Polarization-insensitive local-oscillator-carrier loopback modulation for a cost-effective and high-port-count wavelength-routing optical switch.

A wavelength-routing optical switch uses a wavelength-tunable laser at each input port, and this transmitter implements output port selection by tuning the wavelength that is associated with each output port. With coherent transmission, loopback modulation of a local oscillator (LO) carrier generated at the output port can eliminate the need for a wavelength-tunable laser. However, loopback modulation can be unstable since the power fluctuates because fiber traversal by the light creates polarization rotation. Here, we propose a simple polarization-alignment circuit and verify its effectiveness in creating a high-port-count optical switch system. The proposed circuit consists of passive components and aligns the polarization state of the supplied LO carrier to be linearly polarized along the x-direction of a TE-input dual-polarization (DP) IQ modulator. The circuit is shown to yield stable modulation with Q-variation of less than 0.8 dB, regardless of any birefringence along the transmission path. The proposal's effectiveness is verified in optical switch system experiments with DP-QPSK signals; 1,856 × 1,856 switch scale is achieved with loopback modulation.

Design and verification of a LO bank enabled by fixed-wavelength lasers and fast tunable silicon ring filters for creating large scale optical switches.

The fast and widely tunable wavelength bank is a key enabler in creating wavelength-routing optical switches that do not use fast wavelength tunable lasers. A cost-effective design criterion needs to be developed before it can be applied to intra data center networks. In this paper, we develop a systematic method for designing a wavelength bank that yields high port-count and fast wavelength-routing optical switches for intra data center application. The wavelength bank is created with fixed-wavelength laser sources and wavelength-tunable filters with rapid wavelength selectivity. To optimize the optical switching system that uses the wavelength bank for supplying local oscillator (LO) lights for coherent detection, various parameters are analyzed, including effective bandwidth, laser output power, loss distribution, splitter port count, and optical amplifier gain. We carry out numerical simulations for optimizing the tradeoff between system performance and cost. To verify the designed wavelength bank, a silicon ring filter is newly fabricated with an average fiber-to-fiber insertion loss of 5.3 dB over a 22-nm bandwidth. Using 256-Gb/s DP-QPSK signals, experiments demonstrate a 1,024×1,024 optical switch that uses a fabricated silicon ring filter. The effectiveness of the scalable and fast-tunable LO bank is verified by achieving 262.1-Tb/s switch throughput with switching time under 18 µs.

2 × 2 16-ch silicon photonics wavelength-selective switch based on waveguide gratings.

We demonstrate a 2 × 2 16-ch silicon photonics wavelength-selective switch consisting of contra-directional couplers and thermo-optic Mach-Zehnder switches. The contra-directional couplers are based on sidewall corrugated Bragg gratings with an unlimited free spectral range and thus enable the device to operate over a very wide wavelength range of the C- and L-band. We obtain a fiber-to-fiber insertion loss of 9.2 dB, an on-chip loss of 5.4 dB, a 3-dB bandwidth of 4.2 nm, a bar-port extinction of 15.0 dB, and a cross-port extinction of 23.0 dB, all in 16-ch average. An average trimming power for the bar states and an average switching power for the cross states are 3.8 mW (σ = 2.3 mW) and 15.6 mW (σ = 2.1 mW), respectively. The wavelength dependence of the spectral responses and the resonant effect from the apodization are discussed in detail to show how to further improve the spectral performance.

Ultra-compact silicon photonics switch with high-density thermo-optic heaters.

Miniaturization of silicon photonics switches is essential for both dense integration and low-loss operation. However, it has remained unclear how small the switches can be made while using thermo-optic (TO) element switches. In this paper, the minimum possible distance between adjacent TO phase shifter arms was first examined. Next, the architecture for a switch matrix for the high-density arrangement of TO switches that includes multi-layer electrical wirings for compact electrical wire-out was proposed and demonstrated. As a result, we achieved 1/23 miniaturization of an 8 × 8 silicon photonics switch for the PIC part when compared with our previous design.

SiN/Si double-layer platform for ultralow-crosstalk multiport optical switches.

We experimentally demonstrate a double-layer platform of silicon nitride and silicon for ultralow-crosstalk multiport optical switches. By using a silicon nitride overpass with a large gap of 1.5 µm, we achieve a crosstalk of less than -50 dB and -45 dB almost entirely in the C-band for 4 × 4 and 16 × 16 switches, respectively. To demonstrate the scalability of the platform, we also measured a 32 × 32 passive test device and show that a worst-case crosstalk of less than -50 dB is feasible with appropriate gate switches.

On-demand transfer of trapped photons on a chip.

Photonic crystal nanocavities, which have modal volumes of the order of a cubic wavelength in the material, are of great interest as flexible platforms for manipulating photons. Recent developments in ultra-high quality factor nanocavities with long photon lifetimes have encouraged us to develop an ultra-compact and flexible photon manipulation technology where photons are trapped in networks of such nanocavities. The most fundamental requirement is the on-demand transfer of photons to and from the trapped states of arbitrary nanocavities. We experimentally demonstrate photon transfer between two nearly resonant nanocavities at arbitrary positions on a chip, triggered by the irradiation of a third nonresonant nanocavity using an optical control pulse. We obtain a high transfer efficiency of ~90% with a photon lifetime of ~200 ps.