Angular Alignment Tolerances Research Articles

Expanded-Beam Backside Coupling Interface for Alignment-Tolerant Packaging of Silicon Photonics

We demonstrate an alignment-tolerant backside coupling interface in the O-band for silicon photonics by generating an optimized through-substrate (downward) directionality beam from a TE-mode grating coupler and hybrid integrating the chip with backside silicon microlenses to achieve expanded beam collimation. The key advantage of using such an expanded beam interface is an increased coupling tolerance to lateral and longitudinal misalignment. A 34 $\mu$ m beam diameter was achieved over a combined substrate thickness of 630 $\mu$ m which was then coupled to a thermally expanded core single-mode fiber to investigate the tolerances. A 1-dB fiber-to-microlens lateral alignment tolerance of 14 $\mu$ m and an angular alignment tolerance of $1^{\circ }$ was measured at a wavelength of 1310 nm. In addition, a large $\pm$ 2.5 $\mu$ m 1-dB backside alignment accuracy was measured for the placement of microlens with respect to the grating. The radius of curvature of Si microlens to achieve a collimated beam was 480 $\mu$ m, and a 1-dB longitudinal alignment tolerance of 700 $\mu$ m was measured for coupling to a single-mode expanded core fiber. The relaxation in alignment tolerances make the demonstrated coupling interface suitable for chip-to-package or chip-to-board coupling

Fabry-Pérot interferometry for long range displacement sensing

We investigate different optical configurations of a low-finesse Fabry-Pérot interferometer used for displacement sensing. The different configurations of the Fabry-Pérot cavity are selected in order to achieve large measurement ranges and angular alignment tolerances and to make the interferometer applicable for targets of various reflectivity ranges. The possible working ranges and angular alignment tolerances are characterized with respect to the interference contrast which is a measure for the signal quality. The use of a confocal arrangement enables a measurement range of up to about 0.4 m, or to work with an angular tolerance of more than ±0.2°. In order to predict the optical response of arbitrary configurations of the Fabry-Pérot interferometer, we introduce a simulation method based on the Airy formula and the fiber optic coupling efficiency.

Analysis of the indoor GPS system as feedback for the robotic alignment of fuselages using laser radar measurements as comparison

The alignment of aircraft fuselages in the aerospace sector is currently done either manually or by complex, expensive automated systems. The manual process introduces a significant production delay and the automated systems are purpose-built and have limited flexibility, apart from its financial drawback. This work proposes a low-cost, high-flexibility system and, as part of it, evaluates the performance of a Rotary-Laser Automatic Theodolite (R-LAT) as a feedback source for the adaptive robot control of an anthropomorphic manipulator. In the proposed solution the robot carries a fuselage barrel and aligns it with respect to a second barrel. A high accuracy, frequency-modulated laser equipment is used to generate the reference system for the procedure. The measurements of the R-LAT are then verified with the frequency-modulated laser equipment in order to determine the linear and angular alignment tolerances achieved by the robot/R-LAT closed loop in a predefined work envelope. A throughout, step-by-step analysis of the measuring procedure is carried out to allow the recognition of error sources and thus the determination of an optimized method. These results identify the operation boundaries of the R-LAT within the process and yield its best configuration for the intended purpose. Using the EN ISO 9283 robot evaluation standard, the closed loop system was found to attain the nominal position with an average accuracy of 0.38mm and 0.01^o, contrasting with an average accuracy of 4.53mm and 0.21^o when the robot was operating in an open loop configuration.

Angular tolerances and trapped internal reflections in wedged high refractive index Brewster's angle plates

It is well known that the angular alignment tolerance for Brewster's angle plates is not generally strict. It is often not appreciated that for high index plates however wedge tolerances are small. It is shown that in Brewster angle plates of high refractive index materials, small wedge angles in the fabricated plate can cause internal s-plane reflections to become ‘trapped’ by total internal reflection. A wedge angle acceptable in terms of its p-plane loss can cause such trapped reflections. In laser systems the resulting multiply reflected beams can move sideways in the plate and eventually hit o-ring seals or other mounting arrangements, causing component damage and leaks in unexpected locations. Stray light problems of an unexpected nature can occur from this effect in both laser based and conventional optical devices.

