3 Challenges Facing Silicon Photonics Technology
- Getting light into the Photonic Integrated Circuit (PIC)
- Getting light out of the PIC
- Optimizing the performance of the PIC itself
We’ll look at these each in turn, and describe what the difficulty is in each case, before outlining what companies in the industry are doing to address it.
To begin with, light has to be channeled into the PIC. This calls for precise alignment and coupling between external light sources and the on-chip waveguides.
“Coupling” in this context refers to the process of efficiently transferring light energy between the light source and the PIC. If components are well aligned, the light couples effectively between them, with minimal loss of light energy. This also reduces signal degradation, and enables the desired optical functionalities.
Achieving optimal light-load coupling requires intricate engineering to ensure effective energy transfer and minimize losses.
Common Methods for Getting Light into PICs
Several techniques exist to couple light into the PIC and these can be categorized into the location of the laser – whether it is on the PIC or external to the PIC. We discussed each of these in more detail below.
Coupling external light sources into the PIC
There are multiple methods for coupling a light source into the PIC, each with its own advantages and challenges. Waveguides offer a direct pathway for efficient energy transfer but can experience losses from scattering, absorption, and radiation. Tapers (edge couplers), which gradually narrow waveguide dimensions to reduce mode mismatch, are sensitive to fabrication errors. Grating couplers introduce light from free space into waveguides but have limited bandwidth and are polarization-sensitive. Vertical couplers present similar challenges, focusing on vertically stacked waveguides. Lastly, Spot Size Converters adjust light beam sizes between waveguides, optimizing light coupling efficiency at a low cost, but they require precise alignment and offer limited bandwidth.
Each of these methods requires a laser to be placed externally to the PIC and requires precise alignment between the laser and the coupling mechanism to get good coupling efficiency into the PIC. This often includes several manufacturing and alignment steps adding to cost of the overall product.
Indium Phosphide (InP) Integration
InP integration into Silicon has been the subject of sustained attention from the industry, because of the many benefits it offers.
This technique offers high coupling efficiency, high bandwidth, and low loss. And because InP has a direct bandgap, it allows for efficient emission and detection of light. It can be bonded directly on silicon, making integration straightforward.
But even InP comes with its own challenges. The complex design of these components introduces additional steps to the fabrication process, with implications for cost and scalability. And the laser design is specific to the integrator, which does not provide flexibility in the design or supply chain of the laser.
Integrating external (off the shelf) lasers into the Silicon PIC (L3C)
At DustPhotonics, we have developed a novel alternative that integrates off-the-shelf lasers, enabling advantages in performance, cost, power and scalability. This approach involves sourcing any of a number of external, off the shelf lasers, from a number of the existing laser fabs that exist around the world. We then integrate those lasers directly into a trench built on the PIC using low-cost automated assembly machines. We refer to this method as low-loss laser coupling (L3C). The advantage of this is that we can use best-in-breed lasers from multiple sources providing optimal performance and supply chain flexibility while maintaining excellent coupling efficiency.Getting Light out of the PIC
Efficiently extracting light from a Photonic Integrated Circuit (PIC) is also important for integrated optical devices. Various methods enable effective light extraction from the chip.
Many of the same methods used to introduce light into the PIC are deployed to extract it. For example, grating couplers are effective at directing light outward by diffracting it into free space. Tapers gradually expand the confined waveguide mode, facilitating smooth light coupling to larger structures.
At Dust Photonics, we offer two methods for getting light out of the PIC. One includes a direct fiber attachment to the PIC and the other requires external lenses to steer the light into a fiber array.
Optimizing the Performance of the PIC Itself
A great deal of work has also been invested in Process Development Kits (PDKs). These are standardized design toolkits provided by foundries or semiconductor companies. They offer essential building blocks, design rules, and simulation models that enable researchers and engineers to develop and validate integrated circuits based on specific semiconductor processes, such as silicon photonics.
Typical components inside a PIC include splitters, waveguides, modulators, photodetectors, and lasers.
These kits underpin the creation and improvement of Photonic Integrated Circuits. Over time, PDKs have matured, enabling researchers and engineers to craft more sophisticated PICs. But the demand for higher speeds in communications is growing at a faster rate, pushing us closer to the fundamental limits of silicon photonics technology.
The Limits of Silicon Photonics: What Comes Next?
Simply put, silicon photonics is a complex field. While PIC design may seem simple from the outside, many companies have struggled to develop a PIC that can consistently meet all specifications, while being commercially viable at the same time. As such, many companies (including DustPhotonics) have enhanced the PDKs of the fabs in which they operate with their own additional IP blocks. In the world of silicon photonics as it is today, these enhancements are critical to enabling differentiated products.
This article aimed to shed light on the pressing challenges in silicon photonics and various approaches currently being taken to overcome them. The challenges are significant, but there is no doubt that silicon photonics technology still has a long roadmap of exciting enhancements ahead of it and we at DustPhotonics are committed to helping pave that path forward.
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