Photonics, the science of generating, detecting and manipulating particles of light, forms the basis of many of the technologies we use in our everyday life. In the evolving branch of integrated photonics, increasingly large and complex optical circuits can be built on the surface of a chip. The majority of these circuits are currently designed for specific applications in telecommunications, sensing and other fields. However, photonic technology is finding application in more and more areas. Because of this, we need “‘general purpose’ optical circuits that can be programmed directly by the end user,” according to a press release posted on the website of the Polytechnic University of Milan, a project partner in the EU-funded Super-Pixels project.
All photonic circuits are ASICs at present, if you want to change protocol you need to make a new chip. Photonic chip functionality is usually hard-wired by design, however reconfigurable optical elements would allow light to be routed flexibly, opening up new applications in programmable photonic circuits. A field-programmable gate array (FPGA) is an integrated circuit designed to be programmed or configured by a customer or a designer after manufacturing – hence “field-programmable”— be it for speech recognition, computer vision, cryptography, or something else. In the heart of an FPGA is a large array of logic blocks that are wired up by reconfigurable interconnects, allowing the chip to be reconfigured or programmed via specialized software.
Programmable photonic circuits or reconfigurable photonics are the optical version of field-programmable gate arrays (integrated electronic circuits designed to be configured by a customer after their manufacture). A photonic circuit that can be reconfigured using software to perform different functions. The ability to control light in a silicon chip could enable novel applications in programmable photonic circuits, including applications for optical testing and data communication. The fact that such an optical circuit can be used for many different applications makes the technology more accessible and reduces costs, as well as research and development time.
University of Southampton Researchers have developed a new way to build power efficient and programmable integrated switching units on a silicon photonics chip. The new technology is poised to reduce production costs by allowing a generic optical circuit to be fabricated in bulk and then later programmed for specific applications such as communications systems, LIDAR circuits or computing applications.
Programmable optical chips are a key technology for integrated microwave photonics. They enable flexible and stable signal processing operations on miniaturized chips with ultimate control precision and potential for low-cost fabrication. Integrated microwave photonic filters are an important function for photonic-assisted RF front-ends which opens a path to overcome the bandwidth limitation of the current digital electronics. Programmable optical chips enable such filters with high function flexibility, continuous tunability, and sharp frequency selectivity, which facilitate innovations of a wide range of new applications.
Researchers at the Polytechnic Univeristy of Valencia are at the forefront of a revolution in microwave photonics, bringing us the first all-purpose programmable optical chips (currently laboratory scale only).
Improvements in both silicon photonics and III–V compound semiconductor technology, such as InP and GaAs are now enabling the development of optical equivalent of an FPGA. Researchers are starting to build designs of programmable optical signal processors on a chip by cascading arrays of coupled waveguide structures that feature phase shifters to control the flow of light through the array and thus support reconfigurability.
“Silicon photonics is capable of integrating optical devices and advanced microelectronic circuits all on a single chip,” said research team member Xia Chen from the University of Southampton. “We expect configurable silicon photonics circuits to greatly expand the scope of applications for silicon photonics while also reducing costs, making this technology more useful for consumer applications.”
In 2016 Researchers at the Universitat Politècnica de València (UPV) design a generic optical chip that can be programmed on demand to carry out any number of functions.
In fact, they first proposed the concept of an all-purpose optical processor three years ago, putting them at the forefront of the state-of-the-art in the field of microwave photonics. Today, the team led by Prof. José Capmany at the UPV’s Institute of Telecommunications and Multimedia Applications (iTEAM) are in the process of defining a starter chip that can be programmed to offer two functions: filter and instant frequency measurement. Manufactured from indium phosphide, they expect to be designing chips that incorporate between five and six functionalities by the end of the year.
Campany underlines the importance of this achievement: “This the first step towards a complete overhaul of the telecommunications sector. The programmable optical chip, once available on the market, will trigger an exponential drop in chip manufacturing costs. In the not-too-distant future, we will have generic optical processors with a standard configuration and universal core which will be programmable on demand. Manufacturing processes will be unified -whatever they are used for, the chip will be manufactured in the same way- which will lead to massive savings across the board”.
Besides the economic advantages, the programmable chip will also usher in efficiency gains and all-important versatility: “Let’s imagine, for instance, that I want to manufacture three types of circuits: one that takes a delay line, another to generate signals and a third to filter. Until now we needed to manufacture three different types of chips. Now we can use the same hardware platform for all three functions”.
Programmable photonic circuit technology
According to the press release, the most frequently used strategy for creating these circuits is to set up interconnected optical tracks (mesh) on a photonic chip. Since the mesh’s nodes can be configured and managed using software and algorithms, the light can be made to perform the desired function rapidly and without consuming much energy. Changing the circuit’s function then simply entails reprogramming it.
“With the same optical circuitry, we can choose to carry out mathematical operations, implement artificial intelligence and machine learning systems, create networks of on-chip sensors and imaging systems, and manipulate quantum states of light. Furthermore, the rapid convergence between electronic and photonic technologies will soon lead to all this being possible on the same silicon chip,” stated Polytechnic University of Milan’s Prof. Francesco Morichetti in the same press release.
The potential of such programmable photonic circuits is the focus of a study supported in part by Super-Pixels and the EU-funded projects PhotonICSWARM, MORPHIC, UMWP-CHIP and FPPAs. Published in the journal ‘Nature’, the study discusses recent developments in this emerging technology and potential applications in various fields.
