PCB integrated waveguides Launching light into highly multi mode structures cd.

2. FUNDAMENTALS

2.1. WAVEGUIDE-IN-COPPER TECHNOLOGY

One out of a variety of technologies for the fabrication of board level optical interconnections is the “waveguide-in-copper” technology. It was developed at the Electronics Technology Laboratory of TU Dresden in collaboration with Fraunhofer-Institute for Reliability and Micro­Integration (FhG IZM).All investigations described



in this paper have been performed with samples prepared in the “waveguide-in-copper technology”. For this reason it will be explained more detailed. Yet the obtained results will in general apply to other PCB integrated waveguides. The unique feature of the waveguide-in-copper technology is, that it exclusively deploys manufacturing steps, which are present standard processes in PCB manufacturing. A generic wet etching on a conventional FR4-substrate will result in isotropic removal and herewith a semicircle-shaped cross-section of the channels. Several steps of applying UV-curing polymer with different refractive indices follow to finally form a waveguide as can be seen in figure 1. The sample is now ready to be implemented to a multilayer PCB. The coupling endfaces are prepared with a grinding process.

2.2. ATTENUATION MEASUREMENTS

The optical attenuation or loss is a crucial quality characteristic of an integrated polymer waveguide. This parameter can be used to describe the functional behavior and reliability of the light guiding structure. The measurement will be performed in two steps. The attenuation is calculated from the detected power and the reference reading. Respectively to the technologies and applications there will be fundamental differences compared to fiber assessment, because of broken symmetries, prepared endface, interface properties and so on. These are reasons why the theoretical basis for analyzing these structures is not yet fully available. It is known though that the launching conditions tremendously affect the mode distribution inside the waveguide. Many issues need to be followed in order to achieve repeatable results. To determine the optical loss as a physical parameter of the waveguide it needs to be independent from the sample length [1]. This can be only accomplished in the case of equilibrium mode distribution (EMD) – a steady state of power distribution among the carried modes [5]. As this is hard to gain, the launching will have to be as close to EMD as possible. For this reason a modemixed multimode fiber (MMF) was applied and compared to a measurement with a single mode fiber working at a wavelength of 850 nm (SMF@850 nm) that exhibits a very stable gaussian power distribution. Additionally the coupling conditions for example the quality of endfaces and the index matching become very important. All those effects make the assessment of the waveguide attenuation a challenge.

2.3. SET-UP DESCRIPTION

Figure 2 shows a simplified scheme of the arrangement applied for multiple measurements. It is used for characterization of a number of effects concerning integrated waveguide.

The set-up consists of three blocks:
- Motion controlling,
- Data acquisition (DAQ),
- Optical transmission line.

DC Motion motors allow for adjusting and scanning movement with accuracy of 1 μm. The data acquisition set combines: integrating sphere with InGaAs/Si detector, power meter and PCI DAQ board. Both blocks are automated by a computer system with signal transmission based on BNC and RS232 protocol. DAQ and control of the set-up is achieved with LabVIEW environment. The optical path with laser sources for 850 nm and 633 nm is accomplished with different types of fibers. The above-described set-up is mainly used for spatial scans and can be monitored with a vision system. Because of the modular character it can be easily rebuilt for numerous measurement purposes.