Dual-polarization Quadrature Phase Shift Research Articles

Development of a Technique for Mitigating Four-Wave Mixing Effect in Dense Wavelength Division Multiplexing System Using DP-QPSK with Channel Spacing

A Dense Wavelength Division Multiplexing (DWDM) system is a vital component in the optical network for the transmission of high data over a long distance. However, the non-linear effect of Four-Wave Mixing (FWM), one of the critical non-linear effects, limits the channel capacity and the amount of data that the DWDM system transmits. Intensity Modulation formats and Channel Shuffling algorithms being used to overcome the impact of FWM in DWDM are unable to sufficiently suppress the FWM effect due to limited capacity. This research, therefore, examined the mitigation of the FWM effect in the DWDM system using Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) and channel arrangement. The DP-QPSK was designed and implemented using 100 GHz for both Equal Space Channel Allocation (ESCA) and Unequal Space Channel Allocation (USCA). It was analysed on a 64-channel DWDM system operating at 100 Gbps capacity over a transmission distance of 1000 km. The DP-QPSK-based ESCA and USCA techniques were simulated using OptiSystem 17 simulation software and Python for data analysis. The performance of the system was evaluated using bit error rate (BER), optical signal-to-noise ratio (OSNR) and Electrical Constellation Diagram. Validation was done with other advanced modulation formats: Quadrature Phase Shift Keying (QPSK) and Differential-Quadrature Phase Shift Keying (D-QPSK). The DWDM system using DP-QPSK gave better performance due to the highest OSNR and BER values obtained when compared with ESCA-based QPSK and D-QPSK techniques. The technique can be used to suppress the effect of FWM in optical communication systems.

All-fiber mode exchange of high-speed DP-QPSK signals using mechanical long-period fiber grating

Mode exchange (ME) can effectively improve the utilization of network resources in short-distance mode division multiplexing (MDM) communication systems. In this paper, a mechanical long-period fiber grating (LPFG) mode converter is used to realize the channel exchange of LP01- and LP11-mode dual-polarization (DP) signals, with the channel crosstalk of −14 dB or less. We experimentally tested the receiver sensitivities of two DP channels, which are dependent on channel crosstalk and polarization-dependent loss (PDL), and demonstrated the practicability of the real-time data transmission and exchange for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) in multiple-input multiple-output free (MIMO-free) MDM system. The experimental results show that the ME system can provide the optical power margin of about 25 dB, capable of effectively supporting the metropolitan transmission and exchange between the data centers.

Free space optical communication in Indian cities; channel characteristics and link performance

Abstract Free Space Optical (FSO) communication systems, offering high-speed and high-bandwidth transmission, have emerged as a possible substitute for conventional wired and wireless communication systems. However, FSO links are susceptible to atmospheric deficiencies including attenuation, scintillation, and turbulence, which may severely impact the performance of the communication link. In this research paper, we propose and analyze the performance of a 25 Gbps dual-polarization quadrature phase shift keying (DP-QPSK) coherent optical orthogonal frequency division multiplexing (CO-OFDM)-based FSO communication link with spatial diversity technique under various climatic conditions in four different geographical locations of India, i.e., Delhi, Ahmedabad, Kolkata, and Chennai in the year 2021. The proposed FSO communication system is evaluated through performance analysis and parametric evaluation under various weather conditions such as clear sky, fog, haze, rain, cloud, and thermal gradient. Key performance metrics such as bit error rate (BER), Q-factor, signal-to-noise ratio (SNR), received power, communication range, and reliability are analyzed based on the simulation results. The proposed FSO communication system provides high data rates, improved power efficiency, better resilience to atmospheric impairments, reliable communication links under different weather conditions, and a practical solution for high-speed and high-bandwidth communication in various applications.

Experimental investigation on transmission system of dual-polarization mode-division-multiplexing signals with few-mode erbium-doped fiber amplifiers

The transmission capacity of optical fiber communication can be effectively improved by the combination of dual-polarization (DP) high-order modulation format with mode division multiplexing (MDM) technology. We build up an experimental system for amplification and transmission of DP-MDM signals with orthogonal mode combination (OMC) and identical mode combination (IMC). The MDM transceiver units are composed of mode-selective photonic lanterns (MSPLs) and few-mode polarization controllers (FMPCs). The few-mode erbium-doped fiber amplifier (FM-EDFA) used here is constructed by homemade isolation and wavelength division multiplexers (IWDMs). The amplification and transmission experiments for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) MDM signals are carried out by the commercial optical transport network (OTN) platform. An effective method of optimally receiving DP-MDM signals are presented by appropriately controlling the FMPC in the MDM receiver unit. The experimental results show that, (1) OMC DP-MDM signals have a better performance than IMC cases in differential modal gain (DMG), which provides a new means to reduce the DMG; and (2) under the optimization conditions, OMC and IMC DP-MDM systems possess a close receiver sensitivity, but the former has a larger optical power margin than the latter by 2.2 dB.

