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).     

Influence of spatial mode matching in end-pumped solid state lasers

We present investigations on the influence of mode matching on the efficiency of longitudinally pumped solid state lasers. In a theoretical part we enhance an existing model for four level lasers from idealized cylindrical modes to arbitary pump and laser modes in a random relative position thereby neglecting beam deformation due to thermal effects. The theoretical predictions were confirmed experimentally with an end-pumped Nd:YAG rod operated at 1064 nm. To investigate the effect of misalignment on the efficiency we used a Ti-Sapphire pump laser which was displaced relative to the laser beam. To establish the influence of arbitary pump modes on laser performance a diode laser equipped with coupling optics served as pump source for the same resonator. The resulting decrease in slope efficiency compared to the Ti-Sapphire pumped system could be explained in terms of limited mode overlap due to the characteristic pump field distribution produced by the diode coupling optics.     

Characteristics of Er and Er-Yb-Cr doped phosphate microsphere fibre lasers

In this paper both Er and Er-Yb-Cr doped phosphate microspheres have been successfully created through precise melting of the ends of fibre tapers, drawn, respectively, from Er and Er-Yb-Cr doped phosphate glasses. When coupled with a fibre taper, a microsphere fibre laser cavity can thus be configured creating a system pumped by a 980 nm laser diode and using an optical spectrum analyzer to monitor the spectral characteristics of the laser output. The performance and characteristics of the Er and Er-Yb-Cr microsphere lasers thus created are discussed in detail and cross-compared in this paper. Both lasers have shown low-threshold in terms of the pump power and the laser output wavelengths and a close investigation of the system has shown that the output power and laser stability are closely related to the size of the microsphere, the pump power and the microsphere material composition.     

Optical fibres for material processing lasers

Fibre transmission of high-power radiation has opened the laser a new dimension of material processing applications. The three-dimensional treatment revolutionised a far field of the production technology. One impressed example is the laser welding process in the automotive industry. The Nd–YAG solid state laser in combination with the optical fibre increase the automation and the flexibility of a whole industry. The next generation of Audis aluminium alloy car body is mainly welded by fibre-guided Nd–YAG laser beams. Since the past 10 years the maximum average laser power transmitted through optical fibres increase from some 100 W up to 10 kW. For industrial application only Nd–YAG lasers and recently the diode lasers are economically suited for fibre transmission.     

High-power Yb-doped continuous-wave and pulsed fibre lasers

High-power laser generation using Yb-doped double-clad fibres with conversion efficiencies in excess of 80% have attracted much attention during the last decade due to their inherent advantages in terms of very high efficiency, no misalignment due to in-built intracore fibre Bragg gratings, low thermal problems due to large surface to volume ratio, diffraction-limited beam quality, compactness, reliability and fibre-optic beam delivery. Yb-doped fibres can also provide a wide emission band from ～1010 nm to ～1170 nm, which makes it a versatile laser medium to realize continuous-wave (CW), Q-switched short pulse, and mode-locked ultrashort pulse generation for various applications. In this article, a review of Yb-doped CW and pulsed fibre lasers along with our study on self-pulsing dynamics in CW fibre lasers to find its role in high-power fibre laser development and the physical mechanisms involved in its generation has been described. A study on the generation of high-power CW fibre laser of 165 W output power and generation of high peak power nanosecond pulses from acousto-optic Q-switched fibre laser has also been presented.     

Study on the periodic oscillation of plasma/vapour induced during high power fibre laser penetration welding

High power fibre lasers have recently received much attention because of their inherent advantages such as high output power, high beam quality, compact size, and flexible fibre delivery. Studies on the mechanism behind fibre laser welding systems may further promote their practical application. In this paper, high speed video observations were used to study the characteristics of the plasma/vapour induced during the bead-on-plate welding of ZL114 using a high power CW fibre laser. We also analysed the cause of the periodic oscillation of the plasma/vapour. The results revealed that plasma/vapour induced from high power lasers oscillate periodically at 450–600 μs cycles above the weld pool surface. The use of a shielding gas has little effect on the oscillation cycle. The plasma/vapour absorption is not the main reason for the periodical oscillation of plasma/vapour induced during fibre laser welding. The periodic oscillation of the plasma/vapour can be attributed to the oscillation of the keyhole.     

