Silicon nanophotonic waveguides and their applications

In this paper a novel microwave photonics beamformer device concept, for single side band (SSB) 40 GHz modulated signals, is presented. The proposed device comprises tunable lasers, flat-top arrayed waveguide gratings (AWG), a Mach Zender Modulator (MZM), an all-pass ring resonator and photodetectors. The device can be produced as a photonic integrated circuit. The signals from the lasers (one for each beamformer radiant element) are multiplexed by the first AWG, modulated, and passed through the all-pass ring resonator. The AWG channel spacing and the ring resonator Free Spectral Range (FSR) are both set to be equal to 100 GHz. The signal is demultiplexed by a second AWG and finally photodetected. By tuning each laser within its corresponding AWG passband, the phase difference between the optical carrier and the 40 GHz microwave modulated signal for each beamformer element can be controlled. The difference is determined by the phase response of the all-pass ring resonator. A critical part of the design is the alignment between the resonances of the ring resonator and both AWGs, but this can be alleviated by using a single AWG in fold-back configuration. The power provided to each beamformer element is different due to the intrinsic non-uniform losses of the AWGs and the ring resonators, but this can also be solved either by properly setting the lasers power, or by means of additional optical amplifiers. The presented analysis is independent of the integration technology. In Silicon photonics, the AWGs and ring resonator can be produced, while the (hybrid) integration of lasers, modulator, photodetectors (and eventually amplifiers) is a challenge. The device can be monolithically integrated on semi-insulating InP technology.

A device with two identical arrayed waveguide gratings (AWGs) in a DEMUX-MUX configuration within a ring resonator is proposed and modeled. This allows extracting the insertion loss of a single AWG, independent of the coupling efficiency, with less error than previous methods.

Multistage WDM access architecture employing cascaded AWGs

Monolithic integration of an InP-based O-band 4-channel arrayed waveguide grating (AWG) to a uni-traveling carrier photodiode (UTC-PD) array is realized by the selective area growth (SAG) technique. The passive-active buttjoint design is introduced and experimentally proved to ensure both good compatibility between the PD fabrication process and the SAG technique, and high photodiode quantum efficiency under the complex butt-joint geometry. An extended coupling layer is adopted between the AWG output waveguides and the PD mesa. The extended coupling layer length, the regrowth boundary edge position and the AWG etching edge position relative to the heterogeneous butt-joint boundary, and the refractive indices of the PD collector and coupling layer are optically simulated and optimized by a finite-difference time-domain method. It is found that the extended coupling layer, compared with the un-extended situation, ensures a good matched optical field from AWG to PD and could reduce nearly 30% quantum efficiency loss when connecting seamlessly to the regrown InP AWG top cladding layer. A stable high efficiency around 80% is maintained within an extended layer length from 7.5 m to 15.0 m. The regrowth boundary edge into the coupling region will cause a drastic efficiency oscillation up to 20% period with the increase of distance. The efficiency drop is also attributed to the light scattering at the regrowth boundary edge, caused by the optical field mismatch, while the oscillation comes from the alternative light power concentration between the coupling layer and the core layer, for the light scattering is only obvious when the light power is well concentrated in the coupling layer. The AWG etching edge position deviation from the butt-joint boundary, however, exerts little influence on the PD quantum efficiency, which is believed not to bring obvious coupling loss during device fabrication. The higher UTC-PD collector refractive index is proved to be crucial for further better optical coupling from the coupling layer to the PD, with quantum efficiency rapidly increasing from around 0.1 to 0.8 when the index is increased from 3.20 to 3.42. By comparison, the efficiency is little affected by the coupling layer refractive index from 3.34 to 3.42.All things considered, we select a 10 m extended coupling layer, the refractive indices of both PD collector and the coupling layer to be 3.42, and align both the regrowth boundary edge and the AWG etching edge to the heterogeneous butt-joint boundary, and a PD quantum efficiency of 80% is expected. Owing to the extended coupling layer at the butt-joint, the SAG technique facilitates the PD fabrication process. The overgrown AWG top cladding layer ridge stretches out 4.67 m toward the PD, but not over the mesa yet, hence has little influence on the PD fabrication accuracy. The monolithic chip presents a uniform photodiode quantum efficiency of 76%, which accords well with theoretical value and confirms the butt-joint design. Central wavelengths for the four channels are 1347.0 nm, 1325.0 nm, 1308.0 nm, and 1286.5 nm, respectively. The low crosstalk level (below -22 dB) also indicates a good de-multiplexer performance.

