Chaos dynamics in vertical-cavity surface-emitting semiconductor lasers with polarization-selected optical feedback

We study experimentally and numerically the dynamic states of chaotic oscillations in vertical-cavity surface-emitting semiconductor lasers (VCSELs) with polarization-selected optical feedback. We identify the regimes of fully-developed chaotic states, low-frequency fluctuations (LFFs), and coexistent states of LFFs and stable oscillation for the variations of the bias injection current and the optical feedback ratio. In particular, coexistent states of LFFs and stable oscillations are observed at higher optical feedback ratio and lower bias injection current. We draw maps of dynamic states in the space of the bias injection current and the optical feedback ratio. The qualitative agreement between the theory and the experiment is found.     

Chaos synchronization and communication of the polarization modes for two unidirectionally coupled vertical-cavity surface-emitting lasers

We theoretically investigate the synchronization performance of the polarization modes of two unidirectionally coupled vertical-cavity surface-emitting lasers (VCSELs), where the master VCSEL subjects to polarization-preserved optical feedback and the slave VCSEL subjects to polarization-preserved optical injection. We demonstrate that high synchronization can be achieved between corresponding polarization components of the master laser and the slave laser. We also analyze the influence of the internal mismatched parameters on the synchronization performance. Furthermore, this system is used to dual-channel communication and the different transmitted digital information is respectively recovered at each polarization branch of the slaver laser successfully with the encryption scheme of chaos masking (CM). The results open an opportunity for multichannel chaotic communication by utilizing different polarization components in single-mode or multi-transverse-mode VCSELs.     

Conceal time-delay signature of chaotic vertical-cavity surface-emitting lasers by variable-polarization optical feedback

Time-delay (TD) signatures of chaotic outputs generated by vertical-cavity surface-emitting lasers (VCSELs) are investigated quantitatively by using two proposed estimators, the peak signal to mean ratio (PSMR) calculated based on the auto-correlation function, and the valley signal to mean ratio (VSMR) calculated based on an information-theory-based estimator, the permutation entropy (PE) function. The VCSELs subject to variable-polarization optical feedback (VPOF) are considered, and the effects of polarizer angle, feedback strength, feedback delay and injection current are discussed. When the feedback strength is small, the TD signatures for the fully developed chaotic outputs of VCSELs can be well concealed for all polarizer angles. While for large feedback strength, the TD signatures cannot be concealed for any polarizer angle. However, for moderate feedback strength, the TD signatures of VCSELs with VPOF at intermediate polarizer angles can be better concealed than those of VCSELs with conventional polarization-selected optical feedback. Thus, the VCSELs with VPOF are extremely interesting for the security-enhanced chaotic optical communication systems.     

Locking bandwidth and birefringence effects for polarized optical injection in vertical-cavity surface-emitting lasers

The aim of this paper is to investigate the effects of external optical injection taking account of polarization and electron spin properties in vertical-cavity surface-emitting lasers (VCSELs). Using external polarized injection we seek the locked phases and amplitudes of specific polarized fields in terms of injection level and frequency detuning, taking account of two kinds of distinguishable carrier density (spin-up and spin-down). For the conventional form of optical injection without taking account of spin-polarized fields there are three fundamental equations describing the carrier density, field amplitude and phase. However, by using the spin flip model (SFM), the combined effect of polarized fields along two perpendicular crystal axes and electron spin properties results in six equations. We analyse the conditions for stable locking and also the influence of birefringence effects on the stability map of detuning versus optical injection for both cases of injection polarized parallel and perpendicular to the lasing mode of the solitary VCSEL. For given values of pumping and spin relaxation rate there is a minimum birefringence rate for orthogonal injection. Above this value three regions of elliptical polarization are found in the stability map, namely “quasi-stability” (QS), “coupled limit cycle” (CLC) and “coupled chaos” (CC). The three regions of linear polarization, namely chaos, limit cycle and stability, are reduced in area compared to the case of parallel injection. For orthogonal injection it is found that increased birefringence or reduced spin relaxation rate causes the stable locking region to begin at higher injected power and frequency detuning.     

