Polarization Shift Keying Research Articles

Influence of laser linewidth and polarization modulator length on polarization shift keying for free space optical communication.

Modulating signal with polarization modulator (PolM) is the simplest method for polarization shift keying (PolSK) in free space optical communication. However, this method has an intrinsic drawback on degree of polarization (DOP) reduction for the existence of polarization mode dispersion (PMD) in PolM. In this work, we analyze this change of DOP and its influence on PolSK using coherency matrix. We demonstrate that the decrease of DOP after PolM will generate extra loss and bit error ratio (BER) for PolSK communication, while this loss and BER will aggravate with the increase of laser linewidth and PolM length. For a practical PolSK system, laser linewidth should be less than 0.008nm.

Reconfigurable dual-channel all-optical logic gate in a silicon waveguide using polarization encoding.

A reconfigurable dual-channel all-optical logic gate is proposed and experimentally demonstrated using four-wave mixing in a silicon waveguide for polarization encoding signals. Six logic functions, XNOR, AND, NOR, XOR, AB¯, and A¯B are implemented at two different wavelength channels by adjusting the polarization states of two 10 Gb/s non-return-to-zero polarization-shift keying (NRZ-PolSK) signals modulated by 10-bit on-off keying (OOK) sequences. The eye diagrams of the logic signals are clearly observed, and the logic functions are well demonstrated as the two incident NRZ-PolSK signals are both modulated by the OOK sequences, which originate from 2(31)-1 pseudo-random binary sequences.

Free space optical transmission scheme based on the combination of circular polarization shift keying and differential phase shift keying with atmospheric turbulence

A transmission scheme is proposed which loads differential phase shift keying (DPSK) information and circular polarization shift keying (CPolSK) information on the same optical carrier for free space optical links through an atmospheric turbulent channel. Theoretical models of modulation and demodulation are derived and a design is specified to detect and recover the information with two mutually independent branches in order to meet the requirement for a high real-time transmission capacity and to entirely remove modulation-induced cross talk. Simulation results show that the CPolSK/DPSK system has a high tolerance to intensity scintillation and phase noise caused by atmospheric turbulence. A bit error rate performance comparison of a CPolSK system, a DPSK system, and a CPolSK/DPSK system is also presented to comprehensively demonstrate that the CPolSK/DPSK system outperforms the other two modulation techniques in terms of a high real-time transmission capacity.

Performance Analysis of Polarization Modulated DirectDetection Optical CDMA Systems over Turbulent FSO LinksModeled by the Gamma-Gamma Distribution

This paper proposes a theoretical study to characterize the transmission of optical code division multiple access (CDMA) systems deploying polarization shift keying (PolSK) over a free space optical (FSO) link under the impact of atmospheric turbulence. In our analysis, a novel transceiver architecture for atmospheric OCDMA FSO systems based on polarization modulation with direct detection is proposed and discussed. A detailed analytical model for PolSK-OCDMA systems over a turbulent FSO link is provided. Further, we derive a closed-form bit error ratio (BER) and outage probability expressions, taking into account the multiple-access interference (MAI), optical noise and the atmospheric turbulence effect on the FSO link modeled by the Gamma-Gamma distribution. Finally, the results of this study show the most significant parameters that degrade the transmission performance of the PolSK-OCDMA signal over FSO links and indicate that the proposed approach offers improved bit error ratio (BER) performances compared to the on-off-keying (OOK) modulation scheme in the presence of turbulence.

MIMO free-space optical communication employing coherent BPOLSK modulation in atmospheric optical turbulence channel with pointing errors

An optical wireless communication system is an alternative to radio frequency communication, but atmospheric turbulence induced fading and misalignment fading are the main impairments affecting an optical signal when propagating through the turbulence channel. The resultant of misalignment fading is the pointing errors, it degrades the bit error rate (BER) performance of the free space optics (FSO) system. In this paper, we study the BER performance of the multiple-input multiple-output (MIMO) FSO system employing coherent binary polarization shift keying (BPOLSK) in gamma–gamma (G–G) channel with pointing errors. The BER performance of the BPOLSK based MIMO FSO system is compared with the single-input single-output (SISO) system. Also, the average BER performance of the systems is analyzed and compared with and without pointing errors. A novel closed form expressions of BER are derived for MIMO FSO system with maximal ratio combining (MRC) and equal gain combining (EGC) diversity techniques. The analytical results show that the pointing errors can severely degrade the performance of the system.

