Single-mode Fiber Link Research Articles

Real-time dual-polarization transmission based on hybrid optical wireless communications

Abstract We present experimental work on a gigabit-capable and long-reach hybrid coherent UWDM-PON plus FSO system for supporting different applications over the same fiber infrastructure in the mobile backhaul (MBH) networks. Also, for the first time, we demonstrate a reconfigurable real-time DSP transmission/reception of DP-QPSK signals over standard single-mode fiber (SSMF) and FSO links. The receiver presented is based on a commercial field-programmable gate array (FPGA). The considered communication links are based on 20 UDWDM channels with 625 Mbaud and 2.5 GHz channel spacing. We are able to demonstrate the lowest sampling rate required for digital coherent PON by employing four 1.25 Gsa/s ADCs using an electrical front-end receiver that offers only 1 GHz analog bandwidth. We achieved this by implementing a phase and polarization diversity coherent receiver combined with the DP-QPSK modulation formats. The system performance is estimated in terms of receiver sensitivity. The results show the viability of coherent PON and flexible dual-polarization supported by software-defined transceivers for the MBH.

Fair comparison of complexity between a multi-band CAP and DMT for data center interconnects.

We present, to the best of our knowledge, the first known detailed analysis and fair comparison of complexity of a 56Gb/s multi-band carrierless amplitude and phase (CAP) and discrete multi-tone (DMT) over 80km dispersion compensation fiber-free single-mode fiber links based on intensity modulation and direct detection for data center interconnects. We show that the matched finite impulse response filters and inverse fast Fourier transform (IFFT)/FFT take the majority of the complexity of the multi-band CAP and DMT, respectively. The choice of the multi-band CAP sub-band count and the DMT IFFT/FFT size makes significant impact on the system complexity or performance, and trade-off must be considered.

Image-Free Microwave Photonic Down-Conversion Approach for Fiber-Optic Antenna Remoting

For commercial and military applications, it is highly desired to acquire broadband radio frequency (RF) signals from remotely located receiving antennas to the central office (CO) for centralized signal processing such as down-conversion. In this paper, a photonic approach for image-free microwave frequency down-conversion is proposed for antenna remoting scenarios. The RF signal is captured by the phase modulator (PM) at remote antenna unit and transmitted over a long fiber link to the CO for performing frequency down-conversion. In analog, with the Hartley architecture in the electronic domain, two in-phase (I) and quadrature (Q) intermediate frequency (IF) components are generated by using an electro-optic polarization modulator (PolM) and two polarizers. To achieve large image rejection ratio (IRR), the digital post-processing technique is introduced to accurately compensate the amplitude and phase imbalances between the I and Q IF signals. In the experiments, a 2-km single-mode fiber link is inserted between the PM and PolM. As the local oscillator signals are set as 35 and 3 GHz, two target sinusoidal signals at 35.5 and 3.1 GHz are applied with two image signals, a sinusoidal signal at 34.5 GHz and a broadband RF signal centered at 2.9 GHz, respectively. Then, both real-time analog and off-line digital processing methods are used to process the generated I and Q IF signals for image rejection, yielding an IRR over 45 and 60 dB respectively, when the sinusoidal image signal is applied. The distortions from the broadband 2.9-GHz image RF signal are also effectively suppressed by using the two processing methods. The proposed approach is capable of covering a wide frequency range from 5 to 40 GHz.

Low-Complexity Receiver Using Undersampling for Guard-Band SSB-DDO-OFDM

A guard-band-based and single-side-band (SSB) modulation scheme is usually employed to improve receiver sensitivity and avoid power fading for the direct-detection optical orthogonal frequency-division multiplexing (DDO-OFDM). In this paper, we take advantages of analog-to-digital converter (ADC) undersampling to achieve a low-complexity receiver for guard-band SSB-DDO-OFDM. By using an electrical bandpass filter, the signal-to-signal beating interference is removed from the guard-band, and the desired OFDM signal is sampled at a rate twice its signal bandwidth rather than its highest frequency component. The guard-band will be filled with the aliased components which can be used to fully recover the transmitted data, and the discrete Fourier transform size at the receiver side is reduced by more than half. As a result, the implementation complexity of the receiver can be greatly reduced with the lower ADC sampling rate and reduced digital signal processing complexity. The feasibility of the proposed low-complexity receiver is investigated by numerical simulations. When the quantization noise is not dominant, the simulations show that the proposed low-complexity guard-band SSB-DDO-OFDM has similar error vector magnitude performances with the conventional one over different bias voltages and single-mode fiber links.

