On-chip digitally tunable positive/negative dispersion controller using bidirectional chirped multimode waveguide gratings

A means to calculate the multimodal spectral response of Bragg gratings in general non-circular multimode waveguides is proposed. To illustrate the power of the technique, the spectra of two Bragg temperature sensors are numerically calculated in which coupling between 100 modes is considered. It is shown how the Bragg wavelength in multimode Bragg grating waveguides is affected by the number of modes and energy distribution among them. Good matching of the simulated spectrum of a multimode Bragg grating on a planar inverted rib waveguide to the measured spectrum is seen.

Optical phased arrays (OPAs) have received considerable attention as solid-state beam scanners. However, conventional OPAs that actively control the phase difference between arrays are characterized by excessive power consumption for high-precision beam emission. In this study, we fabricated an OPA comprising Bragg grating and arrayed waveguide grating (AWG). Multi-mode waveguide is used in AWG to reduce the effect of manufacturing error. This device realizes wide and high-resolution two-dimensional beam steering only by sweeping wavelength. FWHM of the emitted beam is 0.534° × 2.27°, and the steering range is 43.9° × 13.5° with 1/64 of the power consumption of conventional OPA.

New wide single-mode strip and grating loaded (SGL) channel waveguides made of silicon nitride on the oxide buffer layer of a planar silicon-on-insulator waveguide are studied. The central 10-lm-wide strip produces a multi-mode channel waveguide and diffraction gratings with a period 0.6 lm built on the structure edges produce mode-dependent additional losses due to radiation to the surrounding medium. The optical properties of these waveguides are discussed using the results of a three-dimensional numerical simulation by the FDTD and BPM methods. It is shown that a wide SGL waveguide is quasi-single-mode one because ithas a small propagation loss (∼ 0.3 dB cm-1) for the fundamental mode and a high (up to -20 dB cm-1) loss forthe higher order modes. The new SGL waveguides are CMOS compatible and can become basic for fabricating new photonic elements, including tunable optical filters and multi-plexers based on the multireflector technology.

A multimode planar grating spectrograph focuses, into the same point, all modes with equal propagation constants. Guided waves with equal propagation constants may have different free-space wavelengths within a bandwidth determined by the numerical aperture of this structure. The consequences for the spectral resolution of a multimode planar grating are quantitatively discussed.

A polarization-insensitive optical filter is proposed and demonstrated with 340-nm-thick silicon photonic waveguides. It consists of a polarization-insensitive mode (de)multiplexer and a polarization-insensitive multimode waveguide grating (MWG). The MWG is designed optimally according to the polarization-insensitive phase-matching condition. Meanwhile, longitudinal apodization is introduced in the MWG to suppress the sidelobes. For the fabricated silicon photonic filter based on a single stage of MWG, the 1-dB bandwidth as large as ∼13 nm, the excess loss is ∼1 dB and the sidelobe suppression ratio is about 10 dB for both polarizations. In particular, the performance is improved greatly by introducing more than one MWGs in cascade, and the fabricated device has a sidelobe suppression ratio as high as >30 dB.

A high‐capacity lithium‐niobate‐on‐insulator (LNOI) optical transmitter for next‐generation passive optical‐networks (PONs) is proposed and demonstrated with ultra‐low temperature‐dependence for the first time by simultaneously introducing Z‐ and Y‐propagation LNOI photonic waveguides. There are five wavelength‐division‐multiplexed channels involved with the center wavelengths of 1270/1300/1342/1537/1577 nm, and the photonic filters are realized by integrating amplitude‐apodized multimode waveguide gratings (MWGs) and mode (de)multiplexers designed with Z‐propagation LNOI photonic waveguides, which are found to be with a low thermo‐optic coefficient and thus enable the athermalization. Meanwhile, high‐speed optical modulators connected at the add ports of the photonic filters for the 1342/1577 nm channels are developed with Y‐propagation LNOI photonic waveguides with the maximal electro‐optic effect. For the fabricated on‐chip optical transmitter, box‐like spectral responses are achieved with ultra‐low excess losses of <0.5 dB and low inter‐channel crosstalk of < −26 dB for all the channels whose bandwidths are respectively ∼16/20/4/15/5 nm as expected. The present LNOI photonic chip is nearly athermal, exhibiting very low temperature‐dependence less than 8.7 pm/°C in the full wavelength range of >300 nm. Finally, high‐capacity data transmissions of 50/100 Gbps on‐off‐keying signals and 100/200 Gbps four‐level‐pulse‐amplitude‐modulated signals are demonstrated, providing a promising solution for next‐generation PONs.