Open Access
Add to BibSonomy
Abstract
This paper proposes a non-orthogonal polarization shift on-off keying (NOPS-OOK) transmission for free-space optical (FSO) communication. At the transmitter end, NOPS-OOK signal is modulated with the non-orthogonal polarizations of bits ‘1’ and ‘0’ characteristics employing a single Mach-Zehnder modulator (MZM). At the receiver end, NOPS-OOK signal is detected using the cascaded polarization-insensitive and gain saturated semiconductor optical amplifier (SOA) and sloped-linear polarizer (SLP) with the polarization perpendicular to bit ‘0.’ The proposed technique is verified in experiments. The experimental results showed that the bit-error-rate (BER) performance of the proposed technique was close to the orthogonal polarization shift OOK (OPS-OOK) signal transmission.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Free-space optical (FSO) communication technique has been widely researched in ground-to-satellite, ground-to-aircraft, and building-to-building high-speed communication, owing to its high data rate, wide bandwidth, power efficiency, light weight, free licensed spectrum, and high security [1]. However, in the vertical and long-distance horizontal atmospheric links, the turbulence-induced beam scintillation effect, which brings a dramatic fluctuation of the received optical irradiance (up to 30 dB), is the major issues of the FSO system distortion [2].
Various mitigation techniques were studied to reduce the scintillation effect. Adaptive optics technique was used to correct the wave front distortion of laser beam using a deformable mirror model in order to reduce the scintillation effect, but, it is effective in the weak turbulence channel [3]. Diversity method was introduced to mitigate the scintillation induced fading by the repeated signal transmission, however, it increases the complexity of the communication system [4]. Decision threshold was adaptively estimated according to the instantaneous channel state information (CSI) in order to compensate the scintillation induced intensity fluctuation, nonetheless, it is difficult to calculate the instantaneous CSI in practice [5]. The state of polarization (SOP) of polarization shift keying (PolSK) signal was measured instead of the signal intensity to suppress the intensity fluctuation due to the stable state of SOPs in the atmospheric turbulence channel, however, the estimation of SOP requires the knowledge of x- and y-polarizations [6]. Orthogonal polarization shift OOK (OPS-OOK) signal was directly detected by the semiconductor optical amplifier (SOA) and linear polarizer (LP) without the SOP estimation to mitigate the scintillation effect with the transmitted power efficiency [7,8]. However, it is difficult to modulate PS-OOK signal with a high polarization orthogonality in practice. Therefore, we introduce a non-orthogonal polarization shift on-off keying (NOPS-OOK) transmission in this study.
In this paper, we propose a NOPS-OOK transmission for FSO communication. At the transmitter end, NOPS-OOK signal is modulated with the non-orthogonal polarizations of bits ‘1’ and ‘0’ employing a single Mach-Zehnder modulator (MZM). At the receiver end, NOPS-OOK signal is detected using the cascaded polarization-insensitive gain saturated SOA and sloped-linear polarizer (SLP). The scintillation effect is mitigated by SOA in the gain saturation state without polarization distortion. The degraded extinction ratio (ER) of NOPS-OOK is recovered by SLP with the polarization perpendicular to bit ‘0’ and large polarization extinction ratio (PER). The performance of the proposed technique is analyzed under various angles of polarization between bits ‘1’ and ‘0’ of NOPS-OOK signal. Besides, the capability of ER recovery is evaluated in the case of SLP with perpendicular and non-perpendicular polarizations to bit ‘0’. The proposed technique was experimentally estimated. Experimental results demonstrated that the bit-error-rate (BER) performance of the proposed technique was close to the OPS-OOK signal transmission.
