Folded Heterogeneous Silicon and Lithium Niobate Mach-Zehnder Modulators with Low Drive Voltage.

Optical modulators were, are, and will continue to be the underpinning devices for optical transceivers at all levels of the optical networks. Recently, heterogeneously integrated silicon and lithium niobate (Si/LN) optical modulators have demonstrated attractive overall performance in terms of optical loss, drive voltage, and modulation bandwidth. However, due to the moderate Pockels coefficient of lithium niobate, the device length of the Si/LN modulator is still relatively long for low-drive-voltage operation. Here, we report a folded Si/LN Mach-Zehnder modulator consisting of meandering optical waveguides and meandering microwave transmission lines, whose device length is approximately two-fifths of the unfolded counterpart while maintaining the overall performance. The present devices feature a low half-wave voltage of 1.24 V, support data rates up to 128 gigabits per second, and show a device length of less than 9 mm.

Reconfigurable silicon bandpass filters based on cascaded Sagnac loop mirrors.

We demonstrate a high-performance reconfigurable bandpass filter implemented by cascaded Sagnac loop mirror (SLM)-based coupled resonator optical waveguides (CROWs) on the silicon-on-insulator platform. By dynamic thermal tuning of the reflectivity in each SLM, the proposed filter can achieve simultaneous 3 dB bandwidth tuning from 8.50 to 20.25 GHz and a central wavelength tuning range of 216.25 GHz. A box-like filtering response with an ultra-high extinction ratio up to 70 dB and an ultra-sharp roll-off of 0.61 are observed in a 6th-order SLM-coupled resonator optical waveguide (SLM-CROW). The proposed reconfigurable SLM-CROW filter can satisfy the demand for next-generation flexible-grid WDM networks.

Ultra-compact broadband polarization diversity orbital angular momentum generator with 3.6 × 3.6 µm2 footprint.

Orbital angular momentum (OAM), one fundamental property of light, has been of great interest over the past decades. An ideal OAM generator, fully compatible with existing physical dimensions (wavelength and polarization) of light, would offer the distinct features of broadband, polarization diversity, and ultra-compact footprint. Here, we propose, design, fabricate, and demonstrate an ultra-compact chip-scale broadband polarization diversity OAM generator on a silicon platform with a 3.6 × 3.6 µm2 footprint. The silicon OAM chip is formed by introducing a subwavelength surface structure (superposed holographic fork gratings) on top of a silicon waveguide, coupling the in-plane waveguide mode to the out-plane free-space OAM mode. We demonstrate in theory and experiment the broadband generation of polarization diversity OAM modes (x-/y-polarized OAM+1/OAM-1) from 1500 to 1630 nm with high purity and efficiency. The demonstrations of an ultra-compact broadband polarization diversity OAM generator may open up new perspectives for OAM-assisted N-dimensional optical multiplexing communications/interconnects and high-dimensional quantum communication systems.

Orbital angular momentum vector modes (de)multiplexer based on multimode micro-ring.

Orbital angular momentum (OAM) multiplexing has emerged as an important method to increase the communication capacities in future optical information technologies. In this work, we demonstrate a silicon integrated OAM (de)multiplexer with a very simple structure. By simply tapping the evanescent wave of two different whispering gallery modes rotating inside a multimodal micro-ring resonator, four in-plane waveguide modes are converted to four free-space vector OAM beams with high mode purity. We further demonstrate chip-to-chip OAM multiplexing transmission using a pair of silicon devices, which shows low-level mode crosstalk and favorable link performance.

Generating and synthesizing ultrabroadband twisted light using a compact silicon chip.

Compact and broadband manipulation of spatial modes is important in applications exploiting the space domain of light waves. Here, we demonstrate chip-scale generation and synthesization of ultrabroadband orbital angular momentum (OAM) modes (twisted light) on a silicon platform. By introducing a subwavelength holographic fork grating on top of a silicon waveguide, the in-plane guided mode is converted to the free-space OAM mode. Inputs from both sides of the waveguide enable the synthesization of OAM modes. We also characterize wavelength-dependent emission efficiency, offset angle, and purity with favorable performance. The chip-scale ultrabroadband OAM generator and synthesizer may find potential applications in multidimensional optical communications and quantum key distribution.

