Gbaud Quadrature Phase Shift Keying Research Articles

High-speed Multi-channel all-optical de-aggregator based on polarization-assisted phase-sensitive amplification in elastic optical networks

An all-optical de-aggregator for four wavelength-multiplexed channels based on polarization-assisted (PA) phase-sensitive amplification (PSA) is proposed, aiming to improve spectral efficiency in elastic optical networks (EONs). Benefiting from a single bi-directional PSA based on the nonlinear-optical loop mirror (NOLM), signals and their conjugates can be extracted and located at two polarization states respectively. To avoid the interference of undesired harmonics sharing the same frequency with input signals, we introduce a spatial dislocation between signal and interference harmonics by sending signals to different input ports of the NOLM. For each channel, the signal and its conjugate are coherently superimposed by PA-PSA. Each of the four 25/50 Gbaud quadrature phase shift keying (QPSK) signals is de-aggregated into two binary phase shift keying (BPSK) signals. We use the same scheme to de-aggregate 25/50 Gbaud 16-ary quadrature amplitude modulation (16-QAM) into 4-level pulse-amplitude-modulation (PAM-4) signals for four channels to verify the tunability of the scheme. The bit-error-rate (BER) and error vector magnitude (EVM) performance of the scheme are investigated and the corresponding OSNRs for error-free signal recovery are obtained respectively.

Experimental demonstration of remotely controlled tunable optical correlators of 10–50 Gbaud QPSK channels using linear and nonlinear components and laser-delivered powers

We experimentally demonstrated two enabling architectures for remotely controlled optical quadrature-phase-shift-keying (QPSK) correlators based on both linear and nonlinear elements. The correlator site was located far from the transmitter site and had no access to the local optical power. In the first demonstration, a remote correlator based on a cascade of Mach–Zehnder interferometers (MZIs) was phase-controlled through optical power sent from the transmitter via an optical fiber link. The issue of power loss because of fiber nonlinearities was further addressed by monitoring and managing the backscattered power. In this manner, the power delivered through the link was boosted by ∼13 dB. Another ∼6 dB gain in the delivered power was obtained by adding a second source of light power. The correlator was shown to be able to identify different target patterns within incoming QPSK signals at different baud rates from 10 to 50 Gbaud. In the second architecture, we experimentally demonstrated a tunable optical correlator for a 10/15-Gbaud QPSK data signal using temperature-controlled nonlinear wave mixing at a remote node. A high-power pump was phase-modulated to overcome the link backscattering effect and was sent along with the signal copies to a remote correlator node through an optical fiber link. The waves generated at the output of the correlator were sent back to the transmitter for the monitoring of the operation. We showed a power boost of more than 3 dB for the correlated signal at a temperature drift of 2 °C. Using remote control and monitoring, improved constellation diagrams with lower error vector magnitudes (EVMs) for a temperature drift range of < 2 °C were obtained.

Microwave photonics‐based channelized receiving and reconstruction for broadband RF signal

A microwave photonics-based channelized receiver for broadband vector signals is proposed and experimentally demonstrated. A 4-line optical frequency comb (OFC) generated by a dual-parallel Mach-Zehnder modulator (DPMZM) is used as local oscillator (LO) signal in the in-phase/quadrature (I/Q) receiver for coherently down-converting the broadband signal into different channelized bands. By properly setting the frequency difference between the OFC comb line and the broadband signal, either homodyne or heterodyne detection of I/Q receiver is realized. To compensate the amplitude difference and phase difference of adjacent channel signals, an overlap ratio of adjacent slices is introduced for channel equalization and signal reconstruction. The simulated result shows the error vector magnitude of 4-channel channelization for a reconstructed 8 GBaud quadrature phase shift keying (QPSK) signal is very close to the original one. In the experiment, 2-channel channelization for a 2 GBaud QPSK is realized and the whole signal is reconstructed successfully. The experimental result also proves the quality improvement of reconstructed signal with an increase of channel overlap ratio.

Accurate OSNR monitoring based on data-augmentation-assisted DNN with a small-scale dataset.

