Intensity Modulation And Direct Detection Research Articles

A 6-LP-mode ultralow-modal-crosstalk double-ring-core FMF for weakly-coupled MDM transmission

Weakly-coupled mode division multiplexing (MDM) technique based on few mode fiber (FMF) with low linear-polarized (LP) modal crosstalk is promising for capacity enhancement of short-reach transmission. In this paper, we propose a double-ring-core FMF for simultaneous 6-LP-mode weakly-coupled MDM transmission. FMF with ring-core structure is theoretically analyzed by deriving from a step-index circular-core FMF with perturbed ring areas. Then simplified method is proposed to define boundaries of ring areas in FMF and increase the minimum effective refractive index difference (min|Δneff|) among all the modes. Fabricated by commercial fabrication technique, the FMF achieves a min|Δneff| up to 1.49×10−3 while its maximum Δn between core and cladding is only 0.828%, which greatly reduces the difficulty of fiber fabrication. Thanks to the ultralow modal crosstalk of the FMF, we successfully demonstrate simultaneous 6-LP-mode (LP01, LP11, LP21, LP02, LP31 and LP12) 4-wavelength 71-km weakly-coupled MDM transmission using 10 Gb/s optical on-off-keying (OOK) intensity modulation and direct detection (IM/DD) without any digital signal processing (DSP). The design method can be extended to weakly-coupled FMFs with more ring areas.

Computational complexity comparison of single-carrier DMT and conventional DMT in data center interconnect.

High peak-to-average power ratio (PAPR) is an inherent defect in intensity modulation and direct detection (IM/DD) discrete-multitone (DMT) system, which will cause serious signal nonlinear distortion over fiber transmission. Single carrier-DMT (SC-DMT), which also refers to the discrete-Fourier-transform spread DMT (DFT-spread DMT), is a promising technology for DMT signal PAPR reduction, but higher computational complexity is required due to the additional DFT/IDFT operations in transceiver. In this paper, we experimentally compare the performance of SC-DMT and conventional DMT (CDMT) signal when the computational complexity of SC-DMT transceiver is lower than CDMT by reducing the FFT size in SC-DMT. The results show that the receiver sensitivity of 20 GHz 1024-point FFT based SC-DMT improves by 0.7 dB than 8192-point FFT based CDMT for both 120 Gb/s 64QAM-DMT and 140 Gb/s 128QAM-DMT signal transmission over 2-km single mode fiber (SMF) at the BER of 3.8 × 10-3 and 2.0 × 10-2, respectively. It is the first time to find that the SC-DMT with lower transceiver computational complexity outperforms CDMT. In addition, fast-Hartley-transform (FHT) technique is employed to replace FFT for further transceiver computational complexity reduction. The results give out that FHT-based SC-DMT shows the same BER performance with FFT-based SC-DMT, while the computational complexity of the transceiver can be reduced by half.

$4\times100$ -Gb/s PAM-4 FSO Transmission Based on Polarization Modulation and Direct Detection

For the first time, we experimentally demonstrate a 400-Gb/s [ $4\times100$ -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.

Performance enhancement of gain saturated SOA based free space optical link using dual-wavelength transmission

This paper proposes the dual-wavelength transmission technique for improving the performance of the gain-saturated semiconductor optical amplifier (SOA)-based free-space optical (FSO) communication link. When the gain saturated SOA is used to suppress turbulence-induced scintillation, extinction ratio (ER) degradation effect occurs owing to the high-dynamic gain frequency characteristics for intensity modulation and direct detection (IM/DD) system. The proposed method uses small-spacing dual-wavelength (signal and supplemental wavelengths) transmission to compensate ER degradation by mitigating the gain difference between the bits ‘1’ and ‘0’. Regarding the supplemental wavelength, continuous wave (CW) with various average powers and differential signal (DS) transmissions are experimentally investigated. The proposed technique is evaluated under various turbulence channel strengths. The experimental results show that the bit-error rate (BER) performance is significantly improved via effective ER degradation compensation and scintillation suppression.

