Advancements in waveguide architectures using high-performance silica-on-silicon platform - INVITED

. Novel applications in optical coherence tomography (OCT) and LiDAR systems have become possible due to performance characteristics of a state-of-the-art silica-on-silicon planar lightwave circuit (PLC) platform. We have achieved ultra-low propagation losses of <0.009 dB/cm with unmatched phase control in a polarization-insensitive way, enabling a range of real-time advanced vision and imaging applications utilizing k-clocks and analog frequency sampling architectures.


Introduction
Over the past several years, we have witnessed a progress in integrated photonics [1] that lead to rapid increase in demand for integrated photonic solutions not only in high-speed communication but also in advanced vision systems.Among the different integrated platforms, silica-on-silicon planar lightwave circuit (PLC) technology stands out as a versatile and low-cost platform with several unmatched characteristics, including ultralow propagation losses, efficient fiber coupling, unparalleled polarization and phase control, excellent reliability and established high-volume fabrication processes.These characteristics enable silica-on-silicon PLCs to be deployed in applications ranging from medical imaging to autonomous driving to precision manufacturing.
In this paper, we review the key performance metrics of our silica-on-silicon PLC platform that were instrumental for the adoption of the technology in advanced vision and imaging applications.We then demonstrate key optical building blocks that are necessary for today's emerging applications.

Silica-on-silicon PLC platform
Our PLC chips are fabricated using standard atmospheric pressure chemical-vapour deposition (APCVD) and reactive ion etching processes.To achieve a good compromise between the strength of optical confinement and optical performance, we focus on a relatively low refractive index contrast of ∆n = 2.0% with typical waveguide dimensions of around 3×3 μm.
A series of design and fabrication optimizations have allowed us to achieve exceptional performance capabilities of fabricated devices.Previously reported results demonstrated 10-meter long spirals [2] with worstcase propagation losses of 0.01 dB/cm.Through further optimizations, we can achieve propagation losses of 0.009 dB/cm or better, with negligible polarization-dependent loss and wavelength-independent operation.Combined with fiber-matched mode converters (0.5 dB per facet) and temperature-stable operation (< 10 pm/°C), consistent performance is achieved in advanced vision applications around both 1300 nm and 1550 nm.In addition, the low absorption and the lack of second-order non-linear properties in silica make silica-on-silicon chips wellsuited for use with high-power scanning laser arrays.
In spite of the relatively low refractive index contrast, we have successfully introduced advanced architectural solutions that allow us to achieve ultra-compact devices.State-of-the-art coarse-wavelength division multiplexing (CWDM) and LAN-WDM multiplexers [3][4] have been achieved using a dense arrangement of arbitrarily-long interferometric structures.Comprehensive optimizations of our process and design parameters allow us to realize a wide variety of devices that rely on coherence effects and precise phase control.In addition to their high optical performance characteristics, the designs have proven to be remarkably robust in volume manufacturing, resulting in their high volume deployments in commercial applications.

High performance optical building blocks
Polarization beam splitters (PBSs) are key enabling blocks in coherent communication systems.PBSs have enabled some of the most spectrally efficient fiber-optic communication systems, such as polarization multiplexed quadrature phase shift keying (PM-DQPSK) [5].Most recently, PLC-based PBSs have emerged as key components in advanced vision systems, such as time-offlight (ToF) and frequency-modulated continuous-wave (FMCW) LiDARs, where they are used to improve overall collection efficiency.Typical LiDAR systems contain from one to a few dozens of PBSs, requiring highly integrated chip-based solutions to make them practical.
The challenge of integrating PBSs on a chip stems from the requirement of achieving precise absolute and relative phase control.Silica-on-silicon PLCs are ideally suited for realizing polarization control on a chip, and we have specifically focused on the development of PLC-based PBSs that are capable of operating in a wide frequency range.While many different PBS architectures have been explored [6], we have chosen an architecture that comprises a pair of Hermitian-conjugated multi-section couplers interjected with a birefringent region formed by lithographically varying the waveguide width.To lock the phase, we applied our end-to-end optimization technique to achieve wavelength-independent couplers and a πphase difference between the two polarizations in the birefringent region, while simultaneously cancelling the absolute phase difference in the principal polarization states.A typical transmission spectrum of such a PBS is shown in Figure 1, where isolation of more than 22 dB has been achieved in the entire operational range of 1510-1575 nm.When used in an arrayed form, these PBSs have dramatically improved the range of LiDAR systems by means of crosstalk suppression and added selectivity with respect to depolarized scattering.Medical imaging, and especially optical coherence tomography (OCT), has also benefited from the low propagation losses and tight polarization control that is possible in silica-on-silicon PLCs [7][8].In such systems, the reference signal is optically delayed on the chip from a few picoseconds to a few nanoseconds to match the round trip pathlength to the studied object, creating interference between the reference and the sample signals.
We leverage the advantages of the PLC platform to realize variable free spectral range k-clocks, whose frequency is inversely proportional to the length of the delay line in a Mach-Zehnder interferometer.This is critical as both OCT and LiDAR systems measure the signal in the momentum (k)-space, which is proportional to the optical frequency of the swept source.The sampled output provides a highly linear optical frequency fiducial marker and facilitates resampling of the signals to allow high-resolution, real-time and in-situ imaging of tissue microstructure in a non-invasive manner [9].The extremely low propagation losses of the PLC platform allow for a wide range of k-clocks to be easily manufactured, from GHz k-clocks typically used in OCT systems to MHz k-clocks used in LiDAR.Figure 2 shows design examples of k-clocks with vastly different frequency responses.

Conclusions
High-speed communication and advanced vision systems have come to rely on the state-of-the-art silica-on-silicon platform due to its unprecedented performance characteristics, including ultra-low propagation losses, efficient fiber coupling, temperature stability, and unmatched polarization and phase control.We have shown advancements in the design of integrated polarization beam splitters and variable frequency kclocks that led to a wide use of the silica-on-silicon platform in OCT and LiDAR applications.

Fig. 2 .
Fig. 2. Examples of K-clock system-on-chip with a frequency sampling rate of 10 GHz (left) and 200 MHz (right).