Optical Alignment Tolerances Research Articles

Symmetric nonconfocal Fabry–Perot cavity with a stable long optical path length and improved tolerance for angular alignment

Engineering of optical alignment tolerance between a laser source and a Fabry–Perot (FP) cavity is of great importance in optimizing a laser analysis system and saving costs in the additional alignment task. We propose a modification of the conventional confocal FP (CFP) cavity–a symmetric nonconfocal FP (SNFP) cavity with two identical spherical mirrors separated by a distance equal to half of the common radius of curvature. Numerical analysis of the two cavities reveals that the SNFP cavity has better off-axis stability and much better angular misalignment tolerance than the CFP cavity, even with a longer optical path length (OPL). The SNFP cavity provides a six-transit path length for a basic cavity mode that supports a 1.5-fold long OPL within the same volume as the CFP cavity, which is favorable for applications requiring limits on the footprint and weight of the cavity. In addition, a ray-tracing simulation with a disk-type source having an angular Gaussian distribution (α = β = 5 deg) reveals that the emission beam profile from the SNFP cavity is narrower than that from the CFP cavity. These results are expected to help reduce cavity loss and to improve alignment efficiency between the laser source and the FP cavity.

106.25-Gbps PAM-4 bidirectional optical sub-assembly module with in-line arrangement of optical and electrical interfaces.

We successfully demonstrate a 106.25-Gbps PAM-4 bidirectional optical sub-assembly for optical access networks, including a driver amplifier and an electro-absorption modulated laser for a transmitter, a photodiode and transimpedance amplifier for a receiver, and an optical filter block. For its implementation, we propose design strategies providing an in-line arrangement of optical and electrical interfaces while ensuring optical alignment tolerance for easy assembly and reducing electrical crosstalk between the transmitter and receiver. Measured receiver sensitivity was <-11.4 dBm for the KP4 forward error correction limit during transmitter operation, and measured power penalty of 10-km single-mode fiber transmission was <0.9 dB.

Avalanche Photodiodes with Dual Multiplication Layers for High-Speed and Wide Dynamic Range Performances

In this work, we demonstrate In0.52Al0.48As top/backside-illuminated avalanche photodiodes (APD) with dual multiplication layers for high-speed and wide dynamic range performances. Our fabricated top-illuminated APDs, with a partially depleted p-type In0.53Ga0.47As absorber layer and thin In0.52Al0.48As dual multiplication (M-) layer (60 and 88 nm), exhibit a wide optical-to-electrical bandwidth (16 GHz) with high responsivity (2.5 A/W) under strong light illumination (around 1 mW). The measured bias dependent 3-dB O-E bandwidth was pinned at 16 GHz without any serious degradation near the saturation current output. To further increase the speed, we downscaled the active diameter and adopted a back-side illuminated structure with flip-chip bonding for batter optical alignment tolerance. A significant improvement in maximum bandwidth was demonstrated (25 versus 18 GHz). On the other hand, we adopted a thick dual M-layer (200 and 300 nm) and 2 μm absorber layer in the APD design to circumvent the problem of serious bandwidth degradation under high gain (&gt;100) and high-power operation which significantly enhanced the dynamic range. Due to dual M-layer, the carriers could be energized in the first M-layer then propagate to the second M-layer to trigger the avalanche process. In both cases, despite variation in thickness of the absorber and M-layer, the cascade avalanche process leads to values close to the ultra-high gain bandwidth product (GBP) of around 460 GHz with a responsivity of 0.4 and 1 A/W at unit gain for the thin and thick M-layer devices, respectively. We successfully achieved a good sensitivity of around −20.6 dBm optical modulation amplitude (OMA) at a data rate of 25.78 Gb/s, by packaging the fabricated APDs (thin dual M-layer (60 and 88 nm) version) with a 25 Gb/s trans-impedance amplifier in a 100 Gb/s ROSA package. The results show that, the incorporation of a dual multiplication (M) layer structure in the APD opens a new window to obtaining the higher GBP in order to meet the requirements for high-speed transmission without the need of further downscaling the multiplication layer.

FSO Receiver With High Optical Alignment Robustness Using High-Speed 2D-PDA and Space Diversity Technique

A free-space optical (FSO) receiver with high optical alignment robustness configured by a single optical lens is presented, in addition to a newly developed 2-D photodetector array (2D-PDA). A 6 × 6 square-shaped aligned arrangement matrix structure in the 2D-PDA provides a large active area for photodetection. In a 1.5-m FSO demonstration, the optical alignment tolerance on the receiver's lens is evaluated to be as large as 12 mm. The multiple-output signals from the 2D-PDA are used for the maximum ratio combining method in space diversity. The system design and alignment tolerance without a mechanical beam-tracking system are presented and the bit error rate contributed by space diversity is discussed.

