dB Alignment Tolerances Research Articles

Experimental Demonstration of the High Alignment-Tolerant Behavior of a Mid-Infrared Waveguide Platform for Evanescent Field Sensing.

Alignment tolerant coupling interfaces are an important feat for mid-IR waveguides when moving closer to real-world sensing applications, as they allow for an easy and fast replacement of waveguides. In this work, we demonstrate the alignment tolerant behavior of a germanium-on-silicon trenched waveguide platform with monolithically integrated microlenses using backside coupling of an expanded beam for evanescent field sensing between 6.5 and 7.5 μm. The chip with a propagation loss of approximately 5 dB/cm was mounted and aligned, using active alignment, in a sample holder that could be moved in all three dimensions to induce misalignments with a precision of the manual actuator of 1.3 μm. Using this setup, the in-plane 1 dB alignment tolerances were measured to be ±16 μm, while the 1 dB alignment tolerances in the longitudinal direction were found to be larger than ±150 μm. Without the addition of the microlenses, we expect an in-plane 1 dB alignment tolerance of ±3 μm based on simulations. Additionally, it could be demonstrated that the integration of the microlenses significantly improves the stability of the broadband grating couplers in regard to misalignment-induced intensity changes in the obtained transmission spectra.

A Feasible Method for Fabricating Vertical Taper Applied to Spot Size Converter

Spot size converter (SSC) can significantly reduce the coupling loss between single mode fiber and waveguide. Therefore, it has been widely used in photonic integrated circuits (PICs). In this paper, an InGaAs/InP vertically tapered SSC has been proposed and fabricated with a length of 1000 μm and a height of 1 μm. We proposed a feasible way of fabricating the SSC by contact lithography. According to the relationship between the exposure dose and the residual photoresist thickness. Choose the appropriate exposure dose, multiple overlays to form a step shape on the photoresist, and the thermal reflow makes it form a slop. This method can be applied to fabricate vertical tapers of various sizes. 1 dB alignment tolerance is enhanced by this vertical taper spot size converter, which achieves 1 dB alignment tolerances ±2.5 μm (horizontal) and ±3.0 μm (vertical). The insertion loss is reduced from 1.8 dB to 1.0 dB.

A Theoretical Sub-0.1 dB Loss Single Mode Fiber-To-Chip Edge Coupler for Silicon Nitride Waveguides

A low loss optical interconnection between optical fibers and photonic integrated circuits is critical for high performance photonic systems. In the past decade, spot size converters, subwavelength waveguide grating (SWG) structures, and different refractive index materials have been applied to allow efficient coupling between the fiber and the photonic chips. However, it is still challenging to achieve low-loss coupling when interfacing high index contrast waveguides such as SiN with SMF-28 fibers. In this work, we report a multilayer edge-coupler using SiOxN materials with different indices to allow for efficient edge coupling between SMF-28 fiber and SiN single mode waveguides. A coupling loss of 0.068 dB for the TM mode was achieved theoretically at a 1550 nm wavelength, with a 1 dB alignment tolerance offset of 2.4 μm.

3D-printed facet-attached optical elements for connecting VCSEL and photodiodes to fiber arrays and multi-core fibers.

Multicore optical fibers and ribbons based on fiber arrays allow for massively parallel transmission of signals via spatially separated channels, thereby offering attractive bandwidth scaling with linearly increasing technical effort. However, low-loss coupling of light between fiber arrays or multicore fibers and standard linear arrays of vertical-cavity surface-emitting lasers (VCSEL) or photodiodes (PD) still represents a challenge. In this paper, we demonstrate that 3D-printed facet-attached microlenses (FaML) offer an attractive path for connecting multimode fiber arrays as well as individual cores of multimode multicore fibers to standard arrays of VCSEL or PD. The freeform coupling elements are printed in situ with high precision on the device and fiber facets by high-resolution multi-photon lithography. We demonstrate coupling losses down to 0.35 dB along with lateral 1 dB alignment tolerances in excess of 10 μm, allowing to leverage fast passive assembly techniques that rely on industry-standard machine vision. To the best of our knowledge, our experiments represent the first demonstration of a coupling interface that connects individual cores of a multicore fiber to VCSEL or PD arranged in a standard linear array without the need for additional fiber-based or waveguide-based fan-out structures. Using this approach, we build a 3 × 25 Gbit/s transceiver assembly which fits into a small form-factor pluggable module and which fulfills many performance metrics specified in the IEEE 802.3 standard.

