Bandwidth-Enhanced PAM-4 Transmissions Using Polarization Modulation and Direct Detection With a Tunable Frequency Range

In the spectral efficient frequency division multiplexing (SEFDM) system, in order to minimize mutual interference between subcarriers, a detector should be introduced at the receiver to handle the interference, but increases the complexity of system. In this paper, we propose a simplified precoding algorithm at transmitter instead of receiver, which can first implement higher 16QAM SEFDM transmission over standard single mode fiber (SSMF). The technique is based on interference information from non-orthogonality between subcarriers and uses nonlinear matrix operations to improve system performance with low complexity. The numerical results show that the precoding scheme achieves SSMF transmission of intensity modulation and direct detection (IM/DD) 16QAM SEFDM system under the premise of achieving higher spectral efficiency, and obtains good bit error rate performance.

As higher symbol rates are utilized in the intensity modulation and direct detection (IM/DD) scheme to meet the unrelenting growth of data traffic, overcoming the inter-symbol interference (ISI) induced by the limited bandwidth has become increasingly crucial. Channel equalization based on digital signal processing (DSP) is an effective solution, where least-mean squares (LMS) algorithm is adopted to adjust tap coefficients. However, the LMS algorithm usually has slow rate of convergence and requires lots of training symbols. This work proposes a novel training sequence to accelerate the LMS-based equalization. A first-order Markov chain (MC) is employed for sequence generation, which introduces correlation between samples and shapes signal spectrum. Compared with the conventional training sequence that consists of independent and identically distributed (i.i.d.) samples and has a white spectrum, the MC sequence enables faster convergence of tap coefficients and mean-squared error (MSE). Moreover, an experimental demonstration of a 43 Gbaud PAM-4 signal shows that the proposed sequence can achieve a lower pre-forward-error-correction (pre-FEC) bit error rate (BER) than that of the i.i.d. sequence with the same length. When the PAM-4 signal is transmitted over a 5-km standard single mode fiber (SSMF) with 6-dB system bandwidth of 10 GHz, more than 70% training sequence length reduction can be attained. When the fiber length is increased to 10 km and the signal suffers from severe power fading, more than 48% reduction can be achieved.

Fronthaul plays a crucial role in bridging mobile and optical communications. Conventional digital radio-over-fiber (RoF) systems, utilizing the common public radio interface (CPRI), require a capacity exceeding an order of magnitude above the user data rate, posing challenges for dense small cell deployments and the growing bandwidth demands of 6 G and beyond. Intermediate-frequency-over-fiber (IFoF) with intensity modulation and direct detection (IM/DD) presents a promising alternative due to its high spectral efficiency and simpler architecture. However, traditional IM/DD systems are susceptible to fiber chromatic dispersion, limiting their application in fronthaul. In this Letter, we utilize a time skew-enabled vestigial sideband (VSB) technique to overcome power fading caused by chromatic dispersion in direct detection-based IFoF mobile fronthaul. We experimentally demonstrate hardware-efficient and high-capacity transmission of 1.536-Tb/s CPRI-equivalent rate with 128-QAM signal over a 10-km standard single-mode fiber (SSMF).

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).

For the first time, we experimentally demonstrate a 400-Gb/s [4×100-Gb/s wavelength-division multiplexing (WDM)] pulse-amplitude-modulation-4 (PAM-4) transmission based on free space optics (FSO) utilizing the polarization modulation (PolM) and direct detection (DD) scheme. Each WDM channel comprises a polarization modulator, a polarizing beam splitter (PBS), and two photodetectors (PDs). Four wavelengths each carrying a 100-Gb/s PAM-4 signal (aggregate 400 Gb/s) are transmitted over the free space. At the receiver side, the PBS separates the two orthogonal polarizations, and then, the two polarized signals are detected by two PDs. Then, the two detected signals are combined by a subtractor, followed by digital signal processing (DSP). The PolM-DD system can achieve higher data rate while maintaining the required forward-error-correction (FEC) threshold compared with intensity modulation and direct detection (IM-DD) scheme. Moreover, owing to the stable polarization state in the free space and the mitigation of the common mode noise by the subtraction, the PolM-DD system provides enhanced tolerance to the amplitude fluctuations attributable to weather turbulence, which makes it more reliable than the conventional IM-DD FSO system. To the best of our knowledge, this is the first experimental demonstration of a 400-Gb/s FSO system using the PolM-DD scheme with the transmission distance of 1.2 m.

Spectral efficient frequency division multiplexing (SEFDM) can offer a higher spectral efficiency (SE) than orthogonal frequency division multiplexing (OFDM). In this work, we propose a diversity technique based on SEFDM for beyond 100-Gb/s optical intensity modulation and direct detection (IM/DD) long reach (LR) applications. We mathematically demonstrate that the self-created inter-carrier interference of SEFDM signals can be reused to achieve a diversity gain on each sub-carrier and, in turn, improve the tolerance to power fading induced by chromatic dispersion (CD) in IM/DD LR links. Based on the proposed diversity technique, we further demonstrated a 112-Gb/s SEFDM transmission over 80-km standard single-mode fiber, using only 28-GHz bandwidth and modulation format of up to 16-QAM. Experimental results show that SEFDM with the proposed diversity technique performs robust against CD effects and outperforms the conventional OFDM with adaptive bit and power loading of the same bandwidth and data rate, which validates the superiority of the proposed SEFDM in optical IM/DD LR transmissions.

