Mode Fiber Laser Research Articles

Rapid micro hole laser drilling in ceramic substrates using single mode fiber laser

This paper presents experimental results on rapid single mode fiber laser drilling of ceramic substrates. The materials under study are alumina and aluminum nitride with thicknesses ranging from 0.25mm and 1.5mm. Both the bore diameter and the drilling speed are optimized by varying the laser focus position, laser power, gas pressure and laser modulation scheme, respectively. Within a design of experiment approach we show that the focus position has the highest influence on the bore diameter and highlight the capability to drill sub-50μm bore diameters with an almost vanishing taper (1.57°) and good circularity (≤1.07) using modulated single mode fiber lasers. The required drilling time for, e.g. 0.25mm alumina is as low as 0.1ms with, in general, longer drilling times for AlN as compared to Al2O3. Using a pre-pigmenting technique the splatter after drilling is completely removed in an ultrasonic bath.

Towards low timing phase noise operation in fiber lasers mode locked by graphene oxide and carbon nanotubes at 1.5 µm.

We investigate the timing phase noise of fiber lasers mode locked by graphene oxide (GO) and carbon nanotubes (CNTs), respectively, integrated in a linear cavity fiber laser in the reflecting operation. Due to the shorter decay time of the GO and CNTs, weaker slow saturable absorber effects are expected and mode-locked lasers based on these two saturable absorbers exhibit low excess timing phase noise coupled from the laser intensity noise. Compared with a reference laser mode locked by semiconductor saturable absorber mirror (SESAM), GO based laser obtains a timing phase noise reduction of 7 dB at 1 kHz and a timing jitter reduction of 45% experimentally whereas CNTs based laser obtains a timing phase noise reduction of 3 dB and a timing jitter reduction of 29%. This finding suggests that saturable absorbers with short decay time have the potential for achieving mode locking operation with low timing phase noise, which is important for applications including frequency metrology, high-precision optical sampling, clock distribution and optical sensing.

Modelling of the Remote Fusion Cutting Process Based on Experiments

Remote fusion cutting (RFC) is an interesting industrial process compared to traditional laser cutting. It is because traditional laser cutting is limiting travel speed and accessibility due to the required positioning of the cutting head just above the workpiece for providing a cutting gas pressure. For RFC this pressure is created by the vapor, which is formed when the laser beam evaporates the cut material. The drawback of RFC compared to traditional laser cutting is a worse cut quality, wide cut kerf and a slower travel speed. The contribution of this paper is an experimental investigation, which determined the process window for RFC in stainless steel with a single mode fiber laser. The process variables: travel speed, focus position, power and sheet thickness were investigated. Based on the results of the experiments and process knowledge the aim of this work was to determine and describe the most important driving mechanisms for understanding and modelling the RFC process. The purpose is to deepen the understanding of the mechanisms in the process and find the factors, which can improve the performance and also determine the limitations. The validation results show that the developed model of the RFC process gives a similar process window as the experimental results for the tested parameters and variation of travel speed and focus position.

Angular Stability Margins for the Remote Fusion Cutting Process

The set of incident angles, that yields stable cuts when conducting remote fusion cutting (RFC), is of great importance when scheduling multiple cuts on a work piece. This is due to their ability to determine how much the laser beam can be moved by angling the cutting head instead of translating it. This paper investigates how the stability of the RFC process is affected by changing the incident angle when processing stainless steel sheets. This investigation was conducted as an experimental study in which the angle of incidence was decomposed into a work angle and a travel angle. The stability was evaluated by an automatic procedure based on images acquired by a programmable microscope and a computer vision algorithm developed in MATLAB. The results showed that the stability of the RFC process was dependent on the work and the travel angle. It was also seen that a coherent region of stable incident angles could be found. All experiments were conducted with a 3kW single mode fiber laser at the laser processing laboratory at Aalborg University.