The edge-coupler of fiber-to-chip with ultra-low coupling loss based on double-layer silicon waveguides

The edge-coupler of fiber-to-chip with ultra-low coupling loss is demonstrated on SOI platform. The edge-coupler is consisted of the cantilevered SiO2 waveguide, the amorphous silicon (α-Si) nano taper and the crystal silicon (c-Si) nano tapers. The thin α-Si layer is deposited on the c-Si layer to improve the pattern matching with fiber. The optical input signal from the optical fiber is launched into the suspended SiO2 waveguide, then coupled into the α-Si nano taper at the center of the SiO2 waveguide, and finally coupled into the c-Si nano taper. We characterized the cantilevered edge-coupler using cleaved single-mode optical fiber with a mode field diameter of 10.5 μm. The measured coupling loss is as low as -1.7 dB per facet for TE mode without index matching liquid at 1550 nm. The 1 dB bandwidth is more than 100 nm with 1 dB alignment tolerances of ±2.0 μm in both horizontal and vertical directions. Besides, potential hybrid optical integration could also be allowed with this results in the future.

Comparison of Fiber-to-Waveguide Couplers in Point Diffraction Interferometer Based on Waveguide Reference Wavefront Source

To improve the power of reference wavefront generated by reference wavefront source (RWS) based on silicon nitride (SiN) waveguide in point diffraction interferometer (PDI), we design the Y-branch coupler and grating coupler, and then compare the maximum coupling efficiency, 3 dB fabrication tolerance, 3 dB alignment tolerance and polarization dependent loss of two couplers. Our results show that grating coupler has higher coupling efficiency, lower etching difficulty and alignment difficulty, while Y-branch coupler has lower coating difficulty. To get the maximum efficiency, mode in the fiber must be TE mode no matter for grating coupler or Y-branch coupler. This paper improves the power of the wavefront by selecting appropriate fiber-to-waveguide couplers for waveguide RWS. PDI based on the power improved waveguide RWS is expected to be used in many measurement fields.

Low-loss fiber-to-chip couplers with ultrawide optical bandwidth

Providing efficient access from optical fibers to on-chip photonic systems is a key challenge for integrated optics. In general, current solutions allow either narrowband out-of-plane-coupling to a large number of devices or broadband edge-coupling to a limited number of devices. Here we present a hybrid approach using 3D direct laser writing, merging the advantages of both concepts and enabling broadband and low-loss coupling to waveguide devices from the top. In the telecom wavelength regime, we demonstrate a coupling loss of less than −1.8 dB between 1480 nm and 1620 nm. In the wavelength range between 730 nm and 1700 nm, we achieve coupling efficiency well above −8 dB which is sufficient for a range of broadband applications spanning more than an octave. The 3D couplers allow relaxed mechanical alignment with respect to optical fibers, with −1 dB alignment tolerance of about 5 µm in x- and y-directions and −1 dB alignment tolerance in the z-direction of 34 µm. Using automatized alignment, many such couplers can be connected to integrated photonic circuits for rapid prototyping and hybrid integration.

Pluggable Single-Mode Fiber-Array-to-PIC Coupling Using Micro-Lenses

Single-mode optical coupling between fiber and photonic integrated circuit (PIC) requires precision alignment and bonding, and significantly adds to the cost of photonic packaging. This article describes how a pair of micro-lens arrays—one on the Si-PIC and the other on the fiber-array—can be used to achieve fiber-to-PIC grating-coupling with an insertion-loss of 1.7 dB (i.e., a coupling efficiency of 68%) at 1300 nm, and a 1 dB alignment tolerance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm ~30~\mu \text{m}$ </tex-math></inline-formula> . Such relaxed tolerances allow for a “pluggable” connector to have a make-break insertion-loss reproducibility of 0.2 dB (one standard deviation).