Deep neural networks (DNNs) have been successfully applied for accurate optical signal-to-noise ratio (OSNR) monitoring. However, the performance of OSNR monitoring substantially degrades when a mega dataset is inaccessible. Here, we demonstrate an accurate OSNR monitoring scheme based on a data-augmentation (DA)-assisted DNN with a small-scale dataset. When a 20 GBaud quadrature phase shift keying (QPSK) signal is transmitted over 400 to 2600 km standard single-mode fiber (SSMF) with an OSNR range from 8 to 14 dB, we experimentally evaluate the minimum dataset size to secure a mean absolute error (MAE) of OSNR monitoring less than 1 dB. The DA-assisted scheme only requires 50% of the raw data, in comparison with the traditional DNN scheme. Thus, the DA-assisted DNN scheme is promising for field-trial accurate OSNR monitoring, especially when the collection of mega datasets is inconvenient.

High-capacity bi-directional full-duplex transmission based on fiber-eigenmode multiplexing over a FMF with 2×2 MIMO.

We propose and experimentally demonstrate symmetrical (homo-modal) and asymmetrical (hetero-modal) full-duplex bi-directional architectures based on dual-vector eigenmodes multiplexing over a 3 km few mode fiber (FMF). Firstly, 4 vector modes (VMs) of 2 mode groups (MGs), l = 0 (HE11o and HE11e modes) and l = +2 (EH11o and EH11e modes), each carrying a 14 GBaud quadrature phase-shift keying (QPSK) signal, are utilized in the up and down links and a 224 Gb/s same-mode bi-directional transmission is successfully realized. The crosstalk between the VMs in l = 0 and l = +2 of this full-duplex system is less than -13.8 dB. To strengthen the immunity to performance degradation induced by connector reflection and back scattering, we propose an effective approach to mitigate impairments by using hetero-modes on two terminals of the bi-directional system. Then, 2 VMs of l = 0 and 2 VMs of l = +2 are respectively employed in the up and down streams. The channel isolation between the VMs in l = 0 and l = +2 of such full-duplex link is larger than 19 dB, which supports a 448 Gb/s bi-directional transmission with 28 GBaud 16-ary quadrature amplitude modulation (QAM) modulation over a 3 km FMF by using 2 × 2 MIMO. Moreover, mode-wavelength division multiplexing including 2 modes and 4 wavelengths in both up and down streams is implemented in the transmission system. A total capacity of the 1.792 Tb/s link with 28 GBaud 16-QAM signal over each channel is successfully realized over the 3 km FMF. The measured bit-error-ratios (BERs) of all channels are below the 7% hard decision forward error correction (FEC) threshold at 3.8 × 10-3. The experimental results adequately indicate that such a scheme has a great potential in high-speed bi-directional point-to-point (P2P) optical interconnects.

Demonstration of terabit coherent on-chip optical interconnects employing mode-division multiplexing.

We experimentally demonstrate a net capacity per wavelength of 1.23 Tb/s with 30 GBaud 16-ary quadrature amplitude modulation (16-QAM) mode-division multiplexing (MDM) signals over a single silicon-on-insulator (SOI) multimode waveguide for optical interconnects employing $11 \times 11$ multiple-in-multiple-out (MIMO) digital signal processing. In order to simplify the receiver architecture for coherent optical interconnects, we further propose and evaluate an on-chip self-homodyne coherent detection (SHCD) scheme. In the experiment, 30 Gbaud quadrature phase shift keying (QPSK) signals carried by 10 waveguide modes are successfully recovered with bit error rates (BERs) below 7% forward error correction (FEC) threshold using the pilot tone delivered by ${{\rm TE}_0}$ mode as a local oscillator. Around 10% penalty on error vector magnitude (EVM) is observed due to modal cross talk compared to homodyne detection.

On the Ring Resonator-Based Dispersion Compensation Method for Analog 5G/B5G Mobile Fronthauling

We present a novel technique for dispersion compensation by exploiting the filtering properties of a fully reconfigurable serially coupled microring resonator (MRR) structure in analog intermediate frequency-over-fiber/mmWave (IFoF/mmWave) fiber/wireless 5G and beyond 5G(B5G)-oriented mobile fronthaul. To this extend, we demonstrate simulation and experimental results on analog photonic processing of 1 Gbaud quadrature phase shift keying (QPSK) modulated radio signal. By tuning filter's bandwidth in the range of 14–69 GHz, and resonance within a resonance free spectral range (FSR) range, dispersion induced power fading mitigation was achieved, for a set of widely adopted intermediate frequencies (IFs) and transmission distances. Analytical calculations carried out in MATLAB environment, highlighting significant improvement of transmission system output power, whereas are verified via transmission system simulations, resulting to error vector magnitude (EVM) improvement up to ∼ 30%. Experimental results verified both analytical and transmission simulation results for a series of IFoF downlink transmission system configurations. For widely used wireless transmission configuration at 8.5 GHz IF, by tuning the filter to appropriate bandwidth of 14 GHz and resonance peak properly, successful fiber wireless transmission for over 25 km of fiber was demonstrated, achieving 16.9% EVM for QPSK, below the 3GPP (<17.5%) requirements for 5G NR.

