Random distributed feedback fibre lasers

The concept of random lasers exploiting multiple scattering of photons in an amplifying disordered medium in order to generate coherent light without a traditional laser resonator has attracted a great deal of attention in recent years. This research area lies at the interface of the fundamental theory of disordered systems and laser science. The idea was originally proposed in the context of astrophysics in the 1960s by V.S. Letokhov, who studied scattering with “negative absorption” of the interstellar molecular clouds. Research on random lasers has since developed into a mature experimental and theoretical field. A simple design of such lasers would be promising for potential applications. However, in traditional random lasers the properties of the output radiation are typically characterized by complex features in the spatial, spectral and time domains, making them less attractive than standard laser systems in terms of practical applications. Recently, an interesting and novel type of one-dimensional random laser that operates in a conventional telecommunication fibre without any pre-designed resonator mirrors–random distributed feedback fibre laser–was demonstrated. The positive feedback required for laser generation in random fibre lasers is provided by the Rayleigh scattering from the inhomogeneities of the refractive index that are naturally present in silica glass. In the proposed laser concept, the randomly backscattered light is amplified through the Raman effect, providing distributed gain over distances up to 100 km. Although an effective reflection due to the Rayleigh scattering is extremely small (∼0.1%), the lasing threshold may be exceeded when a sufficiently large distributed Raman gain is provided. Such a random distributed feedback fibre laser has a number of interesting and attractive features. The fibre waveguide geometry provides transverse confinement, and effectively one-dimensional random distributed feedback leads to the generation of a stationary near-Gaussian beam with a narrow spectrum. A random distributed feedback fibre laser has efficiency and performance that are comparable to and even exceed those of similar conventional fibre lasers. The key features of the generated radiation of random distributed feedback fibre lasers include: a stationary narrow-band continuous modeless spectrum that is free of mode competition, nonlinear power broadening, and an output beam with a Gaussian profile in the fundamental transverse mode (generated both in single mode and multi-mode fibres).     

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.     

Frequency behavior of coherent random lasing in diffusive resonant media

We investigate diffusive propagation of light and consequent random lasing in an amplifying medium comprising resonant spherical scatterers. A Monte-Carlo calculation based on photon propagation via three-dimensional random walks is employed to obtain the dwell-times of light in the system. We compare the inter-scatterer and intra-scatterer dwell-times for representative resonant and non-resonant wavelengths. Our results show that more efficient random lasing, with intense coherent modes, is obtained for a system with intra-scatterer gain. This is also coupled with a larger reduction in frequency fluctuations. We find that such a system can yield almost thresholdless random lasing. Inspired by these results, we discuss a possible practical situation, based on a monodisperse aerosol, wherein frequency controlled coherent random lasing can be obtained. Since our analysis essentially investigates transport of intensity, the results are relevant to coherent random lasers under nonresonant feedback.     

Random laser action in ZnO

We report a direct evidence of random laser in optically pumped ZnO powder. Discrete lasing modes are observed above threshold. The laser emission spectra depend on the angle of observation and are random. The lasing action is attributed to the coherent feedback due to recurrent light scattering in the powder. The lasing threshold intensity depends on the excitation volume. Received: 15 September 1999 / Revised version: 2 February 2000 / Published online: 5 July 2000     

Transition from amplified spontaneous emission to laser action in strongly scattering media

In an active random medium, the combination of multiple scattering with light amplification may lead to random laser action. However, it is crucial but sometimes difficult to distinguish between amplified spontaneous emission and lasing. By varying the amount of scattering in an amplifying random medium, we have observed the transition from amplified spontaneous emission to lasing with coherent feedback. We have found out when the transition occurs through the measurement of the scattering mean free path. Our numerical simulation based on the direct solution to Maxwell equations clearly illustrates the transition from light amplification to laser oscillation due to an increase of the amount of scattering in active random medium.     

Spatial confinement of laser light in active random media

We have observed spatial confinement of laser light in micrometer-sized random media. The optical confinement is attributed to the disorder-induced scattering and interference. Our experimental data suggest that coherent amplification of the scattered light enhances the interference effect and helps the spatial confinement. Using the finite-difference time-domain method, we simulate lasing with coherent feedback in the active random medium.     

Tailoring of random lasing characteristics in dye-doped nematic liquid crystals

Amplified spontaneous emission and random lasing are investigated in random systems with dye-doped nematic liquid crystals. And that temporal stability of random lasing is analyzed. The influence of pumping polarization as well as the multiple scattering and reflection between boundaries on the emission behavior and the formation of coherent feedback is investigated in detail. For freely suspended samples, certain emission wavelength can be obtained by changing the pump wavelength. This feature is useful in making wavelength-tunable lasers. Moreover, as the pumping thickness of wedge sample increases, the emission spectrum is red shifted and the average spacing of adjacent spikes decreases. This property can be applied in laser mode selection, i.e., the number of modes within certain wavelength range can be chosen.     