Low-loss two-dimensional silicon photonic grating coupler with a backside metal mirror.

We design and fabricate a low-loss silicon photonic two-dimensional grating coupler that serves to couple light between standard single-mode fibers and single-mode waveguides in the silicon-on-insulator platform and to split both orthogonal polarization states. The efficiency of the fabricated device is enhanced by a backside metal mirror and reaches a record value of -1.8 dB with a 1 dB bandwidth of 32 nm around 1550 nm. The demonstrated coupling efficiency is 2.2 dB better than the conventional two-dimensional grating coupler without a metal mirror.

Compact tunable electromagnetically induced transparency and Fano resonance on silicon platform.

We propose and demonstrate an on-chip coupling resonant system to generate electromagnetically induced transparency (EIT)-like effect and Fano resonance on silicon platform. It is composed of a microring resonator (MRR) and two cascaded Sagnac-loop mirrors (SLMs) assisted Fabry-Perot (FP) cavity on silicon-on-insulator. According to the coupling conditions of the MRR, two cases are studied theoretically. When the MRR is over coupling, EIT-like transmission can be observed. In contrast, Fano resonances can be generated by the condition of under coupling. In the experiment, the add-drop MRR is under coupling, leading to a sharp asymmetric line shape for Fano resonance. The resonance wavelength of the MRR can be dynamically tuned based on thermal-optic effects by tuning the micro-heater. The experiment results show Fano resonances with maximum extinction ratio (ER) of 23.22 dB and maximum slope rate (SR) of 252 dB/nm. Moreover, the wavelength of Fano resonance can be shifted widely with a tuning efficiency of 0.2335 nm/mW.

Compact high-efficiency perfectly-vertical grating coupler on silicon at O-band.

A compact, high-efficiency grating coupler is demonstrated for interfacing a silicon waveguide and a perfectly-vertical fiber at O-band. The grating lies on a tilted silicon membrane for minimizing the reflections. Circular grating lines are adopted to shorten the overall device length to about 60µm. 57% peak coupling efficiency and >28nm 1-dB coupling bandwidth are obtained experimentally. Back reflections of 1% to the silicon waveguide and the single mode fiber are theoretically estimated. The processing flow to realize the proposed structure is discussed in detail. The fabrication control over the tilted angle of the silicon membrane is investigated. The approach by applying an oxide cladding to improve the stability of the membrane is also introduced. The present grating coupler is compatible to common fabrication processes for silicon photonic chips.

Photonic linear chirped microwave signal generation based on the ultra-compact spectral shaper using the slow light effect.

A novel concept to generate a linear chirped microwave signal is proposed and experimentally demonstrated. The frequency to time mapping method is employed, where the photonic crystal waveguide Mach-Zehnder interferometer structure acts as the spectral shaper thanks to the slow light effect. By optimizing the structural parameters of the photonic crystal waveguide, a linear chirped microwave signal with the time-bandwidth product of about 30 is experimentally obtained. The impact of the slow light photonic crystal waveguide on the generated linear chirped microwave signal is also investigated. The utilization of the slow light effect brings in significant advantages, including the ultra-small footprint of 0.096 mm2 and simple structure to our scheme, which may be of great importance towards its potential applications.

Integrated optical vortex beam receivers.

A simple and ultra-compact integrated optical vortex beam receiver device is presented. The device is based on the coupling between the optical vortex modes and whispering gallery modes in a micro-ring resonator via embedded angular gratings, which provides the selective reception of optical vortex modes with definitive total angular momentum (summation of spin and orbital angular momentum) through the phase matching condition in the coupling process. Experimental characterization confirms the correct detection of the total angular momentum carried by the vortex beams incident on the device. In addition, photonic spin-controlled unidirectional excitation of whispering-gallery modes in the ring receiver is also observed, and utilized to differentiate between left- and right-circular polarizations and therefore unambiguously identify the orbital angular momentum of incident light. Such characteristics provide an effective mode-selective receiver for the eigen-modes in orbital angular momentum fiber transmission where the circularly polarized OAM modes can be used as data communications channels in multiplexed communications or as photonic states in quantum information applications.