Dual-polarization Quadrature Phase-shift Keying Research Articles

Upgraded Architecture of TWDM-PON with Type B Protection for Capacity and Budget Improvement

The next-generation mobile fronthaul system based on technologies beyond 5G requires optical access networks with excellent disaster resistance. For passive optical networks (PONs), also known as economical optical access networks, the Type B protection method that considers disaster resistance is a practical conventional approach to address transceiver and link failure. However, the transmission capacity upgrade and effective utilization of optical power of time wavelength division multiplexing (TWDM)-based-PON with Type B protection have not been extensively studied. In this study, we intend to upgrade the architecture by applying Type B protection to a 40 Gbit/s-class TWDM-PON that uses the uplink C band/downlink L band specified by NG-PON2. To improve the downlink capacity, we have considered a bidirectional asymmetric modulation transmission system to introduce a dual-polarization quadrature phase-shift keying system for the downlink C band. The downlink coherent signal is transmitted by selecting either the primary or secondary link using the optical switch. There are no major drawbacks with either the primary or secondary link. Moreover, we have numerically and experimentally clarified the impact of improving the reception power by receiving a burst single-input multi-output in the uplink under the condition of no link failure. Additionally, we describe the bidirectional transmission feasibility of C-band asymmetric modulation under link failure and reflection. We have found that bidirectional transmission at the same wavelength is possible by allowing a specific penalty even for asymmetric symbol rate/polarization/modulation formats. A principle verification experiment revealed that SIMO reception could be improved by a 1.5 dB budget on the uplink. The method of upgrading the TWDM-PON system can be changed while effectively using NG-PON2 technology until the future coherent PON method is completely introduced.

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.

Enabling cost-effective high-performance vibration sensing in digital subcarrier multiplexing systems.

Enabling communication networks with sensing functionality has attracted significant interest lately. The digital subcarrier multiplexing (DSCM) technology is widely promoted in short-reach scenarios for its inherent flexibility of fine-tuning the spectrum. Its compatibility with large-scale as-deployed coherent architectures makes it particularly suited for cost-sensitive integrated sensing and communication applications. In this paper, we propose a scheme of spectrally integrating the digital linear frequency modulated sensing signal into DSCM signals to achieve simultaneous sensing and communication through shared transmitter. Consequently, this cost-effective scheme has been demonstrated to achieve 100-Gb/s dual-polarization quadrature phase-shift keying (DP-QPSK) and 200-Gb/s dual-polarization 16-ary quadrature amplitude modulation (DP-16QAM) transmission with a distributed acoustic sensing sensitivity of 69 pε/Hz and 88 pε/Hz respectively, at a spatial resolution of 4 m.

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.

Numerical evaluation of bidirectional high-speed data transmission over turbulence tolerable FSO link employing WDM-OAM multiplexing and DP-QPSK modulation techniques

Orbital angular momentum (OAM) mode multiplexing is achieving utmost interest for the enhancement of transmission capacity and spectral efficiency. We have reported a (2 × 8 × 112 Gbps) 1792 Gbps bi-directional FSO link based on WDM-OAM multiplexing and dual-polarization quadrature phase-shift keying (DP-QPSK) modulation technique. An optical frequency comb lines (OFCL) generator, capable to generate 44 comb lines with flatness∼0 dB is employed as a broadband laser source. The efficiency of the system is not only checked for clear weather but also for different turbulence situations (weak, moderate, and strong atmospheric turbulence). According to the best of our knowledge, this is the first one where the effect of atmospheric turbulence on DP-QPSK data carrying OAM modes has been checked for a bidirectional link. The combination of coherent detection with digital signal processing is utilized for compensating wave-form distortion and noise cancellation due to atmospheric turbulence. A very low power penalty of 1.65 dB in clear weather and 4.91 dB in strong atmospheric turbulence at a BER∼10 −9 with open eye and clear constellation diagrams depicts the feasibility of the proposed system.

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.

