However, the development of coherent communication has been delayed for nearly 20 years after that period [ 25 , 26 ]. Coherent optical detection re-attracted the research interests until , since the advanced modulation formats, i. In addition, coherent optical detection allows the electrical mitigation of system impairments. With the two main merits, the reborn coherent detections brought us the enormous potential for higher transmission speed and spectral efficiency in current optical fiber communication systems [ 31 , 32 ].
With an additional local oscillator LO source, the sensitivity of coherent receiver reached the limitation of the shot-noise. Furthermore, compared to the traditional intensity modulation direct detection system, the multilevel modulation formats can be applied using the phase modulations, which can include more information bits in one transmitted symbol than before.
Meanwhile, since the coherent demodulation is linear and all information of the received signals can be detected, signal processing approaches, i. The typical block diagram of the coherent optical transmission system is shown in Figure 1. The transmitted optical signal is combined coherently with the continuous wave from the narrow-linewidth LO laser so that the detected optical intensity in the photodiode PD ends can be increased and the phase information of the optical signal can be obtained.
The use of LO laser is to increase the receiver sensitivity of the detection of optical signals, and the performance of coherent transmission can even behave close to the Shannon limit [ 3 , 12 ].
The development of the coherent transmission systems has stopped for more than 10 years due to the invention of Erbium-doped fiber amplifiers EDFAs [ 1 , 2 ]. The coherent transmission techniques attracted the interests of investigation again around , when a new stage of the coherent lightwave systems comes out by combining the digital signal processing techniques [ 41 — 46 ]. This type of coherent lightwave system is called as digital coherent communication system. In the digital coherent transmission systems, the electrical signals output from the photodiodes are sampled and transformed into the discrete signals using high-speed analogue-to-digital convertors ADCs , which can be further processed by the DSP algorithms.
The phase locking and the polarization adjustment were the main obstacles in the traditional coherent lightwave systems, while they can be solved by the carrier phase estimation and the polarization equalization, respectively, in the digital coherent optical transmission systems [ 47 — 55 ].
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Besides, the chromatic dispersion and the nonlinear effects can also be mitigated by using the digital signal processing techniques [ 56 — 62 ]. The typical structure of the DSP compensating modules in the digital coherent receiver is shown in Figure 2. In this section, the chromatic dispersion compensation, polarization mode dispersion equalization, and carrier phase noise compensation are analyzed and discussed using corresponding DSP algorithms. The characters of these filters are analyzed based on a Gbaud dual-polarization quadrature phase shift keying DP-QPSK coherent transmission system using postcompensation of dispersion.
The TD-LMS filter employs an iterative algorithm that incorporates successive corrections to weights vector in the negative direction of the gradient vector, which eventually leads to a minimum mean square error [ 34 , 38 , 63 — 65 ]. The distribution of the magnitudes of the converged tap weights is plotted in Figure 3 b , and it is found that the central tap weights take more dominant roles than the high-order tap weights [ 34 , 66 ].
Since the complexity is very low for compensating large CD [ 34 , 70 ], the most promising and popular chromatic dispersion compensation filters in coherent transmission systems are the frequency domain equalizers. The transfer function of the frequency domain equalizers is given by the following expression:.
The frequency domain equalizers are generally implemented using the overlap-save OLS and the overlap-add OLA approaches based on the fast Fourier transform and the inverse fast Fourier transform iFFT convolution algorithms [ 71 — 73 ], as described in Figure 5. Due to the random character of the polarization mode dispersion and the polarization rotation, the compensation of the PMD and the polarization rotation are generally realized by the adaptive algorithms such as the least-mean-square LMS and the constant modulus algorithm CMA filters.
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In the electrical domain, the impact of the PMD and the polarization fluctuation can be adaptively equalized using the decision-directed LMS DD-LMS filter [ 36 , 63 ], of which the transfer function is given by:. The influence of the PMD and the polarization fluctuation can also be compensated employing the CMA adaptive filter [ 74 , 75 ], of which the transfer function can be described as:. It can be found that the CMA algorithm is based on the principle of minimizing the modulus variation of the output signal to update its weight vector.
In this section, the analyses on different carrier phase estimation algorithms, involving the one-tap normalized LMS, the differential phase estimation, the block-wise average BWA , and the Viterbi-Viterbi VV methods in the coherent optical transmission systems, will be presented. The one-tap normalized LMS filter can be employed effectively for carrier phase estimation [ 76 — 78 ], of which the tap weight is expressed as:. It has been demonstrated that the one-tap normalized LMS carrier phase estimation behaves similar to the differential phase estimation [ 28 , 53 , 55 , 76 ], of which the BER floor in the m -PSK coherent optical transmission systems can be approximately described by the following analytical expression:.
