levels. Our images clearly display sub-204 m phantoms when placed 3 mm deep within a test scattering medium with total effective attenuation coefficient (mu'_eff) up to 5.8^-1 cm or 2.5 mm deep in chicken tissue tests. Preliminary digital image processing shows the image contrast enhancement and the definition improvement.     

Characteristics of a saturated 18.9-nm tabletop laser operating at 5-Hz repetition rate

We report the characteristics of a saturated high-repetition rate Ni-like Mo laser at 18.9 nm. This table-top soft X-ray laser was pumped at a 5-Hz repetition rate by 8-ps 1-J optical laser pulses impinging at grazing incidence into a precreated Mo plasma. The variation of the laser output intensity as a function of the grazing incidence angle of the main pump beam is reported. The maximum laser output intensity was observed for an angle of 20/spl deg/, at which we measured a small signal gain of 65 cm/sup -1/ and a gain-length product g/spl times/l>15. Spatial coherence measurements resulting from a Young's double-slit interference experiment show the equivalent incoherent source diameter is about 11 /spl mu/m. The peak spectral brightness is estimated to be of the order of 1/spl times/10/sup 24/ photons s/sup -1/ mm/sup -2/ mrad/sup -2/ within 0.01% spectral bandwidth. This type of practical, small scale, high-repetition soft X-ray laser is of interest for many applications.     

High-power/high-brightness diode-pumped 1.9-/spl mu/m thulium and resonantly pumped 2.1-/spl mu/m holmium lasers

We report high power (>36 W) with beam propagation factor M/sup 2//spl sim/2 in a diode end-pumped Tm:LiYF/sub 4/ (Tm:YLF) laser generating output near the 1.91-/spl mu/m region. Using the 1.91-/spl mu/m emission and high brightness achieved with the Tm:YLF laser we resonantly end-pump the Holmium /sup 5/I/sub 7/ manifold in Ho:YAG and demonstrate /spl sim/19 W of continuous-wave (CW) output. The diode-to-Holmium optical to-optical conversion efficiency achieved is /spl sim/18%. Using a CW pumped and repetitively Q-switched configuration, the Tm:YLF pumped Ho:YAG laser achieves >16 W of output power with an M/sup 2//spl sim/1.48 at 15 kHz. A Q-switched frequency range of 9 to >50 kHz is also achieved.     

Antenna-coupled infrared detectors for imaging applications

Infrared focal plane arrays (IRFPAs) are a critical component in advanced infrared imaging systems. IRFPAs are made up of two parts, a detector array and a readout integrated circuit (ROIC) multiplexer. Current ROIC technology has typical pitch sizes of 20/spl times/20 to 50/spl times/50 /spl mu/m/sup 2/. In order to make antenna-coupled detectors suited for infrared imaging systems, two-dimensional (2-D) arrays have been fabricated that cover a whole pixel area with the penalty of increasing the noise figure of the detector and, therefore, reducing its performance. By coupling a Fresnel zone plate lens to a single element antenna-coupled detector, infrared radiation can be collected over a typical pixel area and still keep low-noise levels. A Fresnel zone plate lens coupled to a single-element square-spiral-coupled infrared detector has been fabricated and its performance compared to single element antenna-coupled detectors and 2-D arrays of antenna coupled detectors. Measurements made at 10.6 /spl mu/m showed a two-order-of-magnitude increase in SNR and a /spl sim/3/spl times/ increase in D/sup */ as compared to 2-D arrays of antenna-coupled detectors.     

