All-optical lip-lops based on semiconductor technologies
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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.
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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.
7. 10Gb/s switching operation with no bit loss exploiting the ultra-fast all-
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.
Fig. 28. Output 1 of the 2×2 all-optical switch, when just input 1 is connected (input 2 is
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
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Semiconductor Technologies
370
noise figure, followed by a VOA, a BPF and a photo-receiver, whose input power is kept
constant (by means of the VOA) at -16.7dBm in order to avoid thermal noise. As shown in
Right:, making a comparison with the back-to-back case, the maximum penalty at BER=10
-9
is about 1dB, making the switch driven by the ultra-fast all-optical flip-flop suitable for
cascaded schemes.
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Fig. 29. Left: eye-diagrams of inputs 1 and 2 data frames (a)-(b) and output 1 in BAR (c) and
CROSS (d) configurations of the 2×2 all-optical switch. Right: BER curves in the back-to-
back (B to B) case and at switch output 1 in BAR and CROSS configurations.
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