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Future passive optical networks (PONs) call for more flexibility to support diversified users with various rate demands and link qualities. Using traditional time-division multiplexing (TDM), the concept of a flexible rate PON was proposed to accommodate more users with link diversity by rate adaptation. In this Letter, we reveal the PON coverage can be further extended through frequency-division multiplexing (FDM) in the presence of multiuser diversity, namely, (i) there exist users with frequency-dependent link conditions and (ii) the link conditions exhibit disparity among users. We build a mathematical model and propose an optimization algorithm based on the binary tree search to optimize diversity gain. We experimentally verify its feasibility by studying the diversity gain concerning chromatic dispersion, optical path loss, and signal-to-noise ratio (SNR) variation in a 200G-class intensity-modulation direct-detection (IM-DD) system.

We demonstrate a direct-modulation and direct-detection system with a back-to-back line rate of 411.6 (net bit rate of 337.5) Gb/s using a 65 GHz DFB+R laser. The O-band laser with a chirp parameter of 0.6 supports dispersion-tolerant transmissions up to 15 km without an optical amplifier.

Direct detection attracts much attention for its simplicity compared with coherent detection. In this Letter, we propose for the first time, to the best of our knowledge, a high-dimensional Stokes vector direct detection (HD-SVDD) receiver for mode-division multiplexing transmission in few-mode fibers where the coupled modes can be recovered without resorting to coherent detection. To the best of our knowledge, the first high-dimensional Stokes vector reception based on the proposed HD-SVDD receiver has been successfully demonstrated with a dual-spatial and dual-polarization mode at 60 Gb/s over a 200 m two-mode fiber.

The past decade witnessed the stirring development of advanced optical modulations and digital signal processing, which have been pushing optical transmission systems towards the capacity limit. Recent research has sought to squeeze out the last few bits from bandwidth-limited optical channels. One straightforward path is to expand the signal spectrum beyond the bandwidth limit while keeping the single-carrier modulation, which inevitably induces huge inter-symbol interference. To cope with such penalty, sophisticated digital nonlinear equalization on single-carrier signals should be exploited to reduce the burden of the subsequent forward error corrections (FEC). On the other hand, a more instinctive capacity-approaching method for bandwidth-deficient channels is the well-known water-filling realized by multicarrier modulation. As its approximation, bit loading (BL) has been a well-established algorithm to maximize the bit rate of a discrete multitone (DMT) channel with fixed-rate FEC. Built on probabilistic constellation shaping (PCS), multicarrier entropy loading (EL) goes beyond BL by continuous source entropy adaptation and has proven its superiority over the single-carrier PCS counterpart. In this paper, we reveal the EL advantage over BL on both achievable information rate (AIR) and FEC, aiming to prove EL as the optimum capacity-approaching solution for bandwidth-limited channels with frequency-selective fading. In a 100G direct detection system with a bandwidth-deficient directly modulated laser, EL improves the AIR by 5%-10% over BL using identical FEC overhead. EL will be critical for short-reach interconnects dominated by low-cost optical components to squeeze out the last few bits from the bandwidth-constrained system.

The Kramers-Kronig (KK) receiver has recently attracted significant attention due to its capability of field recovery with direct detection. Under minimum phase condition, the KK receiver may use either single- or multi-carrier modulation formats. In this Letter, we investigate the appropriate modulation formats for both KK and signal-signal beat interference (SSBI) iterative cancellation (IC) receivers. It is shown that for the KK receiver, the single-carrier modulation format is superior to orthogonal frequency division multiplexing (OFDM), because the multi-carrier nature of OFDM signals increases the peak-to-average power ratio, which causes a violation of minimum phase condition. For the IC receiver, SSBI cancellation is more effective when the OFDM modulation format is adopted; thus, OFDM is the better fit for IC receivers than single carrier.

