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Abstract
This paper proposes a polarization-dependent semiconductor optical amplifier (SOA)-based polarization shift keying (PolSK) modulation for turbulence-robust free-space optical (FSO) communication. Based on the nonlinear gain and polarization characteristics, the PolSK signal is modulated by the optical bit inversion using the polarization-dependent SOA and linear polarizer (LP). The capability of scintillation mitigation is analyzed under both balanced and polarization-independent SOA-based detections. The proposed technique is evaluated in experiments. The experimental results demonstrated that the bit-error-rate (BER) performance of the proposed technique was close to the conventional Mach-Zehnder modulator (MZM)-based PolSK modulation under various scintillation effects.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the last few decades, free-space optical (FSO) communication has attracted great attention from the researchers due to the high data rate, wide bandwidth, low power consumption, electromagnetic interference immunity, unregulated spectrum, and high security features [1]. Nevertheless, the system performance is dramatically deteriorated by the serious intensity fluctuations (up to 30 dB) caused by the atmospheric turbulence-induced beam scintillation effect, which is the critical issue for FSO communication [2,3].
On-off keying (OOK) with adaptive threshold decision (ATD) was researched to mitigate the scintillation effect through the symbol-by-symbol decision threshold estimation [4]. However, the acquisition of precise instantaneous channel state information (CSI) is difficult in practice. Phase-shift keying (PSK) was studied to improve the system performance by the measurement of phase information [5]. However, it is difficult to precisely control the one-bit delay interferometer, and it is much serious as to the multi-rate PSK transmission. Regarding to the spatially coherent plane wave, the state of polarization (SOP) keeps a stable state in the atmospheric turbulence channel [6]. Therefore, polarization shift keying (PolSK) was researched to suppress the scintillation effect by estimating the knowledge of SOPs [7].
Various researches have been conducted into PolSK modulation. PolSK signal was modulated by combing mutually inverted OOK signals with orthogonal X- and Y- polarizations generated by Mach-Zehnder modulators (MZMs) [8,9]. Variable wave plate was used to change SOPs of PolSK by controlling the inherent birefringence [10,11]. Semiconductor optical amplifier (SOA) has the attractive characteristics of compact size, capability of monolithic integration, and high nonlinearity [12,13]. Therefore, PolSK signal can be alternatively modulated by SOA with nonlinear gain and polarization features.
In this paper, we propose the polarization-dependent SOA-based PolSK modulation for turbulence-robust FSO communication. The directly modulated OOK signal is split into OOK with orthogonal X- and Y- polarizations. A polarization-dependent SOA is introduced to optically erase the OOK signal with X-polarization and rotate polarization of bits ‘1’ and ‘0’ according to the nonlinear gain and polarization characteristics. Linear polarizer (LP) with X-polarization is used to invert OOK signal and recover rotated polarization by blocking the polarization of bit ‘1’, since the bit ‘1’ has a much larger magnitude of polarization rotation than bit ‘0’. PolSK signal is generated by combining mutually inverted OOK signals with orthogonal X- and Y- polarizations. The modulated PolSK signal is detected by both balanced and polarization-independent SOA-based detections in order to effectively mitigate the scintillation effect. The proposed technique was verified in experiments. The scintillation effect was accommodated using the MZM-based fading simulator. The experimental results illustrated that the bit-error-rate (BER) performance of the proposed technique was close to the conventional MZM-based PolSK modulation under various scintillation effects.
