Abstract
Programmable photonic integrated circuits represent an emerging technology that enables reconfigurable optical systems implemented on a chip. Silicon photonics has emerged as the leading platform for realizing programmable photonics due to its compatibility with CMOS processes. This tutorial provides an introduction to programmable photonics and discusses the principles, technology requirements, applications and design approaches for implementing programmable photonic systems using silicon photonics. The use of PhotoCAD for layout implementation is also presented.
Introduction
The field of photonics has seen tremendous advances with the development of photonic integrated circuits (PICs). PICs implement optical components and systems on a chip, providing advantages like small footprint, stability, scalability and low cost. Silicon photonics has been the driving force behind the growth of PICs due to its compatibility with CMOS fabrication, enabling high volume manufacturing. However, most silicon PICs developed so far have been application specific designs performing fixed functions.
Programmable photonic circuits represent a new paradigm where a PIC's function can be reconfigured electronically through software [1]. This concept was inspired by field programmable gate arrays (FPGAs) in electronics, which can be reconfigured to implement different digital logic functions. Programmable photonics promises similar flexibility for optical systems, along with rapid prototyping and lower development costs. This tutorial provides an overview of programmable photonics and its implementation using silicon photonic integrated circuits.
Principles of Operation
The key idea behind a programmable photonic circuit is the use of a reconfigurable photonic mesh consisting of tunable couplers and phase shifters to route signals between components. As shown in Fig. 1, the tunable couplers and phase shifters form 2x2 optical gates that can mix and route signals arbitrarily between waveguides. The reconfigurable mesh replaces fixed waveguide connections, providing software control over connectivity.
Different topologies exist for implementing the photonic mesh. Forward-only meshes allow light to propagate in one direction from inputs to outputs for linear transformations. Recirculating meshes enable bidirectional propagation, supporting features like delay lines and resonators. The mesh architecture can be optimized based on requirements. Light is processed by other components like modulators and detectors connected to the mesh.
Technology Requirements
Several technology developments have been critical to enabling programmable photonics:
Silicon photonics provides a high index contrast waveguiding platform capable of dense integration of components like tunable couplers and phase shifters [5]. The compatibility with CMOS also enables high volume manufacturing.
Tunable couplers using mechanisms like microelectromechanical systems (MEMS), thermo-optic, electro-optic and nonlinear effects provide reconfigurable power splitting between waveguides.
Tunable phase shifters using techniques like MEMS, thermo-optic, electro-optic, piezo-electric and strain adjustment facilitate dynamic phase control in the photonic mesh.
Control electronics with digital and analog circuits interface with the photonic chip to configure the tunable elements. Feedback monitoring is also utilized.
Packaging and integration techniques connect the optics and electronics, bringing input/output signals on and off chip [9]. Hybrid and monolithic integration are active areas of research.
Programming algorithms and routines configure the tunable photonic mesh to implement functions from simple routing to complex filters.
Applications
Some application areas where programmable photonics can provide value are:
Optical communications: Programmable transmitters, receivers, filters, routers, switches etc.
Microwave photonics: Reconfigurable optical beamforming, filtering, equalization and distribution.
Sensing: Adaptable spectrometers, interferometers, lidars etc.
Optical neural networks: Linear transformations and matrix-vector multiplication.
Quantum information: Linear optical transformations and circuits.
PIC Layout Design Using PhotoCAD
PhotoCAD is the PIC layout tool in PIC Studio platform. It enables rapid circuit prototyping by leveraging a Python 3 framework and parametric layout generators for common components like waveguides, couplers, phase shifters etc. PhotoCAD can be used to efficiently implement programmable photonic circuit layouts.
For example, a triangular MZI mesh as shown in Fig. 2 can be implemented with the following steps:
Define the MZI PCell class with the layout.
Instantiate the MZI components and connect them using the Linked class for waveguide routing.
Add electrical ports for phase shifter controls.
Incorporate other components like detectors.
Export the circuit layout to GDSII format.
The high level circuit description language interface of PhotoCAD makes it easy to experiment with different topologies by modifying the connections. Compact bend styles reduce mesh footprint. PhotoCAD enables rapid realization of silicon photonic programmable PIC prototypes.
Conclusion
Programmable photonic integrated circuits implemented using silicon photonics provide a flexible platform for reconfigurable optical signal processing. The ability to reprogram functionality through software control eliminates the fabrication steps required to change designs. Programmable photonics promises to make optics more accessible to users without specialized photonics expertise. The open-source PhotoCAD toolkit enables rapid layout generation to prototype programmable photonic systems on a silicon chip. Further technology development and adoption of programmable photonics can lead to a thriving ecosystem of accessible photonic solutions.
Comments