Continuous wave mirrorless lasing in cholesteric liquid crystals with a pitch gradient across the cell gap

Despite numerous efforts, continuous wave (CW) lasing in dye doped, one-dimensional (1D) photonic bandgap cholesteric liquid crystal (CLC) structures has not been previously reported, to our knowledge. Here we report on the observation of lasing in such structures under both coherent (laser) and incoherent (LED) CW light excitation. To achieve this effect, we used a 1D-photonic bandgap structure made of a polymer stabilized CLC with a pitch gradient across the cell thickness. A spectral reflectivity profile of such a CLC structure reveals local minima in the area within a photonic stopband and close to it. The realization of lasing pumped by low power CW light sources opens the possibility of all-organic, compact, tunable CW lasers for display and medical applications.     

Nanolasers Enabled by Metallic Nanoparticles: From Spasers to Random Lasers

Owing to exotic optical responses, metallic nanoparticles and nanostructures are finding broad applications in laser science, leading to numerous design variations of plasmonic nanolasers. Nowadays, two of the most intriguing plasmonic nanolasing devices are spasers and random lasers. While a spaser is based on a single metallic nanoparticle resonator with the optical feedback provided by the localized surface plasmon resonance, the operation of a random laser relies on multiple light scattering within randomly distributed metallic nanoparticles. In this paper, an up‐to‐date review on the applications of metallic nanoparticles in spasers and random lasers is provided. Principles of a random spaser, a device combining the features of a spaser and a random laser, are briefly discussed as well. The paper is focused on major theoretical and experimental approaches to control the core metrics of lasing performance, including threshold, resonant wavelength, and emission directionality. The applications of spasers and random lasers in the fields of sensing and imaging are also mentioned. Finally, the challenges and future perspectives in this area of research are discussed.     

Holographic Image Denoising using Random Laser Illumination

In holographic applications, coherent lasers are indispensable source of illumination. Despite high intensity from coherent light sources, they fail in full-field image projection and illustrate speckle images due to high spatial coherence. This article demonstrates speckle-free high contrast computer-generated holographic image projection upon illumination with a perovskite–polystyrene 10 wt%-based random laser. Solvent-engineered efficient and durable perovskite and perovskite–polystyrene 10 wt%-based random lasers are fabricated. Optical characterizations are elucidated and controlled coherence random lasing operation is achieved under room temperature upon addition of polystyrene concentration 10 wt% on perovskite thin film. The addition of 10 wt% polystyrene concentration results in a low far-field divergence angle of ≈10⁰. The controlled coherence in random lasers is necessary to produce a stable interference pattern and to retain the depth of field in holograms. Additionally, the holographic image projection using random lasers reduces diffraction noise, and exhibits high spatial resolution with full-field imaging. Moreover, this study is clear evidence of an effective strategy to achieve high-performance, indigenous designed-controlled coherence in disordered random lasing media and its application to novel holographic image projection.     

Self-synchronization and dissipation-induced threshold in collective atomic recoil lasing

Networks of globally coupled oscillators exhibit phase transitions from incoherent to coherent states. Atoms interacting with the counterpropagating modes of a unidirectionally pumped high-finesse ring cavity form such a globally coupled network. The coupling mechanism is provided by collective atomic recoil lasing, i.e., cooperative Bragg scattering of laser light at an atomic density grating, which is self-induced by the laser light. Under the rule of an additional friction force, the atomic ensemble is expected to undergo a phase transition to a state of synchronized atomic motion. We present the experimental investigation of this phase transition by studying the threshold behavior of this lasing process.     

Low-Threshold Random Laser with One Mirror and Feedbacks in PMMA Nano-Composite Films

A low-threshold random laser with one mirror and feedback is investigated in the PMMA film containing rhodamine 590 and TiO2 nano-particles. Incoherent and coherent laser emission is observed. Effect of particle concentration on light emission is explored, and the optimum particle concentration is obtained. Optical microscopy and scanning probe microscopy are used to investigate the film structure, and the principle of incoherent and coherent laser is analysed.     

Random fiber laser operating at 1,115 nm

We experimentally demonstrate a random fiber laser operating at 1,115 nm using a LD-pumped Yb-doped fiber laser as the pump source. We achieve about 270 mW lasing output in a 50 km standard communication optical fiber with slope efficiency more than 28 %. A new wavelength is provided for the application of random distributed feedback fiber lasers as light sources.     