Evaluation of spherical particle sizes with an asymmetric illumination microscope

A polarized microscope system is used to perform goniometric measurements of light scattered by small particles. The light incident angle on a sample of monodispersed latex microspheres is increased sequentially and a microscope objective lens collects scattered light from the samples. Light is only collected at angles greater than the objective lens numerical aperture (NA) so that only light scattered by the spheres is collected. The experimental results were modeled with a Mie theory-based algorithm. Experiments conducted with microspheres of diameter 1.03, 2.03, and 6.4 /spl mu/m show that, by decreasing the objective lens NA from NA=0.55 to NA=0.0548, a more distinguishable scattering pattern is detectable. From these highly shaped curves, we found that the size of a sphere of nominal diameter 2.03 /spl mu/m was 2.11 /spl plusmn/0.06 /spl mu/m and a 6.4 /spl mu/m sphere was 6.34 /spl plusmn/0.07 /spl mu/m.     

High-speed InSb photodetectors on GaAs for mid-IR applications

We report p-i-n type InSb-based high-speed photodetectors grown on GaAs substrate. Electrical and optical properties of photodetectors with active areas ranging from 7.06/spl times/10/sup -6/ cm/sup 2/ to 2.25/spl times/10/sup -4/ cm/sup 2/ measured at 77 K and room temperature. Detectors had high zero-bias differential resistances, and the differential resistance area product was 4.5 /spl Omega/ cm/sup 2/. At 77 K, spectral measurements yielded high responsivity between 3 and 5 /spl mu/m with the cutoff wavelength of 5.33 /spl mu/m. The maximum responsivity for 80-/spl mu/m diameter detectors was 1.00/spl times/10/sup 5/ V/W at 4.35 /spl mu/m while the detectivity was 3.41/spl times/10/sup 9/ cm Hz/sup 1/2//W. High-speed measurements were done at room temperature. An optical parametric oscillator was used to generate picosecond full-width at half-maximum pulses at 2.5 /spl mu/m with the pump at 780 nm. 30-/spl mu/m diameter photodetectors yielded 3-dB bandwidth of 8.5 GHz at 2.5 V bias.     

Electron mobility in dense argon gas at several temperatures

The mobility /spl mu/ of excess electrons in dense Argon gas was measured as a function of the applied electric field E and of the gas density N at several temperatures in the range 142.6 < T < 200 K, encompassing the critical temperature T/sub c/ = 150.86 K We report here measurements up to N /spl ap/ 7 nm/sup -3/, close to the critical density, N/sub c/ /spl ap/ 8.1 nm/sup -3/. At all temperatures, and up to moderately high densities, the density-normalized mobility /spl mu/N shows the usual electric field dependence in a gas with a Ramsauer-Townsend minimum due to the mainly attractive electron-atom interaction. /spl mu/N is constant and field independent for small E, shows a maximum for a reduced field E/N /spl ap/ 4 mTd, and then decreases rapidly with the field. The zero field density-normalized mobility /spl mu//sub 0/N, for all T > T/sub c/, shows the well known anomalous positive density effect, i.e., /spl mu//sub 0/N increases with increasing N. Below T,, however, /spl mu//sub 0/N does not show the expected effect, but features a broad maximum. This appears to be a crossover behavior between the positive density effect shown for T > T, and the small negative effect previously observed for T /spl ap/ 90 K However, the data at all temperatures confirm the interpretation of the anomalous density effect as being essentially due by the density-dependent quantum shift of the electron ground state kinetic energy in a disordered medium as a result of multiple scattering (MS) processes, although other MS processes influence the experimental outcome.     