A 4.6-pJ/b 200-Gb/s Analog DP-QPSK Coherent Optical Receiver in 28-nm CMOS

Coherent detection with polarization multiplexing is widely used because of its high spectral efficiency. However, it utilizes analog-to-digital converters (ADCs) and digital signal processing (DSP), which consume a large amount of power and thereby hinder its application in power-sensitive short-reach links. In this article, we present a 200-Gb/s analog <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C$ </tex-math></inline-formula> -band dual-polarization quadrature phase shift keying (DP-QPSK) coherent optical receiver (RX), which uses four-way time-interleaved sampling to relax the internal bandwidth limit. The RX utilizes a chromatic dispersion (CD) equalizer, a polarization demultiplexer (Pol. DEMUX), and carrier recovery (CR) to eliminate the intersymbol interferences induced by fiber impairments and the carrier frequency offset. A subsampling scheme is adopted for low-power adaptation. The decrease in the frequency offset detection range caused by subsampling is prevented using a consecutive slice sampling technique. Measurement results show that our RX supports equalization for impairments equivalent to a 10-km fiber and CR for a residual carrier frequency offset of 12 MHz. A bit error rate (BER) of less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1e-10$ </tex-math></inline-formula> is achieved with an energy efficiency of 4.6 pJ/b.

Frequency-Doubled Triangular Pulse Signal Generation with Variable Symmetry Based on a Dual-Polarization Quadrature Phase-Shift Keying Modulator

Frequency-Doubled Triangular Pulse Signal Generation with Variable Symmetry Based on a Dual-Polarization Quadrature Phase-Shift Keying Modulator

Cascaded modulation format identification and optical signal-to-noise ratio estimation method for optical communication system

Modulation format identification (MFI) and optical performance monitoring are important for elastic optical networks. We propose a cascaded module-based MFI and optical signal-to-noise ratio (OSNR) estimation method. Polar coordinates were used as features to distinguish the modulation formats (MFs) and estimate the OSNR. Dilated convolution was adopted to increase the receptive field and reduce computational complexity. Four commonly used MFs were investigated: dual-polarization (DP) quadrature phase shift keying, 8QAM, 16QAM, and 32QAM. A 32G baud DP transmission system is established, and the results are discussed. The results show that MFI accuracy can reach 100% when the OSNR is less than the 7% forward error correction limit, and the accuracy of OSNR monitoring is close to 100%. Two factors, sample length and nonlinearity, were further investigated. The results show that the MFI and OSNR estimators can achieve perfect performance when the sample length is 3000. Simultaneously, the accuracy of the MFI remained at 100%, and the accuracy of the OSNR estimator decreased by 1% to 2% when the launch power changed from 0 to 5 dBm. Furthermore, two modules with the same structure, DC-CNN and CNN, were compared. The results show that the two models can achieve similar accuracies and that the DC-CNN has the least number of parameters. Finally, experimental verification was conducted to ensure the practical applicability of the proposed method.

Enhanced Atmospheric Turbulence Resiliency With Successive Interference Cancellation DSP in Mode Division Multiplexing Free-Space Optical Links

We experimentally demonstrate the enhanced atmospheric turbulence resiliency in a 137.8 Gbit/s/mode mode-division multiplexing free-space optical communication link through the application of a successive interference cancellation digital signal processing algorithm. The turbulence resiliency is further enhanced through redundant receive channels in the mode-division multiplexing link. The proof of concept demonstration is performed using commercially available mode-selective photonic lanterns, a commercial transponder, and a spatial light modulator based turbulence emulator. In this link, 5 spatial modes with each mode carrying 34.46 GBaud dual-polarization quadrature phase shift keying signals are successfully transmitted with an average bit error rate lower than the hard-decision forward error correction limit. As a result, we achieved a record-high mode- and polarization-division multiplexing channel number of 10, a record-high line rate of 689.23 Gbit/s, and a record-high net spectral efficiency of 13.9 b/s/Hz in emulated turbulent links in a mode-division multiplexing free-space optical system.

Filter-free photonics-assisted frequency translator with a tunable phase shift and amplitude based on an integrated DP-QPSK modulator.

