With the beginning of large-scale deployment of 40Gb/s, the industry has emerged a variety of new types of 100G/s modulation encoding format. Face of the code for the transmission of many different characteristics, in the comprehensive consideration of other system design parameters based on industry mainly from the transmission distance, channel spacing, and 40Gb / s and 10Gb / s system compatibility and module cost and transmission performance of balance of comprehensive selection.
With the development of high speed digital signal processing (DSP) and analog to digital conversion (ADC), coherent optical communication has become a hot research topic. The combination of coherent detection and DSP technology can carry out the carrier phase synchronization and polarization tracking in the electric field, and the two major obstacles of the conventional coherent reception are eliminated. Based on DSP of the coherent receiver structure is simple and is transparent to hardware of; in electrical domain compensation of various transmission impairments, and simplify the transmission link, reduce transmission cost; support multi band modulation format and polarization multiplexing, to achieve high spectral efficiency of the transmission. Through the industry for one or two years for the research and development of 100Gb/s module, the 100G/s four phase shift keying (Coherent) module (PM-QPSK) is becoming the main choice of the industry.
Basic principles of coherent optical communication
The coherent optical communication system can be divided into many channels, so that the optical frequency band is fully utilized, that is, multi channel optical fiber communication. The coherent optical communication technology has the advantages of high sensitivity, and the receiving sensitivity of the coherent detection technology is higher than that of the direct detection technique 18dB.
As the carrier of the laser signal through PBS (polarization beam splitter) divided into two X / y, each signal in by two MZ modulator composed of I / Q modulator (I or Q phase difference is 90 DEG) were the 10.7/27.5Gb/s signal modulated carrier, then the X axis and the Y axis optical signal according to the polarization multiplexing combined together through optical fiber to send out through the polarization multiplexer, so as to realize the transmission on a single fiber 40/100Gb/s.
In the receiver, and intensity modulation of one direct detection system, coherent optical fiber communication system in optical receiver increases the heterodyne and homodyne reception required local oscillation source (LO), with the output of the light source of light to receive the modulated light wave in wave fronts meet polarization matching and matching conditions, photoelectric mixing. To change the optical frequency of the laser, the selected channel can be changed, so the linewidth of the laser is very high. After mixing, the output of the signal is proportional to the square of the square of the intensity and the intensity of the light wave. Due to the variation of the difference frequency signals and the variation of the optical signals are the same, and unlike straight detection means of communication that current detection only reflected light intensity, therefore, can be realize the amplitude, frequency, phase and polarization modulation.
Due to the polarization multiplexing signal detection, accept signals through a polarization beam splitter (PBS) PolarizationBeamSplitter decomposed into two orthogonal signals, each of the orthogonal signals are with a local source Lo mixing, the carrier frequency of the local light source control precision for hundreds of kHz. 4 polarization and phase orthogonal optical signals are obtained by mixing, and the is detected by PIN, and the digital signal is converted to digital signal by A/D circuit. Digital signal processing (DSP) chip digital equalization method: timing recovery, signal recovery, polarization and PMD tracking, as well as dispersion compensation.
Main advantages of coherent optical communication
The coherent optical communication has the characteristics of full use of the coherent communication mode with the mixing gain, excellent channel selectivity, and the characteristics of the. The coherent optical communication system has the following unique advantages compared with the IM/DD system:
(1) high sensitivity, long relay distance
One of the most important advantages of coherent optical communication is that it can be coherent detection, so as to improve the sensitivity of the receiver. In the coherent optical communication system, the size of the output photocurrent is proportional to the product of the signal light power and the power of the product. Under the same conditions, coherent receiver sensitivity about 18db higher than ordinary receiver, can achieve close to bulk grain noise limit of high performance, and therefore increase the signal light no relay transmission distance.
(2) good selectivity, communication capacity
(3) the use of the equalization technique of electronics to compensate for the dispersion effect of optical pulses in optical fibers is another major advantage of the coherent optical communication, which can improve the selectivity of the receiver. In direct detection, the receiver is larger, in order to suppress the interference of noise, the detector is usually required to place a narrow band filter, but the band is still very wide. In coherent heterodyne detection, the detection is optical signal and local oscillator light mixing light, so only in the medium frequency band noise can enter the system, and other noises are narrow bandwidth of microwave frequency amplifier filter. Can be seen that the heterodyne detection has a good filtering performance, which will play a major role in the application of coherent optical communication. In addition, because of the excellent wavelength selectivity of coherent detection, coherent receiver can the frequency interval of the frequency division multiplexing system is greatly reduced and dense wave divided multiplexing (DWDM), to replace the traditional recovery technology of large frequency intervals, with frequency division multiplexing to achieve the potential advantages of higher transmission rate.