Enhanced frequency-domain fractionally spaced equalization for coherent optical transmission system with colored noise

An enhanced minimum mean-square-error (MMSE) frequency-domain (FD) fractionally spaced equalizer (FSE) for polarization demultiplexing is proposed in the presence of colored noise. The 2-norm of the estimated channel matrix and polynomial fitting of equalization coefficients are introduced to mitigate the colored noise without requiring knowledge of the optical and electrical filtering. Simulation results show that ∼3-dB improvement of equalization performance is achieved compared to conventional MMSE FD-FSE in an 8×112-Gbit/s wavelength-division multiplexing polarization-division multiplexing-quadrature phase shift keying transmission system over a 1200-km single mode fiber link.

Evaluation of selective control information detection scheme in orthogonal frequency division multiplexing-based radio-over-fiber and visible light communication links

Block-level detection is required to decode what may be classified as selective control information (SCI) such as control format indicator in 4G-long-term evolution systems. Using optical orthogonal frequency division multiplexing over radio-over-fiber (RoF) links, we report the experimental evaluation of an SCI detection scheme based on a time-domain correlation (TDC) technique in comparison with the conventional maximum likelihood (ML) approach. When compared with the ML method, it is shown that the TDC method improves detection performance over both 20 and 40 km of standard single mode fiber (SSMF) links. We also report a performance analysis of the TDC scheme in noisy visible light communication channel models after propagation through 40 km of SSMF. Experimental and simulation results confirm that the TDC method is attractive for practical orthogonal frequency division multiplexing-based RoF and fiber-wireless systems. Unlike the ML method, another key benefit of the TDC is that it requires no channel estimation.

Adaptive Three-Dimensional Optimization for Optical Direct-Detection OFDM

We propose an adaptive three-dimensional (3-D) optimization including bit loading, power loading, and trellis-coded modulation for optical direct-detection orthogonal frequency division multiplexing (OFDM) in the fading channel. This approach could improve the bandwidth of the various devices such as electro-optical components, digital to analog convertors (DACs), and analog to digital convertors (ADCs) effectively. We conduct experiments to verify this 3-D optimization and compare the performance with that of 2-D optimization, i.e., bit loading and power loading only. Our experimental results show that the receiver sensitivity of 3-D optimization can be improved by 2 dB at the BER of 10 –3 compared with that of 2-D optimization in the fading channel. With the simple scheme we present of realizing the optimum 3-D optimization adaptively, the data rate is improved by 32% compared with using adaptive 2-D optimization in the fading channel. With the adaptive 3-D optimization, we also transmit a 16-GHz optical OFDM signal using a 10-GHz Mach–Zehnder (MZ) intensity modulator over a 20-km standard single-mode fiber link successfully. The modulation bandwidth of the MZ modulator is improved by 60% and the data rate is improved by 40%.

SSBI Mitigation and the Kramers–Kronig Scheme in Single-Sideband Direct-Detection Transmission With Receiver-Based Electronic Dispersion Compensation

The performance of direct-detection transceivers employing electronic dispersion compensation combined with DSP-based receiver linearization techniques is assessed through experiments on a 4 × 112 Gb/s wavelength-division multiplexing direct-detection single-sideband 16 quadratic-amplitude modulation Nyquist-subcarrier-modulation system operating at a net optical information spectral density of 2.8 b/s/Hz in transmission over standard single mode fiber links of up to 240 km. The experimental results indicate that systems with receiver-based dispersion compensation can achieve similar performance to those utilizing transmitter-based dispersion compensation, provided it is implemented together with an effective digital receiver linearization technique. The use of receiver-based compensation would simplify the operation of a fiber link since knowledge of the link dispersion is not required at the transmitter. The recently proposed Kramers–Kronig receiver scheme was found to be the best performing among the receiver linearization techniques assessed. To the best of our knowledge, this is the first experimental demonstration of the Kramers–Kronig scheme.

