International Core Journal of Engineering 2020-26 | Page 120

integrated into a rectangular wave by a non-return-to-zero
code pulse trigger and then modulated onto the previous sine
wave by OOK-modulation via the M-Z Modulator. Server A
adds a delay, before the rectangular wave is modulated onto
the carrier, to simulate an asynchronous scenario that the
server may encounter. Two signals pass via a 50m fiber to
the ToR for XOR encoding operation
simulation design of optisystem for the platform. The above-
mentioned Saganac-based all-optical logic gate is described
in detail in [6]. The dynamic wavelength assignment
algorithm for multi-server is described in [7].
The structure of the all-optical XOR gate at ToR is
shown in Fig. 4. The XOR gate has two control ports, one for
the clock signal and another for the ground. The principle of
Saganac-based XOR gate requires signal of the control port
and the input signal polarization orthogonal. Since the
polarization angle of the light source of the input signal has
been set to 90 degrees, the polarization angle of the clock
control signal should be set to 0 degrees. The clock signal is
also implemented by a non-return-to-zero code pulse trigger,
and then the signal is modulated onto a light wave having a
wavelength of 1550 nm and a power of 0.05 mW. The clock
signal and the ground signal pass via the circulator to the X-
type coupler. After coupling, the output signal and the input
signal are respectively entered into the Saganac fiber ring via
the polarization combiner. Since a power difference is
required between the input signal and the control signal, the
input signal is added to EDFA with a gain of 30 dB before
entering the polarization synthesizer. The two signals are
transmitted via a strong birefringent photonic crystal fiber
and separated by a polarization beam splitter before reaching
the coupler. Since the polarization of the polarization beam
splitter is set to 0 degrees, the polarization angle of the output
signal of the upper arm of the polarization beam splitter is 0
degrees. The clock signal is divided into CS and CSS signals
by the coupler, which are respectively modulated by the
input signals IP1 and IP2. After transmission via the fiber
optic ring, a clock signal with a phase difference of " S " is
output from the OP2 terminal, and a phase difference of "0"
is output from the OP1 terminal. Therefore, the output of the
OP2 terminal is an XOR result pulse sequence, that is, the
result of the signal encoded.
Fig. 5. (a)-(g) represent time traces of encoding and decoding processes at
synchronous, asynchronous 0.25-bit and asynchronous 0.5-bit; (h) eye
diagrams representing synchronous and asynchronous operations.
IV. E XPERIMENTAL A ND S IMULATION R ESULTS
The synchronous and asynchronous OPNC scheme was
evaluated exerimentally by a 100 GHz subcarry. Fig.5
depicts time traces and eye diagrams of encoding and
decoding processes at synchronous and asynchronous. Fig.5
(a) and Fig.5 (b) reveal the input time traces of the bit-level
of Data A and Data B at synchronous and asynchronous. We
insert as delay at Data A using sub-bit times-offsets of 12.5
ps (0.25 of the bit duration) and 25 ps (0.5 of the bit
duration). The resultant traces are illustrated Fig.5 (c), where
it is revealed that a logical “1”, when data A and B are
different, and a logical “0”, when data A and B are the same.
The figure can be seen the XOR encoded signal is damaged
at asynchronous. They are no longer complete bits, and have
many very narrow pulse waveforms smaller than 1 bit. Fig.5
(d) shows the decoded data A at the second Saganac-based
XOR gate at synchronous and asynchronous, each generated
by a bitwise XOR between the XOR encoded trace of Fig.5
(c). Fig.5 (e) reveals time trace after photoelectric conversion.
The decoded time trace of Data A is equal to the initial
pattern at synchronous. The trace of Data A can be
successfully retrieved at asynchronous, too. However, the
retrieved time trace of the sub-bit times-offsets of 0.25bit is
worse than the sub-bit times-offsets of 0.5bit. Since the XOR
encoded trace of sub-bit times-offsets of 0.25bit is narrower
than 0.5bit. Narrow pulses are more susceptible to noise. In
turn, Fig.5 (f) show the decoded data A at the second XOR
gate at synchronous and asynchronous, each generated by a
The encoded signal is broadcasted by a power splitter
with a power division ratio of 1:2 and transmitted to the
receiving end of A and B via a 50m optical fiber for
decoding operation. For the sake of convenience, we
simplified the system by sending the pulse signal of Data A
directly to the server A via the optical fiber to replace the
buffer data of the server A. The XOR decoding operation is
performed with the encoding result of the ToR that has via
the 50m fiber. Due to the power loss caused by the device
and the fiber during the encoding operation, we need to use
an EDFA to amplify the encoded signal to 2.08016 mW. The
parameter setting of the XOR gate is the same as the
previous encoding operation during decoding, and the
encoded signal is XORed with the simultaneously
transmitted A signal pulse to obtain the pulse waveform of
the server B. The clock signal is divided into CS and CSS
signals by the coupler, which are respectively modulated by
the input signals IP1 and IP2. After transmission via the fiber
optic ring, a clock signal with a phase difference of " S " is
output from the OP2 terminal, and a phase difference of "0"
is output from the OP1 terminal. The delay at the decoding is
used to implement asynchronous decoding operations. The
signal pulse waveform of the server B is obtained by passing
the decoded signal via a photoelectric conversion and a low
pass filter. Server B's bitstream is available after the
threshold decision. This experiment is based on the
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