Additive manufacturing (AM) is promising to produce complex shaped components, including metals and alloys, to meet requirements from different industries such as aerospace, defense and biomedicines. Current laser AM uses CW lasers and very few publications have been reported for using pulsed lasers (esp. ultrafast lasers). In this paper, additive manufacturing of Tungsten materials is investigated by using femtosecond (fs) fiber lasers. Various processing conditions are studied, which leads to desired characteristics in terms of morphology, porosity, hardness, microstructural and mechanical properties of the processed components. Fully dense Tungsten part with refined grain and increased hardness was obtained and compared with parts made with different pulse widths and CW laser. The results are evidenced that the fs laser based AM provides more dimensions to modify mechanical properties with controlled heating, rapid melting and cooling rates compared with a CW or long pulsed laser. This can greatly benefit to the make of complicated structures and materials that could not be achieved before.
Due to the unique ultra-short pulse duration and high peak power, femtosecond (fs) laser has emerged as a powerful tool for many applications but has rarely been studied for 3D printing. In this paper, welding of both bulk and powder materials is demonstrated for the first time by using high energy and high repetition rate fs fiber lasers. It opens up new scenarios and opportunities for 3D printing with the following advantages - greater range of materials especially with high melting temperature, greater-than-ever level of precision (sub-micron) and less heat-affected-zone (HAZ). Mechanical properties (strength and hardness) and micro-structures (grain size) of the fabricated parts are investigated. For dissimilar materials bulk welding, good welding quality with over 210 MPa tensile strength is obtained. Also full melting of the micron-sized refractory powders with high melting temperature (above 3000 degree C) is achieved for the first time. 3D parts with shapes like ring and cube are fabricated. Not only does this study explore the feasibility of melting dissimilar and high melting temperature materials using fs lasers, but it also lays out a solid foundation for 3D printing of complex structure with designed compositions, microstructures and properties. This can greatly benefit the applications in automobile, aerospace and biomedical industries, by producing parts like nozzles, engines and miniaturized biomedical devices.
A laser-induced breakdown spectroscopy (LIBS) guided smart surgical tool using a femtosecond fiber laser is developed. This system provides real-time material identification by processing and analyzing the peak intensity and ratio of atomic emissions of LIBS signals. Algorithms to identify emissions of different tissues and metals are developed and implemented into the real-time control system. This system provides a powerful smart surgical tool for precise robotic microsurgery applications with real-time feedback and control.
This paper presents the newest development of high energy, high power ultrafast fiber lasers in 1 μm, 2 μm and 3 μm regimes at PolarOnyx Inc. For the 1 μm mJ fiber laser, a Yb-doped PCF optical amplifier was built to boost pulse energy to 1.1 mJ at 100 kHz, with pulse duration of 710 fs. In the kW experiment, up to 1050 W average power was obtained with a repetition rate of 80 MHz and pulse duration of 850 fs. In the Tm-doped fiber laser experiment, an average power of up to 76 W was achieved at wavelength of 2012 nm with a repetition rate of 31 MHz and pulse duration of 870 fs. In the Er:ZBLAN fiber laser experiment, an average power of up to 142 mW was achieved at wavelength of 2784 nm with a repetition rate of 16.4 MHz and pulse duration of 5 ps. This work lays out a foundation for further energy and power scaling of ultrafast fiber lasers.
In this paper, a LIBS guided smart surgical tool using a femtosecond (fs) fiber laser is investigated. This functional
system includes a high energy fs fiber laser system (PolarOnyx Laser, Inc. - Uranus mJ Series) for material ablation,
a 3D controllable motion stage, LIBS signal collecting fiber to a spectrometer and a computer for data analysis and
process control. The laser source employed emits pulses with pulse duration of 750 fs at a repetition rate tunable
from 1 Hz to 1 MHz. The centre wavelength is at 1030 nm and the pulse energy can be up to 500 μJ. General
characteristics like ablation rate and LIBS signal are determined at first. Furthermore the LIBS data is processed and
analyzed for material characterization and differentiation. Comparison methods to identify the different materials
emissions are developed and algorithms are implemented into a real-time control system. This system allows
processing of different materials with real time feedback and capability to the laser parameters (pulse energy and
repetition rate) and processing parameters (speed) and provides a powerful LIBS guided smart surgical tool with fs
fiber laser for delicate surgery applications.
