Hybrid Chalcogenide‐Germanosilicate Waveguides for High Performance Stimulated Brillouin Scattering Applications

Abstract

On‐chip stimulated Brillouin scattering (SBS) in arsenic trisulfide (As2S3) planar waveguides lead to a range of outstanding demonstrations in microwave photonics signal generation and processing. However, the lack of other integrated functionalities, high back reflections, and large in‐ and out‐fiber coupling losses in high index contrast waveguides cause a number of serious impairments and lessen the applicability of microwave photonic devices. In this report, a hybrid integration scheme is demonstrated where As2S3 waveguides optimized for SBS gain are coupled with very low losses via a vertical taper to a high index contrast and versatile germanosilicate (Ge:SiO2) platform. The Ge:SiO2 waveguide is optimally mode‐matched to commercially available high numerical aperture optical fiber to achieve very low coupling losses. The structure has very low back reflection due to the adiabatic nature of the taper and negligible refractive index difference across the fiber‐chip interface. The hybrid architecture exhibits a similar Brillouin gain coefficient to its monolithic counterpart but with an improvement of >3 dB/facet fiber‐to‐chip loss and >20 dB reduction in facet reflectivity. The hybrid structures demonstrated will bring chalcogenide‐based chip scale SBS devices closer to practical application.

Chip-Based Brillouin Processing for Microwave Photonic Phased Array Antennas

“…Although integration of multiple materials enhances functionality, the transitions between different materials must be carefully designed to minimise reflections of the Brillouin pump [94], which increases the spurious content in the RF output [93]. For example, Lai et al measured a 4% reflection from the interface between lensed tip fibre and monolithic As 2 S 3 chip [94], which is lower than the reflection at the interface between air and bulk As 2 S 3 (≈17%) owing to the use of lensed-fibres and the anti-reflection coating at the facet of the As 2 S 3 chip [60]. Further reducing reflections of the Brillouin pump is desirable to minimise spurious content in the output of the MWP link [93].…”

“…Further reducing reflections of the Brillouin pump is desirable to minimise spurious content in the output of the MWP link [93]. To reduce the reflection experienced by the Brillouin pump, a mode-matched and vertically tapered transition between As 2 S 3 and germanosilicate waveguides [94] facilitated an adiabatic transition between materials, as shown in Fig. 6 (e).…”

“…19 (b), which works as follows; an input RF signal is modulated onto the optical carrier by MZM2 at (i), a modulation transformer [157], [158] generates the required modulation format at (ii), on-chip optical amplifiers set the pump and probe power as required before the probe undergoes Brillouin processing at (iii), an optical notch filter suppresses reflections of the Brillouin pump at (iv). Reflections are primarily caused by index changes between different materials and can be greatly reduced in an appropriately designed integrated platform, compared to systems that rely on a combination of chip-scale and fibre components [94]. Finally, the signal is converted back to the RF domain.…”

“…As discussed in Section IV, reflections must be minimised in systems that use backwards Brillouin processing to avoid degradation to the RF link performance [93]. The vertically tapered interface between Ge:SiO 2 and As 2 S 3 that Lai et al demonstrated [94] is an attractive method to achieve low-loss and low reflection interfaces in heterogeneously integrated MWP systems. Demonstrations of similar low-reflection transitions in future integrated Brillouin MWP processing platforms are essential to improve device performance.…”

“…Although RF interferometric processing techniques reduce the pump power required by Brillouin-based MWP processors [73], [93], [115], relatively large pump powers are still required to generate Brillouin gain over a wide bandwidth. For example, approximately 350 mW of coupled pump power was used by McKay et al to generate 4 dB of Brillouin gain over a 400 MHz bandwidth [73] in a Ge:SiO 2 -As 2 S 3 PIC [94]. By analysing (13), the magnitude of Brillouin gain generated for a fixed pump power is related to the figure of merit (FOM), G S BS L e f f , and determines how effective a material platform is at generating Brillouin gain [63].…”