Three-Dimensional Printed Miniature Fiber-Coupled Multipass Cells with Dense Spot Patterns for ppb-Level Methane Detection Using a Near-IR Diode Laser.

Tunable diode laser absorption spectroscopy (TDLAS) based on a multipass cell (MPC) is a powerful analytical tool and is widely applied to air quality monitoring, industrial process control, and medical diagnostics. However, the conventional MPC as a core component in TDLAS devices has a large size, low utilization efficiency of the mirror surfaces, and tight optical alignment tolerances. In this paper, we design and fabricate a mini-MPC with an optical absorption path length of 4.2 m and dimensions of 4 × 4 × 6 cm3 (open cavity), which, to our best knowledge, is the current smallest MPC in terms of the same optical path length. The mini-MPC generates a seven-nonintersecting-circle dense spot pattern on two 25.4 mm spherical mirror surfaces, providing a high fill factor of 21 cm-2. A fiber-coupled collimator and an InGaAs photodetector are integrated into the mini-MPC via a high-resolution three-dimensional printed frame, hence removing the requirement of active optical alignment. Using a 1.65 μm distributed-feedback laser, the performance of this mini-MPC for methane detection was evaluated in terms of linearity, flow response time, stability, minimum detectable limit, and measurement precision. Continuous measurements of methane near a sewer and in the atmosphere were performed to demonstrate the stability and robustness of the highly integrated mini-MPC-based gas sensor. Our analysis shows that a methane minimum detectable limit of 117 ppbv is achieved, paving the way toward a sensitive, low-cost, and miniature trace gas sensor inherently suitable for large-scale deployment of distributed sensor networks and for handheld mobile devices.

Ultra-compact visible light depolarizer based on dielectric metasurface.

With rapid development towards shrinking the size of traditional photonic systems such as cameras, spectrometers, displays and illumination systems, there is an urgent need for high performance and ultra-compact functional optical elements. The large footprint of traditional bulky optical elements, their monofunctional response and the inability for direct integration into nanophotonic devices have severely limited progress in this area. Metasurfaces, consisting of an array of subwavelength nanoscatterers with spatially varying geometries, have shown remarkable performance as ultrathin multifunctional optical elements. Here, based on an all-dielectric metasurface, we propose and experimentally demonstrate a spatial domain optical depolarizer capable of efficiently depolarizing linearly polarized light in the visible spectral band from 450 nm to 670 nm, with a degree of polarization of less than 10 %. Remarkably, it is capable of depolarizing light beam with a diameter down to several micrometers, about two orders of magnitude smaller than commercial liquid crystal depolarizers. Furthermore, the long response time, bulky footprint, tight optical alignment tolerance and large pixel size severely limit the performance and system integration of commercial depolarizers. We envision the metasurface depolarizer to find applications in next generation ultra-compact grating spectrometers and illumination systems.

Low-Cost CWDM Transceiver Module by Spatial Power Combination and Division for Multimode Fiber Links

A cost-effective coarse wavelength division multiplexing optical sub-assembly (OSA) for short-distance optical interconnection is proposed and fabricated. Filter-free transmitter multiplexing is demonstrated by a spatial power combination of multiple-wavelength laser outputs into a fiber. As a result, the OSA is constructed with a reduced burden in optical alignment and packaging owing to the small number of optical elements and large alignment tolerance. Data transmission with a bit rate of 6 Gb/s is achieved through a 100-m-long multimode fiber. No significant degradation of eye patterns is observed during the transmission, and a bit error rate below $10^{{\mathbf {-12}}}$ is attained with the proposed module.

