Similar to consumer electronic devices, optical transceiver modules have followed a path towards size reduction with increased performance and functionality. Driven by data center and service provider applications requiring high-bandwidth, high-density, and low-power optical interconnects, coherent optics suppliers’ goal of meeting these requirements has led to compact, volume-manufacturable designs. These requirements have also helped drive large-scale investments and advancements in silicon photonic (SiPh) opto-electronic integration and packaging. A direct benefit of these efforts includes electrical high-frequency radio frequency (RF) performance improvements inside the module. In this article, we describe how improved RF performance from opto-electronic integration and packaging paves the way for next-generation coherent designs that are expected to operate beyond 100Gbaud, providing network operators an upgrade path without sacrificing reach or stranding network bandwidth when migrating from current-generation solutions.
Importance of high-speed RF interconnects in next-gen optics
Although the two primarily used material systems of silicon (Si) and indium phosphide (InP) for coherent photonic integrated circuit (PIC) designs are both capable of achieving modulation speeds beyond 100Gbaud, the electrical interconnects of the PICs and surrounding RF components such as electrical drivers and transimpedance amplifiers (TIAs) can become a limiting factor in performance and design complexity. Additionally, the RF performance at these speeds is sensitive to impairments along the electrical path between the coherent digital signal processor (DSP) and the PIC along with its surrounding RF components. In traditional designs (Figure 1), the high-speed RF electrical signal may traverse between these components over multiple physical interfaces such as solder ball-grid array (BGA) bumps/balls, substrate, printed circuit board traces, wire bonds, and gold-box lead-frame pins, with each element introducing parasitics and losses that degrade signal integrity. This applies to next-generation optical solutions in general, regardless of whether the transmission technique is coherent or not. Fortunately, techniques to mitigate opto-electronic RF signal impairments already exist in the electronics industry.
Opto-electronic RF interconnect evolution for higher speeds
An advantage of SiPh is its ability to leverage mature, volume-manufacturable processes from the electronics industry, such as component stacking (Figure 2). In component stacking, electrical impairments are reduced due to direct electrical connections between key RF components, creating a robust signal path for extremely high frequency/baud rate operation. In this co-packaged/stacked design, the gold-box packaging is eliminated, the DSP and PIC are tightly co-packaged, and the high-speed Si modulator driver and TIA components are stacked on the PIC.
Figure 2. Illustration of a component stacking configuration.
Figure 3 shows the stacked design has a higher (better) frequency response than the traditional gold-box design. Advanced stacking designs to address interconnect impairments can result in a frequency response that can support well above 100Gbaud coherent transmission.
Architectural comparisons of SiPh and InP
An advantage of InP-based coherent optics is that the laser can be integrated with other optical elements on a common substrate, co-packaged with Si high-speed RF components (Figure 4). In contrast, SiPh-based coherent optics house the thermally sensitive InP laser separately from the more thermally tolerant Si/SiPh components. The co-packaging of the Si/SiPh components enable efficient heat dissipation designs, which can lower overall power consumption. Although the InP laser needs to be located separately, the optical connection is easily achieved using well-known design and manufacturing processes.
Co-packaging of all the Si/SiPh components, as shown in Figure 4, allows stacked high-speed electrical paths that provide high-performance RF interconnects, compared to an InP-based design that may further separate components due to material or thermal mismatches and hermeticity requirements. As previously mentioned, SiPh leverages high-volume manufacturing processes already established in the electronics industry. Acacia’s 3D Siliconization is an example of leveraging these processes to manufacture compact, coherent modules with high-performance electrical RF interconnects.
Continuous integration for higher-speed efficiency
Advancements in SiPh opto-electronic integration and packaging are important for developing the increased performance and functionality needed for data centers and service providers to meet growing bandwidth demands. Analogous to how integration continues to play a large role in the miniaturization of smart phones and watches, the role of integration plays an important role for the next generation of coherent optics to achieve higher speeds in a small footprint. These improvements enable a new era of solutions designed to operate beyond 100Gbaud, providing network operators with the upgrade path they need to compete and thrive in the future.
By Eugene Park at Acacia Communications