75 Gbit/s transmission over single mode fibre links using 8‐PAM and 8‐AACC modulations

Absolute added correlative coding with eight levels has been proposed and demonstrated numerically at bit rates of 30 and 60 Gbps back-to-back, and up to 75 Gbps over 2000 and 3000 m standard single-mode fibre using a single-drive multiple quantum-well Mach–Zehnder modulator. The transmission performance is compared with the conventional eight-level pulse amplitude modulation.

Training Symbol Assisted in-Band OSNR Monitoring Technique for PDM-CO-OFDM System

We proposed and demonstrated an in-band optical signal-to-noise ratio (OSNR) monitoring method for a polarization-division-multiplexed coherent optical orthogonal frequency-division-multiplexing (PDM-CO-OFDM) system. OSNR of the PDM signals on both polarization tributaries can be simultaneously monitored by calculating the electrical SNR with low complexity by taking advantage of the uncorrelation property between the noise and training symbol, which is used for conventional frame synchronization. This OSNR monitoring technique is verified in 103.57-Gb/s 16 quadrature amplitude modulation-based PDM CO-OFDM transmission system with 240—1440-km standard single-mode fiber (SSMF) links. Experimental results show that after calibration, the OSNR estimation error is less than 0.6 dB in a dynamic range of 12—27 dB, the estimation remains accurate even after 1440-km SSMF transmission, regardless of the chromatic dispersion and polarization mode dispersion in the fiber. To further investigate the robustness of the proposed OSNR monitoring method, additional simulation is implemented on VPI and MATLAB platform. It turned out to be insensitive to nonlinear effect within reasonable range as our monitoring technique shows good performance with launching power below −2 dBm after 500-km SSMF transmission. The proposed technique could still accurately monitor the individual OSNR of each polarization tributary of the PDM signal even in the presence of 5-dB polarization-dependent loss.

RF-pilot aided modulation format identification for hitless coherent transceiver.

We propose a RF-pilot aided modulation format identification (MFI) technique to enable a hitless flexible coherent transceiver with fast format switching. For the MFI, modulation format information is encoded to the amplitude of the RF-pilot, which can be simultaneously used for the compensation of both laser phase noise and fiber nonlinearity. The proposed MFI technique is able to identify arbitrary modulation formats including multi-dimensional formats and hybrid QAM formats. The high accuracy of the proposed MFI scheme is experimentally demonstrated without sacrificing the tolerance of both laser phase noise and fiber nonlinearity for various modulation formats up to dual-polarization (DP) 64QAM. Finally, over 2240 km standard single mode fiber (SSMF) link, we experimentally demonstrate a hitless coherent transceiver with a fast block-by-block modulation format switching enabled by the proposed MFI.

Real-Time Generation and Reception of OFDM Signals for $X$ -Band RoF Uplink With Heterodyne Detection

A field-programmable gate arrays-based real-time orthogonal frequency-division multiplexing (OFDM) transceiver is implemented for the uplink of an $X$ -band radio-over-fiber (RoF) system with heterodyne detection for the first time. Digital intermediate-frequency $I/Q$ modulation and demodulation, and forward error correction are applied. Minn’s timing synchronization method is slightly modified with reduced implementation complexity for the proposed OFDM-RoF system. By using Reed–Solomon code with symbol interleaving technique, the real-time transmission of the $X$ -band 16-ary quadrature amplitude modulation OFDM-RoF signal over 10-m wireless and 2.26-km single mode fiber link is successfully achieved with a post-FEC bit error rate below $1\times 10^{-9}$ .