The schematic of the one-tap normalized LMS carrier phase estimation is illustrated in Figure 6. Share your experience with other students. Write review. Become a Data Scientist datacamp. Build and share your own catalog of courses with Class Central's custom lists. This course offers a gradual approach to optical communications with emphasis on latest developments in coherent optical communications.
Starting from a broad introduction to transmitters and receivers, this course covers optical fibers and waveguides, lasers, detectors, optical amplifiers, channel impairments and their mitigation using signal processing algorithms. Matlab models will be discussed. Get personalized course recommendations, track subjects and courses with reminders, and more. Home Subjects Engineering Electrical Engineering.
Third-generation fiber-optic systems operated at 1.
This development was spurred by the discovery of Indium gallium arsenide and the development of the Indium Gallium Arsenide photodiode by Pearsall. Engineers overcame earlier difficulties with pulse-spreading at that wavelength using conventional InGaAsP semiconductor lasers.talilessmidp.gq
Digital Optical Communications
Scientists overcame this difficulty by using dispersion-shifted fibers designed to have minimal dispersion at 1. These developments eventually allowed third-generation systems to operate commercially at 2. The fourth generation of fiber-optic communication systems used optical amplification to reduce the need for repeaters and wavelength-division multiplexing to increase data capacity. The focus of development for the fifth generation of fiber-optic communications is on extending the wavelength range over which a WDM system can operate. The conventional wavelength window, known as the C band, covers the wavelength range 1.
Other developments include the concept of " optical solitons ", pulses that preserve their shape by counteracting the effects of dispersion with the nonlinear effects of the fiber by using pulses of a specific shape. In the late s through , industry promoters, and research companies such as KMI, and RHK predicted massive increases in demand for communications bandwidth due to increased use of the Internet , and commercialization of various bandwidth-intensive consumer services, such as video on demand.
Internet protocol data traffic was increasing exponentially, at a faster rate than integrated circuit complexity had increased under Moore's Law.
Advanced Digital Optical Communications (Optics and Photonics)[PDF] A…
From the bust of the dot-com bubble through , however, the main trend in the industry has been consolidation of firms and offshoring of manufacturing to reduce costs. Modern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send through the optical fiber, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal.
The information transmitted is typically digital information generated by computers, telephone systems and cable television companies. The most commonly used optical transmitters are semiconductor devices such as light-emitting diodes LEDs and laser diodes. The difference between LEDs and laser diodes is that LEDs produce incoherent light , while laser diodes produce coherent light. For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient and reliable, while operating in an optimal wavelength range and directly modulated at high frequencies.
In its simplest form, an LED is a forward-biased p-n junction , emitting light through spontaneous emission , a phenomenon referred to as electroluminescence. However, due to their relatively simple design, LEDs are very useful for low-cost applications. The large spectrum width of LEDs is subject to higher fiber dispersion, considerably limiting their bit rate-distance product a common measure of usefulness.
LEDs have also been developed that use several quantum wells to emit light at different wavelengths over a broad spectrum and are currently in use for local-area WDM Wavelength-Division Multiplexing networks. The narrow spectral width also allows for high bit rates since it reduces the effect of chromatic dispersion.
Roadmap of optical communications
Furthermore, semiconductor lasers can be modulated directly at high frequencies because of short recombination time. Laser diodes are often directly modulated , that is the light output is controlled by a current applied directly to the device. For very high data rates or very long distance links , a laser source may be operated continuous wave , and the light modulated by an external device, an optical modulator , such as an electro-absorption modulator or Mach—Zehnder interferometer.
External modulation increases the achievable link distance by eliminating laser chirp , which broadens the linewidth of directly modulated lasers, increasing the chromatic dispersion in the fiber. A transceiver is a device combining a transmitter and a receiver in a single housing see picture on right. Fiber optics have seen recent advances in technology. The main component of an optical receiver is a photodetector which converts light into electricity using the photoelectric effect.
The primary photodetectors for telecommunications are made from Indium gallium arsenide. The photodetector is typically a semiconductor-based photodiode. Several types of photodiodes include p-n photodiodes, p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal MSM photodetectors are also used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.
Optical-electrical converters are typically coupled with a transimpedance amplifier and a limiting amplifier to produce a digital signal in the electrical domain from the incoming optical signal, which may be attenuated and distorted while passing through the channel. Further signal processing such as clock recovery from data CDR performed by a phase-locked loop may also be applied before the data is passed on. An optical communication system transmitter consists of a digital-to-analog converter DAC , a driver amplifier and a Mach—Zehnder-Modulator. Digital predistortion counteracts the degrading effects and enables Baud rates up to 56 GBaud and modulation formats like 64 QAM and QAM with the commercially available components.