Directly Modulated Laser Research Articles

Review on the Millimeter-Wave Generation Techniques Based on Photon Assisted for the RoF Network System

With the development trend of wireless and broadband in the communication link and even the whole information industry, the demand of high-frequency microwave bandwidth has been increasing. The RoF network system solves the problem of spectrum congestion in low-frequency band by providing an effective technology for the distribution of high-frequency microwave signals over optical fiber links. However, the traditional mm-wave generation technique is limited by the bandwidth of electronic devices. It is difficult to generate high-frequency and low-phase noise mm-wave signals with pure electrical components. The mm-wave communication technology based on photon assisted can overcome the bandwidth bottleneck of electronic devices and provide the potential for developing the low-cost infrastructure demand of broadband mobile services. This paper will briefly explain the characteristics of the RoF network system and the advantages of high-frequency mm-wave. Then we, respectively, introduce the modulation schemes of RoF mm-wave generation based on photon assisted including directly modulated laser (DML), external modulation, and optical heterodyne. The review mainly focuses on a variety of different mm-wave generation technologies including multifrequency vector mm-wave. Furthermore, we list several approaches to realize the large capacity data transmission techniques and describe the digital signal processing (DSP) algorithm flow in the receiver. In the end, we summarize the RoF network system and look forward to the future.

Improved polar decoding for optical PAM transmission via non-identical Gaussian distribution based LLR estimation.

In this paper, an improved polar decoder based on non-identical Gaussian distributions is proposed and experimentally demonstrated for optical pulse amplitude modulation (PAM) interconnection. The principle of the polar coded PAM system is illustrated theoretically and the non-identical Gaussian distributions based log-likelihood ratio (LLR) estimation is introduced in the polar decoder to mitigate nonlinearity. Transmission systems of 28-Gbaud 4-level pulse amplitude modulation (PAM-4) and 8-level pulse amplitude modulation (PAM-8) based on commercial 10-GHz directly modulated laser (DML) are both demonstrated over 10-km standard single-mode fiber (SSMF) in C-band without dispersion compensation. Experimental results show that, aided by the improved polar decoder, the channel nonlinearity can be taken into consideration and additional sensitivity gains of 0.7 dB and 1 dB are respectively achieved compared with traditional polar decoder for PAM-4 and PAM-8 systems.

Mitigation of optical multipath interference impact for directly detected PAMn system.

This paper investigates solutions for mitigating multipath interference (MPI) impact for intensity modulation direct detection systems, particularly for PAM4 systems. We propose a scheme that uses a high pass filter (HPF) at the receiver to remove the MPI induced carrier-carrier beat noise and therefore improve the MPI tolerance. The scheme has been verified via theoretical analysis, numerical simulation, and experiment. For a typical optical transmitter based on Mach-Zehnder modulator (MZM) without frequency chirp, over 5dB MPI tolerance can be improved for 106.4Gb/s PAM4 by using the proposed scheme. Investigation was also extended to electro-absorption modulated laser (EML) and directly modulated laser (DML) based optical transmitters. Results show that transient chirping in EML reduces the effectiveness of proposed MPI mitigation scheme. For EML with chirp factor smaller than 1.5, MPI tolerance gain of 2dB is still achievable. However, the scheme is ineffective for typical DML where the adiabatic chirp significantly broadened the laser linewidth.

Employing a Kramers–Kronig receiver with a directly-modulated laser

The Kramers–Kronig (KK) receiver has been used for dispersion compensation in direct detection systems employing external-modulator-based transmitters. In this paper, we investigate the use of the KK receiver for dispersion compensation in a direct detection system based on a directly-modulated laser. We show the validity of the KK receiver by considering a linear contribution to the optical phase arising from a numerical solution of the laser rate equations as a frequency shift of the optical spectrum. The modulated optical signal can then meet the minimum-phase condition required by the KK algorithm. A system model is presented based on the direct modulation of a 10 Gb/s distributed feedback laser with 16-ary quadrature amplitude modulation (16-QAM) subcarrier modulation. The modulated signal is optically filtered, and a 53.5 Gb/s vestigial sideband signal is transmitted over 65 km of standard single-mode fiber. Dispersion compensation is applied after direct detection with a KK receiver. In addition, the impact of the optical filter parameters and receiver sampling rate on system performance are investigated.

