Optical interconnections and sensors

Over the past years CMST has built up an optical technology platform to allow for the integration of optical waveguides, light sources, detectors, and electronic circuitry with both rigid and flexible substrates. Since recently, also stretchable optical links are under investigation.

Our technology allows the establishment of flexible high-speed optical interconnections, aiming at datacom applications. In addition, it offers an integrated solution to the increasing demand for optical sensors to be applied to irregular surfaces or to be folded into compact modules.

Optical waveguides are patterned using standard lithography (Figure 1), or using KrF excimer laser ablation (Figure 2). Both single- and multi-layer structures can be fabricated (Figure 3), the multi-layer approach allowing for an increased integration density and for more flexible routing schemes. Different commercially available polymers with excellent optical, mechanical, and environmental properties are being investigated: Truemode Backplane^TM Polymer [Exxelis], Ormocer^® [micro resist technology] and LightLink [Rohm & Haas].

Figure 1 Polymer waveguides defined by standard lithography	Figure 2 Polymer waveguide defined by KrF excimer laser ablation	Figure 3 Cross-section of a two layer optical waveguide structure

Experiments have shown that the flexibility of the optical layers is significantly improved by sandwiching them between two spin-coated Polyimide layers, which absorb all stress and pressure during bending (Figure 4). The resulting stack shows good adhesion and excellent flexible behavior, down to bending radii of 3 mm (Figure 5).

Figure 4 Optical waveguides sandwiched between two Polyimide layers	Figure 5 Excellent flexible behavior of the optical waveguide foil

The waveguides show low propagation losses (<0.15 dB/cm) and low bending losses (<0.15 dB/cm for a bending radius of 15 mm, and <0.25 dB/cm for a bending radius of 8 mm), measured at 850 nm wavelength.

Coupling light in and out of the optical waveguides is achieved by using out of plane deflecting micro-mirrors, fabricated using KrF excimer laser ablation (Figure 6). For multi-layer structures, the metal coated 45° micro-mirrors enable coupling of light between different layers (Figure 7).

Figure 6 45° out of plane deflecting micro-mirror	Figure 7 Cross-section of a two layer structure with metal coated micro-mirrors

The integration of opto-electronic devices into the flexible substrate requires the VCSELs and photodetectors to be thinned down to 25-50 µm, in order to become bendable. A thinning process of individual dies has been established, by combining both lapping and polishing.

The 1x4 VCSEL and photodetector array is placed into a pre-structured cavity (Figure 8, Figure 9), and fixed using an underfill adhesive. A micro-via technology is used for electrical contacting of the embedded opto-electronics, based on CO₂ or Nd-YAG laser drilling, and sputtering. The resulting opto-electronic foil has a very low thickness, and a very high flexibility.  

Figure 8 1x4 VCSEL array embedded in the optical waveguide layer	Figure 9 1x4 photodetector array embedded in the optical waveguide layer

Since recently, also stretchable optical links are studied, aiming at optical tactile sensors. Different sensing schemes are under development: based on crossing waveguides (Figure 10), based on VCSELs with optical feedback, and based on embedded Fiber Bragg Gratings (FBG) in a stretchable silicone matrix (Figure 11).

Figure 10 Optical tactile sensor based on the crossing waveguide concept	Figure 11 FBG embedded in a stretchable silicone matrix