Single-transverse-mode 3.4-mW emission of oxide-confined 780-nmVCSELs

The authors have fabricated oxide-confined vertical-cavity surface-emitting lasers with metal apertures exhibiting a spatial mode filtering effect. Single-mode continuous-wave output is enhanced up to 3.4 mW by this effect for AlGaAs-based 780-nm vertical-cavity surface-emitting lasers with a 3.5-μm-diameter oxide aperture. From numerical calculations, the round-trip loss difference between zeroth- and first-order optical modes as a function of metal aperture size indicates that there is an optimum point; a 4-μm metal aperture size is suitable for higher order mode suppression of a 3.5-μm oxide aperture device     

Optical fiber up-tapers modeling and performance analysis

An extensive CRAY-computer-based simulator using the propagating beam method for the accurate evaluation of the performance of optical fiber up-tapers is discussed. Up-tapers with and without straight tips and with arbitrary index profiles and tapering functions can be evaluated. Results of the evaluation of various 1.5-cm-long optical up-tapers followed by a straight tip and drawn from special preforms with various index profiles are quite encouraging. They show that beam expansion ratios in the range of 5-10 are feasible with a corresponding insertion loss per taper of less than 0.01-0.025 dB. Tapers shorter than 1.5 cm with insertion loss in the same range are also feasible but have slightly smaller beam expansion ratios     

Single-mode power dependence on surface relief size formode-stabilized oxide-confined vertical-cavity surface-emitting lasers

We have studied the mode behavior of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with a surface relief for fundamental mode selection. The dependence of single-mode power on the surface relief diameter was measured and compared with numerically calculated values. VCSELs with diameters of 9 and 12 μm were equipped with surface reliefs with diameters in the range 4-10 μm. The results show that there exists an optimum relief diameter for each VCSEL size. A maximum single-mode power of 2.2 mW was achieved for a 9-μm-diameter VCSEL with a 4-μm-diameter surface relief     

High-power temperature-insensitive gain-offset InGaAs/GaAsvertical-cavity surface-emitting lasers

The authors have grown 997 nm vertical-cavity surface-emitting lasers with an offset between the wavelength of the cavity mode and the quantum well gain peak to improve high temperature operation, and with higher aluminum-content barriers around the active region to improve the carrier confinement. They fabricated lasers of 8-15 and 20-μm diameters. The 8-μm-diameter devices exhibited CW operation up to 140°C with little change in threshold current from 15°C to 100°C, and the 20-μm-diameter devices showed CW output power of 11 mW at 25°C without significant heat sinking     

Propagation loss measurements in semiconductor microcavity ring anddisk resonators

We report the measurement of cavity propagation losses in nearly single-mode semiconductor waveguide-coupled ring and disk microcavity optical resonators. Using a novel 10.5-μm-diameter ring resonator, we measure transverse electric (TE) and transverse magnetic (TM) field intensity losses in 0.35-μm-wide ring waveguide cavities in the 1.55-μm-wavelength region. We present the experimental results for nanofabricated AlGaAs-GaAs 10.5-μm-diameter ring and disk resonators to quantify cavity losses and to show the feasibility of these promising and robust submicron-scale devices     

A column-base InP lateral bipolar transistor

An InP lateral bipolar transistor has been successfully fabricated on a semi-insulating substrate by implanting Si⁺ as the emitter and collector contacts and Mg⁺ as the column base. An array of 33 1-μm-diameter columns with 1-μm separation between each was formed between the emitter-collector spacing of 3 μm. A current gain of 290 was obtained at 77 K; it was over 12 at room temperature     

Integration of magnetooptical waveguides onto a III-V semiconductorsurface

Optical waveguides fabricated on a yttrium iron garnet (YIG) substrate are integrated onto a semiconductor surface by using ridge waveguides patterned onto a prefabricated recess in the YIG surface. The recess separates the waveguides from the semiconductor substrate with an air-gap. This structure makes it possible to avoid coupling light within the YIG waveguides into the semiconductor substrate which has a higher refractive index. The excess optical loss due to the coupling can be as low as ⩽0.1 dB/cm with a 1-μm-wide air-gap. The calculated coupling loss is confirmed by comparing the guided TE and TM modes     

Transverse mode characteristics of vertical-cavity surface-emittinglasers buried in amorphous GaAs antiguide layer

We report the transverse mode characteristics of InGaAs-GaAs vertical-cavity surface-emitting lasers (VCSEL's) buried in a low-temperature-deposited amorphous GaAs (a-GaAs) layer. The maximum current maintaining a stable fundamental transverse mode is increased by the antiguide effect of the a-GaAs clad with a high refractive index. For 10- and 15-μm-diameter devices, we attain a stable single-mode emission over a wide range of current. The antiguide effects and transverse mode profiles in vertical cavity lasers buried in the high refractive index clad are calculated using a two-dimensional beam propagation method     

