EO nonlinear function generator.

An electro-optical programmable nonlinear function generator (PNFG) is developed on a multimode waveguide with four parallel thermal electrodes. The current on one electrode is chosen as the input, while the rest serve as function-defining units to modulate the multimode interference. The electro-thermo-optical effects are analyzed step by step and the impact on the eigenmode properties is derived. It shows that the optical output power variation by altered interference, in response to the input current, manifests as a complex ensemble of functions in general. The PNFG aims to find the special setting under which such relation can be simplified into some basic functions. Through an optimization program, a variety of such functions are found, including Sigmoid, SiLU, and Gaussian. Furthermore, the shape of these functions can be adjusted by finetuning the defining units. This device may be integrated in a large-scale photonic computing network that can tackle complex problems with nonlinear function adaptability.

Micro Light Flow Controller on a Programmable Waveguide Engine.

A light flow controller that can regulate the three-port optical power in both lossless and lossy modus is realized on a programmable multimode waveguide engine. The microheaters on the waveguide chip mimic the tunable "pixels" that can continuously adjust the local refractive index. Compared to the conventional method where the tuning takes place only on single-mode waveguides, the proposed structure is more compact and requires less electrodes. The local index changes in a multimode waveguide can alter the mode numbers, field distribution, and propagation constants of each individual mode, all of which can alter the multimode interference pattern significantly. However, these changes are mostly complex and not governed by analytical equations as in the single-mode case. Though numerical simulations can be performed to predict the device response, the thermal and electromagnetic computing involved is mostly time-consuming. Here, a multi-level search program is developed based on experiments only. It can reach a target output in real time by adjusting the microheaters collectively and iteratively. It can also jump over local optima and further improve the cost function on a global level. With only a simple waveguide structure and four microheaters, light can be routed freely into any of the three output ports with arbitrary power ratios, with and without extra attenuation. This work may trigger new ideas in developing compact and efficient photonic integrated devices for applications in optical communication and computing.

Thermally resettable laser transmission induced transparency in polymer waveguides at 635†nm.

Laser transmission induced transparency (LTIT) has been observed in a polymer waveguide using commercial perfluorinated acrylate-based materials when a continuous-wave laser at 635 nm is injected. The transmitted optical power increases continuously and follows a non-linear curve with respect to the laser injection time. Loss reduction over 13 dB is observed within 60 min at a moderate laser power of 5 mW. While higher injection power leads to a quicker change of the waveguide transparency, this loss reduction tends to saturate at a level irrelevant to the injection power. Further experiments demonstrate that a laser injection at 635 nm can also slightly improve the transparency at near-infrared wavelengths from 1500 nm to 1600 nm which is also the target wavelength range for this material. The state after a certain laser injection dose of 635 nm proves to be stable and the transmission characteristics of the polymer waveguide can be maintained and will continue after being stored at room temperature over a long period of time. By baking the waveguide at 200 °C for 20 min, the transparency property can be reset and the waveguide will return to the original high-loss state of 635 nm. These unique properties can be attributed to the photo-induced generation and thermally induced recombination of free radicals in the organic material. Our discovery may trigger interesting applications of polymer waveguides in the development of optical memory, clock, and encryption devices, beyond their target applications in optical communication.

Multibit NOT logic gate enabled by a function programmable optical waveguide.

Multibit logic gates are of great importance in optical switching and photonic computing. A 4-bit parallel optical NOT logic gate is demonstrated by an optical switching/computing engine based on a multimode waveguide. The multimode interference (MMI) patterns can be altered by thermal electrodes because the number of guided modes, their profiles, and propagation constants can all be altered via the thermo-optic effect. Instead of conventional forward design based on time-consuming simulations, the proposed engine can update the thermal electrodes automatically and monitor the change of the interference in a synchronized and rapid way until the desired function is reached, all experimentally. We name the system "function programmable waveguide engine" (FPWE). As opposed to solutions where the phase or amplitude of light is taken as the signal, the input stays in the electronic domain, and the output is converted into optical intensity variations, calculated from a truth table. This simple, low-cost yet powerful engine may lead to the development of a new set of devices for on-chip photonic computing and signal switching.

Multiport all-logic optical switch based on thermally altered light paths in a multimode waveguide.

A generic multiport optical switch capable of generating all-logic outputs is demonstrated by altering the mode profiles and propagation characteristics in a multimode waveguide through a combination of microheaters. The principles and design rules are introduced. As proof of concept, a 3-bit all-logic switch is fabricated on a polymer waveguide platform. The experimental results are in good agreement with the simulations based on a heat solver and the eigenmode expansion method. The device shows polarization insensitive and colorless operation from 1520 to 1600 nm with an extinction ratio between "On" and "Off" states larger than 11.9 dB in all cases. The maximum heat power is 43.9 mW (for (1, 0, 0) state). The simple, compact, and easily scalable device can be used to construct 1×N and M×N switch networks, showing promising applications in on-chip photonic signal processing and computation.

3D integrated wavelength demultiplexer based on a square-core fiber and dual-layer arrayed waveguide gratings.

We present a 3D integrated wavelength demultiplexer using a square-core fiber (SCF) and matched dual-layer arrayed waveguide gratings (AWGs). The SCF works as a 3D fiber multimode interference device, which splits the input light into symmetric four spots. The spots are then coupled to a pitch-matched 4-waveguide network, each connecting an AWG. Interface waveguides are designed to improve the coupling efficiency between the SCF and the dual-layer chip. The four AWGs are designed with different central wavelengths and a large free spectral range (FSR) of 120 nm. To reach a small and uniform insertion loss among different channels, only the central channels of each AWG are used for demultiplexing. The device is fabricated on a polymer platform. The upper and lower layers of the chip are fabricated using the same photolithography mask but rotated 180° so that 4 different AWG designs can be mapped to a single chip. The measured transmission spectra of the four AWGs cover a bandwidth of 112 nm. The insertion loss variation is smaller than 1.4 dB. The designed device can find applications in fiber optic sensing, communication, and astronomy.