First- and second-order Bragg gratings in single-mode planarwaveguides of chalcogenide glasses

First- and second order Bragg reflectors at telecommunication wavelength (1.5 μm) were fabricated in single-mode monolayer (As₂S₃) and multilayer (AsSSe-AsS) chalcogenide glass (ChG) planar waveguides with near bandgap illumination using an interferometric technique. Reflectivities as high as 90% near 1555 nm, and index modulations up to 3×10^-4 were achieved. The volume photodarkening effect is the principal mechanism involved in the formation of the Bragg gratings     

Universal fiber-optic point sensor system for quasi-static absolutemeasurements of multiparameters exploiting low coherence interrogation

A universal fiber-optic high-resolution point sensor system, based upon signal recovery by dual-wavelength low coherence interferometry, has been developed and demonstrated for quasistatic absolute measurements of multiparameters. This system is capable of multiplexing up to 32 fiber-optic point sensors which can be fiber optic interferometers or fiber Bragg gratings or any combination of the two. The topology of this system is based on a spatially multiplexed scheme with low coherence signal recovery that we have reported previously. A range of multiparameter point sensors, including a medium pressure sensor, a high pressure sensor, a miniature temperature sensor, a displacement sensor, and a fiber Bragg grating strain sensor with drift-compensation, have been developed and incorporated into this network and demonstrated A range to resolution of better than 10⁴ :1 and 2×10³:1 has been achieved for the interferometric sensors and the Bragg grating strain sensor, respectively. The interchangeability of the sensors has also been demonstrated, allowing the sensors to be replaced in the event of damage. Due to the universality of the signal interrogation, the instrument can be compatible with any interferometric point sensor which has a similar optical path difference with the transmitting interferometer or any fiber grating sensor whose normal wavelength is within the spectral range of the light source. In addition, as the total sensor number which can be multiplexed is quite large, the average cost for each sensor is reduced considerably. Therefore, this system allows optical fiber sensors to compete with conventional sensors with the additional benefits of fiber-optic sensors     

Ultrafast IR Laser Writing of Strong Bragg Gratings Through the Coating of High Ge-Doped Optical Fibers

Strong fiber Bragg gratings were written through the standard polymer coatings of "off-the-shelf" high numerical aperture single-mode optical fibers after a few seconds exposure with femtosecond pulse durations of infrared radiation through a phase mask. While writing through the acrylate coating, we obtained index modulations of up to 1.4 x 10^-3 and 7 x 10^-4 with and without H₂ -loading, respectively.     

Efficiency and thermal behaviour of cerium-doped fluorozirconateglass fibre Bragg gratings

1.55 μm Bragg gratings have been written in cerium-doped fluorozirconate glass singlemode fibres through UV illumination at 246 nm. Photoinduced refractive index changes up to 4×10^-4 have been achieved, leading to grating efficiencies close to 100%. Preliminary results on the thermal behaviour of the gratings and realisation of a tunable laser including such a grating are also reported. The ZBLALi (ZrF₄-BaF₂-LaF₃-AlF₃-LiF:Ce) glass composition was shown to be the most efficient     

Characterization of phosphorus implanted low pressure chemical vapor deposited polycrystalline silicon

Thin (3000–5000Å) low pressure chemically vapor deposited (LPCVD) films of polycrystalline silicon suitable for microelectronics applications have been deposited from silane at 600°C and at a pressure of 0.25 Torr. The films were phosphorus implanted at 150 KeV and electrically characterized with the annealing conditions and film thickness as parameters, over a resistivity range of four orders of magnitude (10³–10⁷Ω/□). Annealing during silox deposition was found to result in a lower film resistivity than annealing done in nitrogen atmosphere. Resistivity measurements as a function of temperature indicate that the electrical activation energy is a linear function of 1/N(N is the doping concentration), changing from 0.056 eV for a doping concentration of 8.9 × 10¹⁸ cm⁻³ to 0.310 eV for doping concentration of 3.3 × 10¹⁸ cm⁻³. The grain boundary trap density was found to have a logarithmically decreasing dependence on the polysilicon thickness, decreasing from 1.3 × 10¹³ cm⁻² for 2850Å polysilicon film to 8.3 × 10¹² cm⁻² for 4500Å polysilicon film.     

