In the past, several frequency-domain channel-estimation methods were proposed for coherent optical orthogonal frequency division multiplexing systems, which use offset-quadrature-amplitude modulation (CO-OFDM-OQAM). A common challenge for these methods is that the training sequences they use, even though they increase the magnitude of the pseudo-pilots, can make the peak-to-average power ratio (PAPR) of the signal worse. To address this problem, an innovative modified frequency-domain channel estimation method is designed and investigated for a polarization-division-multiplexed CO-OFDM-OQAM system that uses dual-dependent pilots. The operating principle of this new method is described in detail. Then, its PAPR performance and channel-estimation capability are examined using numerical simulations. The proposed method not only ensures a sufficiently large magnitude of the pseudo-pilot but also does not seriously degrade the PAPR of the signal. It is also found that the magnitude of its pseudo-pilot (2.3136) exceeds that of the interference approximation method with real pilots (1.1086), and it is only slightly worse than that of the enhanced IAM with complex pilots (2.6074). The PAPR of the new method (max 11.4 dB) is much lower than that of other frequently used methods. Finally, the channel-estimation capability of the proposed method is validated for both back-to-back and long-distance transmission scenarios. Furthermore, the BER performance of several different channel-estimation methods is also compared. The obtained results confirm the above observations.
In this paper, we propose a wide-range LFO estimation and correction procedure for a coherent optical FBMC/OQAM system. Using a deliberately designed preamble, this method can not only precisely estimate the fractional frequency offset, but also effectively implement the estimation of the integer frequency offset. After a correction of the frequency offset, the final channel responses are acquired by inter-frame averaging and cubic spline interpolation. We investigated the accuracy and stability of this method under the influence of fiber dispersion and nonlinearity. The results from numerical simulation experiments show that the proposed low complexity technique is fully applicable to fiber channel and can significantly improve the performance of CO-FBMC/OQAM systems with respect to the bit-error-rate.
Intermodal Brillouin frequency shift and Brillouin gain spectrum in few-mode fibers are investigated by full vectorial finite element method, and the influences of pump power on the time delay and pulse broadening factor are also simulated. The simulation results show that Brillouin gain of intermodal stimulated Brillouin scattering varies with different modes pairs. Time delay increases with increasing of pump power. Pulse broadening factors decrease with the input signal pulse width but increase with the input pump power. Optimized results show that time delay of LP01 - LP01mode pair is 213.2ns, and the corresponding pulse broadening factors is 1.126.
In this paper, we propose a nanoscale plasmonic structure, which consists of a MIM bus waveguide coupled to a triple-cavity resonator system arranged in a T-shape. Numerical simulations by finite element method (FEM) and CMT are carried out to investigate the transmission spectra and field distribution of this special design. The results show that the PIA effect can be observed in the proposed structure with shoulder-coupling configuration. The group delay time at the center wavelength of PIA window can achieve to -0.232 ps using our proposed structure. We also dicuss the possible methods for controlling fast light characteristics. It is turn out that the PIA effect can be effectively controlled by varying the structural parameters of this plasmonic coupling system.
In this work, we propose a schematic design for the biochemical sensor, which is based on the polyatomic photonic crystal ring resonator (PCRR). Unlike with the conventional approach, the proposed PCRR is constituted by two different branching waveguides (WG), which are all in the same lattice direction but have different optical propagation properties due to the binary nature of the diatomic square lattice. Electromagnetic analysis via PWE and FDTD numerical techniques are employed to investigate the sensing performance and the results show that the proposed sensor can efficiently detect the small changes in the refractive index of sensing area.
In this work, we propose a new scheme of generating high quality frequency quadrupling signal for millimeter-wave wireless communication system. The frequency quadrupling scheme is achieved by using three parallel Mach-Zehnder modulators (MZMs) and an optical phase shifter. The first two MZMs are driven by the RF signals to operate at the maximum transmission point. The third MZM is operated with no RF signal and an extra π-phase difference is introduced for it by the optical phase shifter. The advantage of the proposed scheme is that the optical carrier and the fourth optical sideband can be suppressed simultaneously. The performance of proposed scheme is investigated theoretically and evaluated by simulations. Numerical results show that the radio frequency spurious suppression ratio (RFSSR) higher than 44.18 dB and the optical sideband suppression ratio (OSSR) higher than 21 dB can be obtained without optical filter when the extinction ratio (ER) of the MZM is 30 dB. The impact of the non-ideal RF driven voltage and phase difference of RF driven signal applied to the first two sub-MZMs on OSSR and RFSSR is also discussed and analyzed.
In this paper, a novel fiber Bragg grating (FBG) sensor which can measure the temperature and strain simultaneously is presented. The cladding layer of the proposed FBG sensor is made of a uniaxial crystal material (LiTaO3) and the electric field is applied on the 1/2 area of the sensor. The sensing performance was investigated by the coupled-mode theory and dual-wavelength method. We found that the strain sensitivity and the temperature sensitivity of the 1/2 area with no electric field are 0.841 pm/με and 14.31 pm/°C respectively. If the electric field is increased from 0 to 400×107 v/m, the temperature sensitivity of this device varies from 14.31 pm/°C to 14.12 pm/°C and its strain sensitivity varies from 0.841 pm/με to 0.850 pm/με. So, the obtained results demonstrate that the simultaneous measurement of temperature and strain can be achieved by using this scheme. The proposed sensor has potential applications in optical fiber sensing systems due to small size, high sensitivity and compatible with optical fiber.
In this work, the slow light modes of the polyatomic photonic crystal (PhC) which has multiple different holes in the
smallest unit cell are investigated. The slow light waveguide with nearly constant group index over large bandwidth is
achieved using this new photonic crystal geometry based on square lattice. The feasibility of controlling the dispersion
relation through subtle structural modification is also investigated.
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