Intensity Modulation And Direct Detection Research Articles

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.

Single-Lane 145 Gbit/s IM/DD Transmission With Faster-Than-Nyquist PAM4 Signaling

The performance of faster-than-Nyquist intensity modulation and direct detection (IM/DD) systems can be severely degraded by the inter-symbol interference (ISI) due to the component bandwidth limitation and the faster-than-Nyquist filtering. In addition, nonlinearities caused mainly by optical modulation and square-law detection can further deteriorate signal quality. Here, we propose to incorporate transmitter-side Tomlinson-Harashima precoding to mitigate the ISI and a Volterra nonlinear equalizer (VNLE) to mitigate the nonlinear distortions for PAM4 faster-than-Nyquist systems. We experimentally demonstrate a single wavelength 145 Gb/s PAM4 IM/DD transmission with bandwidth limited commercial components and sub-Nyquist sampling at 70 GSa/s. The optimal VNLE filter length is also investigated to balance the implementation complexity and system performance. In addition, compared with the scheme using transmitter-side duobinary precoding, receiver-side VNLE and maximum likelihood sequence estimation, both performance improvement and implementation complexity reduction are achieved.

PAM-8 IM/DD Transmission Based on Modified Lookup Table Nonlinear Predistortion

In this paper, a modified lookup table (LUT) nonlinear predistortion method is proposed. With the modified LUT method, the number of patterns and the computational complexity can be reduced by half with a similar system performance comparing with normal LUT. We experimentally demonstrate an 8-ary pulse amplitude modulation (PAM-8) intensity modulation and direct detection (IM/DD) transmission over 15-km fiber using a directly modulated laser (DML) based on modified LUT. Around 1-dB receiver sensitivity improvement at bit-error ratio of 3.8 × 10 –3 is achieved by adopting a modified LUT over 15-km fiber transmission at 2 Gbaud based on a commercial DML. In addition, we also experimentally demonstrate Mach–Zehnder modulator based 40 Gbaud PAM-8 IM/DD transmission under back-to-back case, about 1-dB receiver sensitivity can also be improved.

Demonstration of C-band 112G PAM-4 transmission over 80-km stand-single mode fiber based on direct-detection

In this paper, we experimentally demonstrate the intensity modulation and direct-detection (IM/DD) 112-Gb/s single carrier pulse amplitude modulation (PAM)-4 transmission system over 80-km standard signal-mode fiber (SSMF) without any optical chromatic dispersion (CD) compensation at the 3.8×10−3 BER threshold for 7% the hard-decision forward error correction (HD-FEC). The training based pre-equalization (pre-EQ) and the nonlinear look-up-table (LUT) based pre-distortion (pre-DT) in the transmitter (Tx)-side are employed to reduce the computation complexity and deal with the modulation bandwidth and the nonlinearity impairments of electro-optical modulators and detectors. On the other hand, the system without optical CD compensation modules is obviously more simple and flexible. Therefore, the nonlinear LUT pre-DT simultaneously used for CD compensation will further simplify the system. The BER is about 20% without any digital signal processing (DSP) such as equalization, pre-distortion, pre-equalization or compensation. After signal recovery, the BERs of BTB and 80-km SSMF system can achieve 3.07×10−5 and 1.50×10−3 respectively. The principle of the LUT based pre-DT and the pre-EQ are introduced in the paper.

Implementation of optical comb based SBS-resistant IMDD OFDM transmission over 100 km SSMF for long-reach access networks

An optical comb based SBS-resistant intensity modulation and direct detection (IMDD) orthogonal frequency division multiplexing (OFDM) transmission over 100 km SSMF for long-reach access networks, which employs 4-wavelength transmission scheme to alleviate stimulated Brillouin scattering (SBS) effect, is experimentally demonstrated in this paper. The 4-wavelength light is generated by an optical comb generator, and effective SBS suppression can be achieved with optical launching power as high as 18 dBm. 12.5 Gbps OFDM transmission over 100 km SSMF without optical repeaters is successfully demonstrated. Experimental results also show that more than 30 dB system power budget can be achieved under launching power ranges from 15 dBm to 17 dBm by using PIN detector.

