Semiconductor Technologies
352
are depicted in Fig. 6 (b). The set-pulses have an energy of 75fJ and the reset-pulses 190 fJ.
The repetition rate is 1.25GHz and the switch-on time is 75ps. An almost immediate switch-
off time of 20ps has been obtained, which corresponds with the resolution of the optical
scope.
(a)
(b)
Fig. 6. (a): Bistability of an injected DFB laser as a function of the injected power. (b): Results.
(a)
(b)
Fig. 7. (a): Operation principle of the monolithic semiconductor ring laser. (b): Results.
As discussed previously, integrable solutions are preferred since they would allow high-
density packaging, with the possibility of reducing costs, power consumption, and
operation speed. To achieve these results, researchers are investigating novel technologies in
order to reduce as much as possible device dimensions. A possible solution towards this
direction is the use of a monolithic semiconductor micro-ring laser (Trita et al., 2009) which
shows an intrinsic and robust directional bistability between its CW and ACW propagating
modes. If the ring laser is correctly set, injecting a laser pulse in one direction makes the
laser emit in that direction (Fig. 7 (a)). Experiments show a switching time of about 20ps for
both rising and falling edges, with set/reset pulses of 5ps and 150fJ energy.
Another promising technology is nano-photonics, exploited in the realization of photonic
crystals (PCs) and quantum dots (QDs). By combining these technologies one could take
advantage of both the band-gap effect and the highly dispersive property of PCs, and the
high-density of state and high nonlinear property of QDs.
Fig. 8. Schematic diagram of the PC-FF.
A Mach Zehnder-type all-optical flip-flop developed by combining GaAs-based two-
dimensional photonic crystal (2DPC) slab waveguides and InAs-based optical nonlinear
QDs has been proposed in (Azakawa, 2007). The photonic crystal-based flip-flop (PC-FF)
schematic is shown in Fig. 8, and is based on two photonic-crystal-based Symmetric Mach
Zehnder (PC-SMZ) switches. The principle of the PC-SMZ is based on the time-differential
phase modulation caused by the nonlinear-induced refractive index change in one arm of
the two interferometers. 2DPC waveguides are composed of single missing line defects,
while nonlinear-induced phase shift arms are selectively embedded with QDs. The
mechanism of the third-order nonlinear property is an absorption saturation of the QD
caused by a control (pump) pulse. A resultant refractive index change produces a phase
shift for the signal (probe) pulse. A wavelength of the control pulse is set to the absorption
peak of the QD, while a wavelength of the signal pulse is set in the high transmission range
in the 2DPC waveguide with the QD. A single PC-SMZ switch would operate as a pseudo-
flip-flop, meaning that the on-state is limited by the carrier relaxation time in the nonlinear
material (~ 100ps in the experiment). In order to change the pseudo FF into the normal FF
operation, the scheme of Fig. 8 was proposed. An output signal of the PC-SMZ impinges
into an optical AND element (which is another PC-SMZ switch) via a feedback loop, where
another input pulse, i.e., a clock pulse impinges. An output of the AND element is combined
to the set pulse, as shown in the figure. The clock pulse serves as a refresh pulse to expand
the on-state period against the relaxation of the carrier, while the feedback signal restricts
the clock pulse to be controlled by the set and reset pulses. The feasibility of this idea has
been verified only by computer simulation.
3. Flip-flops based on coupled SOA ring lasers: advantages and limitations
In order to investigate advantages and drawbacks of SOA-based solution we consider the
setup shown in Fig. 9. The flip-flop consists of two coupled ring lasers emitting at two
different wavelengths (λ
1
=1550nm and λ
2
=1560nm). In each ring, an SOA acts as the gain
element, a 0.25nm band-pass filter (BPF) is used to as select the wavelength, and an isolator
makes the light propagation unidirectional. Both the SOAs are polarization insensitive
www.intechopen.com
All-optical lip-lops based on semiconductor technologies
353
are depicted in Fig. 6 (b). The set-pulses have an energy of 75fJ and the reset-pulses 190 fJ.
The repetition rate is 1.25GHz and the switch-on time is 75ps. An almost immediate switch-
off time of 20ps has been obtained, which corresponds with the resolution of the optical
scope.
(a)
(b)
Fig. 6. (a): Bistability of an injected DFB laser as a function of the injected power. (b): Results.
(a)
(b)
Fig. 7. (a): Operation principle of the monolithic semiconductor ring laser. (b): Results.
As discussed previously, integrable solutions are preferred since they would allow high-
density packaging, with the possibility of reducing costs, power consumption, and
operation speed. To achieve these results, researchers are investigating novel technologies in
order to reduce as much as possible device dimensions. A possible solution towards this
direction is the use of a monolithic semiconductor micro-ring laser (Trita et al., 2009) which
shows an intrinsic and robust directional bistability between its CW and ACW propagating
modes. If the ring laser is correctly set, injecting a laser pulse in one direction makes the
laser emit in that direction (Fig. 7 (a)). Experiments show a switching time of about 20ps for
both rising and falling edges, with set/reset pulses of 5ps and 150fJ energy.
Another promising technology is nano-photonics, exploited in the realization of photonic
crystals (PCs) and quantum dots (QDs). By combining these technologies one could take
advantage of both the band-gap effect and the highly dispersive property of PCs, and the
high-density of state and high nonlinear property of QDs.
Fig. 8. Schematic diagram of the PC-FF.
A Mach Zehnder-type all-optical flip-flop developed by combining GaAs-based two-
dimensional photonic crystal (2DPC) slab waveguides and InAs-based optical nonlinear
QDs has been proposed in (Azakawa, 2007). The photonic crystal-based flip-flop (PC-FF)
schematic is shown in Fig. 8, and is based on two photonic-crystal-based Symmetric Mach
Zehnder (PC-SMZ) switches. The principle of the PC-SMZ is based on the time-differential
phase modulation caused by the nonlinear-induced refractive index change in one arm of
the two interferometers. 2DPC waveguides are composed of single missing line defects,
while nonlinear-induced phase shift arms are selectively embedded with QDs. The
mechanism of the third-order nonlinear property is an absorption saturation of the QD
caused by a control (pump) pulse. A resultant refractive index change produces a phase
shift for the signal (probe) pulse. A wavelength of the control pulse is set to the absorption
peak of the QD, while a wavelength of the signal pulse is set in the high transmission range
in the 2DPC waveguide with the QD. A single PC-SMZ switch would operate as a pseudo-
flip-flop, meaning that the on-state is limited by the carrier relaxation time in the nonlinear
material (~ 100ps in the experiment). In order to change the pseudo FF into the normal FF
operation, the scheme of Fig. 8 was proposed. An output signal of the PC-SMZ impinges
into an optical AND element (which is another PC-SMZ switch) via a feedback loop, where
another input pulse, i.e., a clock pulse impinges. An output of the AND element is combined
to the set pulse, as shown in the figure. The clock pulse serves as a refresh pulse to expand
the on-state period against the relaxation of the carrier, while the feedback signal restricts
the clock pulse to be controlled by the set and reset pulses. The feasibility of this idea has
been verified only by computer simulation.
3. Flip-flops based on coupled SOA ring lasers: advantages and limitations
In order to investigate advantages and drawbacks of SOA-based solution we consider the
setup shown in Fig. 9. The flip-flop consists of two coupled ring lasers emitting at two
different wavelengths (λ
1
=1550nm and λ
2
=1560nm). In each ring, an SOA acts as the gain
element, a 0.25nm band-pass filter (BPF) is used to as select the wavelength, and an isolator
makes the light propagation unidirectional. Both the SOAs are polarization insensitive
www.intechopen.com