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Applied Physics Letters

Fast coherent all-optical flip-flop memory

Jani Oksanen, Jukka Tulkki

Published May 1, 2006• 7 citations
Publisher

Abstract

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).

DOI
10.1063/1.2199971

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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.
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.
A multi-state optical flip-flop memory based on cascaded lasers is presented. We show that only one of the lasers can lase at a time, thus the state of the optical memory is determined by the wavelength of the dominant laser. The light from the dominant laser suppresses its neighboring lasers through gain saturation, but still receives amplification by the active element of the suppressed lasers, compensating for coupling losses. This light passes through each of the successive lasers, simultaneously suppressing and being amplified, thus all other lasers are suppressed. A five-state optical memory based on this concept is experimentally demonstrated
Semiconductor ring lasers are semiconductor lasers where the laser cavity consists of a ring-shaped waveguide. SRLs are highly integrable and scalable, making them ideal candidates for key components in photonic integrated circuits. SRLs can generate light in two counterpropagating directions between which bistability has been demonstrated. Hence, information can be coded into the emission direction. This bistable operation allows SRLs to be used in systems for all-optical switching and as all-optical memories. For the demonstration of fast optical flip-flop operation, Hill et al. [Nature 432, 206 (2004)] fabricated two SRLs coupled by a single waveguide, rather than a solitary SRL. Nevertheless, the literature shows that a single SRL can also function perfectly as an all-optical memory. In our recent paper [W. Coomans et al., Phys. Rev. A 88, 033813, (2013)], we have raised the question whether coupling two SRLs to realize a single optical memory has any advantage over using a solitary SRL, taking into account the obvious disadvantage of a doubled footprint and power consumption. To provide the answer, we have presented in that paper a numerical study of the dynamical behavior of semiconductor ring lasers coupled by a single bus waveguide, both when weakly coupled and when strongly coupled. We have provided a detailed analysis of the multistable landscape in the coupled system, analyzed the stability of all solutions and related the internal dynamics in the individual lasers to the field effectively measured at the output of the waveguide. We have shown which coupling phases generally promote instabilities and therefore need to be avoided in the design. Regarding all-optical memory operation, we have demonstrated that there is no real advantage for bistable memory operation compared to using a solitary SRL. An increased power suppression ratio has been found to be mainly due to the destructive interference of the SRL fields at the low power port. Also, multistability between several modal configurations has been shown to remain unavoidable.
The fastest known operation of all-optical flip-flop memory was experimentally demonstrated using a 980-nm polarization bistable vertical-cavity surface-emitting laser (VCSEL). Operating conditions of the input signal power and the frequency detuning to achieve the fast optical memory operation were characterized experimentally. At the optimum condition, 1-bit data signals were arbitrarily sampled and memorized from a 2(6)-1 pseudorandom bit sequence return-to-zero signal at 20 Gb/s by using AND gate and memory functionalities obtained from the polarization bistability. In addition, 1-bit memory operation was achieved for a 6-bit non-return-to-zero signal at 40 Gb/s. Both memory operations required 250-microW data signal power and had optical gain. The high potential of all-optical flip-flop memories based on polarization bistable VCSELs for use in ultrafast all-optical future networks was demonstrated.