Experimental Study of 1.55- $\mu$ m EML-Based Optical IM/DD PAM-4/8 Short Reach Systems

We experimentally evaluate high-speed intensity modulation/direct detection (IM/DD) transmissions with a 1.55- $\mu \text{m}$ broadband electro-absorption modulated laser and pulse amplitude modulations (PAM). We demonstrate 80 Gb/s/ $\lambda $ PAM-4 and 96 Gb/s/ $\lambda $ PAM-8 transmissions with low-complexity digital equalizers at the receiver. Performance comparison with different types of equalizers are performed, including linear symbol-spaced feed-forward equalizer (FFE), fractional (half-symbol) spaced FFE and decision feedback equalizer (DFE), with different tap number. It is found that for both cases, a 6-tap symbol-spaced FFE is sufficient to achieve a stable performance with bit-error-rate below the 7% overhead hard decision forward error correction (7%-OH HD-FEC) threshold over a 4 km standard single mode fiber link. Practical considerations including comparison between adaptive and static equalizer implementation and tolerable fiber chromatic dispersion are discussed.

Bandwidth Improvement Using Adaptive Loading Scheme in Optical Direct-Detection OFDM

In current metro and access optical networks, the frequency band beyond the bandwidth of the electro-optical components is always deprecated due to the fading frequency response. On the other hand, an orthogonal frequency division multiplexing (OFDM) system is managed on individual subcarrier and can adapt to the varied channel frequency response by optimally allocating different modulation formats and powers across different subcarriers. Therefore, to reuse the commercial low bandwidth components in high speed networks, we design modified OFDM signals, which are with adaptive power and bit allocation, to improve the bandwidth utilization and maximize the data rate. We successfully transmit 16-GHz optical OFDM signals over a 20-km standard single mode fiber link with a 10-GHz commercial Mach–Zehnder (MZ) intensity modulator and direct detection. Eleven modulation formats are used for bit loading and the maximum data rate can reach 88.8 Gb/s. The modulation bandwidth utilization of the MZ modulator is increased by 60%.

56 Gb/s multi-band CAP for data center interconnects up to an 80 km SMF.

We present the first (to the best of our knowledge) experimental demonstration of a 56 Gb/s multi-band carrierless amplitude and phase modulation (CAP) signal transmission over an 80-km single-mode fiber link with zero overhead pre-FEC signal recovery and enhanced timing jitter tolerance for optical data center interconnects.

Use of polarization freedom beyond polarization-division multiplexing to support high-speed and spectral-efficient data transmission

Increasing the system capacity and spectral efficiency (SE) per unit bandwidth is one of the ultimate goals for data network designers, especially when using technologies compatible with current embedded fiber infrastructures. Among these, the polarization-division-multiplexing (PDM) scheme, which supports two independent data channels on a single wavelength with orthogonal polarization states, has become a standard one in most state-of-art telecommunication systems. Currently, however, only two polarization states (that is, PDM) can be used, setting a barrier for further SE improvement. Assisted by coherent detection and digital signal processing, we propose and experimentally demonstrate a scheme for pseudo-PDM of four states (PPDM-4) by manipulation of four linearly polarized data channels with the same wavelength. Without any modification of the fiber link, we successfully transmit a 100-Gb s−1 PPDM-4 differential-phase-shift-keying signal over a 150-km single-mode fiber link. Such a method is expected to open new possibilities to fully explore the use of polarization freedom for capacity and SE improvement over existing fiber systems.

A High Spectral Efficiency Coherent RoF System Based on OSSB Modulation With Low-Cost Free-Running Laser Sources for UDWDM-PONs

