Pan Ou

Optical generation of binary-phase-coded, direct-sequence ultra-wideband signals by polarization modulation and FBG-based multi-channel frequency discriminator.

In this paper a novel optical generation approach for binary-phase-coded, direct-sequence ultra-wideband (UWB) signals is experimentally demonstrated. Our system consists of a laser array, a polarization modulator (PolM), a fiber Bragg grating (FBG), a length of single mode fiber, and a photo detector (PD). The FBG, designed based on the superimposed, chirped grating, is used as the multi-channel frequency discriminator. The input electronic Gaussian pulse is modulated on the optical carrier by the PolM and then converted into UWB monocycle or doublet pulses sequence by the multi-channel frequency discriminator. The PolM is used so that the desired binary phase code pattern could be simply selected by adjusting the polarization state of each laser, rather than tuning the laser wavelengths. The desired UWB shape, monocycle or doublet, could be selected by tuning the FBG. Based on our proposed approach, four-chip, binary-phase-coded, DS-UWB sequences with different pulse shapes and code patterns are experimentally demonstrated. The impact of the fiber dispersion on the generated UWB pulses is also discussed in our paper.

High-Stability Er-Doped Superfluorescent Fiber Source Incorporating Photonic Bandgap Fiber

A tunable photonic bandgap fiber filter was incorporated in an Er-doped superfluorescent fiber source to improve its mean wavelength thermal stability. This simple filter was able to successfully compensate the mean wavelength to less than 10 ppm from −40 °C to 70 °C. When the environmental temperature was controlled to ±0.5 °C, the SFS exhibited a long-term mean wavelength stability of ±1 ppm over 11 hours. With 60 mW pump power at 974.2 nm, an optimal Er-doped fiber length of 8m was found, and the final output power of the compensated SFS reached 8 mW.

A Generally Optimized FDTD Model for Simulating Arbitrary Dispersion Based on the Maclaurin Series Expansion

An optimized solution to the discrete numerical model for simulating arbitrary dispersion by the finite-difference time-domain (FDTD) method is derived for both the single-pole and two-pole frequency responses of dispersive media. Based on the method of Maclaurin series expansion, the derivation is aimed to suppress the redundant truncation errors into minimum levels by eliminating the first several series terms of difference between the numerical and theoretical solutions to a modeled susceptibility. The accuracy of numerical approximation to theoretical dielectric functions based on our proposed approach is shown to be exactly equivalent to the bilinear transformation model for the single-pole dispersion response of the modeled dispersive material, but higher than those of other two previously reported models for a two-pole dispersion response of a modeled dispersive material. The explicit coefficients of the proposed model for several classical types of dispersive materials are derived and their corresponding dominant truncation errors are given as well. Both the analytical and simulation results obtained from the FDTD modeling of an exemplified material, silver, demonstrate that the new model outperforms the other models when they are incorporated into the fourth-order accurate FDTD algorithm with small numerical dispersion error rather than in the second-order one with large numerical dispersion error.

Narrow-linewidth single-polarization frequency-modulated Er-doped fiber ring laser

We demonstrate a 1550-nm narrow-linewidth fiber ring laser with stable single polarization by using single-mode Er-doped fiber as active fiber and saturable absorber. A polarization-maintaining circulator is used to acquire single-polarization laser light with the degree of polarization of 99.8%-99.9%. The linewidth measured using a delayed self-heterodyne method is less than 0.5 kHz. Frequency of the fiber laser can be modulated by driving the waveguide phase modulator with proper voltage. A Mach-Zehnder interferometer with the optical path difference between two arms of about 36 km is used to study the long-distance coherent detection of the fiber laser for frequency-modulated continuous-wave application.