1.3-/spl mu/m-range GaInNAsSb-GaAs VCSELs

1.3-/spl mu/m-range GaInNAsSb vertical-cavity surface-emitting lasers (VCSELs) with the doped mirror were investigated. GaInNASb active layers that include a small amount of Sb can be easily grown in a two-dimensional manner as compared with GaInNAs due to the suppression of the formation of three-dimensional growth in MBE growth. The authors obtained the lowest J/sub th/ per well (150 A/cm/sup 2//well) for the edge-emission type lasers due to the high quality of GaInNAsSb quantum wells. Using this material for the active media, the authors accomplished the first continuous wave operation of 1.3-/spl mu/m-range GaInNAsSb VCSELs. For the reduction of the threshold voltage and the differential resistance, they used the doped mirror grown by metal-organic chemical vapor deposition (MOCVD). By three-step growth, they obtained 1.3-/spl mu/m GaInNAs-based VCSELs with the low threshold current density (3.6 kA/cm/sup 2/), the low threshold voltage (1.2 V), and the low differential resistance (60 /spl Omega/) simultaneously for the first time. The back-to-back transmission was carried out up to 5 Gb/s. Further, the uniform operation of 10-ch VCSEL array was demonstrated. The maximum output power of 1 mW was obtained at 20/spl deg/C by changing the reflectivity of the front distributed Bragg reflector mirror. GaInNAsSb VCSELs were demonstrated to be very promising material for realizing the 1.3-/spl mu/m signal light sources, and the usage of the doped mirror grown by MOCVD is the best way for 1.3-/spl mu/m VCSELs.     

Comparative study on the spectroscopic properties of Nd:GdVO/sub 4/ and Nd:YVO/sub 4/ with hybrid process

We have proposed a hybrid procedure for determining spectroscopic parameters for uniaxial solid-state laser crystals. Using our procedure, the spectroscopic properties of Nd:GdVO/sub 4/ were evaluated and compared to those of Nd:YVO/sub 4/. As a result, the peaks of absorption and stimulated emission cross sections of Nd:GdVO/sub 4/ in /spl pi/-polarization were determined to be 2.6 and 10.3/spl times/10/sup -19/ cm/sup 2/, respectively, and were smaller than those of Nd:YVO/sub 4/. On the other hand, the fluorescence lifetime of 1 at% Nd:GdVO/sub 4/ was evaluated to be 83.4 /spl mu/s, and was similar to 84.1 /spl mu/s of 1 at% Nd:YVO/sub 4/. Therefore, the product of stimulated emission cross section and fluorescence lifetime (/spl sigma//sub em//spl tau//sub f/ product) of Nd:GdVO/sub 4/ was smaller than that of Nd:YVO/sub 4/ under 1 at% of Nd/sup 3+/ doping concentration. The radiative lifetime of spontaneous emission of Nd:GdVO/sub 4/ was 168 /spl mu/s and was 1.9 times longer than that of Nd:YVO/sub 4/. Because of the low value of radiative quantum efficiency of Nd:GdVO/sub 4/ (50%), careful cavity design is required for creating a well performing solid-state laser with Nd:GdVO/sub 4/, based on the larger /spl sigma//sub em//spl tau//sub f/ product rather than the /spl sigma//sub em//spl tau//sub f/ product of Nd:YAG.     

Near-infrared (NIR) tomography breast image reconstruction with a priori structural information from MRI: algorithm development for reconstructing heterogeneities

A combined magnetic resonance and near-infrared (MRI-NIR) imaging modality can potentially yield high resolution maps of optical properties from noninvasive simultaneous measurement. The main disadvantage of near-infrared (NIR) tomography lies in the low spatial resolution resulting from the highly scattering nature of tissue for these wavelengths. MRI has achieved high resolution, but suffers from low specificity. In this study, NIR image reconstruction algorithms that incorporate a priori structural information provided by MRI are investigated in an attempt to optimize recovery of a simulated optical property distribution. The effect of high levels of tissue heterogeneity are evaluated to determine the limitations of incorporating prior information into a realistic set of patient breast images. We assume absorption coefficient (/spl mu//sub a/) variations near /spl plusmn/40%, and transport scattering coefficient (/spl mu//sub s//sup //) variations near /spl plusmn/20%, in a coronal breast MRI geometry. Changes in tissue pathology due to tumor growth can be observed with NIR tompgraphy, and so the goal here is to determine how best to quantify these tumor-based contrast regions within the presence of high tissue heterogeneity. By applying knowledge of tissue's layered structure in reconstruction through various constraints in the iterative algorithm, quantitative recovery of the tumor optical properties improves from 69% to 74%, and localization improves as well. However, only when the true heterogeneity of the tissue distribution was included was accurate quantification of the tumor region possible. Using a good initial guess of /spl mu//sub a/ and /spl mu//sub s//sup //, derived from the regional structure of the model, quantification of the region reaches 99% of the true value, and spatial resolution retains a similar value to the original MRI image.     

