Optical Waveguides and Fibers

Introduction In the preceding chapters we discussed the characteristics of optical fibers, optical sources, and optical detectors. These form the three basic units of any optical fiber communication system. In this chapter we discuss how these basic elements can be put together to build a simple point-to-point optical fiber communication link. Let us assume that we need to transmit information between two points (see Figure 13.1). The separation between these points could range anywhere from less than a kilometer in the case of computer data links to several thousands of kilometers, for instance, in transoceanic links. In all such links, there would be a transmitter that could be either an LED or a laser diode, the transmission path consisting of optical fibers that could be either multimode or singlemode fibers, and the optical receiver that could be a PIN or APD followed by the detection electronics. The choice of the components would depend on the distance as well as on the bit rate. When the separation between the points is greater than about 50–100 km, then because of attenuation in the link or pulse dispersion, it becomes necessary to use regenerators that consist of a receiver–transmitter combination. These regenerators detect the pulse stream before the power becomes too low or the pulses become unresolvable and retime, reshape, and regenerate (and, hence, the name 3R repeater) a new set of optical pulses to be transmitted over the next part of the link. For links that are limited by loss rather than by dispersion, one can use optical amplifiers in place of regenerators.

Nowadays, the coherent optical communication system plays an important role in communication field because of large capability and bandwidth. A coherent optical communication, based on high-order modulation and digital signal processing technologies, consists of optical transmitters, optical fiber lines, optical amplifiers and optical receivers. In the high-speed coherent optical communication system, the phase noise from the transmitter laser and the local oscillator laser can significantly degrade the performance of the signal transmission and detection, especially for the systems using high-order modulation format, such as m-ary phase shift keying (mPSK) and m-ary quadrature amplitude modulation (m-QAM). Therefore, investigations on laser phase noise compensation algorithm based on digital signal processing technologies has become more and more significant. In this work, a multi-ring carrier phase recovery algorithm is developed for compensating the laser phase noise in optical fiber communication systems using high-order modulation formats. Degradations on the performance of communication systems due to the laser phase noise have been investigated. The system performance using the proposed algorithm and the conventional Viterbi-Viterbi algorithm were also evaluated in 9-channel and 15- channel, 32-Gbaud, Nyquist-spaced QPSK, 16-QAM, 64-QAM and 256-QAM coherent transmission systems with considering the impact of the laser phase noise. It is found that the phase noise leads to stricter constraints on the linewidths of transmitter-side and receiver-side lasers, and it can greatly degrade the achievable information rates in communication systems. Besides, compared to the conventional Viterbi-Viterbi algorithm, which is usually applied in the QPSK system, our proposed algorithm can also well mitigate the laser phase noise in 16-QAM, 64-QAM and 256-QAM optical communication systems.
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Characterization and Modeling of SAGCM InP/InGaAs Avalanche Photodiodes for Multigigabit Optical Fiber Communications

光纤通信已经成为目前通信网络发展的主流方向。但是随着光纤通信系统向着超远距离、超大容量、超高速率的方向发展，光纤色散已经成为其巨大的阻碍。如何有效地控制光通信系统中产生的色散，使光信号在大容量、高速率、长距离传输中无误码地到达接收端，已经成为人们迫切需要解决的问题。对光通信系统中的色散进行监测并进行动态补偿的研究，使得超大容量、超长距离、超高速率的光纤通信系统成为可能，有助于最终实现全光智能自动交换网络。在众多测量光纤色散的方法中，调制相移法具有结构简单，可靠性强等优点。本文中，我们对使用调制相移法测量光纤色散进行了研究和仿真，讨论了在不同光纤长度和不同调制频率下，影响调制相移法测量结果精度的因素。通过适当地选取调制频率，我们可以使用调制相移法测得不同光纤长度下色散的精确值。另外在本文中，我们通过仿真验证了调制相移法可以测量具有任意色散–波长曲线的光纤。因此在实际应用中，调制相移法是可以应用于高精度光纤色散的测量。 Optical fiber communication is the mainstream of communication development at present. However, with the development of optical fiber communication system which trending towards ultra-long length, ultra-large capacity and ultra-high speed, fiber dispersion turned to be a great obstacle. How to control the dispersion created in the optical communication system effectively while force optical signal to reach the receiving end in high-capacity, high-rate and error-free long-distance transmission has increasingly become a research focus of optical fiber communication systems. It is a great significance to research about monitor and dynamic compensate dispersion in the optical communication systems, so that the ultra-large capacity, ultra-long length and ultra-high speed optical fiber communication systems would not be impossible, all-optical intelligent network with automatic switching functions would come true. Among the varieties of methods to measure fiber dispersion, the phase-modulation shift method is the unique one, which has the advantages of simple structure, high reliability. In this paper, we make a research and simulate to use the phase-modulation shift method, meanwhile we also discuss about the factors which affect the accuracy of the results in the different fiber lengths and modulation frequencies. By the means that selecting appropriate modulation frequency, we can use modulation phase shift method to measure the exact value of dispersion comes out from different lengths of fiber. Moreover, this paper also proves that the phase-modulation shift method is able to measure the optical fiber with arbitrary dispersion-wavelength curve, which is already verified by our simulation. Therefore, the phase-modulation shift method is well to be applied to take practice in the high-precision fiber dispersion measurement.

