Claudio Castellan

Design and Implementation of an Integrated Reconfigurable Silicon Photonics Switch Matrix in IRIS Project

This paper aims to present the design and the achieved results on a CMOS electronic and photonic integrated device for low cost, low power, transparent, mass-manufacturable optical switching. An unprecedented number of integrated photonic components (more than 1000), each individually electronically controlled, allows for the realization of a transponder aggregator device which interconnects up to eight transponders to a four direction colorless-directionless-contentionless ROADM. Each direction supports 12 200-GHz spaced wavelengths, which can be independently added or dropped from the network. An electronic ASIC, 3-D integrated on top of the photonic chip, controls the switch fabrics to allow a complete and microsecond fast reconfigurability.

Reflectance Reduction in a Whiskered SOI Star Coupler

In this letter, we evaluate how the background noise is modified within a whiskered-shaped star coupler, optimized to reduce the reflectivity of its internal surfaces to reduce the back-scattered light. This letter is carried out by analyzing the Fabry-Perot interference due to the reflections between the input focal plane of the star coupler and the output facet of the waveguides. By comparing a whiskered star coupler with a standard one, we find a reduction in the star coupler reflectance of more than one order of magnitude. The experimental results are supported by a theoretical model and by numerical simulations.

Automatic alignment of photonic components of massive optical switch to ITU channels (Conference Presentation)

We demonstrate the automatic thermal alignment of photonic components within an integrated optical switch. The WDM optical switch involves switching elements, wavelength de-multiplexers, interleavers and monitors each one needing independent control. Our system manages rerouting of channels coming from four different directions, each carrying 12, 200GHz spaced, wavelengths into eight add/drop ports. The integrated device includes 12 interleavers, which can act either as optical de-interleavers to split the optical signal into odd and even channels or as optical interleavers that recombine the odd and even channels coming from the switching matrix. Integrated Ge photodiodes are placed in key positions within the photonic integrated circuit (PIC) are serve for monitoring. An electronic integrated circuit (EIC) drives the photonic elements by means of dedicated heating circuits (824 on-board heater control cells, 768 for the switching elements and 56 for the interleavers and the mux/de-mux) and reads out the Ge diodes photocurrent through TIAs. We applied a stochastic optimization algorithm to align the spectral response of the interleavers to the ITU grid. We exploit the thermo-optic effect to shift the interleavers pass-band in a desired spectral position. The interleavers are provided with dedicated metallic heaters that can be operated in order to tune the interleaver response, which is typically misaligned due to fabrication inaccuracies. The experimental setup is made of a tunable laser coupled with one input port of optical switch. The optimization algorithm is implemented via a software to drive the EIC till finding the best heating configuration (on the two branches of the interleaver) on the basis of the monitor diode-feedback. This way, the even and odd wavelengths input in the interleaver are directed toward the wanted lines within the switching matrix. Our method has been used for aligning the micro-ring based switching elements in the PIC as well. In that case, the integrated Ge photodiodes have been used to align the photonic components in the PIC in order to enable different pathways for the routing or the broadcasting operation of the optical switch. With no bias applied to the heaters of the switching elements, the optical signal is expected to be maximum at the through port. When the micro-ring heaters are biased, the feedback controller finds the best set of heating values that minimize the optical power at the through port of the switching node. This way, the optical signal is coupled in the drop port and the node is enabled for switching. The algorithm, implemented in LabVIEW, converges over multiple instances and it is robust against stagnation. This work aims at enabling the automatic reconfiguration/restoration of the whole WDW optical switch.

Low crosstalk silicon arrayed waveguide gratings for on-chip optical multiplexing

In this work, we report on the modeling and the experimental characterization of a 6×400 GHz silicon Arrayed Waveguide Grating (AWG). The design of the device is based on the reduction of the background noise. The good characteristics of the AWG demonstrate that unwanted reflections have a detrimental role on its performance. We demonstrate a smoothing of the output channel shape of the AWG, as well as a reduction of the crosstalk level from −20.6(1) dB to −24.4(1) dB.

Tuning the strain-induced resonance shift in silicon racetrack resonators by their orientation

In this work, we analyze the role of strain on a set of silicon racetrack resonators presenting different orientations with respect to the applied strain. The strain induces a variation of the resonance wavelength, caused by the photoelastic variation of the material refractive index as well as by the mechanical deformation of the device. In particular, the mechanical deformation alters both the resonator perimeter and the waveguide cross-section. Finite element simulations taking into account all these effects are presented, providing good agreement with experimental results. By studying the role of the resonator orientation we identify interesting features, such as the tuning of the resonance shift from negative to positive values and the possibility of realizing strain insensitive devices.

