Proton implanted VCSEL's for 3 Gb/s data links

Transverse single-mode and multimode intensity modulated butt-coupled InGaAs vertical cavity surface emitting lasers (VCSEL)s are investigated as a light source for optical fiber communication systems. Data transmission at 3 Gb/s with a bit error rate (BER) of less than 10 ^-11 is reported for both 4.3 km of standard fiber, as well as 0.5 km of multimode graded-index fiber, 10-μm active diameter single-mode VCSELs are shown to have lower mode competition noise requiring 3 dB and 6 dB less power at the front end receiver at a BER of 10^-11 compared to 19-μm and 50-μm active diameter devices, respectively. In data transmission with multimode VCSELs, the dispersion penalty is lower than for single-mode sources since the noise at the receiver is mainly determined by transmitter-mode competition noise     

Spectroscopic properties and efficient diode-pumped laser operationof neodymium-doped lanthanum scandium borate

We report on the spectroscopic properties and laser performance of Czochralski-grown LaSc₃(BO₃)₄ (LSB) crystals with high neodymium concentrations up to 2.5·10²¹ cm^-3. The low-concentration quenching of the upper laser level of neodymium and the polarization dependence of the spectra indicate that LaSc₃(BO₃)₄ crystalizes in a huntite-type structure like Nd³⁺:YAl₃(BO₃)₄ (NYAB). With diode laser pumping at 808 nm, a multimode Nd³⁺:LaSc₃ (BO₃)₄ (NLSB) laser at 1063 nm is demonstrated. Optical slope efficiency was 64% with respect to absorbed pump power. The possibility of second-harmonic generation in the laser crystal is discussed     

Launching condition dependence of graded-index multimode fiber lossand bandwidth

Theoretical and experimental studies of the launching-condition dependence of graded-index 50-μm core, 0.2-numerical aperture multimode fiber transmission loss and baseband bandwidth are discussed. A combination of a SGS mode scrambler and a mandrel wrap mode filter was used as the launching system. It was found that steady-state fiber loss measurements should be carried out at an approximate mandrel curvature of ρ=0.07 mm^-1. This curvature is much smaller than the curvature ρ=0.15 mm^-1 which produces an equilibrium-mode distribution as defined by the International Telegraph and Telephone Consultative Committee (CCITT). Measurement at ρ=0.15 mm^-1 is shown to underestimate steady-state loss. It was also found that reproducible and reasonable bandwidth measurement can be obtained by choosing ρ=0 to 0.07 mm^-1     

Multimode Si-wire waveguides for integrated optical delay lines

A delay line of multimode Si-wire waveguide folded with 45deg mirror bends is described. A 17 cm waveguide with a core size of 1times3 mum² was integrated in an area of 0.85 mm². The waveguide generated a constant group delay of 2 ns in the entire C-band with a typical optical loss of 14 dB. It also exhibited an error-free operation for 10 Gbit/s optical data signals.     

Fabrication of buried channel waveguides in fused silica usingfocused MeV proton beam irradiation

Low-loss channel waveguides have been fabricated in fused silica using a beam of MeV protons focused down to a spot size of several microns. By using a combination of beam and sample scanning, single- and multimode graded index waveguides with lateral dimensions down to approximately 5 μm×5 μm have been fabricated using ion doses in the range (3×10¹⁴)-(6×10¹⁶) ions/cm ². Typical beam currents in the range 100 pA-10 nA were used. Optical mode profiles have been measured at 670 nm and propagation losses of the order of 3 dB/cm measured in unannealed samples. Annealing the substrate for 1 h at 500°C reduced these losses to below 0.5 dB/cm     

Interlaboratory comparison of radiation-induced attenuation inoptical fibers. I. Steady-state exposures

A comparison of the losses induced at 0.85 μm in three all glass-pure silica core optical fibers by steady-state radiation exposures has been made among five laboratories. Both the growth of the attenuation during irradiation and the recovery following exposures of 3000 and 10⁵ rads have been measured. Although a standard set of parameters was attempted by all laboratories, the slight divergences (0.45⩽σ⩽0.99 dB/km) observed in some data indicate sensitivity of the results to factors such as photobleaching, injection conditions, and sample coil diameter     

Error-free 10 Gbit/s transmission using individual Fabry-Perot modes of low-noise quantum-dot laser

A comb-generating laser suitable for DWDM application is demonstrated on the basis of a multimode quantum-dot laser. Bit error rate < 10^-13 at 10 Gbit/s external modulation was measured for 10 longitudinal modes owing to low (<0.3% over 0.001-10 GHz) relative intensity noise of each individual mode.     