Determination of piezoelectric and pyroelectric coefficients and thermal diffusivity of 1-3 PZT/epoxy composites

PZT/epoxy composites with 1-3 connectivity were prepared using the dice-and-fill technique. The samples were poled with an electric field of 10 MV/m for 30 minutes at room temperature. The piezoelectric and pyroelectric coefficients for the composites were measured. From the laser interferometric measurements, it was found that the piezoelectric d/sub 33/ coefficients for the composites were independent of the volume fraction and averaged (190 /spl mnplus/ 10) pm/V, which was about half of the measured value of lead zirconate titanate (PZT) ceramic. Measurements of the pyroelectric coefficient showed that the coefficients increased with the ceramic content and reached values as large as 54 /spl mu/C/m/sup 2/ /spl deg/C. The thermal diffusivity of the composites was also determined using a technique based on the measurement of the phase retardation of a thermal wave passing through the material. The average value for the composites was (2.15/spl mnplus/ 0.05) /spl times/ 10/sup -7/ m/sup 2//s.     

Wafer level reliability and lifetime analysis of InGaAsP/InP quantum-well Fabry-Perot laser diode

This paper investigated the reliability of semiconductor 1.3-/spl mu/m multiquantum-well (MQW) Fabry-Perot laser diodes (LDs) in a quarter 2-in wafer level that are measured to have uniform threshold currents, slope efficiencies, and wavelengths within 4% of the maximum deviation. By performing the accelerated aging test under a constant optical power of 3 mW at 85/spl deg/C for 2100 h, the lifetime of the fabricated optoelectronic devices was estimated, where the failure rate was matched on the fitted line of the lognormal distribution model resulting in the mean-time-to-failure (MTTF) of 2/spl times/10/sup 6/ h operating at room temperature.     

Thermal-stability improvement of a sulfur-passivated InGaP/InGaAs/GaAs HFET

The temperature-dependent characteristics of an InGaP/InGaAs/GaAs heterostructure field-effect transistor (HFET), using the (NH/sub 4/)/sub 2/S/sub x/ solution to form the InGaP surface passivation, are studied and demonstrated. The sulfur-passivated device shows significantly improved dc and RF performances over a wide temperature range (300-510 K). With a 1/spl times/100-/spl mu/m/sup 2/ gate-dimension HFET by (NH/sub 4/)/sub 2/S/sub x/ treatment, the considerably improved thermal stability over dc performances including lower temperature variation coefficients on the turn-on voltage (-1.23 mV/K), the gate-drain breakdown voltage (-0.05 mV/K), the gate leakage current (1.04 /spl mu/A/mm/spl middot/K), the threshold voltage (-1.139 mV/K), and the drain-saturation-current operating regimes (-3.11/spl times/10/sup -4//K) are obtained as the temperature is increased from 300 to 510 K. In addition, for RF characteristics, the sulfur-passivated device also shows a low degradation rate on drain-saturation-current operating regimes (-3.29/spl times/10/sup -4//K) as the temperature is increased from 300 to 400 K. These advantages provide the promise for high-speed high-frequency high-temperature electronics applications.     

High-saturation current wide-bandwidth photodetectors

This paper describes the design and performance of two wide-bandwidth photodiode structures. The partially depleted absorber photodiode utilizes an absorbing layer consisting of both depleted and undepleted In/sub 0.53/Ga/sub 0.47/As layers. These photodiodes have achieved saturation currents (bandwidths) of >430 mA (300 MHz) and 199 mA (1 GHz) for 100-/spl mu/m-diameter devices and 24 mA (48 GHz) for 100-/spl mu/m/sup 2/ area devices. Charge compensation has also been utilized in a similar, but modified In/sub 0.53/Ga/sub 0.47/As-InP unitraveling-carrier photodiode design to predistort the electric field in the depletion region in order to mitigate space charge effects. For 20-/spl mu/m-diameter photodiodes the large-signal 1-dB compression current and bandwidth were /spl sim/90 mA and 25 GHz, respectively.     

