We report on the design and characterization of ring-assisted (RA) few-mode fibers with improved minimum mode effective index difference MinΔn _eff . By introducing a high-index RA structure whose position coincides with the trough of mode power distribution of LP ₀₂ mode, the effective index of the LP ₂₁ mode can be largely elevated while that of LP ₀₂ mode remains almost unaffected. The dependences of both Δn _eff and effective mode area A _eff on different ring parameters have been investigated. We fabricated RA 4-mode fibers with a minimum Δ _neff (1.8 × 10 ^-3 ) which is 2.2 times larger than that of step-index (SI) 4-mode fiber (0.8 × 10 ^-3 ) without obvious degradation on other characteristics. The fabricated RA 4-mode fiber has a low mode-average loss and a low mode dependent loss of 0.23 and 0.02 dB/km at 1550-nm wavelength, respectively, which are comparable with that of an SI 4-mode fiber fabricated for reference purposes. The distributed inter-mode crosstalk of the fibers has been measured using swept-wavelength interferometry with a winding tension of 0.5 N. We observed similar distributed inter-mode crosstalk between the RA-FMF and the SI-FMF with low tension and the results show that no additional deformation was introduced by the high-index ring. As a result, the fabricated RA-FMF with a larger MinΔn _eff is expected to be more robust than SI-FMF in real applications. Moreover, we adopted this method for the design of 7-mode fibers and successfully obtained a minimum Δn _eff as high as 1.7 × 10 ^-3 and an effective area larger than 100 μm ² .

We comprehensively evaluated optical characteristics, environmental stabilities, fiber optics compatibilities, and high-speed performances of multimode polymer waveguides fabricated in small batch with a length of 946 mm. The waveguides exhibit a low transmission loss (0.046 dB/cm at a wavelength of 850 nm), low interchannel crosstalk (≤ -58 dB), and a large misalignment tolerance (±20 μm) between fiber and waveguide. The waveguides show good environmental stability and the increase of propagation loss is as small as 0.005 dB/cm and 0.004 dB/cm after a 1000-h ageing test and five solder reflow cycles, respectively. The waveguides have good compatibilities with fibers having different core diameters and index profiles that may be encountered in the real application environment. High-speed transmission performances were evaluated using both nonreturn to zero (NRZ) transmission at data rates of 25 Gb/s and 30 Gb/s, and PAM4 transmission at data rates of 20 Gb/s and 56 Gb/s, respectively. There is no obvious degradation on the eye diagram due to the insertion of the waveguide. Error free transmission were successfully obtained for NRZ transmission at data rates of both 25 Gb/s and 30 Gb/s. The BER for PAM4 transmission reached a level of 10 ^-4 at data rates of both 10 Gbaud (20 Gb/s) and 28 Gbaud (56 Gb/s), which is well below the forward error correction limitation. Moreover, it is the bandwidth of the test equipment rather than the polymer waveguide itself that limits the high-speed transmission performances in our experiments. The results demonstrate that the polymer waveguides are good candidates for high-speed and meter-scale on-board optical interconnect applications.

We demonstrate a III-V/Si widely wavelength tunable laser covering the entire L-band for the optical communication systems. By carefully designing the silicon ring filter, low threshold current and high output power are expected. Using the standard silicon photonic process, the silicon filter chip is fabricated and compactly packaged with a reflective semiconductor optical amplifier. The low threshold current of 20 mA is achieved at 15 °C, and over 76 mW output power is obtained at 320 mA. The wavelength tuning range from 1565 nm to 1635 nm is realized with the side-mode suppression ratio larger than 50 dB. Furthermore, the intrinsic linewidth narrower than 25 kHz and relative intensity noise below −152 dB/Hz is achieved, which can support the coherent communications.

An adjoint-based shape optimization approach for the inverse design of microwave components is proposed. The proposed approach is a pure postprocessing process that only needs the field solutions. It is independent on numerical methods, system matrix, and mesh information, thus can be used in conjunction with mainstream Maxwell solvers currently available on the market, which makes it an efficient, convenient, and user-friendly method. To be applicable to the shape optimization of complex cases, this approach can calculate shape derivatives at all points in design region with only one simulation by a rigorous analytical solution, which makes it have arbitrarily many degrees of freedom (DoFs) while the reported similar approaches can only deal with several variables. Flexible parameters’ updating is achieved by the level set method. Owing to the speed of calculating shape derivatives at boundary points within adjoint method, coupled with the accurate updating of shape by gradient descent and level set method, our approach normally requires several or dozens of iterations even in cases with numerous DoFs. The developed approach is used to optimize a high-directivity directional coupler and an unequal Wilkinson power divider to verify its validity and robustness.

An ultracompact and broadband silicon wavelength demultiplexer based on SWG-assisted directional coupler is designed and experimentally demonstrated for wavelength splitting of 1310 and 1550 nm. The embedded SWG enables significantly reduced footprint, due to the precise phase matching at 1550 nm and large phase mismatch at 1310 nm. As a result, the fabricated device has a length of only 9 μm, and provides high extinction ratios (ERs) of 23 dB and 19 dB at 1310 and 1550 nm, respectively. Its 3-dB bandwidths are 85 nm for O-band (ER > 15 dB) and 140 nm for C-band (ER > 10 dB).

With continuously increasing demand for broadband services, such as big data, cloud computing, and large-capacity communications, wavelength division multiplexing (WDM) technology has been widely used thanks to its ability to increase the number of channels and the transmission bandwidth.¹ Significant attention is paid to wavelength demultiplexers with features of compact size, low insertion loss (IL), high extinction ratio (ER) and broad bandwidth.² What’s more, silicon on-insulator (SOI) is an attractive platform for wavelength multiplexers due to its high integration density and fabrication compatibility with the complementary metal-oxide-semiconductor technology.