Fully passive-alignment pluggable compact parallel optical interconnection modules based on a direct-butt-coupling structure for fiber-optic applications

A low-cost packaging method utilizing a fully passive optical alignment and surface-mounting method is demonstrated for pluggable compact and slim multichannel optical interconnection modules using a VCSEL/PIN-PD chip array. The modules are based on a nonplanar bent right-angle electrical signal path on a silicon platform and direct-butt-optical coupling without a bulky and expensive microlens array. The measured optical direct-butt-coupling efficiencies of each channel without any bulky optics are as high as 33% and 95% for the transmitter and receiver, respectively. Excellent lateral optical alignment tolerance of larger than 60 μm for both the transmitter and receiver module significantly reduces the manufacturing and material costs as well as the packaging time. The clear eye diagrams, extinction ratios higher than 8 dB at 10.3 Gbps for the transmitter module, and receiver sensitivity of better than −13.1 dBm at 10.3 Gbps and a bit error rate of 10−12 for all channels are demonstrated. Considering that the optical output power of the transmitter is greater than 0 dBm, the module has a sufficient power margin of about 13 dB for 10.3 Gbps operations for all channels.

Low-Cost PCB-Integrated 10-Gb/s Optical Transceiver Built With a Novel Integration Method

A novel integration method for the production of cost-effective optoelectronic printed circuit boards (OE PCBs) is presented. The proposed integration method allows fabrication of OE PCBs with manufacturing processes common to the electronics industry while enabling direct attachment of electronic components onto the board with solder reflow processes as well as board assembly with automated pick-and-place tools. The OE PCB design is based on the use of polymer multimode waveguides, end-fired optical coupling schemes, and simple electro-optic connectors, eliminating the need for additional optical components in the optical layer, such as micro-mirrors and micro-lenses. A proof-of-concept low-cost optical transceiver produced with the proposed integration method is presented. This transceiver is fabricated on a low-cost FR4 substrate, comprises a polymer Y-splitter together with the electronic circuitry of the transmitter and receiver modules and achieves error-free 10-Gb/s bidirectional data transmission. Theoretical studies on the optical coupling efficiencies and alignment tolerances achieved with the employed end-fired coupling schemes are presented while experimental results on the optical transmission characteristics, frequency response, and data transmission performance of the integrated optical links are reported. The demonstrated optoelectronic unit can be used as a front-end optical network unit in short-reach datacommunication links.

프로젝션 기반 무안경 방식 멀티뷰 3D 디스플레이에서 구면 렌티큐라 렌즈 시트를 이용하여 재생된 입체영상의 해상도를 증가시키는 광학적 접근 방법

Multi-view 3D displays based on a limited number of pixels have the problem that the stereo-scopic image has a low resolution because of increasing view number. To solve the problem of low resolution, we propose an optical approaching method that focuses the width of a unit pixel by using a commercial spherical shape lenticular lens sheet and increases the effective resolution by increasing the number of sources of light in the multi-view 3D display system based on projection type. The method was performed in such an order that several main derivable parameters were defined, and, through the theoretical and experimental result, the value of the contractible unit pixel width and the scalable effective resolution was derived in a given system environment. As a result, for the case that the ray of light from the projector transmitted the 25 LPI lenticular lens sheet which has the pitch size 1.016 mm, the focused unit pixel width was 0.19 mm and the scalable effective resolution was, at most, 5 times wider than the original one. In addition, the range of depth of focus was 1.496 mm, which shows us the range of thickness tolerances of commercial spherical shape lenticular lens sheet and sufficient optical alignment tolerances.

Silicon Photonic Circuits: On-CMOS Integration, Fiber Optical Coupling, and Packaging

Silicon photonics is a new technology that should at least enable electronics and optics to be integrated on the same optoelectronic circuit chip, leading to the production of low-cost devices on silicon wafers by using standard processes from the microelectronics industry. In order to achieve real-low-cost devices, some challenges need to be taken up concerning the integration technological process of optics with electronics and the packaging of the chip. In this paper, we review recent progress in the packaging of silicon photonic circuits from on-CMOS wafer-level integration to the single-chip package and input/output interconnects. We focus on optical fiber-coupling structures comparing edge and surface couplers. In the following, we detail optical alignment tolerances for both coupling architecture, discussing advantages and drawbacks from the packaging process point of view. Finally, we describe some achievements involving advanced-packaging techniques.

Cost-effective integration of plastic optical fiber and total internal reflection mirrors in printed circuit boards for parallel optical interconnects

A novel method of integrating a total internal reflection (TIR) mirror into an optical waveguide embedded in a printed circuit board (PCB) was demonstrated for application in chip-to-chip optical interconnects. Plastic optical fiber (POF) was placed into channels that were mechanically machined into FR-4 composite plates. The TIR mirror was created by a second mechanical machining. The mirror loss was measured to be approximately −1.6 dB per reflection. The use of POF resulted in attenuation losses in the waveguide over an order of magnitude lower than what is obtainable with typical planar waveguides that are integrated into PCBs. Monte Carlo ray tracing was used to determine the theoretical efficiency of the system as well as cross talk between channels due to optical device alignment tolerances.