싱글모드 파이버 레이저를 이용한 SUS304와 Cu의 고속 겹치기 용접에서 접합부 및 인장시험 파단부의 특성에 관한 연구

To develop and understand dissimilar metals joining of Stainless steel and Copper, ultra-high speed laser lap welding was studied using single mode fiber laser in this study. SUS304 and Cu have large differences in materials properties, and Cu and Fe have no intermetallic compounds by typical binary phase of Cu and Fe system. In this study, ultra-high speed lap welds of SUS304 and Cu dissimilar metals using single- mode fiber laser was generated, and weldability of the weld fusion zone was evaluated using a tensile shear test. To understand the phenomenon of tensile shear load, weld fusion zone of interface weld area and fracture parts after tensile shear test were observed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis system. And it was confirmed that Cu was easily melting and penetrating in the grain boundaries of SUS304 because of low melting temperature. And high thermal conductivity of copper occurred dissipate heat energy rapidly. These properties cause the solidification cracking in weld zone.

Numerical study on the minimum modulation depth of a saturable absorber for stable fiber laser mode locking

We conducted a numerical study to investigate the minimum modulation depth of a saturable absorber that is essentially required for stable optical pulses to be generated from a passively mode-locked fiber laser cavity. More specifically, an extended nonlinear Schrodinger equation was numerically solved in order to analyze the impact of the cavity group velocity dispersion (GVD), cavity χ(3) nonlinearity, and saturation gain of an active fiber on the minimum modulation depth that is required. The net cavity GVD is shown to significantly influence the required minimum modulation depth level of a saturable absorber. A cavity with little net anomalous dispersion was found to be readily mode locked through the use of a saturable absorber with a very small amount of modulation depth. Furthermore, the reason that a fiber laser cavity with a zero cavity GVD becomes unstable and needs a much higher modulation depth was investigated, and such a condition was found to be associated with the fiber χ(3) nonlinearity. The minimum modulation depth was also shown to vary according to the saturation gain level of an active fiber.

Dissipative vector soliton in a dispersion-managed fiber laser with normal dispersion.

We numerically study the vector dynamics of dissipative solitons (DSs) in a 2μm dispersion-managed fiber laser mode locked by a semiconductor saturable absorber mirror and operated in the normal dispersion regime. It is shown that the effective gain bandwidth is crucial for the DS generation. The steep spectral edges of DSs are the consequence of the interaction among the normal dispersion, fiber nonlinearity, gain and loss, and gain dispersion effect, etc. We numerically duplicate the experimental results and further explore the vector features of the generated DSs. Two DSs formed along the two orthogonal polarization directions which, incoherently coupled with each other, could propagate in the birefringent cavity with the same group velocity.

Dual-wavelength single longitudinal mode fiber laser for microwave generation

Abstract A simple dual-wavelength single longitudinal mode (DW SLM) fiber laser for microwave generation is proposed and demonstrated. Simultaneous oscillation of dual-wavelength with a wavelength spacing of 0.2124 nm is realized, the output peak power of the lasing dual-wavelength is equalized by adjusting polarization controller at room temperature. Birefringent fiber filter and saturable absorber are utilized to guarantee the SLM operation of dual-wavelength fiber laser. A microwave signal at 26.49 GHz is successfully obtained in the experiment, the laser is proved to be effective in the application of microwave generation.

Multiplexed multi-longitudinal mode fiber laser sensor.

A multiplexed multi-longitudinal mode fiber laser sensor system is proposed and demonstrated. By incorporating two matched wavelength division multiplexers (WDMs) and a semiconductor optical amplifier (SOA) into a fiber laser cavity, multiwavelength oscillation is established. Each wavelength corresponding to one channel of WDMs contains multi-longitudinal modes. The multiwavelength output of the laser is directed to another WDM which functions as a demultiplexer. By monitoring the longitudinal mode beat frequency generated at photodetectors following the WDM, the sensing information can be demodulated. Preliminary results for multiplexing two sensors measuring strain and temperature are presented to verify the principle of the system.