If the transmission function of the intermediate frequency filter in the coherent optical communication is just the opposite to that of the optical fiber, the influence of the fiber dispersion on the system can be reduced.
Key technology of coherent optical communication
In order to achieve accurate, effective and reliable coherent optical communication, the following key technologies should be adopted:
(1) light source technology
In the coherent optical fiber communication system, the signal source and the light source are relatively high, which requires narrow spectral line and high frequency stability. Connaught line width of the light source itself will decide which the system can achieve the lowest bit error rate, should try to reduce, and semiconductor laser frequency on temperature and injection current change is very sensitive, changes in the volume of its general around dozens of GHz/ DEG C and dozens of GHz/mA. Therefore, for the frequency stability, in addition to the injection current and temperature stability, also should take other measures to stabilize the frequency, the light frequency remained stable.
(2) receiving technology
The receiving technology of coherent optical communication includes two parts, one part is the receiving technology of the optical receiver, the other is the demodulation technology of the various formats of the intermediate frequency.
Balanced reception method: in the FSK system, because the semiconductor laser in the modulation process, it is inevitable with the additional amplitude modulation noise, the use of balanced reception method can reduce the amplitude modulation noise. Main idea of balance method is when the signal light from entering the optical fiber, the vibration light through a polarization control adapted to the state of polarization of the guarantee and the signal, the vibration signal light and light at the same time after refined direction is divided into two parts, respectively, the input two identical pin photo detector, the two photoelectric detector output is amplitude and phase envelope signal, and then the two signal synthesis, the FM signal is doubled, and parasitic amplitude modulation noise offset each other, also counteract the DC component to eliminate the impact of noise amplitude modulation.
Polarization control technology: the receiving end of the coherent optical communication system must be required to obtain a good mixing effect and improve the receiving quality. Signal light after a long distance transmission in single mode fiber, polarization is random fluctuation, in order to solve this problem, put forward many methods, such as the use of polarization maintaining fiber, polarization controller and a polarization diversity reception method. In the light of ordinary optical fiber transmission, phase and polarization will change randomly, PMF is through the process of the selection of the material and the light phase and polarization maintaining optical fiber special constant, but the fiber loss is large, the price is very expensive; the polarization controller is mainly to signal light and local oscillator light with bias, this the method is slow response speed, loop control requirements are relatively high; polarization diversity is the use of signal light and local oscillator light mixing, by polarizing beam splitter mixing light into two polarized components perpendicular to each other, the local light two vertical polarization component is controlled by a polarization controller, the two components of power equal, so that when the polarization of signal light in random fluctuation may cause a branch of the intermediate frequency signal fading, but if signal is another branch still exists, so this system finally get the demodulated signal and the signal light polarization almost irrelevant, the response speed is faster, more practical, but more complicated.
(3) external optical modulation technique
Because of the semiconductor laser optical carrier of a certain parameter directly modulated always attached parasitic oscillation of the other parameters, such as ask direct modulation accompanied by changes in the phase and modulation depth will be restricted. In addition, the frequency of the characteristics of the uneven and the problem of the delay is not smooth. Therefore, in the coherent optical communication system, in addition to the FSK can be used to direct injection current frequency modulation, the other is the use of external optical modulation.
(4) nonlinear crosstalk control technology
Because of the dense frequency division multiplexing technology in coherent optical communication. Therefore, the nonlinear effect in the optical fiber  may make the signal intensity and phase of the coherent optical communication channel is affected by other channel signals, and the nonlinear crosstalk is formed.
Because in the last few years, in optical devices has made great progress, which the output power of the laser, linewidth, stability and noise, as well as the photodetector bandwidth, power capacity and the common mode inhibition ratio is greatly improved and the performance of microwave electronic devices amplitude increases. These advances have made the commercial use of the coherent optical communication system possible.