Counting of deep-level traps using a charge-coupled device

Quantization in dark current generation has been observed for the first time through the use of a virtual-phase charge-coupled device. Two sites for bulk silicon dark current have been identified with capture cross sections of 1.8 × 10^-15cm²and 5.4 × 10^-16cm², and concentrations of 1.3 × 10⁹cm^-3and 1.5 × 10⁸cm^-3, respectively.     

A comparison of optoelectronic properties of lattice-matched andstrained quantum-well and quantum-wire structures

The k·p formalism is used to study the absorption spectra, material and differential gain in quantum wires as a function of orientation, built-in strain, and wire dimensions. The results for material and differential gain are compared with those for an optimized quantum-well structure. We find that for quantum wires at 300 K, the gain becomes positive at a carrier density of 1.27·10¹⁸ cm^-3, while in quantum wells this density is calculated to be 1.82·10¹⁸ cm^-3. Incorporating tensile strain in the wires reduces the transparency carrier concentration to 0.96·10¹⁸ cm^-3 while compressive strain allows one to obtain positive gain for densities greater than 1.08·10¹⁸ cm^-3. Orienting the wire along the [111] direction reduces the transparency carrier density to 0.60·10¹⁸ cm^-3. The differential gain in quantum-well structures for injections near the threshold is on the order of 10^-14 cm^-4, while for 50 Å·100-Å quantum wires the differential gain near the threshold is found to be on the order of 10^-13 cm^-4 . The differential gain in wires whose wire axis is parallel to the [111] direction has also been found to be on the order of 10^-13 cm^-4 for carrier injections close to the threshold     

Effects of trap-state density reduction by plasma hydrogenation inlow-temperature polysilicon TFT

The reduction of trap-state densities by plasma hydrogenation in n-channel polysilicon thin-film transistors (poly-TFTs) fabricated using a maximum temperature of 600°C has been studied. Hydrogenated devices have a mobility of ~40 cm²/V×5, a threshold voltage of ~2 V, an inverse subthreshold of ~ 0.55 V/decade, and a maximum on/off current ratio of 5×10⁸. The effective channel length decreases by ~0.85 μm after a short hydrogenation which may be attributed to the activation of donors at trap states near the source/drain junctions. Trap-state densities decrease from 1.6×10¹² to 3.5×10¹¹ cm^-2 after hydrogenation, concomitant with the reduction of threshold voltage. Using the gate lengths at which the trap-state densities deviate from the long-channel values as markets for the leading edge of passivation, the apparent hydrogen diffusivity is found to be 1.2×10^-11 cm²/s at 350°C in the TFT structure     

The sublinear relationship between index change and carrier densityin 1.5 and 1.3 μm semiconductor lasers

The carrier-induced index change was measured using a novel injection-reflection technique in combination with differential carrier lifetime data. The observed relation between index change and injected carrier density at bandgap wavelength is nonlinear and is approximately given by δn_act=-6.1×10^-14 ( N)^0.66 for a 1.5-μm laser and δn _act=-1.3×10^-14 (N)^0.68 for a 1.3-μm laser. The carrier-induced index change for a 1.3-μm laser at 1.53-μm wavelength is smaller and is given by δn _act=-9.2×10^-16 (N)^0.72      

Frequency stabilization of the sequence-band CO2 laserusing the 4.3-μm fluorescence method