High-Bandwidth and Large Coupling Tolerance Graded-Index Multimode Polymer Waveguides for On-Board High-Speed Optical Interconnects

Optical interconnects have attracted significant research interest for use in short-reach board-level optical communication links in supercomputers and data centres. Multimode polymer waveguides in particular constitute an attractive technology for on-board optical interconnects as they provide high bandwidth, offer relaxed alignment tolerances, and can be cost-effectively integrated onto standard printed circuit boards (PCBs). However, the continuing improvements in bandwidth performance of optical sources make it important to investigate approaches to develop high bandwidth polymer waveguides. In this paper, we present dispersion studies on a graded-index (GI) waveguide in siloxane materials designed to deliver high bandwidth over a range of launch conditions. Bandwidth-length products of >70 GHzxm and ~65 GHzxm are observed using a 50/125 um multimode fibre (MMF) launch for input offsets of +/- 10 um without and with the use of a mode mixer respectively; and enhanced values of >100 GHzxm are found under a 10x microscope objective launch for input offsets of ~18 x 20 um^2. The large range of offsets is within the -1 dB alignment tolerances. A theoretical model is developed using the measured refractive index profile of the waveguide, and general agreement is found with experimental bandwidth measurements. The reported results clearly demonstrate the potential of this technology for use in high-speed board-level optical links, and indicate that data transmission of 100 Gb/s over a multimode polymer waveguide is feasible with appropriate refractive index engineering.

Mode size converter between high-index-contrast waveguide and cleaved single mode fiber using SiON as intermediate material.

High-index-contrast (HIC) waveguide such as Si and Si3N4 has small mode size enabling compact integration. However, the coupling loss with single mode fiber is also remarkable owning to the mode mismatching. Therefore, mode size converter, as the interface between HIC waveguide and optical fiber, takes an important role in the field of integrated optics. The material with refractive index (RI) between HIC waveguide and optical fiber can be used as a bridge to reduce the mode mismatching loss. In this letter, we employ silicon oxynitride (SiON) with RI about 1.50 as the intermediate material and optimize the structure of the SiON waveguide to match with cleaved single mode fiber and HIC waveguide separately. Combined with inverse taper and suspended structure, the mismatching loss is reduced and the dependence to the dimension of the structure is also released. The coupling loss is 1.2 and 1.4 dB/facet for TE and TM mode, respectively, with 3 dB alignment tolerance of ± 3.5 μm for Si(3)N(4) waveguide with just 200 nm-wide tip. While for Si waveguide, a critical dimension of 150 nm is applied due to the higher index contrast than Si(3)N(4) waveguide. Similar alignment tolerance is realized with coupling loss about 1.8 and 2.1 dB/facet for TE and TM mode. The polarization dependence loss (PDL) for both platforms is within 0.5 dB.

Compact Multimode Polymer Waveguide Bends for Board-Level Optical Interconnects

Multimode polymer waveguides are promising for use in board-level optical interconnects. In recent years, various on-board optical interconnection architectures have been demonstrated making use of passive routing waveguide components. In particular, 90° bends have played important roles in complex waveguide layouts enabling interconnection between non co-linear points on a board. Due to the dimensions and index step of the waveguides typically used in on-board optical interconnects, low-loss bends are typically limited to a radius of ~ 10 mm. This paper therefore presents the design and fabrication of compact low-loss waveguide bends with reduced radii of curvature, offering significant reductions in the required areas for on-board optical circuits. The proposed design relies on the exposure of the bend section to the air, achieving tighter light confinement along the bend and reduced bending losses. Simulation studies carried out with ray tracing tools and experimental results from polymer samples fabricated on FR4 are presented. Low bending losses are achieved from the air-exposed bends up to 4 mm of radius of curvature, while an improvement of 14 μm in the 1 dB alignment tolerances at the input of these devices (fibre to waveguide coupling) is also obtained. Finally, the air-exposed bends are employed in an optical bus structure, offering reductions in insertion loss of up to 3.8 dB.

Regenerative polymeric bus architecture for board-level optical interconnects

A scalable multi-channel optical regenerative bus architecture based on the use of polymer waveguides is presented for the first time. The architecture offers high-speed interconnection between electrical cards allowing regenerative bus extension with multiple segments and therefore connection of an arbitrary number of cards onto the bus. In a proof-of-principle demonstration, a 4-channel 3-card polymeric bus module is designed and fabricated on standard FR4 substrates. Low insertion losses (≤ -15 dB) and low crosstalk values (< -30 dB) are achieved for the fabricated samples while better than ± 6 µm -1 dB alignment tolerances are obtained. 10 Gb/s data communication with a bit-error-rate (BER) lower than 10(-12) is demonstrated for the first time between card interfaces on two different bus modules using a prototype 3R regenerator.