Nonlinear Equalization Based on Artificial Neural Network in DML-Based OFDM Transmission Systems

This article reports the application of an equalizer based on an artificial neural network (ANN), in the form of nonlinear waveform regression, to mitigate nonlinear impairments in directly modulated laser (DML)-based orthogonal frequency-division multiplexing (OFDM) optical transmission. Experiments involving transmission over 0-200 km demonstrate that using an ANN with one hidden layer can greatly reduce nonlinear distortion. The proposed scheme outperformed a Volterra nonlinear equalizer at transmission distances exceeding 25 km. Using a 10G-class DML, the proposed scheme achieved the following data rates: 39.2 Gbps at 100 km (an improvement of 59%) and 33.5 Gbps at 150 km (an improvement of 57%). We also modified the cost function of the ANN during the training procedure to overcome the poor signal-to-noise ratio of the original ANN at low frequencies. This resulted in >30-Gbps transmission over 0-200 km.

Generalized model of optical single sideband generation using dual modulation of DML and EAM.

The dual modulation of a directly modulated laser (DML) and an electro-absorption modulator (EAM) is known to be an effective way to generate an optical single sideband (SSB) signal in highly compact and cost-effective fashions. Two theoretical models were developed for this dual modulation scheme under the assumptions of negligible intensity modulation of DML and/or zero chirp of EAM. In this paper, we develop a generalized model of dual modulation scheme without those assumptions. We show theoretically that the previous models can be unified as special cases of our generalized model. We evaluate the validity of our model using numerical simulations and experiments. The results show that our model estimates the modulation conditions for optical SSB generation accurately over wide ranges of modulation frequency and EAM's chirp. We also show experimentally that we can achieve an optical sideband suppression ratio higher than 40 dB by using our generalized model. This is >25 dB higher than the ratios obtained from the previous models.

Blind nonlinearity mitigation of 10G DMLs using sparse Volterra equalizer in IM/DD PAM-4 transmission systems

Abstract The nonlinear characteristics of the electro-optic components are the major limiting factor to realize a multi-level signal transmission. Cascaded multi-modulus algorithm (CMMA) is a popular blind equalization scheme that can compensate for only linear distortions without using any training sequence. In this paper, a modified CMMA is proposed to blindly adapt the tap weights of a receiver-side nonlinear Volterra equalizer (NLVE) to mitigate both linear and nonlinear distortions. Furthermore, a nonlinear sparse Volterra equalizer (NLSVE) is suggested in this paper that can achieve similar performance with a substantial reduction of complexity, in comparison with an NLVE. Experimental results for a 25 Gb/s PAM-4 transmission in the 1310-nm band over a 20-km single-mode fiber (SMF-28e) using a commercial 10 GHz-class directly modulated laser (DML) for an intensity modulation direct-detection (IM/DD) optical access network verify that the proposed NLSVE can achieve a gain of ~ 2-dB in comparison with a traditional CMMA.

Cascade recurrent neural network-assisted nonlinear equalization for a 100 Gb/s PAM4 short-reach direct detection system.

We propose a novel, to the best of our knowledge, cascade recurrent neural network (RNN)-based nonlinear equalizer for a pulse amplitude modulation (PAM)4 short-reach direct detection system. A 100 Gb/s PAM4 link is experimentally demonstrated over 15 km standard single-mode fiber (SSMF), using a 16 GHz directly modulated laser (DML) in C-band. The link suffers from strong nonlinear impairments which is mainly induced by the mixture of linear channel effects with square-law detection, the DML frequency chirp, and the device nonlinearity. Experimental results show that the proposed cascade RNN-based equalizer outperforms other feedforward or non-cascade neural network (NN)-based equalizers owing to both its cascade and recurrent structure, showing the great potential to effectively tackle the nonlinear signal distortion. With the aid of a cascade RNN-based equalizer, a bit-error rate (BER) lower than the 7% hard-decision forward error correction (FEC) threshold can be achieved when the receiver power is larger than 5 dBm. Compared with traditional non-cascade NN-based equalizers, the training time could also be reduced by half with the help of the cascade structure.

