CTN Issue: October 2023

A note from the editor:

As the fall season descends upon us, our October article takes us on a hayride through the somewhat unfamiliar yet fascinating landscape of photonic integrated circuits (PIC). Our October 2021 article, “Can RF Photonics Save the Wireless Day? Unbounded Beam-Bandwidth Product to the Rescue”, introduced the ComSoc community to the research and development efforts on RF photonic phased arrays, where photons interact with electrons to support extremely high beam-bandwidth applications for sensing and communication, envisaged by, e.g., the beyond-5G roadmap. Two years in, the authors are keen to share progress made in reducing the size, weight, power consumption and cost (SWAP-C) of such systems by way of PIC, bringing the approach closer to reality. This article provides an overview of the burgeoning foundry ecosystem for photonic integration, with a focus on the material platform frontrunners and challenges in heterogenous integration of electronic and photonic components. As always, reader comments and feedback are greatly appreciated.

Xiao Feng Qi, CTN Associate Editor

Introduction

With the commercial implementation of 5G networks in full swing, service providers are striving for higher speed and lower latency data connections, to address the perpetually growing customer demand. Accordingly, higher radio frequencies (RF) and wider bandwidth massive MIMO systems are being developed for operational frequencies extending well into the millimeter wave (mmW) region of the spectrum and beyond for integrated sensing and communication (ISAC).  Furthermore, 3GPP roadmaps have identified major enhancements in the areas of artificial intelligence (AI) and extended reality (XR) that are expected to be implemented in the 2025 time frame [1].

The move to higher frequencies and wider bandwidths is putting significant stress on the corresponding RF components required to implement such advanced systems. To this end, recent developments in the field of radio-frequency-photonics (RF-photonics) may offer a viable alternative to supporting the desired implementations not only in the near-term, but long into the future. This arises with the ultra-wide carrier frequency range of RF-photonic devices and systems and their inherently large bandwidths that serve to enable previously unattainable performance using solely RF technologies. As an example, an RF phased array beamformer employing intermediatory optical processing is proposed in [2]–[4], and illustrated in Figure 1. Optical up-conversion directly at each element in the RF front-end by means of electrooptical modulator (EOM) produces optical outputs that are gathered into a fiber bundle that forms an array to re-launch the up-converted RF signals back into free space, where they re-form the exact beams that were incident on the front-end antenna array. Re-forming of the RF beams in the optical domain is possible due to the spatially coherent up-conversion process, which preserves not only the amplitude and phase of the RF signal at each antenna element, but also the phase relations between the elements. At this point, an optical lens performs a real-time, no-latency, no-power, two-dimensional (2D) spatial Fourier transformation, which focuses each incident wavefront to a unique spot, see Figure 1. Each spot is then coupled through a lenslet to a corresponding fiber that routes the optical signal to a high-speed photodetector (PD, not shown), where it is mixed with an optical local-oscillator for down-conversion to an intermediate frequency (IF) of choice for further digital processing. Due to the extremely wide bandwidth of the EOM and PD, the RF photonic approach offers unprecedented performance in terms of beam-bandwidth product with significantly lower power consumption. Successful applications in sensing (e.g., passive mmWave imaging [3]. PIC based imaging sensor and atmospheric RF spectrometer are also in the works) and wireless communication (e.g., multi-user massive MIMO at mmWave range [4]) have been demonstrated. It is worth noting that while Fourier lens is suitable for far-field beamforming, it is conceivable that the optical upconversion approach can be applied to near-field wavefront manipulation as well, with the lens replaced by more versatile optical processing based on holographic principles.

Silicon/Silicon Nitride (Si/SiN)

The primary silicon photonics platform consists of silicon-on-insulator (SOI) that leverages the large optical index contrast between silicon and silicon dioxide to provide highly confined optical waveguides. Silicon photonics leverages the existing CMOS foundry infrastructure to provide PICs with very high levels of complexity, and in some cases, integration with CMOS electronics has been demonstrated [11]. Accordingly, access to reliable foundries with PDKs for silicon photonics has become more common, and some current choices include: AIM Photonics, IMEC, IME, GlobalFoundries, INPHOTEC, TowerJazz, LioniX, Sandia National Labs, and LIGENTEC [12], [13], to name a few. Silicon nitride (SiN) has been included by extension since Si and SiN are compatible with one another and both can be processed using CMOS technologies. For the listed entities offering a PDK and MPW runs, some offer silicon only, others silicon nitride only, yet others both. The AIM Photonics Manufacturing Institute is one of the newest foundry that has both silicon and silicon nitride with a relatively large PDK offering [14].