In this paper, we propose and demonstrate a novel, to the best of knowledge, filter-free photonics-assisted microwave frequency translator with a tunable phase shift and amplitude. The pivotal component of the proposed scheme is an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator, which is applied to generate a polarization orthogonal carrier-suppressed single sideband modulation signal and frequency shifted optical carrier signal. The polarization-multiplexed optical signal outputs from the DP-QPSK modulator is then sent to a photodetector (PD) via a polarization controller (PC) and a polarizer to implement photoelectric conversion. The electrical signal output from the PD is the desired frequency translated microwave signal, and the amount of frequency shift is determined by the frequency of the sawtooth wave applied to the DP-QPSK modulator. In addition, since the PC can be used to adjust the polarization angle and introduce a phase difference between the two orthogonally polarized optical signals, the phase shift and amplitude of the obtained translated signal can also be easily tuned. A theoretical analysis and simulation experiment are carried out to verify the feasibility of the proposed scheme. The simulation results show that the novel scheme can realize frequency translation with a 360° continuously tunable phase shift and adjustable amplitude for both a single-tone signal and linearly frequency modulated signal with a 50MHz bandwidth. The spurious suppression ratios of the single-tone signal and LFM signal after frequency translation are larger than 48 and 30dB, respectively.

Photonic Generation of a Windowed Phase-Coded Microwave Waveform With Suppressed Spectrum Sidelobes

An approach to photonic generation of a windowed binary phase-coded microwave waveform with suppressed spectrum sidelobes is proposed and demonstrated. An optical double sideband plus carrier signal with the carrier and the two sidebands being orthogonally polarized is generated by a dual-polarization quadrature phase shift keying (DP-QPSK) modulator, with the generated signal sent to a phase modulator (PM) where phase coding is performed. The PM can support phase modulation in two orthogonal polarization directions. The phase-modulated signals are projected to one polarization direction and detected at a photodetector (PD). Switching the polarity of the coding signal, a π phase shift is introduced. Since the amplitude of the generated signal is dependent on the amplitude of the input coding signal, a windowing function is applied to the generated waveform by using a windowed input phase coding signal, to suppress the sidelobes in the spectrum of the generated microwave waveform. The key features of the proposed waveform generator include all-optical generation without the need of electronic components and optical filters, thus ensuring a wide operating frequency range. The proposed phase-coded microwave waveform generator also has the ability to switch between the fundamental and harmonic carrier frequency by controlling the modulator bias voltages. Experimental results show that, using a windowed 64-bit Gaussian pseudo-random binary sequence coding signal, a phase-coded microwave waveform is generated with the sidelobes of the spectrum largely suppressed by 20 dB and a pulse compression ratio as large as 133. Generation of binary phase-coded microwave waveforms at the 2nd harmonic carrier frequencies is also demonstrated experimentally.

Microwave photonic multiform microwave frequency shift keying signal generator.

A photonic approach to realizing multiform microwave frequency shift keying (FSK) signal generation is proposed and demonstrated. In the scheme, a commercial dual-polarization quadrature phase shift keying modulator (DP-QPSKM) is employed to generate two orthogonally-polarized signals containing specific optical sidebands, and a subsequent Sagnac loop structure govern the interference results of these two signals. From a theoretical analysis, when the modulators are properly biased, microwave FSK signal with fixed double relationship or flexibly tunable subcarrier frequencies can be obtained, and high frequency multiplication can be realized in the meantime. Furthermore, a photonic-optimized coherent demodulation structure is designed to recover the binary coding data, which can effectively avoid the electronic bottleneck. Simulation has been performed to investigate the mechanism and the discussions about the robustness to non-ideal parameters including DC bias, phase shift and polarization angle are also given. In the proof-of-concept experiment, microwave FSK signal with subcarrier frequencies of 2/4, 2.2/3.8, 2.4/3.6, 2.6/3.4, 2.8/3.2 GHz are generated, and the correct binary coding data is successfully recovered with the aid of simulation platform. The simulation and experimental results can verify the feasibility of the proposed multiform microwave FSK signal generator, which may find applications in modern radar and communication systems.

Demonstration of 10-channel mode- and polarization-division multiplexed free-space optical transmission with successive interference cancellation DSP.

We experimentally demonstrate 10-channel mode-division multiplexed free-space optical transmission with five spatial modes, each carrying 19.6925-Gbaud dual-polarization quadrature phase shift keying signals. Strong inter-mode cross talk is observed in our commercially available photonic lantern based system when using a complete orthogonal mode set as independent channels. A successive interference cancellation based multiple-input multiple-output digital signal processing (DSP) algorithm is first applied to mitigate the inter-mode cross talk in mode-division multiplexed systems. The DSP also supports unequal transmit and receive channel numbers to further improve the cross talk resiliency. Compared to the conventional minimum mean square error DSP, the required optical signal-to-noise ratio of the successive interference cancellation DSP is decreased by approximately 5 dB at the hard-decision forward error correction limit. As a result, this system demonstrates a record-high independent channel number of 10 and spectral efficiency of 13.7 b/s/Hz in mode-division multiplexed free-space optical systems.