Coherent Dual-Band Radar-Over-Fiber Network With VCSEL-Based Signal Distribution

Today, the advent of radar sensing in applications requiring ever higher precision, such as automotive and industrial monitoring, needs the development of systems with superior resolution, stability and accuracy on 2D/3D spaces. Multiple-input–multiple-output (MIMO) radar systems have been studied for more than twenty years, proving to achieve excellent resolutions by merging simultaneous observations from sparse multiple radars. In addition, long-term frequency/phase coherence among distributed signals to and from remote and widely distributed radar frontends, enables a centralized architecture where, in a common base station (BS) site, all the received signals can be coherently processed in synergy, allowing to further improve detection, localization, and imaging capabilities. Radio-over-fiber (RoF) solutions represent an enabling technology for guaranteeing coherence among all radar signals coherently generated in the BS and distributed/collected to/from the radar heads by means of optical fiber links. Despite the unprecedented frequency-agility provided by photonics-based radiofrequency (RF) up-/down-conversion in the optical domain, for applications requiring massive low-cost production and power efficiency, such as advanced driver-assistance systems, it might be convenient to transmit and receive conventionally generated RF signals, and to limit the use of photonics, as in RoF, to RF signal distribution through cheap high-capacity optical links, in order to avoid EM interference, minimize loss, signal distortion and link encumbrance, still keeping the coherence among distributed signals. In this work, we present a MIMO radar-over-fiber (RaoF) network, where a common BS transmits and receives RF signals through optical standard single-mode fiber (SSMF), to and from two remote radar transmitters (TXs) and four receivers (RXs), all remote with respect to the BS and distributed on a 3 m-long baseline, and employing cheap patch antennas. Electro-optic conversion is achieved through direct modulation (DM) of low-cost, power-effective, and high-speed vertical-cavity surface-emitting lasers (VCSELs) working in the 1.3 µm-wavelength regime. Such a wavelength ensures propagation over SSMF with negligible distortions up to the km-range. The system is tested in a down-scaled indoor scenario and an RF carrier of 8.5 GHz is used, due to the limited frequency response of the employed VCSELs. System performance is evaluated with 1.4 GHz-bandwidth signals, comparing non-coherent and coherent signal elaboration. In addition, dual-band operation in the same RF region is tested, confirming that VCSEL-based optical links are a viable way to guarantee phase coherence among the distributed signals and among multiple frequency bands. Coherent MIMO processing also confirm the potential to achieve superior range and cross-range resolution than non-coherent data fusion.

Amplifier-less transmission of beyond 100-Gbit/s/λ signal for 40-km DCI-Edge with 10G-class O-band DML

In this article, we experimentally demonstrate the amplifier-less transmission of beyond 100-Gbit/s signal with advanced modulation formats including discrete multi-tone (DMT) and pulse amplitude modulation (PAM) in an intensity modulation direct detection (IM-DD) system for 40-km datacenter interconnects edge (DCI-Edge) utilizing a low cost commercial 10G-class O-band directly modulated laser (DML). During the transmission of DMT signal, Chow Cioffi Bingham algorithm-based bit-loading (CCB-BL) and discrete-Fourier-transform spread (DFT-spread) techniques are both discussed in the bandwidth-limited communication system. For the PAM single carrier communication system, look-up table (LUT) pre-distortion and time-domain pre-equalization schemes are jointly employed at the transmitter side to mitigate nonlinear distortions and insufficient bandwidth induced impairments, respectively. The results show that 105-Gbit/s DFT-spread DMT, 115-Gbit/s CCB-BL DMT, and 120-Gbit/s PAM-8 signals are successfully transmitted over 40-km single mode fiber (SMF) with the bit-error-rate (BER) below hard decision forward error correction (HD-FEC) threshold of 3.8 × 10–3. 120-Gbit/s DFT-spread DMT and CCB-BL DMT signals allocated within 20-GHz bandwidth can be successfully transmitted over 20-km SMF, and the receiver sensitivity of DFT-spread DMT outperforms CCB-BL DMT by 1.5-dB at the BER of 3.8 × 10–3. Due to the flexibility of bit-loading technique, when the allocated bandwidth of 120-Gbit/s CCB-BL DMT signal extends from 20-GHz to 25-GHz, the required received optical power (ROP) of CCB-BL DMT can be lower than DFT-spread DMT by 1-dB at the BER of 3.8 × 10–3 over 20-km SMF transmission. In 120-Gbit/s PAM-8 system, LUT pre-distortion technique improves the system receiver sensitivity by more than 1-dB over 40-km SMF transmission. This is the first time to discuss and implement different advanced modulation formats to achieve 100-Gbit/s/λ transmission in a 40-km amplifier-less IM-DD system with a low-cost O-band 10G class DML. Both CCB-BL DMT and LUT pre-distortion assisted PAM-8 can achieve a BER lower than HD-FEC threshold of 3.8 × 10–3 when the net rate per optical carrier is higher than 100-Gbit/s, which are promising solutions for 40-km four-lane 400-GbE DCI-Edge applications.

