PSK format conversion with multicast functionality based on cascaded SOA configuration

Journal Pre-proof All-optical XOR function accompanied with OOK/PSK format conversion with multicast functionality based on cascaded SOA configuration Yaya Mao, Bo Liu, Rahat Ullah, Tingting Sun, Lilong Zhao

PII: DOI: Reference:

S0030-4018(20)30096-1 https://doi.org/10.1016/j.optcom.2020.125421 OPTICS 125421

To appear in:

Optics Communications

Received date : 15 November 2019 Revised date : 26 December 2019 Accepted date : 29 January 2020 Please cite this article as: Y. Mao, B. Liu, R. Ullah et al., All-optical XOR function accompanied with OOK/PSK format conversion with multicast functionality based on cascaded SOA configuration, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125421. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

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*Manuscript Click here to view linked References

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Yaya Mao , Bo Liu , Rahat Ullah , Tingting Sun , Lilong Zhao a.

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All-optical XOR function accompanied with OOK/PSK format conversion with multicast functionality based on cascaded SOA configuration a

Institute of Optoelectronics, Nanjing University of Information Science & Technology, Nanjing, 210044, China.

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Abstract:

We proposed a novel all-optical XOR logic accompanied with OOK/PSK format conversion by using cascaded semiconductor optical amplifier (SOA) configuration. Within each SOA, the cross phase modulation and nonlinear polarization rotation processes are properly controlled by OOK signals. The eye diagrams and obtained spectra witnessed the effectiveness of operation. A multicast function is also explored in the process of XOR function, and error-free operation (BER <10−9) with less than 2.5 dB power penalty is obtained. Key Words

1.

Introduction

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Semiconductor optical amplifier; Nonlinear polarization rotation; Optical logic; Format conversion

All-optical logic gate is a key component to realize all-optical digital signal processing in photonic system [1]. Exclusive-OR (XOR) logic is regarded as one of the fundamental logic gates in signal processing, which plays a

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key role in applications such as bit pattern recognition, pseudorandom bit sequence (PRBS) generation, parity checking and optical computing [2]. All-optical XOR logic based operation for on-off-keyed (OOK) optical signals can be implemented by utilizing photonic crystal fiber[3], micro-ring resonators [4-5] and quantum dot or well materials [6-7]. Furthermore, due to their advantages of high nonlinear effect and easy integration, semiconductor optical amplifiers (SOAs) are always used for XOR logic operation in combination with an optical interferometer, for example, Mach-Zehnder interferometers (MZIs) [8-9], Sagnac interferometer and ultrafast nonlinear interferometer (UNI) [10-12].Recently, newly introduced advance modulation techniques involving phase-encoding such as differential phase shift keying (DPSK) have attracted attention because of their high tolerant behavior toward system impairments and nonlinearities. All-optical logic gates for DPSK based data have been successfully demonstrated in numerous structures such as SOAs [13-15], periodically poled lithium niobate (PPLN) [16] and highly nonlinear waveguides and fiber [17-21]. However, these schemes suffer from the drawbacks of one form or another, such as high control signal power requirements for four-wave mixing (FWM) in SOA, precise temperature control for PPLN, dispersion in highly nonlinear waveguide and long nonlinearity optical fiber demands. Therefore, improved and alternative techniques are of significant interest to circumvent these issues and allow the integration of these modules into real systems. Furthermore, format conversion plays an important role at the network node and both for OOK and phase shift keying (PSK) based signals. All-optical NRZ-OOK/RZ-BPSK format conversion for wavelength multicasting has been demonstrated by using cross-phase modulation (XPM) in

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optical fiber and SOA-MZI wavelength converter [22-23]. However, there is no format conversion during the logic operation as mentioned above. Up to now, to the best of our knowledge, there is only one proposed scheme for XOR gate accompanied by OOK/PSK format conversion by using XPM in optical fiber and has been experimentally demonstrated [24], but the disadvantage with the scheme is its low conversion efficiency and further hard for multicasting.

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In this paper, based on the XPM effect, we proposed and demonstrated a module to simultaneously realize the function of XOR logical gate and format conversion of OOK/PSK using a cascaded SOA configuration. Furthermore, we experimentally demonstrated the proposed all-optical XOR gate function by feeding two NRZ-OOK signals into SOAs, and the converting launched OOK pulse sequence to PSK signal as a result of XOR

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operation. In addition, wavelength multicasting function is also realized based on this structure. Since an XOR gate is not intended for stand-alone use but in combination with identical gates to form more complex all-optical circuits, the fan-in and fan-out capability would be limited by the inordinate increase of the number of SOAs, and even further compromise the system performance regarding bit error rate (BER) and signal-to-noise ratio (SNR), making

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it a far less pragmatic solution for all-optical network bandwidth computing system. But the proposed logic still has the potential to be the key components for an optical packet switching system due to their small feature sizes and low power consumption.

2.

