PHOTONIC INTEGRATED CIRCUITS (PICs) are ready to repeat the success story of electronic integrated circuits (ICs). PICs work with light instead of electrons and will play a key role in tomorrow’s infrastructure in communication, sensing and transportation. Whereas silicon photonics has been around for more than 20 years, new material platforms have been introduced in the past decade that offer additional benefits.
The motivation for using silicon nitride (SiN) waveguides are manifold. Firstly, silicon nitride is a well-known material that is CMOS compatible and already used in the semiconductor industry. This enabled the development of fabrication techniques and process design kits (PDKs) with standard CMOS tools. This was one of the main requirements when scaling a process afterwards to volume or more importantly when using already existing infrastructure to run the process. Secondly, silicon nitride as a material offers new possibilities to the PIC market. If we look, for example, at the application wavelength as one major parameter we can see that in classical silicon photonics, where the optical wave is guided in silicon, transparency starts above one micron. This is perfect for many fiber optical applications, especially communications.
However, there are many more applications which require light propagation at lower wavelengths that cannot be served by silicon photonics. Silicon nitride with its transparency window spanning from the visible to the mid infrared opens the path for new applications. In addition to that, silicon nitride offers extremely low propagation losses compared to silicon or indium phosphide. Last but not least, high power propagation of several Watts of CW laser power is possible due to the large bandgap of silicon nitride. This is why silicon nitride offers superior performance to manage the light in the chip circuitry with unprecedented low propagation losses and highpower handling.
Telecom and datacom industries are one of the largest PIC consumers as of today. Lowering optical losses is getting more important in those domains, since optical loss affects not only the energy consumption, but also the performance of the devices. The crosstalk performance of arrayed waveguide gratings for wavelength division as MUX and DEMUX for example is directly proportional to optical propagation loss. AWGs with high propagation losses are accumulating phase errors in their arms which results in increased cross talk between the channels. Another key parameter is to have low temperature dependence of the AWG to minimize the thermal effects on device performance. Here silicon nitride offers a 10 times lower temperature dependence than silicon. Additionally, a good process control is needed to ensure the right wavelength band selection. Statistical process control guarantees this with LIGENTEC’s fabrication platform. Especially for AWGs LIGENTEC’s proprietary technology provides a competitive advantage, as not only the loss of the waveguides are very low, but also the area size of the arrayed waveguide grating is small due to the small bending radius common to the platform. This is enabled by the high confinement of the optical mode in the waveguide. The LIGENTEC platform offers very low phase errors together with a small footprint.
The above-mentioned advantages are also of great importance to other new applications. To enable fully autonomous driving, for instance, it is expected that the next generation of LiDAR sensors for long distance sensing will be based on coherent detection. Here the reflected beam is mixed with a local oscillator, filtering out all light that is not coming back in reflection from the object. These coherent detection schemes are rather complex and benefit significantly from photonic integration.
Key requirements for such an FMCW LiDAR system are the ability to transmit high optical power, have low propagation loss and low phase errors and last but not least low cross talk between the transmit and receive channels. A key building block here is the delay line interferometer, used to control the modulation of the laser signal. The length of the delay line is a critical performance parameter since it relates directly to the precision of the range measurement. With the low propagation loss and short bend radius, delay lines of 1m on chip are possible. This in combination with low phase noise enables high resolution FMCW LiDAR solutions.