Optical fiber communication is favored by many communication manufacturers because of its low interference, large capacity, long distance and low consumption. Extending the transmission distance of optical fiber communication system will result in attenuation loss. Therefore, it is necessary to improve the incident power of the initial optical fiber, but it will be affected by Raman scattering effect, thus reducing the performance of the overall optical fiber communication system. In order to overcome the above problems, Raman amplifier is used to pump high-power short-wavelength light wave and amplify long-wavelength and low-power signals, but the flattening of signal gain needs to be considered. In this study, the working principle of Raman amplifier is introduced, the Raman pump module is optimized, the system architecture of the module is studied, and the module circuit is designed. It mainly covers the structure design, driving circuit design and auxiliary circuit design of the module. In the experimental process, based on the transmission coupling equation of the fiber Raman amplifier, the distribution of pump and signal power on the fiber is calculated by numerical analysis. By optimizing the pump module, the gain flatness of the amplification system is optimized, and the signal gain of the whole fiber communication system is reduced by more than 80%. It proves that the optimal design of fiber Raman amplifier proposed in this study is effective and can achieve the overall stability of signal reception in the application of optical fiber communication system.

Guest Editorial

In the optical fiber communication system of high-speed and large capacity, the dispersion is one of the main technical indicators. The dispersion causes the light pulse broadening, so it leads to the distortion of signal transmission and BER (bit error rate). The main research is the single-mode optical fiber dispersion compensation. The design and performance of system structure on the dispersion compensation fiber communication system is accomplished by using of the optic system optical fiber communication system simulation. Through the analysis of system structure and the research of the performance of the key components of the system, the output signal waveform is simulated and the influence of the output signal waveform by the system structure and key components performance of the receiving end is compared. The design can reduce the system dispersion.

In the modern world, bandwidth and data rates are two most important parameters under consideration in any communication system. An optical fiber communication system is capable of transmitting the data at high bit rates and has a large bandwidth capacity. This high data rate and bandwidth capacity in an optical fiber communication system is further enhanced by using wavelength division multiplexing (WDM) in conjunction with the information transmission system, in which information from multiple sources are transferred using the same fiber, at the same time, but at different wavelengths. But the high capacity and data rates in optical WDM system is limited by nonlinear effects that occur in optical WDM system, which not only pose a limitation on the channel capacity and data rates but also degrades the performance of the data transmission system. In this paper, analysis has been done to investigate the efficiency of Four Wave Mixing (FWM) effect under the influence of different system parameters such as chromatic dispersion coefficient, channel spacing, transmission power level, effective area of fiber and fiber length in the form of input signals and output signals frequency spectrums.

We propose a method of modulation format identification based on compressed sensing using a high-order cyclic cumulant combined with a binary tree classifier. Through computing the fourth-order cyclic cumulant of the pretreated band signal, which is obtained by compressed sensing with the sampling rate much less than the Nyquist sampling value, the feature vector for classification is extracted. Simulations are carried out in the optical coherent fiber communication system with different modulation formats of multiple phase-shift keying and multiple quadrature amplitude modulation. The results indicate that this method can identify these modulation formats correctly and efficiently. Meanwhile, the proposed method is insensitive to laser phase noise and signal noise.

