High-frequency oscillations and self-mode locking in shortexternal-cavity laser diodes

An effect of compound-cavity mode competition for a laser diode with a short external cavity is investigated analytically and numerically on the basis of a round-trip approximation for emitted photons in the external cavity. Due to the noncoincidence of the compound-cavity modes with possible frequencies of stationary lasing, such competition may cause self-locking of two compound-cavity modes with deep high-frequency oscillations of the laser output for a short (typically ⩽5 mm) external cavity. Simple analytical expressions for critical feedback parameters and the frequencies of self-oscillations are given, as well as a generalized diagram of different dynamic regimes of a short external-cavity laser, which shows these regimes of high-frequency self-oscillations as well as coherence collapse instability. The results may be considered a guideline for the design of high-speed laser diodes with an integrated passive cavity. In particular, such a device may be a very efficient pulse source in the microwave (>20 GHz) region     

Effects of feedback asymmetry in external-cavity semiconductorlaser systems

Presents the first systematic study of the effects of feedback asymmetry, as determined by misalignment of an external mirror, on the characteristics of external-cavity semiconductor lasers. Particular attention has been paid to output power, intensity noise and optical spectrum. The authors have observed and explained undulations in power as a function of external mirror tilt as well as regimes in which complex noise spectra occur     

Spectral characteristics of vertical-cavity surface-emitting laserswith strong external optical feedback

We study the spectral optical properties of vertical-cavity surface-emitting lasers (VCSEL's) in the presence of strong optical feedback from an external reflecting surface. When the feedback strength exceeds 1%, multicompound-cavity-longitudinal-modes are observed. The output spectral width increases significantly as the feedback increases. Besides the normal compound-cavity modes, high-loss compound-cavity modes are also observed for the first time. The experimental observations are well explained by a compound-cavity laser theory     

Tunable operation of a gain-switched diode laser by nonresonantself-injection seeding

We report tunable operation of a gain-switched diode laser by nonresonant self-injection seeding from an uncoated glass slide used as an external cavity reflector. A spectral linewidth reduction from 11 to 0.05 nm has been achieved for picosecond pulses with little effect on other laser characteristics. Good agreement with numerical simulations based on a compound-cavity laser model is also reported     

单瓣近衍射极限输出的带外腔半导体激光器

实验研究了带外腔反馈注入的宽接触条形激光器，并用光线传输矩阵分析了该外腔结构。利用闪耀光栅及耦合输出反射镜对表面未镀增透膜的半导体激光器构成外腔，选择一定模式的激光反馈注入回激光器，从而限制了其他模式在半导体激光器内的振荡，压缩了激光器输出激光的光谱宽度。当激光器驱动电流为2．7倍阈值电流时，获得230mW输出功率，0．6nm谱宽，单瓣近衍射极限的激光输出。用一平面镜代替光栅作为外腔反射镜，获得了320mW输出功率，1．5nm谱宽的单瓣近衍射极限的激光输出。     

Self-pulsations in strongly coupled asymmetric external cavitysemiconductor lasers

Self-pulsations in asymmetric external cavity semiconductor lasers are studied experimentally and are analyzed using improved rate equations which include multiple reflections. These equations are valid for arbitrary levels of coherent external optical feedback. The dependence of self-pulsation frequencies on injection current, external mirror tilt angle and reflectivity, and external cavity length is explained by small-signal analysis of the rate equations. By numerical integration of the rate equations, self-pulsations are demonstrated theoretically and resonant enhancement of intensity noise is shown to occur when the self-pulsation frequency is an integer fraction of the external cavity resonance frequency     

无输出镜外腔光谱合束结构对反馈效率的影响

半导体激光阵列无输出耦合镜外腔光谱合束技术利用光栅的0级和1级衍射光束反馈实现发光单元的波长锁定,避免了0级和1级衍射光束的转储和浪费,可以获得高的合束效率。因此,0级和1级衍射光束的反馈量高低就会决定外腔波长锁定的稳定性,进而影响合束后光束质量的高低甚至光谱合束的成败。针对此种结构,理论研究了两外腔长度、望远镜滤波结构及"Smile"效应对0级和1级衍射光束反馈效率的影响,结果表明：(1)外腔长度会影响反馈功率以及串扰程度;(2)望远镜滤波结构可以有效滤除大偏角杂散光束以及使光束正确反馈回原发光单元;(3)"Smile"效应的程度对反馈效率以及输出光束质量影响尤为严重,需要采取措施进行抑制。     

Single-mode operation characteristics of a self-injection seededFabry-Perot laser diode with distributed feedback from a fiber grating

We study the operational characteristics of a self-injection seeded laser diode in an external cavity consisting of a fiber Bragg grating. The distributed feedback (DFB) of light in the grating provides an additional tolerance of frequency detuning in mode selection. The spectral characteristics at different operating frequencies and feedback rates have been investigated. Wavelength tuning has also been realized by applying compressive and tensile strain to the grating and a tuning range of 11.6 nm has been obtained     

The complex way to laser diode spectra: example of an external cavity laser strong optical feedback

An external cavity laser with strong grating-filtered feedback to an antireflection-coated facet is studied with a time-domain integral equation for the electric field, which reproduces the modes of the oscillation condition as steady-state solutions. For each mode, the stability and spectral behavior is determined by analysis of the location of side modes in the complex frequency plane. The complex frequency diagrams are shown to be a useful tool to determine the self-stabilization effect of mode coupling and its dependence on laser parameters and external cavity design. The model is used to simulate the large signal time evolution after start from unstable modes.     

