Photonic-assisted high-order vector millimeter-wave signal generation enabled by DSM.

We propose an optical dual-single-sideband (dual-SSB) modulated 16384-quadrature amplitude modulation (QAM) photonic vector millimeter-wave (mm-wave) signal generation scheme based on delta-sigma modulation (DSM). With the aid of the DSM, the severe nonlinear distortion of envelope detection for high-order QAM modulation signals in wireless communication can be effectively resolved. For the validation of our proposed scheme, we experimentally demonstrate the generation of a 40 GHz 16384-QAM orthogonal frequency division multiplexing (OFDM) photonic vector mm-wave signal and transmission over a 25-km standard single-mode fiber (SSMF), and a 1-m wireless link with the bit error ratio (BER) reaches the hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10-3.

A 2 µm Wavelength Band Low-Loss Spot Size Converter Based on Trident Structure on the SOI Platform.

A 2 µm wavelength band spot size converter (SSC) based on a trident structure is proposed, which is coupled to a lensed fiber with a mode field diameter of 5 µm. The cross-section of the first segment of the tapered waveguide structure in the trident structure is designed as a right-angled trapezoidal shape, which can further improve the performance of the SSC. The coupling loss of the SSC is less than 0.9 dB in the wavelength range of 1.95~2.05 µm simulated by FDTD. According to the experimental results, the lowest coupling loss of the SSC is 1.425 dB/facet at 2 µm, which is close to the simulation result. The device is compatible with the CMOS process and can provide a good reference for the development of 2 µm wavelength band integrated photonics.

0.017†nm, 143†ps passively mode-locked fiber laser based on nonlinear polarization rotation.

Mode-locked lasers with ultra-narrow spectral widths and durations of hundreds of picoseconds can be versatile light sources for a variety of newly emergent applications. However, less attention seems to be given to mode-locked lasers that generate narrow spectral bandwidths. We demonstrate a passively mode-locked erbium-doped fiber laser (EDFL) system that relies on a standard fiber Bragg grating (FBG) and the nonlinear polarization rotation (NPR) effect. This laser achieves the longest reported pulse width (to the best of our knowledge) of 143 ps based on NPR and an ultra-narrow spectral bandwidth of 0.017 nm (2.13 GHz) under Fourier transform-limited conditions. The average output power is 2.8 mW, and the single-pulse energy is 0.19 nJ at a pump power of 360 mW.

Digital mobile fronthaul based on delta-sigma modulation employing a simple self-coherent receiver.

The coherent digital radio-over-fiber (DRoF) system is a promising candidate for future mobile fronthaul networks (MFNs) due to its high receiver sensitivity and excellent robustness against nonlinearities. However, conventional coherent receivers with complicated structure and heavy algorithms are too expensive and power-hungry for cost-sensitive MFN applications. In addition, currently deployed digital MFNs based on common public radio interface (CPRI) suffer from low spectral efficiency and high data rate. Towards these issues we propose a novel DRoF downlink scheme employing a simple self-coherent receiver. In baseband unit (BBU), the radio signal is converted to a digital bit stream by a band-pass delta-sigma modulator (BP-DSM), which can be simply recovered with the utilization of a band-pass filter at the receiver. In remote radio unit (RRU), an electro-absorption modulated laser (EML) acts as a low-cost coherent homodyne receiver in virtue of injection locking technique. In the experiment, the injection-locked operation of the DSM signal is successfully achieved, and two modified schemes are proposed for the DSM signal to increase the locking range with a tolerable sensitivity penalty. The experimental results demonstrate the superiority of our approach in two aspects: 1) the EML-based coherent receiver outperforms a PIN photodiode in terms of receiver sensitivity; 2) compared to the analog RoF system, a 5-dB improvement in loss budget is obtained when DSM is employed with the aid of a simple equalizer.

SOA Amplified 100 Gb/s/λ PAM-4 TDM-PON Supporting PR-30 Power Budget with >18 dB Dynamic Range

Semiconductor optical amplifier (SOA) is considered an excellent candidate for power amplification at O-band due to its low cost and small footprint. In passive optical networks (PONs), SOA is popular as a booster and pre-amplifier to improve the link power budget. However, whether as a booster or pre-amplifier, SOA will induce different degrees of nonlinearity when the output power is high, which degrades the transmission performance of the system and leads to a limited receiver dynamic range. In this paper, we experimentally demonstrate the feasibility of using SOA in both transmitter and receiver sides for power budget improvement in 100 Gb/s/λ four-level pulsed amplitude modulation (PAM-4) time division multiplexed PON (TDM-PON) system at O-band. For compensating the linear and nonlinear impairments induced by transceivers and SOA, a look-up-table (LUT) pre-compensation at the optical line terminal (OLT) side and a simple feed-forward equalizer (FFE) at the optical network unit (ONU) side are adopted for downstream transmission. For upstream transmission, a 2nd-order Volterra nonlinear equalizer (VNLE) is utilized at the OLT side, and no pre-compensation is used at the transmitter of the ONU, which releases the digital signal processing (DSP) pressure of ONUs in a multi-user scenario. For the soft-decision FEC (SD-FEC) threshold (1 × 10−2), the IEEE PR-30 power budget requirement is met, and >18 dB dynamic range is achieved in both 25 km downstream and upstream transmission.

