Low kappa, narrow bandwidth Si(3)N(4) Bragg gratings

Advances in Waveguide Bragg Grating Structures, Platforms, and Applications: An Up-to-Date Appraisal

A Bragg grating (BG) is a one-dimensional optical device that may reflect a specific wavelength of light while transmitting all others. It is created by the periodic fluctuation of the refractive index in the waveguide (WG). The reflectivity of a BG is specified by the index modulation profile. A Bragg grating is a flexible optical filter that has found broad use in several scientific and industrial domains due to its straightforward construction and distinctive filtering capacity. WG BGs are also widely utilized in sensing applications due to their easy integration and high sensitivity. Sensors that utilize optical signals for sensing have several benefits over conventional sensors that use electric signals to achieve detection, including being lighter, having a strong ability to resist electromagnetic interference, consuming less power, operating over a wider frequency range, performing consistently, operating at a high speed, and experiencing less loss and crosstalk. WG BGs are simple to include in chips and are compatible with complementary metal-oxide-semiconductor (CMOS) manufacturing processes. In this review, WG BG structures based on three major optical platforms including semiconductors, polymers, and plasmonics are discussed for filtering and sensing applications. Based on the desired application and available fabrication facilities, the optical platform is selected, which mainly regulates the device performance and footprint.

Narrow-bandwidth Bragg grating filter based on Ge-Sb-Se chalcogenide glasses

Bragg grating (BG) filters play important roles in integrated photonics such as signal processing and optical sensing. In silicon-based counterpart photonic platforms, the application of narrow-bandwidth (Δλ) filters is often restrained by fabrication limitations. In this study, narrow-bandwidth BG filters based on Ge-Sb-Se chalcogenide materials are investigated. The structure of the filter is designed by optimizing the grating period, corrugation height, and grating number. The large corrugation of chalcogenide BG is more friendly and convenient for manufacturing process. The symmetric and asymmetric corrugation filters are then fabricated and characterized. Experimental results show a half-maximum bandwidth of 0.97 nm and 0.32 nm for symmetric and asymmetric filters, respectively, which demonstrates excellent narrow-bandwidth filtering performance of chalcogenide BG.

Ultra-narrow linewidth laser based on a semiconductor gain chip and extended Si3N4 Bragg grating

We demonstrate ultra-narrow linewidth fixed wavelength hybrid lasers composing a semiconductor gain chip and extended silicon nitride Bragg grating. Fabricated ultra-low κ Bragg gratings provide a narrow bandwidth and high side-lobe suppression ratio. A single-wavelength 1544 nm hybrid extended-distributed Bragg reflector laser with 24 mW output power and a Lorentzian linewidth of 320 Hz is demonstrated, providing a high-performance light source for on- and off-chip applications.

On-chip mode converter based on two cascaded Bragg gratings

We propose an on-chip mode converter via two cascaded Bragg reflection processes. A forward conversion between two guided modes can be achieved with the aid of an additional mode. The proposed structure is theoretically studied and simulated via the rigorous three-dimensional finite-difference time-domain (3D-FDTD) method. The bandwidth and central wavelength of the proposed mode converter can be adjusted according to our theoretical analysis and simulation results. By applying the similar design approaches as fiber Bragg gratings, conversion spectra with different shapes can be obtained. As an example, several mode converters with bandpass and sidelobe-reduced spectra are designed. We also investigate and verify the mode conversion by experiment. Therefore, the proposed method may pave a new path for the mode converters with desired conversion spectra.

Ultra-Low-Loss Silicon Waveguides for Heterogeneously Integrated Silicon/III-V Photonics

Integrated ultra-low-loss waveguides are highly desired for integrated photonics to enable applications that require long delay lines, high-Q resonators, narrow filters, etc. Here, we present an ultra-low-loss silicon waveguide on 500 nm thick Silicon-On-Insulator (SOI) platform. Meter-scale delay lines, million-Q resonators and tens of picometer bandwidth grating filters are experimentally demonstrated. We design a low-loss low-reflection taper to seamlessly integrate the ultra-low-loss waveguide with standard heterogeneous Si/III-V integrated photonics platform to allow realization of high-performance photonic devices such as ultra-low-noise lasers and optical gyroscopes.

Tunable optical-microwave filters optimized for 100 MHz resolution

New continuously tunable RF-spectrum analyzers, RF receivers, and RF signal generators are proposed and analyzed for the silicon-on-insulator integrated-photonic platform at the ~1550 nm wavelength. These RF system-on-a-chip applications are enabled by a new narrowband 2x2 Mach-Zehnder interferometer (MZI) tuned filters for reconfigurable multiplexing, demultiplexing and RF channel selection. The filter can be optimized for ~100 MHz 3-dB bandwidth (BW) by utilizing N closely coupled Bragg-grating resonators to form one effective waveguide resonator in the single-mode silicon nanowire used for each MZI arm. The number of periods M within each individual resonator is selected to engineer BW in the 0.1 to 1 GHz range. Butterworth design is employed. Continuous tuning of the 100 MHz-BW devices over 18.6 GHz has been simulated by using local micron-scale thermo-optical heater stripes on the MZI arms with a temperature rise from 0 to 48K. For the case of N = 3 and 100-nm silicon side teeth, some representative performance predictions are: insertion loss (IL) = -10.7 dB, BW = 80.5 MHz and L = 113 μm for M = 58; while IL = -0.74 dB, BW = 1210 MHz and L = 86 μm for M = 44.

Uniform emission, constant wavevector silicon grating surface emitter for beam steering with ultra-sharp instantaneous field-of-view

We report on uniform emission intensity profile, uniform propagation constant silicon gratings for beam steering application with ultra-sharp instantaneous field-of-view (IFOV). To achieve uniform emission intensity across relatively long emission length, we designed a custom grating with varying Si₃N₄ width and duty cycle while maintaining a uniform propagation constant for relatively narrow divergence emission pattern. We designed and fabricated the custom Si₃N₄/Si grating with the varying Si₃N₄ width/duty cycle together with the reference Si₃N₄/Si grating with a constant 50:50 duty cycle. The custom grating demonstrated the beam steering angle value of 6.6° by sweeping wavelength between 1530 nm and 1575 nm with the emission length over 1 mm. The measured IFOV based on the 3-dB beamwidth values of the far field patterns for the TE polarization are 0.10° and 0.75° for the custom grating and for the reference grating, respectively. The custom grating also indicates mode-selective behavior due to the perturbation of propagation constant for input modes other than TE polarization. The measured TE-mode to TM-mode suppression ratio for the custom grating is approximately 8.2 dB peak-to-peak measured at far field.