Photonic signal processing is nowadays recognized as the solution to overcome electronic limits for processing ultra-wideband signals, among other, energy efficiency and heat dissipation, main environmental hindrances in developing large-scale data centers. We demonstrated a novel device, namely Double Injection (DI) modulator that we propose as a main building block of an optical processor. The device is able to deliver programmable response shaping in time and frequency domains, a property we already showed to relief fabrication tolerances of single modulators and provide enlarging the Free Spectral Range (FSR) in optical filters without resizing the dimensions of the device. Our present and future efforts are directed toward integrating the device in processor arrays, to provide complex ultrafast data processing functions. As a first goal, we aim the building a Fully Reconfigurable Programmable Photonic Signal Processing, namely the optical equivalent of an electronic FPGA
Shaping of the frequency response, which can be re-stated as electrical-to-optical response is realized by means of a racetrack resonator designed and fabricated in the so-called, Double Injection (DI) configuration [1]. Shaping is realized by properly selecting different coupling coefficients that provide a variety of interesting transmissions. Various shapes were already demonstrated: sinusoidal, triangular (linear), square (bandpass), dips and peaks (2 states), interleaver, Fano resonance and a, so-called, 20dB-min Parameters-Insensitive-Response modulator (PIR20). The transmissions were experimentally realized, fabricated in a silicon-on-insulator platform and characterized at wavelengths around 1,550nm. Notably, a device can possess more than two inputs (injection points) and\or outputs, thus forming a multiple-input multiple-output (MIMO) device, DI being a special case. The DI configuration owns a unique property that allows two Free Spectral Range states (regular, 2×regular) to exist for a single racetrack length.
A racetrack-based resonator augmented by a Double Injection mechanism can be configured to provide various responses by merely modifying the coupling coefficients. A key advantage of this design is that it comprises a single ring resonator, thus basically requiring one electrode for modulation operation. Since only one ring is required, the circuit is relatively simple to design and manufacture and there are no cumulative fabrication variances, thus making the device much less sensitive to various parameters in most configurations. This is also significant when considered for a practical receiver/transmitter that incorporates arrays of Electro-Optical (EO) elements on chip with respect to real-estate and power consumption. Placing electrodes over the coupling regions can make the device dynamically programmable, and still simpler than other schemes. 

It is possible to tune the DI device to an FSR value greater than a conventional ring of the same size. This is realized by judicious placement of one injection arms with respect to the other.
Typical Application – parameters-insensitive Resonator (PIR)

A racetrack-shaped modulator based on the Double Injection mechanism has been recently introduced exhibiting enhanced robustness to deviations in parameters. Fabrication process deviations that influence sensitive ingredients of ring-based modulators such as the ring’s waveguide, couplers, and heat that is generated in Silicon Photonics devices due to carrier transport, can degrade the modulator extinction-ratio (ER) and optical modulation depth (OMA). The sensitivity of the, so-called, PIR20 configuration (PIR with ER of 20 dB) to fabrication and temperature deviations compares favorably with other known modulators, the latter being based on either a ring, MZI or Fabry-Pérot resonators. 
Typical Application – Fully Reconfigurable DI Resonator for Programmable Photonic Signal Processing (Optical FPGA)
By placing thermal or electrical electrodes over the input/ring couplers one can control the device parameters thus allowing reconfiguration of it’s response as needed. Consequently, such active DI-based resonators can be utilized for assembling an integrated-photonic grid of far more complex functionality which, furthermore, has dynamic programing capabilities. In recent years, the research in this field has been accelerated to the point where photonic-based “processors” are aimed at integrating with, or in certain cases even replacing, their electrical counterparts. Below is an illustrative example of a basic grid comprising of DI devices that can function as a “programmable photonic signal processor”.

Above, an example of a 2-D array (grid) of interconnected optical MIMO devices. In this scheme each MIMO device may in fact be a DI. Below: Left – a 4-by-4 MIMO array using 4 DI devices; Right – a 6-by-6 MIMO array using 4 triple-injection (3-input/output devices).

[1] Cohen, R.A., Amrani, O. and Ruschin, S., 2018. Response shaping with a silicon ring resonator via double injection. Nature Photonics, 12(11), pp.706-712.