X All-optical flip-flops based on semiconductor technologies
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- 7. 10Gb/s switching operation with no bit loss exploiting the ultra-fast all- optical flip-flop
Semiconductor Technologies 366
equalize these two wavelength components. Using NOT logic gate 2, we invert signal D and thus obtain signal E, at the same time convert it to one single wavelength λ E =1560nm. Signal E is switched on and off by the set and reset pulses respectively, showing fast rising and falling edges.
controller. Signal A is inverted by NOT logic gate 1 obtaining signal C and added with signal B. Signal D (B+C) is inverted by NOT logic gate 2 obtaining signal E.
d1 : delay between set and reset pulses; T d2 : delay between reset and assistant pulses; T on : rising time; T off
: falling time; ΔT=T d1 +T off .
The optical NOT logic gates are implemented exploiting cross gain modulation (XGM) in SOAs. Concerning NOT logic gate 1, in SOA 5 a CW probe light counter-propagates with respect to signal A. The gain of SOA 5 is modulated by the intensity profile of signal A through XGM. In particular, when signal A has a low input power, the gain provided by SOA 5 for the CW probe will be high, whereas when signal A has a high power the CW probe will experience a lower gain. Ultimately the CW probe undergoes the gain variations obtaining the inversion of signal A, i.e. signal C. Signals from A to E are shown in Fig. 26. Since the slow edges of signals B and C do not have a linear behavior, their sum gives rise to a residual peak during the high level of signal D. After NOT logic gate 2 this dynamic is suppressed because of the gain saturation level of SOA 6. CW probe power injected into SOA 6 has been set in order to optimize its saturation level (as CW probe injected into SOA 5). Exploiting input set and reset pulsewidths of 1µs with edge time of 16ps, signal E presents rising and falling times of 18.8ps and 21.9ps respectively, as shown in Fig. 26 (b) and (c) (measured with a total bandwidth of 53GHz), preserving a contrast ratio of 17.5dB. It is possible to obtain a higher contrast ratio just decreasing the CW probe signal powers in SOA 5 and SOA 6, reducing their gain saturation level, with the drawback of slower switching times (Berrettini, 2006, a). Moreover, www.intechopen.com All-optical lip-lops based on semiconductor technologies 367
equalize these two wavelength components. Using NOT logic gate 2, we invert signal D and thus obtain signal E, at the same time convert it to one single wavelength λ E =1560nm. Signal E is switched on and off by the set and reset pulses respectively, showing fast rising and falling edges.
controller. Signal A is inverted by NOT logic gate 1 obtaining signal C and added with signal B. Signal D (B+C) is inverted by NOT logic gate 2 obtaining signal E.
d1 : delay between set and reset pulses; T d2 : delay between reset and assistant pulses; T on : rising time; T off
: falling time; ΔT=T d1 +T off .
The optical NOT logic gates are implemented exploiting cross gain modulation (XGM) in SOAs. Concerning NOT logic gate 1, in SOA 5 a CW probe light counter-propagates with respect to signal A. The gain of SOA 5 is modulated by the intensity profile of signal A through XGM. In particular, when signal A has a low input power, the gain provided by SOA 5 for the CW probe will be high, whereas when signal A has a high power the CW probe will experience a lower gain. Ultimately the CW probe undergoes the gain variations obtaining the inversion of signal A, i.e. signal C. Signals from A to E are shown in Fig. 26. Since the slow edges of signals B and C do not have a linear behavior, their sum gives rise to a residual peak during the high level of signal D. After NOT logic gate 2 this dynamic is suppressed because of the gain saturation level of SOA 6. CW probe power injected into SOA 6 has been set in order to optimize its saturation level (as CW probe injected into SOA 5). Exploiting input set and reset pulsewidths of 1µs with edge time of 16ps, signal E presents rising and falling times of 18.8ps and 21.9ps respectively, as shown in Fig. 26 (b) and (c) (measured with a total bandwidth of 53GHz), preserving a contrast ratio of 17.5dB. It is possible to obtain a higher contrast ratio just decreasing the CW probe signal powers in SOA 5 and SOA 6, reducing their gain saturation level, with the drawback of slower switching times (Berrettini, 2006, a). Moreover, www.intechopen.com Semiconductor Technologies 368
integrated coupled ring lasers would experience a round trip time in the ps range (instead of 100ns as in our experiment), allowing to use an injected pulsewidth in the ps range too.