In the paper, a Yb:YAG single crystal fiber is used for the first time to amplify week image signal. It was longitudinally pumped by a fiber-coupled laser diode with a maximum power of 150W at 940 nm. The image amplifier provided low noise and high gain amplification. A spatially uniform amplification gain of up to 10.2 dB at wavelength of 1030 nm was obtained.
Micro-hole drilling and cutting in ambient air are presented by using a femtosecond fiber laser. At first, the micro-hole drilling was investigated in both transparent (glasses) and nontransparent (metals and tissues) materials. The shape and morphology of the holes were characterized and evaluated with optical and scanning electron microscopy. Debris-free micro-holes with good roundness and no thermal damage were demonstrated with the aspect ratio of 8∶1 . Micro-hole drilling in hard and soft tissues with no crack or collateral thermal damage is also demonstrated. Then, trench micromachining and cutting were studied for different materials and the effect of the laser parameters on the trench properties was investigated. Straight and clean trench edges were obtained with no thermal damage.
We describe in detail a high energy, high power ultrafast thulium-doped fiber laser system. The pulse energy of 156 μJ was realized. The laser system is comprised of a mode-locked 2020-nm seed oscillator and multiple-stage power/energy amplifiers. The seed oscillator output pulses at a repetition rate of 2.5 MHz. The seed pulses were stretched with the anomalous dispersion fiber to the duration of 320 ps. An acousto-optic modulator was used as a pulse picker to lower the repetition rate. A two-stage preamplifier was used to boost the pulse energy to 3 μJ. The pulse energies of up to 156 μJ and the average power of 15.6 W were obtained from the final stage of power amplifier at a repetition rate of 100 kHz with a slope efficiency of 26%. The pulse durations of 780 fs were obtained after pulse compression. High optical signal-to-noise ratio (OSNR) and low background noise were also achieved at this low repetition rate.
In this work, blackening of metals is performed using a commercialized mode-locked femtosecond fiber laser. Different types of surface structures are produced with varying laser scanning conditions (scanning speed and pitch). The modified surface morphologies are characterized using Scan Electron Microscope (SEM) and the blackening effect was investigated both visually and through spectral measurements. Spectral measurements show that the reflectance of the processed materials decreased sharply in a wide wavelength range and varied at different rate for different scanning pitch and speed. Above 95% absorption over the entire visible wavelength range has been demonstrated for the surface structures and the absorption for specific wavelengths can go up to 98.6%. It is found that the enhanced absorption of the black metal is due to light trapping and a variety of micro- and nano-scale surface structures. This study shows the great potential applications such as constructing sensitive detectors and sensors, solar energy absorber and biomedicine. Keywords: Femtosecond laser, fiber laser, blackening, direct writing, nanostructure, light trapping.
This paper reports the studies on time-resolved laser induced breakdown spectroscopy (LIBS) of plasmas produced by a
femtosecond (fs) fiber laser. The temporal behavior of specific ion and neutral emission lines of different materials
(metals, glasses and semiconductors) has been characterized. Sub-spot-size craters are generated with near threshold
pulse energy and it shows the potential for further improved spatial resolution using fs laser for LIBS application. The
decay between the continuum plasma emission and the atomic emission were used as a means to maximize the signal-tonoise
ratio (SNR) of the atomic emission lines for different materials. The SNR can be improved by more than one order
of magnitude with optimal delay and gating. This fiber laser based LIBS can lead to a more compact, reliable, low-cost
and field-deployable detection system for versatile and rapid analysis of chemical and special explosive materials.
Femtosecond (fs) laser becomes more popular in precise material procesing due to the limited heat affected zone (HAZ) over longer laser pulses. In this paper, micro-hole drilling in ambient air with fs fiber laser at 1030 nm is presented. Micro holes were fabricated in both transparent (polymer and glass), and non-transparent materials (metal and tissue). A scanning electron microscopy is used to investigate the hole quality. Both percusiion drilling and trepanning drilling methods were evaluated and compared. High quality micro holes with no visible micro-cracks were demonstrated. Micro-hole drilling in hard and soft tissues with no collateral thermal damage is observed. This study can be extended to MEMS, microfluidic, and micro surgery applications.
