Lian Shan Yan

Wideband Unpredictability-Enhanced Chaotic Semiconductor Lasers With Dual-Chaotic Optical Injections

The unpredictability degree and bandwidth properties of chaotic signals generated by semiconductor lasers subject to dual chaotic optical injections (DCOI) are investigated numerically. The unpredictability degree is evaluated quantitatively via permutation entropy. Compared with the slave laser (SL) subject to single chaotic optical injection, both the chaotic bandwidth and the unpredictability degree can be enhanced significantly for SL with DCOI. The effects of injection strength, frequency detuning as well as feedback strength are considered. It is shown that, with the increase of injection strength, the unpredictability degree of chaotic signals generated by SL increases firstly and then decreases until saturates at a constant level. Positive frequency detuning is preferred to achieve wideband unpredictability-enhanced chaos, and higher bandwidth and unpredictability degree can be further expected by adopting two differently detuned master lasers. The physical mechanisms behind the wideband unpredictability-enhanced chaos are also revealed. The wideband unpredictability-enhanced chaotic signals generated by SL with DCOI are extremely useful for high speed random number generators, as well as for high capacity security-enhanced chaotic communications.

Quantifying Chaotic Unpredictability of Vertical-Cavity Surface-Emitting Lasers With Polarized Optical Feedback via Permutation Entropy

To quantify the unpredictability of chaotic signals generated by vertical-cavity surface-emitting lasers (VCSELs) subject to polarized optical feedback, we numerically investigate the properties of recently introduced quantifier based on information theory, i.e., permutation entropy (PE) H. VCSEL1 subject to polarization-preserved optical feedback and VCSEL2 subject to polarization-rotated optical feedback are considered. The H for X-polarization (XP) mode, Y-polarization (YP) mode, and total output for both VCSELs are discussed and compared in detail. The influences of feedback strengths, feedback delays, and injection currents are focused on. For lower injection current, the 2-D maps illustrate that the trends of H for the XP mode, YP mode, and total output are different for both VCSELs. While for higher injection current, the values of H for both VCSELs increase with the feedback strengths, are not sensitive to feedback delays, and are higher than the counterparts for lower injection current. Moreover, the H increases with injection current for both VCSELs with a decreasing slope. These results show that the PE is an effective tool for quantifying the degree of unpredictability of chaotic signals of VCSELs, and provides valuable information for choosing the proper chaotic carrier.

A Simple Demodulation Method for FBG Temperature Sensors Using a Narrow Band Wavelength Tunable DFB Laser

A simple demodulation method for a fiber Bragg grating (FBG) temperature sensor is proposed and demonstrated, which uses a low cost commercial temperature-controlled wavelength scanning distributed-feedback (DFB) laser, a sensing FBG and a low-speed photodetector (PD). Relative high resolution is achieved by utilizing an FBG with enhanced temperature sensitivity design. Such a method may be suitable for applications requiring moderate resolution and low cost implementations.

Synchronization of Unpredictability-Enhanced Chaos in VCSELs With Variable-Polarization Optical Feedback

The generation and synchronization of unpredictability-enhanced chaos in vertical-cavity surface-emitting lasers (VCSELs) are investigated numerically. The master VCSEL is subject to variable-polarization optical feedback. Two injection schemes, the polarization-preserved optical injection (PPOI) and variable-polarization optical injection (VPOI) are considered for the slave VCSELs. The influences of unpredictability degree on the synchronization quality and the synchronization properties of unpredictability-enhanced chaos are focused on. For the PPOI scheme, the synchronization quality is hardly affected by the unpredictability degree; high-quality synchronization can be achieved between unpredictability-enhanced chaos due to injection locking. However, for the VPOI scheme, the unpredictability degree affects the synchronization quality significantly. High degree of unpredictability leads to low synchronization quality. Only the low-unpredictability chaos can be well synchronized, while the unpredictability-enhanced chaos cannot be synchronized even with large injection strength. The achievement of unpredictability-enhanced chaos synchronization by the PPOI scheme is extremely useful for the security-enhanced VCSELs-based chaos communication systems.

Randomness-Enhanced Chaotic Source With Dual-Path Injection From a Single Master Laser

The randomness and bandwidth enhancement of chaotic signals in optically injected semiconductor lasers (SLs) with dual-path injection from a single master laser are investigated experimentally and numerically. It is shown that both the randomness and bandwidth of chaotic signals generated in a slave SL (SSL) can be enhanced significantly, especially for positive frequency detuning, by simply adding an additional injection path in the conventional master-slave configuration. The region of injection parameter space leading to wideband randomness-enhanced chaos in SSL can be greatly broadened compared to the SSL with single-path injection. This low-cost and simple configuration for wideband randomness-enhanced chaotic source is highly desirable for high-speed random number generators based on chaotic SLs.