2. Operation principle
Figure 1 shows the principle of the proposed NOPS-OOK transmission. A continuous wavelength (CW) generated from the laser diode (LD) is divided into the mutually orthogonal $SO{P_X}$ and $SO{P_Y}$ by polarization beam splitter (PBS). $SO{P_Y}$ is externally modulated into the OOK signal via MZM, and $SO{P_X}$ is attenuated by variable optical attenuator (VOA). NOPS-OOK signal is modulated by recombining the orthogonal $SO{P_X}$ and $SO{P_Y}$ by polarization beam combiner (PBC). The ERs of the modulated NOPS-OOK signal $E{R_{TX}}$ is calculated by
The angle of polarization $SO{P_D}$ between $SO{P_{NOPS - OO{K_{bit1}}}}$ and $SO{P_{NOPS - OO{K_{bit0}}}}$ is given by
$SO{P_D}$ determines the polarization non-orthogonality of NOPS-OOK signal, and it is dependent on G values. NOPS-OOK signal is simply modulated using a single MZM and VOA. NOPS-OOK signal experiences the turbulence-induced scintillation effect in the atmospheric channel due to the variation of temperature and pressure of atmosphere, and it has the cutoff frequencies < few KHz. The magnitude of scintillation effect is represented by the scintillation index $\sigma _I^2 = {{\left\langle {{I^2}} \right\rangle } / {{{\left\langle I \right\rangle }^2}}} - 1$, where I is the normalized signal intensity and $\left\langle \cdot \right\rangle $ is the ensemble average [1]. Therefore, the system performance is seriously distorted by the received intensity fluctuation caused by the scintillation effect. The pre-amplifier SOA with polarization-insensitive characteristics is applied before photodiode (PD) to suppress the intensity fluctuation in the gain saturation state due to the high frequency (10 GHz) nonlinear optical gains [7]. Besides, the intensities of bits ‘1’ and ‘0’ are also equalized by this high dynamic gain frequency. SLP with the perpendicular polarization to $SO{P_{NOPS - OO{K_{bit0}}}}$ was deployed after SOA to recover the ER of NOPS-OOK signal. Figure 2 illustrates the ER recovery of NOPS-OOK signal using SLP. The optics axis of SLP is configured to have a polarization perpendicular between the SOP of SLP $SO{P_{SLP}}$ and $SO{P_{NOPS - OO{K_{bit0}}}}$. Since SLP has a large PER $PE{R_{SLP}}$, the ER of NOPS-OOK signal after SLP $E{R_{RX}}$ is recovered by filtering the polarization of bit ‘0’ using SLP. It is calculated by3. Experiments and results
Figure 3 shows the experimental setup of the proposed technique. The laser beam generated from LD (1550 nm) was polarized into the linear SOP of using polarization controller1 (PC1). It was divided into the orthogonal polarizations using PBS. Lower branch polarization was modulated into the OOK signal using MZM1; upper branch polarization was directly transmitted without the modulation process. Lower and upper branches of polarizations were combined using PBC to generate the NOPS-OOK signal. Variable optical attenuator1 (VOA1) was used to control the degree of non-orthogonality between bits ‘1’ and ‘0’ of the NOPS-OOK signal. MZM-based simulator was applied in experiment to emulate the turbulence channel with the scintillation effect [9]. The time-varying intensity fluctuation signal was generated by arbitrary waveform generator (AWG), and then it was injected into the RF port of MZM2. The optical signal suffered the intensity fluctuation in MZM2. Erbium-doped fiber amplifier (EDFA) was used to increase the input powers at SOA. Optical band pass filter (OBPF) was applied to reduce the ASE noises from EDFA. The scintillation effect was mitigated and intensities of bits ‘1’ and ‘0’ were equalized by the nonlinear gains from the polarization-insensitive SOA. VOA2 was used to control the saturation degree of SOA. PC2 was used to rotate the optical axis of optical signal. LP (GENERAL PHO-TONICS POL-001) with PER of 40 dB was used to recover the ER of NOPS-OOK signal. VOA3 was used to control the input powers at PD. Finally, it was detected using a single PD and decided with FTD without CSI. The data rate was configured to 1.25 Gb/s.
Figure 4 illustrates the measurement of ERs, SOPs, and orthogonality of the modulated NOPS-OOK under various attenuation values of VOA1. In Fig. 4(a), the ERs of modulated NOPS-OOK signal were improved to have a transmitted power efficiency under the increase of attenuation values of VOA1. In Fig. 4(b), the SOPs of bits ‘1’ and ‘0’ of the NOPS-OOK signal were rotated from $SO{P_X}$ and $SO{P_Y}$ under the variation of attenuation values of VOA1. The variation of the angle of polarization between bits ‘1’ and ‘0’ of the NOPS-OOK signal was observed in Fig. 4(c). Therefore, the NOPS-OOK signal was modulated with the non-orthogonal polarization and high ER characteristics using a single MZM.
Fig. 4. Measurement of (a) ERs, (b) SOPs, and (c) orthogonality of the modulated NOPS-OOK signal under various attenuation values of VOA1.
Figure 5 shows the measurement of the recovered ERs of the NOPS-OOK signal. The scintillation effect with $\sigma _I^2$ of 0.25 was adopted by the MZM-based simulator. The optical average input power at SOA was set to 0 dBm in order to effectively mitigate the scintillation-induced intensity fluctuation and equalize the intensities of bits ‘1’ and ‘0’ of the NOPS-OOK signal. As to NP-SLP, the SOP of SLP was set to the position of $SO{P_Y}$. The polarizations of both bits ‘1’ and ‘0’ were filtered by NP-SLP due to the non-orthogonal polarization characteristics of the NOPS-OOK signal. The recovered ERs of NOPS-OOK signal were significantly reduced with the increasing of attenuation values of VOA1. As to P-SLP, the SOP of SLP was set to the polarization perpendicular to bit ‘0’ of the NOPS-OOK signal. An almost constant ER was observed under various attenuation values of VOA1. Consequently, the ERs of NOPS-OOK signal was effectively recovered using SLP with the polarization perpendicular to bit ‘0’ of the NOPS-OOK signal.
Fig. 5. Measurement of the recovered ERs of the NOPS-OOK signal. P-SLP: SLP with the polarization perpendicular to bit ‘0’ of the NOPS-OOK signal, NP-SLP: SLP with the polarization of $SO{P_Y}$.