Efficient delivery of 60 J pulse energy of long pulse Nd:YAG laser through 200 μm core diameter optical fibre

Most of today’s industrial Nd:YAG lasers use fibre-optic beam delivery. In such lasers, fibre core diameter is an important consideration in deploying a beam delivery system. Using a smaller core diameter fibre allows higher irradiances at focus position, less degradation of beam quality, and a larger stand-off distance. In this work, we have put efforts to efficiently deliver the laser output of ‘ceramic reflector’-based long pulse Nd:YAG laser through a 200 μm core diameter optical fibre and successfully delivered up to 60 J of pulse energy with 90% transmission efficiency, using a GRADIUM (axial gradient) plano-convex lens to sharply focus down the beam on the end face of the optical fibre and fibre end faces have been cleaved to achieve higher surface damage thresholds.     

Development of multi-wavelength microsphere fibre laser system for potential sensor applications

A multi-wavelength microsphere laser system, using a chirped fibre Bragg grating and a microsphere resonator as wavelength-selective elements and a high dopant erbium doped fibre as the gain material, has been successfully demonstrated. The multi-wavelength generation of the laser system arises from both the microsphere whispering gallery mode selection and from the additional Raman scattering inside the microsphere cavity when the erbium laser is operating at resonance with the whispering gallery modes. Through an appropriate design and fabrication of a microsphere and of a fibre taper, a selective multi-wavelength fibre laser has been realized when the pump power is above threshold required. The laser output lines created have shown much narrower linewidths than those from conventional fibre lasers and these characteristics are particularly suitable for the range of sensor applications envisaged in the work.     

Development of Transmission Optics for High-Power TEA CO2 Lasers and Methods of Increasing Laser-Radiation Resistance of Materials for High-Power IR Optics

The results of analytical and experimental investigations aimed at increasing laser-radiation resistance of materials for IR optics and developing high-power optics for microsecond TEA CO₂ lasers with energy per pulse up to 12–25 kJ and gas-dynamic CO₂ lasers with energy per pulse up to 130 kJ are presented. It is demonstrated that the integrated approach that combines the improvement of already existing technological methods and the development of novel technological methods for refining the parameters of materials for transmission IR optics (including techniques of growth of single crystals, strain hardening, and laser, ionic, and chemical treatment), the design and optimization of optical units (including the development of segmented transmission optics, the improvement of optical schemes for spatial formation of laser beams, the use of fast-response physical effects to screen optical elements from high-power fluxes of laser radiation) is necessary to solve this problem.     

Optically pumped far infrared lasers

The far-infrared (FIR) region of the electromagnetic spectrum is commonly thought of as the wavelength region ≈ 30μm-≈2 mm. Thus, the FIR wavelength region is located between the more familiar areas of microwaves and optics. Primarily due to the lack of FIR sources and detectors, the FIR region is difficult to access and therefore relatively unexplored and unused. The FIR source problem is presently under attack from neighbouring disciplines; from the microwave side by extending the frequency operating range of classical electron tube oscillators (e.g. backward wave oscillators) and semiconductor devices (e.g. IMPATT and quantum well oscillators) and from the optical side primarily by optically pumped molecular gas lasers.The FIR technology evolution accelerated in the mid 60's with the discovery of the discharge pumped hydrogen cyanide laser, lasing at a handful of lines located at about 330μm wavelength. However, the most important step towards a useful coherent FIR source was the discovery of the optically pumped FIR laser in 1970. In optically pumped FIR lasers a molecular gas (e.g. methyl fluoride methyl alcohol, formic acid) is pumped by an external laser, usually a carbon dioxide laser. The FIR laser transitions typically takes place between adjacent rotational levels in an excited vibrational state. Today, optically pumped FIR lasers cover the full FIR region by more than one thousand discrete laser lines observed in hundreds of FIR laser media. FIR output powers on the order of 1–100 mW are available from a vast number of laser transitions.Despite the rapid development of semiconductor FIR oscillators the optically pumped FIR laser is still the only practical unit that bridge the full frequency-gap between microwaves and optics. The fact that FIR lasers are considered as local oscillators in space born applications, indicate that FIR laser technology has matured considerably.This survey paper discusses optically pumped FIR lasers from the engineer's point of view: principles of operation, design and characteristics.     