Performance Evaluation of High Speed Multicarrier System for Optical Wireless Communication

Optical wireless communication (OWC) in the infrared and visible range is quite impressive solution, especially where radio communication face challenges. Visible light communication (VLC) uses visible light over a range of 400 and 800 THz and is a subdivision of OWC technologies. With an increasing demand for use of wireless communications, wireless access via Wi-Fi is facing many challenges especially in terms of capacity, availability, security and efficiency. VLC uses intensity modulation and direct detection (IM/DD) techniques and hence they require the signals to certainly be real valued positive sequences. These constraints pose limitation on digital modulation techniques. These limitations result in spectrum-efficiency or power-efficiency losses. In this paper, we investigate an amplitude shift keying (ASK) based orthogonal frequency division multiplexing (OFDM) signal transmission scheme using LabVIEW for VLC technology.

Experimental research of UWB over fiber system employing 128-QAM and ISFA-optimized scheme

Abstract In this paper, an optimized intra-symbol frequency-domain averaging (ISFA) scheme is proposed and experimentally demonstrated in intensity-modulation and direct-detection (IMDD) multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband over fiber (UWBoF) system. According to the channel responses of three MB-OFDM UWB sub-bands, the optimal ISFA window size for each sub-band is investigated. After 60-km standard single mode fiber (SSMF) transmission, the experimental results show that, at the bit error rate (BER) of 3.8 × 10−3, the receiver sensitivity of 128-quadrature amplitude modulation (QAM) can be improved by 1.9 dB using the proposed enhanced ISFA combined with training sequence (TS)-based channel estimation scheme, compared with the conventional TS-based channel estimation. Moreover, the spectral efficiency (SE) is up to 5.39 bit/s/Hz.

Efficient Analytical Calculation of Non-Line-of-Sight Channel Impulse Response in Visible Light Communications

This study provides an analytical method to calculate the non-line-of-sight (NLoS) channel impulse response (CIR) in visible light communication (VLC) systems based on intensity modulation and direct detection (IM/DD). In this method, the NLoS channel is decomposed into a number of components with different propagation categories. These propagation categories are defined according to the number of reflections and the reflective surfaces that the light undergoes. The CIR corresponding to each light propagation category is analysed and the overall NLoS CIR is approximated by the combination of the calculated CIR components in different propagation categories. The proposed method has the major advantage of offering accurate results with very low computational complexity. Typically, a NLoS CIR with a time resolution of 0.1 ns can be generated within a second in MATLAB. Furthermore, the analytical results derived herein could be used as an analytical tool for the VLC channel characterisation study in future research.

IM/DD Transmission Techniques for Emerging 5G Fronthaul, DCI, and Metro Applications

For short range optical transmissions, the intensity modulation and direct detection (IM/DD) offers the advantages of low cost and low complexity. In this paper, we will review our recent investigation results about IM/DD schemes such as polybinary and discrete multitone (DMT). Polybinary four-level pulse amplitude modulation (PAM-4) system shows better performance than the conventional PAM-4 system for both multimode and single-mode fibers. If the electrical dispersion compensation (EDC) is conducted with DDMZM, the reachable distance can be extended to 80 km which is suitable for the metro access scenario. Single sideband (SSB) polybinary-PAM-4 is also feasible to greatly increase the spectral efficiency (SE) and system capacity. Also, some DMT architectures for metro applications without using dispersion compensating modules will be introduced. SSB-DMT is very simple, while chromatic dispersion (CD) calculation and pre-compensation are not necessary. EDC double sideband (DSB) DMT owns 3-dB better performance than the SSB-DMT. CD can be calculated according to the power fading and the result is sent back to the transmitter for pre-compensation. Twin-SSB-DMT has the highest SE. More than 200-Gb/s data have been successfully generated and transmitted over 80 km single mode fiber (SMF).

