Compact, Low-Crosstalk, WDM Filter Elements for Multimode Ribbon Fiber Data Links

We have been developing the optical components for a source-routed wavelength division multiplexed (WDM) computer interconnect fabric that uses multi-mode fiber ribbon cable as the transmission medium. We are developing wavelength selectable VCSEL transmitters, interference filters, and a compact broadcast element. Here we report on key results from our interference filter development activities. Our WDM filter approach is based upon post-market machining of the commercially available molded plastic “MT” fiber ribbon connector. We use III-IV semiconductors grown by MBE or MOCVD as the filter materials. The high indices of our thin film materials enable us to use multimode fiber and maintain narrow passbands without the need for micro-optics. We have fabricated both 2-port and 3-port devices based upon this approach. Our current work focuses on 2-port WDM filters suitable for a broadcast and select architecture. Our single-cavity FabryPerot (FP) filters have demonstrated insertion losses of < 2 dB for 4 nm passbands. The maximum crosstalk suppression for the single-cavity FP filters is 18dB To improve crosstalk suppression beyond that attainable with the Lorentzian lineshapes of the single-cavity FP we have investigated some multiple-cavity Fabry-Perot (MC-FP) designs which have a spectral response with a flatter top and sharper passband edges. Filter passband edge sharpness can be quantified by the ratio of the filter 3 dB bandwidth to 18 dB bandwidth This ratio is 0.48 for our multi-cavity filter, three times sharper than the single-cavity FP devices. This device provides a 5 nm tolerance window for component wavelength variations (at 1 dB excess loss) and is suitable for 10 nm channel spacing with 23 dB crosstalk suppression between adjacent channels. The average insertion loss for the MC-l? devices is 1.6 dB. (Average of insertion losses for the 12 fibers in a filter module.) Our current MC-FP filters have a 3-dB width of 7.6nm. Fiber to fiber center wavelength variations within a typical filter module are less than lnm. The MC-FP devices exhibit cross-talk suppression >30dB over a 100nm span. Introduction Parallel optical interconnects based on multimode fiber (MMF) ribbon cable are emerging as a robust, highperformance data link technology which increases channel bandwidth by using linear MMF arrays [l]. While this technology has primarily been implemented as single wavelength point-to-point links, it can be significantly enhanced by wavelength division multiplexing (WDM). WDM enables both increased point-to-point bandwidth as well as more complex interconnect topologies and routing approaches that are particularly attractive for high performance computing platforms [2]. Exploiting the potential richness of WDM interconnects, however, also requires a low-loss routing fabric that includes small footprint wavelength selective filters. Low insertion loss is critical for this technology because the transceivers exhibit link power budgets well below that of telecom WDM systems and because the MMF cabling precludes the use of optical amplifiers. While high performance filters can be realized for single-fiber applications [3,4], achieving highperformance, small footprint devices with ribbon cable is significantly complicated by MMF’s high numerical aperture (NA=0.275) and large core diameter (62.$nn). We have developed both 2-port and 3-port filter modules assembled in simple, robust packages. Our 3-port modules are suitable for add/drop multiplexing while the 2-port devices are suitable for broadcast and select architectures. In this paper we present a summary of our 3-port filter module work [5], and we focus on our recent work with 2-port filter modules. Fig. 1. Schematic of a) 2-port and b) 3-port filter modules Filter Packaging Issues Figure 1 shows the geometry of our filter modules while figure 2 provides a perspective view of a 2-port module to aid in visualization. A high refractive index ( >3.0) interference filter is sandwiched between parallel arrays of 12 MMFs Filter Chip Mounted at 10