A spot-size converter-integrated 1.3 µm TM mode LD for coupling with surface-plasmon polariton waveguides

A 1.3 µm transverse magnetic (TM) mode laser diode (LD) with a spot-size converter was fabricated and its various characteristics were investigated. At 100 mA, the power ratio of 33 dB was obtained between TM and TE mode using 0.68% tensile strained quantum wells. The device shows far field angles of 10.4° and 11.7° in horizontal and vertical directions, respectively, resulting in 3.1 dB coupling loss with a surface-plasmon polariton (SPP) waveguide (WG). The 3 dB alignment tolerance with the SPP-WG is 7.2 µm and 6.8 µm, respectively. 3 dB bandwidth is 6.3 GHz and clear eye-opening is confirmed under 7 Gbps NRZ transmission.

Enhancing alignment tolerance of silicon waveguide by using a wide grating coupler

We fabricate a 20 um wide grating coupler for a single-mode thermally-expanded-core (TEC) fiber, in order to enhance positional tolerance in alignment. The minimal coupling loss is measured at 5 dB per facet and the optical 3 dB bandwidth is measured at 40 nm. The 3 dB alignment tolerance is measured at +-7.5 microm in horizontal direction and +290 microm in vertical direction. The 1 dB alignment tolerance is measured at +-4.2 microm in horizontal direction and +125 microm in vertical direction. The alignment tolerance is enhanced twice in horizontal direction and four times in vertical direction, compared with the coupling of a standard single-mode fiber to a standard 10 microm wide grating coupler which is also fabricated in this experiment.

Spot-size conversion using uniform waveguide sections for efficient laser-fiber coupling

Using numerically simulated results, it is shown that an efficient laser-to-fiber coupling is possible by incorporating a uniform spot-size converter (SSC) with two nonidentical but phase matched optical waveguides. Using a "diluted" waveguide with a low index contrast, the power coupling loss can be significantly reduced from 10.4 dB to only 1.15 dB and at the same time 1.0 dB alignment tolerances can also be improved to achieve /spl plusmn/1.8 /spl mu/m. Besides the improved coupling, the beam divergence of the far-field is also reduced considerably to 9/spl deg/, accompanied by a significant lowering of the reflected power from the fiber interface.

1.55 ?m spot-size converter integrated-laser diode fabricated by selective-area metalorganic vapor-phase epitaxy

A 1.55 μm spot-size converter integrated-laser diode (SSC–LD) with a vertically tapered-thickness waveguide was fabricated by using selective-area metalorganic vapor-phase epitaxy (MOVPE). A tapered-thickness profile and PL wavelength shift as large as 238 nm in the waveguide were obtained by using an exponentially tapered shape mask. The SSC–LD exhibited a high slope efficiency of 0.31 W/A and a beam divergence of 6.9°×12.4° without facet coating. The coupling loss between the LD and SMF was measured to be 2.76 dB, and the 1 dB alignment tolerances were ±2.4 μm for the horizontal direction, ±2.0 μm for the vertical direction, and 13 μm for the longitudinal direction. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 25: 300–302, 2000.

1.3 µm beam-expander integrated laser grown by single-step MOVPE

A new structure for a 1.3 /spl mu/m-wave length beam-expander integrated ridge-waveguide laser is demonstrated that does not require a regrowth step. A narrow beam divergence of 5.4/spl deg/ (lateral) and 18.9/spl deg/ (vertical) enables a laser diode to be coupled to a cleaved singlemode fibre with an efficiency as high as 33% with a 1 dB alignment tolerance of /spl plusmn/2.4 /spl mu/m laterally and +1.5 /spl mu/m vertically. >

Monolithic integration of DFB laser with spot-size transformer for highly efficient laser fibre coupling

The monolithic integration of a mushroom type InGaAs-InGaAsP MQW DFB laser with a spot-size transformer based on a two-layer lateral taper is reported. Highly efficient coupling to a butt-joined dispersion shifted singlemode fibre with a coupling loss down to 0.9 dB and large alignment tolerances was achieved, maintaining the good spectral characteristics of an isolated DFB-laser structure. >

Low-loss fibre-matched diluted multiple quantum well waveguides

InP/InGaAsP diluted multiquantum well waveguides have been developed for butt-coupling with cleaved standard single-mode fibres of 11 µm diameter. Coupling losses per facet as low as -1.2 dB, with -1 dB alignment tolerances of ±2.5 µm, have been measured at 1.55 µm for both TE and TM modes.