Laser linewidth tolerance for nonlinear frequency division multiplexing transmission with discrete spectrum modulation.

Although fruitful investigations of carrier phase estimation (CPE) have been conducted for a traditional coherent fiber optical transmission, there are few studies on the CPE for a nonlinear Fourier transform (NFT) based transmission. A laser linewidth induced phase noise leads to a phase rotation of the nonlinear spectra and the scattering data, which is similar to its effect on the linear spectra. Here, we first identify that both feed forward the M-th power, and the blind phase search (BPS)-based CPE can function well in the nonlinear frequency division multiplexing (NFDM) transmission with discrete spectrum modulation. Then, a performance comparison between two CPE schemes is presented for various modulation formats under the scenario of a single eigenvalue NFDM transmission. Our simulation results indicate that the laser linewidth tolerances of 2 GBaud quadrature phase shift keying (QPSK), 8-phase shift keying (8-PSK), and 16-amplitude phase shift keying (16-APSK) are 2.3 MHz, 1.05 MHz, and 250 KHz, respectively, given a 1-dB optical signal to noise ratio (OSNR) penalty at BER = 10-3. Finally, the BPS algorithm is experimentally verified under the same scenario of a 2 GBaud back-to-back transmission, due to the use of a semiconductor laser with a 100 KHz linewidth.

5 Gbaud QPSK coherent transmission in the mid-infrared

We experimentally demonstrate a high-speed mid-infrared (MIR) transmission system with a 5 Gbaud quadrature phase shift keying (QPSK) modulation over free space optical (FSO) link. The MIR system operates through homemade robust and compact MIR transmitter and receiver devices, which has the ability to achieve wavelength conversion between C-band and MIR based on difference frequency generation (DFG) process. The wavelength and power of MIR signal generated from MIR transmitter are 3.57 dBm and 3594 nm, and the C-band signal regenerated from MIR receiver are −26.12 dBm and 1550.116 nm. Both the constellation diagrams and eye diagrams of regenerated C-band signal are clear when the input power of C-band receiver is higher than −40 dBm. Compared with the back to back (BTB) transmission system, the power penalty of the MIR transmission system is less than 2.8 dB measured at bit error ratio (BER) of 1×10−7. It can be concluded from the experimental results that the MIR transmission system supports high-speed data rate, variable wavelength and high-order modulation format, and has the potential to be applied to FSO communications or satellite communications in atmospheric environments.

Continuous delay tunability using a combination of three types of fiber Bragg gratings, wavelength conversion, and wavelength multicasting with a frequency comb

We experimentally demonstrate a delay subsystem that utilizes a combination of three types of fiber-Bragg gratings (FBGs), wavelength conversion, and wavelength multicasting using a frequency comb source. While the linearly chirped FBG together with a channelized dispersion compensation one, provide the continuous delay, an array of standard FBGs utilizes the multicast copies to extend the delay in steps determined by the array geometry. Following the characterization of the group-delay dispersion performance of the FBGs, a 10 Gbaud quadrature phase-shift keying (QPSK) signal is continuously delayed by 2.2 ns and discretely delayed in 3 steps of ∼2 ns, to provide a total gap-less delay of 8.1 ns. The delayed signal is finally recovered to its original wavelength with an optical signal-to-noise ratio (OSNR) penalty of ∼2.3 dB. Furthermore, we show that the penalty can be reduced to 1.2 dB after equalization and report delaying a 20 Gbaud QPSK signal. The demonstrated high-quality delay performance is quite scalable. The continuous delay can be extended to tens of nanoseconds, limited only by the performance of the used components, while the discrete delay is determined by the number of multicast copies and propagation distance between the reflections in the array of FBGs.