Stokes vector receivers (SVR) bridge the 4-D (i.e. dual-polarization complex signals) coherent detection and the conventional intensity-only 1-D direct detection (DD). By multi-dimensional polarization modulation in Stokes space, SVR maximizes the electrical spectral efficiency (ESE) of DD receivers by recovering at most 3-D signals. However, most 3-D schemes lack the capability of optical field recovery, an essential requirement for digital post-compensation of fiber dispersion that elongates the achievable distance. We propose a 3-D Stokes-space field modulation to enable 3-D signal field recovery, verified by a 3-D 32-Gbaud per dimension probabilistic-constellation-shaped 64-QAM transmission over 260-km fiber at C-band. This sets an ESE record of 16.5 (net ESE of 13.9) bit/s/Hz for DD receivers.

Direct detection is traditionally regarded as a detection method that recovers only the optical intensity. Compared with coherent detection, it owns a natural advantage-the simplicity-but lacks a crucial capability of field recovery that enables not only the multi-dimensional modulation, but also the digital compensation of the fiber impairments linear with the optical field. Full-field detection is crucial to increase the capacity-distance product of optical transmission systems. A variety of methods have been investigated to directly detect the optical field of the single polarization mode, which normally sends a carrier traveling with the signal for self-coherent detection. The crux, however, is that any optical transmission medium supports at least two propagating modes (e.g. single mode fiber supports two polarization modes), and until now there is no direct detection that can recover the complete set of optical fields beyond one polarization, due to the well-known carrier fading issue after mode demultiplexing induced by the random mode coupling. To avoid the fading, direct detection receivers should recover the signal in an intensity space isomorphic to the optical field without loss of any degrees of freedom, and a bridge should be built between the field and its isomorphic space for the multi-mode field recovery. Based on this thinking, we propose, for the first time, the direct detection of dual polarization modes by a novel receiver concept, the Stokes-space field receiver (SSFR) and its extension, the generalized SSFR for multiple spatial modes. The idea is verified by a dual-polarization field recovery of a polarization-multiplexed complex signal over an 80-km single mode fiber transmission. SSFR can be applied to a much wider range of fields beyond optical communications such as coherent sensing and imaging, where simple field recovery without an extra local laser is desired for enhanced system performance.

Semiconductor lasers are versatile optical transmitters in nature. Through the direct modulation (DM), the intensity modulation is realized by the linear mapping between the injection current and the light power, while various angle modulations are enabled by the frequency chirp. Limited by the direct detection, DM lasers used to be exploited only as 1-D (intensity or angle) transmitters by suppressing or simply ignoring the other modulation. Nevertheless, through the digital coherent detection, simultaneous intensity and angle modulations (namely, 2-D complex DM, CDM) can be realized by a single laser diode. The crucial technique of CDM is the joint demodulation of intensity and differential phase with the maximum likelihood sequence estimation (MLSE), supported by a closed-form discrete signal approximation of frequency chirp to characterize the MLSE transition probability. This paper proposes a statistical method for the transition probability to significantly enhance the accuracy of the chirp model. Using the statistical estimation, we demonstrate the first single-channel 100-Gb/s PAM-4 transmission over 1600-km fiber with only 10G-class DM lasers.

The frequency chirp of optical direct modulation (DM) used to be a performance barrier of optical transmission system, because it broadens the signal optical spectrum, which becomes more susceptible to chromatic dispersion induced inter-symbol interference (ISI). However, by considering the chirp as frequency modulation, the single DM simultaneously generates a 2-D signal containing the intensity and phase (namely, the time integral of frequency). This complex modulation concept significantly increases the optical signal to noise ratio (OSNR) sensitivity of DM systems. This Letter studies the duobinary pulse shaping (DB-PS) for chirp enabled DM and its impact on the optical bandwidth and system OSNR sensitivity. DB-PS relieves the bandwidth requirement, at the sacrifice of system OSNR sensitivity. As DB-PS induces a controlled ISI, the receiver requires one more tap for maximum likelihood sequence estimation (MLSE). We verify this modified MLSE with a 10-Gbaud duobinary PAM-4 transmission experiment.

