B. Van Hoe

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.

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.

Photonic skin for pressure and strain sensing

In this paper, we report on the strain and pressure testing of highly flexible skins embedded with Bragg grating sensors recorded in either silica or polymer optical fibre. The photonic skins, with a size of 10cm x 10cm and thickness of 1mm, were fabricated by embedding the polymer fibre or silica fibre containing Bragg gratings in Sylgard 184 from Dow Corning. Pressure sensing was studied using a cylindrical metal post placed on an array of points across the skin. The polymer fibre grating exhibits approximately 10 times the pressure sensitivity of the silica fibre and responds to the post even when it is placed a few centimetres away from the sensing fibre. Although the intrinsic strain sensitivities of gratings in the two fibre types are very similar, when embedded in the skin the polymer grating displayed a strain sensitivity approximately 45 times greater than the silica device, which also suffered from considerable hysteresis. The polymer grating displayed a near linear response over wavelength shifts of 9nm for 1% strain. The difference in behaviour we attribute to the much greater Youngs modulus of the silica fibre (70 GPa) compared to the polymer fibre (3 GPa).

Assembly of optoelectronics for efficient chip-to-waveguide coupling

This paper presents two solutions to achieve efficient optical coupling between vertical cavity surface emitting lasers and planar optical waveguides, by minimizing the free space optical path length. One approach is based on embedding of optoelectronic chips in polymer layers, and the other approach on flip-chip assembly using micro-bumps. These micro-bumps are defined by laser-induced-forward-transfer (LIFT), a technique in which the bumping material can be transferred from a donor carrier to the waveguiding substrate. In both cases, out-of-plane bending of the light is accomplished by using a 45 degrees micro-mirror interface.

Compact coupling and packaging concepts for flexible and stretchable polymer 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 photodiodes (PDs), the coupling concept 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 potentially low-cost, flexible interconnection scheme.

Photonic Incremental Pressure Sensor Based on Optical Feedback in a Polymer Embedded VCSEL

A highly accurate integrated incremental pressure sensor is presented based on optical feedback in a vertical-cavity surface-emitting laser (VCSEL). This laser chip is embedded in a polymer host material and an external cavity, consisting of a compressible transducer material and a reflecting layer, is fabricated on top. The reflecting layer is coupling part of the emitted laser light back into the internal VCSEL cavity causing self-mixing interferometry. By applying pressure and consequently changing the external cavity length, this interference signal adopts a periodic shape corresponding to half the VCSEL wavelength. The use of unpackaged VCSELs limits the sensor dimensions and minimizes the distance between two adjacent sensing points. A proof-of-principle setup is developed and the integrated sensing principle has been demonstrated using a polydimethylsiloxane transducer layer. A 850-nm VCSEL is used and forces up to 300 mN are applied resulting in a 2-mV peak-to-peak variation of the electrical driving voltage.

Polymer integration of optoelectronic devices in on-board and board-to-board optical communication systems

This paper demonstrates a combined packaging and optical coupling scheme for optoelectronic devices in short distance optical communication systems. The proposed scheme allows an ultra short optical path between the optoelectronic component and the optical waveguide entry. This is achieved by embedding the bare die optoelectronics in the substrate of the optical system. The positioning and alignment of the embedded dies is performed with a scalable passive alignment process based on physical alignment studs which are manufactured with standard photolithography combined with the use of Moiré interference patterns for precise alignment.

Polymer Photonic Sensing Skin

A highly flexible sensing skin with embedded polymer optical fibre Bragg gratings is characterised The response to pressure and strain compare favourably to a similar skin instrumented with silica fibre Bragg grating sensors.