2. Operation principle
Figure 1 shows the block diagram of the proposed technique. As to the transmitter end, OOK signal is directly modulated into laser diode (LD). Then, the linearly polarized OOK is divided into two streams of OOK signals with orthogonal X- and Y- polarizations using polarization beam splitter (PBS). Then, OOK of X-polarization undergoes the signal erasure and polarization rotation due to the nonlinear gain and polarization characteristics of polarization-dependent SOA [14]. Polarization-dependent SOA has a high nonlinearity and dynamic gain frequency (around 10 GHz), thus, the simultaneous OOK erasure and polarization rotation with the linear polarization maintenance are available [15]. Then, the erased signal is converted into the bit inverted OOK by LP1 of X-polarization. Figure 2 depicts the details of optical bit inversion. OOK has X-polarization for bits ‘1’ and ‘0’. The SOPs of OOK are transformed into $SO{P_1}^{\prime}$ and $SO{P_0}^{\prime}$ as to bits ‘1’ and ‘0’ by the polarization-dependent SOA with the rotation degrees of ${\theta _1}$ and ${\theta _0}$, respectively. ${\theta _1} \gg {\theta _0}$, since the magnitudes of polarization rotation are dependent on the degrees of the input signal power [11–13]. $SO{P_1}^{\prime}$ and $SO{P_0}^{\prime}$ are converted back to X-polarization and bits ‘1’ and ‘0’ are optically inverted by the polarization filtering using LP1 of X-polarization. The amplitudes of the inverted OOK ${A_1}^{\prime\prime}$ and ${A_0}^{\prime\prime}$ are given by
3. Experiments and results
Figure 3 shows the experimental setup the proposed technique. Reflective semiconductor optical amplifier (RSOA) (CIP-SOA-ROEC-1550) with a high polarization sensitivity was adopted instead of polarization-dependent SOA due to the laboratory limitation. Figure 3(a) shows the experimental setup of the polarization-dependent RSOA-based PolSK modulation. The directly modulated OOK signal was polarized into SOP of $45^\circ $. It was split into downstream and upstream OOK signals with orthogonal polarizations by PBC. Downstream OOK signal was injected into RSOA. RSOA was configured to the bias current of 50 mA and operating temperature of 20$^\circ C$ in order to have a high polarization sensitivity. PC1 was applied to adjust the optical axis of input OOK signal at RSOA. Variable optical attenuator1 (VOA1) was applied to adjust the input optical powers at RSOA so as to control the degrees of signal erasure and polarization rotation. Optical band pass filter1 (OBPF1) was used to reduce the ASE noises. LP1 (GENERAL PHOTONICS POL-001) with a high PER of 40 dB was deployed to block the polarization of bit ‘1’ and recover the rotated polarization. LP1 was configured to have a same optical axis with the OOK signal before RSOA. The output power from RSOA was attenuated by VOA2 in order to have an equal average power of upstream OOK and inverted OOK. Upstream OOK signal was directly transported to PBC. Optical delay line (ODL) was used to match the bit synchronization between upstream OOK and inverted OOK. The SOP of PolSK signal maintains a stable state in the turbulence channel, thus, we focus on the turbulence-induced beam scintillation effect. The turbulence-induced beam wander caused pointing errors and beam spreading caused power loss were neglected. Therefore, MZM-based fading simulator was used to simulate the scintillation effect in this study [16,17]. The time-varying intensity fluctuation signal was modeled from the temporal spectrum of the log-amplitude fluctuations through phase modulation, inverse Fourier transform, and first-order Rytov approximation, and then, it was injected into the RF port of MZM as to vary the intensity of the traveling optical signal. The bias voltage of MZM was set into the quadrature point for the sake of an optimized performance. Figure 4 shows that the scintillation effect simulated by MZM-based fading simulator has the characteristics of the temporal intensity fluctuation and low-frequency components in the temporal spectrum. Besides, it fits well with the lognormal distribution, which is widely used in weak turbulence channel. Thus, MZM-based fading simulator was sufficient for this research.
Fig. 3. Experimental setup of (a) polarization-dependent RSOA-based PolSK modulation, (b) polarization rotation measurement and balanced detection, and (c) polarization-independent SOA-based detection.
Figure 3(b) illustrates the experimental setup of the polarization rotation measurement and balanced detection. PC2 was used to match the optical axis between PolSK signal and PBS. The degrees of polarization rotation from RSOA were measured by calculating the power ratios between VOA3 and VOA4. The scintillation effect was mitigated by the balanced detection through the comparison of orthogonal polarization components of PolSK. Figure 3(c) depicts the experimental setup of the polarization-independent SOA-based detection. The polarization-independent SOA was deployed to suppress the scintillation effect by the nonlinear optical gains. PC3 was applied to match the optical axis between PolSK signal and LP2. The SOPs of PolSK were transformed into different intensity levels by LP2 and decided with FTD. In experiments, the data rate was set to 1.25 Gbps.
Figure 5 illustrates the measured nonlinear gain and nonlinear polarization rotation of RSOA. The signal after OBPF1 was measured using the setup of Fig. 3(b). The optical gains were reduced and the degrees of polarization rotation were improved with the increasement of input powers at RSOA. Therefore, a simultaneous signal erasure and polarization rotation can be achieved by using RSOA.