There has long existed a debate over whether analog or digital optical link is more suitable for wireless convergence applications. Digital link achieves the highest fidelity, with the sacrifice of huge bandwidth due to the high resolution of digitization, and large power consumption due to the exhaustive digital data recovery. Analog link avoids these drawbacks, but it inevitably suffers from the SNR degradation. In this paper, we propose the angle modulation for analog optical link, which successfully breaks the SNR ceiling of amplitude modulation, and achieves ultrahigh link fidelity. Using the digital link (CPRI) equivalent bandwidth, angle modulation exhibits around 30-dB SNR advantage over the conventional amplitude modulation. Combined with its high tolerance on link nonlinearity, angle modulation has great potential in the future SNR-hungry analog optical applications.

Directly modulated lasers (DMLs) have long been employed for short- and medium-reach optical communications due to their low cost. Recently, a new modulation scheme called complex modulated DMLs has been demonstrated showing a significant optical signal to noise ratio sensitivity enhancement compared with the traditional intensity-only detection scheme. However, chirp-induced optical spectrum broadening is inevitable in complex modulated systems, which may imply a need for high-bandwidth receivers. In this Letter, we study the impact of receiver bandwidth effects on the performance of complex modulation and coherent detection systems based on DMLs. We experimentally demonstrate that such systems exhibit a reasonable tolerance for the reduced receiver bandwidth. For 10 Gbaud 4-level pulse amplitude modulation signals, the required electrical bandwidth is as low as 8.5 and 7.5 GHz for 7% and 20% forward error correction, respectively. Therefore, it is feasible to realize DML-based complex modulated systems using cost-effective receivers with narrow bandwidth.

Optical communication using high-speed on-off-keying signal by directly modulated semiconductor lasers (DML) was one of the most significant breakthroughs for telecommunication in 1960s. The wide deployment of 2.5-Gb/s per-channel transoceanic optical fiber links in 1990s drove the internet as a global phenomenon. However, the detrimental frequency chirp of DML prevents its application to the subsequent internet capacity evolution. Today, the state-of-the-art long-haul optical transponder uses external modulators to support high-order complex modulation. In contrast, this paper shows that the "detrimental" chirp effect can be exploited to generate complex modulation with a single DML, which achieves dramatic sensitivity gain of signal-to-noise-ratio compared to the conventional intensity modulation of DML. By using large chirp parameters, complex-modulated DML paves an attractive pathway towards high-order pulse-amplitude modulation with an ultra-low transmitter cost, which has great potential in future medium reach optical communications.

Due to the chirp effect, the modulation information is carried by both intensity and phase of the optical signal generated by a directly modulated laser (DML). Compared to the signal recovery based on intensity alone, the receiver sensitivity is significantly improved when phase is also involved for signal recovery. In this paper, we provide thorough analysis of two schemes to recover the directed modulated signal based on the combined information of intensity and phase. The intensity and phase combined recovery for polarization-division-multiplexed (PDM) directly modulated signal is demonstrated for the first time, achieving a data rate of 40 Gb/s over a transmission distance of 160 km. Compared with the conventional intensity-only recovery, the receiver sensitivity is improved by 5.5 dB and 10 dB respectively by using two intensity and phase combined recovery schemes.

We report the first experiment of digital coherent detection of directly modulated signals that utilizes both phase and amplitude of the signals. A novel algorithm is proposed to take advantage of simultaneous detection of both phase and amplitude of the modulated signal. Using our proposed scheme, the experiment shows 6-dB improvement over the amplitude-only detection scheme.

