Photonic integrated circuits for compact optical displacement sensors
Rachel Jones defended her PhD thesis at the Department of Electrical Engineering on June 26th.
Many of our daily tasks and activities involve electronic devices such as smartphones, cars, computers and smart appliances in our homes. At the core of these devices are integrated circuits (ICs): tiny chips containing billions of components. As the demand for smaller, faster, and more powerful chips grows, the machines that manufacture them, known as photolithographic scanners, must also advance. Though, these required advancements bring significant challenges with them. Rachel Jones explores in her PhD research how integrated photonics made from Indium Phosphide (InP) can offer a solution.
The focus of Rachel Jones’ research is on one critical challenge: the current technology for moving and positioning silicon wafers, the base material for IC’s, is nearing its physical limits. While new actuator types are being explored, they introduce environmental vibrations that threaten the precision required for chip fabrication. To counter these disturbances, the next generation of scanners will require significantly more sensors to measure and correct the slightest deviations in positioning. However, using traditional optical components for this would result in larger, heavier, and more expensive metrology systems.
Integrated photonics could offer a solution for this challenge, because it allows complex optical systems to be built on a chip as small as a 5 cent coin. Jones’ research concentrates on integrated photonics circuits made from Indium Phosphide (InP), a material well suited for generating, manipulating, and detecting infrared light.
Developing a stable light source
Central to any optical interferometric sensor is its light source. An interferometer determines a change in a physical parameter of a system, like displacement, through a measurement of the change in the optical properties of the light returning to the sensor, for example the wavelength. For this reason, it is important that the light source is stable. However, various sources of noise—stemming from fundamental physics or environmental factors like temperature and vibration—cause fluctuations in the laser’s wavelength, affecting the sensor’s measurement accuracy.One of Jones’ key objectives was the development of an integrated light source with a stable wavelength for use in these sensors. This stability was achieved in two ways: first, through simulation and measurement of different laser cavity designs, and second, through active stabilization of the lasers using a feedback control system. The stabilized lasers were then applied in two types of high-resolution optical sensors, one of which demonstrated displacement resolution at the picometre scale.
Potential and technical challenges
The results of this research demonstrate the potential of this emerging technology for compact, high-performance sensors in precision applications. Next to that, the research also highlights the technical challenges of integrated photonics.
Title of PhD thesis: . Supervisors: Dr. Erwin Bente and Prof. Kevin Williams.