A novel power-efficient reconfigurable mode converter is proposed and experimentally demonstrated based on cross-connected symmetric Y-junctions assisted by thermo-optic phase shifters on a silicon-on-insulator platform. Instead of using conventional Y-junctions, subwavelength symmetric Y-junctions are utilized to enhance the mode splitting ability. The reconfigurable functionality can be realized by controlling the induced phase differences. Benefited from the cross-connected scheme, the number of heating electrodes can be effectively reduced, while the performance of the device is maintained. With only one-step etching, our fabricated device shows the average insertion losses and cross talks are less than 2.45 and -16.6dB, respectively, measured with conversions between two arbitrary compositions of the first four TE modes over an observable 60 nm bandwidth. The converter is switchable and CMOS-compatible, and could be extended for more modes; hence, it can be potentially deployed for advanced and flexible mode multiplexing optical networks-on-chip.

A high-efficiency inverse design of “digital” subwavelength nanophotonic devices using the adjoint method is proposed. We design a single-mode 3 dB power divider and a dual-mode demultiplexer to demonstrate the efficiency of the proposed inverse design approach, called the digitized adjoint method, for single- and dual-object optimization, respectively. The optimization comprises three stages: 1) continuous variation for an “analog” pattern; 2) forced permittivity biasing for a “quasi-digital” pattern; and 3) a multilevel digital pattern. Compared with the conventional brute-force method, the proposed method can improve design efficiency by about five times, and the performance optimization can reach approximately the same level. The method takes advantages of adjoint sensitivity analysis and digital subwavelength structure and creates a new way for the efficient and high-performance design of compact digital subwavelength nanophotonic devices, which could overcome the efficiency bottleneck of the brute-force method, which is restricted by the number of pixels of a digital pattern, and improve the device performance by extending a conventional binary pattern to a multilevel one.

In this paper, an integrated compact four-channel directly modulated analog optical transceiver is proposed and fabricated. The 3 dB bandwidth of this optical transceiver exceeds 20 GHz, and the measured spurious-free dynamic range is up to 91.2dB·Hz2/3. The optical coupling efficiency (CE) is improved by using a precise submicron alignment technique for lens coupling in a transmitter optical subassembly, and the highest CE is achieved when the oblique angle of the arrayed waveguide grating using a silica-based planar lightwave circuit (AWG-PLC) in receiver optical sub assembly is set to 42°. Based on the proposed optical transceiver, we have experimentally demonstrated a 6.624 Gbit/s 4×4 multi-input multioutput (MIMO) 16-quadrature amplitude modulation orthogonal frequency division multiplexing (16QAM-OFDM) radio signal over 15.5 km standard single mode fiber, together with 1.2 m wireless transmission in both an uplink and a downlink. To cope with the channel interference and noise of the fiber-wireless transmission system, a low-complexity MIMO demodulation algorithm based on lattice reduction zero-forcing (LR-ZF) is designed. In our experiment, 1.6 dB power penalty is achieved by using the proposed LR-ZF algorithm, compared to the commonly used zero-forcing algorithm. Moreover, this LR-ZF algorithm has much less complexity than the optimal maximum-likelihood sequence estimation (MLSE) at a given transmission performance. These results not only demonstrate the feasibility of the integrated optical transceiver for MIMO fiber-wireless application but also validate that the proposed LR-ZF algorithm is effective to eliminate the interference for hybrid fiber-wireless transmission.

With the rapidly increasing bandwidth requirements of optical communication networks, compact and low-cost large-scale optical switches become necessary. Silicon photonics is a promising technology due to its small footprint, cost competitiveness, and high bandwidth density. In this paper, we demonstrate a 12×12 silicon wavelength routing switch employing cascaded arrayed waveguide gratings (AWGs) connected by a silicon waveguide interconnection network on a single chip. We optimize the connecting strategy of the crossing structure to reduce the switch’s footprint. We develop an algorithm based on minimum standard deviation to minimize the port-to-port insertion loss (IL) fluctuation of the switch globally. The simulated port-to-port IL fluctuation decreases by about 3 dB compared with that of the conventional one. The average measured port-to-port IL is 13.03 dB, with a standard deviation of 0.78 dB and a fluctuation of 2.39 dB. The device can be used for wide applications in core networks and data centers.

We propose and experimentally demonstrate a novel ultracompact dual-mode waveguide crossing based on subwavelength multimode-interference couplers for a densely integrated on-chip mode-division multiplexing system. By engineering the lateral-cladding material index and manipulating phase profiles of light at the nanoscale using an improved inverse design method, a subwavelength structure could theoretically realize the identical beat length for both TE0 and TE1, which can reduce the scale of the device greatly. The fabricated device occupied a footprint of only 4.8 μm×4.8 μm. The measured insertion losses and crosstalks were less than 0.6 dB and 24 dB from 1530 nm to 1590 nm for both TE0 and TE1 modes, respectively. Furthermore, our scheme could also be expanded to design waveguide crossings that support more modes.