All-optical lip-lops based on semiconductor technologies
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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
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