Figure 6 shows eye diagrams of the proposed PolSK modulation. The eye patterns were measured after PBS, LP1, and PBC. A large eye opening of directly modulated OOK signal was observed after PBS. The eye patterns were measured after LP1 with the variation of average input powers at RSOA. Firstly, the eye openings were decreased as to the average input powers at RSOA of – 9 dBm and – 6 dBm compared to the directly modulated OOK due to the insufficient nonlinear gain and polarization rotation. Then, the eye openings were increased as to the average input powers at RSOA of – 3 dBm and 0 dBm due to a large degree of gain saturation and polarization rotation. The average input power at RSOA was optimized to – 3 dBm in order to have a same extinction ratio (ER) between OOK and inverted OOK. Therefore, the optical bit inversion was achieved by RSOA and LP1. VOA2 was used to match the average powers between OOK and inverted OOK. A closed eye pattern was observed after PBC by combining OOK and inverted OOK with orthogonal polarizations. Therefore, PolSK signal was effectively modulated by the proposed modulation technique.
Figure 7 shows the BER performance of the proposed PolSK modulation under the balanced and polarization-independent SOA-based detections. Turbulence channels with $\sigma _I^2$ of 0.11, 0.25, and 0.59 were accommodated using the MZM-based simulator. The performance of the proposed technique was compared to the conventional MZM-based PolSK modulation and OOK modulation. As to the balanced detection, the BERs were measured by comparing orthogonal polarization components of PolSK under the variation of average input powers at PDs. BERs were effectively improved with the increase of average input powers at PDs as to $\sigma _I^2$ of 0.11. However, the BER performance was limited for $\sigma _I^2$ of 0.25 and 0.59, because the fading parts of turbulence channel have a poor signal-to-noise ratios (SNRs). BERs of the proposed PolSK modulation were worse than the MZM-based PolSK modulation due to the ASE noises from RSOA, however, it was close to the conventional OOK with adaptive threshold decision (ATD) due to the balanced detection. As to the polarization-independent SOA-based detection, firstly, the average input power at PD was configured to – 3 dBm. The BERs were calculated using FTD under the variation of average input powers at polarization-independent SOA. A poor BER performance was observed as to the conventional OOK due to the ER degradation issues from the polarization-independent SOA during the scintillation mitigation process. The BERs of the proposed PolSK modulation were enhanced compared to the conventional OOK with the increase of average input powers at polarization-independent SOA due to the effective scintillation mitigation in the gain saturation state. However, the proposed PolSK modulation has a poor BER performance compared to the MZM-based PolSK modulation due to the ASE noises from RSOA. Then, the average input power at PD was increased to 0 dBm as to the proposed PolSK modulation with the polarization-independent SOA-based detection. The BERs of the proposed PolSK modulation were close to the conventional MZM-based PolSK modulation due to the increase of SNR. Polarization-independent SOA has a large output power in the gain saturation state; thus, it is sufficient to provide a large input power at PD. Therefore, it is evident that the proposed PolSK modulation with the polarization-independent SOA-based detection has a similar performance with the conventional MZM-based PolSK modulation. A higher performance can be achieved by using a polarization-dependent RSOA with lower ASE noises.
Fig. 7. BER performance of the proposed PolSK modulation under (a) balanced, (b) polarization-independent SOA-based detections. RSOA-Based-PolSK-H: RSOA-based PolSK modulation with a higher average input power at PD.
4. Conclusion
In summary, we proposed the polarization-dependent SOA-based PolSK modulation for turbulence-robust FSO communication. PolSK signal was modulated using a polarization-dependent SOA and LP on the basis of the nonlinear gain and polarization characteristics. The performance of the modulated PolSK signal was analyzed under the balanced and polarization-independent SOA-based detections. Besides, it was compared to the conventional MZM-based PolSK modulation. The proposed technique was evaluated in experiments under various scintillation effects simulated by the MZM-based fading simulator. The experimental results demonstrated that a similar BER performance was obtained by the proposed technique compared to the conventional MZM-based PolSK modulation. Therefore, it is a highly potential technique for FSO communication systems.
Funding
Institute for Information and Communications Technology Promotion (2019-0-00685).
Acknowledgments
This work was supported by the Institute for Information and Communications Technology Promotion (IITP) grant funded by the Korean government (MSIT; Ministry of Science and ICT) (No. 2019-0-00685, Free-space-optical-communication-based vertical mobile network).
Disclosures
The authors declare no conflicts of interest.
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