Optical alignment tolerances in double-side irradiated self-written waveguide-induced fiber arrays packages

In this paper, we present our experimental study on the optical alignment tolerance between the couplings of single-mode fibers (SMFs) connected with a double-side irradiation-induced self-written waveguide (SWW). The study firstly focuses on the coupling of two SMFs and then on the two fiber arrays (FAs) for parallel optical communication. The SWW was formed in dye-dispersed epoxy materials by the photopolymerization technique. Rhodamine 6G dye was dispersed in epoxy, which is commonly used in the photonic packaging industry as a bonding adhesive. Using double-side irradiated SWW, we found the alignment tolerance for such optical interconnect to relax significantly. All the formed SWWs were evaluated in terms of optical loss. In our study, up to 4 µm misalignment tolerance was allowed for only 1 dB loss penalty. In addition, the optical interconnect formed by this technique was also able to tolerate up to ± 10 µm lateral shift with only 1 dB extra loss. The wavelength-dependent loss (from 1520 to 1610 nm) and polarization-dependent loss were less than 0.4 dB. The double-side irradiated SWW-induced couplings between two FAs also provided low optical loss. They were found to be less sensitive to temperature changes, and no significant distortion in the digital signal transmission test was observed. We believe that the findings are useful and applicable to other dye-dispersed epoxy material systems for relaxing the alignment tolerance of the optical interconnects in various photonic packaging situations.

Analysis of Optical Coupling for SOI Waveguides

Optical coupling loss and alignment tolerance between SOI waveguides and laser diodes were investigated. It was found that the minimum optical coupling loss was about 1.6 dB and alignment tolerance was about ±0.7 μm. An optical input/output interface for optical fibers was also proposed and investigated the coupling loss characteristics.

Alignment tolerance enlargement of a high-speed photodiode by a self-positioned microball lens

By integrating a commercially available microball lens on the InGaAs p-i-n photodiode chip, we have achieved at least 5- and 16-fold improvements of the optical alignment tolerance in the transverse and longitudinal axes, respectively, without sacrificing the diode efficiency. The optical alignment during lens/chip integration is achieved by a self-positioning process, which provides an on-chip ball-lens socket in concentric circles to the photodiode aperture to sustain the drop-in microball lens. The photodiode after lens integration exhibits a 3-dB bandwidth larger than 9 GHz and a responsivity larger than 0.9 A/W, both at 1310-nm wavelength. In addition, the InGaAs photodiode exhibits a dark current density of less than 3 μA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , both before and after lens positioning, indicating a proper integration.

LTCC technology for cost‐effective packaging of photonic modules

PurposeThe aim of the research was to evaluate the concept that utilizes structured planar substrates based on low temperature co‐fired ceramics (LTCC) as a precision platform for the passive alignment of a multimode fiber and wide‐stripe diode laser.Design/methodology/approachPresents the manufacturing process for realisation of 3D precision structures, heat dissipation structures and a cooling channel into the LTCC substrate. The developed methodology for 3D modelling and simulation of the system was used to optimize structures, materials and components in order to achieve optimal performance for the final product and still maintain reasonably low fabrication costs. The simulated optical coupling efficiency and alignment tolerances were verified by prototype realization and characterization.FindingsThe achieved passive alignment accuracy allows high coupling efficiency realisations of multimode fiber pigtailed laser modules and is suitable for mass production.Research limitations/implicationsProvides guidance in the design of LTCC precision platforms for passive alignment and presents a hybrid simulation method for photonics module concept analysis.Practical implicationsThe three‐dimensional shape of the laminated and fired ceramic substrate provides the necessary alignment structures including holes, grooves and cavities for the laser to fiber coupling. Thick‐film printing and via punching can be incorporated in order to integrate electronic assemblies directly into the opto‐mechanical platform.Originality/valueIntroduces the LTCC 3D precision structures for photonics modules enabling passive alignment of multimode fiber pigtailed laser with high efficiency optical coupling. Demonstrates the hybrid simulation methodology for concept analysis.

