Performance enhancement of IM/DD optical wireless systems

Due to the fast development of telecommunications and the great demand of bandwidth, optical wireless communication (OWC) has become a promising trend. OWC offers flexible networks for both indoor and outdoor communications providing high security, high speed and a license free spectrum. Data transmission over indoor OWC systems with diffused channel model at high speed will extend the channel impulse response over several symbol periods with many reflections causing inter-symbol interference. This interference distorts the data symbols being transmitted and affects the signal recovery at the receiver side. This paper studies error control coding for OWC systems over additive white Gaussian noise and ceiling bounce channel models. It considers Hamming coding and channel equalization for unipolar optical orthogonal frequency division multiplexing (OFDM) system. The objective is to mitigate the channel distortion, and hence achieve reliable transmission and correct detection at the receiver side.     

Efficient coding techniques for ADO-OFDM in IM/DD systems

Optical wireless communication (OWC) is an age-long technology, which is based on optical data transmission through free space, and it can be implemented in both indoor and outdoor applications. Asymmetrically clipped DC-biased optical orthogonal frequency division multiplexing (ADO-OFDM) is a modulation scheme that can be utilized in indoor OWC systems. It is based on transmitting DC-biased optical OFDM (DCO-OFDM) for even-frequency subcarriers and asymmetrically clipped optical OFDM (ACO-OFDM) for odd-frequency subcarriers. The ADO-OFDM exhibits better optical power performance than those of the conventional ACO-OFDM and DCO-OFDM. The main disadvantage of ADO-OFDM is the clipping noise, which mainly affects the even subcarriers. So, in this paper, convolutional and turbo coding techniques are investigated to improve the bit error rate performance of the ADO-OFDM over Additive white Gaussian noise and diffuse channels. Simulation results show that the proposed coded ADO-OFDM using convolutional and turbo coding techniques achieves significantly lower BERs compared to that of the uncoded ADO-OFDM. In addition, turbo-coded ADO-OFDM gives the best BER performance.     

Orthogonal space?time block codes for free-space IM/DD optical links

Use of orthogonal space-time block codes (STBCs) with multiple transmitters and receivers can improve signal quality. However, in optical intensity modulated signals, output of the transmitter is non-negative and hence standard orthogonal STBC schemes need to be modified. A generalised framework for applying orthogonal STBCs for free-space IM/DD optical links is presented.     

Limitations in long haul IM/DD optical fibre systems caused by chromatic dispersion and nonlinear Kerr effect

The power penalty induced by dispersion, Kerr nonlinearity and attenuation is numerically investigated for long haul IM/DD systems with cascaded optical amplifiers. The authors demonstrate that the eye opening is critically dependant on the input mean power.<>     

Optimum system parameters for multigigabit CPFSK optical heterodynedetection systems

To obtain a multigigabit continuous-phase frequency-shift-keying (CPFSK) system with a high receiver sensitivity, we theoretically and experimentally investigated the optimum modulation index parameter and IF center frequency, considering the modulation and demodulation baseband widths and the IF bandwidth. In a 6-Gb/s CPFSK experiment, we achieved a receiver sensitivity of -41.6 dBm, or 89 photons/bit. To use the 4- to 13-GHz IF band efficiently, we set the IF center frequency f _IF to 8.7 GHz, or 1.45 times the bit rate. To compensate for the insufficient 5-GHz FM-modulation bandwidth in the transmitter, we set the modulation index parameter β to 1.38, that is, a modulation index m of 0.8     

Chirping-induced waveform distortion in 2.4-Gb/s lightwavetransmission systems

The performance of high-speed lightwave transmission systems operating in the 1.55-μm-wavelength region with conventional single-mode optical fiber can be influenced by the dynamic wavelength chirping exhibited by directly modulated semiconductor lasers. In a recent paper (1989), the authors presented the first detailed analysis of the bit error ratio performance of lightwave systems influenced by chirping-induced waveform distortion. Here, additional numerical results which address the performance implications of the waveform distortion by considering the dependence of the receiver sensitivity on the decision time and decision threshold, the eye pattern, and bit error rate ratio pattern are presented. It is shown that the response of the linear receiver filter can influence the dependence of the receiver sensitivity on the decision time     

A simplified dispersion limit formula for IM/DD systems and itscomparison with experimental results