Towards MIR SPDC generation in strained silicon waveguides

Silicon has a centrosymmetric crystalline structure that leads to a null second order nonlinear χ(2) coefficient. On the other hand, χ(2) effects are highly sought in Si to develop low power and fast electro-optic modulators. Recently, it has been reported that by stressing a Si waveguide with a silicon nitride over-layer, a sizeable χ(2) is induced. In [1] Second Harmonic Generation (SHG) measurements were reported in large area strained Si waveguides. However, the SHG data were affected by different contributions caused by the interfaces, the free carrier induced internal electric fields, the inhomogeneous strain and the uncontrolled modal structure of the waveguides. Here, we present SHG in Silicon On Insulator (SOI) strained Si waveguides, designed by a nonlinear propagation model. The design maximizes the SHG efficiency by a detailed simulation of the waveguide modal structure to achieve proper phase matching: neff(λp) = neff(λp/2), (neff is the effective refractive index and λp is the pump wavelength). Since the refractive index is dispersive, this condition is never satisfied for the same optical mode. However, the phase matching condition can be satisfied by different optical modes (modal phase matching). The cross section of the resulting waveguides is sketched in Fig.1(a). These are SOI waveguides with different widths around 2 μm, an height of 250 nm and a 140-nm-thick overlayer of silicon nitride, which induces a tensile stress of +1.25 GPa in Si. Waveguides of different widths have been measured for different pump powers, polarizations and wavelengths. The laser source was a —100 fs tunable laser operating at 1 kHz repetition rate. The temporal width of the laser pulses was tailored to about 13 ps, by means of a pulse shaper, in order to avoid walk-off effects within the waveguide. The operating wavelengths were (2.4 : 2.6) μm, to eliminate two photon absorption. A SPAD InGaAs detector, externally triggered by the pump laser led to a lowest detectable power of P ∼ 0.01 fW. The experimental results are reported in Fig. 1(b). The SHG generated power scales with the square of the pump power, as expected. The optical modes involved in this process are: TBI for the pump and TM5 for SHG. No interface charge effects were observed and, therefore, we isolated in the measurements the effects of strain. We estimate a low value for the strain induced second order nonlinearity of χ2 = (0.304 ± 0.025) pm/V, which is actually an underestimation due to experimental issues. This value fits with other high frequency observations of the electro-optic effect in strained silicon [2].

From SHG to mid-infrared SPDC generation in strained silicon waveguides

The centrosymmetric crystalline structure of Silicon inhibits second order nonlinear optical processes in this material. We report here that, by breaking the silicon symmetry with a stressing silicon nitride over-layer, Second Harmonic Generation (SHG) is obtained in suitably designed waveguides where multi-modal phase-matching is achieved. The modeling of the generated signal provides an effective strain-induced second order nonlinear coefficient of χ(2) = (0.30 ± 0.02) pm/V. Our work opens also interesting perspectives on the reverse process, the Spontaneous Parametric Down Conversion (SPDC), through which it is possible to generate mid-infrared entangled photon pairs.

Modeling and validation of high-performance and athermal AWGs for the silicon photonics platform

Array waveguide gratings (AWGs) are a key component in WDM systems, allowing for de-multiplexing and routing of wavelength channels. A high-resolution AWG able to satisfy challenging requirements in terms of insertion loss and X-talk is what is needed to contribute to the paradigm change in the deployment of optical communication that is nowadays occurring within the ROADM architectures. In order to improve the performances and keep down the footprint, we modified the design at the star coupler (SC) and at the bending stages. We evaluated how the background noise is modified within a whiskered-shaped SC optimized to reduce the reflectivity of the SOI slab and keep down back-scattered optical signal. A dedicated heating circuit has also been designed, in order to allow for an overall tuning of the channel-output. A high-performance AWG has also to cope with possible thermal-induced environmental changes, especially in the case of integration within a PIC. Therefore, we suggested a way to reduce the thermal-sensitivity.

Modeling and validation of high-performance and a-thermal AWGs for the silicon photonics platform

Array waveguide gratings (AWGs) are a key component in WDM systems, allowing for de-multiplexing and routing of wavelength channels. A high-resolution AWG able to satisfy challenging requirements in terms of insertion loss and X-talk is what is needed to contribute to the paradigm change in the deployment of optical communication that is nowadays occurring within the ROADM architectures. In order to improve the performances and keep down the footprint, we modified the design at the star coupler (SC) and at the bending stages. We evaluated how the background noise is modified within a whiskered-shaped SC optimized to reduce the re ectivity of the SOI slab and keep down back-scattered optical signal. A dedicated heating circuit has also been designed, in order to allow for an overall tuning of the channel-output. A high-performance AWG has also to cope with possible thermal-induced environmental changes, especially in the case of integration within a Photonic Integrated Circuit (PIC). Therefore, we suggested a way to reduce the thermal-sensitivity.