Sandeep Kalathimekkad

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.

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.

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.

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.

Fluorescence-based optochemical sensor on flexible foils

This paper describes the implementation of a low-cost technology platform for fluorescence-based optochemical sensors made up of arrays of multimode waveguides and coupling structures integrated onto a flexible substrate. Such a configuration is ideal for multi-analyte detection owing to a possibility of future integration of different dyes in each waveguides. The presence of light sources, fluorescent sensing elements and photodetectors in a foil platform makes it a compact optochemical sensor, which has wide-range of applications in medical, biochemical, and environmental diagnostics. Flexible lightguides fabricated using soft-lithography based replication techniques, are used in combination with 45° micromirror coupling structures, having a loss of 0.5dB. Fluorescent dyes are incorporated with the lightguides enabling a detection of shift in fluorescence-peaks in contact with gases, which are read-out at the detection. Initial measurements yielded promising results of the waveguides mixed with fluorescent dyes showing response to toluene.

Foil-based optical technology platform for optochemical sensors

This paper describes the development of a low-cost technology platform for fluorescence-based optochemical sensors. These sensors were constructed by incorporating fluorescent sensing elements in the core of multimode waveguides or lightguides, and have applications in medical, biochemical and environmental diagnostics. Flexible lightguides were fabricated either with silsesquioxane-based or PDMS-type optical polymers using photolithography or soft-lithography based replication techniques respectively. Spectral transmission characteristics were measured along with loss values obtained by cut-back measurements for several wavelengths from visible to mid-IR. Propagation losses as low as 0.14dB/cm were measured for 50 x 50 μm2 waveguides. For coupling light in and out of the waveguides, different types of coupling structures, e.g. 45° micromirror plugs were investigated.

Alcohol vapor sensor based on fluorescent dye-doped optical waveguides

This paper presents an alcohol vapor sensor realized using stretchable optical waveguides doped with commercially available fluorescent dyes. The fabrication technology is based on a cost-efficient replication method, employing polydimethylsiloxane materials mixed with the dye Nile red. Upon introduction of ethanol vapors, the fluorescent emission was found to have a wavelength shift of ~20 nm with a response time of ~10 s. Observing the fluorescence intensity of the shifted emission spectrum in a periodically varying environment inside a gas-sensing setup showed a respective variation with introduction of ethanol vapor. The intensity variation also showed the reversibility of the sensor. The sensing platform is found to hold much promise for further integration and multiplexing.