Demonstration of a 50G-PON with a 45-dB power budget using an IQ-interleaved coherent detection scheme.

The application of traditional coherent detection technology to optical access networks has been undermined due to its high complexity and high cost. In this paper, we propose a novel IQ-interleaved detection method which uses the preset frequency offset of the lasers at the transmitter and receiver to obtain the in-phase and quadrature components of the received signal. It keeps the simple structure of heterodyne detection and avoids the down-conversion process. Without Nyquist pulse shaping, the received signal bandwidth of the proposed scheme is theoretically 0.5B smaller than that of heterodyne detection for signal with a symbol rate of B. The 50-Gb/s NRZ transmission experiment proves that by using the proposed scheme, the receiving sensitivity and the frequency drift tolerance can be improved by ∼1 dB and 1 GHz compared with heterodyne detection under strong bandwidth limitation. Without pulse shaping, the receiving sensitivity, frequency drift tolerance (1-dB sensitivity penalty) and link power budget for 20-km fiber transmission are -31.8 dBm, 11 GHz and 43.5 dB, respectively. A higher power budget of 45 dB can be achieved when Nyquist pulse shaping is applied. The proposed scheme provides a low-complexity potential solution for a next-generation coherent PON.

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.

Carrier-less phase retrieval receiver leveraging digital upsampling.

Phase retrieval (PR) receivers can reconstruct the full electrical field of the signal using only intensity measurements without any optical carrier. In this Letter, we investigate the requirement of digital upsampling and receiver bandwidth of the PR receiver based on alternative projection employing a dispersive element. An iteration scheme averaging the interleaved upsampled symbols to maintain two samples per symbol for the estimated complex-valued signal is proposed and experimentally demonstrated with fast algorithm convergence. The PR uses a modified Gerchberg-Saxton algorithm. Experimentally, we measure Nyquist-shaped 30-GBaud quadrature phase shift keying signals after 55-km single-mode fiber transmission using only 110 and 250 iterations to reach, respectively, the 20% and 7% forward-error correction threshold levels.

Temporal and spectral coding over amplified spontaneous emission for secure optical coherent communications.

We demonstrate secure optical coherent communications employing low-coherence matched detection based on the randomness of amplified spontaneous emission (ASE) noise. Two-level physical-layer optical encryption is achieved through temporal and spectral coding over a broadband ASE source. An ASE-carried signal and unmodulated carrier are polarization multiplexed, transmitted over a same single-mode fiber (SMF), and separated with the aid of polarization tracking before having matched detection at the receiving side. The impact of chromatic dispersion on the low-coherence matched detection system is analyzed and experimentally investigated. We experimentally realize optically coded 20 Gbaud QPSK and 8-PSK signals transmission over a 43 km SMF span with a maximum line rate of 60 Gbits/s.

Transfer learning assisted deep neural network for OSNR estimation.

We propose a transfer learning assisted deep neural network (DNN) method for optical-signal-to-noise ratio (OSNR) monitoring and realize fast remodel to response to various system parameters changing, e.g. optical launch power, residual chromatic dispersion (CD) and bit rate. By transferring the hyper-parameters of DNN at the initial stage, we can fast response to the channel variation with fewer training set size and calculations to save consumptions. For feature extraction processing, we use amplitude histograms of received 56-Gb/s QPSK signals as the input for DNN at the initial stage, which shows the root mean squared error (RMSE) of OSNR estimation is less than 0.1 dB with the OSNRs ranging from 5 to 35 dB. Then, we change several system parameters and find superior capabilities of fast remodeling and data resource saving with the proposed method. The required training epochs have about four times reduction, and the required training set size is only one-fifth compared to retraining the network without any accuracy penalty. The DNN assisted by transfer learning can save resources and will be beneficial for real-time application on OSNR estimation.

Demonstration of symmetrical 50-Gb/s TDM-PON in O-band supporting over 33-dB link budget with OLT-side amplification.

Single wavelength 50 Gb/s passive optical network (PON) is an excellent candidate for meeting high capacity requirements. In this paper, we experimentally investigate a symmetrical 50 Gb/s time division multiplexed passive optical network (TDM-PON) system in the O-band based on 25G optics. Semiconductor optical amplifier (SOA) is used in optical line terminal (OLT) side to improve link power budget. We initially investigate the performances of SOA as a booster amplifier with different gain in the downstream and make a trade-off between receiver sensitivity and power budget. The performances of 50 Gb/s non-return-to-zero (NRZ) in the downstream with avalanche photodiode (APD) receiver and upstream with SOA-PIN receiver with different equalization schemes are evaluated. Experimental results show that up to 34.97 dB link power budget is achieved in the downstream direction with 7-tap feed forward equalization (FFE), and 33.76 dB link power budget is achieved in the upstream direction with only 3-tap FFE filtering.