Experimental demonstration of quasi-synchronous CDMA-VLC systems employing a new OZCZ code construction.

In this paper, we propose a new construction of optical zero correlation zone (OZCZ) code set for quasi-synchronous code division multiple access (QS-CDMA) visible light communication (VLC) system. The proposed code set includes a pair of unipolar and bipolar code sets, which is more suitable for intensity modulation and direct detection (IM/DD) and maintains dimming values at 50% regardless of user numbers for CDMA-VLC system. A QS-CDMA-VLC system adopting the proposed code set is investigated by numerical simulation and experimental verification. The system bit error rate (BER) performance is evaluated with different user numbers, sample rates, delay user numbers and transmission distances. In a 32-user CDMA-VLC system, the 250 MS/s transmission at a distance of 1.5 m can be achieved successfully with the BER less than the forward-error-correction (FEC) threshold. And the BER varies slightly taking account of small delay time. The results indicate that the new OZCZ code construction can effectively overcome non-perfect synchronous problem when the time delay among users does not exceed the zero correlation zone length.

Frequency-resolved adaptive probabilistic shaping for DMT-modulated IM-DD optical interconnects.

For decades, advanced modulation techniques have been proposed to increase the capacity for intensity-modulation and direct-detection (IM-DD) optical fiber interconnection systems. Typically, the frequency-resolved discrete multi-tone (DMT) modulation was proposed by loading modulations with different bit numbers to fit the channel's frequency response. Capacity can thus be better improved through finer use of the signal-to-noise-ratio (SNR) distribution in the frequency domain. For conventional DMT, the constellations loaded on individual subcarriers are all equip probability distributed. In this work, we propose a probabilistically shaped DMT (PS-DMT) modulation with adaptively loaded entropies referring to channel frequency response for short-reach optical interconnects. Achievable information rate (AIR) improvements of PS-DMT with both Maxwell-Boltzmann and dyadic distribution are investigated based on generalized mutual information (GMI). Moreover, the proposed PS-DMT has been realized experimentally over a multimode optical link using vertical-cavity surface-emitting lasers (VCSELs) with 100-m-long multimode fiber (MMF) transmission. This method can significantly improve the signaling capacity since two significant benefits are simultaneously utilized: 1) the shaping gain of PS at limited SNR condition and 2) the frequency-resolved continuous entropy loading for better fitting to the channel frequency response. Improved capacity, in terms of AIR, can thus be expected for a practical channel when using PS-DMT. This method can potentially be extended to a wide range of application scenarios, including both multimode and single-mode IM-DD fiber-optic communications.

Comparison of null-subcarriers reservation and adaptive notch filter for narrowband interference cancellation in intra-data center interconnect with DMT signal transmission.

In this paper, we experimentally compare the performance of two different narrowband interference suppression schemes in 120 Gb/s intensity modulation and direct detection (IM/DD) system with discrete multi-tone (DMT) signal transmission for intra-data center interconnect (Intra-DCI). Digital pre-equalization and DFT-spread techniques are applied for system bandwidth limitation induced signal distortion compensation and signal peak to average power ratio (PAPR) reduction, respectively. Null-subcarriers reservation (NSR) and adaptive notch filter (ANF) techniques are compared during the suppression of digital-to-analog convertor (DAC) clock leakage induced narrowband interference. 1.2 dB and 1.8 dB DMT receiver sensitivity improvements can be achieved at a bit-error rate of 3.8 × 10-3 in optical back-to-back (OBTB) transmission when optimized NSR and ANF schemes are applied for narrowband interference cancellation, respectively. After 2-km single mode fiber (SMF) transmission, the required received optical power (ROP) of DMT signal with optimized NSR and ANF for narrowband interference cancellation at BER of 3.8 × 10-3 are -6.5 dBm and -7.1 dBm, respectively. Obviously, ANF outperforms NSR scheme in narrowband interference cancellation for DMT system.