Wavelength tunable single longitudinal mode fiber laser pinned to 25 GHz spacing

ABSTRACTA wavelength tunable single longitudinal mode (SLM) fiber laser is demonstrated with a high power Er‐Yb codoped fiber amplifier (EDFA) used as gain medium. A fiber Fabry‐Perot filter and a bandpass filter ensure single wavelength lasing. To realize SLM operation, a section of unpumped EDF in a Sagnac loop is adopted as a saturable absorber, which results in a self‐induced FBG (frequency band gap) with an ultranarrow bandwidth. The proposed laser is very stable and in SLM operation. The signal to noise ratio is measured about 65 dB. The output laser has a tuning range over 30 nm from 1535 to1565 nm. The tuning step of the proposed laser is 0.20 nm pinned to the ITU grid. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2404–2406, 2014

120 W subnanosecond ytterbium-doped double clad fiber amplifier and its application in supercontinuum generation

We report a 120 W high average power all-fiber subnanosecond master oscillator power amplifier (MOPA) system with a two-stage amplifier chain. The seed source is an ytterbium-doped single mode fiber laser passively mode-locked by nonlinear polarization rotation (NPR). Its pulse width is tunable from 0.59 ns to 1.2 ns by changing the length of the laser cavity. The average output power of 91.5 mW is obtained at 26.3 MHz with pulse width of 0.59 ns. A large mode area double cladding high-absorption ytterbium-doped fiber is introduced into the master power amplifier to suppress the nonlinear effects. Finally, 120 W average output power is obtained with single pulse energy of 4.56 μJ, and its optical-to-optical conversion efficiency is up to 83.9%. In addition, we demonstrate 19.5 W average-power supercontinuum generation in photonic crystal fiber (PCF) using this subnanosecond pulsed fiber amplifier.

Design Optimization for High Manufacturability of Optical Fiber Endcap for Range Detection LADAR

High power fiber laser module of the transmitting channel for the designed LADAR (LAser raDAR) system has an angled endcap to avoid the direct reflection fed back to the source laser unit. However, the reference endcap design shows low manufacturability caused by the technical challenge in splicing and fabrication. We explored four alternative design solutions exhibiting similar performances. The end cap design compatibility was achieved with the transmittance and beam shape analysis using a non-sequential ray tracing program, ASAP. Specially, the analysis was focused at the zoom lens position and the endcap position. Among the four designs, the best solution shows the transmittance of about 89% of the Reference Design with 0.14° LOS (Line of Sight) shift. We believe that 11% in absorption by the mechanical components and can be further improved with the minor mechanical change and that the LOS shift is within the accommodation range of the wedge compensators in the transmitting channel.

Semiconductor and Ceramic Microstructure Made by Single Mode Fiber Laser

In the paper the results of micromachining of 3D microstructures of microsystems made from silicon and alumina ceramic using a single mode fiber laser (1064 nm) are presented. The quality of obtained structures and its smallest dimensions with acceptable maintained quality were examined. The influence of variable parameters of laser processing with changing of mapping scale on geometrical features of structures was identified.

Lasers: Coupled Polymer Microfiber Lasers for Single Mode Operation and Enhanced Refractive Index Sensing (Advanced Optical Materials 3/2014)

A single-mode laser is obtained by H. D. Sun and co-workers from coupled whispering gallery mode fiber lasers (two bright rings) via the Vernier effect under optical excitation (green light). Application of the coupled structure for refractive index sensing of ethanol solution (molecules), as shown on page 220, exhibits a doubled sensitivity compared with the single structure. The background shows the magnetic field distribution of a typical coupled cavity.

Effect of welding speed on microstructural and mechanical properties of laser lap weld joints in dissimilar Al and Cu sheets

Conventional fusion welding of aluminium and copper dissimilar materials is difficult because of poor weldability arising from the formation of brittle intermetallic compounds on the weld zone as well as different chemical, mechanical and thermal properties of welded joints. Joining of Al and Cu plates or sheets offers a metallurgical challenge due to unavoidable formation of brittle intermetallic compounds. Therefore, it is necessary to effectively suppress the formation and growth of Al–Cu intermetallic compounds. For welding of dissimilar Al and Cu sheets, no systematic work has been conducted to reduce these defects. Thus, this paper focuses on the effect of welding speed on the quality of a lap weld joint in the Al and Cu sheets with a single mode fibre laser. It was found that consequently sound strong weld joints could be produced by suppressing the formation of intermetallic compounds in the interface zone at extremely high speeds.

