Experimental phase-error extraction and modelling in silicon photonic arrayed waveguide gratings

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.

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.

We present a numerical and experimental study of the impact of phase errors on the performance of large, high-resolution arrayed waveguide gratings (AWG) for applications in astronomy. We use a scalar diffraction model to study the transmission spectrum of an AWG under random variations of the optical waveguide lengths. We simulate phase error correction by numerically trimming the lengths of the optical waveguides to the nearest integer multiple of the central wavelength. The optical length error distribution of a custom-fabricated silica AWG is measured using frequency-domain interferometry and Monte-Carlo fitting of interferogram intensities. In the end, we give an estimate for the phase-error limited size of a waveguide array manufactured using state-of-the-art technology. We show that post-processing eliminates phase errors as a performance limiting factor for astronomical spectroscopy in the H-band.

We describe a method for measuring the phase error distribution of an arrayed waveguide grating (AWG) in the frequency domain when the free spectral range (FSR) of the AWG is so wide that it cannot be covered by one tunable laser source. Our method is to sweep the light frequency in the neighborhoods of two successive peaks in the AWG transmission spectrum by using two laser sources with different tuning ranges. The method was confirmed experimentally by applying it to a 160 GHz spaced AWG with a FSR of 11 THz. The variations in the derived phase error data were very small at +/-0.02 rad around the central arrayed waveguides.

A novel liquid crystal on silicon (LCOS)-based wavelength selective switch (WSS) is proposed, fabricated, and demonstrated. It employs a multilayered arrayed waveguide grating (AWG) as a wavelength multiplex/demultiplexer. The LCOS deflects spectrally decomposed beams channel by channel and switches them to desired waveguide layers of the multilayered AWG. In order to obtain the multilayered AWG with high yield, phase errors of the AWG is externally compensated for by an additional phase modulation with the LCOS. This additional phase modulation is applied to the equivalent image of the facet of the AWG, which is projected by a relay lens. In our previously-reported WSS configuration, somewhat large footprint and increased cost were the drawbacks, since two LCOSs were required: one LCOS was driven for the inter-port switching operation, and the other was for the phase-error compensation. In the newly proposed configuration, on the other hand, both switching and compensation operations are performed using a single LCOS. This reduction of the component count is realized by introducing the folded configuration with a reflector. The volume of the WSS optics is 80 × 100 × 60 mm3, which is approximately 40% smaller than the previous configuration. The polarization-dependent loss and inter-channel crosstalk are less than 1.5 dB and -21.0 dB, respectively. An error-free transmission of 40-Gbit/s NRZ-OOK signal through the WSS is successfully demonstrated.

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 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

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.

The authors present a system performance analysis of an optical single sideband system (OSSB) when phase errors occur in the modulating signal driving the dual-arm Mach–Zehnder modulator (MZM). The performance is evaluated in terms of the optical sideband suppression, received subcarrier power, frequency response, and power-penalty measurements. Experiments were carried out and a mathematical model of the OSSB system, incorporating the phase error, was developed. Both the theoretical and experimental results show that, although optical sideband suppression reduces with phase errors, the received subcarrier power can increase as high as 3 dB for positive phase errors. However, for transmission over longer fiber, the frequency response of the OSSB system is not only dependent on the phase error, but also on the phase shift due to chromatic dispersion (CD). The power penalty was found to be dependent on the frequency response of the OSSB system. At any frequency-response null point, the power-penalty slope is reversed as the received subcarrier power is reduced. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1574–1578, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21724

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 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.

All-optical packet switching can overcome limitations of electronic switches in terms of power consumption, speed, cost, and footprint. Switch architectures combining wavelength converters and fiber delay lines provide tunable routing and contention resolution when used with an N×N arrayed waveguide grating (AWG), a key passive optical component to bypass electronic processing limitations. An AWG passively routes either single or multiple input port wavelengths to output ports. A single wavelength per port strategy reduces crosstalk within the AWG, but drastically increases the dimensionality of the device. AWG design constraints due to bandwidth limitations and fabrication processes limit the port number for the foreseeable future to under 100. In order to scale optical switches to emerging network requirements, we must use multiple wavelengths per port. In this paper, we examine several optical router architectures for data center applications using multiple wavelengths per port, and quantify the physical layer impairments. We consider not only the AWG crosstalk, but also Q-factor degradation caused by the multiple wavelength conversions occurring when a packet is buffered for contention resolution. We present the results as a function of the number of recirculations for on–off-keying (OOK) signal formats. While previous work has addressed this issue in terms of accumulated loss, we focus on accumulated intensity noise due to crosstalk and amplified spontaneous emission (ASE). We compare the routing performance of each architecture, and we point out that the AWG crosstalk and accumulated ASE noise during packet recirculation are both critical to the routing performance.