Performance analysis of precoded layered ACO-OFDM for visible light communication systems

Optical-orthogonal frequency division multiplexing (O-OFDM) is regarded as an effective scheme for intensity modulation and direct detection (IM-DD) based visible light communication (VLC) systems. State-of-the-art O-OFDM approaches complying with IM-DD constraints are; direct-current (DC) biased O-OFDM (DCO-OFDM) and asymmetrically clipped (AC)O-OFDM. ACO-OFDM has half the spectral efficiency (SE) of DCO-OFDM, howbeit, its SE can be augmented towards that of DCO-OFDM using layered ACO-OFDM (LACO-OFDM). Nevertheless, LACO-OFDM suffers from a high peak-to-average power ratio (PAPR). Forthright extension of PAPR reduction techniques devised for ACO-OFDM to LACO-OFDM can be unwieldy and complex. Hence, precoding can be an alternative to counterbalance the high PAPR without a considerable increase in complexity. As our contribution, we comprehensively investigate the performances of Fourier transform (FT) and Hartley transform (HT) precoding on LACO-OFDM. The bit error rate (BER) is analyzed in additive white Gaussian noise (AWGN) and time dispersive channels. Furthermore, the impact of superimposition of layers, PAPR, electrical-to-optical power conversion efficiency and system complexities are also investigated.

Adaptive modulation scheme for DML-based IM/DD FOFDM systems using overlap frequency-domain equalization

Abstract To further investigate the optical fast orthogonal frequency-division multiplexing (FOFDM) with sub-channel spacing equal to half of the symbol rate, an adaptive modulation scheme for FOFDM is proposed and experimentally demonstrated in low-cost short-reach intensity-modulated and direct-detection (IM/DD) systems using a directed modulated distributed feedback laser (DML). The experimental results and analyses indicate, it is feasible to transmit an adaptively modulated optical FOFDM signal over 20-km and 50-km standard single mode fibers (SSMFs). Compared with a single modulation format of 16-PAM, after 20-km SSMF transmission, the receiver sensitivity of the adaptive modulation scheme can be improved by about 2.3 dB at the bit error ratio (BER) of 2.4 × 10−2. Moreover, after 50-km SSMF transmission, the adaptive modulation scheme can still under 2.4 × 10−2 when received optical power (ROP) is larger than −1 dBm, while the 16-PAM modulation scheme cannot. In addition, an overlap frequency-domain equalization (OFDE) scheme is employed after sub-carrier de-multiplexing at the receiver to enable the effects of inter-symbol interference (ISI) to be effectively suppressed even though a cyclic prefix (CP) is not employed. Experimental results show that the FOFDM systems employing OFDE scheme have almost no performance degradation when compared to employing conventional FDE with an additional CP, but the spectral efficiency (SE) can be significantly improved by 12.37%.

Detection and Equalization of Set-Partitioned Offset-QAM OFDM in IMDD Systems

We design the detection algorithm for orthogonal frequency division multiplexing (OFDM) based on set-partitioned offset quadrature amplitude modulation (SP-offset-QAM) in adaptively-loaded intensity modulation and direct detection (IMDD) systems. The algorithm mitigates signal-signal beating interference (SSBI), improves the equalization capability of conventional one-tap equalizers, and reduces the complexity and the required length of training sequence in the decoding of multi-dimensional SP-QAM signals. We experimentally demonstrate the adaptively-loaded SP-offset-QAM OFDM over 50-km single-mode fiber to verify the proposed algorithm. It is shown that the modified equalization significantly improves the performance while the SSBI mitigation brings benefits after transmission. The proposed decoding scheme reduces the complexity comparable to that in conventional QAM. It is also shown that the SP-offset-QAM OFDM outperforms the SP-based conventional QAM and conventional offset-QAM OFDM both at back-to-back and after 50-km transmission, when the algorithm is applied to all schemes.