Operation principle

The schematic presentation of the proposed all-optical XOR logic is shown in Fig. 1 that is mainly consists of two SOAs. Two steps are involved to perform XOR process, at the first step the continuous wave (CW) light and

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OOK1 signal are launched into SOA1 as probe light and control pulse, respectively. The phase of the probe light is modulated due to the XPM induced by the control pulse. Then during second step, the probe light and the other OOK2 control signal are launched into SOA2 synchronously. SOA parameters and wavelength allocation of the probe and control pulses are carefully designed and the launched powers of the control pulses are adjusted to satisfy that the mark symbol both of OOK1 and OOK2 induces π phase shift in the probe pulse. After passing through the SOAs, the probe signal can be converted to PSK signal in which phase is modulated depending on the XOR

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operation with OOK1 and OOK2. Initially, when OOK1 and OOK2 are both “0”, the probe light traveling through the two SOAs acquires a phase shift of “0” when it recombines at the output port, thus the output is “0”. When OOK1 and OOK2 are “1” and “0” respectively, the probe light traveling through the SOAs due to the cross phase modulation (XPM), the output has an additional phase shift “ π” which results in an output “1”. The same phenomenon happens if OOK1 is 0 and OOK2 is 1. However, when OOK1 and OOK2 are both “1”, than the phase has 2π additional phase change, hence the output is correspondingly “0”. Consequently, the probe CW light carries the result of the XOR operation of the two OOK signals at the output of scheme.

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Fig. 1 Schematic presentation of the proposed all-optical XOR gate

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Experiment setup and results

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Fig. 2 Structure of the wavelength converter based on cascaded SOAs. DFB: distributed feedback laser; PC: polarization controller; PPG: pulse pattern generator; MZM: Mach-Zehnder modulator; SOA: semiconductor optical amplifier; EDFA: erbium doped fiber amplifier; AWG: array waveguide grating; OSC: oscilloscope DI: optical delay

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interferometer.

Fig. 2 shows the experimental system of the multicasting XOR logical gate performing OOK to PSK format conversion. Four distributed feedback (DFB) lasers generate probe lights with four different wavelengths from λ1 to λ4. All the probe lights are combined with an arrayed waveguide grating (AWG) and then injected into SOA1 as probe light. Four polarization controllers (PCs) are employed, before injecting into AWG, to adjust the state of polarization (SOP) of the probe lights with respect to the SOA1 layers. Two other DFB lasers with the

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wavelengthλ5 and λ6 are modulated by Mach-Zehnder modulators (MZMs), to generate a 10 Gb/s OOK1 and OOK2 signals. The two signals are then relatively delayed by integral bit periods compared to each other using an optical tunable delay line (ODL). Then the two signals are amplified by erbium-doped fiber amplifiers (EDFAs) respectively, and launched into SOAs as control signal through circulators. The phase shift of “π” and “0” for the probe light can be achieved under the control signal “1” and “0” in SOAs, respectively. Therefore, the probe light picks up the XOR results with OOK1 and OOK2 signals. Nevertheless, during changing the phase of the probe light, the amplitude of the probe light is also changed due to cross gain modulation (XGM), and it is necessary to equalize the power of the probe light. Due to the non-linear polarization rotation (NPR), the SOP of the probe light rotates under control signal of “1”. Therefore, we can achieve power balance by adjusting the polarizer behind SOA. At receiving end, the converted PSK signals are separated by the AWG which is demodulated by another delay line interferometer. An optical spectrum analyzer sampling oscilloscope is used to measure the spectra and eye diagrams of the converted signals, respectively.

At first, we perform the XOR gate without multicasting function, and only one channel probe light is used in the experiment. The SOA (ModelISPAD1501, InPhenix) being used in the proposed scheme is a commercially available device with 22 dB small signal gain, 10 dBm saturated output power and 25 ps carrier recovery time. The average power of the probe light and the input OOK signals are -7 dBm and -3 dBm respectively, and the SOAs are biased at 180 mA. The temporal waveforms and the eye diagrams of the two fixed input data bits and one channel

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of the output XORed signal are captured on a sampling oscilloscope, as shown in Fig. 3. The left column shows the waveforms of OOK1, OOK2, the output PSK signal after XOR operation and its demodulated signal from top to bottom. The OOK1 and OOK2 signals are 10 bits which are given as “1 0 1 0 1 0 0 1 0 0” and “1 0 0 1 0 1 0 0 1 0” and shown in Fig. 3(a) and (b) respectively. They are converted to the PSK signal with the phase information of “0 0 π π π π π 0 π π 0” as shown in Fig. 3 (c). Since the polarization-maintaining fiber is not used and the original

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OOK signal has a rising and falling edge regarding its waveform, the intensity level of the converted signal is fluctuating due to the remaining XGM. Fig. 3 (d) shows the demodulated PSK signal processed by one-bit delay interferometer. Due to the transmitter defects, a power jitter exists at the front of the OOK pulse, which results in an overshoot as shown in the demodulated PSK signal, and if we use the new-type transmitter such as mode-locked

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SOA fiber ring laser as shown in Ref. 25, higher quality conversion can be obtained. The right column shows the respective eye diagram in correspondence to their respective signals given at left side. The eye diagram in the last row shows the demodulated XOR signal which is clearly open and highlights the effective all-optical XOR operation.