Analysis of different sub-carrier allocation of M-ary QAM-OFDM downlink in RoF system

In this paper, the performance of a 60 GHz radio over fiber (RoF) system with 4/16/64 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) downstream signals is studied. Delivery of 10 Gbit/s M-ary QAM (MQAM) OFDM signals through the 20-km-long single-mode fiber (SMF) is complicated in terms of intensity modulation and direct detection (IM/DD). Using self-homodyne method, the beating of two independent light waves generating the millimeter-wave at the photodetector can be down-converted to baseband in the electrical domain. Meanwhile, three kinds of sub-carrier arrangement schemes are compared and discussed, and the simulation results show that lower peak-to-average power ratio (PAPR) can be obtained adopting the adjacent scheme. At bit error rate (BER) of 10-3, the receiver sensitivity using 4QAM-OFDM sub-carrier signal is almost enhanced by 4 dB and 9 dB compared with those of 16QAM-OFDM signal and 64QAM-OFDM signal.

Simplified demultiplexing scheme for two PDM-IM/DD systems utilizing a single Stokes analyzer over 25-km SMF

We propose a four-linear state of polarization multiplexed intensity modulation and direct detection (IM/DD) scheme based on two orthogonal polarization division multiplexing (PDM) on-off keying systems. We also experimentally demonstrate a simple demultiplexing algorithm for this scheme by utilizing only a single Stokes analyzer. At the rate of 4×10 Gbit/s, the experimental results show that the power penalty of the proposed scheme is about 1.5dB, compared to the single PDM-IM/DD for back-to-back (B2B) transmission. Compared to B2B, just about 1.7dB power penalty is required after 25km Corning LEAF optical fiber transmission. Meanwhile, the performance of the polarization tracking is evaluated, and the results show that the BER fluctuation is less than 0.5dB with a polarization scrambling rate up to 708.75deg/s.

Single channel 224 Gbit/s (56‐GBaud) PAM‐16 transmission using linear digital pre‐distortion

With linear digital pre-distortion (DPD) and 8-bit digital to analogue converter (DAC), a single channel 224 Gbit/s (56-GBaud) 16-level pulse amplitude modulation (PAM-16) signal is experimentally transmitted over a 100 m standard single-mode fibre (SSMF) for the first time using intensity modulation and direct detection (IM/DD). The 3 dB bandwidth of the whole system is around 22 GHz, which is far below the bandwidth of the 56-GBuad PAM-16 signal. The strong inter-symbol interference caused by the limited system bandwidth is pre-compensated by the proposed linear DPD technique. The experimental results indicate that the linear DPD process can effectively avoid the noise enhancement by the aggressive feed-forward equalisation at the receiver, and also improves the system performance at the cost of additional finite impulse response filter at the transmitter.

Silicon-organic hybrid (SOH) modulators for intensity-modulation / direct-detection links with line rates of up to 120 Gbit/s.

High-speed interconnects in data-center and campus-area networks crucially rely on efficient and technically simple transmission techniques that use intensity modulation and direct detection (IM/DD) to bridge distances of up to a few kilometers. This requires electro-optic modulators that combine low operation voltages with large modulation bandwidth and that can be operated at high symbol rates using integrated drive circuits. Here we explore the potential of silicon-organic hybrid (SOH) Mach-Zehnder modulators (MZM) for generating high-speed IM/DD signals at line rates of up to 120 Gbit/s. Using a SiGe BiCMOS signal-conditioning chip, we demonstrate that intensity-modulated duobinary (IDB) signaling allows to efficiently use the electrical bandwidth, thereby enabling line rates of up to 100 Gbit/s at bit error ratios (BER) of 8.5 × 10-5. This is the highest data rate achieved so far using a silicon-based MZM in combination with a dedicated signal-conditioning integrated circuit (IC). We further show four-level pulse-amplitude modulation (PAM4) at lines rates of up to 120 Gbit/s (BER = 3.2 × 10-3) using a high-speed arbitrary-waveform generator and a 0.5 mm long MZM. This is the highest data rate hitherto achieved with a sub-millimeter MZM on the silicon photonic platform.