Demonstration of Low-Cost EML Based 240 Gbit/s DFT-Spread DMT Signal Transmission Over Few-Mode Fiber With Cylindrical Vector Beam Multiplexing

In this paper, we experimentally demonstrated the high-speed low-cost mode-division-multiplexed (MDM) discrete multi-tone (DMT) signal transmission based on cylindrical vector beam (CVB) multiplexing over few-mode fiber (FMF) with intensity modulation and direct detection (IM/DD) utilizing electro-absorption modulated laser (EML). Two CVBs (TM01 and TE01 modes) are employed for signal transmission simultaneously with the minimum mode isolation of 16.7 dB between them after transmission over 5 m 4-mode FMF. The digital discrete Fourier transform-spread (DFT-spread) and pre-equalization techniques are implemented in transmitter to improve the system performance. By adopting the proposed schemes, the 240 Gbit/s DFT-spread DMT signal has been transmitted successfully over 5 m FMF below the bit-error-ratio (BER) of $3.8\times 10^{-3}$ which is the hard-decision forward-error-correction (HD-FEC) threshold. The required received optical powers (ROPs) of the two CVBs (TM01 and TE01 modes) are -7.27 dBm and -7.15 dBm, respectively. The experimental results show that CVB-based multiplexing technology combined with advanced modulation format could be a potential candidate for high-speed low-cost server backplane optical interconnection.

Machine Learning for 100 Gb/s/<italic>λ</italic> Passive Optical Network

Responding to the growing bandwidth demand by emerging applications, such as fixed-mobile convergence for fifth generation (5G) and beyond 5G, 100 Gb/s/ λ access network becomes the next research focus of passive optical network (PON) roadmap. Intensity modulation and direct detection (IMDD) technology is still considered as a promising candidate for 100 Gb/s/ λ PON attributed to its low cost, low power consumption, and small footprint. In this paper, we achieve 100 Gb/s/ λ IMDD PON by using 20G-class optical and electrical devices due to its commercial availability. To mitigate the system linear and nonlinear distortions, neural network (NN) based equalizer is used and the performance is compared with feedforward equalizer (FFE) and Volterra nonlinear equalizer (VNE). We introduce the rules to train and test the data while using NN-based equalizer to guarantee a fair comparison with FFE and VNE. Random data have to be used for training, but for test, both random data and pseudorandom bit sequence are applicable. We found that the NN-based equalizer has the same performance with FFE and VNE in the case of linear distortion, but outperforms them in a strong nonlinearity case. In the experiment, to improve the loss budget, we increase the launch power to 18 dBm, achieving a 30-dB loss budget for 33 GBd/s PAM8 signal at the system frequency response of 16.2 GHz, attributed to the strong nonlinear equalization capability of NN.

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

We propose a polarization-modulation and direct-detection (PolM-DD) bandwidth-enhanced pulse-amplitude modulation (PAM-4) transmission system with a tunable frequency range and demonstrate, for the first time, its performance after transmissions through a standard single mode fiber (SSMF). By adjusting the polarization angle between one principal axis of the PolM and polarizer, the frequency notch, caused by the spectral dispersion inherent in SSMF, can be shifted to higher frequencies. As a consequence, the available 3-dB bandwidth at the baseband is much more enhanced than it would be in the traditional case of an intensity-modulation and direct-detection (IM-DD) system. Since the output of PolM modulation module are intensity modulated signals with a tunable chirp, the system is free of polarization-mode dispersion. From the analysis in this paper, we conclude that under ideal conditions, the PolM-DD system gains approximately 41.4% in bandwidth over its counterpart IM-DD system. The optimal polarization angle is independent of optical wavelengths and fiber distances, so it can be easily applied in wavelength-division-multiplexing systems. In our experiment, 100-Gbps, 84-Gbps, 68-Gbps, 58-Gbps, and 52-Gbps PAM-4 signals are successfully transmitted by our PolM-DD system over distances of 5-km, 10-km, 15-km, 21-km, and 25-km SSMF, respectively, without dispersion compensation component or single sideband modulation. The experimental results are consistent with our analysis and simulations results, and support more than 41% data rate gain compared to the IM-DD system, given comparable component bandwidths.

