PHOTONIC INTEGRATED CIRCUITS RESEARCH AND DEVELOPMENT AT RAYTHEON

Photonic integrated circuit (PIC) technologies promise miniaturization and large-scale implementation of optical and electro-optical devices to meet the performance requirements and complexity of next generation radio frequency (RF) and optical system architectures.  This is witnessed by large and diverse investments in recent years through commercial industry and government funding in a wide range of application areas including sensors, timing/metrology (e.g. chip-scale optical clocks), signal processing, communications, and computing. In this article we review current PIC development efforts in Raytheon domains of interest and focus on potential applications that provide increased capabilities to meet customer requirements.

Since its inception in the 1960s, PIC technology has made significant progress — particularly at near-infrared (IR) and telecom band spectra with major focus on applications in data communication and interconnects. Semiconductor optical materials such as silicon (Si) and III-V compounds (e.g. indium phosphide and gallium arsenide) with their technology developed in microelectronics have been the main driving factor for implementing photonic components for these applications. A key advancement in PIC technology is the emerging area of silicon photonics, a field that substantially leverages complementary metal–oxide–semiconductor (CMOS) microelectronics, optical fiber communications technology, and advances in nanofabrication techniques to enable ultra-low-loss Si PIC components. The heterogeneous integration of different PIC materials on a Si photonic platform and the ability to couple the light between the waveguide structures in these different materials has added more degrees of freedom to take advantage of each material component, creating the specific functionality needed to meet overall device performance requirements. Figure 1 shows the major milestone achievements in silicon photonic technology.

Figure 1: Milestone achievements in silicon (Si) photonic integrated circuit (PIC) technology.

PIC Technology Application Areas

Raytheon is actively participating in this rapidly growing PIC technology to realize complex, system-level solutions to support customer needs and demanding requirements by executing a variety of internal and external PIC-based projects. An example is the Raytheon Core Research  funded project, focusing on maturing Si-based PIC device technologies (including silicon nitride) as well as new emerging III-Nitride (e.g., GaN) PIC technologies. These different material technologies are being investigated to address both near-term and long-term business solutions and customer needs based on these cutting edge applications in both the RF and optical domains. 

Figure 2: Benefits of PIC technology span a diverse group of application areas.

A diverse group of application areas investigated by Raytheon engineers and scientists have potential to benefit from PIC technologies (Figure 2). Examples of Si PIC technology development at Raytheon include fundamental Si PIC building block design such as waveguides and a compact microresonator coupled to a Si waveguide (Figure 3), and the example PIC chip layout and experimental testbed shown in Figure 4. Included with the development of these technologies is the analysis and modeling of how these PIC components perform as part of an entire system level processing chain, thus gaining acceptance with improved understanding by the system engineering community. These goals will all contribute to and strengthen the establishment of a Raytheon PIC technology ecosystem that will enable the insertion of PIC technologies into Raytheon product lines.

Figure 3: An example of fundamental Si PIC building blocks including waveguides (left) and a compact microresonator coupled to a Si waveguide (right).
Figure 4: A typical PIC chip layout with many different devices (left) and a PIC experimental testbed with edge coupling using optical fiber (right).

The American Institute for Manufacturing Integrated Photonics (AIM Photonics) is an industry-driven, public-private national consortium supporting the advancement of integrated photonic technology throughout U.S. industry, government and academia. Started in 2015 with a fabrication facility located at the State University of New York (SUNY) Polytechnic Institute campus in Albany, New York, the consortium has made great progress establishing both silicon wafer processing and package assembly capabilities. Raytheon is a member of AIM Photonics and has been involved in a number of consortium-funded projects.

Developing system on a chip (SoC) solutions using silicon photonic technologies may soon provide implementations that can overcome some of the main challenges in cryogenic-based imaging, computing and Positioning, Navigation and Timing (PNT) systems. Raytheon has been developing cryogenic PIC transceivers to transfer massive data from cooled focal plane arrays (FPAs) to room temperature processors. We are also involved in a team project funded by AIM Photonics and the Air Force Research Laboratory (AFRL) to integrate cryogenic PIC transceivers with FPAs. On the applications of PIC for cryogenic computing, Raytheon is a performer on a program funded by the Intelligence Advanced Research Project Agency (IARPA) to design and demonstrate cryogenic PIC microsystems and transceivers scalable to petabit/s data transfer rates from a 4 Kelvin superconducting computing system to room temperature over optical fibers, replacing the traditional copper cable technology. For PNT applications, Raytheon has been collaborating with the National Institute of Standards and Technology (NIST) in developing PIC-based chip-scale optical clocks operating at cryogenic temperatures with improved performance and stability.

PIC multichip-module (MCM) interconnects is another application area that can be an enabler for next generation radar and multifunction systems. Under the DARPA Photonics in the Package for Extreme Scalability (PIPES) Program, Raytheon is investigating the application of photonic-enabled MCMs to address the data movement bottle neck and enhance the performance of digital beam forming arrays.

A promising direction that has been actively pursued recently by NASA, the Department of Defense and the commercial sector is deploying PIC technology for free-space optical communications. In this area, Raytheon is developing silicon PIC phased array systems for free-space communication funded through external and internal programs.    

