Michael Tryson and Erin Byrne, TE Connectivity
Big Data, a term that seems to encompass many of today’s technology buzzwords including the Internet of Things (IoT), machine to machine (M2M), wireless communication, cloud computing, and more is driving the need for increased bandwidth. Among the problems that became apparent even at 10Gbps is the power consumption of copper-interconnect signals within systems—and the problem is compounded at 25Gbps data rates.
The advent of faster data rates to keep pace with the demands for data poses a number of technical challenges. While this is not a new radical thought, the radical conversation on how to combat these challenges—specifically when it comes to energy consumption and thermal management— is imperative to fuel this data deluge.
Mid-board optics offer a solution
Innovative architectures such as mid-board optics (MBO) have been developed to allow system designers to embed optical transceiver technologies inside computer and communication systems. Embedding high-speed optical transceiver technologies onto traditional server line cards or switch fabrics allows system architects to achieve much-improved airflow and thermal management at the faceplate. Along with higher input/output densities, the MBO architecture can also result higher energy efficiency.
Conventional interconnects providing the interface between high-speed electronics and optical components occur at the faceplate, i.e., optical transceivers and active optical cables. Active optical cables (AOCs) improve the cost-effectiveness of this interconnect by placing the optical engine in the connector and eliminating optical connections to a pluggable transceiver. While this is an important improvement, it doesn’t alleviate the faceplate density problem associated with pluggable modules.
MBO-inside systems mitigate the extra electrical losses encountered at the 25G signaling rate. Figure 1 shows a comparison of faceplate density when optics are moved from the faceplate and onto the system’s printed circuit board. The top of the image shows maximum pluggable I/O density, based upon 400 G active optical cable assemblies where the transition from optical signals to electrical signals occurs. The bottom shows the benefit if I/O is based upon optical connectivity. The optical solution results in substantially higher electrical I/O density while eliminating the cooling problem at the faceplate.
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Figure 1. Faceplate airflow as well as power and heat requirements provide a driving force for intra-chassis MBO that delivers increased signaling density and front panel capacity.
The mid-board optics module shown in Figure 2 provides the key to eliminating the faceplate density problems and the attendant heat management problems. The TE MBO module shown is a 12-channel transceiver capable of transmitting and receiving 300 Gb/s. The electrical interface is provided through a land grid array (LGA) socket on the optical module side and a ball grid array (BGA) on the host board, and allows modules to be placed on a 1-inch grid. The high-speed inputs are DC-coupled to a floating input termination. On the Rx side, the incoming optical signal is converted to a current by the PIN diode. The output stage is current-mode-logic and provides 50 Ohm back-terminations.
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Figure 2. Mid-Board Optics modules shown here provide the key to a high-density interface. The integrated circuit in the center of this MBO assembly is TE Connectivity’s CoolBit optical engine.
Coolbit optical engine technology
Advancing the capabilities of fiber optics to accommodate the demands for 25Gbps and beyond is the challenge that requires the application of advanced optical engine technology. An example of the latest advances is the Coolbit optical engine developed by TE Connectivity. Optical engines, like the CoolBit engine can satisfy both high density and high bandwidth requirements while running at about two thirds the power of conventional solutions.
Improvements in data rate performance and power consumption are achieved by the integration of all of the optical engine elements into a single module. This process begins with the semiconductor fabrication of the VCSEL and Photodiode ICs, moves to the automated wafer assembly of the VCSEL, photodiode and other ICs and ends with operational testing of the wafer resulting in the final engine. Using in-house, automated wafer-level assembly, TE’s Coolbit optical engines are manufactured to semiconductor quality and reliability levels with passive self-alignment of VCSEL, photodiodes and ICs.
Faster and denser pluggable optical modules
While MBO provides the “ultimate solution” to data density, the transition from copper to fiber utilizing active optical cables (AOCs) provides significant advantages, as well. Active optical cable assemblies embed the high-speed optics (see Figure 3) behind two transceiver ends and deliver an electrical interconnection to the other system electronics. This design enables very high speed and high aggregate data rate links at costs significantly below those of separate transceivers and fibers. AOCs offer the benefits of optical with the ease-of-use of copper.
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Figure 3. Active Optical Cables (AOCs) like this replace cooper interconnects with a fiber optic system with optical engines embedded in the connectors.
In addition to having a higher data density and longer transmission distances, a fiber optical link represents significant size and weight savings. Fiber interconnects represent about 1/40th the weight of twinax cabling and consume 1/5 the packaging volume per unit length . Couple these advantages with the large bandwidth (MM >400GHz & SM >10 THz) fiber solutions represent multigenerational future-proof architectures. Fiber can accommodate future generation technology insertion of faster optical transceiver and more data per fiber via wavelength division multiplexing (WDM).
System level thermal advantages take two forms. First, the lower power consumption needed for optical transmission (power/bit-meter) is the primary thermal advantage. Less power dissipation results in less power needed in the conversion to drive the optics, and less heat that needs to be extracted by system cooling mechanisms. Second, the high-density packaging low cross-section of optical fiber produces less obstruction to system cooling, thus cooling schemes are more efficient compared to solutions that encounter airflow restriction with high-density copper cables.
The demands of big data are driving developments in optical communications technology to accommodate the increased bandwidth requirements while addressing the efficiency and thermal management issues. Developments like the TE Coolbit optical engine are enabling systems and architecture advances to address these problems.