A high energy, high power ultrafast Tm doped fiber laser system was successfully developed. Pulse energy of 156 μJ and average power of 15.6 W were achieved. The laser system consisted of a mode-locked 2020 nm seed oscillator and multiple-stage power amplifiers. The seed included 30 m-long dispersion compensating fiber and emitted slightly chirped pulses with spectrum bandwidth of 8 nm. The mode-locking was stable and self-started. Repetition rate of seed oscillator was 2.5 MHz. The seed pulses were stretched with anomalous dispersion fiber to the duration of 320 ps. An AOM was used as a pulse picker to lower the repetition rate. A two-stage pre-amplifier was used to boost the pulse energy to 3 μJ. Pulse energies of up to 156 μJ were obtained from the final stage of power amplifier at a repetition rate of 100 kHz. Pulse durations of 780 fs were obtained after pulse compression. High OSNR and low background noise were also achieved at this low repetition rate.
Femtosecond fiber lasers are becoming an enabling technology for biomedical imaging and diagnostics from the bench to the bedside. Techniques used for achieving mode locked fiber lasers are discussed. These techniques include polarization shaping, pulse shaping, and spectral shaping. Mode locked fiber lasers operating at 1 μm, 1.55 μm, 2 μm and their harmonic generations (780-800 nm, 515-532 nm, 344-355 nm, 257-266 nm) are discussed. By using dispersion managed amplifiers, amplification and compression of 100 fs femtosecond pulses to 10 watts were demonstrated. These femtosecond fiber lasers are packaged in compact and robust modules and passed long term operation test without any degradation, and proved to be reliable light sources for clinic applications.
Micro- and nano-processing are performed by using a high energy mode-locked femtosecond (fs) fiber laser. The laser beam has 1030 nm center wavelength, 750 fs pulse duration and up to 10 μJ pulse energy. Firstly, direct writing of optical waveguide inside glass materials is presented and a 3-D curved waveguide is demonstrated. Secondly, by taking advantages of fs laser deterministic damage threshold, micrometer and sub-micrometer features are obtained for surface ablation. Thirdly, fs UV (FHG, 258 nm) laser processing is investigated and nanometer features are obtained. Periodic structures in good order are also found and the patterns extend coherently over many overlapping laser pulses and scanning tracks. It has 100 nm period, 50 nm width and 50 nm depth. Such micro and nano processing method suggests a possible technique to produce nanogratings, microelectronics, or nanopatterned surfaces of micro-sensors for space optoelectronics.
This paper reports the LIBS studies on elemental composition detection and identification by employing a femtosecond
(fs) fiber laser. High quality LIBS spectra were obtained in air using near-infrared fs fiber laser coupled with a
broadband high sensitivity spectrometer without gating control. Specific ion and neutral emission lines of different
materials have been characterized by line scanning, including metals, glasses and even explosive materials. Different
laser parameters including pulse energy, repetition rate, scanning speed and integration times have been investigated to
optimize the sensitivity. Results show that faster scanning speed and higher pulse energies can greatly enhance the signal
level and reduce the integration time. The LIBS spectra are highly reproducible at different repetition rates up to 1 MHz.
Furthermore, detection of explosive materials was also achieved and both the constituent elemental emission and the CN
and C2 molecules emission were collected. Compared with conventional LIBS, fs fiber laser based LIBS system have
advantages of less sample heating and damage, better spatial resolution and signal to background ratio, compact, reliable
and cost-effective. This shows a potential portable LIBS system for versatile and rapid analysis of chemical and special
Development of techniques for joining and welding materials on a micrometer scale is of great importance in a number
of applications, including life science, sensing, optoelectronics and MEMS packaging. In this paper, methods of welding
and sealing optically transparent materials using a femtosecond fiber laser (1 MHz & 1030 nm) were demonstrated
which overcome the limit of small area welding of optical materials from previous work. When fs laser pulses are tightly
focused at the interface of the materials, localized heat accumulation based on nonlinear absorption and quenching occur
around the focal volume, which melts and resolidifies, thus welds the materials without inserting an intermediate layer.
The welding process does not need any preprocessing before the welding. At first, single line welding results with
different laser parameters was studied. Then successful bonding between fused silica with multi line scanning method
was introduced. Finally, complete sealing of transparent materials with fs laser was demonstrated. Scanning electron
microscopy (SEM) images of the sample prove successful welding without voids or cracks. This laser micro-welding
technique can be extended to welding of semiconductor materials and has potential for various applications, such as
optoelectronic devices and MEMS system.
In the paper, a 2 μm high energy fs fiber laser is presented based on Tm doped fiber at a low repetition
rate. The seed laser was designed to generate pulse train at 2 μm at a pulse repetition rate of 2.5 MHz.