IM/DD (Intensity Modulation and Direct Detection) systems are widely used in short-haul optical transmission for its low complexity and low cost. IM/DD systems with PAM (Pulse Amplitude Modulation) can be applied to intra- and inter-DC (data center) which has an exponentially growing transmission rates demand. Meanwhile, PS (Probabilistic Shaping), as a channel coding method, can shape the probability of symbols in different amplitude in order to maximize the transmission rate to be close to the Shannon limit. Nonlinear compensation based on DSP (Digital Signal Processing) plays an important role in compensating the degradation of the channel. In this paper, a method which combines with PS and 2nd-order VNLE (Volterra Nonlinear Equalizer), is proposed to reduce signal degradation while VNLE is used for nonlinear compensation at the receiver. The compensation performances by (1) VNLE and FFE (Feedforward Equalizer) with PS, (2) VNLE with PS and without PS are compared in a DML (Directly Modulated Laser)-based IM/DD systems. PS is implemented with CCDM (Constant Composition Distribution Matching) which is used to shape the symbols. In a 10km PS based IM/DD systems, the SER (Symbol Error Rate) of the transmitted symbol obviously decreases compared to that without PS. The introduction of VNLE method gets a lower SER than that using FFE method. In summary, the proposed method is proved to have a better performance and is supposed to be implemented in the future IM/DD systems.

We experimentally demonstrate a high-speed intensity modulation and direct-detection (IM/DD) system with four wavelength division multiplexing (WDM) channels. With the aid of semiconductor optical amplifier (SOA) and probabilistic shaping (PS) technique, 1T (280Gbit/s×4) PS-PAM8 signals can be successfully transmitted over 40-km standard single mode fiber (SSMF). As a more precise performance criteria, the normalized generalized mutual information (NGMI) should be used to report the performance of PS shaped signals. In this experiment, each WDM channel can support 280Gbit/s/channel PS-PAM8 signals transmission with a NGMI larger than the threshold of a soft-decision forward error correction (SD-FEC) code of 0.83. Considering 25% FEC overhead, the net bit rate of each WDM channel is 220Gbit/s/channel. Therefore, the net transmission rate of the WDM system is 880Gbit/s, which can support the demand of 800G data connection. To the best of our knowledge, it is the first time to realize over 1Tbit/s PAM signal transmission over 40km in a four-lane IM/DD system.

Cascaded inverse fast Fourier transform/fast Fourier transform (IFFT/FFT)-based multi-channel aggregation/de-aggregation offers a promising solution in constructing highly desirable flexible optical transceivers for considerably improving optical networks' elasticity, flexibility, and adaptability. However, the multi-channel aggregation operation unavoidably results in generated signals having high peak-to-average power ratios (PAPRs). To solve this technical challenge, this paper first explores the PAPR characteristics of the corresponding flexible transceivers in optical back-to-back (B2B) and 20 km intensity modulation and direct detection (IMDD) transmission systems, and then numerically investigates the feasibility and effectiveness of utilizing the conventional clipping techniques in reducing their PAPR reductions. The results show that the last IFFT operation size is the primary factor determining the PAPRs rather than the channel count and modulation format. For a given last IFFT operation size, the optimal clipping ratio can be identified, which is independent of channel count. With the identified optimal clipping ratio, when the channel count is >4, every two-channel increase in the channel count can only lead to <1.2 Gb/s decreases in the maximum aggregated signal transmission capacity.

Intensity modulation and direct detection (IM/DD) schemes with pulse amplitude modulation (PAM) is utilized to economically realize short reach transmission systems, such as data center (DC) applications. In the next-generation Ethernet, the transmission rates are expected to reach over 800 Gb/s, a higher baudrate is desirable due to the current demand on DC data traffic. Digital signal processing (DSP) technology is used to increase tolerance against inter-symbol interference (ISI) caused by narrow band components to achieve high speed data rates. The signal-signal beating terms produced by the square-law detection at the receiver side contribute most significantly to the system performance, which are the 2nd-order nonlinear distortions. Volterra nonlinear equalizers (VNLEs) have been proposed for the nonlinear impairments compensation.In this paper, a nonlinear differential coding based spectral-shaping technology and quadratic VNLE are used in directly modulated laser (DML)-based IM/DD systems. The tolerance to bandwidth limitations (BWL) can be increased with nonlinear differential coding PAM (NLDCP), the signal spectrum can also be narrowed. We verified this method in a 10 km system, and the results show that in DML-based IM/DD systems, NLDCP in combination with VNLE used can provide more than 3.8 dB and 1.7 dB BER gain when baud rate is 90 G at O-band and C-band, respectively. The optimal extinction ratio (ER) of the systems with NLDCP is about 0.3 dB and 0.1 dB lager than that in systems without NLDCP at O-band and C-band, respectively. We also investigate the performance dependency on the number of taps in the nonlinear equalizers.

