Abstract
This paper proposes a semiconductor optical amplifier (SOA)-based multilevel polarization shift on–off keying (MPS-OOK) transmission for free-space optical (FSO) communication. At the transmitter end, the MPS-OOK signal is modulated with a constant linear state of polarization (SOP) at the high-intensity level and various SOPs at the low-intensity level in order to improve the spectral efficiency (SE) with the transmitted power efficiency. At the receiver end, first, a polarization-independent SOA was introduced to optically suppress the turbulence-induced scintillation effect and equalize the intensities of the various SOPs in the deep gain saturation state without polarization distortion. Then, a linear polarizer (LP) with a high extinction ratio (ER) was deployed to convert the equalized SOPs into a known intensity. Finally, the converted MPS-OOK signal was detected using a single photodiode (PD) and distinguished using a multilevel fixed-threshold decision (M-FTD). The proposed technique was evaluated using experiments. A Mach–Zehnder modulator (MZM)-based fading simulator was introduced to emulate the turbulence-induced scintillation effect. The experimental results demonstrated that the scintillation effect was effectively mitigated and the SE was improved by up to 2 bit/s/Hz using the proposed four-level polarization shift on–off keying (4PS-OOK) transmission.
Highlights
Free-space optical (FSO) communication has been widely researched in recent decades due to its characteristics of a high data rate, wide bandwidth, low power consumption, less mass, an unlicensed spectrum, and high security [1]
The proposed multilevel polarization shift on–off keying (MPS-on–off keying (OOK)) modulation was verified without the scintillation effect
The proposed semiconductor optical amplifier (SOA)-based MPS-OOK detection was compared to the conventional OOK detection under different scintillation effects
Summary
Free-space optical (FSO) communication has been widely researched in recent decades due to its characteristics of a high data rate, wide bandwidth, low power consumption, less mass, an unlicensed spectrum, and high security [1]. The spectral efficiency (SE) of PS-OOK is the same as the conventional OOK due to the application of two SOPs. Higher-order modulation techniques, such as pulseamplitude modulation 4 (PAM-4), multilevel polarization shift keying (MPolSK), and multilevel phase-shift keying (MPSK), have been researched to improve the capacity of FSO communication systems [5,7,10]. It is preferable to improve the SE of PS-OOK via the transmission of multilevel SOPs. In this study, we proposed a semiconductor optical amplifier (SOA)-based multilevel polarization shift on–off keying (MPS-OOK) transmission for FSO communication. The experimental results illustrated that the SE was enhanced by up to 2 bit/s/Hz with effective scintillation mitigation using the proposed four-level polarization shift on–off keying (4PS-OOK) transmission
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