Optical fiber up-tapers with high beam expansion ratios forcomponent fabrication

Optical fibre up-tapers with beam expansion ratios of 10, about twice the previously achieved value for the same taper outer diameter, have been fabricated and characterized. The increased beam expansion is achieved with a near step-index profile for the core of the taper. The excess coupling loss between two tapers is less than 0.1 dB. A lateral offset of more than 20 μm or an axial displacement of 4 mm between tapers caused an excess loss of 1 dB. The angular tolerance for this loss is a manageable 0.3°. These tapers permit, for a given beam expansion ratio, the manufacture of smaller outer diameter, more compact structures of self-aligned beam expansion. They also permit the insertion of optical elements into the expanded beam for the fabrication of inline single-mode passive fiber components     

Influence of quantum-well width on device performance ofAl0.30Ga0.70As/In0.25Ga0.75As (on GaAs) MODFETs

An experimental study in which the quantum well width (W) is varied from 45 to 200 Å is discussed. Optimum device performance was observed at a well width of 120 Å. The 0.2-μm×130-μm devices with 120-Å quantum-well width typically exhibit a maximum channel current density of 550 mA/mm, peak transconductance of 550 mS/mm, and peak current gain cutoff frequency ( f_T) of 122 GHz. These results have been further improved in subsequent fabrications employing a trilevel-resist mushroom-gate process. The 0.2-μm×50-μm devices with mushroom gate exhibit a peak transconductance of 640 mS/mm, peak f _T of 100 GHz, and best power gains cutoff frequency in excess of 200 GHz. These results are among the best ever reported for GaAs-based FETs and are attributed to the high two-dimensional electron gas (2DEG) sheet density, good low-field mobility, low ohmic contact, and the optimized mushroom gate process     

Temperature-dependent characteristics and single-mode performanceof AlGaInP-based 670-690-nm vertical-cavity surface-emitting lasers

We report on temperature dependent characteristics and single mode performance of one-wave cavity, planar implanted, AlGaInP-based vertical-cavity surface emitting lasers. By optimizing the overlap between the gain peak and the cavity mode of the structure, we demonstrate record device performance, including 8.2 mW maximum output power and 11% power conversion efficiency for multimode operation and 1.9 mW and 9.6% power conversion efficiency for single mode operation at 687 nm. Improved performance at elevated temperatures is also achieved, with 1.5 mW output power demonstrated at 50°C from a 15-μm-diameter device     

Single-mode operation of mushroom structure surface emitting lasers

Mushroom structure vertical cavity surface emitting lasers with a 0.6-μm GaAs active layer sandwiched by two Al_0.6Ga_0.4 As-Al_0.08Ga_0.92As multilayers as top and bottom mirrors are discussed. The lasers exhibit a 15-mA pulses threshold current at 880 nm. Single longitudinal and single transverse mode operation was achieved on lasers with a 5-μm-diameter active region of current levels near 2×l_th. The light output above threshold current was linearly polarized with a polarization ratio of 25:1     

Amorphous-silicon-on-glass field emitter arrays

An amorphous-silicon (a-Si) field emitter array (FEA) has been fabricated on a glass substrate and characterized, At first, a 0.3-μm-thick Cr film was deposited on the glass by vacuum evaporation technique and subsequently a 1-μm-thick Si film was deposited on the Cr film by conventional RF sputtering technique with an undoped Si target at room temperature. The sputtered Si film was identified as amorphous from X-ray diffraction patterns and had a resistivity of 3-5 kΩ-cm. The FEA consists of 1-μm-high emitter tips and a gate electrode with 1.8-μm-diameter openings. This a-Si FEA with 5×5 (=25) tips exhibited a threshold voltage of 30 V and an emission current of 2 μA at a gate voltage of 100 V. Structure and emission characteristics are discussed     

Low-threshold 1.5 μm compressive-strained multiple- andsingle-quantum-well lasers

Design considerations for low-threshold 1.5-μm lasers using compressive-strained quantum wells are discussed. Parameters include transparency current density, maximum modal gain, bandgap wavelength, and carrier confinement. The optical confinement for a thin quantum well in the separate-confinement heterostructure (SCH) and the step graded-index separate-confinement heterostructure (GRINSCH) are analyzed and compared. 1.5-μm compressive-strained multiple- and single-quantum-well lasers have been fabricated and characterized. As a result of the compressive strain, the threshold current density is loss limited instead of transparency limited. By the use of the step graded-index separate-confinement heterostructure to reduce the waveguide loss, a low threshold current density of 319 A/cm² was measured on compressive-strained single-quantum-well broad-area lasers with a 27 μ oxide stripe width     