Figure 6 shows the BER performance of the proposed technique. The FTD without CSI was used to estimate BERs under various signal-to-noise ratios (SNRs). The SOA was applied to mitigate the scintillation effect in the gain saturation state. The proposed technique was evaluated under the NOPS-OOK signal detection using both P-SLP and NP-SLP. BERs were measured in case of various attenuation values of 4 dB, 10 dB, and 16 dB of VOA1. It was also compared to the conventional-OOK and OPS-OOK transmission. As to the conventional-OOK transmission, a poor BER performance was observed due to the ER of conventional-OOK was degraded by the gain saturated SOA. Both OPS-OOK and NOPS-OOK have a better BER performance than the conventional-OOK transmission due to the recovery of the degraded ER. As to the NOPS-OOK transmission, P-SLP detection has a similar BER performance under various attenuation values of VOA1 comparing with OPS-OOK transmission, since the ER of NOPS-OOK signal was effectively recovered by the SLP with large PER via filtering the polarization of bit ‘0.’ However, NP-SLP detection has a poor BER performance under large attenuation values of 10 dB and 16 dB due to the ineffective ER recovery. Therefore, the NOPS-OOK signal can be effectively detected by the gain saturated SOA with P-SLP under the scintillation effect.
Fig. 6. BER performance of the proposed technique. NOPS-OOK-NP-SLP-4dB: NOPS-OOK detection using SLP with the polarization perpendicular to bit ‘0’ of the NOPS-OOK signal and 4 dB attenuation from VOA1, NOPS-OOK-P-SLP-4dB: NOPS-OOK detection using SLP with the polarization of and 4 dB attenuation from VOA1.
Figure 7 shows the power efficiency of the NOPS-OOK signal under various attenuation values of VOA1. The transmitted power efficiency of the NOPS-OOK signal was improved by increasing the attenuation values of VOA1. Besides, NOPS-OOK signal with P-SLP under the attenuation value of 16 dB has a similar BER performance with that of OPS-OOK. Consequently, the NOPS-OOK signal was effectively modulated using a single MZM and VOA1 of 16 dB attenuation and detected by the cascaded polarization-insensitive and gain saturated SOA and SLP with the polarization perpendicular to bit ‘0’ in FSO communication.
4. Conclusion
In summary, a NOPS-OOK transmission was proposed for FSO communication. NOPS-OOK signal transmission was analyzed under various angles of polarization between bits ‘1’ and ‘0’. The capability of the ER recovery was discussed under SLP with the polarization perpendicular and non-perpendicular to bit ‘0’. The proposed technique was experimentally verified using the MZM-based simulator. Experimental results demonstrated that the BER performance of the proposed technique was close to the OPS-OOK signal transmission. Therefore, this technique is highly potential for various FSO systems.
Funding
Natural Science Foundation of Liaoning Province (20180520022).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. H. Kaushal and G. Kaddoum, “Optical communication in space: challenges and mitigation techniques,” IEEE Commun. Surv. Tutorials 19(1), 57–96 (2017). [CrossRef]
2. X. Zhu and J. M. Kahn, “Free-space optical communication through atmospheric turbulence channels,” IEEE Trans. Commun. 50(8), 1293–1300 (2002). [CrossRef]
3. H. Yuksel, S. Milner, and C. Davis, “Aperture averaging for optimizing receiver design and system performance on free-space optical communication links,” J. Opt. Netw. 4(8), 462–475 (2005). [CrossRef]
4. X. Zhang, Z. Ghassemlooy, S. Rajbhandari, W. O. Popoola, and C. G. Lee, “Coherent heterodyne multilevel polarization shift keying with spatial diversity in a free-space optical turbulence channel,” J. Lightwave Technol. 30(16), 2713–2717 (2012). [CrossRef]
5. M. L. B. Riediger, R. Schober, and L. Lampe, “Fast multiple-symbol detection for free-space optical communications,” IEEE Trans. Commun. 57(4), 1119–1128 (2009). [CrossRef]
6. S. Benedetto and R. Gaudino, “Direct detection of optical digital transmission based on polarization shift keying modulation,” IEEE J. Select. Areas Commun. 13(3), 531–542 (1995). [CrossRef]
7. Y. Q. Hong, W. H. Shin, and S. K. Han, “Enhancement of SOA-based scintillation mitigation by PS-OOK transmission in FSO communication,” IEEE Photonics J. 12(4), 1–10 (2020). [CrossRef]
8. Y. Q. Hong, D. H. Kwon, J. Y. Choi, I. H. Ha, W. H. Shin, and S. K. Han, “SOA-based multilevel polarization shift on–off keying transmission for free-space optical communication,” Photonics 8(4), 100 (2021). [CrossRef]
9. S. Takashi, T. Morio, and T. Hideki, “Fading simulator for Satellite-to-ground optical communication,” J. Nat. Inst. Inform. Commun. Technol. 59(1/2), 999105 (2012). [CrossRef]
