## 1 Introduction  
In order to accommodate spiraling digital data consumption and maximize resource utilization, data centers need to embrace more flexible and scalable topologies, which allow greater disaggregation of compute, storage and memory resources. As a consequence of the resulting need for ubiquitous, distance-agnostic, communication links, future data center systems will increasingly need to accommodate a wider range of different optical communication profiles associated with the different optical interconnect tiers in the data center. At the higher switching tiers of the data center, investment in single-mode fiber infrastructures is increasing, as it is considered future-proof and aligns well with emerging silicon photonic integrated circuit based single-mode optical transceiver and switching technologies. A parallel movement is underway at the sub-rack level with low-cost, commodity transceivers enabling a rack or pod-localized multimode infrastructure to emerge. The proliferation of low-cost multimode midboard transceivers now offers the renewed prospect of optical links migrating into traditionally cost sensitive data center sub-system enclosures. Although, in the early stages of their deployment, these transceivers would be internally connected by commercial fiber jumpers, they will set the crucial precedent for system-embedded optical interconnect in high volume commercial systems. This, in turn, will be expected to provide a fresh entry point for electro-optical printed circuit board (OPCB) technologies whereby optical signals may be conveyed along waveguides embedded within the PCB itself. However, the optimal choice of waveguide type will vary depending on the characteristics of the optical communication signal in question, in particular the wavelength and data rate.  
OPCB technology has advanced considerably over the past 15 years with new optical waveguide materials and fabrication techniques providing enhanced performance including reduced signal dispersion through graded refractive index profiles and lower material absorption in different wavelength bands. In particular, two distinct classes of planar graded index multimode waveguide have recently emerged based on polymer [1; 2] and glass [3] materials.  
This paper provides a review the state of the art in planar graded index waveguides. We report on the fabrication of planar graded index polymer waveguides using the Mosquito method [1] devised by Keio University and the photo-addressing method devised by Sumitomo Bakelite [2], and the fabrication of planar graded index glass waveguides using ion diffusion on thin glass foils as developed by Fraunhofer IZM [4]. We comparatively characterize their insertion loss at both 850 nm and 1310 nm and signal integrity at 10.3 Gb/s to assess their suitability with respect to two emerging transceiver classes based on directly modulated multimode 850 nm VCSELs and single-mode 1310 nm silicon photonic integrated circuits.  
The different classes of waveguide were temporarily inserted into an optically disaggregated data storage system using the Serial Attached SCSI protocol and the system performance was characterized to assess their suitability within real future data center environments.  
Early comparative research between polymer and glass planar waveguides was reported in [5], and is taken further in this paper to include all leading varieties of embedded planar graded index waveguide.