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Fig. 3 Waveforms and eye diagrams of (a) OOK1 signal, (b) OOK2 signal, (c) the output PSK signal, (d) the demodulated signal.

The multicasting function is performed with four different channels of probe lights, which consist of four CW lights with the wavelengths ranging from 1554.0 nm to 1556.4 nm with an interval of 0.8 nm. The wavelengths of the two OOK signals are 1552.0 nm and 1552.4 nm, respectively, and the signals are programmed with PRBS of length 231-1. The optical power of probe light for each channel is -13dBm and the input OOK signal power is 3 dBm in each SOA. Fig. 4(a)-(d) show the measured eye diagrams of the output XOR channels. Due to the limitation of the rising edge and the falling edge of the OOK signals, the power of the converted signal fluctuates slightly. Although slight signal jitter can be seen from the converted signal, this can be alleviated by improving the synchronization of two injected signals into the SOA2. The demodulated signals are shown in Fig. 4(e)-(h). With the influence of spontaneous emission noise in the cascaded SOA, the eye openings of the converted signals are narrow, and the converted signal will be quality-improved by using low-noise SOA.

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Fig. 4 Four-channel eye diagrams of (a-d) the XOR PSK signals and (e-h) the demodulated converted signals.

The measured spectra of the input probe lights and the converted signals are shown in Fig.5 (a) and (b), respectively. It is obvious that the spectra of all converted signals are broadened due to the modulation of the

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control signal. It can also be seen from the spectrum that there is a depression at the frequency interval of ± 10GHz near the center wavelength, which is caused by signal modulation, and the frequency interval at the distance depression of the center wavelength is equal to the value of modulation rate. In Fig. 2 (a), the frequency interval of 0.08nm is 10GHz, which is equal to the value of modulation rate.

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Fig. 5 Measured spectra for the (a) input probe light and (b) converted XOR output.

In order to realize XOR function accompanied with OOK/PSK format conversion, the phase shift of π for the probe light should be received under the control OOK signal. Fig. 6(a) shows the BER curve versus input level of OOK signal under the probe light power of -7 dBm. For all the cases of different wavelengths, the received signal power is -11 dBm, and the BER of the converted signal shows a trend of first decreasing and then increasing as the control signal power is gradually increased from 0 to 5 dBm. For the wavelength of 1556.4 nm, when the control

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optical power is 3 dBm, the BER reaches the minimum (below 10-9). Since the phase shift for different wavelengths in SOA is slightly different, for the wavelength of 1554.8 and 1555.6 nm, the minimum BER is obtained when the control signal 3.5 dBm. And for the wavelength of 1554 nm, the minimum BER is obtained when the control optical power is 4 dBm.

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To characterize the performance of the proposed scheme for four output channels, BER curves versus the received optical power for each output channel is measured and shown in Fig. 6(b). The back-to-back (B2B) performance of the two input OOK signals are also presented as references. The results in Fig. 6 indicate that error-free operations are obtained for the multicasting channels. Compared to the B2B case, the maximum power

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penalty of the output wavelength multicasting channels is 2.5 dB. The error-free operation clearly demonstrates that our scheme can be used for XOR operation at 10 Gb/s data rates. Furthermore, due to the wide gain bandwidth, the operation can be applied in the entire C-band. -4 1554.0nm 1554.8nm 1555.6nm 1556.4nm

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Fig. 6 (a) BER curves versus input control signal power and (b) BER curves of all the multicasting outputs.

Conclusion

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4.

We experimentally demonstrated an all optical XOR logic gate accompanied with OOK/PSK format conversion with multicast functionality. The XOR logic is successfully implemented and the two input OOK signals are converted to PSK signals with wavelength multicasting. For different wavelengths, the converted signals are capable of obtaining power penalties ranging from 0.8 to 2.5 dB at a BER of 10-9 compared with original signal. Limited by the carrier recovery time of the SOA, the XOR gate is operated at 10 Gb/s in this paper, and if we use SOA with shorter carrier recovery time such as 25 ps, logic operation with 40 Gb/s signal of can be realized. So this device can operate at even higher data rates by using new-type SOAs such as quantum-dot SOAs which have short carrier recovery time.

Funding

The financial supports from National Key Research and Development Program of China (No. 2018YFB1801703), the National NSFC of China (No. 61835005, 61822507, 61522501, 61475024, 61675004, 61705107, 61727817, 61775098, 61720106015, 61875248); Beijing Young Talent (No. 2016000026833ZK15);and the Fund of State Key Laboratory of IPOC (BUPT).

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*Credit Author Statement

CrediT authorship contribution statement

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Y. Mao: Methodology, Conceptualization, Writing - original draft, Software. B. liu: Investigation, Project administration, Visualization. R. Ullah: Investigation, Writing - review & editing. T. Sun: Validation, Supervision. L. Zhao: Writing - review & editing.