基于TiO_2纳米粒子薄膜的低阈值随机激光器的动力学研究

本文利用共轭聚合物(MEH-PPV)覆盖TiO_2纳米粒子薄膜制作随机激光器。随机TiO_2纳米粒子薄膜的激光辐射阈值比平面MEH-PPV薄膜的放大自发辐射阈值缩小了9倍。这是由于TiO_2纳米粒子诱导的多重散射造成的。进一步的飞秒荧光上转换实验表明,随机激光器中,光在增益介质里的停留时间有所增加,这直接证实了光在随机激光器结构中的多重散射引起光的传播路径增加。因此,这会促进更多的光发生辐射,从而降低随机激光器的阈值。     

Random lasing from dye-doped chiral nematic liquid crystals in oriented and non-oriented cells

These are the experimental results describing random lasing in dye-doped chiral nematic liquid crystals. A novel random lasing emission is studied in this article based on the helical domains of dye-doped chiral nematic liquid crystals in oriented and non-oriented cells. Under frequency doubled 532 nm Nd:YAG (yttrium aluminum garnet) laser-pumped optical excitation, we carefully observed and analysed random lasing from dye-doped chiral nematic liquid crystals with wavelength ranges from 600 nm to 620 nm. In addition, the line-width of multi-mode peaks is less than 0.2 nm. The difference between the two random lasing behaviours in the oriented and non-oriented cells arises from the fact that random lasing appearing in the oriented cell results from stronger multiple scattering of light generated by the spiral domains of the liquid crystal molecules. Furthermore, chiral nematic liquid crystal micro-domains with different orientations can induce variation of the diffusion constant, thereby resulting in a decrease or increase in the lasing intensity of the random lasers, and an increase or decrease in their energy thresholds. In addition, a detailed comparison of the two experimental results is also presented in the article, showing the dependence of the lasing threshold and the number of lasing modes on the transport mean free path, the excited area, and the sample size. This process allows us to obtain a random laser by changing the structure of the sample, realising tunable random lasers at low cost.     

软X射线激光汤姆逊散射实验尝试

汤姆逊散射是诊断高温稠密等离子体状态参数的重要方法之一, 受到广泛的关注. 但是目前用于进行汤姆逊散射的探针光波长多局限于可见光或紫外光, 能够诊断的区域电子密度远低于驱动激光的临界密度. 相比较而言, 以软X射线激光作为探针, 有希望诊断更高密度区域的等离子体. 利用“神光Ⅱ”高功率激光装置产生的类氖锗软X射线激光作为探针, 开展了软X射线激光汤姆逊散射实验的尝试. 根据散射的条件, 分别进行了非相干散射和相干散射的实验, 但均未能获得明显的散射谱. 理论分析表明, 主要原因可能是实验中作为探针的类氖锗软 X射线激光的聚焦功率密度不够, 通过优化实验条件, 有希望在今后的研究中获得相干汤姆逊散射的结果. 关键词： 等离子体诊断 软X 射线激光 汤姆逊散射     

Lasing action due to the two-dimensional quasiperiodicity of photonic quasicrystals with a Penrose lattice

We have fabricated photonic quasicrystal lasers with a Penrose lattice that does not possess translational symmetry but has long-range order, and observed coherent lasing action due to the optical feedback from quasiperiodicity, exhibiting a variety of 10-fold-symmetric lasing spot patterns. The lattice constant dependence of lasing frequencies and spot patterns show complicated features very different from photonic crystal/random lasers, and we have quantitatively explained them by considering their reciprocal lattice. Unique diversity of their reciprocal lattice opens up new possibilities for the form of lasers.     

Secure communication scheme using chaotic laser diodes subject to incoherent optical feedback and incoherent optical injection

We propose a secure communication scheme based on anticipating synchronization of two chaotic laser diodes, one subject to incoherent optical feedback and the other to incoherent optical injection. This scheme does not require fine tuning of the optical frequencies of both lasers as is the case for other schemes based on chaotic laser diodes subject to coherent optical feedback and injection. Our secure communication scheme is therefore attractive for experimental investigation.     

Random fiber laser

We investigate the effects of two-dimensional confinement on the lasing properties of a classical random laser system operating in the incoherent feedback (diffusive) regime. A suspension of 250 nm rutile (TiO2) particles in a rhodamine 6G solution was inserted into the hollow core of a photonic crystal fiber generating the first random fiber laser and a novel quasi-one-dimensional random laser geometry. A comparison with similar systems in bulk format shows that the random fiber laser presents an efficiency that is at least 2 orders of magnitude higher.     

Photon statistics of random lasers with resonant feedback

We have measured the photon statistics of random lasers with resonant feedback. With an increase of the pump intensity, the photon number distribution in a single mode changes continuously from Bose-Einstein distribution at the threshold to Poisson distribution well above the threshold. The second-order correlation coefficient drops gradually from 2 to 1. By comparing the photon statistics of a random laser with resonant feedback and that of a random laser with nonresonant feedback, we illustrate very different lasing mechanisms for the two types of random lasers.