In October 2022, the company acquired the MicroLED fab and engineering team from Nanosys to accelerate the development and manufacturing of high-speed GaN MicroLEDs for parallel multi-Tbps interconnects. Now, it is partnering with Osram to develop high-volume manufacturing capabilities for those chips.
Avicena’s LightBundle architecture uses densely packed arrays of Gallium Nitride (GaN) microLEDs to create highly parallel optical interconnects with throughputs exceeding 1 Terabit per second (Tb/s) at an energy consumption of less than 1 picojoule per bit. This technology offers a high-bandwidth and energy-efficient alternative to traditional optical interconnects based on LEDs or other technologies.
The LightBundle cable utilizes a highly multicore multimode fiber that connects a GaN microLED transmitter array to a matching array of silicon photodetectors (PDs). Hundreds or thousands of these microLED and PD arrays can be easily integrated with standard CMOS ICs, enabling optical interconnects to be integrated with electrical circuits at a very close level.
LightBundle links use simple Non-Return-to-Zero (NRZ) modulation instead of Pulse Amplitude Modulation 4 (PAM4) modulation, which is commonly used in many modern optical links. The use of NRZ modulation leads to lower power consumption and reduced latency, making it suitable for low-latency applications such as High-Performance Computing (HPC) and Artificial Intelligence/Machine Learning (AI/ML) applications.
NRZ modulation is a simple and commonly used modulation format in digital communications, where a signal is represented as a series of voltage levels, each representing a binary value of 0 or 1. In contrast, PAM4 modulation uses four voltage levels to represent two bits of data, making it more complex but allowing for a higher data rate. However, the increased complexity and higher power consumption of PAM4 modulation make it less suitable for low-latency applications.
LEDs, MicroLEDs, and VLC
GaN-based LEDs are commonly used in Visible Light Communication (VLC) due to their high efficiency and wide bandgap, which allows for higher output power and modulation bandwidth. GaN-based LEDs are manufactured through epitaxial growth techniques such as Metal-Organic Chemical Vapor Deposition (MOCVD), where thin layers of GaN and other materials are deposited onto a substrate.
Recent experiments have shown the effectiveness of high-efficiency Gallium Nitride (GaN) based MicroLEDs when employed with modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) or NRZ. These techniques have achieved data transmission rates of Gigabits per second (Gbps). Non-polar GaN-based MicroLEDs are particularly attractive for VLC applications due to their extraordinary high bandwidth at a low current density.
On the other hand, AlGaInP LEDs offer another option for data transmission in MicroLED geometries since they don’t possess internal electric fields associated with polarization. Researchers have demonstrated the feasibility of red-emitting aluminum gallium indium phosphide MicroLEDs for VLC applications through micro-transfer printing. These MicroLED platelets were printed on glass and diamond substrates, with an emission wavelength of 630 nm. The epitaxial structure of these MicroLEDs is similar to typical AlGaInP epistructures produced on Gallium Arsenide (GaAs).
The printing process involved the use of Polydimethylsiloxane (PDMS) stamps to imprint the MicroLED platelets onto both substrates after they had been picked up from the temporary sapphire substrate. One significant advantage of micro-transfer printing is the ability to transfer MicroLEDs onto a range of substrates, enabling the use of a wide variety of materials for different applications. Additionally, the process provides high precision and repeatability, resulting in uniform and high-quality MicroLED arrays. Overall, the use of micro-transfer printing and encapsulation techniques, coupled with the unique properties of GaN-based and AlGaInP MicroLEDs, provides exciting possibilities for the development of high-performance VLC devices for a wide range of applications.
Researchers in Taiwan have found that MicroLEDs face challenges in achieving both high efficiency and high speed due to factors like QCSE, size dependence, and efficiency drop. QCSE is a phenomenon that occurs in semiconductor materials when an electric field is applied to them, causing the energy levels of electrons to shift, resulting in a change in the material’s optical properties. In MicroLEDs, this effect can negatively impact their performance by reducing their efficiency and limiting their modulation bandwidth. Researchers are working to minimize the impact of QCSE and other factors to improve the efficiency and speed of micro-LEDs.
To address these challenges, researchers have studied high-speed MicroLEDs emitting yellow-green to red wavelengths, and explored structural optimization approaches to enhance modulation bandwidth. One potential solution is using blue MicroLEDs coated with quantum dots for color conversion, which can produce a wider range of colors with high efficiency and stable conversion. However, this approach has challenges such as limited stability of quantum dots and the complexity of depositing them onto the MicroLED surface.
While MicroLEDs have shown superior performance in VLC, there is still room for further improvement, particularly in achieving both high efficiency and high modulation bandwidth simultaneously. Potential solutions include enhancing efficiency droop, minimizing etching damage, and achieving high bandwidth at low current. Additionally, advanced modulation formats appropriate for VLC applications must be developed to increase the modulation bandwidth for MicroLEDs and their array structure.