The explosive growth of the traffic between data centers has led to an urgent demand for high-performance short-reach optical interconnects with data rate beyond 100G per wavelength and transmission distance over hundreds of kilometers. Since direct detection (DD) provides a cost-efficient solution for short-reach interconnects, various advanced modulation formats have been intensively studied to improve the performance of DD for high-performance short-reach optical interconnects. In this paper, we report the recent progress on the advanced DD modulation formats that provide superior electrical spectral efficiency (SE) and transmission reach beyond that of simple direct modulation (DM) based direct detection (DM/DD). We first provide a review of the current advanced modulation formats for high-performance short-reach optical interconnects. Among these formats, Stokes vector direct detection (SV-DD) achieves the highest electrical spectrum efficiency, presenting itself as a promising candidate for future short-reach networks. We then expound some novel algorithms to achieve high-performance SV-DD systems under severe impairments of either polarization mode dispersion (PMD) or polarization dependent loss (PDL).

We report nondisruptive measurement of distributed mode coupling along a five-mode fiber (5MF) using a reconfigurable Brillouin optical time domain reflectometer (OTDR) based on programmable spatial light modulators (SLMs). Mode coupling between LP01 mode and higher order modes LP11a,b and LP21a,b is successfully characterized with a spatial resolution of 50 m.

We propose a novel detection scheme called Stokes vector direct detection (SV-DD) to realize high electrical spectral efficiency and cost-effective optical communication for short and medium reach. With SV-DD, the signal is modulated in only one polarization and combined with the carrier in the orthogonal polarization for fiber transmission. At reception, the combined signal is detected in Stokes space by three or four photo-detectors. Compared with conventional DD technique, SV-DD is resilient to both chromatic dispersion and signal-to-signal beat noise. Furthermore, SV-DD does not require polarization tracking or narrow band optical filtering for carrier extraction. In this paper, we present for the first time the numerical analysis and experimental demonstration of single-carrier SV-DD. We report 62.5-Gb/s data rate single-carrier SV-DD transmission over 160-km SSMF using 12.5-Gbaud 32-QAM modulation.

We propose the Stokes vector direct detection (SV-DD) scheme which simultaneously achieves receiver phase diversity and the cancellation of photodetection nonlinearity. An 80  Gb/s single-polarization modulated SV-DD signal is successfully received after 160 km SSMF transmission with 11.64  bit/s/Hz electrical spectrum efficiency.

We propose signal-carrier interleaved (SCI) optical OFDM for direct detected transmission systems. Such a scheme can be considered as a variation of self-coherent detection where the carrier and signal are supplied at the transmitter and extracted at the receiver for coherent-like detection. This provides high OSNR sensitivity while maintaining very low carrier-to-signal power ratio (CSR). Our experiment results show that with 0 dB CSR, 43.2 Gb/s 16 QAM OFDM signal can be successfully delivered over 80 km standard single mode fiber (SSMF) with 24 dB OSNR requirements at 7% FEC limit.

We propose a scheme for a direct-detection optical orthogonal frequency-division multiplexing (OFDM) system based on blockwise signal-phase-switching (SPS). Experimental demonstration of 61 Gbits/s SPS direct-detection optical OFDM signal transmission over 80 km standard single-mode-fiber was successfully achieved with single polarization and a single photodetector.

We propose phase switching for either main-carrier or subcarriers of two consecutive signal blocks to achieve fading-free double-sideband direct-detection (DD). The proposed approach has twice of the electrical spectral efficiency (SE) of offset OFDM, and the same electrical SE as single-side band (SSB) OFDM. With this scheme, 40-Gb/s DD-OOFDM is successfully received over 80-km SSMF with single polarization and single detector.

We propose a fiber structure based on a single-mode-multimode-single-mode one. A theoretical analysis model based on leaky mode expansion along with a new concept of leaky mode interference is presented to facilitate and enlighten the analysis for the leaky structure. Then we focus on transmission characteristics of the structure in response to surrounding refractive indices (RI) that are higher than that of the silica fiber. In our simulation, output intensity of the structure increases monotonically for RI from 1.46 to 1.55, and changes over 2.0 dB for a change of 0.01 in RI range from 1.46 to 1.48, which is well verified by the following experiments.