Multi-Carrier 5G-Compliant DML-Based Transmission Enhanced by Bit and Power Loading

The introduction of 5G enhanced mobile broadband (eMBB) services has brought unprecedented bit-rate demands to the optical transport infrastructure, which can hardly be supported through cost-effective digital fronthauling solutions. This opens up a whole new opportunity for bandwidth-efficient radio-over-fiber (RoF) analog fronthaul transmission, which avoids the well-known bandwidth multiplication issue associated with the digitization of radio signals. Using a low-cost directly-modulated laser (DML) with less than 3 GHz bandwidth and a standard PIN photodetector, we demonstrate the transmission of a carrier-aggregated 5G downlink signal over up to 25 km of single-mode fiber (SMF). Resorting to the use of intermediate-frequency-over-fiber (IFoF), we enable the transmission of 5G-compatible signals composed of up to 12 aggregated 400 MHz component carriers (CCs), resulting in a total radio bandwidth of 4.8 GHz, corresponding to >300 Gbps CPRI-equivalent downlink data-rate. 5G-compliant EVM performance across all CCs is achieved through the optimization of power and bit loading between component carriers, enabling an aggregated end user data-rate of 15.6 Gbps.

RZ Line Coding Scheme With Direct Laser Modulation for Upgrading Optical Transmission Systems

Abstract This study reports the efficient use of a direct laser modulated response measured with the return-to-zero coding scheme in optical transmission systems. The measured direct laser modulated response has a bit rate of 1.2 Gbps for an optical fiber cable of a transmission length of 10 km. The eye diagram analyzer is used to calculate the maximum quality factor and minimum bit error rate of the proposed model. The maximum quality factor is 531.2, and the minimum bit error rate tends to zero for the same optical fiber channel length compared with that of the previous model. The proposed model provides better results than the previous model. The figures of the proposed model are more stable than those of its previous counterpart. Bit error rate approximately tends to zero in the proposed model.

Demonstration of 4 × 100 Gbit/s PAM-4 Transmission Over 40 km in an IM/DD System Based on Narrow Band DMLs

We demonstrate a four-lane wavelength division multiplexing (WDM) intensity modulation and direct detection (IM/DD) system at O-band. The 3-dB bandwidth of directly modulated lasers (DMLs) in this experiment is only 15 GHz. To support 100-Gbit/s/lane PAM-4 transmission in each lane, digital pre-equalization and advanced receiver-side DSPs are adopted to compensate for bandwidth limitation. Moreover, semiconductor optical amplifier (SOA) is utilized at the receiver to compensate for the optical power loss during fiber transmission so that 40-km transmission distance can be achieved. To the best of our knowledge, it is the first time to successfully transmit 400-Gbit/s PAM-4 signals over 40-km SSMF with 15-GHz DMLs.

Unsupervised Learning for Neural Network-Based Blind Equalization

For the first time, an unsupervised learning training method was proposed for neural network (NN)-based blind equalization in intensity modulation and direct detection (IMDD) system. The whole scheme can directly train an NN-based equalizer with only the received signal rather than original symbols as reference. Besides, the unsupervised learning can also help a well-trained NN to keep its performance in face of varying system status, such as wavelength shift and bias fluctuation, in practical applications. We evaluate the performance of the proposed scheme in a 50 Gb/s IMDD system. The experimental results confirm that the proposed unsupervised learning can train fully-connected NN-based equalizer as well as convolutional neural network (CNN)-based equalizer to reach the same performance as the one trained by supervised learning. Besides, in face of signals with different bias current of the directly-modulated laser (DML), the unsupervised learning method can train the NN to keep the best performance. It can be proved that the scheme can help maintain the performance of NN-based equalizer against the continuous system parameter variations.

Latency-efficient code-division multiplexing (CDM) based carrier aggregation for 5G NR mobile fronthaul

We propose and experimentally verify a latency-efficient code-division multiplexing (CDM) based carrier aggregation (CA) scheme with simultaneous mitigation of peak-to-average power ratio (PAPR), for the scenario of 5G new radio (NR) mobile fronthaul (MFH). We utilize the least squares approximation (LSA) algorithm to reduce 36% number of iterations for achieving the same PAPR mitigation, in comparison with the traditional CDM based CA scheme. Finally, we experimentally demonstrate the proof-of-concept transmission of 16×100 MHz 5G NR signals mapped with 64 quadrature amplitude modulation (QAM) having the common public radio interface (CPRI) equivalent date rate of 78.6 Gb/s, based on the use of a directly modulated laser (DML). After 10 km standard single mode fiber (SSMF) transmission, we can secure both the latency reduction of CA processing and the PAPR mitigation with almost no penalty of receiver sensitivity.

