Library of single-etch silicon nitride grating couplers for low-loss and fabrication-robust fiber-chip interconnection

Abstract

Silicon nitride (Si3N4) waveguides become an appealing choice to realize complex photonic integrated circuits for applications in telecom/datacom transceivers, sensing, and quantum information sciences. However, compared to high-index-contrast silicon-on-insulator platform, the index difference between the Si3N4 waveguide core and its claddings is more moderate, which adversely affects the development of vertical grating-coupled optical interfaces. Si3N4 grating couplers suffer from the reduced strength, therefore it is more challenging to radiate all the waveguide power out of the grating within a beam size that is comparable to the mode field diameter of standard optical fibers. In this work, we present, by design and experiments, a library of low-loss and fabrication-tolerant surface grating couplers, operating at 1.55 μm wavelength range and standard SMF-28 fiber. Our designs are fabricated on 400 nm Si3N4 platform using single-etch fabrication and foundry-compatible low-pressure chemical vapor deposition wafers. Experimentally, the peak coupling loss of − 4.4 dB and − 3.9 dB are measured for uniform couplers, while apodized grating couplers yield fiber-chip coupling loss of − 2.9 dB, without the use of bottom mirrors, additional overlays, and multi-layered grating arrangements. Beside the single-hero demonstrations, over 130 grating couplers were realized and tested, showing an excellent agreement with finite difference time domain designs and fabrication-robust performance. Demonstrated grating couplers are promising for Si3N4 photonic chip prototyping by using standard optical fibers, leveraging low-cost and foundry-compatible fabrication technologies, essential for stable and reproducible large-volume device development.

High-Efficiency Metamaterial-Engineered Grating Couplers for Silicon Nitride Photonics

“…The calculated 1 dB and 3 dB bandwidths are 30 nm and 50 nm, respectively. These findings mark a significant improvement over the performance achievable with SiN-only couplers [27]. Furthermore, the efficiency can be further enhanced by increasing the fiber-grating overlap through coupler apodization, as discussed in the following section.…”

“…To date, SOI and Si 3 N 4 photonic integrated circuits are now capable of achieving close to 1 dB efficiency through careful optimization of the directionality and field overlap between the radiated beam and the optical fiber mode. The fiber-grating field overlap, typically limited to 80% for uniform structures [24], can be enhanced through near-field apodization [25][26][27] or the beam-focalizing technique [28,29]. Near-field apodization is generally implemented by varying the geometry of the grating coupler in order to control the coupling length, i.e., to regulate the amount of waveguide power that is radiated by each grating period.…”

Repeatable Passive Fiber Optic Coupling of Single-Mode Waveguides in High-Precision Disposable Photonic Biosensors

“…One solution to mitigate coupling losses and enhance tolerance is the implementation of vertical coupling grating structures in combination with waveguide tapers [9]. Grating couplers are described by the Bragg condition, as expressed in Equation (1), where N is an integer representing the diffraction order, λ0 is the free-space wavelength, neff is the effective refractive index of the grating, Λ is the grating period, and θ denotes the angle of light diffraction [18]. The light emitted by the grating exhibits an exponentially decaying power profile along the propagation direction [19].…”

High-efficiency off-chip grating couplers for silicon nitride photonics

“…The Si3N4 takes advantages of a broadband transparency, well-established deposition techniques, and a lower index contrast. So far, a wide range of chip-integrated building blocks has been developed and demonstrated, including ultra-low-loss and dispersion-engineered waveguides [4,5], splitters [6], resonators [7], filters [8], modulators [9] and fiber-chip grating and edge couplers [10][11][12][13], to name a few. The index contrast between the Si3N4 core and the claddings is low, therefore it is still an important design objective to realize efficient connections between the outside world and Si3N4 chips.…”