A silicon probe with integrated microheaters for thermal markingand monitoring of neural tissue

This paper describes a microheater structure and its integration on a silicon microprobe. The 30-μm-diameter microstructure can be used to heat local areas of tissue or to measure local tissue temperature with an accuracy of <0.3°C. The polysilicon microheater is suspended on a dielectric membrane to reduce undesired heat conduction to the probe substrate. The heating efficiency is 4.4°C/mW in still water and 2.2°C/mW in guinea pig cortex. Six milliwatts applied for 2 min in cortex produces a temperature of 50°C, creating a well-defined 50-μm-wide lesion for determining probe position histologically. Fabrication of the heaters requires no additional masking or processing steps in addition to those normally used for recording or stimulating probes     

Wide-frequency fiber-optic phase modulator using piezoelectricpolymer coating

The highly sensitive response of a fiber-optic phase modulator that is coated with 80-90-μm-thick vinylidene fluoride/trifluoroethylene copolymer on an 80-μm-diameter single-mode fiber and made piezoactive by radial poling is demonstrated over a wide frequency range, from 500 Hz to 50 MHz. In the range of 10 kHz up to 2 MHz, a relatively flat response is measured, whereas at high frequencies between 6 and 50 MHz, the phase modulation exhibits multiple peaks dominated by radial resonances of the fiber-jacket composite. Theoretical considerations on the phase shift and experimental results are given     

The future of wires

Concern about the performance of wires wires in scaled technologies has led to research exploring other communication methods. This paper examines wire and gate delays as technologies migrate from 0.18-μm to 0.035-μm feature sizes to better understand the magnitude of the the wiring problem. Wires that shorten in length as technologies scale have delays that either track gate delays or grow slowly relative to gate delays. This result is good news since these “local” wires dominate chip wiring. Despite this scaling of local wire performance, computer-aided design (CAD) tools must still become move sophisticated in dealing with these wires. Under scaling, the total number of wires grows exponentially, so CAD tools will need to handle an ever-growing percentage of all the wires in order to keep designer workloads constant. Global wires present a more serious problem to designers. These are wires that do not scale in length since they communicate signals across the chip. The delay of these wives will remain constant if repeaters are used meaning that relative to gate delays, their delays scale upwards. These increased delays for global communication will drive architectures toward modular designs with explicit global latency mechanisms     

Room-temperature operation of VCSEL-pumped photonic crystal lasers

Room-temperature operation of two-dimensional photonic crystal lasers optically pumped by a vertical-cavity surface-emitting laser emitting at 860 nm is reported. The photonic crystal membrane is surrounded by air on both sides and consists of four compressively strained quantum wells as the active region. The incident threshold pump power of an approximately 2.6-μm-diameter hexagonal defect cavity laser operating at 1.6 μm is 2.4 mW     

RF crosstalk in multiple-wavelength vertical-cavitysurface-emitting laser arrays

We present the crosstalk measured in multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) arrays under RF modulation. The array consists of eight bottom-emitting VCSEL's arranged with a “pie”-like configuration within a 60-μm-diameter circle with a 3-μm spacing for coupling into a multimode fiber. The crosstalk is investigated by measuring a cross-modulation response of two VCSEL's in the array using a monochromator in an optical spectrum analyzer as a receiver. Excess crosstalk is minimized by flip-chip mounting the array and introducing a proper bias condition. The crosstalk observed In the adjacent VCSEL's is less than -40 dB at the modulation frequency below 700 MHz and it Increases at 20 dB/decade above this frequency when both VCSEL's are biased at 5.0 mA     

Integrated fabrication of polysilicon mechanisms

The integrated fabrication of planar polysilicon mechanisms incorporating lower and higher kinematic pairs (or joints) is described. The two lower kinematic pairs (revolute and prismatic) commonly used in macrorobotic systems are compatible with silicon microfabrication technology. The mechanisms are fabricated by surface micromachining techniques using polysilicon as the structural material and oxide as the sacrificial material. Turbines with gear and blade rotors as small as 125 μm in diameter and 4.5 μm in thickness were fabricated on 20-μm-diameter shafts. A clearance as tight as 1.2 μm was achieved between the gear and the shaft. Gear trains with two or three sequentially-aligned gears were successfully meshed. A submillimeter pair of tongs with 400-μm range-of-motion at the jaws was fabricated. This structure incorporates a single prismatic joint and two revolute joints, demonstrating linear-to-rotary motion conversion