Development of a multigrid FDTD code for three-dimensionalapplications

Numerical modeling of realistic engineering problems using the finite-difference time-domain (FDTD) technique often requires more detail than is possible when using a uniform-grid FDTD code. We describe the development of a three-dimensional (3-D) multigrid FDTD code that focuses a large number of cells of small dimensions in the region of interest. The detailed solution procedure is described and some test geometries are solved using both a uniform-grid and the developed multigrid FDTD code to validate the results and check the accuracy of the solution. Results from these comparisons as well as comparisons between the new FDTD code and another available multigrid code are presented. In addition, results from the simulation of realistic microwave-sintering experiments in large multimode microwave cavities are given to illustrate the application of the developed method in modeling electrically large geometries. The obtained results show improved resolution in critical sites inside the 3-D multimode sintering cavity while keeping the required computational resources manageable. It is shown that it is possible to simulate the sintering of ceramic samples of 0.318-cm wall thickness in a cylindrical multimode microwave cavity with a diameter of 74 cm and a length of 112 cm using 2.24×10⁶ total FDTD cells. For comparison, a total of 102×10⁶ cells would have been required if a uniform-grid code with the same resolution had been used     

Fiber-optic transmissions of microwave 8-phase-PSK and 16-aryquadrature-amplitude-modulated signals at the 1.3-μm wavelengthregion

Transmissions of a 6-GHz 8-phase phase-shift-keyed (8φ-PSK) signal over a 12.5-km single-mode fiber with a 5-dB power margin and bit-error-rate (BER) of 10^-10 and a 6-GHz 16-ary quadrature-amplitude-modulated (QAM) signal over the same distance with a 2-dB power margin and BER of 10^-12 were demonstrated. The 8φ-PSK digital modem operated at 78Mb/s and the 16-QAM digital modem operated at 90 Mb/s. A high-speed multimode InGaAsP laser diode and a high-speed p-i-n diode were used in both fiber-optic transmission systems. Floor characteristics and power penalties observed in the BER performances of both systems were found to be caused by the intensity noise of the laser diode, particularly the reflection-induced intensity noise     

Carbon-doped base GaAs-AlGaAs HBT's grown by MOMBE and MOCVDregrowth

High-quality GaAs-AlGaAs heterojunction bipolar transistors (HBTs) in which the carbon-doped base layers (p=10¹⁰-10²⁰ cm^-3, 400-800 Å thick) and Sn-doped collector and subcollector layers are grown by metalorganic molecular-beam epitaxy (MOMBE) and a subsequent regrowth using metalorganic chemical vapor deposition (MOCVD) is used to provide the n⁺ AlGaAs emitter and GaAs/InGaAs contact layers are discussed. A current gain of 20 was obtained for a base doping of 10¹⁹ cm^-3 (800 Å thick) in a 90-μm-diameter device, with ideality factors of 1.0 and 1.4 for the base-collector and emitter-base junctions, respectively, demonstrating the excellent regrowth-interface quality. For a base doping of 10²⁰ cm^-3 (400 Å thick), the current gain decreased to 8     

Analysis of optical crosstalk in coherence multiplexed systemsemploying a short coherence laser diode with arbitrary power spectrum

It is shown that the crosstalk level in coherence multiplexed systems is closely dependent on the profile of the emission spectrum of the laser diodes used. The cases of Gaussian, Lorentzian, and cos² power spectra are considered. The results demonstrate that crosstalk may very by several tens of decibels when using laser diodes operating with the same coherence length but exhibiting different spectral profiles. Illustrations in the area of optical communications are given for superluminescent diodes and multimode laser diodes with a Lorentzian and a cos² power spectrum. Results indicate that Gaussian sources are preferable     

Low-loss polymeric optical waveguides fabricated with deuteratedpolyfluoromethacrylate

Deuterated polyfluoromethacrylate which has high transparency, low birefringence and good processability was newly synthesized for use as optical waveguide materials, and both single-mode and multimode optical waveguides were fabricated using the polymer. The propagation loss and waveguide birefringence of the single-mode waveguides were as low as 0.10 dB/cm and -5.5×10^-6 at 1.31 μm, respectively. The propagation losses of the multimode waveguides were less than 0.02 dB/cm at both 0.68 and 0.83 μm, and 0.07 dB/cm at 1.31 μm     

Fabrication of p-i-n photodiodes on LPE-grown substrates

Epitaxial growth of a thick heavily doped silicon layer on a highly resistive silicon wafer by the yo-yo solute feeding method and its application to p-i-n photodiodes are discussed. An abrupt transition of the impurity profile is obtained between the n⁺ layer (1.95×10¹⁹ cm^-3, 450 μm) and the n^- layer (7.0×10¹¹ cm^-3, 80 μm). It is possible to use the thick intrinsic layers as the active region of power devices     

Process and device characterization of high voltage gallium arsenide P-i-N layers grown by an improved liquid phase epitaxy method

GaAs P-i-N layers with an i-region net doping of less than 10¹² cm⁻³ were grown on P⁺ and N⁺ substrates by a modified liquid phase epitaxy (LPE) method. Doping profiles and structural data obtained by varius characterization techniques are presented and discussed. A P⁺-P-i-N-N⁺ diode with a 25 μm-wide i-region exhibits a breakdown voltage of 1000 V, a t_rr of 50 ns, and reverse current densities (at V_R = 800 V) of − 3 × 10⁻⁶ A/cm² at 25°C and 10⁻² A/cm² at 260° C.     