Model analysis of supermode generation in active 5-core optical fibre

In the paper, the phase-locked emitters in multi-core optical fibres for high power fibre lasers are presented. The influence of the normalized frequency and diameters of the cores on the shape of the pattern in the Fraunhofer diffraction region has been analysed. The simulation of coupling coefficient between cores influences on phase-locking and in the consequence on the far-field pattern of the fibre laser, based on the analyzed multi-core optical fibre, were performed. In the analyzed fibre laser, while exchanging power of the generated radiation between two adjacent cores on the fibre length (L = 5 m) within the range of 4–10%, the radiation becomes phased. Having satisfied this condition, in the far-field low-divergence, high-power laser beam (supermode) is attained. 5-core double clad optical fibre doped with neodymium ions was fabricated. Luminescence spectra of the manufactured fibre were measured.     

Spectrum control and shaping of optical pulses in optical fibre interferometers

This paper discusses the use of single-mode fibre ring interferometers for injection laser line narrowing and for enhancement of optical emission power. The efficient launching into fibre ring interferometers of multi-frequency external cavity injection laser emission, in both the c.w. and mode-locking regime, has been achieved. Matching the optical lengths of the external cavity and fibre interferometer allowed the measurement of mode width of such a laser. A method is proposed for shaping optical pulses in fibre ring interferometers from constant intensity frequency modulated emission. The mode-locking regime of all-fibre external cavity injection lasers has been achieved.     

Theoretical study and optimization of a high power mid-infrared erbium-doped ZBLAN fibre laser

Based on the rate equations describing the erbium-doped fluoride glass(ZBLAN) fibre lasers with different pumping configurations being taken into account,this paper presents theoretical calculations related to the dynamic population density and the operation performance of a high power mid-infrared all-fibre erbium-doped ZBLAN fibre laser.It shows that the ground-state absorption,excited-state absorption and energy-transfer-upconversion processes co-exist and produce a population balance,causing the laser to operate stably at a continuous wave state.A good agreement between the theoretical results and recent experimental measurement is obtained.Furthermore,the laser structure parameters including fibre length,reflectance of output fibre Bragg grating and pumping configurations are quantitatively optimised to achieve the best performance.The results show,as expected,that the slope efficiency of the fibre laser can be improved significantly through optimisation,which then provides an important guide for the design of high-performance mid-infrared erbium-doped ZBLAN fibre lasers.     

用于激光二极管抽运固体激光器热畸变补偿的微变形镜特性研究

在很多高功率的激光器中,由于热效应的缘故会导致激光器的光束质量大幅下降。为了改善激光光束的动态畸变,自适应光学是一种有效的方法。作为自适应光学系统的核心部件,设计了一种可用于大功率激光二极管抽运固体激光器(DPL)热畸变补偿的新型可变形反射镜,并且通过工艺流片实际制造出有效反射面积为30 mm×30 mm,拥有49个静电驱动单元的可变形反射镜。对该微机电系统(MEMS)微变形镜的变形特性进行了详细的理论分析和模拟研究,并和实际测量结果进行了比较,得到了满意的结果。为了实现闭环控制,对微变形反射镜的光学影响函数矩阵进行了全面测量,并用得到的电压矩阵对畸变激光光束的波前进行了校正。实验结果表明,微变形反射镜可以有效地改善激光光束质量。     

Holograms produced with double-heterojunction laser illumination

Off-axis holograms have been constructed with a double-heterojunction (Ga, Al) As-GaAs semiconductor laser, and image reconstruction has been accomplished using a semiconductor and a He-Ne laser. The reconstructed real images were quite clear, and the signal-to-noise ratio of those images was about 10. Experiments suggest that semiconductor lasers can successfully be applied to coherent optics.     

