Periodic Reporting for period 1 - LIQUORICE (LIQuid-crystal enabled Universal Optical Reconfigurable Integrated Circuit Engineering)
Okres sprawozdawczy: 2022-07-01 do 2023-12-31
In LIQUORICE, we build an operational proof-of-concept generic programmable photonic processor that can eventually be used as a flexible off-the-shelf alternative to difficult-to-design application-specific photonic chips. With this prototype, we will implement a variety of commonly used optical test and measurement functions, targeting the market of scientific and educational lab instrumentation.
Photonic circuits today are custom-designed and fabricated for specific applications. This makes the costly and complicated for new developers to use in new applications. Today, photonics does not have an equivalent to electronic programmable chips (e.g. FPGAs), which can enable rapid prototyping and innovation.
Programmable photonic chips, also called photonic processors, can change this situation. Using a network of on-chip waveguides and electrically tunable elements (so-called optical gates) the paths of light can be reconfigured at run-time to perform different functions. The problem with these new programmable photonic chips is that the electro-optic actuators used consume a lot of power, because they are based on on-chip heaters.
In LIQUORICE, we want to make use of liquid crystals to enable this reconfigurability. Liquid crystals have already been integrated with silicon waveguides before, but never on a large scale. We therefore combine two chip-based technologies to achieve large-scale tuning of photonic circuits. On one hand, we have silicon photonic waveguides, which perform the actual optical function. On the other hand, we take a liquid crystal microdisplay ( liquid-crystal-on-silicon or LCOS chip), where the state of each pixel can be electrically controlled. We bring both together in a sandwich, where the pixels of the LCOS chip can now locally tune the behaviour of the optical waveguides.
Once we have a tunable system, we can program new functionality by sending a different ‘grayscale image’ to the LCOS microdisplay, which will then reconfigure the circuit, so we can test it on different functions (e.g. wavelength filtering, sensing, …)
Photonic circuits today are custom-designed and fabricated for specific applications. This makes the costly and complicated for new developers to use in new applications. Today, photonics does not have an equivalent to electronic programmable chips (e.g. FPGAs), which can enable rapid prototyping and innovation.
Programmable photonic chips, also called photonic processors, can change this situation. Using a network of on-chip waveguides and electrically tunable elements (so-called optical gates) the paths of light can be reconfigured at run-time to perform different functions. The problem with these new programmable photonic chips is that the electro-optic actuators used consume a lot of power, because they are based on on-chip heaters.
In LIQUORICE, we want to make use of liquid crystals to enable this reconfigurability. Liquid crystals have already been integrated with silicon waveguides before, but never on a large scale. We therefore combine two chip-based technologies to achieve large-scale tuning of photonic circuits. On one hand, we have silicon photonic waveguides, which perform the actual optical function. On the other hand, we take a liquid crystal microdisplay ( liquid-crystal-on-silicon or LCOS chip), where the state of each pixel can be electrically controlled. We bring both together in a sandwich, where the pixels of the LCOS chip can now locally tune the behaviour of the optical waveguides.
Once we have a tunable system, we can program new functionality by sending a different ‘grayscale image’ to the LCOS microdisplay, which will then reconfigure the circuit, so we can test it on different functions (e.g. wavelength filtering, sensing, …)
The work in LIQUORICE brings together know-how from different fields, much of which was developed during the ERC consolidator grant PhotonicSWARM. Within this proof-of-concept project we made important progress towards realizing the desired proof of concept, but we did not yet achieve a successful conclusion at the end.