A Highly Accurate FDTD Model for Simulating Lorentz Dielectric Dispersion

A highly accurate and numerically stable model of Lorentz dielectric dispersion for the finite-difference time-domain (FDTD) method is presented. The coefficients of the proposed model are optimally derived based on the Maclaurin series expansion (MSE) method and it is shown that the model is much better than the other four reported models in implementing the Lorentz dielectric dispersion with error of relative permittivity several orders lower. The models stability and performance are also analyzed when it is incorporated into the practical second- and fourth-order accurate FDTD algorithms for an exemplified Lorentz medium. Interestingly, we find that all the mentioned models show nearly the same performance in the second-order algorithm due to its large intrinsic numerical dispersion and the superiority of the proposed MSE model begins to be manifested in the higher-order, say, fourth-order FDTD algorithms as implied by the governing numerical dispersion equations.

Generation of an extended depth of focus using diffractive micro-lenses with binary structures in the non-paraxial domain

Diffractive micro-lenses (DMLs) with binary structures are analyzed and designed to generate an extended depth of focus (DOF) based on rigorous electromagnetic theory in the non-paraxial domain. The DOF can be extended by setting a positive preset DOF and by setting a lower height for the binary profile of the DML, respectively. Compared with the paraxial approximation (scalar theory), the focal characteristics of the designed DMLs, such as the actual DOF, the ratio between the DOF of the designed DML and the DOF of the conventional DML, and the spot size of the central lobe at the actual focal plane, for different f-numbers and for different heights of the binary profile, have been studied by using the rigorous vector method (FDTD method). Numerical and graphic results show that the designed DMLs indeed have an extended DOF while maintaining high transverse resolution by modulating the distribution and height of the binary structures of a DMLs profile. The phenomenon that the variation of the height of the patterns can improve the DOF is also given an initial interpretation.

Application of fiber interferometer in coherent Doppler lidar

A novel coherent Doppler lidar (CDL) system based on single-mode fiber (SMF) components and instruments is presented to measure the speed of target. A fiber interferometer used in CDL system is reported. This fiber mixer is employed as a coherent receiver to resolve the shifts of signal-to-noise ratio (SNR) and mixing efficiency induced by backscattered fields wavefront error. For a certain wavelength, the maximum coupling efficiency between signal and SMF is determined by the ratio of pupil diameter to focal length of the coupling lens. The legible interference patterns and spectrum signals show that fiber interferometer is suitable to compensate for amplitude and phase vibrations. This robust coherent receiver can achieve improved CDL system performance with less transmitter power.

A Full-Spectrum Numerical-Dispersion Compensation Technique for the Pseudospectral Time-Domain Method

In the standard pseudospectral time-domain (PSTD) method, the approximation of temporal derivatives based on the central difference formula is only of second-order accuracy, which results in large numerical dispersion errors when the simulated structures are of electrically large size. In this letter, a numerical technique according to the principle of finite impulse response digital filtering is applied to reduce the errors by numerically modifying the material parameters, and therefore the updating equations, for the constitutive relations of arbitrarily materials. It is not only shown to be simple and stable, but also efficient in the reduction of numerical dispersion errors as demonstrated by the results obtained from both the theoretical derivation and the PSTD simulations of numerical examples.

Performance of third-order ghost imaging with second-order intensity correlation

The third-order ghost imaging with the second-order intensity correlation is theoretically and experimentally demonstrated. The resolution and visibility of the reconstructed image are discussed, and the relationship between resolution and visibility is analyzed. The theoretical results show that a tradeoff exists between the visibility and resolution of the reconstructed image; the better the image resolution, the worse the image visibility. Numerical simulations are carried out to verify this theory, and a ghost imaging experiment is conducted to validate our calculations. The experimental results agree with the theoretical predictions.

Using Translation Invariant Discrete Wavelet Transform to Improve Bias Stability of Fiber Gyroscope

The method using translation invariant discrete wavelet transform arithmetic to improve gyroscope bias stability is described. On the basis analyzing the noise characteristic of fiber gyroscope signal, the suitable wavelet vector was selected to extract the noise from signal. The TIDWT, which can avoid the Pseudo-Gibbs phenomena, had be adopted to construct the denosing arithmetic, then the arithmetic was used to filter the real gyroscope signal, the result indicated that the bias stability was improved about 0.6% effectively.