In this paper, we demonstrate a newly designed multi-source domain transfer learning (MST) scheme to reduce the training cost of deep neural network (DNN) based equalizer in intensity-modulation and direct-detection (IMDD) systems. Different from a common transfer learning algorithm, in this scheme, data with different channel parameters is selected and proportionally used to construct a multi-source domain dataset. This allows training the source domain in a single task while ensuring the model's generalization ability and stability. In an 80Gb/s PAM-4 IMDD short reach system, our proposed MST equalizer was proven effective. The corresponding results demonstrate that, compared to a conventional DNN equalizer, the proposed MST equalizer can achieve a bit error rate that meets the hard decision-forward error correction threshold while saving 87% of the iteration epochs and 65% of the training data.

Photonic computing and signal processing are rapidly regaining attention, capitalizing on the saturation of digital electronics capabilities, owed to the ending of the Moore's law era. Optical communications are among the fields in which digital signal processing (DSP) struggles to offer a viable long-term solution, as its power consumption rises significantly in the latest generation optical links. For example, the traditional intensity modulation and direct detection (IM/DD) systems suffer from challenging problems such as the chromatic dispersion induced power fading effect, which necessitates the use of powerful digital equalizers in the 800G and 1.6T optical transceivers. Here, we experimentally validate our previous numerical works in recurrent optical spectrum slicing (ROSS) accelerators for dispersion compensation in IM/DD links. For this, we utilize recurrent filters implemented both through a waveshaper and by exploiting novel programmable photonic platforms. The ROSS accelerators exploit frequency processing in order to mitigate the power fading effect, which hinders the scalability of IM/DD systems. By equalizing even 80 km of 64 Gb/s PAM-4 transmission in C-band, we prove that our system can offer an appealing solution in highly dispersive channels. We employ the simplest digital equalization in the form of a feed-forward equalizer (FFE), avoiding throughput, latency, and complexity restrictions other digital equalizers impose. We achieve to reduce the bit error rate (BER) from 0.11 to less than 1 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> with the use of only two filter nodes, whereas BER approaches 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> when three nodes are incorporated. These results correspond to almost two orders of magnitude BER gain.

Advanced modulation formats with intensity modulation and direct detection (IM/DD) have shown promise in a short-reach optical communication system like data center interconnect. However, the conventional fiber transmission spectral window is mostly located at 850, 1310, and 1550 nm. Here, we experimentally demonstrated an amplifierless IM/DD link at the 2-μm spectral band. The signal is encoded with pulse amplitude modulation (PAM) and 3-D carrier-less amplitude and phase modulation (3-D-CAP). The encoded signal is transmitted through a 100 m-long solid-core fiber designed for single mode at 2-μm. We achieve a transmission with bit-rate of 40 Gbit/s using PAM-4 and 24 Gbit/s using CAP16. The measured bit error rates of both formats are below the forward error correction (FEC) limit of $3.8\times 10^{-3}$ . This study is the first demonstration of PAM4 and CAP transmission at 2-μm, and the results show a significant potential of advanced modulation for 2-μm optical communication.

In this letter, the authors investigate spatial diversity techniques for free-space optical (FSO) links with intensity modulation and direct detection (IM/DD) over log-normal atmospheric turbulence-induced fading channels. We restrict our attention to the deployment of orthogonal space-time block codes (OSTBCs) and repetition codes both of which have been recently proposed for FSO links. Our performance analysis demonstrates that, although both schemes are able to extract full diversity, repetition codes outperform OSTBCs. The performance gap increases with the increasing number of transmit apertures. Our findings clearly point out that deployment of OSTBCs is not necessary for a FSO IM/DD link.

Adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM) using non-linear signal compression (NSC) (AMOOFDM-NSC) is proposed and numerically explored, for the first time, in intensity modulation and direct detection (IMDD) single-mode fibre transmission systems for cost-sensitive optical access networks. AMOOFDM-NSC utilises a simple cosine operation to compress digital electrical OFDM signals prior to performing digital-to-analogue conversion and optical intensity modulation in the transmitter. Compared with conventional AMOOFDM, AMOOFDM-NSC can not only significantly lower requirements on minimum quantisation bits and sampling rates of digital-to-analogue/analogue-to-digital convertors, but also simultaneously doubles the signal transmission capacity for quantisation bits of < 4. In addition, AMOOFDM-NSC also shows excellent system flexibility and has good performance robustness to variations in system characteristics. AMOOFDM-NSC may have great potential for providing a cost-effective technical solution for practical implementations in cost-sensitive access networks.