The impact of a large bandgap drift region in long-wavelength metamorphic photodiodes

A comparative study of two types of metamorphic double heterojunction long-wavelength photodiodes on GaAs substrates is performed in terms of their bandwidths and responsivities. A p-i-i-n heterostructure with a large bandgap drift layer (I-InAlAs) at the cathode end of the photoabsorption region (i-InGaAs) is compared experimentally and theoretically to a p-i-n structure without a drift layer. Both types of photodiodes were fabricated using an InGaAs-InGaAlAs-InAlAs double heterostructure design to simultaneously achieve high bandwidths and high responsivities. The inclusion of an I-InAlAs drift region resulted in p-i-i-n photodiodes with larger bandwidths than p-i-n photodiodes with the same areas, or conversely a p-i-i-n photodiode can be made larger than a comparable p-i-n photodiode, but achieve the same bandwidth. Therefore, p-i-i-n photodiodes provide larger optical fiber alignment tolerances and better coupling efficiency than p-i-n photodiodes with the same bandwidths, p-i-i-n photodiodes with 10-/spl mu/m-diameter optical windows typically exhibited low dark currents of 500 pA at 5-V bias, responsivities of 0.6 A/W, and a -3-dB bandwidth of 38 GHz for 1.55-/spl mu/m operation.

Optical Alignment Tolerances and Techniques for Particle Image Velocimetry

Toensuregoodparticleimagevelocimetrycorrelations,itisnecessarytocharacterizetheimportantvariablesthat control image acquisition and image analysis. Among these, image spot size as it changes across the entire imaging surfacebearsinvestigation.Therefore, wedescribeanerroranalysisforsubpixel resolutionofcorrelation peaksus- ingWhittaker' smethod.Theseresultsseta toleranceonoptimumimagespotsizes.Typicallensesarethenanalyzed usingray-tracesoftware,toascertainwhatopticalparametersandcameramisalignmentsareallowablewithinthese optimum tolerances. We then describe an alignment procedure that allows us to set a camera to those tolerances. O acquire high-quality particle image velocimetry (PIV) data, it is necessary to use a cross-correlation technique, for exam- ple, two-color or image-shifted PIV, to align carefully the optical system and use an optimized data analysis package. In recent work performed at the Colorado School of Mines (CSM), two PIV inves- tigations resultedin1 )an assessmentofthe effects ofparticleimage spot size on correlation accuracy, 2 ) determination of the minimum spot size for least error, and 3 ) the development of an experimental system that can reliably generate this minimum spot size. One of these projects involved the development of PIV analy- sis software that generates vector data from PIV images (Ref. 1; T. Drouillard and M. Linne, Experiments in Fluids, manuscript in preparation ). In that work, several techniques for resolving cor- relation peaks were evaluated. These included the basic centroid method, Whittaker' s reconstruction, an analytical three-pointGaus- sian e t, and a numerical e ve-point Gaussian e t using the Marquardt method. An assessment of reconstruction errors was included. The least error was produced by the e ve-point Gaussian e t using Mar- quardt nonlinear e tting, whereas Whittaker' s reconstruction was a close second. Furthermore, it was found that the size of the cor- relation peak (directly related to particle image spot size ) has the greatest effect on the accuracy of the reconstruction algorithm. This sets a tolerance on the image spot size. That work is discussed in detail by Drouillard and Linne (manuscript in preparation ). Asecond projectinvolvedthe development of a method by which a PIV experimental apparatus may beconsistently and reproducibly aligned to meet this tolerance on spot size. 2 This topic is of primary importance to chemical e lm-based techniques, where turnaround time is very long. It also has implications for digital systems, espe- cially when it is necessary to align a camera across a large system, from within a wind tunnel, for example. A portion of this inves- tigation included modeling photographic lens performance under ideal and nonideal conditions using computer-based geometric ray tracing.These resultsprovide insightinto howvariousexperimental cone gurations can lead to spot size variation, which in turn affects the accuracy of PIV analysis. Based on such results, a system for alignment of the experiment was developed. That work is the pri- mary focus of this paper.

Optical alignment tolerances and techniques for particle image velocimetry

Optical alignment tolerances and techniques for particle image velocimetry

A self-aligned optical subassembly for multi-mode devices

A completely self-aligned optical sub-assembly is described that meets the optical alignment tolerances needed for low-loss LED and PIN photodiode OSA's. Solder reflow aligns the optical device (LED or PIN photodiode) to a silicon "header," which is mechanically aligned to a silicon fiber stub with fiducial cavities and alignment spheres. This assembly is in turn mounted in a plastic ferrule. Devices made using this alignment scheme have demonstrated losses as low as 0.5 dB relative to a butt-coupled fiber measurement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>