Next-gen silicon photonics

Doris Knauer reports on automatic testing of photonics components

Silicon photonics set the pace on the fibre optic data highway. However, a number of challenges must be overcome for production and testing of the latest generation of silicon photonics wafers. Automated solutions must function reliably under industrial conditions and work around-the- clock with great accuracy.

With silicon photonics (SiP), electronic logic circuits (such as those used for computing) and optical transceivers are integrated into the same wafer. The resulting low-power microchips are then able to transmit data at bandwidths in the Tbit/s range and with low heat generation.

However, because the value of a packaged SiP circuit goes up considerably compared to the raw circuit on a wafer, it is critical to introduce wafer-level testing of each circuit before dicing and packaging. Optical inputs and outputs must be tested with highest precision. They need to be aligned precisely to the optical fibres connected to the test equipment. That is definitely not a trivial matter, because optical waveguides in silicon wafers normally have a core diameter of only 150 to 200nm, and to avoid losses, alignment equipment must have at least 10 times higher resolution.

As a solution supplier for drive technology and precision positioning systems, PI (Physik Instrumente) has taken a close look at this and developed a new complete system for fast fibre alignment, which can align, test and optimise the light input and output at each input and output coupling point in less than one second. This guarantees fast data throughput rates during industrial production.

Application-specific controller

The system consists of two compact identical positioning units that, with a small footprint of 100 × 100mm, require very little installation space. The heart of the system is the application- specific controller with a variety of built-in alignment and tracking algorithms required for optical alignment and which therefore lightens the burden on the higher level control. The host computer communicates with the controller via Ethernet, USB 2.0 or an RS-232 interface.

Positioning is done in two phases. A three-axis positioning stage driven by closed-loop DC motors takes care of the initial task of coarse adjustment; it travels to the respective coupling point over travel ranges of several tens of millimetres with an accuracy of up to 50nm. An additional three-axis positioning unit is then responsible for fine positioning of the fibres.

The system is driven by piezo actuators and achieves resolutions of 1nm with response times in the microsecond range and travel ranges of 100×100×100μm.Thehigh- performance piezo actuators are integrated into guiding systems with FEA-optimised flexure joints and are free of both friction and backlash.

Because actuators, guides and sensor work without wear, the systems are extremely reliable and durable, and are therefore perfectly suitable for industrial requirements in multiple-shift operation (24 hours, seven days a week).

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Doris Knauer is with Physik Instrumente (PI).

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