Dual-wavelength-band subwavelength grating coupler operating in the near infrared and extended shortwave infrared

High-efficient subwavelength structure engineered grating couplers for 2-µm waveband high-speed data transmission

The 2-µm waveband is becoming an emerging window for next-generation high-speed optical communication. To enable on-chip high-speed data transmission, improving the signal-to-noise ratio (SNR) by suppressing the coupling loss of a silicon chip is critical. Here, we report grating couplers for TE and TM polarized light at the 2-µm waveband. With a single-step fully etched process on the 340 nm silicon-on-insulator (SOI) platform, the devices experimentally demonstrate high coupling efficiency of -4.0 dB and 1-dB bandwidth of 70 nm for the TE polarized light, while -4.5 dB coupling efficiency and 58 nm 1-dB bandwidth for the TM polarized light. For comprehensive performance, both of them are among the best grating couplers operating in the 2-µm waveband so far. We also demonstrate 81Gbps high-speed on-chip data transmission using pulse amplitude modulation 8-level (PAM-8) signals.

High-speed 4 × 4 silicon photonic plasma dispersive switch, operating at the 2 µm waveband

The escalating need for expansive data bandwidth, and the resulting capacity constraints of the single mode fiber (SMF) have positioned the 2-μm waveband as a prospective window for emerging applications in optical communication. This has initiated an ecosystem of silicon photonic components in the region driven by CMOS compatibility, low cost, high efficiency and potential for large-scale integration. In this study, we demonstrate a plasma dispersive 4 × 4 photonic switch operating at the 2-μm waveband with the highest switching speed. The demonstrated switch operates across a 45-nm bandwidth, with 10-90% rise and 90-10% fall time of 1.78 ns and 3.02 ns respectively. In a 4 × 4 implementation, crosstalk below -15 dB and power consumption lower than 19.15 mW across all 16 optical paths are indicated. This result brings high-speed optical switching to the portfolio of devices at the promising waveband.

High-efficiency mid-infrared on-chip silicon grating couplers for perfectly vertical coupling

We present, to our knowledge, the first experimental demonstration of two on-chip gratings for perfectly vertical coupling at wavelengths of 3350 nm and 3550 nm, respectively. An anti-backreflection unit containing a fully etched trench and a subwavelength pillar is introduced in each grating period, together with a binary-approximated blazed unit, interleaving fully and shallow-etched slots in 500-nm thick silicon film. Both gratings show a strong ability to eliminate backreflection and provide predicted directionality of around 80%. The physical theoretical analysis is applied during further apodization for mitigating the computation of the optimization algorithm, improving the efficiency and optimization reliability, and increasing the fabrication robustness. The measured coupling efficiencies (CEs) of the gratings are -5.58 dB and -4.34 dB at wavelengths of 3350 nm and 3550 nm, with a 3-dB bandwidth of at least 87 nm and 210 nm, respectively.

Mid-infrared polarization-insensitive grating coupler

Mid-infrared (Mid-IR) (2-20 µm) silicon photonics has attracted much attention in the past few years due to its application potential in free-space optical communications, light detection and ranging, and molecular analysis. The grating coupler technology is one of the most widely employed approaches for light coupling between optical fibers and waveguides. In the mid-IR spectral region, due to the lack of reliable chalcogenide-fiber or ZBLAN-fiber polarization controllers, grating couplers usually suffer from huge insertion losses induced by the arbitrary polarization states of light coupled out of mid-IR fibers. As a result, it is significant to explore polarization-insensitive grating coupling techniques in mid-IR wavelengths. However, the study is currently still in its infancy. Here, we demonstrate an ultra-thin mid-IR polarization-insensitive grating coupler. The grating coupler has a maximum coupling efficiency of -11.5 dB at a center wavelength of ∼2200 nm with a 1-dB bandwidth of ∼148 nm. Compared with conventional subwavelength grating couplers, the polarization-dependent loss was improved from 9.6 dB to 2.1 dB. Moreover, we demonstrated a polarization-insensitive grating coupler at 2700-nm wavelength with a maximum coupling efficiency of -12.0 dB. Our results pave the way for the development of mid-IR photonic integrated circuits.

Single-step etched two-dimensional polarization splitting dual-band grating coupler for wavelength (de)multiplexing

Diffractive periodic-structure-based grating couplers (GCs) are the most widely used devices for light coupling between optical fibers and integrated photonic devices. However, conventional GCs have limited wavelength operation and are polarization specific, which is due to the intrinsic radiation angle dependency on both wavelength and polarization. Here we propose and experimentally demonstrate a polarization-splitting dual-band grating coupler (PS-DBGC) for polarization diversity and wavelength division (de)multiplexing (WDM) operation. The four-port two-dimensional PS-DBGC is based on a periodically arranged structure with square holes, and requires only a single etch step in a 340-nm silicon-on-insulator platform. The simulation predicts that the maximum coupling efficiency (CE) of the proposed PS-DBGC is -2.8 dB and -4.6 dB for the O- and C-band, respectively. The measured peak CEs of the fabricated device are -4.7 dB at 1280 nm and -8.4 dB at 1522 nm. We anticipate that this PS-DBGC could potentially improve the performance of any future integrated WDM passive optical network.

