Optical Interconnects

Optical interconnects are used in telephone (voice) and TV applications, as well in data communications, sensing, and high-speed computing. Optical interconnections for telephony, data, and TV are used in a world-wide fiber optic network that includes industrial park, educational campuses, office buildings, and residences. Intermediate and short-range optical interconnections are also available. These optical interconnects have a range from microns to meters to many tens of meters. They are used to span instrumentation, as well as in rack-to-rack, circuit board, and chip-to-chip applications.

Intermediate-range optical interconnects with a range of meters to many tens of meters use glass or polymer optical fibers. Short-range optical interconnections can be used at the board level, or packaged as stand-alone components that provide transmission and receiver multi-channel-based waveguide links. They may also perform functions such as combining, mixing, or splitting. These links and functions have been configured with optical fibers; increasingly, however, planar polymer waveguides are used because they provide a more versatile media choice.

Short-range optical interconnects are used in high-speed computing, aerospace, and other EMI-sensitive applications; communication systems such as cell phones and laptop cover-to-base links; and as optical sensors for a wide range of measurements (including biosensing). Most of these applications use larger or multimode waveguides in the 20 - 100 micron range. Extremely short-range optical interconnects are used with semiconductors or crystals, and are classified as nanophotonics because they use nanoscale silicon photonics for very high-speed computing. For extremely short-range applications, predominantly small or single-mode waveguides are used, typically in the 2 - 9 micron range.

High computing clock cycles that enable data rates to approach 20 gigabits per second (Gb/S) are a major driver in the implementation of short-range optical interconnects. Above these rates, optics are needed because of EMI. In addition, dense chip I/O that approaches thousands-per-chip compounds heat-dissipation issues. At such high data rates, copper-based electrical connections also experience EMI because of severe signal integrity issues. Optical off-board transmitter/receivers for data transmissions are available for EMI -sensitive links sold commercially. Uses for these optical interconnects include aerospace, ships, and other military vehicle applications where energy, weapons, size, and weight issues are important factors.

Most optical links that span distances of tens of centimeters to hundreds of meters use glass optical fibers. In some cases, polymer optical fibers are used. At shorter lengths, optical fibers are embedded between polymer sheets for multiple-channel board-level routing and distribution. Commercially, these products are referred to as Optiflex links. In some cases, optical fibers are used for a direct interface to light sources and photodetectors. Fiber links with design configuration versatility have very high density, small millimeter-range radius of curve, stackability, ease of assembly, and manufacturability. Such links are desirable for short-range interconnectivity, and are cost-effective when manufactured in high volumes. Polymer waveguides that are imaged in film sheets are increasingly important.

For short-range optical interconnections, there are a number of factors and specifications to be considered. Typically, the required ones for component and/or board/chip level optical-link connectivity include optical system loss (in dB) over the wavelength of interest. This involves waveguides as well as connectivity/coupling losses. Waveguide array density or pitch, connectivity options and connector styles, substrate-bound or flexible, self-supporting guide structures are also important considerations with optical interconnects.

In addition to system loss, performance requirements for optical interconnects include system stability, reliability, lifetime issues under thermal cycling and moisture / humidity ranges, and thermal operational range and projections made using elevated temperatures. Some applications require compatibility with standard industrial electronic assembly processes (like IR solder reflow) to bond the detectors and semiconductor components near, on, or under the waveguides. Waveguide considerations include modal noise, which is sometimes due to waveguide propagation fluctuation or to poor connector coupling and permissible radius of curvature by design and relative to bending needs. There are other specifications are for specific applications. These include polarization sensitivity, open-eye transmission at high data rates, radiation hardness and out-gassing for space applications, high power capability, and physical and mechanical robustness.

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Part Numbers for Optical Interconnects