Performance analysis of FSO links under strong atmospheric turbulence conditions using various modulation schemes

Strong atmospheric turbulence is a major hindrance in wireless optical communication systems. In this paper, the performance of a wireless optical communication system is analyzed using different modulation formats such as, binary phase shift keying-subcarrier intensity modulation (BPSK-SIM), differential phase shift keying (DPSK), differential phase shift keying-subcarrier intensity modulation (DPSK-SIM), M-ary pulse position modulation (M-PPM) and polarization shift keying (PolSK). The atmospheric channel is modeled for strong atmospheric turbulences with combined effect of turbulence and pointing errors. Novel closed-form analytical expressions for average bit error rate (BER), channel capacity and outage probability for the various modulation techniques, viz. BPSK-SIM, DPSK, DPSK-SIM, PolSK and M-PPM are derived. The simulated results for BER, channel capacity and outage probability of various modulation techniques are plotted and analyzed.

Employing circle polarization shift keying in free space optical communication with gamma–gamma atmospheric turbulence channel

A novel theoretical model of a circular polarization shift keying (CPolSK) system for free space optical links through an atmospheric turbulence channel, is proposed. Intensity scintillation and phase fluctuation induced in atmospheric turbulence, from weak to strong levels, are specifically researched with respect to circular polarization control error caused by the system design. We derive closed form expressions of the bit error rate (BER) and outage probability for evaluating the BER performance and communication interruption in the Gamma–Gamma distributed channel model. Simulation results show that atmospheric turbulence and circular polarization control error have significant effects on the BER performance and interruption of communication in the CPolSK system. The deterioration in BER performance, caused by intensity scintillation and phase fluctuation, is augmented by the power penalty conditioned by the circular polarization control error. This consequently adds to the demand for emissive power from the CPolSK system. Furthermore, we demonstrate that controlling the circular polarization control error below 8° as well as the normalized threshold within 8dB, 9dB and 10dB in turbulent scenarios from weak to strong levels can significantly reduce the probability of communication interruption occurring. This study provides reference material for further design of the CPolSK system.

Single SOA based ultrafast all-optical wavelength conversion for PolSK signal

A novel ultrafast tunable all-optical wavelength conversion for polarization-shift-keying (PolSK) signal based on dual-pump four-wave mixing (FWM), including parallel dual-pump FWM and orthogonal dual-pump FWM, in a single semiconductor optical amplifier (SOA) is proposed. A comprehensive broad-band dynamic model of the all-optical wavelength converter based on parallel dual-pump FWM and orthogonal dual-pump FWM in SOA is developed, for the first time to our knowledge. By numerical simulation, the proposed all-optical wavelength conversion is realized at 40 Gbit/s. The effects of the powers of input waves and wavelength detuning on the conversion efficiency of the wavelength converter are theoretically investigated in detail. The results indicate that, due to using the PolSK modulation format, this scheme is free of pattern effect. In addition, a wide tuning range can be obtained. The proposed all-optical wavelength conversion for PolSK signal is robust and potential for applications in future optical networks in terms of simple implementation and high speed operation.

Simulation research of medium-short distance free-space optical communication with optical amplification based on polarization shift keying modulation

Circular polarization shift keying (CPolSK) modulation technique has many advantages such as excellent BER performance and freedom from the alignment of polarization coordinates of the transmitter and the receiver, etc., and it turns out to be a good choice to FSO system. In this paper, a FSO system using CPolSK modulation is studied by simulation; it is found that the communication performance of the system is excellent in most weather condition. Additionally, three ways of optical signal amplification are proposed, and contrastive analysis on performance of corresponding optical amplification systems is carried out by examining SNR、BER and transmission distance with different specific attenuation. The results show that the system with optical amplifier at the transmitter have the optimum performance, and then the system with optical amplifier at the both ends with the same total gain, it is worst for the system with optical amplifier at the receiver. In addition, the safety factor for high emission power induced by optical amplification is also considered in this paper for practical application. The study above may be utilized in the system design for enhancing performance.

Phase-locked all-optical differential polarization demodulation

A novel scheme of differential polarization demodulation is presented and demonstrated based on a polarized asymmetrical Mach—Zehnder interferometer configuration with polarization control. To enhance the stability of the demodulator, a phase-lock device is designed, and it is composed of a symmetric 3 × 3 coupler and a feedback circuit. For further establishing a differential polarization-shift keying (DPolSK) transmission system, we successfully carry out the demodulation experiments on 10-Gb/s DPolSK optical signals for the first time. Due to the all-optical structure with phase-lock, our scheme is available to realize the DPolSK optical communication in practical optical fiber systems.

