Optical-chip and board-level interconnections are becoming increasingly interesting for computing applications.1 Optical interconnections have advantages in comparison to their conventional electrical counterparts over metallic wires. Namely, they can help alleviate architectural as well as physical limitations due to signal delays and inductive coupling.2 For data rates of 10Gb/s and over, optical interconnections that use vertical-cavity surface-emitting laser sources (the most common transmitter device for optical gigabit communication), suitable optical setups, and fast detectors are an attractive option for transmission lengths down to a few centimeters. This opens the door for novel applications. Examples include the distribution of high-definition TV video signals, switching and routing in high-performance data centers, and fiber-based data transmission in the automotive sector.
One approach to implementing the optics is the concept of ‘planar-integrated free-space optics’ (PIFSO),3 which we have pursued and demonstrated at the FernUniversität in Hagen for several years.4 In PIFSO, a 3D optical system is folded in such a way that the optical elements (lenses, beam splitters, and so forth) can be placed in a 2D arrangement on one or both surfaces of a substrate. This substrate—glass or any other transparent material—is typically several millimeters thick. The micro-optical elements may be reflective or reflective-diffractive and are fabricated by lithography or other appropriate methods. Within a PIFSO setup, the light propagating inside the substrate follows a zigzag path that can be visualized by photography: see Figure 1. In the example shown, we used a luminescent plastic substrate and a UV laser beam whose path is clearly visible in fluorescent-green light.
Figure 1. Folded laser beam inside a planar-optical mirror-coated glass substrate.
Recently, we demonstrated the combination of a PIFSO system with a microelectromechanical-systems (MEMS) chip made of silicon. The MEMS chip is a Texas Instruments digital mirror device (DMD), which is typically employed in projectors. We chose this device not only because of the large number of individual switching elements that it offers but also because its use allows two technological concepts—PIFSO and MEMS—to be combined. Because the mirrors in a DMD chip can be individually rotated to an on or off state, the chip may seem a less than optimal choice. However, the on/off-switch property makes it possible to perform several interesting studies. For example, in our experiment we used it to demonstrate a type of crossbar interconnection. This process implies connecting any input in an array (a 1D array, in our case) with any position in an output array (also 1D in our experiment).
Figure 2 illustrates the implementation of a PIFSO-MEMS crossbar system. The input comes from a 1D fiber array and propagates inside the PIFSO system between a static mirror (top) and the dynamically switchable DMD plane (bottom). In this configuration, the beam is coupled to the well-defined position of the output fiber matrix. Figure 3 shows the experimental setup with several propagating and switched beams. The optical paths may be optimized so that nearly the same propagation delay occurs for all signals.5
Figure 2. Scheme of light-beam propagation inside the planar-optical geometry between a switchable digital mirror device (DMD1) and a static mirror plane (MIR1). L1, L2: Lenses. G1, G2: Gratings. S1: Substrate (glass). x/y: Horizontal/vertical spatial coordinates.
Figure 3. Top view of the planar-integrated free-space optics setup with multiple-crossed optical channels.
The micro-optical system we have described here has the advantage of optical robustness, especially regarding the alignment and adjustment of its integrated components. Moreover, the large number of micro-mirrors of the DMD allows higher flexibility in beam deflections. Additionally, in a system with a high number of optical channels, the optical approach used offers smart solutions for the signal-crossing problem (preventing crossed beams from interacting).
A prospective next step of our project is micro-optical integration of a 32×32 optical-crossbar switch based on the PIFSO-MEMS combination. We plan to develop such a prototype in cooperation with Euromicron AG, Germany.
We thank Hannes Bauer from Microsens GmbH for supporting the development of the prototype used in our experiments.
Ulrich Lohmann, Jürgen Jahns
FernUniversität in Hagen
Ulrich Lohmann is a member of the Optical Information Technology group. He received his MSc in electrical engineering in 2008 and is currently doing a PhD in the area of micro-optical integration of high-performance optical interconnects.
Jürgen Jahns studied physics at the University of Erlangen where he finished a doctorate in 1982. He worked four years at Siemens in Munich, Germany, and seven years at AT&T Bell Laboratories in Holmdel, NJ. Since 1994, he has been a full professor and chair at the Optical Information Technology group. Together with two colleagues, he recently won the 2010 Rudolf Kingslake award from SPIE.
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