High-average power EUV light source for the next-generation lithography by laser-produced plasma

Laser-produced plasma is expected to fulfill the strict requirement as an extreme ultraviolet (EUV) light source for the next-generation lithography with 115-W average power at the intermediate focus, in terms of stability, minimum contamination, and cost of ownership. A liquid xenon micro jet is employed in our experimental facility to confirm the scalability up to the 115-W clean output power. The present experimental device is composed of a 1-kW 10-kHz 6-ns Nd:YAG laser with a xenon jet of up to 50-/spl mu/m diameter of 35 m/s speed inside a vacuum chamber, combined with a xenon recirculation system. The observed EUV power is 4 W at 13.5 nm (2% bandwidth, 2/spl pi/sr) from the plasma source with 0.5% stability (1 /spl sigma/, 50-pulse moving average). Debris mitigation and contamination control is now studied for fast ions by time-of-flight measurements, and confinement and exhaust by a magnetic field is shown to be effective. Xe/sup +/ to Xe/sup 13+/ ions were observed with Xe/sup 2+/ being the main charged state. Experimental study is presented on these subjects and further parametric study on the laser wavelength and pulsewidth is reported, discussing the scalability toward the realization of a 115-W system.     

High-power continuous-wave intracavity frequency-doubled Nd:GdVO/sub 4/-LBO laser under diode pumping into the emitting level

The continuous-wave high power laser emission of Nd:GdVO/sub 4/ at the fundamental wavelength of 1.06 /spl mu/m and its 531-nm second harmonic obtained by intracavity frequency doubling with an LBO nonlinear crystal is investigated under pumping by diode laser at 808 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 5/2/ transition) and 879 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 3/2/ transition). It is shown that, in spite of a lower absorption at 879 nm, the infrared emission is comparable under these two wavelengths of pump. The green emission performances were, however, improved by the 879 nm pump: 5.1 W at 531 nm with M/sup 2/=1.46 and 0.31 overall optical-to-optical efficiency was obtained from a 3-mm-thick 1-at.% Nd:GdVO/sub 4/ laser medium and a 10-mm-long LBO nonlinear crystal in a Z-type cavity for 16.5 W pump power. In similar conditions, the maximum green power for the 808 nm pump was 4.4 W, with 0.26 overall optical-to-optical efficiency and M/sup 2/=3.40 beam quality; at this pump wavelength the green emission shows evident saturation for pump power in excess of 9.9 W. This behavior is connected with the enhanced heat generation under 809-nm pumping, as evidenced by the increased thermal lensing of the fundamental emission. A careful alignment of the laser enables emission almost free of chaotic intensity fluctuations.     

Boundary effects on the optical properties of InGaN multiple quantum wells

We examine the issues of spontaneous and piezoelectric polarization discontinuity on the optical properties of 3.0-nm-thick indium gallium nitride (InGaN) multiple quantum wells (MQWs). A quench of band-edge emission from the cap GaN layer is observed when the photoexcitation source is changed from a 355- to a 248-nm laser. The interband transitions from the InGaN wells exhibit a linear dependence on the 1) spectral blue shift of /spl sim/8.5/spl times/10/sup -18/ meV /spl middot/ cm/sup 3/ and 2) change of the internal field of /spl sim/3/spl times/10/sup -14/ meV /spl middot/ cm/sup 2/ with the injected carrier density up to N/sub inj//spl sim/10/sup 19/ cm/sup -3/ at 77 K. These observations are attributed to the redistribution of photogenerated carriers in the InGaN wells due to the polarization discontinuity at the QW interface and the surface band bending effect. By incorporating an additional boundary condition of surface Fermi-level pinning into the Poisson equation and the band-structure analysis, it is shown the emission from the InGaN-GaN MQWs is dominant by the recombination between the high-lying subbands and the screening of internal field effects.     