The low repetition rate seed oscillator eliminated extra devices such as AO pulse picker. Two-stage
fiber amplifier was used to boost pulse energy to 0.65 μJ with chirped pulse amplification.
Focused femtosecond laser pulses from a 1 MHz fiber laser were used to create modifications in Er-
Yb doped zinc phosphate glass. Two glasses with similar phosphate glass networks but different
network modifiers were investigated. To understand the resulting changes caused by the
femtosecond laser pulses various characterization techniques were employed: glass structural
changes were investigated with confocal Raman spectroscopy, defect generation as well as local Er
and Yb environment were investigated with confocal fluorescence spectroscopy, and elemental
segregation resulting from heat accumulation effects was ascertained by scanning electron
There is a great deal of interests and efforts in the area of femtosecond (fs) laser direct writing of transparent materials,
which shows promise to be a powerful and flexible technique for rapid fabrication of photonic micro-device, such as
gratings, waveguides and optical amplifiers. Waveguide properties depend critically on the sample material properties
and writing laser characteristics. In this paper, we present results on the micro-fabrication of waveguide and photonic
micro-devices using fs fiber laser direct writing technique. Single line writing of different types of glasses with respect to
the focused laser beam at different pulse energies and writing speeds has been investigated at first. Then the waveguide
properties were characterized in terms of their shapes and transmission. It was found that specific consideration of the
pulse energy, repetition rate and writing speed should be taken into account in order to fabricate low loss positive index
guiding waveguide devices in a specific type of glass. Furthermore, a coupler-like guiding structure in glasses has also
been demonstrated. The modified regions in both waveguides were checked by scan electron microscope (SEM) to
reveal possible cracks and non-refractive structural defects. This technique can be used to produce micro photonic
devices and applied to fabricate a single glass chip 3D photonic devices.
An ultra-low repetition rate high energy 200 ns Er:Yb co-doped fiber laser has been developed by using a master
oscillator power amplifier configuration at an eye-safe 1.53 μm wavelength . A modulated pump scheme was used to
suppress ASE accumulation between pulse intervals. Combined with pulse shaping technology to mitigate pulse
narrowing effect and SBS effect, a maximum of 480 μJ pulse energy was obtained. In the stable, long-term running
mode, pulse energy up to 204 μJ, were obtained with pulse durations of 200 ns at Hz level.
For broad applications in biomedical research involving functional dynamics and clinical studies, a photoacoustic microscopy system should be compact, stable, and fast. In this work, we use a fiber laser as the photoacoustic irradiation source to meet these goals. The laser system measures 45×56×13 cm3. The stability of the laser is attributed to the intrinsic optical fiber-based light amplification and output coupling. Its 50-kHz pulse repetition rate enables fast scanning or extensive signal averaging. At the laser wavelength of 1064 nm, the photoacoustic microscope still has enough sensitivity to image small blood vessels while providing high optical absorption contrast between melanin and hemoglobin. Label-free melanoma cells in flowing bovine blood are imaged in vitro, yielding measurements of both cell size and flow speed.
Fiber laser is becoming an enabling technology for coherent Lidar applications and free space communications due to its
high efficiency, compact size and reliable operation. In these applications, high extinction rate and high OSNR are
musts for a fiber laser. However, current Q-switched fiber lasers and solid state lasers cannot meet these criteria.
Innovative approach has to be conceived to meet the requirements.
In this paper, we will discuss our researches on high energy/power ns pulsed fiber lasers. Modulation schemes for seed
laser in getting various optical waveforms to accommodate pulse narrowing in high power amplification and reduce SBS,
high power operation of fiber amplifiers, nonlinearity mitigation in high power fiber lasers, and trade-offs among the
parameters such as OSNR, power/energy scaling, extinction ratio, interpulses background noise (contrast ratio), and
efficiency will be discussed.
Microjoule pulse energies are achieved from a single stage upconversion fiber amplifier for the first time in this demonstration of chirped pulse amplification using a multimode Tm:ZBLAN fiber. A Ti:sapphire laser system provides the seed pulse for the upconversion fiber amplifier which produces subpicosecond pulse trains with energies as great as 16 (mu) J at repetition rate of 4.4 kHz. The compressed pulse peak power is more than 1 MW, and the pulse is characterized both temporally and spatially.