Analysis of PolSK based FSO system using wavelength and time diversity over strong atmospheric turbulence with pointing errors

Free space optics (FSO) or wireless optical communication systems is an evolving alternative to the current radio frequency (RF) links due to its high and secure datarates, large license free bandwidth access, ease of installation, and lower cost for shorter range distances. These systems are largely influenced by atmospheric conditions due to wireless transmission; requirement of line of sight (LOS) propagation may lead to alignment problems in turn pointing errors. In this paper, we consider atmospheric turbulence and pointing errors are the major limitations. We tried to address these difficulties by considering polarization shift keying (PolSK) modulated FSO communication system with wavelength and time diversity. We derived the closed form expressions for estimation of the average bit error rate (BER) and outage probability, which are vital system performance metrics. Analytical results are shown considering different practical cases.

Use of state of polarization transparent scheme for polarization shift keying signal as optical phase conjugator to improve the transmission performance

Transmission characteristics of polarization shift keying (PolSK) signal is experimentally demonstrated in this work. Performance degradation due to transmission is improved using optical phase conjugation (OPC) technique. The wavelength converter based on the proposed state of polarization (SOP) transparent scheme for PolSK signal is effective to use it as mid-link optical phase conjugator and which is difficult to achieve using conventional wavelength conversion scheme. It is found that for fiber transmission, the performance of both of the orthogonal SOPs of the PolSK signal is different for their respective chirping characteristics which can be optimized to be similar using proposed OPC technique.

Coherent Polarization Modulated Transmission through MIMO Atmospheric Optical Turbulence Channel

An optical signal suffers from irradiance and phase fluctuations when propagating through the free space optical (FSO) turbulence channel, thus resulting in the degradation of the bit error rate (BER) performance. The BER performance can be improved by adopting the multiple-input multiple-output (MIMO) scheme. In this paper, we propose a coherent binary polarization shift keying (BPOLSK) modulation scheme with MIMO employing maximum ratio combining and equal gain combining diversity techniques to mitigate the turbulence effect. The gamma-gamma statistical channel model is adopted for all the turbulence regimes. The BER performance for the proposed BPOLSK-MIMO FSO link is compared with the single-input single-output and ON-OFF-keying systems by means of computer simulation. The optical power gain is investigated and demonstrated under different turbulence regimes for a number of transmitters/receivers.

Bit-rate transparent DPSK demodulation scheme based on injection locking FP-LD

We propose and demonstrate a bit-rate transparent differential phase shift-keying (DPSK) demodulation scheme based on injection locking multiple-quantum-well (MQW) strained InGaAsP FP-LD. By utilizing frequency deviation generated by phase modulation and unstable injection locking state with Fabry–Perot laser diode (FP-LD), DPSK to polarization shift-keying (PolSK) and PolSK to intensity modulation (IM) format conversions are realized. We analyze bit error rate (BER) performance of this demodulation scheme. Experimental results show that different longitude modes, bit rates and seeding power have influences on demodulation performance. We achieve error free DPSK signal demodulation under various bit rates of 10Gbit/s, 5Gbit/s, 2.5Gbit/s and 1.25Gbit/s with the same demodulation setting.

Coherent Heterodyne Multilevel Polarization Shift Keying With Spatial Diversity in a Free-Space Optical Turbulence Channel

This paper proposes a coherent multilevel polarization shift keying (MPOLSK) modulation scheme using the spatial diversity detection in a free-space optical (FSO) turbulence channel. The symbol error probability (SEP) analysis for the proposed MPOLSK system under various turbulence regimes has been carried out. The power penalty caused by the phase tracking error has been investigated. For instance, the error floors of 1.3×10-5 and 3.4×10-3 are observed for 8POLSK with the phase tracking variances of σΔ = 0.3 and 0.5, respectively. The maximum ratio combining technique has been applied in the MPOLSK receiver in order to improve the SEP performance. To achieve a SEP of 10-9, the spatial diversity gains required for a 16POLSK-FSO employing four detectors are ~5.8, ~15.8, and ~13.7 dB in weak, moderate, and strong turbulence regimes, respectively.

