We propose a device based on self-induced phase shifting to create a non-linear optical transfer function with a single optical access. The device is designed around the Nonlinear-Optical Loop Mirror (NOLM) principle. The device is a loop formed by four SOAs with a splitter/recombiner at one of the junctions for optical access. The device also includes a mirror inside one of the SOAs such that part of the light is transmitted around the loop and part is reflected. The dimensions of formed loop are kept below 4mm (1mm/SOA) to the requirement for integration. It is assumed the loop is based on a standard ridge waveguide design with InGaAsP/InGaAs quantum wells yielding a refractive index of 3.88. Also, the width of the waveguide is kept constant at 2μm to ensure single-moded operation.
We present simulations results obtained with VPITransmissionMakerTM from VPIPhotonics. The software allows the simulation of optical modules such as Lasers, SOAs, Bragg Grating. The SOA and Laser modules exploit on the Transmission Line Laser Model (TLLM) model for solving the standard laser rate equations. For the Multi-Quantum Wells (MQW) SOAs, another set of equations are used to model the effects of carriers entering and exiting the quantum wells. The model is used to explore the tunability of the design and manufacturing parameters for optimal performance of the non-linear optical loop mirror. Design parameters include the size of the loop, drive current of each SOA, position and reflectivity of the mirror, number and size of the quantum wells and separate confinement height. To provide an efficient way of comparing different values for a given parameter, three figures of merits are chosen. The first one is the input dynamic range of the device in its current configuration, which corresponds to the area of the transfer function where the input signal will experience regeneration. The second parameter is the peak to trough ratio corresponding to the maximum possible output swing i.e. the maximum point of the transfer function less its corresponding minima. The final parameter named the regeneration slope is the division of the peak to trough ration by the input dynamic range.
The particularity of this loop is the mirror etched into one of the active waveguides to create self-induced phase shifts leading to non-linear transfer functions with a single optical input. Optimisation is explored for various design parameters that would need to be decided prior to manufacturing such a device. It is believed that such optimisation can provide a way to create all-optical signal processing devices created for a single application.
With the continuous advent of new multimedia technologies, the local network bandwidth is getting closer and closer to limits set by electronic switching constraints. All-optical networks have long been demonstrated in the laboratory and rely on nonlinear switching devices such as Michelson Interferometers (MIs) for all-optical routing and all-optical digital processing. Hybrid integrated MIs allow for a greater electro-optical integration and thus easier packaging. It was recently published that multi-contact optical amplifiers provide a greater ease of use due to their greater flexibility in injecting current into the device. We have therefore investigated the optimisation of twin-contact SOAs for use in one arm of a Michelson device in order to provide the highest possible optically induced phase shift sine qua non for interferometry. The SOA section length as well as the corresponding injection currents were optimised and it was found that non-symmetrical sections (i.e. of different lengths and injection currents) produces best results with phase shifts up to 8.6 radians for 541μm (333μm + 208μm) long devices. This is explained by the non-symmetrical gains saturation effects created by the co-propagating pump and probe signal when passing through the various SOA sections. Multi-contact SOAs are hoped to provide a new ways of designing hybrid integrated interferometric devices by allowing greater control over the optical amplification process within the device.