Analysis and optimization of coupling to external cavities in feedback experiments with vertical-cavity surface-emitting lasers

The feedback strength is a crucial parameter for feedback experiments using semiconductor lasers. In this article, the coupling efficiency of the field of vertical-cavity surface-emitting lasers (VCSELs) to external cavities containing one collimating lens has been analyzed in detail using ABCD-matrix methods. It is found that for a given set of parameters there are two distinct, experimentally realizable positions of the collimating lens which allow for optimal coupling, if the cavity length is sufficiently small. The predictions are verified in experiments using single-transverse-mode VCSELs. The obtained coupling efficiencies exceed 70%.     

Complete chaotic synchronization characteristics of the linear-polarization mode of vertical-cavity surface-emitting semiconductor lasers with isotropic optical feedback

Based on the SFM model [M. San Miguel, Q. Feng, J.V. Moloney, Phys. Rev. A. 52 (1995) 1728.], complete chaotic synchronization characteristics of linear-polarization (LP) Mode of the vertical-cavity surface-emitting semiconductor lasers (VCSELs) with isotropic optical feedback are numerically investigated. When the propagation time τ_c of light from the transmitter to the receiver equals to the external cavity round-trip time τ, and the central frequency of the T_VCSEL matches that of the R_VCSEL, in large scale region of the feedback coefficient and the injection current where the x-polarization mode as well as the y-polarization mode obtains high synchronization quality at the same time. However, in the central frequency mismatch regions where the x- and y-polarization mode can obtain inferior synchronization quality at same time. But for τ_c ≠ τ, each LP mode of the mixed polarization modes achieved inferior synchronization quality at the same time. Besides, with big enough injection current, the output of the system with the same central frequency in two lasers is entirely governed by the y-polarization mode when the feedback coefficient and Δτ(=τ_c − τ) fixed at a certain value. So the system can realize steadily good synchronization of the completely dominant y-polarization mode. But the synchronization quality of the completely dominated y-polarization mode can be deteriorated by increasing absolute frequency detuning value. Besides, in positive frequency mismatch regions where the influence of the same frequency detuning value on the synchronization quality of the entirely governed y-polarization mode becomes smaller than in negative frequency detuning regions. At last, when τ is given and the central frequency of the T_VCSEL matches that of the R_VCSEL, the influence of the τ_c is not a simple time-shift in the time-evolution of the receiver VCSEL, which has serious influence on the complete synchronization quality of each LP mode.     

Polarization instabilities in a multi-transverse-mode vertical-cavity surface-emitting laser with polarized optical feedback

We have experimentally studied polarization instabilities in the multi-transverse-mode regime of a vertical-cavity surface-emitting laser subject to polarized optical feedback. A dynamical regime that is similar to the transition from the so called low frequency fluctuation (LFF) to coherence collapse (CC) in the single-transverse-mode VCSEL is found and investigated. The role of higher order transverse modes is to increase the irregularity of the dynamics as shown experimentally and numerically.     

Temperature behaviour of top mirror reflection spectrum in intra-cavity-contacted oxide-confined vertical-cavity surface-emitting lasers

The thermal dependence of mirror reflective properties in oxide-confined vertical-cavity surface-emitting lasers is investigated. The temperature distribution over a cavity versus applied current is then calculated. The refractive indexes of the mirror layers are subsequently redefined using the temperature coefficients, and changes in radial reflectance spectra of the mirror are analysed. Results show a shift of stop-band and Bragg wavelengths of the mirror, as well as a shift of laser operation wavelength at different injection currents and the formation of a temperature lens in the mirror due to a decrease in temperature close to the edge of the oxide windows.     

Light polarization fingerprints on nonlinear dynamics of vertical-cavity surface-emitting lasers

In this paper we summarize the recent results on nonlinear dynamics of vertical-cavity surface-emitting lasers (VCSELs) which are bearing the fingerprints of the particular polarization properties of VCSELs. Due to the surface emission and cylindrical symmetry, VCSELs lack strong polarization anisotropy and may undergo polarization switching and polarization mode-hopping. This provides new specificities to the rich nonlinear dynamics induced in VCSELs by an external perturbation. We unveil both experimentally and theoretically these new specificities for the case of optical feedback, optical injection, and unidirectional synchronization between VCSELs.     