Machine Learning-Based Optical Performance Monitoring for Super-Channel Optical Networks

In this paper, and for the first time in literature, optical performance monitoring (OPM) of super-channel optical networks is considered. In particular, we propose a novel machine learning OPM technique based on the use of transformed in-phase quadrature histogram (IQH) features and support vector regressor (SVR) to estimate different optical parameters such as optical signal-to-noise ratio (OSNR) and chromatic dispersion (CD). Two transformation methods, the two-dimensional (2D) discrete Fourier transform (DFT) and 2D discrete cosine transform (DCT), are applied to the IQH to extract features with a considerably reduced dimensionality. For the purpose of simulation, the OPM of a 7 × 20 Gbaud dual-polarization–quadrature phase shift keying (DP-QPSK) is considered. Simulations reveal that it can accurately estimate the various optical parameters (i.e., OSNR and CD) with a coefficient of determination value greater than 0.98. In addition, the effectiveness of proposed OPM scheme is examined under different values of polarization mode dispersion and frequency offset, as well as the utilization of different higher order modulation formats. Moreover, proof-of-concept experiments are performed for validation. The results show an excellent matching between the simulation and experimental findings.

Terabyte capacity-enabled (10 x 400 Gbps) Is-OWC system for long-haul communication by incorporating dual polarization quadrature phase shift key and mode division multiplexing scheme

Inter-satellite optical wireless communication (Is-OWC) links can become promising solutions to realize the next-generation high-speed communication services. The operation of Global Navigation Satellite Systems can be improved with the use of Is-OWC links through ranging and communication services. However, the key challenge in Inter-satellite link (ISL) is its effective range which is limited due to pointing errors. In this work, we propose to develop a high-capacity and long-reach Is-OWC link by incorporating hybrid mode division multiplexing (MDM) and wavelength division multiplexing (WDM) schemes to transmit ten independent channels over 40000kms Is-OWC link. Each channel is capable of carrying 400Gbps data which is encoded by the dual polarization quadrature phase shift key technique with required signal to noise ratio (SNR) and received power. The proposed Is-OWC link satisfies the enhanced communication within Geostationary Earth Orbit (GEO) and Low Earth Orbit (LEO) satellites. The proposed Is-OWC is further evaluated under the impact of space turbulences, particularly transmitter and receiver pointing errors. The result reported that the proposed Is-OWC link can transmit 4Tbps data over 16000kms with the transmitter pointing error of 2μrad and receiver pointing error of 1μrad.

1000 Gbps MDM-WDM FSO link employing DP-QPSK modulation scheme under the effect of fog

In this work, a dual polarization-quadrature phase shift keying (DP-QPSK) modulation scheme has been used for analysing the performance of terrestrial MDM-WDM FSO link at a data rate of 1000 Gbps. The proposed link has been investigated considering linearly polarized (LP) 01 and 02 modes with a channel capacity and spacing of 10 × 100 Gpbs and 10 GHz respectively. The performance of the system has been evaluated in terms of received optical power and signal to noise ratio (SNR) for clear weather, light, medium and heavy fog conditions over a propagation range varying from 1.2 km to 20 km.

Investigation of Power Efficiency Changes in DWDM Systems Replacing Erbium-Doped Amplifiers By Semiconductor Optical Amplifiers

Abstract To evaluate potential utilization of semiconductor optical amplifiers (SOAs) as a wideband amplification alternative to erbium doped fibre amplifiers (EDFAs) in dense wavelength division multiplexed (DWDM) coherent systems, the authors discuss changes in power consumption levels required for a single bit transmission. The research evaluates the power efficiency parameter for WDM transmission systems using both amplification schemes – EDFAs that utilise standard C-band and SOAs assuming 75 nm amplification spectral band. The power efficiency levels have been estimated for five transmission spans with maximal distance of 640 km. The standard 50 GHz channel spacing has been chosen for both system configurations to allocate 100 Gbps dual-polarization quadrature phase shift keying (DP-QPSK) optical signals. The simulation schemes are described along with the critical parameters, derived from the recent relevant studies that should be taken into account considering usage of SOAs as in-line amplifiers.