Ultra-thin mid-infrared silicon grating coupler

Mid-infrared (mid-IR) silicon photonics has been attracting great attention due to its tremendous potential applications in nonlinear optics, ranging, sensing, and spectroscopy. To date, mid-IR silicon devices have usually been developed based on silicon wafers with top-layer silicon thicknesses of hundreds of nanometers. Compared with the thick silicon devices, tens-of-nanometers thin silicon devices can provide giant evanescent-field energy proportions and optical mode areas, being significant for many biochemical sensing and nonlinear optics applications. However, ultra-thin mid-IR silicon devices have seldom been studied due to the difficulty of light coupling. Here, we demonstrated an ultra-thin focusing subwavelength-grating coupler for mid-IR ultra-thin suspended subwavelength-grating-cladding waveguide coupling. The results show that the grating has a maximum coupling efficiency of -7.1 dB at a center wavelength of 2200 nm with a 1-dB bandwidth of ∼115 nm and back reflection of -19.9 dB. We also measured the fiber alignment tolerance of 12 µm for 3-dB coupling efficiency reduction and bending optical loss of 0.25 dB/90°. Our results pave the way to developing mid-IR ultra-thin photonic integrated circuits.

Inverse design of multi-band and wideband waveguide crossings

Photonic integrated circuits for wideband and multi-band optical communications will need waveguide crossings that operate at all the wavelengths required by the system. In this Letter, we use the modified gradient decedent method to optimize the dual-wavelength band (DWB) crossings on both single- and double-level platforms. On the single-level platform, the simulation results show insertion losses (ILs) less than 0.07 and 0.11 dB for a crossing working at a DWB of 1.5-1.6 and 1.95-2.05 µm. ILs are less than 0.1 and 0.2 dB for a crossing operating in the DWB of 1.5-1.6 and 2.2-2.3 µm. On the double-layer platform, the simulated results show IL less than 0.08 dB across the wavelength range of 1.25-2.25 µm. We experimentally demonstrate the DWB crossing operating at 1.5-1.6 and 2.2-2.3 µm to have IL less than 0.3 and 0.4 dB and crosstalk of -28 and -26dB in the two bands, respectively.

Dual-band fiber-chip grating coupler in a 300 mm silicon-on-insulator platform and 193 nm deep-UV lithography

Surface grating couplers are fundamental building blocks for coupling the light between optical fibers and integrated photonic devices. However, the operational bandwidth of conventional grating couplers is intrinsically limited by their wavelength-dependent radiation angle. The few dual-band grating couplers that have been experimentally demonstrated exhibit low coupling efficiencies and rely on complex fabrication processes. Here we demonstrate for the first time, to the best of our knowledge, the realization of an efficient dual-band grating coupler fabricated using 193 nm deep-ultraviolet lithography for 10 Gbit symmetric passive optical networks. The footprint of the device is 17×10µm². We measured coupling efficiencies of -4.9 and -5.2dB with a 3-dB bandwidth of 27 and 56 nm at the wavelengths of 1270 and 1577 nm, corresponding to the upstream and downstream channels, respectively.

Grating Couplers on Silicon Photonics: Design Principles, Emerging Trends and Practical Issues

Silicon photonics is an enabling technology that provides integrated photonic devices and systems with low-cost mass manufacturing capability. It has attracted increasing attention in both academia and industry in recent years, not only for its applications in communications, but also in sensing. One important issue of silicon photonics that comes with its high integration density is an interface between its high-performance integrated waveguide devices and optical fibers or free-space optics. Surface grating coupler is a preferred candidate that provides flexibility for circuit design and reduces effort for both fabrication and alignment. In the past decades, considerable research efforts have been made on in-plane grating couplers to address their insufficiency in coupling efficiency, wavelength sensitivity and polarization sensitivity compared with out-of-plane edge-coupling. Apart from improved performances, new functionalities are also on the horizon for grating couplers. In this paper, we review the current research progresses made on grating couplers, starting from their fundamental theories and concepts. Then, we conclude various methods to improve their performance, including coupling efficiency, polarization and wavelength sensitivity. Finally, we discuss some emerging research topics on grating couplers, as well as practical issues such as testing, packaging and promising applications.