Figure 1: End view of OPCB showing (bottom to top) -
FR4 weave, lower cladding, core and upper cladding
layers. Waveguide spacing is 250 µm.

Polymer planar optical waveguides fabricated onto electrical printed circuit boards are a promising technology for high-speed data transfer on computer backplanes. The integrated manufacture of these optical printed circuit boards, OPCB, is currently under investigation[1] and the optical performance of the waveguides is an important aspect in refining the manufacturing process. NPL has developed a system to measures the attenuation and isolation between waveguides with the ability to vary the launch spot size and numerical aperture to fully explore the capability of these multimode waveguides.

The optical measurement system is shown in Figure 2 below. A fibre coupled VCSEL provides the input illumination. The launch optics allows the variation of the launch spot size, which is defined by a range of interchangeable pinholes, P, ranging from 5 to 100 μm. The numerical aperture is controlled by a variable aperture, A, that provides values in the range 0.01 to 0.30. The output from the waveguide under test is imaged onto a CCD array in order to measure the total transmitted power.

Figure 2: Layout of the optical system

The system has been characterised for:

• CCD Linearity
• Magnification
• Spot size and profile
• Spot numerical aperture

The two plots below, with their inserts show the radial profile of the spatial and angular spot intensity. These show reasonable top hat profiles in both spatial and angular domains.

Figure 3: Radial profile of the spatial intensity profile

Figure 4: Radial profile of the angular intensity profile

Measurements have been made on a set of photolithographic waveguides supplied by Dow Corning using their 4140/4141 silicone materials. The waveguide attenuation is separated from the end-face coupling loss by measuring the insertion loss as a function of waveguide length. In the results shown the waveguide attenuation is 0.3 dB/cm with a combined input and output end-face coupling loss of 0.7 dB. Figure 6 shows the cross sectional intensity profile across the centre of three waveguides.

Figure 5: Measured insertion loss vs waveguide length

Figure 6: Intensity profile across the centre of three waveguides, only the left most waveguide was directly illuminated

Acknowledgments and References

Developed in partnership with the industrial and academic members of the IeMRC ‘Flagship Project, Integrated Optical and Electronic Interconnect PCB Manufacturing (OPCB)’[2], the system can be used to fully characterise the loss of planar optical waveguides as a function of the launched spot size and numerical aperture.

[1] D R Selviah, F A Fernández, I Papakonstantinou, K Wang, H Bagshiahi, A C Walker, A McCarthy, H Suyal, D A Hutt, P P Conway, J Chappell, S S Zakariyah and D Milward, 'Integrated optical and electronic interconnect printed circuit board manufacturing', Circuit World, vol. 34, pp. 21-26, 2008.

[2] Integrated Optical and Electronic Interconnect PCB Manufacturing (OPCB) (132 KB)

For further information, please contact: Robert Ferguson