Influence of external optical feedback on threshold and spectralcharacteristics of vertical-cavity surface-emitting lasers

We report a study of the effects of external optical feedback on the threshold and spectral characteristics of VCSELs with feedback levels in the range from -25 dB to 0 dB. A large threshold current reduction and multi-extended-cavity-mode operation have been observed. The experimental results are in good agreement with the theoretical calculations based on a compound cavity laser model     

Effect of external optical feedback on spectral properties of external cavity semiconductor lasers

The effects of external optical feedback on the spectral properties of single-mode external cavity semiconductor lasers that use a graded-index (GRIN) lens in the optical cavity have been investigated. Mode rejection ratio and intensity drop-out rate of the dominant mode as a function of optical strength have been measured. These measurements show that a laser with a short (160 ?m) GRIN-lens external cavity can tolerate optical feedback as large as ?20 dB without significant penalty. This minimum optical feedback can be larger when lasers with shorter cavity length are used.     

光纤光栅外腔反馈的高功率半导体激光器光谱特性的研究

对光纤光栅外腔反馈的高功率半导体激光器的光谱线宽特性进行了理论与实验研究。理论分析表明，根据不同的外腔参数，激光器的线宽能被压缩或展宽。外腔反射率对光谱线宽的影响要比外腔腔长灵敏得多。实验实现了激光器的线宽由６ｎｍ到０．８ｎｍ的压缩。光纤光栅外腔反馈不仅能改善高功率半导体激光器的光谱特性，而且可提高激光器主模对次模的抑制比。     

Optical resonators in the approximation of Gauss

A general theory of optical resonators based on the concept of complex point-eikonal is presented. The analysis is limited to the approximation of Gauss. The modes of resonance of open resonators formed by two spherical mirrors facing each other have been obtained in previous works by fitting the wavefronts of Gaussian beams to the mirror surfaces. This method becomes complicated when the resonator contains focusing elements or dielectric slabs. The approach proposed in this paper is more straightforward than previous approaches dealing with this problem and is applicable to resonators containing anisotropic media. The round-trip point-eikonal is first evaluated on the basis of the conventional laws of Gaussian optics. The presence of apertures with Gaussian transmissivity, and of lasers with quadratic transverse variation of the gain, are accounted for by introducing complex round-trip point-eikonals. The modes ψm(x) of the resonator are obtained from a power series expansion of the Green function of a mode-generating system related to the round-trip point-eikonal S of the resonator. The resonance frequencies and the round-trip losses are given by simple and general formulas. The mode fields are described by Hermite-Gauss functions with complex arguments, explicitly in terms of S. For resonators that do not contain apertures, the wavefronts are the same for all the modes and they are plane at planes of symmetry. In reciprocal resonators, clockwise and counter-clockwise modes have the same losses and resonant frequencies, but different mode pattents. These modes are shown to be mutually adjoint. Adjoint modes, with < ψ^†m, ψn> = 0 if m ≠ n, are useful to evaluate the response of optical resonators to incident fields. Results applicable to resonators containing nonorthogonal astigmatic elements are also discussed.     

Tuning characteristics of a tunable-single-frequencyexternal-cavity laser

The lasing characteristics of a tunable-single-frequency laser operated in conjunction with a tunable external cavity are investigated. The effect of phase error in the external cavity on the lasing frequency, linewidth, and output power is calculated by deriving the characteristic equation for the FP cavity as a traveling amplifier with an effective mirror concept. The analysis suggests that the diode laser and the external cavity may be successfully tuned open-loop if the feedback strength is appropriate and the tuning nonlinearities in the active and passive cavities are not too severe. This is demonstrated by maintaining a linewidth of 4 MHz while tuning continuously over 34 GHz     

Two-segment cavity theory for mode selection in semiconductor lasers

A simple three-mirror composite laser cavity is analyzed and optimum structures proposed in terms of the position of the central reflector and the value of its reflectivity in order to give single longitudinal mode operation. Two optimum structures are discussed, the first corresponding to relatively long cavities where a large period in the gain modulation is required in order to avoid a jump to the next mode with a high effective reflectivity, usually in a second mode group (as defined by a neighboring period of the gain modulation). The second corresponds to shorter lasers where jumps to neighboring modes should be avoided, since the spectral gain variation prevents jumps to other mode groups. The effects of mode discrimination due to external feedback have been analyzed and minimum values of reflectivity for the third mirror derived in order to preserve the internal mode selectivity.     