The saturated 1.3-μm fluorescence stabilization method is applied to the 00⁰2-[10⁰1, 02⁰1]_I,II sequence band CO₂ laser transitions. For this purpose, the 4.3-μm fluorescence is observed using an external longitudinal CO₂ absorption cell heated to 300°C. The dependence of the frequency stability upon the gas temperature and pressure in the cell as well as laser parameters have been carried out in absolute frequency scale with the help of a two-channel heterodyne system. Under optimal conditions, the standard deviation of the beat note frequency between sequence band and regular band lasers for 30 s averaging time is less than 20 kHz, and the long-term stability and reproducibility is achieved at about 10 kHz     

High reflectivity Te-doped GaAsSb/AlAsSb Bragg mirror for 1.5 μmsurface emitting lasers

A high reflectivity GaAsSb/AlAsSb Bragg mirror lattice matched to InP is reported. Operation at 1.5 μm is obtained owing to a large Burstein-Moss shift of GaAsSb absorption caused by a strong N-doping up to 10¹⁹ cm^-2. Peak reflectivity up to 94% has been measured with only 11.5 periods     

Electrical, structural, and optical characterization of free-standing GaN template grown by hydride vapor phase epitaxy

Electrical, structural, and optical properties of a free-standing 200 μm thick n-type GaN template grown by hydride vapor phase epitaxy have been investigated. Hall mobilities of 1100 and 6800 cm²/V s have been obtained at room temperature and 50 K, respectively. Quantitative analysis of acceptor concentration, donor concentration and donor activation energy has been conducted through simultaneous fitting of the temperature dependent Hall mobility and carrier concentration data which led to a donor concentration of 2.10×10¹⁶ cm⁻³ and an acceptor concentration of 4.9×10¹⁵ cm⁻³. The resultant donor activation energy is 18 meV. The analysis indicates that the dominant scattering mechanism at low temperatures is by ionized impurities. The extended defect concentrations on Ga- and N-faces were about 5×10⁵ cm⁻² for the former and about 1×10⁷ cm⁻² for the latter, as revealed by a chemical etch. The full width at half maximum of the symmetric (0 0 0 2) X-ray diffraction peak was 69″ and 160″ for the Ga- and N-faces, respectively. That for the asymmetric (10–14) peak was 103″ and 140″ for Ga- and N-faces, respectively. The donor bound exciton linewidth as measured on the Ga- and N-face (after a chemical etch to remove the damage) is about 1 meV each at 10 K. Instead of the commonly observed yellow band, this sample displayed a green band, which is centered at about 2.45 eV.     

Efficient grating writing through fibre coating with femtosecond IR radiation and phase mask

Bragg gratings have been written through the polymer coating into hydrogen loaded SMF-28 fibres with ultrafast 800 nm radiation and a phase mask. Index modulations of up to 3.5times10^-4 were induced with fibre strengths remaining at ~85% of the pristine fibre value     

Studies of diffused boron emitters: saturation current, bandgapnarrowing, and surface recombination velocity

The emitter saturation current density, J₀ was measured on diffused boron emitters in silicon for the case in which the emitter surface is passivated by a thermal oxide and for the case in which Al/Si is deposited on the emitter surface. The oxide-passivated emitters have a surface recombination velocity, s, which is near its lowest technologically achievable value. In contrast, the emitters with Al/Si on the surface have surface recombination velocities which approach the maximum possible value of s. From the J ₀ measurements, the apparent bandgap narrowing as a function of boron doping was found. Using this bandgap narrowing data, the surface recombination velocity at the Si/SiO₂ interface was extracted for surface boron concentrations from 3×10¹⁷ to 3×10¹⁹ cm^-3     

Deformation of single-mode optical fibers under static longitudinal stress

The deformation of single-mode fibers resulting from a longitudinally applied static force has been measured experimentally by means of high resolution heterodyne interferometry and analyzed theoretically using the second-order theory of elasticity and the photoelastic effect. Both the elongation of the fiber and the phase change of light propagating through the fiber have been measured as a function of tensile force. The values of the elastic constants measured for fibers with pure silica core and B2O3doped cladding areE = 6.41 times 10^{10}N/m²for the Young's modulus,delta = -4.0for the nonlinearity constant of the longitudinal strain, andbeta = -2.3for the nonlinearity constant of the transverse strain. For unit elongations up to 0.3 percent, no creep, hysterisis, or relaxation effects have been observed within a resolution of one part in 10⁴.     

