G. Van Steenberge

MT-compatible laser-ablated interconnections for optical printed circuit boards

Integration of optical interconnections on a printed circuit board (PCB) is very challenging, since compatibility should be maintained with standard PCB manufacturing technology. This paper describes the use of laser ablation, a technique already used in PCB manufacturing for drilling microvias, as a suitable technique for the fabrication of multimode polymer waveguides, micromirrors, alignment features, and microlenses. A frequency-tripled Nd-YAG laser and a KrF excimer laser are used, both mounted on the same stage, resulting in very high alignment accuracies. This paper demonstrates a parallel optical link over approximately 5-cm-long PCB integrated waveguides, fully connected using a standard MT-based connector. This proves that laser ablation can be a key technology in optical board manufacturing to reach the stringent coupling tolerances.

Ultrathin Optoelectronic Device Packaging in Flexible Carriers

This paper presents the development of an advanced packaging technique for commercially available optoelectronic devices. Vertical cavity surface emitting laser (VCSEL) diodes and photodiodes are thinned down to 20 μm thickness, and are embedded in flexible carriers, resulting in a 75-μm-thin package, which can be bent down to a bending radius of 2 mm. Electrical, optical, and mechanical characterization addresses the influence of thinning and embedding of bare optoelectronic chips on their main properties. Next to the embedded optoelectronics, also electrical ICs like amplifiers and drivers can be housed in the same thin flexible package, using an identical technology and layer build-up. Finally, this new packaging approach is demonstrated in two different integrated sensor applications and in an integrated optical interconnection. For the latter application, also waveguides and optical out-of-plane coupling elements are integrated in the package and the complete system reliability is assessed by accelerated aging tests.

Highly Reliable Flexible Active Optical Links

We present a process to embed commercially available optical material layers into a flexible foil. Patterning of the embedded layers results in highly transparent low loss flexible waveguides. Bending of the foil down to a bending radius of 5 mm causes no additional optical propagation losses. Vertical-cavity surface-emitting laser diodes (VCSELs) and photodiodes are thinned down to 20 ¿m and embedded inside the cladding layer of the waveguides. They are optically coupled with the use of embedded micro-mirrors. The result is a thin foil of 150- ¿m thickness with embedded active optical low-loss links. Accelerated aging tests prove the reliability of the embedded optical links exposed to humidity and temperature cycling.

Laser Ablated Micromirrors for Printed Circuit Board Integrated Optical Interconnections

Optical interconnections offer a possible solution to the bandwidth problems associated with future electrical interconnections. Optics has proven its potential for long-haul communication networks, where it is today a well accepted standard. The integration towards shorter distances is challenging. Compatibility with technologies used in printed circuit board manufacturing is required to implement optical interconnections on board-level in the near future in a cost-effective way. Especially coupling structures, which are used to deflect the light beam over 90deg, pose problems. We propose the use of metallized 45deg micromirrors which are fabricated with the use of laser ablation. This letter gives an overview of the fabrication process and shows experimental results. The root-mean-square surface roughness of the mirror facet is 70 nm or better, depending on the used polymer material. The 45deg angle can be ablated with an accuracy of plusmn1 deg and has a high reproducibility. The mechanical properties of the micromirrors were maintained after a Telcordia 85/85 stability test

Tolerance Analysis for Multilayer Optical Interconnections Integrated on a Printed Circuit Board

Polymer multilayer optical interconnections have gained interest over the past few years in view of their ability to increase the integration density, increase the routing flexibility, and make full use of the characteristics of 2D optoelectronic elements. The alignment between the functional elements in the different optical layers has to be sufficiently accurate in ensuring a high overall efficiency of the system. Numerical simulations have been used as a tool to determine whether laser ablation can be used as an alternative technology for the structuring of the functional elements of optical interconnections into a polymer optical layer. The experimentally achievable alignment accuracies are compared to tolerance ranges for an excess loss ¿ 1 dB obtained from the numerical study. The experimental achievements show that the alignment accuracies fall within the numerical tolerance ranges and have a good reproducibility. Experimental realizations of a two-layer multimode waveguide and inter- and out-of-plane- coupling structures are discussed and shown.

Flip-chip assembly of VCSELs to silicon grating couplers via laser fabricated SU8 prisms.

This article presents the flip-chip bonding of vertical-cavity surface-emitting lasers (VCSELs) to silicon grating couplers (GCs) via SU8 prisms. The SU8 prisms are defined on top of the GCs using non-uniform laser ablation process. The prisms enable perfectly vertical coupling from the bonded VCSELs to the GCs. The VCSELs are flip-chip bonded on top of the silicon GCs employing the laser-induced forward transfer (LIFT)-assisted thermocompression technique. An excess loss of < 1 dB at 1.55 µm measured from the bonded assemblies is reported in this paper. The results of high speed transmission experiments performed on the bonded assemblies with clear eye openings up to 20 Gb/s are also presented.

Flip-chip bonding of vertical-cavity surface-emitting lasers using laser-induced forward transfer

This letter reports the use of the Laser-Induced Forward Transfer (LIFT) technique for the fabrication of indium micro-bumps for the flip-chip (FC) bonding of single vertical-cavity surface-emitting laser chips. The FC bonded chips were electrically and optically characterized, and the successful functioning of the devices post-bonding is demonstrated. The die shear and life-time tests carried out on the bonded chips confirmed the mechanical reliability of the LIFT-assisted FC bonded assemblies.

Laser cleaving of glass fibers and glass fiber arrays

An ultraviolet (UV)-excimer-laser-based cleaving procedure for silica fiber has been developed that enables automated cleaving for high-volume production of fiber-optic assemblies. A selective ablation of the glass in the form of a small rectangular cavity serves as a fracture initiator when the fiber is put under stress. The position of the UV-excimer-laser-induced scratch is very precise. The system provides high-quality cleaves on single-fiber and ribbon configurations. The end angle of the cleaved optical fiber is measured using a noncontact optical interferometer system and was 0.85/spl deg/ for perpendicular cleavages. Insertion loss after splicing is in the range of 0.03 dB, which is compatible with mechanical-cleaved fibers.

Laser ablation as an enabling technology for opto-boards

The use of optical interconnections for short distance interconnections requires the integration of optical layers and structures on printed circuit boards. The paper describes the use of laser ablation as a versatile technique for the fabrication of a wide variety of shucmes and as an enablmg technology for the fabrication of such opto-hoards. It is shown that waveguides, deflecting optics, coupling structures, alignment features and micro-optical elements can all be fabricated in a single process. These processing steps are compatible with the standard process flow of an FR4-based PCB.

Novel coupling and packaging approaches for optical interconnects

We present the design and fabrication of a complete optical interconnection scheme including the optoelectronic package, containing driving Vertical Cavity Surface Emitting Lasers (VCSELs) and read-out photodiode (PDs), the coupling scheme of the fiber or waveguide interconnect and the fabrication technology of the waveguide structures itself. Both the optoelectronic package and the waveguide part are fabricated using polymer materials resulting in a low-cost, flexible interconnection scheme. The optoelectronic package consists of an ultra-thin (20 μm) chip embedded in a flexible polymer stack, connected through metalized microvias using thin film deposition steps. A 45° deflecting micromirror is used to couple this optoelectronic package to an optical fiber or an optical waveguide. The waveguiding structures can be integrated with the coupling plug leading to a 1 step alignment process which significantly reduces the coupling losses. Flexible and stretchable multimode polymer waveguides are also developed to end up with a fully flexible optical interconnect for short (waveguide) or long distance (fiber) communication or for application in sensing.