Synchronization of polarization mode of two unidirectionally coupled vertical-cavity surface-emitting laser and its application in communication

We theoretically investigate the synchronization performance of polarization mode of two unidirectionally coupled vertical-cavity surface-emitting laser (VCSEL) and propose an encryption scheme, where the phase of the polarization states of the master laser are modulated. It is demonstrated that high synchronization can be achieved between corresponding polarization components of the master laser and the slave laser. Moreover, two distinct synchronization regimes exist, complete synchronization and injection locking. The transmitted digital information is successfully recovered at each polarization branch of the slaver laser. The results open an opportunity for multichannel chaotic communication by utilizing different polarization components in single-mode or multi-transverse-mode VCSELs.     

Optical simulation of vertical-cavity surface-emitting lasers with non-cylindrical oxide confinement

Two extensions of weighted index method are presented in order to calculate the splitting of degeneracy for non-circular vertical-cavity surface-emitting lasers. Both of them automatically satisfy which transverse optical modes split, and give a reliable approximation for the resonant frequencies, threshold gains and confinement factors. The intuitive effective radius method traces back the splitting for different effective aperture radii which are defined according to the transverse intensity distributions. The hybrid analytical axial and numerical lateral method is more precise from a mathematical point of view, and provides the mode patterns as well as the optical data.     

Polarization switching and synchronization of mutually coupled vertical-cavity surface-emitting semiconductor lasers

Polarization switching (PS) dynamics and synchronization performances of two mutually coupled vertical-cavity surface-emitting lasers (VCSELs) are studied theoretically in this paper. A group of dimensionless rate equations is derived to describe our model. While analysing the PS characteristics, we focus on the effects of coupling rate and frequency detuning regarding different mutual injection types. The results indicate that the x-mode injection defers the occurrence of PS, while the y-mode injection leads the PS to occur at a lower current. Strong enough polarization-selective injection can suppress the PS. Moreover, if frequency detuning is considered, the effects of polarization-selective mutual injection will be weakened. To evaluate the synchronization performance, the correlation coefficients and output dynamics of VCSELs with both pure mode and mixed mode polarizations are given. It is found that performance of complete synchronization is sensitive to the frequency mismatch but it is little affected by mixed mode polarizations, which is opposite to the case of injection-locking synchronization.     

长波长垂直腔面发射激光器研究

垂直腔面发射激光器（简称VCSEL）近年来受到了国内外科技界和企业界的高度关注，它在光通信领域具有潜在的应用．文章主要介绍了长波长垂直腔面发射激光器的发展历史、基本应用、器件结构、材料和制备工艺，最后分析了研制长波长VCSEL所遇到的问题及相应解决方法，并进行了展望．     

长波长垂直腔面发射激光器研究

垂直腔面发射激光器（简称VCSEL）近年来受到了国内外科技界和企业界的高度关注,它在光通信领域具有潜在的应用.文章主要介绍了长波长垂直腔面发射激光器的发展历史、基本应用、器件结构、材料和制备工艺,最后分析了研制长波长VCSEL所遇到的问题及相应解决方法,并进行了展望.     

Micromechanical tunable vertical-cavity surface-emitting lasers

We report the study on a short wavelength-tunable vertical-cavity surface-emitting laser utilizing a monolithically integrated bridge tuning microelectromechanical system. A deformable-bridge top mirror suspended above an active region is utilized. Applied bridge-substrate bias produces an electrostatic force which reduces the spacing of air-gap and tunes the resonant wavelength toward a shorter wavelength (blue-shift). Good laser characteristics are obtained: such as continuous tuning ranges over 11 nm near 940 nm for 0--9 V tuning bias, the peak output power near 1 mW and the full-width-half-maximum limited to approximately 3.2--6.8 nm. A detailed simulation of the micromechanical and optical characteristics of these devices is performed, and the ratio of bridge displacement to wavelength shift has been found to be 3:1.     

