Specific mode output from multimode fiber oscillators by designing rare earth doping profiles

Based on rate equations, a theoretical model of a fiber oscillator with a multimode gain fiber was built. We studied the effect of the rare earth doping profile in the core on the output characteristics of the multimode fiber oscillator. The results indicated that a pure fundamental mode can be obtained by partly doping the core of the large mode area （LMA） ytterbium doped fiber （YDF） in the fiber laser. Furthermore, a sole specific high-order mode can also be implemented by tailoring the rare earth doping profile according to the simulations. The mode coupling effect was also taken into account in the model. In spite of the mode coupling effect, the specific mode was able to dominate in the output of the fiber laser by utilizing the designed LMA YDF.     

Generation of a broadband single-mode supercontinuum in a conventional dispersion-shifted fiber by use of a subnanosecond microchip laser

We report the experimental generation, simply by use of a subnanosecond microchip laser at 532 nm and a conventional dispersion-shifted fiber, of a supercontinuum that spans more than 1100 nm. We show by detailed spectral analysis that this supercontinuum originates from a preliminary four-wave mixing process with multimode phase matching and subsequent double-cascade stimulated Raman scattering and is transversely single mode as a result of Raman-induced mode competition. This technique is believed to be the simplest configuration that allows one to generate a stable supercontinuum.     

Mechanism for stable,ultra-flat multiwavelength operation in erbium-doped fiber lasers employing intensity-dependent loss

We study a mechanism to attain stable and ultra-flat multiwavelength oscillations in erbium-doped fiber lasers (EDFLs). The key concept is to introduce intensity-dependent loss (IDL) into the laser cavity, which can effectively suppress the mode competition in the homogeneously broadened gain medium and ensure a uniform power distribution over wavelengths via the gain-clamping effect. The technique was successfully demonstrated by employing a nonlinear optical loop mirror (NOLM) in erbium-doped fiber laser cavity. Based on the experimental results, further experimental investigation and theoretical analysis are carried out to show the effectiveness of the gain-clamping mechanism in realizing the multiwavelength operation of the EDFL.     

Multimode fiber laser beam cleanup based on stochastic parallel gradient descent algorithm

We present experimental research on multimode fiber laser beam cleanup based on a stochastic parallel gradient descent (SPGD) algorithm. The multimode laser is obtained by injecting a 1064 nm central wavelength single mode fiber laser into a multimode fiber and the system is setup by using phase only liquid crystal spatial light modulators (LC-SLM). The quality evaluation function is increased by a factor of 10.5 and 65% of the laser energy is encircled in the central lobe when the system evolves from open-loop into close-loop state. Experimental results indicate the feasibility of the multimode fiber laser beam cleanup by adaptive optics (AO).     

Remote multiplexing of holograms with random patterns from multimode fiber bundles

We propose the remote multiplexing of holograms with random patterns from a multimode fiber bundle used as the reference beams. The random pattern reference beam is characterized by the superposition and concatenation of propagation modes of multimode fiber and free space. For angle, shift, and wavelength remote multiplexing we compare two methods of laser coupling to the fiber bundle, i.e., direct coupling and lens coupling. A theoretical discussion that uses mode orthogonality is provided to describe multiplexing characteristics, and the theory is verified by experimental results. These remote-multiplexing methods can be applied to general multimode waveguide arrays for construction of compact and integrated optical systems in which multiplexing can be controlled remotely.     

Power spectra and spatial pattern dynamics of a ring laser

Experimental measurements of power spectra of a single longitudinal and transverse mode ring dye laser are presented that reveal the critical slowing down of the laser near threshold. External pump noise serves as a probe of the frequency response of the dye laser. Detailed comparisons of the spectral characteristics with computer simulations and an approximate analytic theory are given. The dynamics of spatial pattern formation in a multimode dye laser is examined through measurements of first-passage-time distributions. A comparison of the experiments with computer simulations based on a simple theoretical model of the two-mode laser shows qualitative agreement. These measurements indicate that there are a variety of complex phenomena associated with the transverse mode pattern formation dynamics that need to be addressed theoretically and studied further experimentally.     