Differential Phase-Shift QKD in a 2:16-Split Lit PON with 19 Carrier-Grade Channels

We investigate the practical network integration of differential phase shift quantum key distribution following a cost-optimized deployment scheme where complexity is off-loaded to a centralized location. User terminal equipment for quantum state preparation at 1 GHz symbol rate is kept technologically lean through use of a directly-modulated laser as optical encoder. Integration in a passive optical network infrastructure is experimentally studied for legacy and modern optical access standards. We analyze the implications that result from Raman scattering arising from different spectral allocations of the classical channels in the O-, S-, C- and L-band, and prove that the quantum channel can co-exist with up to 19 classical channels of a fully-loaded modern access standard. Secure-key generation at a rate of 5.1 times 10e-7 bits per pulse at a quantum bit error ratio of 3.28 percent is obtained over a 13.5 km reach, 2 to 16 split passive network configuration. The high power difference of 93.8 dB between launched classical and quantum signals in the lit access network leads to a low penalty of 0.52 percent in terms of error ratio.

Nonlinear Equalizer Based on Absolute Operation for IM/DD System Using DML

The performance of high-speed intensity-modulation (IM)/direct-detection (DD) system implemented by using O-band directly modulated laser (DML) could be limited by the waveform distortions induced by nonlinear modulation dynamics of DML. The conventional polynomial nonlinear equalizer (PNLE) can be used at the receiver to compensate for such waveform distortions. However, this nonlinear equalizer suffers from high implementation complexity. In this letter, we propose nonlinear equalizers based on the absolute operation for IM/DD system using DML. We replace the square operation in PNLE with the absolute operation to lower the implementation complexity. The effectiveness of the proposed equalizers is evaluated over 100-Gb/s 4-ary pulse amplitude modulation transmission system implemented by using 1.31- $\mu \text{m}$ DMLs. The results show that the proposed equalizers perform similar to the PNLEs, but lower the implementation complexity.

Impact of Analog and Digital Pre-Emphasis on the Signal-to-Noise Ratio of Bandwidth-Limited Optical Transceivers

The ever-growing machine-to-machine traffic in data centers has stimulated the increase of transceiver data rate from 25 Gb/s/λ to 100 Gb/s/λ and beyond. It is believed that advanced modulation formats and digital-to-analog converters (DACs) will be employed in next generation short-reach transceivers. Digital pre-emphasis techniques are widely employed in DAC-based transceivers to compensate for the high frequency roll-off due to the RF and optoelectronics components in optical transceivers. However, digital pre-emphasis essentially reduces the magnitude of the signal low frequency components for a flat frequency response, which unavoidably increases quantization error, reducing the overall signal-to-noise ratio. This trade-off between SNR and bandwidth conflicts with the high SNR requirement of advanced modulation formats such as the Nyquist-shaped pulse amplitude modulation (PAM). To mitigate the quantization error induced SNR degradation, we show that analog pre-emphasis filters can be used in conjunction with digital pre-emphasis for improved system performance. To understand the impact of the analog pre-emphasis filter on system performance, we analyze the relationship between the flatness of the system frequency response and the SNR degradation due to digital pre-emphasis, and demonstrate 1.1-dB increase of receiver sensitivities, for both 64-Gb/s and 128-Gb/s intensity-modulation direct detection (IM-DD) PAM4 signals, respectively employing a directly modulated laser (DML) and an electroabsorption modulator (EAM).

35 km amplifier-less four-level pulse amplitude modulation signals enabled by a 23 GHz ultrabroadband directly modulated laser

The 4-level pulse amplitude modulation (PAM4) based on an 23 GHz ultrabroadband directly modulated laser (DML) was proposed. We have experimentally demonstrated that based on intensity modulation and direct detection (IMDD) 56 Gbps per wavelength PAM4 signals transferred over 35 km standard single mode fiber (SSMF) without any optical amplification and we have achieved the bit error rate (BER) of the PAM4 transmission was under 2.9 × 10−4 by using feed forward equalization (FFE).

Nonlinearity Mitigation of Multi-Level Optical Transmission Using Spectrum-Split Digital Pre-Distortion

To overcome the frequency-dependent nonlinear characteristics of a directly modulated laser (DML) in a multi-level optical transmission, the spectrum-split digital pre-distortion (SS-DPD) is proposed and experimentally demonstrated to mitigate the frequency-dependent nonlinear impairments. The SS-DPD is employed in a 5 Gb/s PAM-4 optical transmission using a DML over a 5 km standard single-mode fiber at 1550 nm. We experimentally compared the SS-DPD with the DPDs based on the Volterra series and the memory polynomial (MP). The results demonstrate that the SS-DPD improved approximately 1 dB in terms of the receiver sensitivity compared with the MP-DPD. Moreover, the SS-DPD achieves 0.4 dB receiver sensitivity gain compared with the Volterra DPD.