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%.     

Using polarization properties to enhance performance of chaos synchronization communication between vertical-cavity surface-emitting lasers

Enhanced chaos synchronization communication between vertical-cavity surface-emitting lasers (VCSELs) is investigated numerically, by adopting a polarization communication protocol where one polarization mode is used as the chaotic carrier and the orthogonal polarization mode is used as the disturbed signal. The chaos synchronization quality for the receiver laser (RL) and attacker laser (AL) are analyzed in terms of the cross-correlation coefficient. There are two distinct regions of good synchronization quality in the injection parameters space. In the first region, good synchronization quality can be ensured for both the RL and AL. In the second region, only the RL can realize chaos synchronization. We further discuss the security enhancement by comparing the communication performance between RL and AL for injection parameters within the second region. The results show that successful and robust decoding is only achievable for the RL.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

1.55-μm InGaAs/InGaAlAs MQW vertical-cavity surface-emitting lasers with InGaAlAs/InP distributed Bragg reflectors

High-performance InGaAs/InGaAlAs multiple-quantum-well vertical-cavity surface-emitting lasers (VCSELs) with InGaAlAs/InP distributed Bragg reflectors are proposed for operation at the wavelength of 1.55 μm. The lasers have good heat diffusion characteristic, large index contrast in DBRs, and weak temperature sensitivity. They could be fabricated either by metal-organic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE) growth. The laser light-current characteristics indicate that a suitable reflectivity of the DBR on the light output side in a laser makes its output power increase greatly and its lasing threshold current reduce significantly, and that a small VCSEL could output the power around its maximum for the output mirror at the reflectivity varying in a broader range than a large VCSEL does.     

Linewidth narrowing caused by optical feedback in a multi-mode vertical-cavity surface-emitting laser

We have studied experimentally spectral characteristics of a multi-mode vertical-cavity surface-emitting laser that is subject to optical feedback. Appropriate alignment of the feedback mirror can suppress higher-order modes and significantly decrease the spectral linewidth of the laser.     

垂直腔面发射激光器的电路模拟

通过分析垂直腔面发射激光器的速率方程,并结合电子电路的特点,建立了等效电路模型。采用数值计算和电路模拟两种方法给出了垂直腔面发射激光器的频率响应特性。通过分析,证明了电路模型的正确性。通过电路模拟得出了脉冲调制的响应特性,验证了激光器固有的电光延迟和张驰振荡现象。     

Graded index profiles and loss-induced single-mode characteristics in vertical-cavity surface-emitting lasers with petal-shape holey structure

The 850-nm oxide-confined vertical-cavity surface-emitting lasers with petal-shape holey structures are presented.An area-weighted average refractive index model is given to analyse their effective index profiles,and the graded index distribution in the holey region is demonstrated.The index step between the optical aperture and the holey region is obtained which is related merely to the etching depth.Four types of holey vertical-cavity surface-emitting lasers with different parameters are fabricated as well as the conventional oxide-confined vertical-cavity surface-emitting laser.Compared with the conventional oxide-confined vertical-cavity surface-emitting laser without etched holes,the holey vertical-cavity surface-emitting laser possesses an improved beam quality due to its graded index distribution,but has a lower output power,higher threshold current and lower slope efficiency.With the hole number increased,the holey vertical-cavity surface-emitting laser can realize the single-mode operation throughout the entire current range,and reduces the beam divergence further.The loss mechanism is used to explain the single-mode characteristic,and the reduced beam divergence is attributed to the shallow etching.High coupling efficiency of 86% to a multi-mode fibre is achieved for the single-mode device in the experiment.     

光子晶体垂直腔面发射850nm波长激光器研究

文章报道了国内首次研制成功的光子晶体垂直腔面发射850nm波长激光器，实现了连续电注入激射．发现器件能否激射直接依赖于光子晶体结构参数，而激光器的阈值、输出功率、输出模式等与光子晶体的晶格常数、占空比、腔的大小等因素有关．     

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.     

Numerical simulation of phase modulation method used as high sensitive self-mixing vibrometry in vertical-cavity surface-emitting laser

The self-mixing interference technique is a compact sensing technique that has widely used for measuring vibrations. For purpose of increasing the accuracy of the vibration measurement system, a new phase modulation technique based on the self-mixing interferometer is presented. The phase modulation is obtained by varying the length of external cavity periodically. The vibration information of the external target is determined by the Fourier analysis method. Theoretical analysis and numerical simulations of phase modulation method used as high sensitive self-mixing vibrometry based on vertical-cavity surface-emitting laser are given in this paper.     

Analysis on thermal characteristics of annular beam pumped vertical-external-cavity surface-emitting lasers by finite element method

The annular beam as a pump light of VECSELs is presented for the first time in this study. The thermal characteristics of VECSELs were calculated and simulated by finite element method, and the influence of the annular beam's annulus radius, the annulus width, and the pumping power to the VECSELs’ thermal characteristics was analyzed in detail. The results indicated that the annular beam can improve the VECSELs’ thermal characteristics efficiently, and provide a favorable term to generate laser in the central part of the annular. The theoretical results provided theoretical reference and experimental study for the design of high-power optically pumped VECSELs.     

The improved output performance of a broad-area vertical-cavity surface-emitting laser with an optimized electrode diameter

The output performance of a 980-nm broad-area vertical-cavity surface-emitting laser(VCSEL) is improved by optimizing the p-electrode diameter in this study.Based on a three-dimensional finite-element method,the current density distribution within the active region of the VCSEL is optimized through the appropriate adjustment of the p-electrode diameter,and uniform current-density distribution is achieved.Then,the effects of this optimization are studied experimentally.The L-I-V characteristics under different temperatures of the VCSELs with different p-electrode diameters are investigated,and better temperature stability is demonstrated in the VCSEL with an optimized p-electrode diameter.The far-field measurements show that with an injected current of 2 A,the far-field divergence angle of the VCSEL with an optimized p-electrode diameter is 9°,which is much lower than the far-field angle of the VCSEL without this optimization.Also the VCSEL with an optimized p-electrode diameter shows a better near-field distribution.