利用振动光纤减弱激光散斑的研究

在激光投影显示技术中,引入振动光纤的方法能够以较小的能量损失减弱激光散斑.本文利用广义范西特-泽尼克定理对激光投影显示中采用振动光纤抑制散斑做了理论分析.结果表明,散斑衬比度的大小取决于光纤的模式数和投影系统的光瞳大小,对于相同大小的像面,收集更多的光纤模式数和增大孔径均可减弱散斑衬比度.用两种不同的多模光纤所做的对比实验定性证实了这一结论.研究结果对光纤的选择和投影系统的设计有一定的指导意义.     

Polarization switching in a quasi-isotropic microchip Nd:YAG laser induced by optical feedback

This paper demonstrates the influence of external optical feedback on the polarization state of longitudinal modes in quasi-isotropic microchip Nd:YAG lasers. Under optical feedback, the polarization state of longitudinal modes in quasi-isotropic lasers relies strongly on the intracavity anisotropy loss and mode competition. When the intracavity anisotropy loss is small, external optical feedback can cause polarization switching and strong mode competition between two orthogonal linearly polarized eigenstates of one laser longitudinal mode, which leads to the distortion of laser intensity modulation waveform. The polarization switching is independent of the initial external cavity length. By increasing the intracavity anisotropy loss, one polarization eigenstate can be suppressed and the laser works in single-polarization state. A theoretical analysis based on the compound cavity model is presented, which is in good agreement with the experimental results. The results offer guidance to the development of laser feedback interferometers.     

超高斯光束抽运调Q固体激光器仿真模型研究

仿真计算一直是研究激光器的重要手段,而激光理论是仿真计算的基础.虽然半经典激光理论能够精确地描述激光的产生过程,但是其复杂性导致仿真需要庞大的计算资源和计算时间.为了能够更加高效地对激光器进行仿真,提出了一种调Q固体激光器的仿真模型.基于此模型,研究了影响主动调Q激光产生过程的因素.这些因素包括抽运功率、抽运光分布和模式竞争.仿真计算结果与对照实验测量数据相符合,说明了模型的有效性.     

Single-mode Raman fiber laser based on a multimode fiber

Single-mode operation of a Raman fiber laser based on a multimode fiber has been demonstrated for what is to our knowledge the first time. Single-mode operation was achieved through the beam cleanup effect of stimulated Raman scattering in multimode fibers. The Raman laser, consisting of a 50-microm-core multimode fiber and a multimode Nd:YAG laser pump, produced an output beam corresponding to the fundamental mode of the fiber.     

Single polarization, single transverse mode, and widely tunable narrow-linewidth laser from a multimode vertical cavity surface-emitting laser by use of a coupled cavity

A simple method for achieving a widely tunable narrow-linewidth, single polarization state and single transverse mode lasing oscillation from a commercially available multimode vertical cavity surface-emitting laser by use of a coupled cavity is reported. The laser has a wavelength tuning range of more than 5 nm along 850 nm, a side-mode suppression ratio of approximately 15 dB, and an output power of approximately 0.3 mW from a 50-microm multimode optical fiber at a bias current of 15 mA. The laser was operated in a single state of polarization lasing oscillation without any polarization switching.     

Influence of mode distortion on the transverse mode competition in large-mode-area amplifiers

In large mode area fiber amplifier, it is difficult to avoid the bend induced mode distortion. With the existence of mode distortion taken into account, a previous transverse mode competition model has been improved. Based on this model and the finite element difference-beam propagation method, the behaviors of bent mode competition are demonstrated and compared with the straight mode competition where the mode distortion is neglected, the comparison and quantitative results show that serious mode distortion can change the appearance of transverse mode competition in bent fiber amplifiers and even drastically degrade the amplifier performance.     

Stability of continuously pumped atom lasers

A multimode model of a continuously pumped atom laser is shown to be unstable below a critical value of the scattering length. Above the critical scattering length, the atom laser reaches a steady state, the stability of which increases with pumping. Below this limit the laser does not reach a steady state. This instability results from the competition between gain and loss for the excited states of the lasing mode. It will determine a fundamental limit for the linewidth of an atom laser beam.     