Prof. Andrea Melloni, who is head of the Photonic Devices Lab at the same university, observed: “While it is still premature to think of photonic devices so advanced that they operate on light in a similar way to current electronic processors, we have come very close to the possibility of creating programmable photonic co-processors that are capable of processing data in the optical domain to perform classes of operations extremely efficiently.” The Super-Pixels (Super-Pixels: Redefining the way we sense the world.) project intends to co-develop a new-generation sensor platform that will revolutionise the way we process light. The project ends in August 2022.
Configurable circuit technology poised to expand silicon photonic applications
In The Optical Society (OSA) journal Optics Express, researchers led by Graham Reed demonstrate the new approach in switching units that can be used as building blocks to create larger chip-based, programmable photonic circuits. “The technology we developed will have a wide range of applications,” said Chen. “For example, it could be used to make integrated sensing devices to detect biochemical and medical substances as well as optical transceivers for connections used in high-performance computing systems and data centers.”
The new work builds on earlier research in which the investigators developed an erasable version of an optical component known as a grating coupler by implanting germanium ions into silicon. These ions induce damage that changes silicon’s refractive index in that area. Heating the local area using a laser annealing process can then be used to reverse the refractive index and erase the grating coupler.
In the Optics Express paper, the researchers describe how they applied the same germanium ion implantation technique to create erasable waveguides and directional couplers, components that can be used to make reconfigurable circuits and switches. This represents the first time that sub-micron erasable waveguides have been created in silicon. “We normally think about ion implantation as something that will induce large optical losses in a photonic integrated circuit,” said Chen. “However, we found that a carefully designed structure and using the right ion implantation recipe can create a waveguide that carries optical signals with reasonable optical loss.”
Building programmable circuits
They demonstrated the new approach by designing and fabricating waveguides, directional couplers and 1 X 4 and 2 X 2 switching circuits, using the University of Southampton’s Cornerstone fabrication foundry. Photonic devices from different chips tested both before and after programming with laser annealing showed consistent performance.
Because the technique involves physically changing the routing of the photonic waveguide via a one-time operation, no additional power is needed to retain the configuration when programmed. The researchers have also discovered that electrical annealing, using a local integrated heater, as well as laser annealing can be used to program the circuits.
The researchers are working with a company called ficonTEC to make this technology practical outside the laboratory by developing a way to apply the laser and/or electrical annealing process at wafer scale, using a conventional wafer prober (wafer testing machine), so that hundreds or thousands of chips could be programmed automatically. They are currently working on integrating the laser and electrical annealing processes into such a wafer-scale prober — an instrument found in most electronic-photonic foundries — being testing at the University of Southampton.
Phase change technology
Phase change technology offers proven nonvolatile electronic programmability; however, the materials used to date have shown prohibitively high optical losses, which are incompatible with integrated photonic platforms.
The fundamental benefits of using nonvolatile phase change materials (PCMs) in reconfigurable photonics have resulted in their extensive exploration for photonic modulation and resonance tuning, with Ge2Sb2Te5 (GST) and, more recently, Ge2Sb2Se4Te1 (GSST) being the most considered materials. The multiple nonvolatile states these materials offer provide unparalleled energy per bit operation in a highly stable platform.
In several recent studies, the antimony-based chalcogenides Sb2S3 and Sb2Se3 have been identified as a family of highly promising ultralow-loss PCMs for photonic applications. The materials exhibit no intrinsic absorption losses (k < 10−5) in either phase over the telecommunications transmission band and show low switching temperatures around 200°C for Sb2Se3 and 270°C for Sb2S3 while remaining nonvolatile at operating temperatures.
The transition between crystalline and amorphous phases changes the arrangement of the chemical bonds in the material, which in effect results in the change of optical properties such as the complex index of refraction. Furthermore, the proximity of their refractive index to that of silicon allows straightforward direct integration of PCM patches onto standard silicon-on-insulator (SOI) integrated photonic platforms with excellent mode matching to the SOI waveguide.
Researchers from Southampton, UK. led by Matthew Delaney have demonstrated the capabilities of the new phase-change technology in providing low-loss programmable optical phase control in PICs. This approach is expected to open many new applications in post fabrication device tuning, programmable weight banks, unitary matrix operations, and, ultimately, all-optical field-programmable arrays
Nonvolatile PCM-based approaches may hold an energy advantage compared to active devices, which require driving voltages to maintain a configuration and hence could offer opportunities for reduced power consumption PICs. The use of nonvolatile programmable MZI with large optical phase shifts exceeding 2π is of interest. Referred to as optical true time delay (OTTD), programmable optical path lengths are of interest in microwave photonics , optical fast Fourier transforms, Fourier transform sensors, and integrated quantum circuits. In microwave photonics and emerging terahertz (6G) applications, OTTDs are used for beam formation using phase array antennas and in optical communications for signal synchronization, equalization, buffering, and time division multiplexing. In quantum optics, precise tuning of optical path length differences is critical for maintaining high multiphoton coherence, write the authors.
The authors concluded that new PCM Sb2Se3 allows the decoupling of optical phase control from amplitude modulation seen in the conventional PCMs. The advances in device footprint and energy consumption of this approach compared to conventional cascaded switch fabrics could enable a range of complex photonic circuits needed for applications such as on-chip light detection and ranging, photonic quantum technology, artificial intelligence hardware, or optical tensor cores of the future while providing a powerful postfabrication programming technique for high-volume PIC ecosystems. Our demonstrated technique provides a general approach that could be easily extended to larger devices and could ultimately achieve a platform for a universal optical chip technology.
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