Subcarrier Index-Power Modulated-Optical OFDM With Dual Superposition Multiplexing for Directly Modulated DFB-Based IMDD PON Systems

A low-signal-modulation format-enabled highly spectrally efficient new transmission technique termed subcarrier index-power modulated optical OFDM with dual superposition multiplexing (SIPM-OOFDM-DSPM) is proposed and investigated, for the first time, in intensity modulation and direct detection (IMDD) passive optical network (PON) systems incorporating directly modulated DFB lasers (DMLs). The DSPM operation passively adds two low-signal-modulation format-encoded complex numbers and assigns the sum to subcarrier index-power modulation (SIPM)-generated low- and high-power subcarriers. Detailed numerical explorations of SIPM-OOFDM-DSPM performances over additive white Gaussian noise channels are undertaken, based on which optimum key transceiver parameters are identified. In comparison with previously published techniques, over simple DML-based IMDD PON systems, SIPM-OOFDM-DSPM considerably improves the signal transmission capacity, system power budget, and performance tolerance to both chromatic dispersion and fiber nonlinearity. The work suggests that SIPM-OOFDM-DSPM is a promising signal transmission technique capable of providing cost-sensitive PON systems with improved transmission capacity, performance flexibility, and adaptability.

Power budget of 25-Gb/s digital orthogonal filter multiple access PONs with 10G-class optics

Abstract Power budget is a key factor that should be considered in intensity modulation and direct detection (IM/DD) passive optical networks (PONs). Based on the low-cost 10G-class electro-absorption modulated laser (EML) operating at C-band, we not only experimentally and theoretically investigate the interplay between the chromatic dispersion, the chirp of EML, and the self-phase modulation (SPM), but also explore the influences of the resolution and timing jitter of the analog to digital converter (ADC). Both theoretical and experimental results show that high power budget can be effectively achieved by making full use of the high launch power induced SPM effect and EML-induced frequency chirp. Based on the 10-G transmitter optical sub-assembly (TOSA) and receiver optical sub-assembly (ROSA), we successfully realize 25 Gb/s signal transmission at C-band over 25 km standard single mode fiber (SSMF) in IM/DD PON system. Even the worst performing bands can still achieve 25 dB power budget. The proposed scheme in this work has a potential application in future cost-effective optical access networks.

Weakly-coupled mode division multiplexing over conventional multi-mode fiber with intensity modulation and direct detection

Multi-mode fiber (MMF) links are expected to greatly enhance capacity to cope with rapidly increasing data traffic in optical short-reach systems and networks. Recently, mode division multiplexing (MDM) over MMF has been proposed, in which different modes in MMF are utilized as spatial channels for data transmission. Strongly-coupled MDM techniques utilizing coherent detection and multiplex-input-multiplex-output (MIMO) digital signal processing (DSP) are complex and expensive for short-reach transmission. So the weakly-coupled approach by significantly suppressing mode coupling in the fiber and optical components has been proposed. In this way, the signals in each mode can be independently transmitted and received using conventional intensity modulation and direct detection (IM-DD). In this paper, we elaborate the key technologies to realize weakly-coupled MDM transmission over conventional MMF, including mode characteristic in MMF and weakly-coupled mode multiplexer/demultiplexer (MUX/DEMUX). We also present the up-to-date experimental results for weakly-coupled MDM transmission over conventional OM3 MMF. We show that weakly-coupled MDM scheme is promising for high-speed optical interconnections and bandwidth upgrade of already-deployed MMF links.