Crystal growth, frequency doubling, and infrared laser performanceof Yb3+:BaCaBO3F

Yb:BaCaBO₃F(Yb:BCBF) has been investigated as a new laser crystal with potential for self-frequency doubling, Yb³⁺ in BCBF exhibits a maximum absorption cross section at 912 nm of 1.1×10^-20 cm² with a bandwidth (FWHM) of 19 nm. The maximum emission cross section at 1034 nm is 1.3×10^-20 cm² with a transition bandwidth of 24 nm. The measured emission lifetime of Yb³⁺ is 1.17 ms. An Yb:BCBF laser has been demonstrated with a Ti:sapphire pump source, and a measured slope efficiency of 38% has been obtained for the fundamental laser output. Single crystal powders of BCBF have been compared with KD ⁺P for a relative measure of the second-harmonic generating potential, yielding d_eff(BCBF)~0.26 pm/V. The phasematching angle has been estimated from the refractive index data for type I second-harmonic generation of 0.517 μm light; the predicted angle is 37° from the c-axis. The growth, spectroscopy, laser performance, and linear and nonlinear optical properties of Yb:BCBF are reported     

High-Q LaAlO3 dielectric resonator shielded byYBCO-films

We have investigated a dielectric resonator consisting of a single crystalline LaAlO₃-cylinder shielded by a cylindrically shaped copper cavity with endplates made from epitaxial films of YBa₂Cu₃O₇ or niobium. For YBa₂ Cu₃O₇ films unloaded quality factors Q₀ of 4.5·10⁵ at 10 K and 1.3·10⁵ at 77 K were achieved at 11.6 GHz using a compact shielding cavity with a diameter of 15 mm and a height of 3.8 mm. The loss contributions of the dielectric resonator, the normal conducting cylinder wall, and the superconducting endplates, with one of them being separated by a small distance h from the dielectric cylinder, were calculated by modeling the electromagnetic fields of the TE_0νμ-modes. The dielectric loss tangent of the LaAlO ₃-cylinders was found to be 10^-6 at 4.3 K and f=11.6 GHz and to increase slightly with temperature. Moreover, the calculations indicate the tunability of the resonance frequency by changing h over a range of 1 GHz without significant degradation of Q ₀. These resonators are considered to be useful devices for stable oscillators and narrowband filters     

Heterostructure injection lasers

A state-of-the-art discussion of heterostructure injection lasers is presented. These lasers are first considered as configurations of uniformly pumped semiconductor slabs that constitute the transmission medium of multilayered slab waveguides, whose boundaries are trapless heterojunctions. Emphasis is on lasers whose electrically active and waveguiding layers are AlxGa1-xAs or GaAs, although several other semiconductors are discussed. Single, double, and separate confinement hetrostructure lasers, all of which yield carrier and optical confinement normal to the plane of the heterojunctions, are described. Lateral electrical and optical confinement in various stripe geometry double-hetrostructure lasers is illustrated. In addition to the more common Fabry-Perot cavity lasers, there are now also several new varieties that use corrugated surfaces as Bragg gratings for internal distributed feedback or reflection. Some of these heterostructure lasers have permitted the demonstration of CW-room temperature injection lasers with low threshold current densities (< 1000 A/cm²), low dc current (∼15 mA), long lives (>10⁵) h), and a considerable degree of control over modes and beam divergence. In addition, the use of heterostructures has permitted the demonstration of the first integratedoptical structures that include both active light generating and passive elements on a single chip.