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.     

Modelling of GaN quantum dot terahertz cascade laser

In this paper GaN based spherical quantum dot cascade lasers has been modelled, where the generation of the terahertz waves are obtained. The Schrödinger, Poisson, and the laser rate equations have been solved self-consistently including all dominant physical effects such as piezoelectric and spontaneous polarization in nitride-based QDs and the effects of the temperature. The exact value of the energy levels, the wavefunctions, the lifetimes of electron levels, and the lasing frequency are calculated. Also the laser parameters such as the optical gain, the output power and the threshold current density have been calculated at different temperatures and applied electric fields.     

Ultrafast switching dynamics of nonlasing modes of vertical-cavity surface-emitting lasers

Single-mode vertical-cavity surface-emitting lasers (VCSELs) have been investigated with ultrafast optical modulation. After injection of a subpicosecond laser pulse into the VCSEL cavity, nonlasing modes were excited and produced optical beating in the emission of the VCSEL. After these oscillations have died down, when the VCSEL is operating at high power and when the injected laser pulses exceed a power threshold as well, the injected laser pulses can induce the lasing to switch to a normally nonlasing mode with crossed polarization. Our measurements of the decay time of the nonlasing modes at low injected pulse energy are consistent with the interpretation of this switching as enabled by increased lifetime of the nonlasing modes at high power.     

新型垂直腔面发射半导体激光器阵列

设计出四次质子注入工艺制备垂直腔面发射激光器(VCSEL)阵列的方法,实现了对阵列中单元器件间的隔离以及对单元器件注入电流限制的分别作用。一方面通过对VCSEL外延片上分布布拉格反射镜(DBR)两次较浅的质子注入形成高电阻区域实现对阵列中单元器件间的隔离,另一方面通过再次的两次较深度的可以达到有源区上表面的质子注入形成高电阻区域实现对单元器件注入电流的限制。由瞬态热传导方程对阵列中单元器件间的热相互作用进行了理论分析。采用四次质子注入工艺实现了2×2、3×3简单的二维GaAs/AlGaAs量子阱VCSEL阵列,并对器件的激射近场、光谱特性及功率等进行了测量。     

Polarization-bistable vertical-cavity surface-emitting lasers: application for optical bit memory

We summarize recent results on polarization-bistable vertical-cavity surface-emitting lasers (VCSELs) and their application to optical buffer memory. All-optical flip-flop operation with very low switching energies and high repetition rates is achieved. An optical buffer memory consisting of a two-dimensional array of polarization-bistable VCSELs, in which the bit state of the optical signal, “0” or “1”, is stored as a lasing linear polarization state of 0 or 90°. Input data stored as the polarization states of the first VCSEL are transferred to the polarization states of the second VCSEL. In our experiments with 980 nm polarization-bistable VCSELs, 10 Gbit/s optical buffering, 2-bit optical buffering, and a shift register function have been successfully demonstrated.     

Light transport and correlation length in a random laser

A random laser is a strongly disordered, laser‐active optical medium. The coherent laser feedback, which has been demonstrated experimentally to be present in these systems beyond doubt, requires the existence of spatially localized photonic quasimodes. However, the origin of these quasimodes has remained controversial. We develop an analytical theory for diffusive random lasers by coupling the transport theory of the disordered medium to the semiclassical laser rate equations, accounting for (coherent) stimulated and (incoherent) spontaneous emission. From the causality of wave propagation in an amplifying, diffusive medium we derive a novel length scale which we identify with the average mode radius of the lasing quasi‐modes. We show that truly localized modes do not exist in the system without photon number conservation. However, we find that causality in the amplifying medium implies the existence of a novel, finite intensity correlation length which we identify with the average mode volume of the lasing quasimodes. We show further that the surface of the laser‐active medium is crucial in order to stabilize a stationary lasing state. We solve the laser transport theory with appropriate surface boundary conditions to obtain the spatial distributions of the light intensity and of the occupation inversion. The dependence of the intensity correlation length on the pump rate agrees with experimental findings.     

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.     

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.     

A review of progress on nano-aperture VCSELInvited Paper

This paper reviews the progress on nano-aperture vertical-cavity surface-emitting lasers (VCSELs). The design, fabrication, and polarization control of nano-aperture VCSELs are reviewed. With the nanoaperture evolving from conventional circular and square aperture to unique C-shaped, H-shaped, I-shaped, and bowtie-shaped aperture, both the near-field intensity and near-field beam confinement from nanoaperture VCSELs are significantly improved. As a high-intensity compact light source with sub-100-nm spot size, nano-aperture VCSELs are promising to realize many new near-field optical systems and applications.     

Influence of polarization mode competition on the synchronization of two unidirectionally coupled vertical-cavity surface-emitting lasers

We analyze theoretically the effect of polarization mode competition on the synchronization of two unidirectionally coupled vertical-cavity surface-emitting lasers (VCSELs). Chaos in the master laser is induced by delayed optical feedback, and the slave laser is subject to isotropic optical injection from the master VCSEL. We show that the synchronization quality can be enhanced when the chaotic regime in the master VCSEL involves both fundamental orthogonal linearly polarized modes.     

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%.     

Suppression of polarization switching in vertical-cavity surface-emitting lasers by use of optical feedback

The polarization properties of vertical-cavity surface-emitting lasers (VCSELs) subject to optical feedback are studied experimentally. It is thereby demonstrated that polarization-selective optical feedback can be utilized to entirely eliminate VCSEL polarization switching over the entire device operating range.     