Ultra-low-loss multi-layer 8 × 8 microring optical switch

Microring-based optical switches are promising for wavelength-selective switching with the merits of compact size and low power consumption. However, the large insertion loss, the high fabrication, and the temperature sensitivity hinder the scalability of silicon microring optical switch fabrics. In this paper, we utilize a three-dimensional (3D) microring-based optical switch element (SE) on a multi-layer Si3N4-on-SOI platform to realize high-performance large-scale optical switch fabrics. The 3D microring-based SE consists of a Si/Si3N4 waveguide overpass crossing in the bottom and the top layers, and Si3N4 dual-coupled microring resonators (MRRs) in the middle layer. The switch is calibration-free and has low insertion loss. With the 3D microring-based SEs, we implement an 8×8 crossbar optical switch fabric. As the resonance wavelengths of all SEs are well aligned, only one SE needs to be turned on in each routing path, which greatly reduces the complexity of the switch control. The optical transmission spectra show a box-like shape, with a passband width of ∼69 GHz and an average on-state loss of ∼0.37 dB. The chip has a record-low on-chip insertion loss of 0.52–2.66 dB. We also implement a non-duplicate polarization-diversity optical switch by using the bidirectional transmission characteristics of the crossbar architecture, which is highly favorable for practical applications. 100 Gb/s dual-polarization quadrature-phase-shift-keying (DP-QPSK) signal is transmitted through the switch without significant degradation. To the best of our knowledge, this is the first time that 3D MRRs have been used to build highly scalable polarization-diversity optical switch fabrics.

Real-Time Polarization Demultiplexing by Multi-Thread Constant Modulus Algorithm for Fully Softwarized Access Networks

Optical access networks are replacing dedicated hardware-based architectures with architectures centered on general-purpose servers based on network function virtualization (NFV) and the software-defined network (SDN) in order to efficiently support various new services. To maximize the advantage of this new approach, studies are now targeting the fully softwarized access network which requires softwarization of digital signal processing (DSP). However, polarization demultiplexing includes sequential processing, and it is difficult to softwarize high-speed filter updates in a constant modulus algorithm (CMA) that supports real-world state of polarization (SOP) change. This paper proposes a novel multi-thread CMA that can track fast SOP changes by using multiple threads to implement filter convolution. It tracks faster SOP changes with the same filter update cycle as the conventional method by calculating multiple filters on multiple threads and averaging them. Its graphics processing unit (GPU) implementation adopts parallel reduction for averaging the filters and parallelizes the update of filters for tributaries X and Y in order to take advantage of many GPU threads. We evaluate the real-time bit error rate (BER) performance achieved by implementing the DSP functions with the proposal in a 5-Gb/s dual-polarization-quadrature phase-shift keying (DP-QPSK) system. The demonstration results show that the proposed algorithm supports 2,000 rd/s real-time performance for optical systems and offers high feasibility for application to the fully softwarized access network.

Real-Time Demonstration of 100 GbE THz-Wireless and Fiber Seamless Integration Networks

We propose and experimentally demonstrate a real-time fiber-THz-fiber 2 × 2 multiple-input multiple-output (MIMO) seamless integration system at 340–510 GHz using commercial digital coherent optics (DCO) modules for baseband signals processing, which realizes a record net rate of 103.125 Gb/s dual polarization quadrature-phase-shift-keying (DP-QPSK) signals delivery over two spans of 20 km wireline single-mode fiber-link and 3 m wireless 2 × 2 MIMO link without using THz power amplifier under 15% soft-decision forward-error-correction threshold (SD-FEC) of 1.56 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> . Two kinds of THz-to-optical conversion schemes based on an integrated dual-polarization Mach-Zehnder modulator (DP-MZM) and two discrete intensity-modulators (IMs) are extensively investigated and compared. To the best of our knowledge, this is the first real-time demonstration of THz-wireless and 100 GbE fiber-optic networks seamless integration transmission, which is compatible with IEEE 802.3 and ITU-T G.798 specifications. It is a promising scheme to meet the demands of future fiber-wireless-integrations communication for low power consumption, low cost and miniaturization.

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.

Hitless Wavelength Switching of Semiconductor Optical Amplifier-integrated Reflection-type Transversal Filter Laser with Suppressed Frequency Error

