China
Africa
Explore chapters and articles related to this topic
Coherent Systems
Published in Jerry D. Gibson, The Communications Handbook, 2018
In the derivation of Eq. (53.3), we assumed the SOPs of the signal and LO light fields are matched. In practice, however, the SOP of the signal fluctuates in a fiber due to the birefringence change caused by the external stress or the temperature change. There are four countermeasures: (1) use of a polarization-maintaining (PM) fiber such as a PANDA (Polarization-maintaining AND Absorption-reducing) or a bow-tie fiber as the system fiber, (2) use of an active polarization controller at the receiver, (3) use of a polarization-scrambling scheme in which the SOP of the signal is scrambled within one bit at the transmitter, and (4) use of a polarization-diversity receiver. Among these countermeasures, the fourth is considered to be superior and is implemented in many receivers. Figure 53.7 shows the polarization-diversity receiver. It uses an optical circuit similar to the 90° optical hybrid. The LO light is set to be a 45° linear SOP and split by the PBS to be x and y linear SOPs having the same amplitude. The PBS splits the signal to be k:1 - k according to the power ratio of x to y elements. Then, the IF signals detected by the x and y branches have the amplitude ratio of Vk:«/l - k. Just like the phase-diversity receiver, they are fed to demodulators having square-law characteristics and combined to be baseband signal independent of the value of k. The polarization diversity scheme can be combined with phase diversity and balanced receivers.
Polarization Control
Published in Kenichi Iga, Yasuo Kokubun, Encyclopedic Handbook of Integrated Optics, 2018
Polarization scrambling: Another important class of applications for optical polarization controllers involve rapid randomization of a fixed or slowly varying SOP, which is known as polarization scrambling. This random polarization modulation has the effect of “depolarizing” the optical signal on the timescale of the polarization changes, which often helps to mitigate polarization-dependent effects in the fiber-optic transmission line. One example of such an effect is the so-called polarization hole burning in EDFA, which impairs the optical signal-to-noise ratio for polarized optical signals. Although this effect is small in a single EDFA, it can rapidly grow in long strings of concatenated amplifiers and, hence, cause severe impairments in transoceanic fiber-optic cables [22]. It turns out that rapid scrambling of the polarization state of the launched optical signal with just a single polarization controller efficiently suppresses polarization hole burning throughout the entire chain of amplifiers [23,24].
Next Generation Transmission Systems Enabling Technologies, Architectures, and Performances
Published in Iannone Eugenio, Telecommunication Networks, 2017
Polarization scrambling technique is quite effective in reducing penalty fluctuations due to the random nature of the PMD, with the additional advantage of being able to process a whole WDM comb without demodulation, but while the bit rate increases, it is more and more difficult to perform a perfect scrambling, that is, scramble at a so high speed that almost all the polarizations states are traversed during the bit time. Let us imagine that the scrambling speed is not sufficient, in particular, let us assume that it is 20% smaller than the speed needed to span all the possible states of polarization.
Bidirectional coherent optical communication system combining unidirectional optical signal amplification
Published in Journal of Modern Optics, 2020
Shiwen Jin, Shuqiang Chen, Miao Yan, Yuanyuan Jiang
A standard steady state numerical model [14,15] is used to simulate the evolution of signals and pumps in the amplifier. We separate the pump light, modulated signal and ASE light to facilitate analysis and solution. The power evolution of the pump light or modulated wave can be described by Eqs. (1) and (2) as [12]: where Ps represents the modulated signal power, and Pp the pump power. αs or αp is the modulated signal or pump transmission loss. Here, the gain factor is defined as: g = g0/(Aeff·Keff), where Aeff is the effective area of the fiber, and Keff the polarization scrambling factor between pump light and modulated signal (Keff varies from 1 to 2 and Keff = 2 for depolarized case). Neglecting the pump depletion, the Raman ON–OFF gain expression of the modulated wave can be derived analytically as: With or without the Fiber Bragg grating, the ASE noise spectral density can be obtained by Eqs. (4) and (5). where L is total DCF length. Plank's constant and centre frequency are represented by h and υ respectively. is the passive fiber loss, thus the net gain from distance z1 to z2 is .
Synthetic Diagnostic for Interpreting the ECE Spectrum on EAST
Published in Fusion Science and Technology, 2018
Tianfu Zhou, Yong Liu, Ang Ti, Lorenzo Figini, Hailin Zhao, Zeying Zhu, Bili Ling
The spectral domains, for which the plasma is optically thin, are sensitive to the wall reflection and polarization scrambling effects. The reflections of the microwaves at the inner wall (molybdenum) of EAST are modeled, assuming that the plasma lies between two parallel walls with multiple passes of the beam occuring along the same path in the plasma and that the walls have an effective reflection coefficient r and a depolarization coefficient p. This model was developed in Ref. 26 and extended in Ref. 22. The emission detected at O- or X-mode is calculated from the single-pass intensity and optical thickness as . To obtain a reasonable description of these effects, the reflection and polarization transfer fraction coefficients are adjusted empirically. Finally, a good agreement between the simulation results and the experimental results for most ohmic cases is achieved with the fixed values (r = 0.7, p = 0.32). An example of the comparison of the EAST pulse 48600 and time 2.9 s is shown in Fig. 2, and the main plasma parameters are plasma current , central electron density , central electron temperature , and magnetic field strength at the major radius 1.85 m. The simulation results fit with the measurements fairly well.