Jeroen Missinne

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.

Stretchable optical waveguides

We introduce the concept of mechanically stretchable optical waveguides. The technology to fabricate these waveguides is based on a cost-efficient replication method, employing commercially available polydimethylsiloxane (PDMS) materials. Furthermore, VCSELs (λ = 850 nm) and photodiodes, embedded in a flexible package, were integrated with the waveguides to obtain a truly bendable, stretchable and mechanically deformable optical link. Since these sources and detectors were integrated, it was possible to determine the influence of bending and stretching on the waveguide performance.

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.

Flexible Shear Sensor Based on Embedded Optoelectronic Components

Tactile shear stresses play an important role in the medical field and robotics. To monitor these stresses in situ, there is a need for unobtrusive flexible sensors that can be wrapped around curved surfaces or moving body parts. The presented sensor is based on changing coupling of optical power between a vertical-cavity surface-emitting laser (VCSEL) and a photodiode facing each other and separated by a deformable transducer layer. The required optoelectronic components were embedded in a polymer foil of only 40 μm thick, yielding a very thin and flexible total sensor stack of 250 μm thick. In the linear part of the range (between 2 and 5.5 N), the sensitivity of the prototype was -350 μA/N; the maximum measurable force was 5.5 N. However, by selecting the appropriate deformable sensor transducer material, the sensitivity and range can be tuned for a specific application.

Ultra Small Integrated Optical Fiber Sensing System

This paper introduces a revolutionary way to interrogate optical fiber sensors based on fiber Bragg gratings (FBGs) and to integrate the necessary driving optoelectronic components with the sensor elements. Low-cost optoelectronic chips are used to interrogate the optical fibers, creating a portable dynamic sensing system as an alternative for the traditionally bulky and expensive fiber sensor interrogation units. The possibility to embed these laser and detector chips is demonstrated resulting in an ultra thin flexible optoelectronic package of only 40 μm, provided with an integrated planar fiber pigtail. The result is a fully embedded flexible sensing system with a thickness of only 1 mm, based on a single Vertical-Cavity Surface-Emitting Laser (VCSEL), fiber sensor and photodetector chip. Temperature, strain and electrodynamic shaking tests have been performed on our system, not limited to static read-out measurements but dynamically reconstructing full spectral information datasets.

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.

Free-Form Optics Enhanced Confocal Raman Spectroscopy for Optofluidic Lab-on-Chips

We present an optofluidic lab-on-chip for confocal Raman spectroscopy, which can be used for the analysis of substances. The device strongly suppresses unwanted background signals because it enables confocal detection of Raman scattering thanks to a free-form reflector embedded in the optofluidic chip. We design the system using non-sequential ray-tracing combined with a mathematical code to simulate the Raman scattering behavior of the substance under test. We prototype the device in polymethyl methacrylateby means of ultraprecision diamond tooling. In a proof-of-concept demonstration, we first show the confocal behavior of our Raman lab-on-chip system by measuring the Raman spectrum of ethanol. In a next step, we compare Raman spectra measured in our lab-on-chip with spectra measured with a commercial Raman spectrometer. Finally, to calibrate the system we perform Raman measurements on urea solutions with different concentrations with our proposed experimental proof-of-concept setup. We achieved a detection limit that corresponds to the noise equivalent concentration of 20 mM.

Curing kinetics of step-index and graded-index single mode polymer self-written waveguides

A low-loss polymer medium to interconnect 2 single mode optical fibers is developed and characterized. It consists of a so-called self-written waveguide (SWW) formed by illuminating a photosensitive polymerization mix with light emanating from the fiber, after which the exposed part polymerizes. Depending on the material system used, this waveguide can have a step index or graded refractive index profile. The fabrication process and its effect on the waveguide performance are explained using an empirical model and afterwards experimentally verified. This approach enables easy process monitoring and optimization, effectively resulting in total insertion losses below 0.3 dB for a single mode fiber-SWW-fiber transition at 1550 nm.

Embedded flexible optical shear sensor

Monitoring shear stresses is increasingly important in the medical sector, where the sensors need to be unobtrusive, compact and flexible. A very thin and flexible sensor foil is presented based on the shear stress dependent coupling change of optical power between a laser and photodiode chip that were separated by a deformable sensing layer. These opto-electronic components were embedded in a very thin foil of only 40µm thick. The sensitivity and measurement range can be modified by selecting the material properties of the sensing layer. The sensor response showed to be reproducible and the influence of normal pressure on the sensor was very limited.

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.