0 5 10 15 20 25 30 35 40 45 0 0.5
1 A 0 5 10 15 20 25 30 35 40 45 0 0.5
1 B 0 5 10 15 20 25 30 35 40 45 0 0.5
1 C 0 5 10 15 20 25 30 35 40 45 0 1 2 D 0 5 10 15 20 25 30 35 40 45 0 0.5 1 time (us) E Fall Time: 21.3ps Rise Time: 16.1ps (a)
(b) (c)
Fig. 26. (a): Signal A, B, C, D and E. Signal C = NOT (signal A); signal D=signal B + signal C; signal E = NOT (signal D). (b)-(c): Signal E rising (a) and falling (b) edges.
optical flip-flop
Fast dynamics (rising and falling times of 20ps) and high extinction ratio (17.5dB) make the ultra-fast all-optical flip-flop suitable to be exploited to control a 2×2 SOA-based all-optical switch (Berrettini, 2006, b). The experimental setup is shown in Fig. 27. The switching operation is based on XGM effect in two different SOAs. Depending on the high or low intensity level of the control signal (pump), in one SOA the gain is strongly reduced while the other SOA is not saturated. The two input signals are generated by splitting a single 10Gb/s Non-Return-to-Zero (NRZ) continuous data stream. The stream is generated by modulating a CW laser at λ IN =1550nm by means of a Mach Zehnder modulator driven by a 10Gb/s pattern generator running in (2 31 -1)-long PRBS mode. At the same time the ultra-fast flip-flop output is used as pump signal of the optical switch and controls the switch state (BAR or CROSS). The inverted pump signal needed for switching operation is obtained within the optical switch block through signal inversion by means of XGM in an SOA. The data streams average power at the switch inputs are set to -7dBm, while the high pump level is 11.5dBm. We have chosen continuous data streams instead of packet traffic to demonstrate and point out that it is possible to obtain a switching operation without any bit loss, exploiting the 20ps-fast dynamics of the flip-flop. Indeed, as can be observed in Fig. 28, we can confirm a fast switching operation (faster than the 10Gb/s single bit edge), connecting only input 1
(disconnecting input 2) of the switch and visualizing output 1 on a sampling oscilloscope, switching the output data signal on and off within one bit time.
Fig. 27. All-optical switching operation experimental setup using a 2×2 SOA-based optical switch controlled by the ultra-fast all-optical flip-flop.
disconnected). Insets shows the fast switching-on and switching-off transitions.
Contrast ratio between switched on and switched off signal is about 14dB. This way we avoid any distorted transition bit between switched on and switched off output signals, and vice-versa. Connecting both inputs 1 and 2 of the switch, high or low intensity level of the input pump signal sets the switch in BAR or CROSS state. During BAR state, input 1 of the switch is routed to output 1 (and input 2 is routed to output 2), while during CROSS state input 2 is routed to output 1 (and input 1 is routed to output 2). Fig. 29 (left) shows both input data eye-diagrams and output 1 eye-diagrams in BAR and CROSS configurations, measured by a wide-band photodiode and a sampling oscilloscope. As it can be noticed, the output signal is not affected by pattern effects, showing clearly open eye-diagrams, confirming the effectiveness of the scheme. Right: (right) shows the BER measurements at output 1 of the switch, in both BAR and CROSS configurations. The used receiver is composed by an optical pre-amplifier with 5dB www.intechopen.com Yüklə 0,55 Mb. Dostları ilə paylaş:
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