Bifurcation to polarization self-modulation in vertical-cavity surface-emitting lasers

Experiments have yielded polarization self-modulation in vertical-cavity surface-emitting lasers (VCSELs) subject to a pi/2 polarization-rotating optical feedback. The phenomenon has been simulated numerically, but its bifurcation has never been explained. We show that polarization self-modulation results from a Hopf bifurcation mechanism that can be analyzed in terms of the laser feedback parameters. Our analysis predicts other bifurcations for low values of the feedback rate, which explain why more-complex time-dependent outputs have been observed as alternatives to polarization self-modulation.     

Circularly polarized lasing over wide wavelength range in spin-controlled (1 1 0) vertical-cavity surface-emitting laser

We investigated the emission wavelength dependence of the lasing polarization in a (1 1 0)-oriented vertical-cavity surface-emitting laser (VCSEL) with GaAs/AlGaAs quantum wells under optical spin injection at room temperature. Lasing was observed in the one circularly polarized mode over a wide wavelength range from 838 to 857 nm, in which a degree of circular polarization higher than 0.8 was maintained. The optical gain spectrum that contributed to the circularly polarized lasing is discussed based on the optical selection rules and the measured polarization-resolved photoluminescence spectra of the active layers.     

宽调谐范围垂直腔面发射激光器特性分析及设计

运用光学传输矩阵和有限元方法对波长可调谐垂直腔面发射激光器(VCSELs)的波长调谐范围进行了研究.对中心波长为980nm的可调谐VCSELs的波长调谐特性和微电子机械系统(MEMS)悬臂梁结构进行了设计，并进行了实验研究.结果表明，MEMS可调谐VCSELs调谐特性同时受到光波谐振腔结构和悬臂梁最大位移的共同影响.在悬臂梁几何尺寸和激光器有源区结构一定的条件下，通过优化可调谐VCSELs的牺牲层厚度可实现大范围波长调谐.同时，对可调谐VCSELs整体结构进行了设计，计算结果显示波长调谐范围达到30nm以 关键词： 悬臂梁 可调谐垂直腔面发射激光器     

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.     

Wideband multiwavelength erbium-doped fiber ring laser with frequency shifted feedback

Wideband multiwavelength erbium-doped fiber ring lasers with frequency shifted feedback are described. The use of an intra-cavity gain flattening filter (GFF) was proposed in order to increase the lasing spectral bandwidth, leading to a demonstration of 34 lasing wavelengths in 28 nm bandwidth in C-band. The GFF induced spectral output power fluctuation is discussed. Multiwavelength operation was also demonstrated for the first time in L-band, where wideband laser operation was obtained without a GFF. Optical bistability and Kerr effect induced pulsation were determined to be limiting factors to stable operation range in this kind of multiwavelength lasers.     

Nitride VCSEL design for continuous-wave operation of higher-order optical modes

Anticipated performance of possible designs of nitride vertical-cavity surface-emitting lasers (VCSELs) has been analyzed and compared with the aid of an advanced 3D optical–thermal–electrical self-consistent simulation. It has been revealed in the simulation that, to achieve room-temperature (RT) continuous-wave (CW) VCSEL threshold operation, uniformity of both carrier-concentration and temperature-increase distributions within VCSEL active regions are equally important. Besides, mostly because of problems with an effective confinement of an optical field in a radial direction, RT CW lasing in standard nitride VCSELs is usually easier to achieve on higher-order transverse modes than on lower-order ones. Therefore, a novel design of nitride VCSELs with double (annular and central dot) p-side contacts has been intentionally constructed to enhance an excitation of low-threshold high-order transverse optical modes. Anticipated RT CW performance characteristics of the novel design have been proved to be definitely more promising than those of other possible configurations of nitride VCSELs. In particular, our calculations have confirmed that RT CW lasing is possible in this new nitride VCSEL. Besides, this device has been found to be less sensitive (than other nitride VCSELs) to possible crystal imperfections created around an active region while high-resistive areas are formed. PACS 42.55.Px; 85.30.De; 85.60.Jb     

High speed intracavity-contacted vertical cavity surface emitting lasers with separated quantum wells

In this work, the simulation of the 980 nm InGaAs intra-cavity-contacted oxide-confined vertical-cavity surface-emitting lasers (ICOC VCSELs) with separated triplets of quantum wells (STQW) is presented. We analyze the thermal, electrical and optical properties of such devices. Results of simulations show the larger optical power efficiency and higher modulation bandwidth for devices with included STQW.     

影响AlGaN/GaN量子级联激光器性能的因素研究

在已有理论基础之上,采用严格的计算方法对激光器实现太赫兹(THz)波的辐射进行了可能性分析。利用传递矩阵法,通过Matlab软件计算了基于AlGaN/GaN材料体系的三能级量子级联激光器导带子能级与电子波函数的分布,详细分析了由该材料特有的极化效应所产生的极化场,得出了在近共振条件下偶极跃迁元、外加电场、垒层Al组分及导带子能级能级差之间的关系,并研究了它们对激光器性能的影响。分析结果表明,实现受激辐射的条件非常严格,Al组分取0.15或0.16时较为适宜,同时外加电场需大于63kV/cm,但不能过大,这样才能满足近共振条件,实现粒子数反转达到太赫兹量子级联激射。在Al组分为0.15,外加电场为69.0kV/cm时激光器的偶极跃迁元最大,表明跃迁几率也最大,对激光器的性能有利,可以为量子级联激光器构造较好的有源区。