Performance Tolerance of IMDD DFMA PONs to Channel Frequency Response Roll-Off

The impacts of the channel frequency response roll-off effect on upstream optical OFDM transmission performance in digital filter multiple access (DFMA) passive optical networks (PONs) based on intensity modulation and direct detection (IMDD) are investigated, in terms of BER versus received optical power performance, optical network unit (ONU) transmission capacity, ONU launch power variation (LPV) range, and inter-ONU sample timing offset. The effect of signal bandwidth is also explored by comparing tolerance with the channel roll-off effect for two different signal bandwidths. Over all of the aforementioned aspects, good agreements are obtained between numerical simulations and experimental measurements. The results provide important insights into channel roll-off-induced performance dependencies to facilitate cost-effective designs of both DFMA transceivers and IMDD DFMA PONs.

OLT-centralized sampling frequency offset compensation scheme for OFDM-PON.

We propose an optical line terminal (OLT)-centralized sampling frequency offset (SFO) compensation scheme for adaptively-modulated OFDM-PON systems. By using the proposed SFO scheme, the phase rotation and inter-symbol interference (ISI) caused by SFOs between OLT and multiple optical network units (ONUs) can be centrally compensated in the OLT, which reduces the complexity of ONUs. Firstly, the optimal fast Fourier transform (FFT) size is identified in the intensity-modulated and direct-detection (IMDD) OFDM system in the presence of SFO. Then, the proposed SFO compensation scheme including phase rotation modulation (PRM) and length-adaptive OFDM frame has been experimentally demonstrated in the downlink transmission of an adaptively modulated optical OFDM with the optimal FFT size. The experimental results show that up to ± 300 ppm SFO can be successfully compensated without introducing any receiver performance penalties.

Effects of diversity schemes and correlated channels on OWC systems performance

The salient features and advantages of free-space optical (FSO) system are very appealing for different applications in a number of communication network sectors. In spite of the advantages and diverse applications of FSO communication, its extensive use is hindered by the atmospheric turbulence-induced fading in real-life scenarios. Spatial diversity technique is one of effective means of mitigating turbulence-induced fading and, consequently, improves the system performance. In this paper, we study the spatial diversity schemes for mitigating turbulence-induced fading in the FSO communication systems using the bit error rate (BER) as a performance metric. The schemes considered are based on orthogonal space–time block codes and repetition codes (RCs). We derive simple approximate closed-form expressions for the error probability of the log-normal FSO links with intensity modulation and direct detection (IM/DD). Furthermore, we also investigate the effects of spatial correlation between the transmit apertures on the system performance. We achieve this using the exponential model for determining the correlations between the apertures. We observe that the proposed BER expressions are able to quantify the effects of spatial diversity schemes and spatial correlations on the system.

An improved diversity combining receiver for layered ACO-FOFDM in IM/DD systems

In this paper, an improved receiver based on diversity combining is proposed to improve the bit error rate (BER) performance of layered asymmetrically clipped optical fast orthogonal frequency division multiplexing (ACO-FOFDM) for intensity-modulated and direct-detected (IM/DD) optical transmission systems. Layered ACO-FOFDM can compensate the weakness of traditional ACO-FOFDM in low spectral efficiency, the utilization of discrete cosine transform in FOFDM system instead of fast Fourier transform in OFDM system can reduce the computational complexity without any influence on BER performance. The BER performances of layered ACO-FOFDM system with improved receiver based on diversity combining and DC-offset FOFDM (DCO-FOFDM) system with optimal DC-bias are compared at the same spectral efficiency. Simulation results show that under different optical bit energy to noise power ratios, layered ACO-FOFDM system with improved receiver has 2.86dB, 5.26dB and 5.72dB BER performance advantages at forward error correction limit over DCO-FOFDM system when the spectral efficiencies are 1 bit/s/Hz, 2 bits/s/Hz and 3 bits/s/Hz, respectively. Layered ACO-FOFDM system with improved receiver based on diversity combining is suitable for application in the adaptive IM/DD systems with zero DC-bias.