Message Encoding/Decoding Using Unpredictability-Enhanced Chaotic VCSELs

Successful message encoding/decoding in an unpredictability-enhanced chaotic vertical-cavity surface-emitting lasers (VCSELs) system is achieved numerically via the chaos-shift-keying technique. Both VCSELs are subject to variable-polarization optical feedback (VPOF), and polarization-preserved optical injection is adopted. Better decoding performance is achieved by choosing proper polarizer angle, and the security is, to some extent, enhanced owing to the VPOF.

Unpredictability-Enhanced Chaotic Vertical-Cavity Surface-Emitting Lasers With Variable-Polarization Optical Feedback

Variable-polarization optical feedback (VPOF) induced unpredictability enhancement in a vertical-cavity surface-emitting laser (VCSEL) is investigated numerically based on the spin-flip model. The chaotic unpredictability is evaluated quantitatively via an information-theory-based quantifier, the permutation entropy (PE). The role of polarizer angle on the chaotic unpredictability is focused on. The influences of feedback strengths, feedback delays, and injection currents are also considered. A critical polarizer angle, at which the PE reaches its maximum, is existed for relatively high feedback strength and injection current. The representations on Poincaré sphere are further given to provide physical insight into the unpredictability enhancement. Besides, larger feedback strength leads to lower critical polarizer angle, while larger injection current contributes to higher critical polarizer angle. These results show that, by selecting critical polarizer angles, the unpredictability of chaotic signals of VCSELs with VPOF can be enhanced significantly, which is extremely useful for VCSELs-based chaotic communication systems.

Influence of Variable-Polarization Optical Feedback on Polarization Switching Properties of Mutually Coupled VCSELs

The polarization switching (PS) properties of mutually coupled vertical-cavity surface-emitting lasers (VCSELs) subject to variable-polarization optical feedback (VPOF) are investigated numerically. Two injection schemes, parallel-polarization optical injection (PPOI), and orthogonal-polarization optical injection (OPOI) are considered. The effects of VPOF on PS properties are focused on, and the roles of feedback strength, injection strength, and frequency detuning are also discussed. The outputs in the time domain and the polarization evolution through the representation on the Poincaré sphere are further presented to explain the dynamics during the PS process. For PPOI, both VCSELs exhibit similar PS properties, abrupt PS can be observed with the variation of polarizer angle, and are hardly affected by frequency detuning. For OPOI, both VCSELs exhibit mixed polarization for all polarizer angles when without frequency detuning. In particular, frequency detuning induced PS can also be observed for a given polarizer angle, and are affected by the leader-lagged relationship in terms of injection locking effect in mutually coupled VCSELs. These results provide an effective tool to obtain controllable PS in mutually coupled VCSELs, and give more insight into the polarization dynamics.

Analysis for reflection peaks of multiple-phase-shift based sampled fiber Bragg gratings and application in high channel-count filter design

An analytical expression for calculating the reflection-peak wavelengths (RPWs) of a uniform sampled fiber Bragg grating (SFBG) with the multiple-phase-shift (MPS) technique is derived through Fourier transform of the index modulation. The new expression can accurately depict the RPWs incorporating various parameters such as the duty cycle and the DC index change. The effectiveness of the derived expression is further confirmed by comparing the RPWs estimated from the expression with the simulated reflective spectra using the piecewise uniform method. And the reflective spectrum has been well optimized by introducing the Gaussian apodization function to suppress the sidelobes without any wavelength shift on the RPWs. Then, a high-channel-count comb filter based on MPS is proposed by cascading two or more SFBGs with different Bragg periods but with the same RPWs. Noticeably, the RPWs of the new structured SFBG can also be accurately calculated through the expression. Furthermore, the number of spectral channels can be controlled by choosing gratings with specified difference Bragg periods.

Study on the dynamic and unpredictability properties of an optical microring resonator with modulation

The unpredictability properties of an optical microring resonator with modulation are investigated quantitatively, by adopting a recently introduced quantifier, the permutation entropy (PE). We can obtain rich nonlinear dynamics, such as period, bifurcation, and chaos by changing the modulation coefficients and the input power. The effect of the nonlinear refractive index is also considered. Moreover, we adopt the permutation entropy to quantify the unpredictability of the resonators output. The results not only qualitatively agree with the dynamical changes, but also indicate the unpredictability of the chaotic regimes. The chaotic carrier with high PE values would contribute to the unpredictability-enhanced optical microring resonator-based chaotic communication.