An over 10-Gb/s transmission experiment using a p-type delta-dopedInGaAs-GaAs quantum-well vertical-cavity surface-emitting laser

We have fabricated a p-type delta-doped InGaAs-GaAs quantum-well (QW) vertical-cavity surface-emitting laser (VCSEL) with a low-resistance GaAs-AlAs distributed Bragg reflector (DBR). The threshold was as low as 700 μA for 10×10 μm² devices. A penalty-free 10-Gb/s transmission experiment with a 100-m-long multimode fiber was performed using fabricated VCSELs. The modulation speed was up to 12 Gb/s, which was limited by an RC constant. Further threshold reduction and high-speed operation can be expected by controlling the doping concentration in p-type delta-doped layers     

Optical interconnection using VCSELs and polymeric waveguidecircuits

We investigated the waveguide loss and transmission characteristics for optical interconnection using vertical-cavity surface-emitting lasers (VCSELs) and multimode polymeric waveguide circuits with crossings. The excess loss with 100 crossings is 2.2 dB when the image magnification from a VCSEL to a waveguide is 2.3. We obtained error-free (i.e., bit error rate <10^-11) optical interconnection at 1.0625 Gbps regardless of the number of crossings or the magnification. These results suggest the practicality of large-scale optical interconnection between VCSEL-based smart-pixel chips using multimode waveguides with more than 100 crossings     

Fabrication of planar optical waveguides in LiB3O5 by 2 MeV He+ ion implantation

The fabrication of planar optical waveguides in LiB₃O ₅ is discussed. Using 2-MeV ⁴He⁺ implantation with a dose of 1.5×10¹⁶ ions/cm² at 300 K, the refractive indexes of a 0.2-μm-thick layer 5.1 μm below the crystal surface are reduced to form optical barrier guides. For this ion dose the maximum change from the bulk values of refractive index at a wavelength of 0.488 μm are 1.5%, 5.25%, and 4% for n_x, n_y, and n_z, respectively. The refractive indexes of the guiding region change by less than 0.02% from the bulk values. The dose dependence of the optical barrier height has been measured. A threshold ion dose of about 0.75×10¹⁶ ions/cm² is required to form a refractive index barrier and ion doses higher than about 2.5×10¹⁶ ions/cm². saturate the refractive index decrease. Waveguide propagation losses for annealed single energy implants of dose 1.5×10¹⁶ ions/cm² are dominated by tunneling and are estimated to be ~8.9 dB/cm for the z-cut waveguides used. Multiple energy implants broaden the optical barrier, and losses of <4 dB/cm have been observed     

Monolithic photovoltaic PbS-on-Si infrared-sensor array

The growth of epitaxial narrow-gap PbS-on-Si substrates using a stacked CaF₂-BaF₂ intermediate buffer layer and the fabrication of linear arrays of photovoltaic infrared (IR) sensors in the PbS layer are discussed. The sensors of the array exhibit resistance-area products at zero bias of 3 Ω-cm² at 200 K (3.4-μm cutoff wavelength) and 2×10⁵ Ω-cm ² at 84 K (4-μm cutoff), with corresponding detectivities of 2×10¹⁰ and 1×10¹³ cm-√Hz/W, respectively     

Thermal Boundary Resistance Between GaN and Substrate in AlGaN/GaN Electronic Devices

The influence of a thermal boundary resistance (TBR) on temperature distribution in ungated AlGaN/GaN field-effect devices was investigated using 3-D micro-Raman thermography. The temperature distribution in operating AlGaN/GaN devices on SiC, sapphire, and Si substrates was used to determine values for the TBR by comparing experimental results to finite-difference thermal simulations. While the measured TBR of about 3.3 x 10^-8 W^-1 ldr m² ldr K for devices on SiC and Si substrates has a sizeable effect on the self-heating in devices, the TBR of up to 1.2 x 10^-8 W^-1 ldr m² ldr K plays an insignificant role in devices on sapphire substrates due to the low thermal conductivity of the substrate. The determined effective TBR was found to increase with temperature at the GaN/SiC interface from 3.3 x 10^-8 W^-1 ldr m² ldr K at 150degC to 6.5 x 3.3 x 10^-8 W^-1 ldr m² ldr K at 275degC, respectively. The contribution of a low-thermal-conductivity GaN layer at the GaN/substrate interface toward the effective TBR in devices and its temperature dependence are also discussed.