Arrayed waveguide gratings (AWG) are widely used as wavelength division multiplexers (MUX) and demultiplexers (DEMUX) in optical networks. Here we propose and demonstrate that conventional AWGs can also be used as integrated spectral filters to realize a Fast Fourier transform (FFT) and its inverse form (IFFT). More specifically, we point out that the wavelength selection conditions of AWGs when used as wavelength MUX/DEMUX also enable them to perform FFT/IFFT functions. Therefore, previous research on AWGs can now be applied to optical FFT/IFFT circuit design. Compared with other FFT/IFFT optical circuits, AWGs have less structural complexity, especially for a large number of inputs and outputs. As an important application, AWGs can be used in optical OFDM systems. We propose an all-optical OFDM system with AWGs and demonstrate the simulation results. Overall, the AWG provides a feasible solution for all-optical OFDM systems, especially with a large number of optical subcarriers.

This paper presents a novel NMOS-based switching cell architecture specially developed for semicustom smart power ICs that can be implemented even using low cost, standard CMOS technologies. This approach is highly suitable either for cost effective smart power ICs or for discrete high power devices, since the NMOS transistor cell can be used to implement medium power switches or intermediate interface stages. This work also presents an overview on integration strategies and layout considerations, using different widely available processes of standard low cost CMOS technology, with no extra processing steps. Optimized NMOS devices were associated in special structures in a matrix arrangement, conveniently designed to be easily programmed using top metal masks, according to the required functionality, in order to implement topologies for commutation, drive, protection, amplification, sensing and control, capable to handle high voltage signals. The proposed power array allows the fast prototyping of a wide range of applications. As an example a therapeutic stimulation microsystem was obtained through the configuration of this power array fabricated using a standard CMOS technology. A boost converter was implemented as the actuator. Experimental results are presented and overall performance is discussed. These strategies can be extended to smart power and other power integration dedicated technologies, when customized ICs are in scope, to reduce low series production cost and production cycle.

We present a compact 4x8 wavelength-routing switch that monolithically integrates fast tunable wavelength converters (TWCs) and an arrayed-waveguide grating (AWG) for optical packet switching. The TWC consists of a double-ring-resonator-coupled tunable laser which allows rapid and stable switching, and an optical gate based on a parallel amplifier structure which prevents an input optical signal from being routed through the AWG (filter-free operation). A deep-ridge waveguide technology, employed for the AWG and ring resonators, facilitates the fabrication of the switch and makes the device compact. The filter-free TWCs achieve low crosstalk of the input optical signal of less than -22 dB. The wavelength routing operation of a non-return-to-zero (NRZ) signal at 10 Gbit/s is achieved with a switching time of less than 5 ns.

A conventional Arrayed Waveguide Grating (AWG) has been modified, without output receiver waveguides, for nonconventional applications such as Astrophotonics and spectroscopy sensing where the input signal can have information over the entire band and a continuum of light/spectrum. The material system chosen for the AWG design is siliconnitride/ SiO 2 /Si (Si 3 N 4 -SiO 2 -Si) for its relatively high refractive index, which for a given channel spacing allowing a more compact device than Silicon-on-Silica. Further, CMOS compatibility and the presence of high non-liner optical coefficient would be an added advantage to design and fabricate densely integrated photonic sub-systems, such as calibration source and AWG, for astrophotonics and spectroscopy. The proposed AWG utilizes a flat image plane optimized for minimal aberration. An analytical calculation, based on Gaussian beam approximation, was used to determine the optimal flat plane position where the non-uniformity in 1/e electric field widths is minimal. This plane can be used as the dicing plane to re-image the entire output of the AWG onto a detector array to sample the entire spectrum. Tailored AWG, with flat image-plane, designed to resolve 48 spectral channels with 0.4nm (50GHz) resolution and adjacent channel cross-talk level within a 0.2nm window (ITU-grid) ~ -28dB. Calculated insertion loss non-uniformity is close to 3dB. The foot-print of high index contrast (Δn=23%) IPS is ~ 12x8.5 mm 2 . The modelled mean spectral resolving power, R, at the flat image-plane is ~ 7,600. The design principle could be utilised for devices using other material systems with different parameters.