Diode-laser-to-waveguide butt coupling

A diode-laser-to-waveguide butt-coupling model is described. The model takes into consideration the Fabry-Perot reflection and transmission of the étalon formed by the laser front facet and the waveguide entrance facet. The model predicts coupling efficiency and the coupled-power fluctuations that occur versus the separation between the laser and the waveguide. Calculations performed for Fabry-Perot-laser-to-KTP waveguide coupling show that the transverse and the angular alignment tolerances of the waveguide can be increased when the waveguide entrance facet is antireflection coated. The longitudinal alignment tolerance of the waveguide can be increased by use of an index-matching gel between the laser and the waveguide.

Spatial resolution and angular alignment tolerance in radiometric analysis of alveolar bone change.

This pilot study was undertaken to determine the effect of x-ray beam alignment and spatial resolution on quantification of alveolar bone using radiometric techniques. Six (6) dry mandibles were radiographed at 70 kVp, 10 mA, 0.6 seconds using D-speed film, with a bone chip (2.64, 4.10, or 6.07 mg) present or absent at 7 x-ray beam alignments (0 degree, 2 degrees horizontal, 2 degrees vertical, 4 degrees horizontal, 4 degrees vertical, 6 degrees horizontal, 6 degrees vertical). This resulted in 28 radiographs per mandible. Radiographs were digitized using 50- and 200-microns pixel spatial resolution. Image gray levels were standardized using a simple look-up table shift. Regions of interest (ROIs) were positioned on the alveolar bone where the bone chips had been placed. Cumulative percent histograms (CPH) were calculated for those ROIs. Regression analysis was used to evaluate the relationships between CPH changes and bone chip size as x-ray beam angulation and spatial resolution was varied. The resulting R2 values for angulation ranges of 0 degree to 1.4 degrees, 1.5 degrees, to 2.4 degrees, and 2.5 degrees to 5.5 degrees were: 0.983, 0.941, 0.891 for 50-microns pixel images and 0.869, 0.909, and 0.774 for 200-microns pixel images. We conclude that 50-microns pixel spatial resolution is apparently superior to 200-microns pixel images if radiometric data is to be evaluated. With 50-microns pixel spatial resolution, alignment variations up to 5 degrees may be acceptable in clinical studies, depending on the magnitude of bone change that is to be detected.

Alignment tolerances of short-external-cavity InGaAsP diode lasers for use as tunable single-mode sources

The side-mode suppression ratio (SMSR) and single-mode continuous tuning range were measured for planar short-external-cavity semiconductor diode lasers to examine the positional and angular alignment tolerances of the external cavity mirror relative to the laser facet. For inverted rib waveguide lasers, the SMSR was found to be </= -25 dB for external cavity lengths of 40-200 microm and </= -23 dB for rotational misalignments of +/-8 degrees . For parallel alignment of the external cavity mirror and laser facet, continuous single-mode tuning ranges of 1.0 nm, or 110% of a mode spacing, were obtained. At external cavity lengths of <100 microm, the total amount of continuous single-mode tuning, summed over all modes, was ~72 cm(-1), which corresponds to ~75% spectral coverage.

Method of calculating the alignment tolerance of a Porro prism resonator

A method of calculating the angular alignment tolerance of a Porro prism laser oscillator is presented. The reduction in mode volume due to misalignment is found by a ray optical approach. An analytical expression for alignment tolerance, in terms of resonator length L, prism azimuth angle theta, rod location L(2), and rod aperture radius alpha is obtained. An experiment was set up to measure the laser output reduction due to reflector tilting. Results obtained are in good agreement with the theoretical predictions.

Interference-filter-tuned, alignment-stabilized, semiconductor external-cavity laser

We compared the angular alignment tolerance and the tuning range of a novel interference-filter-tuned semiconductor external-cavity laser based on a degenerate resonator with characteristics similar to those of a conventional grating-tuned external-cavity laser using a 1300-nm In GaAsP Fabry-Perot laser with an antireflection-coated facet as the gain medium. The interference-filter cavity had a 260-fold greater alignment tolerance (+/-26 versus +/-0.1 mrad) and nearly the same tuning range (90 versus 110 nm) as the grating-tuned cavity.

Compact optical data processor employing holographic reflective lenses

A compact optical data processor is described that employs holographic reflective lenses. The processor is inexpensive and requires one-half of the length of the optical bench as that required for using glass lenses. The fabrication of the holographic lenses is described, and the results of optical processing reported. The results show that an inexpensive data processor employing holographic lenses is a feasible project. The processor may find use for onboard optical processing on spacecrafts and satellites. The most distinguishing characteristic of such a processor is its extreme light weight. The angular alignment tolerances for holographic lenses are very low. The processor must, therefore, be made rugged, designed, and mounted to withstand vibrations, shocks, and other environmental problems associated with spacecrafts and satellites.