Ultraintense lasers: relativistic nonlinear optics and applications

Traditional optics and nonlinear optics are related to laser–matter interaction with eV characteristic energy. Recent progresses in ultrahigh intensity makes it possible to drive electrons with relativistic energy opening up the field of relativistic nonlinear optics. In the last decade, lasers have undergone orders-of-magnitude jumps in peak power, with the invention of the technique of chirped pulse amplification (CPA) and the refinements of femtosecond techniques. Modern CPA lasers can produce intensities greater than 10²¹ W/cm², one million times greater than previously possible. These ultraintense lasers give researchers a tool to produce unprecedented pressures (terabars), magnetic fields (gigagauss), temperatures (10¹⁰ K), and accelerations (10²⁵ g) with applications in fusion energy, nuclear physics (fast ignition), high-energy physics, astrophysics, and cosmology. They promote the optics field from the eV to the GeV.     

Efficient Tm, Ho-co-doped silica fibre laser diode pumped at 1150 nm

High power and highly efficient operation of a Tm³⁺, Ho³⁺-doped silica fibre laser that is pumped with diode lasers operating at 1150 nm is demonstrated. Internal slope efficiencies approaching the Stokes limit were produced and the maximum output power was 2.9 W. High power diode lasers operating at 1150 nm are valuable pump sources for a range of fibre lasers offering output in the shortwave infrared spectrum.     

Lasers for materials processing: specifications and trends

An overview is given of the types of lasers dominating the field of laser materials processing. The most prominent lasers in this field are the CO₂ and the Nd: YAG laser. The domain of CO₂ lasers is applications which demand high laser powers (up to 30 kW are available at present), whereas the domain of Nd:YAG lasers is micro-machining applications. In the kilowatt range of laser output power, the two types of lasers are in competition. New diffusion-cooled CO₂ laser systems are capable of output laser powers of several kilowatts, with good beam qualities, while still being quite compact. The output power and beam quality of Nd:YAG lasers has been improved in recent years, so that Nd:YAG lasers are now an alternative to CO₂ lasers even in the kilowatt range. This is especially true for applications that demand optical fibre transmission of the laser beam, which is possible with Nd:YAG laser light but not with the longerwavelength light emitted by CO₂ lasers. The main problem in solid-state lasers such as Nd:YAG is the thermal lensing effect and damage due to thermal stresses. In order to reduce thermal loading, cooling has to be enhanced. Several alternative geometries have been proposed to reduce thermal loading and, by this, thermal lensing effects. There are now slab and tube geometries which allow much higher output powers than the conventionally used laser rods. A very new scheme proposes a thin slab whose cooled side is also used as one of the laser mirrors, so that thermal gradients occur mainly in the direction of the beam propagation and not perpendicular to it, as is the case in the other geometries. As well as CO₂ and Nd:YAG lasers, semiconductor laser diodes are very promising for direct use of the emitted light or as pump sources for Nd:YAG and other solid-state lasers. When packaging together thousands of single laser diodes, output powers of several kilowatts can be realized. Major problems are collimation of the highly divergent laser beams and cooling of the laser diode bars.     

Multiple beams from Rogowski- profile solid electrode TEA CO2 lasers

Three and five beams were observed to emerge with angular symmetry from Rogowski- profile solid electrode TEA CO₂ lasers. A simple geometrical optics theory of the external reflection guided-modes within the rectangular cavity formed by the laser mirrors and electrodes has successfully accounted for the number and angular separation of these multiple beams, and has established criteria for their elimination.     

GHz optical communication experiments using lead-salt laser diodes in the 3.5–5.5μm wavelength region

The modulation performance of lead-salt laser diodes suitable for mid-infrared fibre optic communication systems has been investigated. The electron and photon lifetime for a PbCdS laser have been determined to 3.1 ns and 3.3 ps, respectively. Moreover, an experiment using short lengths of fluoride fibres shows the feasibility of GHz fibre optic communication links in the mid-infrared region using lead-salt lasers if the potential ultra-low loss of mid-infrared optical fibres can be realized.