Power dissipation in oxide-confined 980-nm vertical-cavity surface-emitting lasers

We presented 980-nm oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with a 16-μm oxide aperture. Optical power, voltage, and emission wavelength are measured in an ambient temperature range of 5℃ C-80℃. Measurements combined with an empirical model are used to analyse the power dissipation in the device and the physical mechanism contributing to the thermal rollover phenomenon in VCSEL. It is found that the carrier leakage induced selfheating in the active region and the Joule heating caused by the series resistance are the main sources of power dissipation. In addition, carrier leakage induced selfheating increases as the injection current increases, resulting in a rapid decrease of the internal quantum efficiency, which is a dominant contribution to the thermal rollover of the VCSEL at a larger current. Our study provides useful guidelines to design a 980-nm oxide-confined VCSEL for thermal performance enhancement.     

Investigation polarization characteristics of vertical-cavity surface-emitting lasers

We have improved transmission line laser model to study the polarization characteristics by incorporating into spin-flip model (SFM). Theoretical studies on the influence of the Bragg reflectivity on the polarization switching of vertical-cavity surface-emitting laser (VCSEL) have been discussed in this paper. We have demonstrated that reflectivity of Bragg reflectors can be optimized to provide suitable hysteresis loops width which indicates the tolerance of the input power.     

基于光反馈的单向耦合注入垂直腔表面发射激光器的矢量混沌同步特性研究

基于自旋反向模型(SFM)，数值研究了基于光反馈的单向耦合注入垂直腔表面发射激光器的矢量混沌同步特性，研究结果表明：当外部光反馈时间等于光从发射系统到接收系统传输时间时， x偏振模和y偏振模都能接收最好的完全混沌同步质量.若外部光反馈时间不等于传输时间且注入电流接近阈值电流时，占主导的y偏振模能暂时实现较好的完全混沌同步质量.相比较而言，占主导地位的x偏振模至始至终获得很差的同步质量，另外，当系统输出为混合偏振模时，混合偏振模中的每一个线性偏振模获得很差完全同步质量.然而，当注入电流远大于阈值电流时，系统输出仅为y偏振模，这时y偏振模能稳定地实现最好的完全混沌同步质量.最后，当接收激光器受到线性偏振模的强注入时，每一个注入线性偏振模能与接收激光器输出的对应的线性偏振模实现很好的注入锁定同步.然而，每一个占主导的线性偏振模比另一被抑制的线性偏振模获得更差注入锁定同步质量、如果有相等的能量的两个线性偏振模同时存在，这两个线性偏振模获得差不多一致的注入锁定混同步质量， 换句话说，能量较少的线性偏振能获得较高的注入锁定同步质量. 关键词： 线性偏振态 垂直腔表面发射激光器 完全矢量混沌同步 强注入锁定矢量混沌同步     

基于1550 nm垂直腔面发射激光器的长距离双向双信道光纤混沌保密通信研究

基于同一混沌信号光注入下两个1550 nm垂直腔面发射激光器 (VCSELs) 中两对应线性偏振模式之间的混沌同步, 提出了一种可实现信息的长距离双向双信道光纤混沌保密通信的系统模型, 并对该系统的同步、双向双信道通信以及光纤信道对信息传输的影响等性能进行了相关仿真研究. 结果表明: 在由驱动混沌激光器所产生的同一混沌光注入下, 两响应激光器中对应的两线偏振模式之间均可实现高质量的等时混沌同步, 且驱动激光器与两响应激光器间的同步系数较低; 基于两响应激光器之间对应线偏振模式的高质量混沌同步, 可实现双向双信道混沌通信; 采用单模保偏光纤 (或色散位移保偏光纤) 作为通信信道, 2.5 Gbit/s信息在传输60 km (或200 km) 后解调信息的Q因子能保持在6以上. 关键词： 垂直腔面发射激光器 (VCSELs) 双向双信道 混沌通信 光纤     

Bidirectional chaos synchronization and communication in semiconductor lasers with optoelectronic feedback

In this paper, a bidirectional chaos communication system based on the optoelectronic feedback semiconductor lasers (SLs) is presented, and the synchronization characteristics and the communication performances of such system are simulated numerically. The results show that the high-quality synchronization with zero lag time and simultaneous bidirectional message transmission can be realized under the suitable system parameters; the internal parameter mismatches of the lasers have influence on the synchronization quality and communication performance, but this system still possesses good robustness to parameter mismatches. Meanwhile, the security can be ensured through a suitable encoding-decoding technique.