A coherent radio over fiber (RoF) system based on optical single-sideband modulation with no optical carrier (OSSB) for ultradense wavelength division multiplexing passive optical networks (UDWDM-PONs) is proposed and experimentally demonstrated. For one channel of the UDWDM-PONs, at the transmitter, the OSSB RoF signal is generated using a dual parallel Mach–Zehnder modulator (DP-MZM) with a real-valued precoded microwave vector signal which is applied to the DP-MZM via the two electrodes with one having a 90° phase shift. Then, the OSSB RoF signal is sent to a coherent receiver over a single-mode fiber (SMF). The coherent receiver with a free-running laser source as a local oscillator (LO) is used to perform the coherent detection. To recover the standard microwave vector signals from the real-valued precoded microwave vector signal that is embedded in a strong phase noise introduced by both the transmitter laser source and the LO laser source, a digital signal processing algorithm is developed to perform phase noise cancellation. An experiment is performed. The transmission of a 1.25-Gbps quadrature phase shift keying, a 1.875-Gbps eight-phase shift keying (8-PSK), and a 2.5-Gbps 16 quadrature amplitude modulation (16-QAM) microwave vector signals with a channel spacing as narrow as 3 GHz over a 25-km SMF is experimentally demonstrated. For the transmission of the 2.5-Gbps 16-QAM microwave vector signal, the received optical sensitivity at forward error correction level over a 25-km SMF link is −24.8 dBm, while for the 1.875-Gbps 8-PSK microwave vector signal, it can reach −29.8 dBm which leaves an enough margin to accommodate the splitting losses in the optical distribution networks.

Field Trial of Long-Reach and High-Splitting λ-Tunable TWDM-PON

This paper presents the world's first field trial of a 40-km reach and over 512-split symmetric-rate 40-Gbit/s λ-tunable time and wavelength division multiplexing passive optical network (TWDM-PON) that utilizes our newly developed high-speed λ-tunable optical network unit transceivers and automatic gain controlled semiconductor optical amplifier-based PON extender for repeater and central office use. The upstream signals for C-band and downstream signals for L+-band in our developed λ-tunable TWDM-PON comply with the wavelength plan of ITU-T G.989 series. In a field trial, we successfully demonstrate error free λ-tunable transmission for the 40-km reach and 1024-split (maximum number) symmetric-rate 40-Gbit/s λ-tunable TWDM-PON over installed standard single-mode fiber links, and advanced network functions such as dynamic load balancing, incremental system upgrade, optical subscriber unit protection, and OLT equipment power saving are also confirmed.

1.55-μm Long-Wavelength VCSEL-Based Optical Interconnects for Short-Reach Networks

We demonstrate the generation and transmission of a 28-Gb/s intensity modulated optical signal over single mode fiber (SMF) links up to 1, 2, 5, and 10 km employing a 18-GHz 3-dB bandwidth monolithic vertical-cavity surface-emitting laser (VCSEL) based on buried tunnel junction technology with regrown n -doped InP material. Inexpensive technologies such as nonreturn-to-zero on–off keying modulation format, direct detection as well as digital signal processing-based receivers make the transmission feasible for chromatic dispersion (CD) uncompensated SMF links up to 10 km at the 7% hard-decision (HD) forward error correction (FEC) limit. Three different equalizers for the receiver side are investigated in this paper in terms of performance optimization, i.e., the maximum likelihood sequence estimation equalizer (MLSE), the feed-forward equalizer (FFE) and a hybrid scheme that comprises of an FFE and an MLSE equalizer (FFE/MLSE) in cascaded form. Our experimental results indicate that MLSE scheme outperforms the other two counterparts in all transmission scenarios. Performance optimization in terms of MLSE complexity is also presented. Less complex implementations like FFE equalizer can provide reliable transmission below the 7% HD-FEC limit for links up to 5 km. The FFE/MLSE scheme provides similar performance as MLSE for links up to 5 km, while its performance deteriorates for 10 and 15 km links as signal distortion due to CD becomes too severe to recover the signal. A timing recovery unit is employed before the equalizers in order to compensate for phase offsets and improve the signal quality. Our proposed scheme proves a promising candidate to enable VCSEL-based short reach optical interconnects in data centers and metro-access area.

Non-orthogonal optical multicarrier access based on filter bank and SCMA.

This paper proposes a novel non-orthogonal optical multicarrier access system based on filter bank and sparse code multiple access (SCMA). It offers released frequency offset and better spectral efficiency for multicarrier access. An experiment of 73.68 Gb/s filter bank-based multicarrier (FBMC) SCMA system with 60 km single mode fiber link is performed to demonstrate the feasibility. The comparison between fast Fourier transform (FFT) based multicarrier and the proposed scheme is also investigated in the experiment.