A simplified formula for the dispersion limit of single-mode-fiber intensity modulation/direct detection (IM/DD) systems is derived for arbitrary given normalized pulse width U, eye opening penalty X and source linewidth enhancement factor α. From the comparison with published theoretical analyses, computer simulation and experimental data, its validity and the limits of existing formulae are shown. Using this simple and general formula, one can easily obtain the dispersion limit of various fiber types under different working conditions in IM/DD systems     

A PAPR reduction technique using Hadamard transform combined with clipping and filtering based on DCT/IDCT for IM/DD optical OFDM systems

In Intensity Modulator/Direct Detection (IM/DD) optical OFDM systems, the high peak-to-power average ratio (PAPR) will cause signal impairments through the nonlinearity of modulator and fiber. In this paper, a joint PAPR reduction technique based on Hadamard transformation and clipping and filtering using DCT/IDCT transform has been proposed for mitigating the impairments in IM/DD optical OFDM system. We then experimentally evaluated the effect of PAPR reduction on the bit error rate (BER) performance and the results show the effectiveness of the proposed technique. At a bit error rate (BER) of 1 × 10⁻³, the receiver sensitivity of the proposed 2.5 Gb/s IM/DD optical OFDM system after 100-km standard single-mode fiber transmission has been improved by 0.8 dB, 1.3 dB and 3.1 dB for a launch power of 6.4 dBm, 8 dBm and 10 dBm respectively when compared with the classical system.     

Techniques for multigigabit coherent optical transmission

It has been anticipated that the main roles for coherent optical techniques will be to provide increased capacity both for long-haul transmission and wide-band signal distribution. This could be achieved by the combination of high-data-rate transmission and fine-grain WDM. To this end, heterodyne detection experiments have recently been extended into the gigabit regime but difficulties not present at lower data rates have been encountered. This paper discusses a range of technical solutions to these problems, including broad-band receivers, homodyne detection, and phase diversity reception, and reviews their current status.     

Optoelectronic components for multigigabit systems

Gigabit data rates are becoming relevant for several applications areas, including computer interconnections, trunk telecommunications, and phased-array radar control. LED's, lasers, p-i-n FET's, photoconductors, and avalanche photodiodes are candidate components. Silicon NMOS, bipolar, GaAs FET, and heterojunction bipolar logic IC technologies are all appropriate and no obstacles are apparent to prevent direct modulation tosim10-20Gbit/s. Wavelength multiplexing will impact strongly in several applications enabling complex new system architectures. Increasing speed and complexity will drive technology to higher optoelectronic integration levels.     

Optoelectronic components for multigigabit systems

Gigabit data rates are becoming relevant for several applications areas, including computer interconnections, trunk telecommunications, and phased-array radar control. LED's, lasers, p-i-n FET's, photoconductors, and avalanche photodiodes are candidate components. Silicon NMOS, bipolar, GaAs FET, and heterojunction bipolar logic IC technologies are all appropriate and no obstacles are apparent to prevent direct modulation to ∼ 10-20 Gbit/s. Wavelength multiplexing will impact strongly in several applications enabling complex new system architectures. Increasing speed and complexity Will drive technology to higher optoelectronic integration levels.     

外部弱光反馈对半导体激光器IM谐波失真的抑制

本文研究了已调外腔半导体激光器(LD)谐波产生的原因,指出了在适当的条件下,外部光反馈可以改善LD的谐波失真。     

IM/DD高速光纤通信技术

高速大容量光纤通信是光纤通信的发展趋势。本文就光强度调制—直接检波(IM/DD)光纤传输方式的几个主要技术课题:高速光源、光调制器、光检波器、光放大器以及光纤色散均衡进行了讨论。     

A generalized suboptimum unequally spaced channel allocationtechnique. I. In IM/DD WDM systems

Four-wave mixing (FWM) is the most serious fiber nonlinearity associated with low-input optical power levels in long-haul multichannel optical systems employing dispersion-shifted fiber. To reduce the crosstalk due to FWM, a generalized suboptimum unequally spaced channel allocation (S-USCA) technique is proposed and investigated. Even though the developed technique is useful in combating FWM crosstalk in wavelength division multiplexing (WDM) lightwave systems with up to 12 channels, its main virtue is in designing multichannel WDM lightwave systems with more than 12 channels. Comparisons of power penalty due to FWM between equal channel spacing (ECS) systems and the S-USCA systems are presented. It is shown that for an intensity modulation/direct detection (IM/DD) transmission system operating in an optical bandwidth of 16 nm with 0 dBm (1 mW) peak optical input power per channel, while a conventional ECS WDM system with 0.84-nm channel spacing cannot even achieve a bit-error rate (BER)=10^-9, the suboptimum technique developed in this paper, for the same minimum channel spacing, can achieve a BER=10^-9 with an FWM crosstalk power of less than 1 dB at the worst channel in a 20-channel WDM system     

