Terahertz (THz) systems represent one of the next frontiers in electronics. THz applications are expected to include automotive advanced driver assistance systems (ADAS) for autonomous vehicles, next-generation telephony like 5G and 6G, augmented and virtual reality (AR & VR) technologies for the metaverse, and more. There will be numerous challenges to overcome in the development of THz electronics, and connectors and interconnection technologies will be one of the keys.
The THz band is often defined as frequencies from 0.1 to 3.0 THz, sitting between optical frequencies at the top end and microwave frequencies at the bottom. This FAQ will review the convergence of micro coaxial, waveguide, and fiber optic connections for THz systems and look at how those connectors will be used to bridge between different systems, within systems, and even within integrated circuits and system in package designs (Figure 1).
As frequencies continue to climb upward, using copper interconnects becomes more challenging. One way to increase data rates over copper is to limit the interconnect length. That’s not an uncommon strategy; Ethernet over copper data rates are doubling about every four years. At the same time, the maximum interconnect lengths for Ethernet over copper are being halved. That’s a necessary consequence of the inherent limitations of copper interconnects, but it raises a question about the efficacy of the doubling data rates.
Copper, waveguides, and fiber
While traditional copper interconnects are expected to remain dominant at the most common system frequencies, waveguides and fiber interconnects are gaining in usage. Waveguides can offer an alternative to copper at THz frequencies, especially at the lower end of the band. While waveguides are lossier than fiber interconnects, they can provide lower attenuations than copper. Even at frequencies as low as 75 GHz, an optimized waveguide has been shown to have over 90 dB lower losses than a 1-meter copper backplane. The disadvantages of waveguides, like larger and more costly structures, mostly preclude their use at relatively low frequencies.
As frequencies increase, the relative cost-performance gap between waveguides and copper shrinks. As frequencies approach the threshold that can justify using optical interconnects, waveguides can provide an option that is more robust to misalignment and cost-effective. At the upper end of the THz spectrum, a waveguide interconnect can have several orders of magnitude more tolerance to misalignment compared with optical interconnects and provide much lower losses compared to copper.
The boundaries between copper, waveguides, and optical interconnects are expected to continue shifting, with waveguides becoming more common. One key would be the commercial development of low-cost waveguide fabrication technologies similar to those currently used for twin axial cables. Of course, there is no perfect interconnect technology that can provide a panacea for all combinations of data rates, efficiencies, costs, and other requirements.
Packaging and interconnects
Interconnects are an important dimension of packaging. Other aspects of packaging include a mechanical platform for the constituent parts, varying levels of environmental protection, EMI shielding, thermal management, and so on. For ICs operating at frequencies below the THz range, molded plastic packages with metal balls or beam leads for input/output (I/O) pins provide the needed levels of signal integrity and performance. What works for microwaves will not necessarily work for THz devices. THz packages and interconnects have more constraints related to signal loss, dimensional stability, and fabrication than microwave devices (Figure 2). THz interconnects still emerging and evolving. Solutions based on ceramic technologies are being considered, but the cost is also a factor, and micromachining or 3-D printing may provide lighter-weight and more cost-effective alternatives.
Coax performance and limitations
Simple metal wire interconnects, or even coaxial connections that work well at microwave and lower frequencies, become problematic at THz frequencies. Transmission line structures that are nondispersive and have controlled impedance at high frequencies are needed. It’s also likely that waveguide structures will be required for THz operation. That’s a new area and is accompanied by concerns about the thickness of PC boards, minimum signal line spacing, and dimensions of the signal interface structures in connectors.
For example, conventional 50-Ω coaxial connectors may not be practical at THz frequencies even with modifications. Figure 3 illustrates the expected dimensions of 50-Ω coaxial connectors with respect to the cutoff frequency of the TE11 mode. TE11 mode is important since TE11 mode has the lowest cutoff frequency and is the dominant mode in a circular waveguide. Even with the dielectric between the inner and outer connectors eliminated, the center pin diameter is expected to be 0.2 mm for a 300 GHz cutoff frequency. Such a small center pin in a 50-Ω coaxial connector would not be reliable or durable in practical installations (Figure 3).
On the other hand, rectangular waveguides offer low losses in a highly durable, reliable, and repeatable mating system. Those waveguides may be the preferred connector geometry for THz systems, even though they are relatively bulky and typically have limited operating bandwidths. Waveguides can provide the best tradeoff between size and robustness at THz frequencies. For example, waveguide flange designs used in scientific and military systems have been optimized for repeatability and accuracy of mating. Applying those waveguide flange design concepts to commercial connectors could enable a new generation of THz connectors.
Photonics connectors for THz systems
Cost can be a limiting factor for the commercialization of integrated photonics devices, especially the cost of efficient optical interfaces. A plug-and-play connector has been developed that uses a three-dimensional (3D) polymer structure to connect a fiber and nanophotonic waveguide while achieving mechanical and optical alignment with a tolerance better than ±10 μm. 3D nano printing was used to fabricate the prototype funnel connector directly on foundry-produced diffraction grating couplers.
The funnel walls control light leakage by minimizing the fiber length, and the polymer waveguide is mode matched with the fiber. The light is coupled to a silicon waveguide using a total internal reflection (TIR) mirror and the grating coupler. The TIM mirror is fabricated at the same time as the funnel. The angled facet and the refractive index difference between the polymer and air are used by the TIR mirror to redirect the light into the grating coupler at the needed diffraction angle. The funnel is also a passive mechanical support and routing structure that optically aligns the fiber for edge coupling with the waveguide. By routing the fiber into the funnel independently of its exact position relative to the funnel’s center, wide alignment tolerances are supported (Figure 4).
In addition to the inherent grating coupler loss, the connector exhibited about 0.05 dB excess coupling loss between a high confinement silicon waveguide and a single-mode fiber waveguide. The resulting connector platform is expected to be scalable for various THz applications.
Testing demonstrated that the funnel connector design is robust for fiber mode field diameter (MFD) variations up to ±2 μm over a temperature range of 20 and 100 °C. Loss variations were within 0.6 dB over the entire temperature range. That compares very well with commercial photonic connectors, typically rated to a maximum of 70 °C. In addition, the dimensions and placement requirements of the proposed funnel connector are compatible with current high-volume microelectronic production tools. The platform fabrication can be done at the wafer level and does not require a cleanroom environment.
The THz band occupies 0.1 to 3.0 THz and sits between optical frequencies at the top end and microwave frequencies at the bottom. Depending on the operating frequency and system architecture, micro coaxial, waveguide, and fiberoptic connections can provide links between microwave systems, THz systems, and optical systems. New THz interconnect techniques are being developed to support robust, low-cost, scalable, and high-performance photonics systems.
Broadband Terahertz Metal-Wire Signal Processors, MDPI photonics
Innovations in Terahertz Interconnects, IEEE Microwave Journal
Packages for Terahertz Electronics, Proceedings of the IEEE
Plug-and-play fiber to waveguide connector, Optics Express
Terahertz integrated electronic and hybrid electronic–photonic systems, Nature electronics