Digital OFDM-PON Based on Delta–Sigma Modulation Employing Binary IM-DD Channels

In this paper, a delta–sigma modulation is proposed for the orthogonal frequency division multiplexed-passive optical network (OFDM-PON) to enable transmission of OFDM signals via binary intensity modulation and direct detection (IM-DD) channels. The delta–sigma modulation is an independent digital signal processing block that converts the analog OFDM signal into binary bit sequence. The OFDM signal can be simply retrieved from the bit sequence, using a low-pass filter at the receiver. Neither the analog-to-digital converter (ADC)/digital-to-analog converter (DAC) nor the linear modulator/detector are required, which means the proposed system can be fulfilled by employing cost-effective 10- or 40-Gb/s binary optics and electronics. Besides, the delta–sigma-modulated OFDM-PON is able to offer excellent robustness against noises and nonlinear distortions. In the experimental demonstration, the performance of proposed delta–sigma-modulated OFDM-PON system has been investigated. It is found that around a 4-dB improvement in receiver sensitivity is achieved, compared to a traditional analog OFDM-PON scheme.

Optical filter requirements in an EML-based single-sideband PAM4 intensity-modulation and direct-detection transmission system.

The feasibility of a single sideband (SSB) PAM4 intensity-modulation and direct-detection (IM/DD) transmission based on a CMOS ADC and DAC is experimentally demonstrated in this work. To cost effectively build a >50 Gb/s system as well as to extend the transmission distance, a low cost EML and a passive optical filter are utilized to generate the SSB signal. However, the EML-induced chirp and dispersion-induced power fading limit the requirements of the SSB filter. To separate the effect of signal-signal beating interference, filters with different roll-off factors are employed to demonstrate the performance tolerance at different transmission distance. Moreover, a high resolution spectrum analysis is proposed to depict the system limitation. Experimental results show that a minimum roll-off factor of 7 dB/10GHz is required to achieve a 51.84Gb/s 40-km transmission with only linear feed-forward equalization.

Ultra-high capacity WDM-SDM optical access network with self-homodyne detection downstream and 32QAM-FBMC upstream.

Towards 100G beyond large-capacity optical access networks, wavelength division multiplexing (WDM) techniques incorporating with space division multiplexing (SDM) and affordable spectrally efficient advanced modulation formats are indispensable. In this paper, we proposed and experimentally demonstrated a cost-efficient multicore fiber (MCF) based hybrid WDM-SDM optical access network with self-homodyne coherent detection (SHCD) based downstream (DS) and direct detection optical filter bank multi carrier (DDO-FBMC) based upstream (US). In the DS experiments, the inner core of the 7-core fiber is used as a dedicated channel to deliver the local oscillator (LO) lights while the other 6 outer cores are used to transmit 4 channels of wavelength multiplexed 200-Gb/s PDM-16QAM-OFDM signals. For US transmission, 4 wavelengths with channel spacing of 100 GHz are intensity modulated with 30 Gb/s 32-QAM-FBMC and directly detected by a ~7 GHz bandwidth receiver after transmission along one of the outer core. The results show that a 4 × 6 × 200-Gb/s DS transmission can be realized over 37 km 7-core fiber without carrier frequency offset (CFO) and phase noise (PN) compensation even using 10 MHz linewidth DFB lasers. The SHCD based on MCF provides a compromise and cost efficient scheme between conventional intradyne coherent detection and intensity modulation and direct detection (IM/DD) schemes. Both US and DS have acceptable BER performance and high spectral efficiency.