An all-optical encryption-decryption method using an exclusive-or gate based on the cross-phase modulation between O-band and C-band light waves is proposed. The feasibility of the encryption-decryption technique is verified by handling binary signals at 2.5 Gbps, with less than 3 dB penalty of extinction ratio and 1 dB polarization dependent loss.
A type of fiber-based orbital angular momentum (OAM) modulator is designed according to transformation relation between OAM beam and optical fiber vector mode, together with mode-coupling theory, which is based on the combination of multimode fiber structure and chirally-coupled-cores structure. Instead of applying external force or grating etching to the fiber in the system, chirally-coupled-cores fiber can realize the modulation of any optical OAM by using single fiber at 1550 nm. Therefore, the test system is relatively simple. From the equation <inline-formula><tex-math id="M1000">\begin{document}${\rm{OAM}}_{ \pm l,n}^{ \pm \sigma } = {\rm{HE}}_{l + 1,n}^{{\rm{even}}} \pm {\rm{i}} \times {\rm{HE}}_{l + 1,n}^{{\rm{odd}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20182036_M1000.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20182036_M1000.png"/></alternatives></inline-formula>, it can be seen that the OAM mode generated by long period chirally-coupled-cores fiber depends on the higher-order modes supported by the central fiber core. Therefore, the generation and modulation of any order OAM beam can be realized by changing the diameter of the central fiber core in theory. Through theoretical analysis and numerical simulation, the effects of different structure parameters on OAM modes are analyzed, including mode purity, mode transmission loss and effective refractive index. By keeping the propagation constants of the center core and side cores unchanged, the number of side cores has no effect on mode purity nor effective refractive index, but which is not for mode transmission loss. The loss of mode transmission increases with the increase of the number of side cores. However, it does not mean that the less number of side cores is a better case, in that the fiber symmetry and processing technology should also be considered. And the pitch calculated by the formula of phase matching condition can change in value within a certain numerical range without strongly affecting the mode purity and mode transmission loss. Pitch has a great influence on the effective refractive index of modes, therefore the pitch can be under control to change the difference in effective refractive index between OAM modes and reduce crosstalk between disparate modes. The distance between the center core and side cores of fiber has little effect on mode purity, great effect on mode transmission loss, but no effect on effective refractive index. Theoretically, the mode purity and mode transmission loss perform better with the distance between two kinds of cores increasing. But it will be limited by the fiber integration level.
The research of dipole source localization has great significance in both clinical research and applications. For example, the EEG recording from the scalp is widely used for the localization of sources of electrical activity in the brain. This paper presents a closed formula that describes the electric field of dipoles at arbitrary position, which is a linear transformer called the transfer matrix. The expression of transfer matrix and its many useful characteristics are given, which can be used for the analysis of the electrical fields of dipoles.This paper also presents the closed formula for determining the location and magnitude of single dipole or multi-dipoles according to its electrical field distribution. A calculation result for a single dipole shows that the dipole will be located at the midpoint of a line segment if there are equivalent fields at its two ends.
The slow light technology of the rectangle signal propagating in erbium-doped fiber (EDF) has potential applications in the fields of all optical communication and optical fiber sensing. The method of using harmonics fractional delay to evaluate the slow/fast light of rectangle signal propagating in the EDF is proposed, and the characteristics of phase delay for fundamental and high order harmonics components are analyzed for the first time based on the rate equations and the theory of the coherent population oscillations (CPO). We experimentally demonstrate the dependences of fundamental fractional delay on input power and optical gain. The maximum fractional delay 20% is obtained when the input power is about 8 mW without pump. The negative fractional delay-20% is also achieved and it will increase with the rising of the optical gain. The Nth-order fractional delays (N=1, 3, 5, 7) of rectangle signal propagating in EDF without pump are investigated. Their maximum fractional delays are all about 0.07 and the corresponding fundamental modulation frequencies are 22, 7, 5 and 3 Hz, respectively. What is more, the Nth-order fractional delays (N=1, 3, 5, 7) with pump are also investigated. Their maximum fractional delays are all about-0.135 and the corresponding fundamental modulation frequencies are 175, 58, 35 and 25 Hz, respectively. The experiments indicate that the maximum Nth-order fractional delays are equal and they will be achieved at the frequency f/N (the fundamental harmonic fractional delay is maximum at the modulation frequency f). The results show good agreement with CPO and the frequency is also located in the spectral burning hole.
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