Communication technologies such as wireless and optical fiber are vital components of today's environment. As demand for increased data transfer and new technologies increases, novel research must be performed to combine the benefits of both technologies. In this case, optical communication technologies serve as the backbone of wireless technology. Currently, only a few studies have been conducted on Free Space Optical (FSO) communication employing Field Programmable Gate Arrays (FPGAs) and VerticalCavity SurfaceEmitting Lasers (VCSELs). When VCSEL is paired in conjunction with FPGA, there are only very few existing designs. These published researches cover many subjects yet are exceedingly complex and fragmented. The principal purpose of this Systematic Literature Review (SLR) is to identify the research trends and emerging themes in the development of FPGA-based Optical Hybrid Fiber - FSO Communication Systems. This review incorporates articles from prestigious journals that were indexed in scientific databases such as Science Direct, Shodh Ganga, IEEE Xplore, Google Scholar, and Scopus. This SLR exhibits the reliability and compatibility of FPGA-based FSOs when used with a lowcost optical source VerticalCavity SurfaceEmitting Laser Diode VCSEL), a range of technologies, communication methods, and approaches. This FPGA-based FSO is designed for optical communication applications that require highly secure transmission schemes, low radiation noise, a low bit error rate (BER), and increased data transfer rates. Additionally, VCSEL has a current threshold that is low, maximum conversion efficiency of power, and least consumption of power. Findings of this study can be used to direct future research in the area of hybrid Fiber-FSO optical communication systems development utilizing VCSEL and FPGA.

As an important application of laser technology, optical fiber communications are key enablers for the current information age. With the emergence of new technologies including Internet of Things, big data, cloud computing, virtual reality, and artificial intelligence, there are growing needs in the society for high capacity information transmission, which puts forward higher requirements for the optical fiber communication technology. Based on a systematic review of the development status of the technology in China and abroad, this paper analyzes the problems and challenges it faces in the application of specific scenarios, and prospects the development of some related technologies in the future. Through analysis, the challenges in optical fiber communication systems with an ultra-large capacity can be solved by increasing the transmitted power, raising the bandwidth of optical amplifiers, and conducting research on low-loss optical fibers and technologies related to space division multiplexing. The cost problems of the systems in other scenarios are also considered. In general, the optical fiber communication technology is evolving toward the direction of ultra-large capacity, intelligence, and integration. It is hoped to realize intelligent monitoring of network parameters, as well as ultra-long distance and ultra-large capacity transmission. With the development of devices and integration technology, the optical fiber communication industry continues to move forward toward the goal of high performance and low cost.

At present, optical fiber communication systems are developing in the direction of large capacity, high speed, long distance, etc., making nonlinear effects and polarization mode dispersion (PMD) and other optical fiber performance defects that were negligible for low-rate systems become the main factors limiting system upgrades and transmission distances. Polarization mode dispersion is an important factor affecting the performance of optical fiber communication systems, and it has become one of the main obstacles to the development of high-speed and long-distance optical fiber communication systems. It can cause pulse widening in the digital transmission system, thereby limiting the transmission rate of the system. Therefore, the influence of polarization mode dispersion on the transmission system and the compensation method are analyzed in this paper.

We study the information capacity for an multi-span optical fiber transmission system with losses, dispersion, nonlinearity and amplified spontaneous emission (ASE) noise. A perturbation technique is developed to examine the information capacity, and a semi-analytic expression of the conditional probability distribution function is determined initially. A novel semi-analytical form of the information capacity was obtained, which can be extended to the wavelength division multiplexing (WDM) optical fiber communication systems. Computer simulations are executed for various parameters of the transmission system. The studied results indicate that the capacity does not monotone increase with the input power and is affected by the nonlinear coefficient prominently.

A fiber optics is a transparent, flexible fabric made of plastic or glass with a diameter slightly bigger than human hair. It's mostly used to transmit light from one fiber end to the other. This kind of transmission is often used in fiber optic communication, where an optic fiber link delivers data over a longer distance and with a higher bandwidth than an electrical line. The optical fiber communication system and network is now undergoing a significant transformation from a static, scalable, energy-efficient, and flexible design to a dynamic, scalable, energy-efficient, and adaptable approach. We look at the evolution of optical network technologies and methods in the fifth century in this article. The fibre optics biosensor is extremely important in the production of biosensors since it helps to quickly miniaturise and integrate those targets. Fiber optics has advanced dramatically since the fourth generation, with data rates reaching 40-60 tbps, repeater spacing reaching 35000 km, and wavelengths reaching 1.5 micrometres.

The upside down taper lens (UDTL) optical fiber ends are formed by a heating method. This new type of microlens can be used to make optical fiber connector between two fibers. In this paper, using plastic optical fiber, a single installation which are needed in the making UDTL fiber ends is described. The making method of this type plastic optical fiber end is introduced. Theoretically, using 1D heat conductor model, we analyze the forming tendency during the heating of the end of a plastic fiber. Experiments of the coupling between two plastic optical fibers with UDTL ends are given. The testing results of the connector shows that this novel optical fiber device can improve the coupling efficiency and provided a potentially inexpensive, simple and direct coupling method for fiber optic communication and fiber sensor systems.