How Microwave Photonic Systems Are Changing Electronics

Key Takeaways

• PCB and IC designers are familiar with microwave signals, but microwave photonics extends the use of microwaves to new domains.

• Microwave photonic systems aim to exploit phenomena that occur between individual photons and matter for practical applications.

• Microwave photonic systems must interface with other optical systems and electronics as part of systems design by bringing microwave functions onto the chip and board level.

Microwave photonic systems are actively being researched by optical systems designers and electronics engineers.

Optical and radio sciences have seen huge progress over the past hundred years, particularly in communications. This is all enabled by electronics advances for the generation, manipulation, and detection of electromagnetic waves using a variety of technologies. Today, further advances go beyond simply broadcasting and receiving microwaves or sending an ON/OFF stream of pulses down an optical fiber. Newer electronics and optoelectronics systems will take advantage of fundamental interactions between photons and electrons to solve practical problems, including at microwave frequencies.

This is the basis of microwave photonic systems, which involve the generation, manipulation, and reception of light at the board and chip levels. These systems have been with us for some time in different forms (e.g., lasers, Er-doped fiber amplifiers, and beamforming antennas), but there is much more innovation to be seen thanks to board-level and chip-level integration. Here are some recent trends in microwave photonic systems and what electronics designers can expect to see in the near future.

Overview of Microwave Photonic Systems

At first glance, it may seem that electronics designers have been doing microwave photonics for decades. All electromagnetic waves are made of photons, and RF designers are adept at working with electronics systems that make use of microwaves. So what makes microwave photonics different from typical radio and electronics systems, and what can be done with these different systems?

There are three important points that separate microwave photonic systems from electronics that just happen to operate at microwave frequencies:

1. Analog signal processing applied to light. Analog signal processing is something of a relic in electronics until you start looking at new transistor and memtransistor architectures for machine learning/AI. A key component of microwave photonic systems is to perform some processing of optical signals directly.

2. Taking advantage of light-matter interactions. A central part of microwave photonic systems is making use of interactions between light and matter, including quantum phenomena, for practical applications.

3. Greater integration. Photonic integrated circuit designers would like to shrink complex laser systems on optical tables down to the chip level. This follows the same progress like that seen in electronics over the last 150 years.

Shrinking this pump-probe measurement down to the board-level and chip-level is one goal in developing microwave photonic systems.

Success in the above three areas requires developing chip-level and board-level architecture to support generation, transmission, processing, and reception of microwave signals.

Microwave Photonic Signal Generation

There are multiple methods for generating photonic signals for use in microwave photonic systems. Some representative examples include:

1. Generation via up or down conversion. A high or low energy photon source can be frequency multiplied to produce the desired wavelength, which is one approach taken in lasers. 

2. Parametric oscillation. An optical parametric oscillator is one device used in lasers to sweep through a range of possible emission frequencies. 

3. Use interference in an optical mode comb. The generation of mode combs in infrared mode-locked lasers at the chip level provides a simple way to create clocks for microwave circuits. Individual modes can be interfered to create a beat signal at high GHz or THz frequencies for use in microwave circuits. 

4. Generate microwave photons directly. Microwave and THz single-photon sources are still a major research area, which is ironic considering the first lasers operated at sub-mmWave wavelengths. 

Processing with Microwave Signals

Processing information with microwave signals is a simple form of analog signal processing that happens at the integrated circuit level between coherent waves. The structures used for these applications resemble their counterparts in RF PCBs, except they are scaled down to smaller sizes. Processing microwave signals occurs through interference between photons or is mediated by light-matter interactions in microwave circuits.

Reception of Microwave Signals

One way to think of reception is as the inverse of generation. Chip-level structures used for microwave generation are required as more of the microwave section in a system moves inside electronic-photonic integrated circuits (EPICs) or fully-photonic integrated circuits (PICs). These structures can be as simple as dipole nanoantennas or they can involve electro-optic conversion to generate an electric current, and vice versa.

Board-Level and Chip-Level Design

When we look at the board level, many of the difficulties that are typically encountered in RF systems can be solved when working with microwave photonics. Treating microwaves as optical waves, just as one would with visible light, requires working with waveguides embedded in the PCB substrate or integrated onto a semiconductor die. Microwave sources (e.g., VCSELs and QD lasers) and detectors need to be fabricated directly on the same die as optical signal processing elements to enable the kind of integration required in microwave photonic systems.

At the board level, interconnects need to be reimagined as waveguides to support microwave signals. In effect, microwave signals need to be passed through some type of deposited waveguide structure that behaves as an optical fiber to route microwave signals around a system. This provides low-loss transmission down to low photon flux levels, which can then be used in areas like sensing.

These optical components are shrunk and placed directly in ICs and PCBs when designing microwave photonic systems.

So what will the architecture for these systems look like? Only time will tell how microwave photonic systems advance and what standards are developed by the industry. PCBs will not be going away anytime soon as not every system needs to be photonic. However, those that need to interface with EPICs and PICs will not be doing so with copper traces. It is likely that polymer materials, nanoflake/nanotube materials (e.g., graphene), or good old-fashioned glass will be used to help route signals between EPICs and PICs on a PCB. Electronic integrated circuits will also still be relevant for providing interfaces between regular electronic and photonic systems.

One possible architecture for microwave photonic systems is a chiplet architecture with multichip modules where entire devices and their interconnects are fabricated on a III-V or II-VI wafer and integrated into a substrate. This is one way to provide a routing and processing architecture on a single device. For the time being, designers will need PCB and IC design tools that help them integrate both types of components on a circuit board for building more advanced electronics, optical, and electro-optical devices.

Designing microwave photonic systems takes a variety of design and layout tools to create new technology. Corner the microwave PCB market and integrate photonic and electronic components onto circuit boards with stronger analytical capabilities in your layout and design tools. 

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