A spurious wavelength (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">spurious</sub> ) from a tunable laser during tuning is an issue in optical systems based on dynamically controlled laser wavelengths. Output shuttering synchronized with tunable-laser tuning is promising way to suppress λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">spurious</sub> . A semiconductor optical amplifier (SOA) is an ideal shutter for laser outputs with λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">spurious</sub> in terms of low-wavelength-independent extinction ratio. We developed an electro-optically tunable reflection-type transversal filter (RTF) laser monolithically integrated with an SOA. Thanks to the nanosecond scale tuning speed of the RTF laser, the required shuttering time of the integrated SOA is also a corresponding nanosecond scale, which leads to little temperature change in the laser cavity. As a result, the laser frequency drift after turning off/on the SOA is suppressed to within +/- 2.5 GHz without any additional laser control algorithm to compensate for the thermal crosstalk from the SOA. Using the laser, we demonstrated a “hitless” wavelength-switching subsystem based on a coherent format. Namely, outputs from an SOA-RTF laser coded with 32-GBd dual-polarization quadrature phase-shift keying (128 Gbps) are dynamically switched with λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">spurious</sub> eliminated by SOA shuttering. In addition to practical bit error rates (BERs) of wavelength channels from the SOA-RTF laser, we observe no interference with the BER of a wavelength channel from another monitor laser in the wavelength-switching subsystem.

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

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.

Demonstration of 144-Gbps Photonics-Assisted THz Wireless Transmission at 500 GHz Enabled by Joint DBN Equalizer.

The THz wireless transmission system based on photonics has been a promising candidate for further 6G communication, which can provide hundreds of Gbps or even Tbps data capacity. In this paper, 144-Gbps dual polarization quadrature-phase-shift-keying (DP-QPSK) signal generation and transmission over a 20-km SSMF and 3-m wireless 2 × 2 multiple-input multiple-output (MIMO) link at 500 GHz have been demonstrated. To further compensate for the linear and nonlinear distortions during the fiber–wireless transmission, a novel joint Deep Belief Network (J-DBN) equalizer is proposed. Our proposed J-DBN-based schemes are mainly optimized based upon the constant modulus algorithm (CMA) and direct-detection least mean square (DD-LMS) equalization. The results indicate that the J-DBN equalizer has better bit error rate (BER) performance in receiver sensitivity. In addition, the computational complexity of the J-DBN-based equalizer can be approximately 46% lower than that of conventional equalizers with similar performance. To our knowledge, this is the first time that a novel joint DBN equalizer has been proposed based on classical algorithms. It is a promising scheme to meet the demands of future fiber–wireless integration communication for low power consumption, low cost, and high capacity.

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.

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.

Real-time demonstration of 103.125-Gbps fiber-THz-fiber 2 × 2 MIMO transparent transmission at 360-430 GHz based on photonics.

In this Letter, we experimentally demonstrate the first real-time transparent fiber-THz-fiber 2 × 2 multiple-input multiple-output (MIMO) transmission system with a record line rate of 125.516 Gbps at 360-430 GHz based on photonic remote heterodyning, hybrid optoelectronic down-conversion, and commercial digital coherent modules. The 103.125-Gbps net data rate using dual-polarization quadrature phase-shift keying (DP-QPSK) modulation is successfully transmitted over two spans of 20-km standard single-mode fiber (SSMF) and 60-cm wireless distance under 15% soft-decision forward error correction (SD-FEC) for a pre-FEC bit error ratio (BER) threshold of 1.56 × 10-2 (post-FEC BER < 10-15). The optical signal to noise ratio (OSNR) margin and the stability of the transmission system are extensively investigated. To the best of our knowledge, this is the first time to realize >100-Gbps real-time transparent fiber-THz-fiber link transmission at beyond the 350-GHz band, making it a promising scheme to pave the way towards a practical seamless integration of a fiber-THz-fiber link to the future 6G mobile communication system.

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.

Wideband Tunable Frequency Converter Based on an OEO Using a DP-QPSK Modulator

Flexibility and tunability are critical for frequency conversion in antenna, radar and modern communication systems. In this study we propose and experimentally demonstrate a wideband tunable frequency converter by using an optoelectronic oscillator (OEO) to generate high-purity local oscillator (LO) signal for flexible frequency conversion. In the proposed frequency converter, a dual-polarization quadrature phase shift-keying (DP-QPSK) modulator is utilized to receive the signal to be converted and also to form a tunable OEO based on a dispersion-induced microwave photonic filter. By adjusting the bias voltage of the DP-QPSK modulator, an LO signal with accurate adjustable frequency is generated by the OEO. The measured phase noise of generated LO signal can reach -110 dBc/Hz at 10 kHz offset. An experiment is performed to verify the feasibility of this proposed frequency converter. The 7.6-GHz radio frequency (RF) signal is freely converted to different intermediate frequency (IF) signals, respectively. Moreover, the proposed microwave photonic converter is applied into wireless transmission systems. A 1-Gb/s NRZ wireless signal centered at 6.5 GHz is down-converted to baseband successfully.