Interconnecting data based on vortex beams by adjusting the ellipticity of a ring-core fiber.

In order to exchange data in a space-division multiplexing (SDM) system, a novel vortex-beam-based data interconnection concept, which is achieved by adjusting the ellipticity of a ring-core fiber, is proposed. A new ring-core fiber is also designed and fabricated for exchanging and propagating the data carried by first- or second-order vortex (orbital angular momentum) beams. The proposed scheme is not only analyzed and simulated in principle, but is also verified through experiments. The numerical results demonstrate that the vortex beams can be exchanged by appropriately adjusting the phase difference (with respect to the ellipticity of a ring-core fiber) between the even and odd vector modes. A new experimental platform is designed and established for the sake of investigating the feasibility of the proposed scheme. The experimental results are consistent with the results of the simulation, and demonstrate that the data carried by the first- or second-order vortex beams can be successfully switched with acceptable bit error rates (BERs) between the first-order vortex beams (L=1 or -1) or between the second-order vortex beams (L=2 or -2, left or right circular polarization), respectively. The measured BERs and constellation diagrams of 16-QAM are employed to evaluate the data exchange performance with respect to different cases (i.e., data exchange once or twice, and data exchange with or without crosstalk). The measured BERs and constellation diagrams also demonstrate that the performance degrades with increase in topological charge or crosstalk. The proposed scheme is flexible, simple, and reliable for data exchange in a SDM system.

Optical single side-band Nyquist PAM-4 transmission using dual-drive MZM modulation and direct detection.

We present the design and optimization of the optical single side-band (SSB) Nyquist four-level pulse amplitude modulation (PAM-4) transmission using dual-drive Mach-Zehnder modulator (DDMZM)modulation and direct detection (DD), aiming at the C-band cost-effective, high-speed and long-distance transmission. At the transmitter, the laser line width should be small to avoid the phase noise to amplitude noise conversion and equalization-enhanced phase noise due to the large chromatic dispersion (CD). The optical SSB signal is generated after optimizing the optical modulation index (OMI) and hence the minimum phase condition which is required by the Kramers-Kronig (KK) receiver can also be satisfied. At the receiver, a simple AC-coupled photodiode (PD) is used and a virtual carrier is added for the KK operation to alleviate the signal-to-signal beating interference (SSBI).A Volterra filter (VF) is cascaded for remaining nonlinearities mitigation. When the fiber nonlinearity becomes significant, we elect to use an optical band-pass filter with offset filtering. It can suppress the simulated Brillouin scattering and the conjugated distortion by filtering out the imaging frequency components. With our design and optimization, we achieve single-channel, single polarization 102.4-Gb/s Nyquist PAM-4 over 800-km standard single-mode fiber (SSMF).

Investigation on the equalization techniques for 10G-class optics enabled 25G-EPON.

High speed modulation based on bandwidth limited devices is desired for cost-effective PON capacity upgrade. In this paper, we investigate the equalization techniques for enabling 25-Gb/s transmission with 10G-class optics. A comparison between FFE and DFE based equalizer and MLSE based digital equalizer is made, where 13-tap FFE and 3-tap DFE are required to obtain similar performances with MLSE based detection. In addition, to verify the cost introduced by the ADC, the demand for the ADC parameters in the MLSE based detector, including the sampling rate, resolution, and timing jitter is investigated. Experimental results show that using a 25-GS/s ADC with 4-bit resolution, 25-Gb/s transmission is realized using 10-G TOSA and ROSA, and 28-/30-dB loss budget can be achieved in C-/O-band respectively.

Low-complexity joint symbol synchronization and sampling frequency offset estimation scheme for optical IMDD OFDM systems.

A low-complexity joint symbol synchronization and SFO estimation scheme for asynchronous optical IMDD OFDM systems based on only one training symbol is proposed. Numerical simulations and experimental demonstrations are also under taken to evaluate the performance of the mentioned scheme. The experimental results show that robust and precise symbol synchronization and the SFO estimation can be achieved simultaneously at received optical power as low as -20dBm in asynchronous OOFDM systems. SFO estimation accuracy in MSE can be lower than 1 × 10-11 under SFO range from -60ppm to 60ppm after 25km SSMF transmission. Optimal System performance can be maintained until cumulate number of employed frames for calculation is less than 50 under above-mentioned conditions. Meanwhile, the proposed joint scheme has a low level of operation complexity comparing with existing methods, when the symbol synchronization and SFO estimation are considered together. Above-mentioned results can give an important reference in practical system designs.