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Optics Express

Optical bistability and flip-flop function in feedback Fano laser

Shih Lun Liang, Jesper Mørk, Yi Yu

Published Feb 8, 2024• 3 citations

Abstract

Optical bistability has the potential to emulate the capabilities of electrical flip-flops, offering plenty of applications in optical signal processing. Conventional optical bistable devices operate by altering the susceptibility of a nonlinear medium. This method, however, often results in drawbacks such as large device size, high energy consumption, or long switching times. This work proposes an optical bistable device incorporating strong optical feedback into a Fano laser. This leads to multiple stable states and introduces a region of bistability between the inherent Fano mode and a feedback-induced Fabry-Perot mode. Unlike conventional bistable devices, the Fano system exploits strong field localization in a nanocavity to control the properties of one of the laser mirrors. This configuration means that switching states can be achieved by modulating the mirror's loss rather than changing the susceptibility of the active medium. Importantly, modulation can be implemented locally on a nanocavity, bypassing the need to adjust the entire laser system. This leads to fast flip-flop actions with low energy consumption. The feedback Fano laser can be embodied in a compact microscopic structure, thus providing a promising approach towards integrated all-optical computation and on-chip signal processing.

DOI: 10.1364/oe.510599

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Optical bistability has the potential to emulate the capabilities of electrical flip-flops, offering plenty of applications in optical signal processing. Conventional optical bistable devices operate by altering the susceptibility of a nonlinear medium. This method, however, often results in drawbacks such as large device size, high energy consumption, or long switching times. This work proposes an optical bistable device incorporating strong optical feedback into a Fano laser. This leads to multiple stable states and introduces a region of bistability between the inherent Fano mode and a feedback-induced Fabry-Perot mode. Unlike conventional bistable devices, the Fano system exploits strong field localization in a nanocavity to control the properties of one of the laser mirrors. This configuration means that switching states can be achieved by modulating the mirror's loss rather than changing the susceptibility of the active medium. Importantly, modulation can be implemented locally on a nanocavity, bypassing the need to adjust the entire laser system. This leads to fast flip-flop actions with low energy consumption. The feedback Fano laser can be embodied in a compact microscopic structure, thus providing a promising approach towards integrated all-optical computation and on-chip signal processing.

Optical bistability (OB) can be defined as the existence of two stable states for one set of optical input conditions. It can be seen in many different systems which generally combine some microscopic nonlinearity (e.g., intensity-dependent refractive index) with some macroscopic feedback (e.g., a Fabry-Perot cavity). OB shows not only hysteresis but also switching transitions. By adjusting the degree of feedback various related effects are also possible (e.g., optical limiters, optical transistors, and several kinds of oscillation phenomena).

We have proposed a scheme for all-optical switching and flip-flop based on controllable optical bistability in a simple three-level V-type atomic system assisted by an incoherent pump field. Due to the auxiliary incoherent pump, the atomic population is redistributed in the three levels, and then the absorption-dispersion property and the Kerr nonlinearity of the medium are different from those of the conventional V-type electromagnetically induced transparency system. The results show that both the thresholds and the area (or shape) of the hysteresis loop can be controlled efficiently via the adjustment of either the incoherent pump field or the coupling field. As a result, with a constant cavity input, the output of the optical cavity can be switched between the low- and high-power states of different bistable curves by tuning the optical fields. Based on this effect, we realize all-optical steady-state switching and set-reset flip-flop operation by adding a pulse sequence (positive pulse or set-reset pulse) onto either the incoherent pump field or the coupling field. Such a simple and effective technique of optical switching and flip-flop can find application in all-optical signal processing systems.

Coherent nonlinear feedback is shown to eliminate the most critical transient problems in the known active integrated all-optical flip-flop structures, potentially increasing the operating speed by an order of magnitude. The nonlinearity is based on the interference of two coherent optical fields and on operating the pertinent laser amplifiers only above the laser threshold. Therefore the lasers have an approximately constant carrier density and an ever present photon population in the laser mode. This enables fast switching. Numerical analysis based on coupled rate equations demonstrates switching times below 25ps (40GHz).

Since there is an increasing demand for fast networks and switches, the electronic data processing imposes a severe bottleneck and all-optical processing techniques will be required in the future. All-optical flip-flops are one of the key components because they can act as temporary memory elements. Several designs have already been demonstrated but they are often relatively slow or complex to fabricate. We demonstrate experimentally fast flip-flop operation in a single DFB laser diode which is one of the standard elements in today's telecommunication industry. Injecting continuous wave light in the laser diode, a bistability is obtained due to the spatial hole burning effect. We can switch between the two states by using pulses with energies below 200 fJ resulting in flip-flop operation with switching times below 75 ps and repetition rates of up to 2 GHz.