Proposal of Inverse Pulse Position Modulation for Downstream Signal in Remodulation PON With PolSK-Modulated Multicast Overlay

Inverse pulse position modulation (PPM) is proposed for downstream data modulation in an optical network unit-source-free wavelength-division-multiplexed passive optical network, while optical amplitude shift keying (ASK) modulation is used for upstream data remodulation. In such a PON, polarization shift keying (PolSK) modulation overlay can be supported on the downstream signals for multicasting services. Our experimental results show that the inverse PPM optical ASK modulation for downstream data has small channel crosstalk with PolSK multicast data and ASK-remodulated upstream data. Compared with the approaches based on the Manchester code or inverse return-to-zero code, the proposed scheme based on inverse PPM has a better power margin, indicating a longer transmission distance and a higher tolerance to link loss.

High-speed polarization-shift keying: experimental and numerical investigation

Polarization-shift keying (PolSK), which is performed based on two orthogonal polarization states, is investigated in detail by experimental and numerical simulation for data rates of 10 and 40 Gb/s. Using a general Mach-Zehnder intensity modulator, a new type of PolSK transmitter is presented and demonstrated by polarization coupling technology. And then the PolSK transmission is experimentally demonstrated in a 50-km single mode fiber system, whose eye diagrams show that the PolSK transmitter works well at 10 Gb/s and the power fluctuation is ~8%. Further simulation reveals that the PolSK has great potential in long distance transmission with data rate of 40 Gb/s, while its error-free distance goes beyond 2 100 km.

Design of ultrafast all-optical PolSK DMUX based on semiconductor optical amplifiers

A novel ultrahigh-speed all-optical demultiplexer (DMUX) with polarization-shift-keying (PolSK) modulation input signals is proposed. This design is based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). For analyzing each amplifier, we use finite-difference method (FDM) based on solution of the traveling wave coupled equations. Using numerical simulation, the all-optical DMUX is theoretically realized at 40 Gb/s. We also study the relation between optical confinement factor and thickness of active layer of the SOA section successfully, and investigate the increasing effect of confinement factor on the DMUX optical output power. With this work, the confinement factor is increased from 0.3 to 0.48, and as a result, the output power approximately twice ofits initial value is achieved. Moreover, the effects of polarization dependence of SOA on the output performance of all-optical DMUX for PolSK signal are theoretically investigated in detail.

Experimental investigation of polarisation modulated free space optical communication with direct detection in a turbulence channel

Binary polarisation shift keying (BPOLSK) has been proposed to mitigate the atmospheric turbulence-induced fading in free space optical (FSO) communication systems. In this study, the Q-factors obtained for the BPOLSK-FSO system are verified in conjunction with theoretical results to confirm the validity of the proposed scheme. The analytical bit error rate (BER) for the BPOLSK and non-return-to-zero on–off keying (NRZ-OOK) schemes are presented. The authors show that the BPOLSK scheme with direct detection offers improved BER performances compared to NRZ-OOK in the presence of weak turbulence, which is inferred from the experimental Q-factor and theoretical BER. For a turbulence variance σ21 of 0.003 and the transmitted optical power of −16.8 dBm, values for Q-factor are ∼11 and ∼8.5 for BPOLSK and NRZ-OOK schemes, respectively. The authors show that the predicted signal-to-noise ratio (SNR) for BPOLSK and NRZ-OOK schemes are ∼13.5 and ∼15 dB, respectively, for a BER of 10−6 and σ21 of 0.01. When σ21 increases to 0.1, ∼8 dB lower values of SNR is required for BPOLSK compared with NRZ-OOK.

High spectral-efficiency 100Gbit/s transmission using DRZ, DQPSK, and PoISK three dimension orthogonal modulation

We propose and demonstrate a new 4-bit/symbol optical data format based on simultaneously modulating dark return-to-zero (DRZ), differential quadrature phase-shift keying (DQPSK) and polarization-shift-keying (PoISK) for 100Gbit/s applications. Through numerical simulation, we show the differential tolerance of the three dimension modulation format toward polarization-mode dispersion (PMD) and chromatic dispersion. The receiver sensitivities of three different combined modulation formats, with those of DRZ, DQPSK and PoISK, are compared respectively, while extinction ratio of DRZ is changed. The proposed three dimension orthogonal modulation format has a more compact spectrum and is more tolerant to phase noise compared with DQPSK for 100Gbit/s transmission.