Another application area of interest is using photonics for the distribution and processing of analog RF/microwave signals, providing higher bandwidth and lower losses versus current approaches. Raytheon has been developing RF-over-fiber applications since the 1990s and has successfully integrated discrete photonic devices in fielded systems that have met performance requirements, demonstrating to the customer community the feasibility of implementing photonic technologies. 

While discrete photonic component devices have enabled unique solutions to specific product implementations, Raytheon researchers recognize the potential benefits of PIC technologies to reduce the size, weight, power and cost to enable the integration of complex photonic-based RF systems. Integrated photonics has had significant success in the digital domain. However, there still remain some issues with PIC components that limit device performance for RF applications. These limitations may be fundamental to the properties of the silicon-based materials or may be a result of variations in the nanofabrication processing capabilities at the foundry compared to the modelling parameters used to design the devices into integrated photonic circuits. The optical waveguide used to make photonic integrated circuits has nonlinear properties at relatively low optical powers, as well as optical loss, two main factors that can affect dynamic range and performance for RF applications. For example, the silicon-based modulators that modulate RF signals onto an optical carrier have relatively low conversion efficiency, and designing these modulators for wide bandwidth has been challenging. In recent years, the heterogeneous integration of high electro-optic coefficient III-V materials (e.g. Indium Phosphide) on silicon has been pursued to improve the conversion efficiency of these modulators, though still more effort is needed to bring performance comparable to that of lithium niobate modulators. In addition to modulators, the linearity and power handling of high-speed optical detectors implemented on silicon photonics is another factor that impacts the utility of these devices for RF applications.

There continues to be progress in improving and optimizing the design kits for both passive and active photonic circuit elements provided by the different foundries. Despite the challenges of PIC devices for RF applications, Raytheon scientists and engineers are developing PIC implementations for the distribution of clock signals and other RF-over-fiber applications. Raytheon is working on a DARPA program investigating the use of PIC-based photonic systems for Electronic Warfare (EW) applications and the potential for miniaturizing these systems.

There are other emerging application areas where current silicon-based PIC technologies may not provide the required capability. One such area is for applications within the visible wavelength spectrum in which Si is absorptive. Although the integration with materials that are transparent in the visible spectrum and are compatible with silicon-based CMOS platforms such as silicon nitride or Al2O3 can provide a working implementation, the passive nature of these materials inhibits making fast active and tunable components. An alternative approach — other wide bandgap materials such as III-Nitrides that are transparent down to UV — has been pursued by Raytheon and the research community. Raytheon recently had a program with IARPA to develop GaN and AlGaN PIC platforms for UV-visible applications. In addition, Raytheon technologists are collaborating on internal programs to expand the capabilities of the company’s GaN foundry to develop these III-Nitride-based PIC devices. 

PIC fabrication and packaging 

In recent years, silicon-based microelectronic foundries in the U.S. (e.g. AIM Photonics, TowerJazz, Global Foundries, Intel, IBM) and worldwide have incorporated silicon photonics fabrication into their process lines and successfully demonstrated fundamental building block components, both passive and active. These foundries have also demonstrated large scale PIC systems with performance metrics that can meet a wide range of digital datacom and interconnect application requirements. Several Defense Advanced Research Projects Agency (DARPA) Microsystems Technology Office (MTO) programs have used AIM Photonics‘ foundry for making photonic microsystems including chip-scale Light Detection and Ranging (LiDAR) systems. 

Although there are a large number of material platforms currently being utilized by the research community for the fabrication of PIC devices, from a technical and cost standpoint, silicon-based PIC foundries are the most mature. This requires designers who intend to implement novel concepts in a PIC platform to consider the feasibility of the fabrication from both a technical and cost perspective. While proof-of-concept prototype devices can be fabricated at a small-scale foundry such as a university foundry, any PIC design to be included in a manufactured product needs to be capable of being fabricated at an established foundry that can provide both the required material processing technologies needed to meet the technical requirements and the manufacturing capability to produce a reliable wafer run to enable high device yields. In addition to designing and manufacturing the PIC itself, the realization of PIC-based solutions in manufactured products will only occur when these PIC chips can be successfully integrated and packaged in robust modules. 

Developing these PIC packaging capabilities for defense applications is not on the current AIM Photonics road map. The AIM Photonics Test, Assembly and Packaging (TAP) facility is located in Rochester, New York, and is primarily focused on the packaging of PIC devices for commercial applications. Raytheon is starting to look closely at developing the in-house expertise to package PICs in modules that meet mil-spec environmental requirements. Developing these PIC packaging capabilities and competencies for Raytheon products requires the maturing of several packaging and manufacturing technologies, including optical, electrical, thermal, mechanical, and advanced manufacturing processes. Raytheon’s long history with the manufacturing and packaging of monolithic microwave integrated circuit (MMIC) products at our Advanced Product Center (APC) can be leveraged to develop these needed packaging technologies. Packaging techniques to couple light into and out of these PIC devices at the chip level of integration with minimal optical loss and mechanical integrity are also needed. 

Path forward

With all the advances occurring in emerging PIC technologies, Raytheon researchers and engineers are actively pursuing both internal research and customer-funded development programs to investigate applications of PIC technology in next generation products, as well as to develop lower cost, higher yield PIC packaging and manufacturing processes.  

— Richard Belansky
— Mo Soltani