One key component we worked on was the photonic chip with the programmable waveguide mesh. For this, we realized multiple suitable circuit designs, fabricated in IMEC’s iSiPP50G silicon photonic process, as well as variations in more experimental process flows. We also developed the necessary postprocessing steps for these chips, to implement a ‘tank’ that can contain the liquid crystal after the photonic chip has been combined with the LCOS chip
The second key element in the system is the LCOS chip itself. This is a component which we sourced commercially. We identified possible suppliers that were willing to provide bare LCOS chips in sufficient quantity, and support us with the associated drivers and test boards. However, here we encountered a setback, as the supplier with who we started the collaboration discontinued their work on LCOS displays (as the market for microdisplays is rapidly shifting from LCOS to micro-LEDs). We did manage to secure sufficient chips and supporting material to continue working on the proof-of-concept, but for future developments we will have to explore alternative partners.
The main work in LIQUORICE relates to the assembly process, essentially making a liquid crystal cell with the LCOS chip on one side, and the silicon photonics on the other. This imposes multiple challenges. The silicon chip is not flat, and needs additional metallization to provide a good reference ground. The assembly itself also introduced challenges to get good adhesion between the two layers, and make sure the cell is hermetically sealed. We tested multiple materials and bonding schemes, and reached a process that we deem satisfactory. This was not without obstacles, and we encountered significant delays in this development.
Once the cell is assembled, it will need filling with liquid crystal. This is performed in vacuum. Because the cell geometry of our device is substantially different from standard liquid crystal cells, we decided to rethink the filling process to make it more reliable. This is currently still work in progress.
In parallel, we tested how we could electrically control the LCOS display, looking into different driving schemes that control the resolution, refresh rate and precision of the display. These will affect how well we can control our programmable photonic chip.
The work on this proof-of-concept is therefore not finished yet, and we hope to demonstrate its functionality, even after the end of the project.
One key component we worked on was the photonic chip with the programmable waveguide mesh. For this, we realized multiple suitable circuit designs, fabricated in IMEC’s iSiPP50G silicon photonic process, as well as variations in more experimental process flows. We also developed the necessary postprocessing steps for these chips, to implement a ‘tank’ that can contain the liquid crystal after the photonic chip has been combined with the LCOS chip
The second key element in the system is the LCOS chip itself. This is a component which we sourced commercially. We identified possible suppliers that were willing to provide bare LCOS chips in sufficient quantity, and support us with the associated drivers and test boards. However, here we encountered a setback, as the supplier with who we started the collaboration discontinued their work on LCOS displays (as the market for microdisplays is rapidly shifting from LCOS to micro-LEDs). We did manage to secure sufficient chips and supporting material to continue working on the proof-of-concept, but for future developments we will have to explore alternative partners.
The main work in LIQUORICE relates to the assembly process, essentially making a liquid crystal cell with the LCOS chip on one side, and the silicon photonics on the other. This imposes multiple challenges. The silicon chip is not flat, and needs additional metallization to provide a good reference ground. The assembly itself also introduced challenges to get good adhesion between the two layers, and make sure the cell is hermetically sealed. We tested multiple materials and bonding schemes, and reached a process that we deem satisfactory. This was not without obstacles, and we encountered significant delays in this development.
Once the cell is assembled, it will need filling with liquid crystal. This is performed in vacuum. Because the cell geometry of our device is substantially different from standard liquid crystal cells, we decided to rethink the filling process to make it more reliable. This is currently still work in progress.
In parallel, we tested how we could electrically control the LCOS display, looking into different driving schemes that control the resolution, refresh rate and precision of the display. These will affect how well we can control our programmable photonic chip.
The work on this proof-of-concept is therefore not finished yet, and we hope to demonstrate its functionality, even after the end of the project.
We are not aware of anyone following a similar approach as we are taking to large-scale programmable photonics. While we have not achieved a successful result yet, we have not yet encountered showstoppers, and we hope that we can still demonstrate the viability of our concept in the near future.
No direct publications have resulted from this project (yet), although parallel work on liquid crystal phase shifters for silicon photonics has yielded spectacular performance, which is currently being prepared for publication.
No direct publications have resulted from this project (yet), although parallel work on liquid crystal phase shifters for silicon photonics has yielded spectacular performance, which is currently being prepared for publication.