强度调制／直接检测（IM／DD）光纤通信系统的色散限制

本文讨论了光纤色散对ＩＭ／ＤＤ光纤通信系统再生中继距离的限制，导出了适用于各种眼图恶化量、归一化脉宽和光源线宽展宽因子，且便于工程计算的Ｂ２Ｌ表达式。通过与其它理论的比较、计算机仿真研究和对实际系统实验数据的分析，说明了该公式所得结论的正确性以及其它结果的限制条件。     

Upper-bounding the capacity of optical IM/DD channels withmultiple-subcarrier modulation and fixed bias using trigonometric momentspace method

We consider the channel capacity of an optical intensity-modulated direct-detection (IM/DD) system using multiple-subcarrier modulation (MSM) with fixed bias. The channel is modeled as an additive white Gaussian noise (AWGN) channel with nonnegative input waveform. The mean of the nonnegative input waveform is the average transmitted optical power. The mean of the waveform during a symbol period is called the d.c. bias of the symbol. In this work, a fixed bias is used for all symbols. Therefore, the power used for each symbol is constant and equals the average transmitted power. The main result of this correspondence shows that, because the input waveforms during each symbol period are nonnegative and have fixed mean, their Fourier coefficients must lie inside certain trigonometric moment spaces. These moment spaces are characterized both algebraically and geometrically. Through the geometrical characterization, we determine the volumes of these moment spaces. The channel capacities of quadrature amplitude modulation (QAM) and pulse amplitude modulation (PAM) systems are shown to be upper-bounded by sphere-packing Gaussian noise in the respective moment spaces     

A nonlinear equalization for a PAM4 IM/DD system using MZM

The nonlinear distortion caused by the Mach-Zehnder modulator(MZM) is one of the main limiting factors for the improvement of the transmission performance of the intensity modulation and direct detection(IM/DD) optical communication system. In order to solve the problem, an improved Volterra nonlinear equalization(VNLE) method is proposed. Compared with the traditional VNLE that uses the least mean square(LMS) to calculate the tap coefficients, the improved VNLE uses the least square(LS) method ...     

基于时间透镜的高速光纤频域IM/DD传输系统

探讨了采用时间透镜来实现时间-频率间的傅里叶变换原理,给出了时间透镜的系统构成,设计了基于时间透镜的正交频分复用系统。仿真实验表明,基于时间透镜的高速光纤频域IM/DD传输系统,在传输速率大于10Gb/s时,比传统的IM/DD系统抗色散性能优势明显、性价比高且实用性强。     

Two and three level optical PCM transmitter design for multigigabit systems using a relaxation oscillation technique

A theoretical method based on gain switching of semiconductor lasers is described which optimizes the bandwidth and modulated output power from a given laser for application in optical PCM systems. Both two- and three-level systems are considered. The type of laser used to demonstrate the technique is typical of an AlGaAs BH laser. A three-level optical system based on only two electrical drive levels is evaluated. Numerical results based on the single-mode rate equations, including a diffusion term, predict bit rates of 5 Gbit/s for a two-level system and 8 Gbit/s for the three level system. The technique was also applied to short cavity and quaternary lasers. For a short-cavity (100-μm) laser bit rates of 9 and 14 Gbit/s were predicted for the two- and three-level systems, respectively. For an In-GaAsP etched mesa BH laser bit rates of 10 and 16 Gbit/s were predicted for the two- and three-levels systems, respectively. The bandwidth was found to be limited by the laser parameters and catastrophic mirror damage which limits the maximum allowed drive current.     

Low-threshold four-wavelength DFB laser array for multigigabit/s high-density WDM systems applications

A high-speed four-wavelength distributed-feedback (DFB) laser array with narrow wavelength spacings ( approximately 3 nm), very low-threshold currents ( approximately 5 mA), and all wavelengths, and operating (1540-1550 nm) within the erbium-doped fiber amplifier bandwidth has been fabricated. Electrical crosstalk of less than -25 dB up to 5 GHz was measured and a small wavelength drift caused by heating effects was observed for adjacent lasers. However, neither the electrical crosstalk nor the thermal effect causes any sensitivity penalties in a high-speed wavelength-division multiplexing transmission system.<>