Integrated Photonic Filters in Support of Converged 5G Mobile Fronthaul & Midhaul Transport Layers

ABSTRACTA Wavelength Division Multiplexing (WDM)-enabled analog mobile fronthaul architecture is presented supporting enhanced flexibility by exploiting the fine filtering properties of a fully reconfigurable, silicon photonics-based filtering element. Numerical studies are presented emphasizing on the flexibility, low-energy consumption, and the compatibility of our proposed chip-scale implementations with the analog Radio over Fiber (RoF)-based mobile transport. Proof-of-concept analog photonic processing of 1 Gbaud Quadrature Phase Shift Keying (QPSK)-modulated radio is successfully demonstrated through RoF transmission experiments. The integration of the analog mobile fronthauling segment in a WDM-enabled, Reconfigurable Optical Add/Drop Multiplexers (ROADM)-assisted fronthaul/midhaul infrastructure supporting low-latency edge computing is also discussed.

A novel in-band OSNR monitoring technique based on fractional Fourier transform of LFM signal

We propose a novel in-band optical signal to noise ratio (OSNR) monitoring technique in digital coherent receiver based on fractional Fourier transform (FrFT), which employs the power aggregation property of linear frequency modulation (LFM) signal in fractional domain. In our method, the LFM signal with extremely low power is added on the data signal. Thus, the received signal exists a peak in fractional domain, which can be used to indirectly differentiate the data signal power from noise thereby calculating the OSNR. Meanwhile, the proposed technique does not occupy additional spectrum resource and avoids any additional hardware on top of the standard digital coherent receiver. Experimental results of a 10 Gbaud quadrature phase-shift keying (QPSK) system show that the proposed technique is highly accurate, with the monitoring error less than 0.55 dB in the OSNR range of 9–29 dB for back-to-back transmission and 0.9 dB in the OSNR range of 11–25 dB for 960 km standard single mode fiber (SSMF) transmission.

All-optical OFDM demultiplexing with optical partial Fourier transform and coherent sampling.

We propose a novel scheme with a "time-lens"-based partial optical Fourier transform (OFT) and coherent sampling for high-speed complex orthogonal frequency-division multiplexing (OFDM) signal detection. Compared with all-optical OFDM demultiplexing with a matched optical filter, our proposed method replaces specialized optical filters with commercially available equipment, which relaxes stringent manufacturing and operational requirements. Our simulation shows that even with a partial OFT, theoretically, close to inter-channel interference-free performance is possible. In addition, we performed a proof-of-concept experiment of 16×10 Gbaud quadrature phase-shift keying (QPSK) all-optical OFDM detection, with all the bit error rates far below the 7% hard-overhead forward error correction limit.

Phase-sensitive QPSK channel phase quantization by amplifying the fourth-harmonic idler using counter-propagating Brillouin amplification

We experimentally demonstrate all-optical phase-sensitive quantization of a 10–20 Gbaud quadrature phase-shift keying (QPSK) signal’s phase without an active phase-locked loop (PLL) by utilizing Brillouin amplification (BA) to amplify the fourth-harmonic idler without path separation, allowing the pump, signal, third-harmonic idler and amplified fourth-harmonic idler to remain synchronized in the phase. We observe up to 65% reduction in phase noise and 4.6 dB gain at a bit-error rate (BER) of 10−4 over 36 min of continues operation.

Frequency-agile vector signal generation based on optical frequency comb and pre-coding

In this paper, we experimentally demonstrate the generation of frequency-agile vector signals based on an optical frequency comb (OFC) and unbalanced pre-coding technology by employing a dual-driven Mach-Zehnder Modulator (DD-MZM) and an intensity modulator (IM). The OFC is generated by the DD-MZM and sent to the IM as a carrier. The IM is driven by a 5 GHz 2 Gbaud quadrature phase-shift keying (QPSK) vector signal with unbalanced pre-coding. The −1st order sideband of one OFC line and the +1st order sideband of another OFC line are selected by a programmable pulse shaper (PPS), after square-low photodiode detection, the frequency-agile vector signal can be obtained. The results show that the 2 Gbaud QPSK vector signals at 30 GHz, 50 GHz, 70 GHz and 90 GHz can be generated by only pre-coding once. It is possible to achieve a bit-error-rate (BER) below 1e-3 for wireless transmissions over 0.5 m using this method.

Full quadrature regeneration of QPSK signals using sequential phase sensitive amplification and parametric saturation.