Beam Shaping to Control of Weldpool Size in Width and Depth

The introduction of high power single mode fiber lasers has given deeper and narrower laser welds than seen previously. In some cases the weld becomes too narrow and must be expanded to fit the geometrical shape of a given weld task. It was suggested that instead of using only one beam, the beam was split into multiple beams placed in a pre-specified pattern. In this way the dimensions of the weld pool could be controlled. This gave the ability to control the final weld face width and penetrationdepth independently of each other with minimum heat input.

Comparison of High Rate Laser Ablation and Resulting Structures Using Continuous and Pulsed Single Mode Fiber Lasers

This paper compares high rate laser ablation and resulting structures of aluminum by using both a continuous wave and a ns-pulsed single mode fiber laser of high average laser power. Two different scan technologies were applied for fast deflection of the laser beams. In this work, 2.5D laser processing was studied by using a high aperture galvanometer scanner with a maximum scan speed of 18 m/s. By contrast, considerably higher scan speeds up to 1,000 m/s were achieved by using the in-house developed polygon scanner system. The ablation rates and the processing rates per unit area were analyzed by means of the depths of line-scan ablation tracks and laser processed cavities. In addition, SEM photograph of the machining samples will be presented in order to evaluate the machining quality. Finally the feasibility of this high rate technology for industrial application is demonstrated by machining examples.

Laser Welding of Copper Using Multi Mode Fiber Lasers at Near Infrared Wavelength

Due to the increasing electrification of automotive drives and the expansion of decentralized renewable energygeneration, the consumption of copper for the fabrication of electrical components such as electric motors or conducting paths increases. To jointhese components, laser welding is more frequently used since it represents a flexible and fully automatable joining process. Because of the high thermal conductivity, the low absorption coefficient forinfrared wavelength of common laser beam sources and the resulting limited process efficiency, welding of copper alloys represents a major challenge for laser assisted processes. In this paper, experimental investigationsare presented to identify arising process limits during laser welding of pure copper materials with multi-mode fiber lasers at near infrared wavelength depending on the applied laser power and welding velocity. In addition, a potential stabilization of the welding process by shielding gas support was examined. Further investigations were focused on the influence of shielding gas on the molten pool geometry.

Passively mode-locked fiber laser sensor for acoustic pressure sensing

The development is reported of a multi-longitudinal mode fiber laser sensor based on passive mode locking employing carbon nanotubes in the laser cavity. A polymer membrane is employed beneath the pre-strained erbium-doped fiber (EDF) to convert the sound pressure disturbance into axial strain, alter the cavity length, and induce a shift of the longitudinal modes beat. Hence, acoustic pressure measurement can be carried out by detecting the shift of the beat frequency. Experimental results show comparable strain and sound pressure sensitivity of ~0.5 kHz/με and 147.2 Hz/Pa, respectively. The proposed sensor is an alternative for the measurement of acoustic pressure and possesses the advantages of good stability and ease of interrogation.

Investigation of superfast deposition of metal oxide and Diamond-Like Carbon thin films by nanosecond Ytterbium (Yb+) fiber laser

Metal oxide (MOx, M: titanium, magnesium) and Diamond-Like Carbon (DLC) thin films were synthesized by Pulsed Laser Deposition (PLD) at room temperature and low vacuum of 2Pa for MOx and vacuum of 4×10−3Pa for DLC films. A fiber based Ytterbium (Yb+) laser operating in the nanosecond regime at a repetition rate of 20kHz was used as an ablation source. Dense and smooth thin films with a thickness from 120 to 360nm and an area of up to 10cm2 were deposited on glass and stainless steel substrates at high growth rates up to 2nm/s for a laser intensity of 10–12J/cm2. The thin films synthesis was compared for two fiber laser modes of operation, at a repetition rate of 20kHz and with an additional modulation at 1kHz. The morphology, chemical composition and structure of the obtained thin films were evaluated using optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and Raman spectroscopy. The morphology of the MOx thin films and the deposition rate strongly depend on the fiber laser mode of operation. Very smooth surfaces were obtained for the metal oxide thin films deposited at lower deposition rates in the modulation mode at 1kHz. The effect of the substrate on the DLC film structure was studied. The films deposited on dielectric substrates were identified as typical tetrahedral (ta-C) DLC with high sp3 content. DLC films on metal substrates were found typical a-C amorphous carbon films with mixing sp2/sp3 bonds.