The combination of readily accessible development options supporting high levels of complexity but limited bandwidth uniquely positions silicon PICs for handling RF-photonic system functionalities such as the optical processing in a RF-photonic phased array system. The optical processing functionality benefits significantly from high complexity, which supports future scaling to systems with high channel counts, while it does not require high frequency operation (the most demanding bandwidth requirement is the photodetectors, which only need to support direct down conversion to IF).

Indium Phosphide (InP)

The most popular III-V material platform of interest for a majority of PIC applications including RF-photonics is Indium Phosphide (InP), which supports light generation capabilities near 1.55 µm. Clearly, the main advantage of the InP platform is the ability to monolithically integrate lasers at the chip-scale [15]. Owing to this ability, an industry and corresponding infrastructure has been developed for wafer-scale processing of InP that includes all the necessary fundamental photonic device building blocks [16], [17]. Much of the developed high performance device building blocks (i.e., lasers, modulators, and photodetectors) is attributable to the optoelectronic industry; however, many of the prescribed devices have been developed individually. The photonic InP platform is based on epitaxial growth of materials specific to the desired device (i.e. a different type of epitaxial growth each for lasers, modulators and detectors), which makes high levels of integration and complexity extremely difficult and costly to achieve [18]. As such, the availability of InP-based photonic PDKs and MPW runs is considerably scarcer in comparison to silicon. The joint European platform for photonic integrated components and circuits (JePPIX) offers photonic InP MPW services, and more recently, Sandia National Labs has an offering that may be accessible through AIM Photonics [19].

The lack of access to MPW options for development of PICs in InP is the main reason for the red color code under the accessibility category in Table 1. Beyond the accessibility hurdle, the performance and device capabilities do provide an option for development of PICs in InP for RF-photonic applications.

Lithium Niobate and Thin Film Lithium Niobate

Lithium Niobate has long been a preferred material for analog optical RF applications, and specifically for EOMs, due to its superior electro-optic performance [20]. This performance has led to the development and commercial availability of optical modulators with bandwidth performance that is unmatched by other material systems and, furthermore, an industry infrastructure for producing such devices. Although several companies maintain lithium niobate specific foundries for their own product development, no MPW services currently exist. The lack of available MPW services presents a major obstacle to PIC development in lithium niobate.

The optical upconversion modulator within the RF-photonic phased array system benefits from development as an individual device due to the architecture breakdown of the system (as mentioned above). More specifically, positioning the modulator as closely as possible to the antenna while also allowing it to be separate from the remainder of the optical processing hardware maximizes the benefits of switching from RF processing to optical processing by minimizing the amount of RF components required and supporting flexibility in the configuration of the RF front-end (again, especially at lower frequencies where phased array antennas may be large).

The optical upconversion performance benefits of lithium niobate modulators have fueled the demonstration of RF photonic phased arrays systems, like the one described above, as well as passive mmW imagers, multi-channel transmit and receive systems, and RF photonic links. Nonetheless, the continued demand for better performance and higher levels of integration has led to a transition into the thin-film lithium niobate device development to pursue better performance and higher levels of integration.

The limitations on dense integration of bulk lithium niobate are due mainly to the low index contrast waveguides. Thin-film lithium niobate overcomes this limitation by combining lithium niobate with an oxide (typically silicon dioxide) to enable the development of higher index contrast waveguides. Higher index contrast waveguides in lithium niobate not only support higher levels of complexity in integration but also provide significant performance improvements to modulator devices. The reduction in the size of the optical mode allows for better overlap with the applied electric field for modulation, which leads to an improvement in the device efficiency (effectively a reduction of the half-wave voltage for modulation), and the increased presence of oxide improves the index matching between the RF and optical signals leading to improved modulation bandwidth [10], [21].

Although accessibility to fabrication capabilities is typically limited for lithium niobate, recent research and development, including heterogeneous integration of lasers, has shown that thin-film lithium niobate may be a considerably competitive platform for future RF-photonic systems [20], [22], [23].

An RF-Photonic Beamforming PIC

Based on the evaluation of system functionality and compatibility with available mature PIC development platforms, the demonstration of an optical processing PIC for a RF phased array system has been made possible using silicon photonics, as presented in Figure 2 [24], [25]. Figure 2(a) shows a schematic of an RF imaging system with optical processing that is receiving two different RF waves. The optical processing of the two RF waves produces spots at two different locations at the output dependent on their angles of arrival. Figure 2(b) and (c) show measured infrared images of the output of the optical processing PIC corresponding to two different RF waves as shown in Figure 2(a) noting that each incoming RF wave maps to a different output waveguide of the PIC. The optical processing PIC enabling this demonstration is shown in Figure 2(d), which only occupies a footprint of 0.25 by 0.15 inches. Figure 2(e) shows the same PIC after packaging with optical fibers and connections to electronic controls in a 2.1- by 1.6-inch aluminum housing.

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