Comparison of DSP-based TDMA and FDMA channel aggregation techniques in mobile fronthauling

Abstract Cloud Radio Access Network (C-RAN) is perceived as a future essential technology to satisfy the ever-increasing demand of mobile data traffic. Considerable research efforts are expending in the optimization of C-RAN architecture. In this paper, we perform a comparison of two DSP-based fronthauling techniques for aggregation of radio waveforms: time division multiple access (TDMA) and frequency division multiple access (FDMA), in terms of error vector magnitude (EVM), spectral bandwidth efficiency and digital signal processing (DSP) complexity as a performance metrics. The two techniques are compared by means of simulation and validated experimentally on an intensity modulation and direct detection (IM-DD) optical fronthaul link capable of aggregating 48 and 96 LTE-A (20 MHz) channels. Moreover, we made simulation comparison on 24 (100 MHz) new radio (NR) waveforms which will be used in the upcoming 5G applications. We reveal that there is ∼50% and ∼20% spectral efficiency gain by TDMA aggregation on LTE-A and NR waveforms respectively. Hence TDMA gives slightly better performance in the case of 96 LTE-A channels which is attributed to slightly better linearity over the optical channel frequency response for larger number of channel. In addition, we show that TDMA is more efficient in terms of complexity than FDMA system that requires an additional pre-emphasis technique to equalize the overall per channel performance.

Optical Transmitters without Driver Amplifiers—Optimal Operation Conditions

An important challenge in optical communications is the generation of highest-quality waveforms with a Mach–Zehnder modulator with a limited electrical swing (Vpp). For this, we discuss, under limited Vpp, the influence of the waveform design on the root-mean-square amplitude, and thus, the optical signal quality. We discuss the influence of the pulse shape, clipping, and digital pre-distortion on the signal quality after the electrical-to-optical conversion. Our simulations and experiments, e.g., suggest that pre-distortion comes at the expense of electrical swing of the eye-opening and results in a lower optical signal-to-noise ratio (OSNR). Conversely, digital post-distortion provides operation with larger eye-openings, and therefore, provides an SNR increase of at least 0.5 dB. Furthermore, we find that increasing the roll-off factor increases the electrical swing of the eye-opening. However, there is negligible benefit of increasing the roll-off factor of square-root-raised-cosine pulse shaped signals beyond 0.4. The findings are of interest for single-channel intensity modulation and direct detection (IM/DD) links, as well as optical coherent communication links.

Experimental study of single channel 100 Gbit/s PAM4 transmission over 40 km using 17 GHz EML and APD at O band

Abstract Single channel 100 Gbit/s PAM4 transmission at O band is experimentally studied based on low cost intensity modulation and direct detection (IM-DD). By the use of advanced digital signal processing algorithms, we successfully achieve a receiver sensitivity of −17.3 dBm at 7% HD-FEC threshold of 3.8E-3 over 40 km un-amplified SSMF transmission with 17 GHz (3 dB bandwidth) commercial electric absorption modulated laser (EML) and avalanche photodiode (APD). To the best of author’s knowledge, we hereby report the best receiver sensitivity for 100 Gbit/s/λ IM-DD system. Due to the low cost and high sensitivity merits, the proposed scheme will be a promising way for high speed short-haul optical transmissions in the future.

100 Gbit/s PAM4 signal transmission and reception for 2-km interconnect with adaptive notch filter for narrowband interference.

In this paper, we experimentally demonstrate the transmission of 56 Gbaud four-level pulse amplitude modulation (PAM4) signal over 2-km single mode fiber (SMF) with intensity modulation and direct detection (IM/DD) scheme, while the bit-error-ratio (BER) of the PAM4 signal is under hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3. Linear pre-equalization is implemented in the transmitter side with a 3-tap finite-impulse-response (FIR) filter to compensate for the intersymbol interference (ISI) induced by system bandwidth limitation. Receiver side equalization is realized with training sequence (TS) based feed-forward equalizer (FFE) with decision-feedback equalizer (DFE). Furthermore, an Adaptive Notch Filter (ANF) is proposed to suppress the digital-to-analog converter (DAC) clock leakage induced narrowband interference for the first time, and the bandwidth of the ANF is optimized to achieve the best BER performance.