The impact of LOC structures on 670-nm (Al)GaInP high-power lasers

We investigate the potential of large optical cavity (LOC)-laser structures for AlGaInP high-power lasers. For that we study large series of broad area lasers with varying waveguide widths to obtain statistically relevant data. We study in detail I/sub th/, /spl alpha//sub i/, /spl eta//sub i/, and P/sub max/, and analyze above-threshold behavior including temperature stability and leakage current. We got as expected for LOC structures minimal /spl alpha//sub i//spl les/1 cm/sup -1/ resulting in /spl eta//sup d/=1.1 W/A for 64/spl times/2000 /spl mu/m/sup 2/ uncoated devices. We obtain total output powers /spl ges/3.2 W (qCW) and /spl ges/1.5 W (CW) at 20/spl deg/C.     

Noise figure of parametric wavelength conversion with quasi-phase-matched LiNbO/sub 3/ waveguide

An analytical model of the noise accumulation in a chain of parametric wavelength converters is proposed. Signal-to-noise electrical power ratio is analytically given as a function of node number k in a chained transparent node system that consists of optical amplifiers, parametric wavelength converters, and several loss elements including optical transmission fiber with parameters of pump light excess noise /spl beta//sub p/, and average photon numbers per unit time of pump light and input signal , and , respectively, and spontaneous emission factor of optical amplifier n/sub sp/. The signal-to-noise degrades inversely proportional to node number k with the coefficient defined by NF/sup (1)/=2n/sub sp/+/spl beta//sub p// when k is lower than /Bo, where B/sub o/ represents optical bandwidth. The noise figure dependence on pump light quality /spl beta//sub p// and average photon number of input light in a single stage configuration are experimentally evaluated using Er-doped fiber amplifiers and quasi-phase-matched lithium niobate waveguide parametric wavelength conversion.     

Gimbal-less monolithic silicon actuators for tip-tilt-piston micromirror applications

In this paper, fully monolithic silicon optical scanners are demonstrated with large static optical beam deflection. The main advantage of the scanners is their high speed of operation for both axes: namely, the actuators allow static two-axis rotation in addition to pistoning of a micromirror without the need for gimbals or specialized isolation technologies. The basic device is actuated by four orthogonally arranged vertical comb-drive rotators etched in the device layer of an silicon-on-insulator wafer, which are coupled by mechanical linkages and mechanical rotation transformers to a central micromirror. The transformers allow larger static rotations of the micromirror from the comb-drive stroke limited rotation of the actuators, with a magnification of up to 3/spl times/ angle demonstrated. A variety of one-axis and two-axis devices have been successfully fabricated and tested, in all cases with 600-/spl mu/m-diameter micromirrors. One-axis micromirrors achieve static optical beam deflections of >20/spl deg/ and peak-to-peak resonant scanning of >50/spl deg/ in one example at a resonant frequency of 4447 Hz. Many two-axis devices utilizing four rotators were tested, and exhibit >18/spl deg/ of static optical deflection at <150 V, while their lowest resonant frequencies are above 4.5 kHz for both axes. A device which utilizes only three bidirectional rotators for tip-tilt-piston actuation achieves -10/spl deg/ to 10/spl deg/ of optical deflection in all axes, and exhibits minimum resonant frequencies of 4096 and 1890 Hz for rotation and pistoning, respectively. Finally, we discuss the preliminary results in scaling tip-tilt-piston devices down to 0.4 /spl times/ 0.4 mm on a side for high fill-factor optical phased arrays. These array elements include bonded low-inertia micromirrors which fully cover the actuators to achieve high fill-factor.