SOI-based arrayed waveguide grating with extended dynamic range for fiber Bragg grating interrogator

Photonic spectrographs offer a highly miniaturized, flexible, and stable on-chip solution for astronomical spectroscopy and can be used for various science cases such as determining the atmospheric composition of exoplanets to understand their habitability, formation, and evolution. Arrayed Waveguide Gratings (AWGs) have shown the best promise to be used as an astrophotonic spectrograph. We developed a publically-available tool to conduct a preliminary examination of the capability of the AWGs in spectrally resolving exoplanet atmospheres. We derived the Line-Spread-Function (LSF) as a function of wavelength and the Full-Width-at-Half-Maximum (FWHM) of the LSF as a function of spectral line width to evaluate the response of a discretely- and continuously-sampled low-resolution AWG (R $\sim$ 1000). We observed that the LSF has minimal wavelength dependence ($\sim$5\%), irrespective of the offset with respect to the center-wavelengths of the AWG channels, contrary to the previous assumptions. We further confirmed that the observed FWHM scales linearly with the emission line width. Finally, we present simulated extraction of a sample molecular absorption spectrum with the discretely- and continuously-sampled low-resolution AWGs. From this, we show that while the discrete AWG matches its expected resolving power, the continuous AWG spectrograph can, in principle, achieve an effective resolution significantly greater ($\sim$ 2x) than the discrete AWG. This detailed examination of the AWGs will be foundational for future deployment of AWG spectrographs for astronomical science cases such as exoplanet atmospheres.

Optoelectronic integration technologies based on silicon-on-insulator (SOI) can bring revolutionary change to on-chip arrayed waveguide grating (AWG) demodulation systems. In this study, we present several ultrasmall 1 x N AWGs for an SOI-based AWG demodulation integration microsystem of different scales. The core sizes of the fabricated AWGs are smaller than 400 x 600 μm 2 . Experimental results match the simulation results, indicating that AWGs have a good transmission spectrum of low crosstalk below -20 dB and low insertion loss below -6.5 dB. The fabricated AWGs can be perfectly applied to improve the integration level and performance of the SOI-based AWG demodulation integration microsystem.

A statistical model is proposed for estimating the chromatic dispersion in the passband of a filter using an arrayed waveguide grating (AWG) with random phase errors. The model is derived without assuming the independence of random errors and can be applied to AWGs with frequency-dependent array amplitude coefficients as well as conventional AWGs. The average and variance of the chromatic dispersion in the passband can be calculated by using simple expressions without random computation once the standard deviation and correlation of random phase errors are given. The behavior of the chromatic dispersion in a synchronized-router-based flat-passband filter consisting of a multiple-input AWG and cascaded Mach-Zehnder interferometers (MZIs) was investigated. The standard deviation of the chromatic dispersion monotonically increases with the standard deviation of random phase errors, and has a distinctive dependence on frequency. The average and standard deviation of the chromatic dispersion in the passband obtained by the model are in good agreement with those obtained by random simulation for both a synchronized-router-based flat-passband filter and conventional AWGs.

A linear tapered double S-shaped arrayed waveguide grating (AWG) was designed as an alternative to a U-shaped AWG, and a complete transmission spectrum for 18 channels of coarse wavelength-division multiplexing (CWDM) was demonstrated. The silicon-on-insulator based AWG with a rib waveguide structure with a broad channel spacing of 20 nm was designed to serve as a multiplexer/demultiplexer. A beam propagation method modeling simulation under transverse electric mode polarization over a free spectrum range of 700 nm was used for the design process. The geometrical dimensions of the AWG rib structure were optimized to achieve the lowest reported insertion loss of 1.07 dB and adjacent crosstalk of −38.83 dB. The influence of different etching depths on the top Si layer of the AWG for a constant core width of 0.6 μm as well as birefringence effects were also investigated. A transmission spectrum response at the output port close to the standard CWDM wavelength grid range of 1271 to 1611 nm with an average channel spacing of 2485 GHz was obtained.

We have presented a simultaneous WDM channel wavelength monitoring method by periodically modulating the arrayed waveguide grating (AWG) temperature. This method provides crossover-wavelengths with very small attenuations compared with adjacent transmission peaks. Thus the monitoring error owing to the ASE can be decreased substantially. Moreover, one WDM channel can be monitored using one AWG output port enabling the use of standard AWGs. The temperature modulated AWG can work also as a multi-channel wavelength locker as well as the multi-channel wavelength monitor.

Directional couplers (DCs) have been playing an important role as a basic element for realizing power exchange. Previously most work was focused on symmetric DCs and little work was reported for asymmetric directional couplers (ADCs). In recently years, silicon nanophotonic waveguides with ultra-high index contrast and ultra-small cross section have been developed very well and it has been shown that ADCs based on silicon-oninsulator (SOI) nanophotonic waveguides have some unique ability for polarization-selective coupling as well as mode-selective coupling, which are respectively very important for polarization-related systems and modedivision-mulitplexing systems. In this paper, a review is given for the recent progresses on silicon-based ADCs and the applications for power splitting, polarization beam splitting, as well as mode conversion/(de)multiplexing.