We demonstrate all-optical regeneration of both the phase and the amplitude of a 10 GBaud quadrature phase shift keying (QPSK) signal using two nonlinear stages. First we regenerate the phase using a wavelength converting phase sensitive amplifier and then we regenerate the amplitude using a saturated single-pump parametric amplifier, returning the signal to its original wavelength at the same time. We exploit the conjugating nature of the two processing stages to eliminate the intrinsic SPM distortion of the system, further improving performance.

Graphene-assisted nonlinear optical device for four-wave mixing based tunable wavelength conversion of QPSK signal

We fabricate a nonlinear optical device based on a fiber pigtail cross-section coated with a single-layer graphene grown by chemical vapor deposition (CVD) method. Using such graphene-assisted nonlinear optical device, we experimentally demonstrate tunable wavelength conversion of a 10 Gbaud quadrature phase-shift keying (QPSK) signal by exploiting degenerate four-wave mixing (FWM) progress in graphene. We study the conversion efficiency as functions of the pump power and pump wavelength and evaluate the bit-error rate (BER) performance. The observed optical signal-to-noise ratio (OSNR) penalties for tunable QPSK wavelength conversion are less than 2.2 dB at a BER of 1 × 10(-3).

Repetition rate optimization of passively mode-locked fiber laser for high-speed linear optical sampling

Optical fiber communication systems are going to adopt the use of advanced modulation formats. It is thus important to develop measurement techniques and solutions capable of quantifying such signals. Linear optical sampling is an effective technique to characterize the quality of an advanced modulation format for high-speed optical signal with high fidelity, while the passively mode-locked fiber laser is an enabling module to implement the linear optical sampling. In this paper, we obtain a trade-off relationship between the repetition rate of passively mode-locked fiber laser and the linewidth of high-speed signal under test, after the introduction of operation principle for linear optical sampling. It is found that, for the quadrature phase shift keying (QPSK) signal, when the ratio of the linewidth of the signal under test to the repetition rate of passively mode-locked fiber laser is less than 1.5×10-3, the linear optical sampling-induced impairments can be ignored when there occurs phase noise. Therefore, the phase estimation can be successfully made by using the Viterbi-Viterbi algorithm applied to the block of samples corresponding to the modulation format phase states. Next, we use an optical sampling pulse with a repetition rate of 95.984 MHz, and carry out the optical linear sampling to a 28 Gbaud QPSK signal with a linewidth of 100 kHz. The error vector magnitude (EVM) has long been a commonly used parameter for quantifying the quality of advanced modulation signals. Using the standard coherent detection algorithm, we can successfully recover the constellation with the error vector magnitude (EVM) error less than 1%. Theoretical investigations agree well with the experimental characterization. Such a conclusion is helpful to optimize the design of passively mode-locked fiber laser for optical sampling application.

Low overhead slipless carrier phase estimation scheme.

Two slipless schemes are compared with application to single carrier 30 Gbaud quadrature phase shift keying (QPSK) system. An equivalent linewidth model considering the phase noise induced by both the laser linewidth and fiber nonlinearity is applied in the performance analysis. The simulation results show that it is possible to mitigate cycle slip (CS) using only 0.39% pilot overhead for the proposed blind carrier phase recovery (CPR) + pilot-symbols-aided phase unwrapping (PAPU) scheme within 1 dB signal-to-noise ratio (SNR) penalty limit at the bit error ratio (BER) of 10(-3) with 4 MHz equivalent linewidth.

Coherent correlator and equalizer using a reconfigurable all-optical tapped delay line

We experimentally demonstrate a reconfigurable optical tapped delay line in conjunction with coherent detection to search multiple patterns among quadrature phase shift keying (QPSK) symbols in 20 Gbaud data channel and also to equalize 20 and 31 Gbaud QPSK, 20 Gbaud 8 phase shift keying (PSK), and 16 QAM signals. Multiple patterns are searched successfully on QPSK signals, and correlation peaks are obtained at the matched patterns. QPSK, 8 PSK, and 16 QAM signals are also successfully recovered after 25 km of SMF-28 with average EVMs of 8.3%, 8.9%, and 7.8%. A penalty of <1 dB optical signal to noise penalty is achieved for a 20 Gbaud QPSK signal distorted by up to 400 ps/nm dispersion.