All-optical lip-lops based on semiconductor technologies
349
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.
2. State of the art
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. 2. Spectral output of two states 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
Semiconductor Technologies
350
close to the lasing wavelength and polarization can be injected into the waveguide
connecting the lasers. This light will set both lasers simultaneously lasing in either the CW
or ACW direction. The different states can be distinguished by the different power levels at
the two outputs. The power level at the output associated with the locked laser will be three
times that of the other output. Additionally, the lasing wavelengths of the lasers may be
different, allowing the states to be distinguished by the wavelength of the light output.
Another scheme recently proposed (Malacarne et al., 2007) exploits absorption and
fluorescence of few meters of erbium–ytterbium (Er–Yb)-doped fiber. This solution suffers
from slow switching times and high set/reset input powers, and since it doesn’t exploit
semiconductor devices, it will not be studied in depth here.
In (Liu et al., 2006) a solution that offer the advantage of being fully packaged, was
presented. It is based on an hybrid integrated circuit consisting of two coupled Mach-
Zehnder interferometers (MZIs), each having one SOA in one arm. The schematic of the
circuit is shown in Fig. 4.
Fig. 4. Schematic diagram of optical flip-flop memory proposed in (Liu et al., 2006).
Each MZI (MZI 1 and MZI 2 in the figure) has an SOA in one arm. A laser emits a
continuous-wave (CW) bias light at wavelength
1
that is fed into MZI 1. The MZI 1 output
is sent into MZI 2, which has the same structure, but biased by a CW light with a different
wavelength,
2
. The system has two possible states: in state 1, the MZI 1 output suppresses
output from MZI 2, so
1
dominates the output; in state 2, the MZI 2 output suppresses
output from MZI 1, and then
2
is dominant. When the CW light with
1
is injected into MZI
1, MZI 1 is biased in such a way that the light out of MZI 1 goes mostly into the low branch
of the 50/50 coupler output. This light then flows into MZI 2 via the 50/50 coupler in MZI 2,
and affects the gain and phase shift for light propagating through it. The MZI 1 light
perturbs the SOA 2 properties so that the CW bias 2 light (
2
) propagating through SOA 2
and phase shifter 2 goes mostly into the top output of the 50/50 coupler in MZI 2. Then the
CW bias 2 light (
1
) does not travel into the MZI 1, and does not affect the properties of SOA
1. Actually, the MZI 1 output suppresses output from MZI 2. The states of the system can be
switched by sending a light pulse (via Set or Reset port) into the MZI that is currently
dominant. This light will switch the MZI output away from suppressing the other MZI,
allowing the other MZI then to become dominant.
An optical flip-flop based on two-mode bistability in a multimode interference bistable laser
diode (MMI-BLD) has also been reported (Takenaka et al., 2005). A schematic view of the
MMI-BLD is shown in Fig. 5 (a). All waveguides including the 2x2 MMI coupler consist of
active materials. Saturable absorbers are located at the end of the output ports to obtain
hysteresis. The 2x2 MMI is designed as a cross coupler, so that only two cross-coupled
lasing modes can exist as illustrated in the insets of Fig. 5 (a). Two-mode bistability between
these two lasing modes will occur due to cross gain saturation and the saturable absorbers if
the injection current is within the hysteresis loop (Takenaka & Nakano, 2003).
(a)
(b)
Fig. 5. (a): Schematic view of the MMI-BLD. Two cross-coupled lasing modes are
illustrated in the insets. (b): All-optical flip-flop operation of the MMI-BLD.
A set signal injected into the set port saturates the absorption to Mode 1, causing Mode1 to
start lasing. At the same time, cross-gain saturation and the absorption to Mode 2 by the
saturable absorber suppress Mode 2. In a similar manner, a reset signal switches the lasing
mode from Mode 1 to Mode 2. Therefore, all-optical flip-flop operation is achievable with
the MMI-BLD, because external light injection to each input port will select the mode to lase.
The corresponding operation, showing the optical power at one of the waveguide output
when set and reset pulses are applied is depicted in Fig. 5 (b).
In (Huybrechts et al., 2008) a single DFB laser diode has been used to realize a flip flop. A
DFB laser injected with CW light shows two different stable states: one in which the laser is
lasing and another one where it is switched off. When the laser is lasing, the gain will be
clamped and relatively small. Therefore, the injected light experiences only a small
amplification and has almost no influence on the laser light. In the second state, the laser is
switched off and the injected light experiences a high amplification. This results in a rising
power progression throughout the cavity and therefore a non-uniform distribution of the
carriers, known as spatial hole burning. This will affect the refractive index, leading to a
distortion of the Bragg reflections in the laser diode. The losses inside the cavity will become
higher and the threshold for lasing will rise. Eventually the laser will stay switched off. The
two states are equally possible for a range of input powers of the injected light and this gives
a bistability in the lasing power (Fig. 6 (a)). This bistability can be exploited to obtain flip-
flop operation by injecting short optical pulses: a pulse injected at the same side as the CW
light will move the DFB laser out of the hysteresis curve and will switch off the laser; to
switch the laser on again, a pulse is injected from the other side, since this will restore the
uniformity of the carrier distribution. In the experiment, the set and reset pulses were
obtained from an ultra-short pulse source generating 7ps-long pulses. The obtained results
www.intechopen.com