Switchable instantaneous frequency measurement by optical power monitoring based on DP-QPSK modulator

We propose and analyze an instantaneous frequency measurement system by using optical power monitoring technique with improved resolution. The primary component adopted in the proposal is a dual-polarization quadrature phase shift keying (DP-QPSK) modulator which is used to modulate the microwave signal that has a designed time delay and phase shifting. The generated optical signal is sent to polarization beam splitter (PBS) in DP-QPSK modulator. Owing to the complementary transmission nature of polarization interference introduced by PBS, the frequency information is converted into the optical power and the relationship between the amplitude comparison function (ACF) and microwave frequency to be measured is established. Thus, the frequency of the microwave signal can be easily measured through monitoring the optical powers of the two output ports of the PBS. Furthermore, by adjusting the direct current (DC) biases of the DP-QPSK modulator instead of changing the electrical delay, the measurement range and resolution can be switched. In this paper, the basic principle of the instantaneous frequency measurement system is derived in detail, and simulation has been performed to investigate the resolution, the measurement range, and the influence of imperfection devices. The proposed scheme is wavelength-independent and its measurement range is switchable, which can avoid the laser wavelength drifting problem and thus greatly increasing the system flexibility.

Wideband microwave Doppler frequency shift measurement based on acousto-optic frequency shift and DP-QPSK receiver

Microwave Doppler frequency shift (DFS) measurement is one of the backbone parts in modern radar systems. In this paper, a simple and novel optical approach to implement broadband microwave DFS measurement is proposed. Two carrier-suppressed single-sideband modulation (CS-SSB) modules, which are constituted by two Mach Zehnder modulators (MZMs) and two optical band pass filters (OBPFs) are employed for modulating both transmitted signal and echo signal. Due to the frequency shift by an acousto-optic modulator (AOM), the proposed method converts the DFS signals to the intermediate-frequency (IF) domain for simultaneous estimation of DFS value and direction, which helps to avoid low-frequency interference. Moreover, an integrated dual-polarization quadrature phase shift keying (DP-QPSK) receiver is used to obtain the in-phase and quadrature (I/Q) DFS terms and improve the system stability. In the experiment, the microwave DFS is estimated with a clear direction and a maximum measurement error of 4.9 Hz over an ultra-wide operation frequency from 10 to 40 GHz. The micro-Doppler frequency shift (mDFS) measurement is also carried out to demonstrate the advantage of low-frequency DFS measurement. Excellent system stability is also observed in the experiment, where the corresponding I/Q amplitude and phase imbalance are maintained lower than 2.5 dB and 4.8 degrees during 60 min.

A Cost-Effective Solution for Testing High-Performance Integrated Circuits

Testing of high-speed integrated circuits that enable 100-Gb Ethernet is a complex and expensive proposition. It becomes increasingly difficult to carry out measurements with probes if the test chip’s pad count is very high. A full-fledged packaging solution to test a chip requires a lot of resources and incurs a considerable cost. Other approaches, such as using flip-chip-on-board, prove to be expensive as well. On-board electrical interconnects, if appropriately designed, can be used with wire-bonded dies to perform high-speed measurements using ordinary methods. This article discusses the design and development of a platform to test a high-speed equalizer chip designed for the 100-Gb/s dual polarization-quadrature phase shift keying-based optical links. It also shows how careful optimization of the test platform addressing the aspects of signal delivery from connectors to chip pads through broadband transmission lines and bond-wire transitions and thermal management facilitates the use of low-cost methods to test high-performance chips. Detailed descriptions of the test platform and its validation using $S$ -parameter measurements and the experimental results with an optical system are also presented.

Photonic Generation and Application of a Bandwidth Multiplied Linearly Chirped Signal With Phase Modulation Capability

Photonic generation of a bandwidth multiplied linearly chirped signal with phase modulation capability is proposed and its applications in communication and radar systems are demonstrated. In the generator, a dual-polarization quadrature phase shift keying modulator is used to simultaneously perform bandwidth multiplication and phase modulation. The generator can be functionally expanded to a transceiver for wireless communication, which can be used to perform optical generation and coherent demodulation of a phase modulated linearly chirped signal. Meanwhile, the generator can be used to increase the radar detection distance by improving the signal time bandwidth product. The signal generation and application are verified by simulation. A linearly chirped signal with an instantaneous frequency of 8–12 GHz or 16–24 GHz is obtained by using a 4–6 GHz electrical driving signal. The proposed transceiver is demonstrated to perform 156.25-bit/s wireless communication. When the emitting power is 11 dBm and the communication distance is 1 km, the bit error rate can be better than 10−4. While for radar detection, the peak to sidelobe ratio of the compressed pulse is increased from 1.1 dB to 9.2 dB, through which the distance improvement can be verified.