非准直外腔He-Ne激光自混合干涉

从三镜法布里-珀罗(F-P)腔出发,建立了非准直外腔三重回馈白混合干涉的理论模型,并进行了数值模拟和实验验证.分析了3种情况下影响自混合干涉光强调谐特性的因素:1)准直外腔二,三重回馈;2)非准直外腔二,三重回馈;3)外腔加入λ/4波片时的回馈.实验结果与理论分析相吻合.研究表明,该自混合干涉系统的分辨率达到λ/6;自混合干涉输出信号与腔反射镜的反射率、外腔的准直性以及腔内置入镜片有关;外腔中加入λ/4波片时的自混合干涉信号与非准直外腔自混合干涉信号类似;二,三重回馈自混合干涉信号携带了反射体位移方向信息,因而更易实现位移方向判断.     

Noise in an AlGaAs semiconductor laser amplifier

The noise characteristics in a Fabry-Perot (FP) cavity type semiconductor laser amplifier, biased at just below its oscillation threshold current, have been studied theoretically and experimentally. Quantum mechanical multimode rate equations containing a Langevin shot noise source and an input signal term were numerically solved for an exponential band-tail model with nok-selection rule. Noise power calculated using this rate equation was compared with a simpler photon statistic master equation method. The experimental results on noise power for an AlGaAs laser amplifier are in reasonable agreement with the two different theoretical predictions. Dominant noise powers in a semiconductor laser amplifier are beat noise powers between signal and spontaneous emission, and between spontaneous emission components. Noise characteristics in a Fabry-Perot cavity type laser amplifier can be improved both by the reduction of the facet mirror reflectivities and by use of an asymmetric cavity configuration with low-input and high-output mirror reflectivities. Two beat noise powers are expressed in simple analytic form by introducing an equivalent noise bandwidth and an excess noise coefficient as figures of merit in an optical amplifier.     

Noise in an AlGaAs Semiconductor Laser Amplifier

The noise characteristics in a Fabry-Perot (FP) cavity type semiconductor laser amplifier, biased at just below its oscillation thresh-old current, have been studied theoretically and experimentally. Quantum mechanical multimode rate equations containing a Langevin shot noise source and an input signal term were numerically solved for an exponential band-tail model with no k-selection rule. Noise power calculated using this rate equation was compared with a simpler photon statistic master equation method. The experimental results on noise power for an AlGaAs laser amplifier are in reasonable agreement with the two different theoretical predictions. Dominant noise powers in a semiconductor laser amplifier are beat noise powers between signal and spontaneous emission, and between spontaneous emission components. Noise characteristics in a Fabry-Perot cavity type laser amplifier can be improved both by the reduction of the facet mirror reflectivities and by use of an asymmetric cavity configuration with low-input and high-output mirror reflectivities. Two beat noise powers are expressed in simple analytic form by introducing an equivalent noise bandwidth and an excess noise coefficient as figures of merit in an optical amplifier.     

Investigation of polarization properties of VCSELs subject to optical feedback from an extremely short external cavity-part I: theoretical analysis

A two modes rate equation model for vertical-cavity surface-emitting lasers (VCSELs) subject to optical feedback from an extremely short external cavity (ESEC) is presented. By making use of it we develop a map of bistability to investigate the parametric dependence of polarization properties of VCSELs in such configuration, finding out a periodic dependence of the polarization switching (PS) currents on the ESEC length. By increasing the external mirror reflectivity we can make this periodic dependence stronger and strongly asymmetric providing the possibility to prevent PS for any injection current achieving polarization stabilization in VCSELs. Further numerical simulations with isotropic and non isotropic feedback show how parameters as the compression coefficients, the frequency splitting between the linearly polarized (LP) modes, the differential gain and the top mirror reflection coefficient, affect the map of bistability.     

Spatial feedback effects in narrow-stripe index-guidedsemiconductor lasers

We demonstrate experimentally that spatial effects of feedback are important in determining the operating characteristics of index-guided semiconductor lasers. It is in fact the inflexibility of the index-guided mode that makes the laser susceptible to small misalignments of the external cavity, which can induce power variations of nearly 40% of the solitary laser power with feedback levels of less than 2%.(-17 dB). With a simple model, we calculate the modification of the feedback wavefront as a function of the external cavity length and show that the power variations are induced through changes in the cavity loss brought on by the interference of the feedback field with the field reflected off the laser facet. By experimental comparison to a partly gain-guided laser, we conclude that these large-scale variations will be significantly reduced or absent in purely gain-guided lasers