X All-optical flip-flops based on semiconductor technologies
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| All-optical lip-lops based on semiconductor technologies 347
All-optical lip-lops based on semiconductor technologies Antonella Bogoni, Gianluca Berrettini, Paolo Gheli, Antonio Malacarne, Gianluca Meloni, Luca Potì and Jing Wang
semiconductor technologies
Antonella Bogoni 1 , Gianluca Berrettini 2 , Paolo Ghelfi 1 ,
Antonio Malacarne 2 , Gianluca Meloni 1 , Luca Potì 1 and Jing Wang 3
Consorzio Nazionale Interuniversitario per le Telecomunicazioni (CNIT), Pisa Italy 2 Scuola Superiore Sant’Anna, Pisa Italy 3 Department of Electronic Engineering, Tsinghua University, Beijing China
Optical technologies represent the main bet for future communication systems. Among the others, digital subsystems for optical processing are of great interest thanks to their intrinsic properties in terms of bandwidth, transparency, immunity to the electromagnetic interference, cost, power consumption, as well as robustness in hostile environment. Key basic functions are represented by logic gate, logic function, flip-flop memories, optical random access memories, etc.. Research in this field is in its very early stages even if some interesting techniques have been already theoretically addressed and experimentally demonstrated. Here we review the state of the art for all-optical flip-flop based on semiconductor technologies: best result will be highlighted in terms of transition speed, switching energy, complexity and power consumption; we will then discuss some new achievement we have recently reached. All-optical packet switching seems to be the most promising way to take advantage of fiber bandwidth to increase routers forwarding capacity, being able to achieve very high data rate operations. All-optical flip-flops have been widely investigated mainly because they can be exploited in all-optical packet switches, where switching, routing and forwarding are directly carried out in the optical domain. Some examples concerning optical packet switches are shown in (Dorren et al., 2003; Liu et al., 2005; Bogoni et al., 2007; Herrera et al., 2007), where an optical flip-flop stores the switch control information and drives the switching operation. Former solutions for all-optical flip-flops have been demonstrated exploiting discrete devices (Dorren et al., 2003) or Erbium-doped fiber properties (Malacarne et al., 2007) which suffer from slow switching times and high set/reset input powers. Several integrated or integrable solutions (Hill et al., 2004; Liu et al., 2006) present a switching energy in the fJ range and switching times of tens of ps at the expenses of poor contrast ratios. On the other hand in (Hill et al., 2005) an integrated scheme exhibiting a very high contrast ratio value but with transition times in the ns range is reported. In any case a 15 www.intechopen.com Semiconductor Technologies 348
trade off between contrast ratio and edges speed must be found as a function of the flip-flop application. Micro-resonators-based bistable element has been demonstrated (Van et al., 2002) presenting high optical operating power, pJ switching energies and microsecond switching times, theoretically reducible down to the order of tens of ps. Making a comparison with electronics, recent large-scale integration (LSI) circuits (Keyes, 2001) show switching energies of 1fJ even though with slower switching speeds. In (Dorren et al., 2003), a solution based on coupled ring lasers is proposed. This solution offers a certain number of advantages: it can provide high contrast ratios between states; there is no difference in the mechanisms for switching from state 1 to state 2 and vice-versa, making symmetric set and reset operations; it presents a large input light wavelength range and a controllable switching threshold. Moreover, considering an integrated version of this kind of flip-flop, through numerical analysis a switching energy in fJ range has been demonstrated. Here we will describe the above mentioned solutions underlining the main benefits, drawback, limitation and perspectives. We will then present our activities on clocked flip- flops, and an example of their use in an all-optical counter. Finally, we will present an SOA- based flip-flop which is able to switch with very short rising and falling edges, and we use it in a realistic switching operation. Integrability of our solutions is also discussed.
One of the simplest way that was originally proposed to implement an optical flip-flop includes two coupled lasers (Hill et al., 2001), as depicted in Fig. 1 (a). The system can have two stable states. In state 1, light from laser 1 suppresses lasing in laser 2. In this state, the optical flip-flop memory emits CW light at wavelength 1 . Conversely, in state 2, light from laser 2 suppresses lasing in laser 1, and the optical flip-flop memory emits CW light at wavelength 2 . To change states, lasing in the dominant laser can be inhibited by injecting external light with a different wavelength and opportune power. The output pulse of an optical header processor can be used to set the optical flip-flop memory into the desired wavelength. From the theory it also follows that laser driving currents and coupling coefficient determines the required switching light power. This flip-flop has also been implemented in a ring configuration based on Semiconductor Optical Amplifiers (SOA), as shown in Fig. 1 (b) (Dorren et al., 2003). Two SOAs act as the lasers gain media. Fabry–Pérot filters (FPF) with a bandwidth of 0.18nm have been used as wavelength selective elements. Optical pulses were used to set and reset the flip-flop. The optical spectrum of the flip-flops’ output states is shown in Fig. 2. The switching time between the two lasing modes is inversely proportional to the length of the laser cavities. Thus, in order to allow switching times in the range of picoseconds, an integrated solution has to be adopted. This was realized in (Hill et al., 2004), where a photonic flip-flop based on two coupled micro-ring lasers with dimensions of 20x40 m 2
was reported, exhibiting a switching time of 18ps and a switching energy of a few fJ. The micro-ring lasers were fabricated in active areas of the integrated circuit containing bulk 1.55nm bandgap InGaAsP in the light guiding layer. Separate electrical contacts allowed each laser’s wavelength to be individually tuned by adjusting the laser current. Passive waveguides connected the micro-ring lasers to the integrated circuit edges (Fig. 3). Micro- ring lasers typically have two inherent lasing modes; laser light traveling in the clockwise (CW) direction, and laser light in the anticlockwise (ACW) direction.
(a)
(b) Fig. 1. (a): Arrangement of two coupled identical lasing cavities forming a flip-flop, showing the two possible states. (b): Implementation of the optical flip-flop memory
Fig. 3. Two micro-ring lasers coupled via a waveguide to form an optical flip-flop. In state A, CW light from laser A is injected via the waveguide into laser B. The light from laser A will undergo significant resonant amplification in laser B if the resonant frequencies of the two laser cavities are close. This injected light competes with the laser B self- oscillations for available power from the laser gain medium. If sufficient light is injected into laser B, then the laser B gain will be decreased below threshold. This extinguishes the laser B self-oscillation, and laser A captures or injection-locks (Buczek et al., 1971) laser B, forcing light to circulate only in the CW direction. To set the system in one state or another, light www.intechopen.com
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