多模光纤光栅温度传感特性的实验研究

利用氢敏化处理的多模光纤制作了多模光纤光栅,并对多模光纤光栅的温度传感特性进行了实验研究与理论分析,表明这种光栅三个反射峰的布拉格波长随温度变化均呈现出良好的线性关系,并且重复性相当好,同一光栅的各反射峰的理论温度灵敏度系数都等于0．01nm／℃,实验测得的温度灵敏度系数为0．0098nm／℃或0．0099nm／℃,与理论分析相当吻合,这些特性与单模光纤光栅的温度传感特性接近相同。因此可以用多模光纤光栅代替单模光纤光栅开发光纤光栅传感器,以降低成本;这一实验结果还可以作为对多模光纤光栅进一步深入研究的参考。     

Longitudinal mode competition in semiconductor lasers under optical feedback: Regime of short-external cavity

This paper introduces a theoretical study of longitudinal mode competition in semiconductor lasers subject to optical feedback. The study is based on a model of time-delay multimode rate equations taking into account both symmetric and asymmetric suppressions of modal gain. The model is numerically solved and applied to the case of a short-external cavity. Mode competition is characterized along the feedback-induced period-doubling route to chaos as well as under chaotic dynamics. Contributions of symmetric and asymmetric gain suppressions to both mode dynamics and modal operation under OFB are clarified. The results show that under chaotic dynamics, mode competition induces multimode hopping giving rise to asymmetric multimode output spectra. In regimes of continuous-wave operation, mode competition supports single-mode oscillation, and the side-mode suppression ratio improves with the increase of feedback. In the regime of strong feedback, the lasing mode moves to either long- or short-wavelength side in a seemingly random fashion, which is strongly related to asymmetric gain suppression.     

Origin of temperature dependence of microbending attenuation in fiber optic cables

Abstract

Temperature-induced changes in the attenuation of multimode optical fiber cables are shown to be caused by mismatch between the thermal expansion coefficients of the fiber and the cabling materials. A quantitative theoretical model of low temperature loss, based on the formation of fiber microbends by microvariations in the jacket concentricity, is described. This model applies to tightly jacketed, soft buffered cable designs. An equation relating the low temperature optical attenuation to cable parameters is derived using this model. Good agreement is obtained between this theoretical prediction and experimental results. The theoretical model is used to compare the effectiveness of different cable designs on reducing excess loss at low temperature.     

多模Ar^＋激光时间相干性与输出功率的关系

本文就多纵模Ar~+激光时间相干性(即相干长度)与输出功率的关系进行了实验观测,得到了相干长度与输出功率关系近似为双曲线函数的实验结果,并根据简单的Smith模型,对多纵模激光的时间相干性与输出功率进行了理论分析与数值计算.研究结果表明,理论与实验结果基本吻合.     

Spatiotemporal mode-locked multimode fiber laser with dissipative four-wave mixing effect

The high degree of freedom and novel nonlinear phenomena of multimode fiber are attracting attention. In this work, we demonstrate a spatiotemporal mode-locked multimode fiber laser, which relies on microfiber knot resonance (MKR) via dissipative four-wave-mixing (DFMW) to achieve high-repetition-rate pulses. Apart from that, DFMW mode locking with switchable central wavelengths can also be obtained. It was further found that high pulse energy induced nonlinear effect of the dominant mode-locking mechanism transforming from DFMW to nonlinear Kerr beam cleaning effect (NL-KBC). The experimental results are valuable for further comprehending the dynamic characteristics of spatiotemporal mode-locked multimode fiber lasers, facilitating them much more accessible for applications.     

Influence of mode competition on beam quality of fiber amplifier

Theoretical and experimental studies of the influence of the mode competition on the output beam quality of fiber amplifiers are presented. Rate equations and modal decomposition method are used in the theoretical model. In the experiment, the output beam-quality factor of a fiber amplifier, which is based on a Yb-doped double-clad large mode area fiber as a function of the seed beam quality and the pump power of the amplifier, is measured. The experimental results are consistent with the theoretical analysis.     

双波长环形腔掺铒光纤激光器建模及模式竞争的仿真

利用Giles模型建立双波长环形腔掺铒光纤激光器模型,根据增益损耗匹配条件对1550 nm与1555 nm双波长的模式竞争进行了仿真。结果表明,在均匀增益加宽模型下,损耗谱的微小变化对激光的模式竞争过程有较大影响。当两模式各自的损耗与增益相匹配时,它们的输出功率相当; 但0.1 dB的随机损耗波动将导致两个模式输出功率的相对波动幅度分别达到9.59％和9.52％。当损耗长时间偏离匹配状态0.05 dB时,一个模式将逐渐减弱并最终湮灭,另一个模式得到增强。