Opto-Electronics

Opto-electronics (O/E), which combine photonics and high-speed electronics to generate, transmit, and process signals from microwave to terahertz frequencies, are becoming more prominent in today’s high-speed designs. O/E devices are an integral part of 5G and IoT systems and networks, and are key elements in 6G R&D. The reason? They provide almost unlimited bandwidth, low loss, low interference, short- and long-distance coverage, low power consumption, and high-speed data transmission capability to high bandwidth data transmissions and high-speed interconnects.

To integrate O/E devices into emerging high-speed complex designs, they have become extremely compact. Miniaturization is necessary because more and more O/E devices are being placed on a single wafer. Further, designs are now incorporating much more complexity on-wafer, as Receiver Optical Sub Assemblies (ROSA), Transmission Optical Sub Assemblies (TOSA), and Bi-Directional Optical Sub Assemblies (BOSA) are also packed onto a wafer. The result is optics and electronics are moving closer to each other in silicon photonic (SiPh) designs. As these two domains come together on a small CMOS chip, verifying performance becomes more daunting.

Emerging Use Cases

O/E is used in numerous applications, including low-phase noise O/E oscillators; carrier envelope phase-locked lasers; photonic-based complex radar signal generators; phased-array antenna architectures; photonic analog-to-digital converters, electro-optic sampling, and terahertz signal generation. Many of these applications are used in emerging use cases. Some examples are listed in Table 1.

Table 1: Use cases utilizing O/E technology

Testing and Verifying O/E Devices

Phase measurements are more important as designs incorporate higher frequencies. Wavelengths are shorter, making a vector network analyzer (VNA) central to any test solution to verify O/E devices. For testing and characterizing devices like TOSA, ROSA, BOSA, and even on-wafer SipH devices, an opto-electronic VNA must cover a microwave broadband frequency range.

One key benefit of a VNA is that it measures magnitude and phase characteristics in the form of S-parameters. This capability provides distinct advantages compared to how lightwave component analyzers (LCAs) conduct measurements, especially in high-speed designs. Small errors in phase controlling or measurement will lead to errors, making proper VNA selection critical to gaining design confidence.

Accurate device characterization requires the VNA to conduct wideband sweeps from near DC to well beyond 150 GHz. This allows for greater accuracy and repeatability when conducting measurements to the 3rd and 5th harmonics. This is especially true for O/E verification, which often requires long measurement sessions.

A complete O/E test solution consists of a VNA with optical modulators (sometimes with a laser source, depending on the application) and reference photodetector. Traceability is very important and becomes a greater challenge as frequency rises. The modules should be characterized to NIST standards. De-embedding is also critical to prevent passivity and causality errors.